Try to bend your hand backwards and touch your thumb to your forearm. If you can do this, you are “double jointed.” But what does that mean? Would you really have two joints where most people only have one? The answer is no. Instead, you could have really loose or stretchy ligaments holding your bones together, or you might have very shallow depressions (fossae) where bones fit into other bones. Then again, you might have a combination of these features. Being double- jointed simply means a person can move the bones at certain joints to a greater degree than most people can.
A joint, also called an arthrosis (ar-THRŌ- sis; arthr, joint), or articulation (ar-tik-ū-LĀ- shun; “fit together”), is where two or more bones come together, and the bones at a joint are said to articulate with one another. While you may have 206 bones in your skeleton, you have well over 200 joints because many of your bones articulate with more than one other bone. For example, the sphenoid bone that you learned about in the previous chapter articulates with fourteen other skull bones!
Joints serve three major purposes. First, some joints allow the skeleton to bend, making it possible to move parts of the body voluntarily. In fact, nearly everything you do voluntarily, including walking, chewing, and even turning pages in this book, requires bones to move at joints. Second, some joints allow adjacent bones to stick to one another so tightly that virtually no movement is possible. This is important in places where bones surround and protect soft internal organs. Third, some joints allow the skeleton to grow. Specifically, epiphyseal plates are special joints where growth of cartilage causes bones to grow.
While it is not practical to describe every joint in the skeleton, this chapter focuses on the structural design and functional roles of the major types of joints. Following an overview describing how joints are named and classified, we will examine each major type more closely. We will conclude by describing selected joints that play important roles in the movement of the upper and lower limbs.
NAMING AND CLASSIFYING JOINTS
Joints are injured more often than any other part of the skeleton, which make them a frequent topic of conversation in clinics and rehabilitation centers.
As a result, it is beneficial for anyone, but especially those pursuing a career in the health profession, to learn the names of joints. If you learned the names of bones in the previous chapter, you will undoubtedly recognize the names of most joints, because most joints are named for their articulating bones. For example, your radioulnar joint is between the radius and ulna in your forearm, and your tibiofibular joint is between the tibia and fibula in your leg. In addition to using names of articulating bones, some joints are more commonly known by their location. For instance, your glenohumeral joint, which connects the glenoid fossa of your scapula to the head of your humerus, is better known as your shoulder joint. A few joints have names based on guidelines other than their articulating bones or location. The sagittal suture (SAJ-i-tal; arrow), the joint between the two parietal bones of the skull, is named for its orientation along the body’s sagittal plane. Another joint, the lambdoidal suture (lam- DOYD-al), which is located between the occipital bone and parietal bones, is so named because it looks like lambda (Λ), the 11th letter in the Greek alphabet.
In addition to naming joints, we can classify them according to (1) their structural design, and (2) the amount of movement possible for the articulating bones.
Classification of Joints by Structure
To understand how different types of joints serve different functions, we need to know about their anatomy. Two characteristics to consider when describing the general structure of a joint include (1) the presence or absence of a fluid-filled joint cavity, the synovial cavity, which encloses the bone’s articular surfaces, and (2) the type of connective tissue holding the articulating bones together. Depending on their general structure, joints are fibrous, cartilaginous, or synovial.
- Fibrous joints do not have a synovial cavity, but they are filled with dense, fibrous con- nective Consequently, the articulating bones united at fibrous joints do not have “free” (unbound) articular surfaces.
- Cartilaginous joints (kar-ti-LAJ-e-nus) lack a synovial cavity, but they are filled with cartilage connective tissue. Like at fibrous joints, the articulating bones at cartilaginous joints do not have free articular surfaces.
- Synovial joints (si-NŌ-vē-al) have a fluid- filled cavity between their articulating bones, meaning each bone has a free articular surface at the joint.
Over time, certain fibrous joints and cartilaginous joints ossify; that is, osseous tissue replaces the flexible connective tissue between the articulating bones. After this happens, the joint is called a synostosis (sen-o-STŌ-sis; syn, together; osis, condition), or bony joint. In short, a synostosis fuses two bones to make one bone. The cartilaginous epiphyseal plate in a developing long bone ultimately becomes a synostosis, after which time we call the joint an epiphyseal line.
CLASSIFICATION OF JOINTS BY MOVEMENT
In order for a person to perform a variety of activities, the skeleton needs different types of joints with varying degrees of mobility. A joint’s mobility refers to how much movement the articulating bones can experience at the joint. Depending on its range of mobility, we will classify a joint as a synarthrosis, amphiarthrosis, or diarthrosis.
- Synarthrosis (sen-ar-THRŌ-sis): an immov- able In reality, no joint is completely immovable, but in synarthroses, the amount of bone movement is negligible. As the pre- fix “syn-” implies, the articulating bones are virtually “together.” Structurally, synarthrotic joints are either fibrous or cartilaginous. In either case, connective tissue virtually fills the gap between the bones, preventing them from moving freely. In fibrous synarthroses, the collagen fibers are too short to allow much bone movement. In cartilaginous syn- arthroses, the binding connective tissue is hyaline cartilage, which lacks the flexibility of fibrocartilage.
- Amphiarthrosis (am-fē-ar-THRŌ-sis; amphi, both sides): slightly movable Like synarthrotic joints, amphiarthrotic joints are filled with either fibrous or cartilaginous connective tissue. In fibrous amphiarthroses, the collagen fibers are longer and more flexible than those in fibrous synarthrotic joints. In cartilaginous amphiarthroses, the binding tissue is fibrocartilage, which is more flexible than hyaline cartilage found at cartilaginous, synarthrotic joints.
- Diarthrosis (dī-ar-THRŌ-sis): freely movable Unlike articulating bones at fibrous and cartilaginous joints, the articulating bones at diarthroses have a “free” articular surface. Instead of being “glued” together tightly by fibrous tissue or cartilage, the articular surfaces at diarthroses are enclosed within a fluid-filled cavity, the synovial cavity. In turn, the synovial cavity allows the articular surfaces of the opposing bones room to move freely. On the other hand, tight bands of connective tissue around the synovial cavity may in fact greatly restrict the ability of the articulating bones to move. Consequently, not all diarthroses are equally movable.
ANATOMY OF FIBROUS JOINTS
Fibrous joints are the simplest joints in terms of structural design and mobility. They contain dense connective tissue consisting of fibroblasts and numerous collagen fibers. The tough fibers hold the articulating bones together tightly, restricting their movement. The skeleton contains three types of fibrous joints: sutures, gomphoses, and syndesmoses.
Sutures
A suture (SŪ-chur; seam) is a type of fibrous joint found only in the skull. Sutures are so named because most of them look like zigzag seams (stitches) made with a sewing machine. The fibrous connective tissue in a suture contains short collagen fibers called sutural ligaments. These tiny ligaments firmly anchor the skull bones to one another, preventing them from moving; therefore, sutures are synarthrotic. Sutures allow flat cranial bones to remain in place so they can form a rigid wall of protection around the delicate brain.
