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Chapter 10: Joints
Chapter Introduction
With the exception of the hyoid bone in the neck, eve bone in the human body is connected to at least one other bone. e locations where bones come together, joints or a iculations come in many formats. Some joints, like your knee, allow for movement between the bones. Other joints may bind the bones together so tightly that the bones do not move apa from each other, such as the joints among the bones in your skull. As you can imagine, bones that are tightly bound to each other with connective tissue provide little or no movement are highly stable. Conversely, joints that provide the most movement between bones are the least stable. Understanding the relationship between joint structure and function will help to explain why pa icular types of joints are found in ce ain pa s of the body.
10.1Classi cation of Joints
A joint is any place where adjacent bones or bone and ca ilage come together to form a connection; in these locations we often describe the bones as a iculating with each other. Joints are classi ed both structurally and functionally. Structural classi cations of joints take into account whether the adjacent bones are directly anchored to each other by brous connective tissue or ca ilage, or whether the adjacent bones a iculate with each other within a uid- lled space called a joint cavity. Functional classi cations describe the degree of movement available between the bones, ranging from immobile, to slightly mobile, to freely moveable joints. e amount of movement is directly related to the function of the joint. For example, immobile or slightly moveable joints se e to protect
internal organs or give stability to the body. Freely moveable joints, on the other hand allow for much more extensive movements of the body and limbs. As a general rule, the more motion is possible at the joint, the less stable and more prone to inju that joint is. For example, the shoulder is the most mobile joint of the human body, it is also the most frequently dislocated.
10.1aStructural Classi cation of Joints
e structural classi cation of joints is based on whether the a iculating su aces of the adjacent bones are directly connected by brous connective tissue or ca ilage, or whether the a iculating su aces are connected via a uid- lled joint cavity (see the feature “Anatomy of a Joint Structure”). A brous joint is where the adjacent bones are united by brous connective tissue. At a ca ilaginous joint, the bones are joined by hyaline ca ilage or broca ilage. At a synovial joint, the a iculating su aces of the bones are not directly connected, but instead connect within a joint cavity that is lled with a lubricating uid. Synovial joints allow for free movement between the bones and are the most common joints of the body.
10.1bFunctional Classi cation of Joints
Whereas the structural classi cation is primarily concerned with what is between the joints (i.e. uid, ca ilage, connective tissue) the functional classi cation of joints is all about how the joint moves, speci cally the amount of mobility found between the adjacent bones. Joints are thus functionally classi ed as a syna hrosis if no motion is possible at the joint, an amphia hrosis if slight movement is possible at the joint, or as a dia hrosis, if a lot of motion is possible at the joint. As always, structure and function are intimately tied. e brous type of joint structure may be functionally classi ed as a syna hrosis (immobile joint) or an amphia hrosis (slightly mobile joint). Ca ilaginous joints are also functionally classi ed as either a syna hrosis or an amphia hrosis joint. All synovial joints are functionally classi ed as a dia hrosis joint.
Syna hrosis
Syna hroses are immobile or nearly immobile joints. e immobile nature of these joints provides for a strong union between the a iculating bones. is is impo ant at locations where the bones provide protection for internal organs. Examples include the sutures of the skull; the brous joints unite the skull bones to protect the brain (Figure 10.1).
Figure 10.1Suture Joints of the Skull
e suture joints of the skull are examples of syna hroses and brous joints. ey are immobile and in these joints two skull bones are tied together directly with a sheet of connective tissue. Later in life (older adulthood), the connective tissue ossi es and these joints become solid.
Anatomy of…
Joint Structure
A uniaxial joint only allows for a motion in a single plane (around a single axis). e elbow joint, which only allows for bending or straightening, is an example of a uniaxial joint (Figure 10.3). A biaxial joint allows for motions within two planes. An example of a biaxial joint is the joint at the base of your nger, where a nger meets the palm of the hand. Examine one of these joints and bend your nger. is is motion in one plane, like the elbow joint. But now spread your ngers away from each other and bring them together. is is motion in a second plane at that same joint. ese joints, the metacarpophalangeal joints, are biaxial. A joint that allows for the several directions of movement is called a multiaxial joint. e shoulder and hip joints are multiaxial joints. ey allow the limbs to move in a circle, a much greater range of motion than the uniaxial or biaxial joints.
