Saladin 5e Extended Outline Chapter 9 Joints I. Joints and Their Classification (pp. 286–290)

A. Any point where two bones meet is called a joint (articulation). (p. 286) (Fig. 9.1)

1. The study of musculoskeletal movement is kinesiology, a branch of biomechanics, which deals with many movements and mechanical processes of the body.

2. The name of a joint is derived from the names of the bones involved, as in atlanto-occipital joint.

3. The four main categories of joints are bony, fibrous, cartilaginous, and synovial.

B. A bony joint, or synostosis, is an immovable joint formed when the gap between two bones ossifies. (p. 287)

1. Bony joints can form by ossification of either fibrous or cartilaginous joints.

2. An infant has right and left frontal and mandibular bones at birth, but these soon fuse into a single frontal bone and mandible.

3. The epiphyses and diaphyses of long bones are jointed by cartilaginous joints that become synostoses in early adulthood.

C. A fibrous joint (synarthrosis or synarthrodial joint) has adjacent bones that are bound by collagen fibers; they consist of sutures, gomphoses, and syndesmoses. (p. 287–289) (Fig. 9.2)

1. Sutures are immovable or only slightly movable fibrous joints that bind the bones of the skull; they can be classified as serrate, lap, and plane sutures. (Fig. 9.3)

a. Serrate sutures appear as wavy lines along which bones firmly interlock, similar to a dovetail joint; examples are the coronal, sagittal, and lambdoid sutures of the parietal bones.

b. Lap (squamous) sutures occur where two bones have overlapping beveled edges, similar to a miter joint; an example is the squamous suture between the temporal and parietal bones. (Fig. 8.10)

c. Plane (butt) sutures occur where two bones have straight, nonoverlapping edges, similar to a butt joint; an example is the suture between the palantine processes of the maxillae.

2. A gomphosis is like a nail hammered into wood; an example is the attachment of a tooth (which is not a bone) in its socket.

a. The tooth is held in place by a fibrous periodontal ligament that extends from the bone matrix of the jaw into the dental tissue. (Fig. 9.2b)

b. Teeth are able to move or give a little under the stress of chewing.

3. A syndesmosis is a fibrous joint at which two bones are bound by longer collagenous fibers that those of a suture or gomphosis, giving the bones more mobility.

a. The joint between the distal ends of the tibia and fibula is a less movable syndesmosis.

b. The syndesmosis between the radius and ulna shafts is more movable, allowing pronation and supination of the forearm. (Fig. 9.2c)

D. In a cartilaginous joint (amphiarthrosis, or amphiarthrodial joint), two bones are linked by cartilage; the two types are synchondroses and symphyses. (p. 289–290) (Fig. 9.4)

1. A synchondrosis is a joint in which the bones are bound by hyaline cartilage; an example is the temporary joint between the epiphysis and diaphysis of a long bone in a child. (Fig. 9.4a)

2. In a symphysis, two bones are joined by fibrocartilage.

a. Examples are the pubic symphysis and the joint between the bodies of two vertebrae; these are joined by cartilaginous discs.

A. The most familiar type of joint is the synovial joint (diarthrosis, or diarthrodial joint). (p. 290)

1. Many are freely movable, whereas others have more limited mobility.

2. They are the most structurally complex type of joint and are most likely to develop uncomfortable dysfunctions.

3. Mobility of synovial joints affects quality of life.

B. The general anatomy of a synovial joint includes several components. (p. 290–291)

1. The facing surfaces of the two bones are covered with articular cartilage, a layer of hyaline cartilage about 2 or 3 mm thick.

Insight 9.1 Exercise and Articular Cartilage

2. A narrow space, the joint (articular) cavity, lies between the bones surfaces, filled with slippery synovial fluid. (Fig. 9.5)

3. A joint (articular) capsule of connective tissue encloses the cavity.

a. It has an outer fibrous capsule continuous with the periosteum of adjacent bones.

b. The inner synovial membrane is composed of fibroblast-like cells that secrete synovial fluid and is populated by macrophages.

4. In a few synovial joints, fibrocartilage grows inward from the capsule and forms a pad between the articulating bones.

a. In the jaw (temporomandibular) and distal radioulnar joints, and at both ends of the clavicle, the pad crosses the entire capsule and is called an articular disc. (Fig. 9.23c)

b. In the knee, two cartilages extend from left and right but do not entirely cross the joint; each is called a meniscus. (Fig. 9.29d)

5. Accessory structures include tendons, ligaments, and bursae.

a. A tendon is a strip or sheet of tough collagenous connective tissue that attaches a muscle to a bone.

b. A ligament is a similar tissue that attaches one bone to another.

c. A bursa is a fibrous sac filled with synovial fluid, located between adjacent muscles, between where a tendon passes over a bone, or between bone and skin. (Fig. 9.24)

i. Tendon sheaths are elongated, cylindrical bursae wrapped around a tendon; they are especially seen in the hands and feet. (Fig. 9.6)

C. A lever is any elongated, rigid object that rotates around a fixed point called the fulcrum; long bones act as levers to enhance the speed or power of movements (pp. 291–298) (Fig. 9.7)

1. The portion of a lever from the fulcrum to the point of effort is called the effort arm; the portion from the fulcrum to the point of resistance (load) is called the resistance arm.

a. In skeletal anatomy, the fulcrum is a joint; the effort is applied by a muscle, and the resistance can be an object, the limb itself, or the tension in an opposing muscle.

2. The advantage conferred by a lever can either 1) exert more force to an object than the force applied to the lever, as in a crowbar prying up an object, or 2) move the object farther or faster than the effort arm is moved, as in rowing a boat with an oar.