Sutures form when the growing edges of skull bones meet during intramembranous ossification. The fibrous tissue within a suture is the remnant of a fibrous “membrane” that served as a model for the developing skull bones. The skull bones originate as ossification centers, and grow as osseous tissue is laid down along a leading edge. As the bones grow, the soft fibrous membrane becomes smaller. At birth, fontanels (“soft spots”) are all that remains of the membranous skull model, and within a few years, even they become fibrous sutures.
Not all sutures look alike; most have jagged edges, but others have relatively straight edges. An example of a jagged suture is the sagittal suture, located along the sagittal plane between the right and left parietal bones. Here, the irregular edges of the parietal bones interlock, preventing them from shifting anteriorly or posteriorly. You can demonstrate this effect by clasping your hands and interlocking your fingers. It is more difficult to pull your hands apart now than it is when your hands are simply pressed together flat. An example of a relatively straight suture is the internasal suture, located between the right and left nasal bones.
While most sutures remain fibrous throughout life, some sutures found in children eventually become synostoses. For example, an infant has a temporary frontal (metopic; me-TOP-ik, “forehead”) suture between right and left frontal bones. In early childhood, however, the frontal suture usually becomes a synostosis, after which the child has only one frontal bone.
Gomphoses
A gomphosis (gom-FŌ-sis; gomph, bolt) is a fibrous joint found only between a tooth and an alveolar socket of the maxilla or mandible. The anatomical name for this particular joint is the dento-alveolar joint. A gomphosis is so named because the tooth fits into the alveolar socket like a bolt fits into a hole in a piece of wood. The fibrous tissue in a gomphosis consists of extremely short collagen fibers called periodontal ligaments. These ligaments connect the periosteum of the jawbone to the outer covering of the tooth’s root (the part beneath the gum line). Since teeth typically do not move within their sockets, gomphoses are synarthrotic.
Syndesmoses
A syndesmosis (sen-dez-MŌ-sis; desmo, bind) is a fibrous joint in which collagen fibers hold two parallel bones together. This is like holding two pencils together side by side with rubber bands. The collagen fibers in a syndesmosis are significantly longer than those in a suture or gomphosis, therefore the articulating bones in a syndesmosis are slightly movable (amphiarthrotic).
One type of syndesmosis found between the distal ends of the tibia and fibula is the cordlike tibiofibular ligament. The collagen fibers in this ligament are arranged in bundles, which helps the ligament to resist stretching. Consequently, very little movement is permitted at this joint. Essentially, the tibiofibular ligament allows the tibia and fibula to experience compression, tension (pulling), and torsion (twisting) forces without their distal ends separating. This is like holding the two pencils together with a single rubber band at one end.
Syndesmoses are also found between the diaphyses (shafts) of the tibia and fibula, and between the diaphyses of the radius and ulna. Here, parallel collagen fibers span the gap between the diaphyses to form an interosseous membrane. This membrane-like syndesmosis provides stability to the forearm and leg by preventing their long bones from separating from one another during normal activity. This is like holding the pencils together with many rubber bands along the entire length of the pencils.
ANATOMY OF CARTILAGINOUS JOINTS
Cartilaginous joints are only slightly more complex than fibrous joints, and are filled with either hyaline cartilage or fibrocartilage. The type of cartilage present determines the amount of movement possible at these joints. The skeleton has two types of cartilaginous joints: synchondroses and symphyses.
Synchondroses
A synchondrosis (sen-kon-DRŌ-sis) is sometimes referred to as a temporary joint, because later in life most synchondroses are replaced by bone tissue; that is, they become synostotic. In synchondroses, a layer of hyaline cartilage “glues” articulating bones together, sort of like a layer of hardened mortar holds bricks together. Like the mortar, which prevents the bricks from moving, the hyaline cartilage holds the bones together so tightly that virtually no movement occurs; for this reason, synchondroses are synarthrotic.
The most common example of asynchondrosis is an epiphyseal plate, or growth plate, located between the epiphysis and diaphysis of a developing bone. Bone growth occurs as the new cartilage is added to the epiphyseal side of the growth plate. However, ossification occurs continually along the diaphysis side of the plate, until eventually the epiphyseal plate becomes solid bone; that is, a synostosis (bony joint). The 1st sternocostal joint, located between the first rib and the manubrium, is also a synchondrosis. This rigid joint can be compared to the joint between a coffee cup and its handle, and in the elderly, it usually becomes synostotic.
Symphyses
Symphyses (SIM-fi-sēz) contain a pad of fibrocartilage that protects articulating bones against compression forces. Fibrocartilage is extremely tough, yet it is more flexible than hyaline cartilage. Thus, a pad of fibrocartilage wedged in between two bones can work like a pillow on a chair. While the pillow can act as a shock absorber when you sit, it doesn’t prevent you from changing your seating position on the chair. Likewise, a pad of fibrocartilage can cushion adjacent bones without totally restricting bone movement.
The slight amount of movement at symphyses classifies these joints as amphiarthrotic.
Most symphyses are intervertebral joints, located between the bodies of vertebrae in the vertebral column. These joints are filled with a fibrocartilaginous pad called an intervertebral disc. The disc contains a soft, jelly-filled center called the nucleus pulposus, which is a remnant of the notochord present during embryonic development. The nucleus pulposus is surrounded by the annulus fibrosus, which is made of tough fibrocartilage. Intervertebral discs cushion the vertebrae against compression forces when the spine is in an upright position. This cushioning role is especially important during walking and running, when compression increases dramatically.
The only other example of a symphysis is the pubic symphysis, located between the right and left pubic bones in the pelvis. This joint contains a fibrocartilaginous pad called an interpubic disc. During walking and running, the femurs send compression shockwaves through the pelvis, but the interpubic disc prevents the pubic bones from crushing each other.
ANATOMY OF SYNOVIAL JOINTS
Synovial joints are the most complex joints, but they come in an assortment of styles, making it possible for the skeleton to have a wide range of different types of movement. We will first examine the structural features that all synovial joints have in common. Features of synovial joints include articular cartilages, a joint cavity, an articular capsule, and synovial fluid (Figure 12-1).
Articular Cartilage
The articular cartilage is a thin layer of cartilage that covers the end of an articulating bone at a synovial joint. The smooth surface of the articular cartilage helps reduce friction as the cartilages of opposing bones slide past one another. Articular cartilages are also resilient, able to spring back to their original shape after being compressed; in essence, articular cartilages can serve as springy shock absorbers. This function is especially important in the hip, knee and ankle joints, where compression forces increase then decrease repeatedly when a person walks or runs.
In most synovial joints, the articular cartilage is hyaline cartilage, but in a few joints, such as the temporomandibular joint, it is more fibrous, much like fibrocartilage. Most cartilaginous tissue in the body receives oxygen and nutrients from blood vessels in the perichondrium, a membrane on the surface of the cartilage. Articular cartilages, however, are not covered with perichondrium. Consequently, their chondrocytes rely on synovial fluid and blood vessels in the synovial membrane for their nutrients.
Joint Cavity
The joint (synovial) cavity is a virtual space between the ends (articular cartilages) of the articulating bones. The joint cavity is a “virtual” space, because unlike a true cavity, which is empty, the joint cavity is filled with fluid, the synovial fluid. The fluid-filled joint cavity provides room in which the articulating bones at synovial joints can move freely. Only synovial joints have a joint cavity (recall that fibrous and cartilaginous joints are filled with connective tissue, which restricts bone movement).