Figure 10.3Joint Movement Classi cation
Joints can be classi ed by the number of anatomical planes or directions in which they allow movement.
10.2Fibrous Joints
At a brous joint, the adjacent bones are directly connected to each other by brous connective tissue; thus, the bones do not have a cavity between them, but form a sandwich with a sheet of connective tissue as the lling. ere are three types of brous joints (Figure 10.4). A suture is the narrow brous joint found between most bones of the skull. At a syndesmosis joint, the bones are more widely separated but are held together by a sheet of brous connective tissue. is type of brous joint is found between the shaft regions of the long bones in the forearm and in the leg. Lastly, a gomphosis is the brous joint between the roots of a tooth and the bony socket in the jaw into which the tooth ts.
Figure 10.4Types of Fibrous Joints
Bones joined by a sheet of connective tissue are called brous joints. Examples include:
(A) Uniaxial joints, such as the elbow joint, allow movement in only one plane, a back-and-fo h motion. (B) Biaxial joints, such as the metacarpophalangeal joints, allow movement in two directions—in this case forward and backward as well as side to side. (C) e hip joint, which is a multiaxial joint, also allows motion in a circle.
(A) sutures between the skull bones, (B) an interosseous membrane, which spans the distance between the shafts of two bones, such as the radius and ulna of the forearm, and (C) a gomphosis, which is a specialized brous joint that anchors a tooth in its bony socket.
10.2aSuture
All the bones of the skull, except for the mandible, are joined to each other by a brous joint called a suture. e brous connective tissue found at a suture strongly unites the adjacent skull bones and thus helps to protect the brain and form the face. ese sutures are not straight lines but are convoluted, like a river, forming a tight union that prevents most movement between the bones (Figure 10.4A). us, skull sutures are functionally classi ed as a syna hrosis, although some sutures may allow for slight movements between the cranial bones.
In newborns and infants, the areas of connective tissue between the bones are much wider, especially in the areas on the top and sides of the skull that will become the sagittal, coronal, squamous, and lambdoid sutures. ese broad areas of connective tissue are called fontanelles (Figure 10.5). During bi h, the fontanelles provide exibility to the skull, allowing the bones to push closer together or to overlap slightly, thus aiding movement of the infant’s head through the bi h canal. After bi h, these expanded regions of connective tissue allow for rapid growth of the skull and enlargement of the brain. e fontanelles greatly decrease in width during the rst year after bi h as ossi cation of the skull bones continues. When the connective tissue between the adjacent bones is reduced to a narrow layer, these brous joints are now called sutures and the fontanelles no longer exist. At some sutures, the connective tissue will ossify and be conve ed into bone, causing the adjacent bones to fuse to each other later in life.
Figure 10.5 e Newborn Skull
Human infants are born with wide soft areas of connective tissue among the bones of the skull. ese are called fontanelles.
10.2bSyndesmosis
10.3aSynchondrosis
A synchondrosis (“joined by ca ilage”) is a ca ilaginous joint where bones are joined together by hyaline ca ilage, or where bone is united to hyaline ca ilage. Synchondroses are found in eve long bone early in life at the epiphyseal plate (growth plate). e epiphyseal plate is the region of growing hyaline ca ilage that unites the diaphysis (shaft) of the bone to the epiphysis (end of the bone) (Figure 10.6A). During an individual’s late teens and early twenties, the epiphyseal plate in each bone is ossi ed, the ca ilage is completely replaced by bone. Once this occurs, bones cannot grow longer and the synchondrosis is no longer present.