Figure 12-1
Articular Capsule
The articular capsule is a double-layered container that encloses the joint cavity. The capsule’s superficial layer, or fibrous capsule, consists of dense irregular connective tissue that is continuous with the periosteum of each articulating bone. The tough collagen fibers within the fibrous capsule help prevent the articulating bones from pulling too far apart. The deep layer of the articular capsule is the synovial membrane. It consists mostly of areolar connective tissue, and its cells include fibroblasts, macrophages, mast cells, and adipocytes. The synovial membrane is thin and pink, and covers the deep surface of the fibrous capsule, but it terminates at the articular cartilages. The primary function of the synovial membrane is to secrete synovial fluid into the synovial cavity.
The articular capsule is the site where the cardiovascular and nervous systems interact with a synovial joint. An interlacing network of small blood vessels envelops the articular capsule, while smaller vessels from this network penetrate the fibrous capsule. Internally, the tiny vessels spread out between the fibrous capsule and synovial membrane. Nerves penetrate the fibrous capsule and respond to several types of stimuli. First, tissue damage causes pain signals to be sent to the brain, which lets you know something is not right with the joint. Second, stretching a fibrous capsule initiates nerve signals that inform your brain about which joints are experiencing tension. In turn, the brain uses this information to decide which muscles need to contract to help you maintain your balance.
The articular capsule is a double-layered container that encloses the joint cavity. The capsule’s superficial layer, or fibrous capsule, consists of dense irregular connective tissue that is continuous with the periosteum of each articulating bone. The tough collagen fibers within the fibrous capsule help prevent the articulating bones from pulling too far apart. The deep layer of the articular capsule is the synovial membrane. It consists mostly of areolar connective tissue, and its cells include fibroblasts, macrophages, mast cells, and adipocytes. The synovial membrane is thin and pink, and covers the deep surface of the fibrous capsule, but it terminates at the articular cartilages. The primary function of the synovial membrane is to secrete synovial fluid into the synovial cavity.
The articular capsule is the site where the cardiovascular and nervous systems interact with a synovial joint. An interlacing network of small blood vessels envelops the articular capsule, while smaller vessels from this network penetrate the fibrous capsule. Internally, the tiny vessels spread out between the fibrous capsule and synovial membrane. Nerves penetrate the fibrous capsule and respond to several types of stimuli. First, tissue damage causes pain signals to be sent to the brain, which lets you know something is not right with the joint. Second, stretching a fibrous capsule initiates nerve signals that inform your brain about which joints are experiencing tension. In turn, the brain uses this information to decide which muscles need to contract to help you maintain your balance.
Synovial Fluid
Synovial fluid is a clear to pale yellow fluid that fills the joint cavity. This fluid consists mostly of filtered blood plasma that leaks out of blood vessels in the synovial membrane. In fact, synovial joints are so named because its fluid looks and flows like uncooked egg white. However, the consistency of synovial fluid is negatively affected by temperature; that is, when the fluid gets warmer, it becomes less viscous. For this reason, synovial fluid circulates more freely after the articulating bones have been moving for a while and generating heat. Synovial fluid serves five important roles:
At the two most freely movable joints, the shoulder and hip, a fibrocartilaginous lip called a labrum (LĀ-brum; “lip”) helps hold a long bone within a deep socket. In the shoulder, the glenoid labrum helps hold the humeral head within the glenoid fossa of the scapula. In the hip, the acetabular labrum helps hold the femur’s head within the deep acetabulum of the pelvis. A good analogy for the way a labrum holds the long bone’s head within the socket is a button-type snap on a shirt or the small snaps on the back of an adjustable baseball cap.
- Lubricates moving parts: Synovial fluid acts as a lubricant that reduces friction inside the joint In the same way that motor oil lubricates the gears of a car’s engine, synovial fluid lubricates articular cartilages and other structures within the synovial joint. As modi- fied blood plasma, synovial fluid is mostly water, which is, by itself, a good lubricant. In addition, however, fibroblasts in the synovial membrane add organic compounds to the flu- id. The most important of these compounds is hyaluronic acid, a glycosaminoglycan, which makes the synovial fluid very slippery and reduces friction even more.
- Nourishes cartilages. Cartilages in a synovial joint are avascular, which means they must be nourished by nutrients diffusing into the cartilage from the surrounding Since synovial fluid is derived from nutrient-rich blood plasma, it can serve as an important source of nutrients for the cartilages. Articu- lar cartilages are very porous and can act like a sponge to absorb the synovial fluid. Syno- vial fluid enters cartilage fastest when the cartilage is expanding after being squeezed, much as an expanding sponge absorbs water. Compression, on the other hand, squeezes sy- novial fluid out of the cartilage and back into the synovial cavity like wringing a sponge; this phenomenon is called weeping lubrica- tion.
- Dissipates heat. Since synovial fluid is mostly water, it has a high specific heat. This means synovial fluid can absorb significant quantities of heat without a significant increase in temperature. Joints heat up as articulating bones move, but the circulating synovial fluid quickly absorbs and dissipates how circulating coolant keeps a car engine from overheating. Just imagine, without synovial fluid, a marathon runner’s knees might burst into flames before the end of the race! (Not really.)
- Distributes compression forces. Fluids cannot be compressed; therefore, when compression forces are exerted on synovial fluid, the forces are dispersed to all surfaces in contact with the fluid.
- Keeps the joint clean: Macrophages from the synovial membrane may enter the synovial fluid, where they phagocytize foreign par- ticles, such as bacteria, and bits and pieces of joint structures that break free.
Accessory Structures at Synovial Joints
Inaddition to the structural features just described, some synovial joints have accessory structures that either stabilize the joint or cushion the bones. Accessory structures found at certain synovial joints include ligaments, tendons, labra, menisci, articular fat pads, bursae, and tendon sheaths.
Ligaments
A ligament is a band of dense regular connective tissue that holds articulating bones together at a joint. Ligaments work sort of like suspenders hold- ing up a loose pair of pants. Without suspenders, the pants wouldn’t stay up; without ligaments, some articulating bones wouldn’t stay together.
Within the fibrous capsule of certain synovial joints, collagen fibers collect into thick bundles called capsular (intrinsic) ligaments. Capsular ligaments are loose while in anatomical position, but become taut whenever the fibrous capsule stretches as the bones move. In this way, intrinsic ligaments strengthen the fibrous capsule, preventing it from splitting open. In addition, synovial joints that allow a significant amount of bone movement have extrinsic ligaments that are not part of the fibrous capsule. These include extracapsular ligaments, which are superficial to the fibrous capsule, and intracapsular ligaments, which are deep to the fibrous capsule. Extrinsic ligaments prevent the articulating bones from moving in awkward directions that could injure the joint.
Tendons
A tendon is a narrow band of dense regular connective tissue that attaches a muscle to a bone. In order for a muscle to move a bone, the muscle’s tendon must cross a joint. Because they stretch over joints, tendons can also act like suspenders to keep articulating bones together.
Labra
At the two most freely movable joints, the shoulder and hip, a fibrocartilaginous lip called a labrum (LĀ-brum; “lip”) helps hold a long bone within a deep socket. In the shoulder, the glenoid labrum helps hold the humeral head within the glenoid fossa of the scapula. In the hip, the acetabular labrum helps hold the femur’s head within the deep acetabulum of the pelvis. A good analogy for the way a labrum holds the long bone’s head within the socket is a button-type snap on a shirt or the small snaps on the back of an adjustable baseball cap.
Menisci and Fat Pads
Synovial joints that withstand intense compression may contain one or more accessory structures that cushion the articular surfaces of the bones. Menisci (me-NIS-sī; meniscus, crescent), or articular discs, are crescent-shaped discs of fibrocartilage that allow opposing articulating bones to fit together more evenly. The fibrocartilaginous discs are like the rubber foam insoles that make your foot and shoe fit together more evenly. Menisci reduce friction and distribute the compression forces over a greater area of the joint. Like the articular cartilages, menisci are not covered by the synovial membrane, but they are lubricated and nourished by weeping lubrication of synovial fluid. Menisci are found in the knee joints, temporomandibular joints, sternoclavicular joints, acromioclavicular joints, and the distal radioulnar joints.
In many joints, adipocytes gather around certain parts of the synovial membrane and form globules of adipose tissue called articular fat pads. Like menisci, fat pads act as springy cushions and help distribute compression forces across the articular surfaces. Moreover, stored triglycerides within the fat pads serve as a source of cellular energy.
Bursae and Tendon Sheaths
Some joints are so movable that they need ac- cessory structures that can reduce friction and minimize the wear and tear on the bones and on the surrounding tissues. Bursae (BER-sē; bursa, purse) are flattened, fluid-filled pouches located between a bone and the overlying skin, ligament, tendon, or muscle at certain joints. Think of a bursa as a mushy, water-filled balloon you can hold between your hands. You can roll the balloon back- and-forth, deforming it in many directions, without causing much friction on either hand. Likewise, a bursa can deform in different directions to reduce friction on a bone and another structure. A bursa has a fibrous outer covering that is lined internally with a thin synovial membrane. The bursa’s syno- vial membrane secretes fluid into the bursa cavity.
At certain joints, bursa-like structures called tendon sheaths reduce the friction between tendons and bones. The tendon sheath envelops the tendon, as a scabbard envelops a sword, (the word sheath translates “scabbard”). Whereas a bursa is flattened, a tendon sheath is more tubular, like a soda straw. Tendon sheaths are found in the wrist and ankle, where tendons and bones press tightly against one another. As a muscle contracts and relaxes, its tendon moves back-and-forth at a joint, but the tendon sheath prevents it from rubbing on bones.
JOINT STABILITY AND RANGE OF MOTION
While the shapes of articular surfaces influence the type of movement possible at a synovial joint, they also affect the joint’s stability and its range of motion. Joint stability, or a joint’s strength, refers to the joint’s ability to function normally without having its articulating bones pull too far apart. A joint’s range of motion is the maximum distance that the opposing articular surfaces can move. Considering all joints, there is generally an inverse relationship between range of motion and stability. Fibrous and cartilaginous joints are very stable, but they have limited ranges of mo- tion. In contrast, synovial joints are less stable, but they have the greatest range of motion. Three factors affect the stability and range of motion at a synovial joint:
- Shape of articular surfaces. Joints in which the articulating surfaces interlock are more stable than joints where bones make contact along relatively flat surfaces. For example, the rounded head of the femur and the ac- etabulum of the pelvis lock together, much like pieces of a jigsaw puzzle. But unlike the pieces of a puzzle, which remain rigid, the head of the femur can move In fact, the ball-and-socket design of the hip joint allows the femur to move in many directions without separating from the acetabulum. Bones with flattened articular surfaces also tend to have a more restricted range of motion than bones that interlock. Flattened surfaces may slide past one another easily, but movements that cause the bones to separate will also cause associated ligaments to tighten and limit the range of motion.
- Tension of ligaments. The stability of a joint is positively correlated with the tautness (tight- ness) of its accessory Compare this to the tautness of large mooring ropes that anchor a ship to a dock. If the ropes are too loose or too stretchy, the ship can drift away from the dock. Likewise, a loose ligament may allow the articulating bones to move too far apart or move in awkward directions. Although tighter ligaments give a joint more stability, they can restrict the joint’s range of motion, just like tight mooring ropes prevent a ship from moving.
- Tension in tendons. The stability of a joint is also positively correlated with the tautness of tendons that cross the joint. When a skeletal muscle contracts, the tendon connecting it to a bone develops The harder the muscle pulls on a bone, the greater the tension in the muscle’s tendon. This is similar to pulling on the ship’s mooring ropes in order to tighten them. Indeed, pulling on tendons hard enough causes bones to move. However, maintaining a lesser amount of tension on tendons while the bones are not moving can help hold the bones together. Even at rest, skeletal muscles are in a slight state of contraction known as muscle tone. In turn, muscle tone maintains tension on tendons, allowing the tendons to function like ligaments. You can increase the stability of a synovial joint by strengthening (increasing the tone of) the muscles that have tendons crossing that joint.
CLASSIFICATION OF SYNOVIAL JOINTS
While the shapes of articular surfaces influence the stability of a synovial joint, they also determine the type of bone movement possible at the joint. We can classify a synovial joint into four types, according to the number of stationary axis lines around which its articular surfaces can move.
- Uniaxial joints (ū-nē-AKS-sē-ul; uni-, single), or monaxial joints (mon-AKS-sē-ul; mono-, one), allow movement around one axis. For example, folding a piece of paper in half forms a crease that acts as a single stationary axis along which you can fold the paper back-and- forth.
- Biaxial joints (bī-AKS-sē-ul; bi-, two) allow movement around two For example, fold a piece of paper lengthwise and crosswise and it forms two creases intersecting at right angles. Now you have two axes along which you can fold the paper back and forth.
- Multiaxial joints (multi-, many) allow move- ment around three axes; for this reason, mul- tiaxial joints are sometimes called triaxial joints (trī-AKS-s-ul; tri-, three). Stick a pencil through the center of the paper just described and you will have a third axis around which to rotate or tilt the paper.
- Nonaxial joints allow a sliding motion; there- fore, the movement does not revolve around an Simply sliding two sheets of notebook paper past one another can be performed in several directions without being oriented around a crease (“axis”).
In addition to classifying synovial joints according to the type of movement possible, we also classify them according to their basic structural design. The structural design is based on the shape of the articular surfaces. It includes plane, hinge, pivot, condylar, saddle, and ball-and- socket joints.
Plane Joints
At a plane joint, two relatively flat articular surfaces can slide-back and-forth past one another. The sliding can be in any direction, but along one sectional plane. (Remember a sectional plane is like a piece of paper, while an axis is like a line drawn on the paper). Since there is no single axis line around which the movement occurs, we classify plane joints as nonaxial. Examples of plane joints include the intercarpal (between carpals), intertarsal (between tarsals), acromioclavicular (between the acromion process and clavicle), sternoclavicular (between the sternum and clavicle), vertebrocostal (between a vertebra and rib), and sacroiliac joints (between the sacrum and ilium).