Synchondroses are present in other bones as well. In the young skeleton, synchondroses are present at the sites where the ilium, ischium, and pubic bones meet. When body growth stops, these sites of ca ilage disappear and are replaced by bone, forming the single right and left hip bones of the adult.
e joints between the ribs and costal ca ilages are also synchondroses and are consistently present throughout life. At these joints, the anterior end of the rib joins to the sternum with a long bar of costal ca ilage between. While the sternal end of all but the rst ribs is a synovial joint, which you will learn about in Section 9.4, the joint between the rib and its costal ca ilage is a synchondrosis. Due to the lack of movement between the bone and ca ilage, all synchondroses are functionally classi ed as syna hroses.
10.3bSymphysis
A joint in which the two bones meet at a broca ilage pad is called a symphysis. Fibroca ilage is ve strong because it contains numerous bundles of thick collagen bers, thus giving it a much greater ability to resist pulling and bending forces compared with hyaline ca ilage. is gives symphyses the ability to strongly unite the adjacent bones, but can still allow for some limited movement to occur. us, a symphysis is functionally classi ed as an amphia hrosis.
Examples of symphysis joints include the pubic symphysis, the manubriosternal joint, and the inte e ebral symphysis. In addition to providing a strong attachment, broca ilage pads at all these locations provide cushioning between the bones, which is impo ant when car ing heavy objects or during high-impact activities such as running or jumping.
10.4Synovial Joints
Synovial joints are the most common type of joint in the body (see the “Anatomy of a Typical Synovial Joint” feature). Synovial joints are unique because they feature an enclosed cavity that is lled with uid. e uid bathes the ends of the bones, providing both nourishment and lubrication. Also, unlike brous or ca ilaginous joints, the bone su aces within a synovial joint do not directly connect to each other, allowing for a greater degree of movement.
Anatomy of…
A Typical Synovial Joint
10.4aStructural Features of Synovial Joints
Synovial joints are characterized by the presence of a uid- lled joint cavity (see the “Anatomy of a Typical Synovial Joint” feature). e walls of this space are formed by the a icular capsule, a brous connective tissue structure that is attached to each bone just outside the area of the bone’s a iculating su ace. e a icular capsule has two layers: the ligament that ties the bones together and the inner synovial membrane. e cells of this membrane secrete synovial uid, a thick, slimy uid that provides lubrication to fu her reduce friction between the bones of the joint. e bones of the joint are tied together by tough ligaments, but they do not touch within the joint cavity. Friction and stress on the bones is eased by the synovial uid as well as by the presence of the a icular ca ilage, a thin layer of hyaline ca ilage that covers the su ace of each bone. e a icular ca ilage allows the bones to move smoothly against each other without damaging the underlying bone tissue. Hyaline ca ilage lacks both blood vessels and ne es, so movements of the bones past each other are painless and there is no risk of damage. e synovial uid provides nourishment to the a icular ca ilage, which, without its own blood vessels, needs to get nourishment externally. Each time the joint moves, the synovial uid is compressed and moves through the a icular ca ilage; think of water moving through a sponge when it is squeezed. erefore, regular movement at synovial joints is impo ant for their health. An overly sedenta lifestyle compromises joint health because without regular movement the a icular ca ilage may not get enough nutrients from the synovial uid and the chondrocytes may su er. Each synovial joint is functionally classi ed as a dia hrosis.
10.4bCushioning and Suppo Structures Associated with Synovial Joints
Student Study Tip
Use alliteration. “Ligaments” connect “like” structures to one another, so you can remember they connect bone to bone, while tendons connect bone to muscle.
Ligaments and Tendons
Outside of their a iculating su aces, the bones are connected together by ligaments, which are strong bands of brous connective tissue. e ligaments form the outer layer of the a icular capsule. ese strengthen and suppo the joint by anchoring the bones together and preventing their separation. Ligaments are classi ed based on their relationship to the brous a icular capsule. An extrinsic ligament is located outside of the a icular capsule,
10.4cTypes of Synovial Joints
Synovial joints are subdivided based on the shapes of the a iculating su aces of the bones that form each joint. e six types of synovial joints are pivot, hinge, condyloid, saddle, plane, and ball-and-socket joints (Figure 10.8).