Hinge Joints
A hinge joint has a convex, bony process (a condyle) of one bone interacting with a concave surface (fossa) of another bone. Movement of the articulating bones is possible along one axis; therefore, hinge joints are uniaxial. Hinge joints are so named because they work like the hinge of a door. Examples include the elbow, knee, and interphalangeal joints (between phalanges).
Pivot joints
A pivot joint has a knobby, bony projection fitting through a “ring” formed partly by a shallow de- pression of another bone and partly by a ligament. The rotating movement of the knobby projection is around one axis, like the paper spinning on the pencil in our model; therefore, pivot joints are uni- axial. Examples of pivot joints include the proximal radioulnar joint and the joint between the atlas (1st cervical vertebra) and the dens (projection on the axis or 2nd cervical vertebra).
Condylar joints
A condylar joint (KON-di-lar; conyl-, knuckle), or ellipsoidal joint (il-IP-soy-dul) unites a convex surface with an oval-shaped depression. Move- ment at this joint can occur along two axes, like our paper model folded two ways; therefore, the joint is biaxial. Examples include the distal radiocarpal joint (between the radius and several carpals), the 2nd-5th metacarpophalangeal joints (between the metacarpals and proximal phalanges), and the metatarsophalangeal joints (between the meta- tarsals and proximal phalanges).
Saddle joints
At a saddle joint, two convex parts on each articu- lating bone fit into concave parts on the opposing bone. You can show this by forming a “U” with each hand (fingers together opposing the thumb), then interlocking the hands. In this way, each hand fits into the other hand like someone sitting in a saddle. The articulating bones at a saddle joint can move along two axes; therefore, it is biaxial. Examples of saddle joints include the carpometacarpal joint of the thumb (between the trapezium and 1st metacar- pal) and the talocrural (between the talus and tibia).
Ball-and-socket joints
A ball-and-socket joint has a large, rounded knob (the head) of one bone fitting into a cuplike fossa of another bone. This type of joint allows the articulating surfaces to move along all three axes; therefore, it is multiaxial. The shoulder joint and the hip joint are examples of ball-and-socket joints.
BODY MOVEMENTS AT SYNOVIAL JOINTS
Knowing the names for different body movements is necessary to prevent misunderstandings when discussing the function of a joint. This is especially true for physical and occupational therapists who deal frequently with injured joints. We can classify the movement at a synovial joint into one of four major categories: gliding, angular, rotational, or special movement. Furthermore, several of these major groups include a number of more specific movements. See these movements illustrated in Figure 12-2 and read their descriptions that follow.
Figure 12-2. Movements at synovial joints
Figure 12-2. continued
Gliding Movement
In gliding joints, the bones may slide in any direction (side-to-side and back-and-forth), but the angle between the articular surfaces does not change; for this reason, gliding movement is uniaxial. As a comparison, think of a hockey puck and the ice beneath it as two articulating bones. The hockey puck can slide in any direction on the ice while remaining flat and the angle between the bottom of the puck and the surface of the ice does not change. However, the puck moves in only one plane, in this case, along a horizontal surface. Gliding movements occur at intercarpal joints and intertarsal joints. These subtle movements aren’t easily noticed because they occur at the same time more obvious angular movements occur at the wrist and ankle.
Angular Movement
Unlike gliding movements, angular movements may occur in one or more planes. The directional terms we use to describe angular movements are in reference to anatomical position. The major types of angular movement include flexion, exten- sion, hyperextension, abduction, adduction, and circumduction.
Flexion, Extension, and Hyperextension
Flexion (FLEX-shun; flex-, to bend) occurs when the angle between articulating bones decreases in the sagittal (anterior-posterior) plane. During flexion, the nonarticulating ends of these bones move closer together. For example, if you bend your elbow joint to move your forearm anteriorly from anatomical position, you are flexing your forearm. As this occurs, the angle between your arm bone (humerus) and your forearm bones (radius and ulna) decreases. In addition, the distal ends of your forearm bones move closer to the proximal end of your humerus. Other examples of flexion include bending your fingers to make a fist, and curling your toes beneath your soles. No joints are flexed while the body is in anatomical position.
Extension (eks-TEN-shun; exten-, to stretch) is the opposite of flexion, and increases the angle between articulating bones that are flexed. During extension, the nonarticulating ends of these bones move farther apart. If you straighten your elbow joint to return your forearm to anatomical position after it has been flexed, you are extending your forearm. While this happens, the distal ends of your forearm bones move farther away from the proximal end of your humerus. All extendable joints are extended when the body is in anatomical position.
Hyperextension occurs when you extend a bone beyond its anatomical position. For example, flexing your neck tilts your head forward allowing you to look at your toes; extending your neck raises your head allowing you to look straight ahead (in anatomical position); hyperextending your neck tilts your head backward allowing you to look straight up. While it is a normal movement at some joints, hyperextension at other joints is beyond the normal extension limit. For example, hyperextension is the most common cause of damage to the elbow and knee joints.
Abduction and Adduction
Abduction (ab-DUKT-shun; abduct-, take away) occurs when a bone moves away from its midline along a coronal (frontal) plane. Abducting the humerus raises the arm laterally. Spreading your fingers also involves abduction. In this case, digits 1 and 2 (thumb and index finger, respectively) move laterally, while digits 4 and 5 (ring finger and pinky) move medially.
Adduction (ad-DUKT-shun; adduct-, bring toward) is the opposite of abduction, and moves a bone toward its midline along a coronal plane. In other words, adduction moves the bone back to its anatomical position after abduction. Lowering your arm back to your side or pulling your spread fingers back together are examples of adduction.
Circumduction
Circumduction (sir-kum-DUC-shun; circ-, circle) occurs when the distal end of an appendage moves in a circular manner. If you move your arm like a spinning propeller, you are circumducting your humerus. Circumduction combines flexion, abduction, extension and possibly hyperextension, and adduction.
Rotational Movement
This is similar to turning a key in a lock. Rotation (rō-TA-shun; “to revolve”) of the limbs may occur toward the body’s midline or away from the midline. The anterior surface of the bone serves as a starting point of reference. For example, if you twist the anterior side of your right arm to the right (laterally), you are performing lateral rotation of your right humerus. If you twist the anterior side of your left arm to the right (medially), you are performing medial rotation of your left humerus. Turning your head or trunk involves right rotation and left rotation, depending on which direction the face or chest turns from anatomical position.
Special Movements
Special movements include those that do not easily fit into the descriptions of joint movements described earlier. We will describe opposite movements together.
Depression and Elevation
Depression (dē-PRESH-un; “push down”) occurs when a bone moves inferiorly, such as when you lower your mandible to open your mouth. You depress your scapulae when lowering your shoulders. Elevation (el-e-VĀ-shun; “lift up”) is the opposite of depression and occurs when a bone moves superiorly. You elevate your mandible to close your mouth, and you elevate your scapulae when shrugging your shoulders.