Figure 10.8Types of Synovial Joints
e six types of synovial joints are specialized to accomplish di erent types of movement.
Pivot Joint
At a pivot joint, a rounded po ion of a bone is enclosed within a ring formed pa ially by the a iculation with another bone and pa ially by a ligament (Figure 10.8A). e bone rotates within this ring. Since the rotation is around a single axis, pivot joints are functionally classi ed as a uniaxial dia hrosis type of joint. An example of a pivot joint is the atlantoaxial joint, found between the C 1 (atlas) and C 2 (axis) ve ebrae. Here, the upward
(A) Pivot joints feature rotation around an axis, such as between the rst and second ce ical ve ebrae, which allows for turning of the head side to side (as you might when you say “no”). (B) Hinge joints feature the rounded end of one bone that another bone moves around within one plane. (C) Condyloid joints feature one rounded bone end that is cupped within a bowl-like depression of another bone. (D) Saddle joints feature two cu ed bones, like saddles, that t into each other complementarily. (E) Plane joints (such as those between the tarsal bones of the foot) feature attened, a iculating, bony su aces on both bones, which allow for limited gliding movements. (F) Ball-and-socket joints (such as the hip and shoulder joints) feature a rounded head of a bone moving within a cup-shaped depression in another bone.
projecting dens of the axis a iculates with the inner aspect of the atlas, where it is held in place by a ligament. Rotation at this joint allows you to turn your head from side to side. A second pivot joint is found at the proximal radioulnar joint. Here, the head of the radius is largely encircled by a ligament that holds it in place as it a iculates with the radial notch of the ulna. Rotation of the radius allows for forearm movements.
Digging Deeper:
Bursitis
e su x - itis means in ammation. Bursae function to help joints move more uidly by sheltering ligaments and tendons from ction against the bones of the joint. Bursitis is the in ammation of a bursa. is will cause pain, swelling, or tenderness of the bursa and surrounding area, and may also result in decreased mobility at the joint. Bursitis is most commonly associated with the bursae found at or near the shoulder, hip, knee, or elbow joints. At the knee, in ammation and swelling of the bursa located between the skin and patella bone is prepatellar bursitis (“housemaid’s knee”), a condition more commonly seen today in roofers or oor and carpet installers who do not use knee pads. At the elbow, olecranon bursitis is in ammation of the bursa between the skin and olecranon process of the ulna. e olecranon forms the bony tip of the elbow, and bursitis here is also known as “student’s elbow.”
Bursitis can be either acute (lasting only a few days) or chronic. It can arise from mechanical issues such as muscle overuse, trauma, and excessive or prolonged pressure on the skin. Other issues such as rheumatoid a hritis (which you read about in the “Apply to Pathophysiology: Rheumatoid A hritis” feature), gout, or infection can all lead to in ammation of the bursae as well. In these cases, only addressing the prima issue (such as the infection if that is the cause) will alleviate the bursitis. Any type of anti-in ammato agent should help to relieve the in ammation until the source can be found.
Student Study Tip
Hinge joints operate similarly to doors. e only way a door can move while on its hinges is by opening or closing. Place your arm straight out in front of you to ex and extend your elbow, and you can see that the movement resembles a door opening and closing.
Hinge Joint
In a hinge joint, the convex end of one bone a iculates with the concave end of the adjoining bone (Figure 10.8B). is type of joint allows only for bending and straightening motions along a single axis, and thus hinge joints are functionally classi ed as uniaxial joints. A good example is the elbow joint, with the a iculation between the trochlea of the humerus and the trochlear notch of the ulna. Other hinge joints of the body include the knee, ankle, and interphalangeal joints between the phalanx bones of the ngers and toes.
Condyloid Joint
At a condyloid joint (ellipsoid joint), the shallow depression at the end of one bone a iculates with a rounded structure from an adjacent bone or bones (Figure 10.8C). e knuckle (metacarpophalangeal) joints of the hand between the distal end of a metacarpal bone and the proximal phalanx bone are condyloid joints. Functionally, condyloid joints are biaxial joints that allow for two planes of movement. One movement involves the bending and straightening of the ngers or the anterior-posterior movements of the hand. e second movement is a side-to- side movement, which allows you to spread your ngers apa and bring them together, or to move your hand in a medial/lateral direction.
Saddle Joint
At a saddle joint, both of the a iculating su aces for the bones have a saddle shape, which is concave in one direction and convex in the other (Figure 10.8D). is allows the two bones to t together like a rider sitting on a saddle. Saddle joints are functionally classi ed as biaxial joints. e best example in the human body is the rst carpometacarpal joint, between the trapezium (a carpal bone) and the rst metacarpal bone at the base of the thumb. is joint provides the thumb the ability to move in almost a complete circle. e thumb can move within the same plane as the palm of the hand, which you can see if you put your hand at on a desk and move your thumb close to and away from the palm. In a second plane of motion, the thumb can move anteriorly, almost laying
For severely a hritic joints, a surge to replace the hyaline ca ilage may be an option. is type of surge involves replacing the a icular su aces of the bones with a i cial materials. For example, in hip replacement, the worn or damaged pa s of the hip joint, including the head and neck of the femur and the acetabulum of the pelvis, are removed and replaced with a i cial joint components (see Figure B). e pa s, which are always built in advance of the surge , are sometimes custom-made to produce the best possible t for a patient.
Gout is a form of a hritis that results from the deposition of uric acid c stals within a body joint. Usually only one or a few joints are a ected, such as the big toe, knee, or ankle. e attack may only last a few days, but may return to the same or another joint. Gout occurs when the body makes too much uric acid or the kidneys do not properly excrete it. A diet with excessive fructose has been implicated in raising the chances of a susceptible individual developing gout.
Other forms of a hritis are associated with various autoimmune diseases, bacterial infections of the joint, or unknown genetic causes. In all forms of a hritis, there is a decrease in the thickness of the a icular ca ilage, and the inne ated bones begin to respond both with pain as well as in ammation and structural changes.
10.5Movements at Synovial Joints
Synovial joints allow the body a tremendous range of movements. e type of movement that can be produced at a synovial joint is determined by its structural type. While the ball-and-socket joint gives the greatest range of movement at an individual joint, in other regions of the body, several joints may work together to produce a pa icular movement. Remember as we discuss body movements in this section, we are using directional terms and anatomical position of the body. If you need a refresher on these words, reviewing Chapter 2 will help make the following section much easier to follow.
10.5aFlexion and Extension
e simplest way to think of exion and extension is that exion reduces the angle of the joint from its resting position and extension returns the joint to its resting position (Figure 10.9A and 10.9B). For example, sta with your body in anatomical position. Now examine your elbow joint. It is a straight-line, or 180 degrees. Now bend your elbow halfway. e angle of the joint is reduced, from 180 degrees to 90 degrees. If you return your elbow back to its straight-line sta ing position, you are increasing the angle of the joint back to 180 degrees and extending it. For a more complex example, we can examine the ve ebral column. Flexion is an anterior (forward) bending of the neck or body, while extension involves a posterior-directed motion, such as straightening from the exed position. Hyperextension is increasing the joint angle beyond 180 degrees (Figures 10.9C and 10.9D). Most joints are not capable of hyperextension, but one example is bending the spine backward as you do if you are seated and you look up toward the ceiling. Since most joints are not capable of hyperextension, this motion sometimes results in injuries. Hyperextension injuries are common at hinge joints such as the knee or elbow. Lateral exion is the bending of the neck or body toward the right or left side.