Protraction and Retraction
Protraction (prō-TRAK-shun: “draw forth”) occurs when a bone moves anteriorly along a transverse plane. You protract your mandible when you stick out your lower jaw, and you protract your scapulae (shoulderblades) when you reach out to hug someone. Retraction (rē-TRAK-shun; “draw back”) is the opposite of protraction, and occurs when a bone moves posteriorly along a transverse plane. Retraction returns a protracted bone to its anatomical position.
Excursion
Excursion (eks-KUR-shun; “run out”) is a side-to- side movement of the mandible. Lateral excursion occurs when the mandible moves laterally; medial excursion occurs when the mandible moves medially to return to anatomical position. Protraction, retraction, and excursive movements of the mandible are common movements when chewing.
Pronation and Supination
Pronation (prō-NĀ-shun; “bending forward”) occurs only when the forearm twists so that the palm faces posteriorly. Pronation causes the shaft of the radius to cross over the shaft of the ulna. Supination (sū-pi-NĀ-shun; “turning backward”) is the opposite of pronation and occurs when the forearm twists to make the palm face anteriorly. The forearm is supinated in anatomical position, and the radius and ulna are parallel to one another. If the elbow is flexed, pronation turns the palm down, and supination turns the palm up. Remember supination allows you to hold soup in your palm (the first three letters in SUP-ination are pronounced like “soup”).
Lateral flexion
Lateral flexion is a bending of a segment of the vertebral column in a coronal plane away from the body’s midline. Tilting your head laterally to bring your ear closer to your shoulder is an example of lateral flexion.
Inversion and Eversion
Inversion (in-VER-shun; “turn inward”) is a foot movement, and occurs when bending the ankle joint turns the sole in a medial direction, toward the body’s midline. In effect, inversion is adduction of the foot. Excessive inversion is the cause of most ankle sprains. Eversion (ē-VER-shun; “turn outward”) is the opposite of inversion and occurs when your sole turns laterally, away from the midline. In effect, eversion is abduction of the foot.
Dorsiflexion and Plantar flexion
Dorsiflexion (dor-si-FLEK-shun; “bend upward”) is a foot movement, and occurs when bending the ankle elevates the anterior aspect of the foot. This action causes you to stand on your heels. Plantar flexion is the opposite of dorsiflexion and occurs when bending the ankle elevates the heel; that is, when you stand on your toes.
Opposition and Reposition
Opposition (op-ō-ZISH-un; “go against”) is a thumb movement in which the thumb moves anteriorly and medially to touch the ends of the other digits on the same hand. Recall that all primates, including humans, have an opposable thumb that allows the hand to grasp and hold objects more effectively. Reposition (rē-pō-ZISH- un; “replace”) is the movement of the thumb to its anatomical position following opposition.
CHARACTERISTICS OF SELECTED JOINTS
We conclude our study of the skeletal system by integrating information about different types of joints with information from lecture. Table 2 summarizes the classification of major joints by structure and movement, beginning at the head and ending at the foot. In addition, we will look more closely at the structure of the six complex joints that are responsible for movements of the upper and lower limbs.
THE SHOULDER JOINT
Of all the joints, the shoulder joint, or glenohu- meral joint, is the most movable, but with this freedom of movement comes less stability (Figure 12-3).
The shoulder joint is multiaxial, allowing flexion, extension, abduction, adduction, rotation, and circumduction of the arm (humerus). However, to allow for such diverse movements, the supporting ligaments around the shoulder joint must remain reasonably loose. Consequently, the articulating bones at the shoulder joint may pull apart (dislocate) more easily than bones at other, more stable joints. Dislocation injuries are especially prevalent in contact sports such as football and rugby. In addition, ligaments and tendons are frequently strained or torn in sports such as baseball and tennis, which involve frequent throwing or swinging motions.
Most superficially, we see that four capsular ligaments help hold the humeral head within the glenoid cavity. The coracohumeral ligament (KOR-uh-kō-HŪ-mer-ul) is the strongest of the four and is found on the superior aspect of the articular capsule. It extends between the coracoid process of the scapula to the greater tubercle of the humerus. Due to its superior position, the coracohumeral ligament supports much of the weight of the upper limb. Three less conspicuous glenohumeral ligaments (GLE-nō-HŪ-mer- ul) (superior, middle, and inferior) are found on the anterior aspect of the articular capsule. They extend from the rim of the glenoid cavity to the lesser tubercle and neck of the humerus. An important extracapsular ligament at the shoulder joint is the coracoacromial ligament (KOR-uh-kō-a-KRŌ-mē-ul), extending between the coracoid and acromial processes of the scapula. This ligament, along with the two bony processes it connects, prevents the humeral head from being pushed superiorly out of the glenoid cavity.
While the articular capsule and capsular ligaments provide some support, muscle tendons provide most of the stability for the shoulder joint. Four tendons, collectively called the rotator cuff, extend from shoulder muscles on the scapula to the greater and lesser tubercles of the humerus. One of these, the tendon of the subscapularis muscle, can be seen extending across the anterior surface of the joint. Muscle tone maintains tension on these tendons, helping to hold the humeral head securely in the glenoid cavity. A tendon from the long head of the biceps brachii muscle, located on the anterior portion of the arm, also helps stabilize the shoulder joint. It extends through the joint cavity and attaches immediately superior to the glenoid labrum. The shoulder joint is weakest on its inferior aspect, since no tendons cross over at that point.
Also located superficially, we see a number of fluid-filled accessory structures that reduce friction between the articulating bones and surrounding tissues at the shoulder joint. A tendon sheath protects the biceps brachii tendon where it passes through the joint. In addition, four bursae reduce friction between the bones and the muscles at the shoulder joint.
As we remove the superficial layers of tissue, we see that the shoulder joint is a synovial, diarthrotic, ball-and-socket joint. The “ball” is the head of the humerus, and it fits into the shallow glenoid cavity (the “socket”) of the scapula. The surfaces of the humeral head and the glenoid fossa are covered with smooth hyaline cartilage (articular cartilages), which reduce friction. An articular capsule extends from the rim of the glenoid cavity to the head and neck of the humerus, and encloses the fluid-filled synovial cavity. The capsule has a loose fit around the joint, allowing the humerus to move freely without overstretching the capsule. The glenoid labrum (LĀ-brum; “ring”) is a ring of fibrocartilage extending around the rim of the glenoid cavity. The labrum slightly deepens the cavity and forms a liquid seal around about one-third of the humeral head. The moist seal functions like a small suction cup to help hold the humeral head in the glenoid cavity. Think of it as being like a moist rubber suction disc on the end of a toy arrow that holds tightly to a smooth surface.
Figure 12-3. The shoulder joint
THE ELBOW JOINT
Moving distally from the shoulder joint, the next major joint of the upper limb is the elbow joint, which allows flexion and extension of the forearm (Figure 12-4). The elbow joint is a compound joint, meaning it contains more than one articulation site. In this case, articulation occurs between the humerus and ulna, and between the humerus and radius.
Superficially, a thin articular capsule encloses the elbow joint and contains two capsular ligaments that help stabilize the joint. The ulnar collateral ligament (cō-LAT-er-ul) consists of three cordlike bands that connect the medial epicondyle of the humerus to the proximal, medial side of the ulna. This ligament prevents the ulna from bending laterally at the elbow. The triangular radial collateral ligament connects the lateral epicondyle of the humerus to the proximal, lateral end of the radius. This ligament prevents the radius from bending medially at the elbow. A number of tendons also provide stability to the elbow joint. These include the biceps brachii tendon, which crosses the anterior surface of the joint, and the triceps brachii tendon, which crosses the posterior surface.
A single bursa, the olecranon bursa, lies between the subcutaneous tissue and the olecra- non process of the ulna. This bursa cushions your olecranon process when you prop your elbow on a desk. It also reduces friction between the olec- ranon process and the overlying subcutaneous tissue during flexion and extension of the forearm.
As we look deeper into the elbow joint, we see that the trochlea of the humerus articulates with the trochlear (semilunar) notch of the ulna; this is the humeroulnar joint (HŪ-mer-ō-UL- nar). This joint works like a hinge, allowing the ulna to swing anteriorly and posteriorly like a door. For this reason, the humeroulnar joint is a synovial, diarthrotic, uniaxial, hinge joint. Smooth articular cartilage covers the trochlea and the trochlear notch, reducing friction when the ulna moves. In anatomical position, the ulna is in full extension with its olecranon process fitting into the olecranon fossa of the humerus; this prevents hyperextension of the ulna. During full flexion, the coronoid process of the ulna fits into the coronoid fossa of the humerus.
The proximal, concave end of the radial head fits onto the rounded, convex capitulum of the humerus at the humeroradial joint (HŪ-mer- ō-RĀ-dē-ul). Like the humeroulnar joint, the humeroradial joint works like a hinge, allowing flexion and extension of the radius. For this reason, we classify the humeroradial joint as a synovial, diarthrotic, uniaxial, hinge joint.
Another joint, the proximal radioulnar joint, is so closely associated with the elbow joint that we will consider it briefly here. At this joint, the radial head articulates medially with the radial notch of the ulna. The only movement possible at this joint is rotation; therefore, the proximal radioulnar joint is a synovial, diarthrotic, uniaxial, pivot joint. Rotation of the radius allows you to pronate and supinate your forearm. The radial annular ligament attaches at both ends to the ulna while it arches over the head of the radius. This ligament holds the radius in place during pronation and supination of the forearm.
Figure 12-4. The elbow joint
Figure 12-5. The hip joint
THE WRIST JOINT
The wrist joint, or radiocarpal joint connects the distal end of the radius to the carpal bones. This is a condylar joint with biaxial movement. The distal end of the ulna does not touch the carpal bones, but articulates with a meniscus (articular disc) that separates the ulna from the lunate and triquetral bones. Two major ligaments span the wrist joint between the radius and carpal bones; these are the radiocarpal ligaments.
Two ligaments prevent excessive side-to-side movement of the hand at the wrist. The radial col- lateral carpal ligament connects the radius to the scaphoid bone and prevents excessive abduction of the hand. The ulnar collateral carpal ligament connects the styloid process of the ulna to the tri- quetral and pisiform bones. This ligament prevents excessive adduction of the hand. Sometimes the word “carpal” is left off of these names, but then they can be confused with the collateral ligaments of the elbow. Intercarpal ligaments bridge adja- cent carpal bones preventing them from pulling away from one another when the hand moves.
THE HIP JOINT
The hip joint, or coxal joint, is the joint between the head of the femur and the acetabulum (hip socket) of the pelvis (Figure 12-5). This joint is second in movability (mobility) only to the shoulder joint. Actions possible at this joint include flexion, extension, abduction, adduction, rotation, and circumduction of the thigh (femur). Although the supporting ligaments around the hip joint must remain reasonably loose to allow multiaxial movement, the hip socket is much deeper than the shoulder socket; thus, the the hip joint is much more stable than the shoulder joint.
Superficially, we see a thick articular capsule consisting of three major capsular ligaments that help hold the femoral head securely within the acetabulum. The iliofemoral ligament (IL-ē-ō- FEM-o-rul) extends from the anterior inferior iliac spine and rim of the acetabulum to the in- tertrochanteric line of the femur. This ligament prevents the femoral head from moving superi- orly. The pubofemoral ligament (PŪ-bō-FEM- u-rul) extends from the anterior portion of the acetabular labrum to the neck of the femur, and prevents the femoral head from moving anteri- orly or inferiorly. The ischiofemoral ligament (IS-kē-ō-FEM-u-rul) extends from the ischial portion of the acetabulum rim to the neck of the femur, and prevents the femoral head from moving posteriorly.
Looking deeper into the hip joint, we see that it is a synovial, diarthrotic, multiaxial, ball-and- socket joint. The “ball” is the head of the femur, and it fits into the deep acetabulum (the “socket”) of the hipbone. Articular cartilage covers the en- tire surface of the femoral head, but it is lunate (C-shaped) within the acetabulum. An articular capsule extends from the rim of the acetabulum to the head and neck of the femur, and encloses a fluid-filled synovial cavity. The capsule fits loosely around the joint, allowing the femur to move freely. The acetabular labrum (AS-e-TAB- ū-lar) is a ring of fibrocartilage extending almost completely around the rim of the acetabulum. The labrum deepens the cavity and forms a seal around femur’s neck, making it difficult to dislocate the femoral head from the acetabulum. This works like two button-type snaps fastening together on a jacket. A transverse acetabular ligament is a noncartilaginous band that forms the inferior rim of the labrum.
A strong intracapsular ligament called the ligamentum teres ( (lig-uh-MEN-tum TER-ēz; “round”) connects the head of the femur to the acetabulum. It extends between the fovea capitis, a small pit in the femoral head, to the transverse acetabular ligament to form the inferior rim of the acetabulum. This ligament does little to stabilize the hip joint, but it contains an artery that nourishes the head of the femur. Numerous hip muscles also cross the hip joint, adding stability to the already highly stable joint.
Figure 12-6. The knee joint
THE KNEE JOINT
Although the knee joint lacks the mobility of the shoulder or hip joints, it is the most complex joint in the body. The knee joint is largely uniaxial, allowing flexion and extension of the leg, but during these movements, it also allows slight leg rotation. Like the elbow joint, the knee joint is compound, having more than one articulation site. In the knee joint, articulation occurs between the femur and tibia, and between the femur and patella (Figure 12-6).
A thin, fibrous articular capsule is present only on the posterior and lateral aspects of the knee joint; therefore, other structures are nec- essary to reinforce the anterior aspect. On the anterior aspect, the quadriceps tendon extends inferiorly from the large, anterior thigh muscles and envelops the patella, holding it in place. This tendon extends inferiorly from the patella as the patellar ligament, which inserts on the tibial tuberosity.
Two extracapsular ligaments stabilize the lateral aspects of the knee joint. The tibial col- lateral ligament connects the medial condyle of the femur to the medial condyle of the tibia. This ligament prevents medial displacement of the tibia. The fibular collateral ligament connects the lateral condyle of the femur to the head of the fibula, and prevents lateral displacement of the tibia. Both of these ligaments are taut when the knee is in full extension (anatomical position), but become loose when the knee bends.