Figure 10.9Movements of the Body (Pa s A–G)
When turning the hand so that the palm faces posteriorly, the radius must rotate over the ulna, a movement called pronation.
Student Study Tip
Abduction is moving away from the body, so you can think of when someone has been abducted, they’ve been taken away.
10.5bAbduction and Adduction
Abduction and adduction motions occur on the limbs. Abduction is a motion that pulls a limb, nger, toe, or thumb away from the middle of the body, while adduction is the opposing movement that brings the limb back toward the
(A–B) Flexion decreases a joint angle and extension brings the joint back to its resting position (usually back to a straight line). (C–D) Extension beyond the straight line, increasing the joint angle to greater than 180 degrees, is called hyperextension. (E) Abduction (moving the structure away from the body, or spreading the ngers and toes) and adduction (bringing the structure toward the body or bringing the ngers and toes together) are motions of the limbs, hands, ngers, or toes. Circumduction (moving a structure in a circular pattern) combines exion, adduction, extension, and abduction. (F) Turning of the head or twisting of the trunk is described as rotation. e limbs are capable of medial and lateral rotation if they turn the anterior side toward or away from the midline. (G) e radius and ulna are parallel in anatomical position, which is called supination.
10.5gInversion and Eversion
Inversion and eversion are complex movements that involve the multiple plane joints among the tarsal bones of the posterior foot. us, inversion and eversion are motions of the foot, not the ankle joint. Inversion is the turning of the foot to angle the bottom of the foot toward the midline, while eversion turns the bottom of the foot away from the midline. ese motions are impo ant motions that help to stabilize the foot when walking or running on an uneven su ace and aid in the quick side-to-side changes in direction used during spo s such as basketball and soccer (Figure 10.9I).
10.5hProtraction and Retraction
Protraction and retraction are anterior-posterior movements of the mandible or scapula. For the mandible, protraction occurs when the lower jaw is pushed forward, to stick out the chin, while retraction returns the lower jaw to its resting position (Figure 10.9J). Protraction of the scapula occurs when the shoulder is moved forward; you can demonstrate this motion by pushing against a wall or pushing a shopping ca forward. Retraction is the opposite motion, with the scapula being pulled posteriorly and medially, toward the ve ebral column.
(J) Protraction of the mandible pushes the chin forward as the mandible moves forward, and retraction returns the mandible to its resting position. (K) e opening and closing of the mouth is accomplished through depression of the mandible (opening) and elevation (closing). (L) Opposition moves the thumb so that it can contact the ngers, and reposition restores anatomical position.
10.5iDepression and Elevation
Depression and elevation are downward and upward movements of the scapula or mandible. e upward movement is elevation, while a downward movement is depression. ese movements are used to shrug your shoulders. Similarly, elevation of the mandible is the upward movement of the lower jaw used to close the mouth or bite on something, and depression is the downward movement that produces opening of the mouth (Figure 10.9K).
10.5jExcursion
Excursion is the side-to-side movement of the mandible. Lateral excursion moves the mandible away from the midline, toward either the right or left side. Medial excursion returns the mandible to its resting position at the midline.
10.5kOpposition and Reposition
Opposition is the thumb movement that brings the tip of the thumb in contact with the tip of a nger (Figure 10.9L). is movement is produced at the rst carpometacarpal joint, which is a saddle joint formed between the trapezium carpal bone and the rst metacarpal bone. umb opposition is produced by a combination of exion and abduction of the thumb at this joint. Returning the thumb to its anatomical position next to the index nger is called reposition.
10.6Anatomy of Selected Synovial Joints
is section will examine the anatomy of selected synovial joints of the body. Anatomical names for most joints are derived from the names of the bones that a iculate at that joint, such as the radiocarpal joint between the radius and carpal bones. Other joint names that are more familiar to you, such as the elbow, hip, and knee joints, are exceptions to this naming scheme. In this section we will examine the diverse structure function relationships in ve joints of the body: the temporomandibular joint, the elbow joint, the hip joint, the knee joint, and the three individual joints that contribute to the shoulder (glenohumeral, sternoclavicular, and acromioclavicular joints).
10.6aTemporomandibular Joint
e temporomandibular joint (TMJ) is the joint that allows for depression/elevation, excursion, and protraction/retraction of the mandible. In other words, the TMJ is responsible for opening and closing your mouth, as well as side-to-side and front-back motions during chewing or grinding of your teeth. If we zoom in to the bone features that contribute to the joint, we can examine the a iculation formed when the condyle (head) of the mandible ts into the mandibular fossa of the temporal bone. Located between the bones is a exible a icular disc (Figure 10.10). is disc se es to smooth the movements between the temporal bone and mandibular condyle. Just anterior to the mandibular fossa, the temporal bone has a smooth at su ace called the a icular tubercle. is bone feature becomes impo ant in the motion at the TMJ as well.
Figure 10.10Temporomandibular Joint
e temporomandibular joint is the hinge joint of the mouth, where the condyle of the mandible ts within the mandibular fossa of the temporal bone.
Student Study Tip
e names of many ligaments are simply a combination of the structures that join together. Use this to help you remember the locations of the ligaments. e glenohumeral ligament is found where the glenoid fossa holds the head of the humerus.
e large range of motion at the shoulder joint is due in pa to the size di erence between the two bones that contribute to this joint. e head of the humerus is ve large and rounded, but the glenoid cavity of the scapula is quite small and shallow, surrounding only about one-third of the su ace of the humeral head. e perimeter of the glenoid cavity has a slightly raised ridge of broca ilage called the glenoid labrum. e a icular capsule that surrounds the glenohumeral joint is relatively thin and loose to allow for large motions of the upper limb. Some structural suppo is provided by ligaments that thicken of the wall of the a icular capsule. ese include the coracohumeral ligament, running from the coracoid process of the scapula to the anterior humerus, and the three glenohumeral ligaments that strengthen the anterior and superior sides of the joint.
Compared to other joints, the shoulder joint has less su ace area between the bones and less suppo from ligaments. e prima structural suppo for the shoulder joint is provided by muscles crossing the joint, pa icularly the four rotator cu muscles. ese muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) join the scapula to the humerus and reinforce the joint while maintaining its exibility. As these muscles cross the shoulder joint, their tendons encircle the head of the humerus and become fused to the anterior, superior, and posterior walls of the a icular capsule. As with other joints, bursae help to prevent friction between the rotator cu muscle tendons and the scapula as these tendons cross the glenohumeral joint. By constantly
adjusting their strength of contraction to resist forces acting on the shoulder, these muscles se e as “dynamic ligaments” and thus provide the prima structural suppo for the glenohumeral joint.
10.6cElbow Joint
e elbow joint is a hinge joint formed by the humerus, the radius, and the ulna. A single a icular capsule surrounds the ends and a iculation of all three bones. Figure 10.12 depicts a lateral view of the elbow joint through the medial sagittal section.
Figure 10.12Elbow Joint
e elbow is a hinge joint formed by the trochlea of the humerus and the trochlear notch of the ulna.
e a icular capsule of the elbow is strengthened by two ligaments that prevent side-to-side movements and hyperextension. On the medial side is the triangular ulnar collateral ligament. e lateral side of the elbow is suppo ed by the radial collateral ligament. e annular ligament encircles the head of the radius. is ligament surrounds the head of the radius at the proximal radioulnar joint. is is a pivot joint that allows for rotation of the radius during supination and pronation of the forearm.
10.6dHip Joint
e hip joint is a multiaxial ball-and-socket joint between the head of the femur and the acetabulum of the hip bone (Figure 10.13). e weight of the upper body rests on the hips during standing and walking; therefore, strength and stability is favored over mobility and the range of motion at the hip is more limited than at the shoulder joint.
Figure 10.13Hip Joint
e hip joint is a ball-and-socket joint that provides greater stability, but a more limited range of motion, than the shoulder.