A number of structures cushion the knee joint, including a large joint capsule, numerous bursae, and several fat pads. The fluid-filled joint capsule at the knee joint is the largest and most elaborate in the body. It is most extensive on the anterior surface of the knee, where it extends su- perior to the patella to terminate as the suprapa- tellar bursa. This bursa reduces friction between the femur and quadriceps muscle tendon. The joint capsule attaches to the edges of the menisci and extends posteriorly to cover the femoral and tibial condyles. The prepatellar bursa lies between the patella and the skin, while the infrapatellar bursa lies between the patellar ligament and joint capsule. A thick infrapatellar fat pad lies at the anterior surface of the knee joint, immediately inferior to the patella. It cushions the femoral condyles and helps prevent them from slipping anteriorly.
Two intracapsular ligaments prevent exces- sive anterior and posterior movements of the tibia. These ligaments are called cruciate ligaments (KRŪ-shē-āt; cruci-, cross) because they cross each other within the intercondylar notch of the femur. The anterior cruciate ligament (ACL) extends from the posterior-medial surface of the femur’s lateral condyle to insert on the anterior portion of the tibial condyles. This ligament becomes taut when the knee is in full extension, thereby preventing the tibia from hyperextending. The posterior cruciate ligament (PCL) extends from the anterior-lateral side of the femur’s medial con- dyle. This ligament prevents the tibia from sliding posteriorly and the femur from sliding anteriorly when flexing the leg. If you could slide down a cruciate ligament, whichever side of the knee you are on when you reach the bottom is the name of the ligament. For example, after sliding down the ACL, you will be on the anterior side of the knee. Deep to the articular capsule, we can see the weight-bearing part of the knee, the tibiofemoral joint (TIB-ē-ō-FEM-u-rul), which is a synovial, diarthrotic, uniaxial, condylar joint. Here, the convex lateral and medial condyles of the femur articulate with concave, fibrocartilaginous me- nisci (articular discs). The nearly circular lateral meniscus rests on the lateral condyle of the tibia, while the C-shaped medial meniscus rests on the medial condyle. The flexible menisci change shape as the knee bends, allowing the femoral con- dyles to transfer the body weight over a broader area on the tibia. Menisci cushion the femoral condyles when a person stands, and they act as shock absorbers when the person walks or runs. Smooth articular cartilage covers the surfaces of the femoral and tibial condyles, reducing friction when the knee bends. The edges of the menisci extend slightly superiorly around the edges of the femoral condyles, helping to prevent the condyles from sliding side-to-side.
Have you ever noticed that when standing for a long time, you can “lock” your knees so that your thigh muscles don’t get tired. What’s happening when a knee locks? When the leg is flexed (bent), each femoral condyle makes minimum contact with its corresponding meniscus; at this time, the collateral ligaments and the anterior cruciate ligaments are loose. However, because the lateral femoral condyle is smaller and less rounded than the medial condyle, it makes maximum surface contact with the lateral meniscus before leg extension is complete. As a way of making the smaller, lateral condyle “wait” on the larger medial condyle to make full contact with the medial meniscus, the femur medially rotates slightly. This action allows the leg to extend further while both menisci flatten to act like suction cups to provide maximum support for each femoral condyle. At this time, the collateral and anterior cruciate ligaments become taut and help “lock” the femur condyles in place. Before the leg can flex, the small popliteus muscle on the knee’s posterior surface must cause the femur to rotate laterally ever so slightly, but this effectively “unlocks” the knee.
The articulation between the femur and patella is the femoropatellar joint (FEM-o- rō-pu-TEL-ur), which is a synovial, diarthrotic, nonaxial, plane joint. Flexing the knee joint causes the patella to glide over the patellar surface of the femur. In full flexion, the patella is anterior to the femur’s intercondylar notch.
Figure 12-7. Popping a synovial joint
Table 12-1. Classification of Joints
Table 12-2. Types of Bone Movements at Synovial Joints
Table 12-3. Classification and Actions of Selected Joints
TOPICS TO KNOW FOR CHAPTER 12
(Joints)
1st sternocostal joint
abduction
acetabular labrum
ACL
acromioclavicular joint
adduction
amphiarthrosis
angular movement
ankle joint
annulus fibrosus
anterior cruciate ligament
arthrosis
articular capsule
articular cartilage
articular disc
articular fat pad
articulation
atlantoaxial joint
atlanto-occipital joint
ball-and-socket joint
biaxial joint
bony joint
bursa
capsular ligament
carpometacarpal joint
cartilaginous joint
circumduction
classification of joints by movement
classification of joints by structure
condylar joint
coracoacromial ligament
coracohumeral ligament
coxal joint
deltoid ligament
dento-alveolar joint
depression
diarthrosis
distal tibiofibular joint
dorsiflexion
elbow joint
elevation
ellipsoidal joint
epiphyseal plate
eversion
excursion
extension
extracapsular ligament
extrinsic ligament
femoropatellar joint
fibrous capsule
fibrous joint
fibular collateral ligament
flexion
glenohumeral joint
glenohumeral ligament
glenoid labrum
gliding movement
gomphosis
hinge joint
hip joint
humeroradial joint
humeroulnar joint
hyaluronic acid
hyperextension
iliofemoral ligament
infrapatellar bursa
infrapatellar fat pad
intercarpal joint
intercarpal ligament
interosseous membrane
interphalangeal joint
interpubic disc
intertarsal joint
intervertebral disc
intervertebral joint
intracapsular ligament
intrinsic ligament
inversion
ischiofemoral ligament
joint
joint cavity
joint mobility
joint stability
knee joint
labrum
lateral flexion
lateral ligament
lateral meniscus
lateral rotation
left rotation
ligament
ligamentum teres
manubriosternal joint
medial meniscus
medial rotation
meniscus
metacarpophalangeal
metatarsophalangeal joint
monaxial joint
multiaxial joint
nonaxial joint
olecranon bursa
opposition
patellar ligament
PCL
periodontal ligament
pivot joint
plane joint
plantar flexion
popping a joint
posterior cruciate ligament
prepatellar bursa
pronation
protraction
proximal radioulnar joint
proximal tibiofibular joint
pubic symphysis
pubic symphysis
pubofemoral ligament
radial annular ligament
radial collateral carpal ligament
radial collateral ligament
radiocarpal joint
radiocarpal ligament
range of motion
reposition
retraction
right rotation
rotational movement
sacroiliac iliac
saddle joint
shoulder joint
sternoclavicular joint
sternocostal joint
supination
suprapatellar bursa
sutural ligaments
suture
symphyses
synarthrosis
synchondroses
syndesmosis
synostosis
synovial cavity
synovial fluid
synovial joint
synovial membrane
talocrural joint
tarsometatarsal joint
temporomandibular joint
tendon
tendon sheaths
tibial collateral ligament
tibiofemoral joint
tibiofibular ligament
transverse acetabular ligament
triaxial joint
ulnar collateral carpal ligament
ulnar collateral ligament
uniaxial joint
vertebrocostal joint
weeping lubrication
wrist joint
xiphisternal joint
EOC Questions