3. The mechanical advantage (MA) of a lever is the ratio of its output force to its input force.

a. If L_E is the length of the effort arm, and L_R is the length of the resistance arem, then MA = L_E/L_R.

b. If MA is greater than 1, the lever produces more force, but less speed or distance, than the force exerted on it.

c. If MA is less than 1, the lever produces more speed or distance, but less force, than the input.

i. The forearm has a resistance arm longer than its effort arm, so MA must be less than 1; in fact, it is 0.15. (Fig. 9.8)

ii. Most, but not all, musculoskeletal levers operate with an MA less than 1.

d. Sometimes two or more muscles act on a single joint in the same way, but the insertions of these muscles produce different mechanical advantages.

4. The three classes of levers are first class, second class, and third class. (Fig. 9.9)

a. In a first-class lever, the fulcrum is in the middle, like a seesaw (RFE); an example is the atlanto-occipital joint of the neck.

b. In a second-class lever, the resistance is in the middle, like a wheelbarrow (FRE); an example is the mandible when the digastric muscle opens the mouth.

c. In a third-class lever, the effort is in the middle, like rowing with an oar (REF); most musculoskeletal levers are third class.

i. The lever classification of a joint can change as it makes different actions.

ii. The forearm is a third-class lever when it flexes the elbow but a first class lever when extends the elbow.

iii. The mandible is a second-class lever when we open the mouth and a third-class lever when we close it to bite.

5. Range of motion (ROM), or flexibility, is one aspect of joint performance and affects quality of life; ROM is normally determined by three factors: structure of the articular surfaces; strength and tautness of ligaments and joint capsules; and action of muscles and tendons.

a. In many cases, joint movement is limited by surface structure; for example, the elbow cannot be straightened beyond 180° or so because the fossa of the humerus prevents the olecranon of the ulna from moving farther.

b. Some bone surfaces impose little limitation on movement, but ligaments restrict the joint’s range; for example, ligaments of the knee prevent it from extending beyond 180° or so.

i. Gymnasts, dancers, and acrobats increase the ROM of joints by gradually stretching the ligaments during training.

c. Muscles and tendons limit motion; for example, extension of the knee is also limited by the hamstring muscles on the posterior thigh.

i. Even resting muscles maintain a state of tension called muscle tone, which stabilizes a joint.

ii. The nervous system continually monitors and adjusts joint angles and muscle tone.

6. The mutually perpendicular x, y, and z axes of geometry correspond to the transverse, frontal, and sagittal planes of the body.

a. A moving bone has a stationary axis of rotation that passes through the bone in a direction perpendicular to the plane of movement.

b. The shoulder joint can demonstrate the relationship between movement in a plane and axes of rotation.

i. When the arm is raised to the side of the body, the arm rises in the frontal plane, but its axis of rotation is in the sagittal plane. (Fig. 9.10)

ii. If the arm is raised in front, it moves through the sagittal plane, but its axis of rotation is on the frontal plane.

iii. If the arm is swung in a horizontal arc, it moves in the transverse plane but its axis of rotation is vertical.

c. Because the arm can move in all three anatomical planes, the shoulder joint is said to have three degrees of freedom, or is multiaxial.

7. There are six fundamental classes of synovial joints, distinguished by the shapes of the articular surfaces and degrees of freedom; one type is multiaxial, three are biaxial, and two are monaxial. (Fig. 9.11)

a. Ball-and-socket joints (multiaxial) include the shoulder and hip joints; in both cases, one bone has a smooth hemispherical head that fits into a cuplike socket on the other bone.

b. Condylar (ellipsoid) joints exhibit an oval convex surface on one bone that fits into a depression on the other; examples are the radiocarpal joint of the wrist and metacarpophalangeal joints at the bases of fingers.

c. In the biaxial saddle joints, both bones have a saddle-shaped surface; one example is the trapeziometacarpal joint at the base of the thumb, and another is the sternoclavicular joint.

d. Plane (gliding) joints, which have limited movement, have bone surfaces that are flat or only slightly concave and convex; examples are found between the carpal and tarsal bones of the wrist and ankle, and the articular processes of the vertebrae.

e. Hinge joints, which are monaxial joints, move freely in one plane with little movement in any other; examples are the elbow, knee, and interphalangeal joints.

f. Pivot joints are monaxial joints in which a bone spins on its longitudinal axis; examples are the atlantoaxial joint between the first two vertebrae and the radioulnar joint at the elbow.

g. Some joints cannot be easily categorized into these six types, such as the temporomandibular joint; this has aspects of condylar, hinge, and plane joints.

D. The movements of synovial joints are described with a specific vocabulary that is used in medicine, kinesiology, physical therapy, and other scientific fields. (pp. 298–305)

1. When a person is standing in anatomical position, each joint is said to be in its zero position; movements are described as deviation from the zero position or returning to it.

2. Flexion is a movement that decreases a joint angle, usually in the sagittal plane. (Fig. 9.12)

a. Flexion is particularly common at hinge joints but occurs in other types of joints as well.

b. Flexion in ball-and-socket joints mean to raise the limb in front of you, such as pointing at something in front or continuing upward to the sky.

3. Extension straightens a joint and generally return a body part to the zero position.

a. In stair climbing, both the hip and knee extend when lifting the body to the next higher step.

4. Hyperextension is extreme extension beyond the zero position.

a. Each backswing of the lower limb when you walk hyperextends the hip.

5. Abduction is the movement of a body part in the frontal plane away from the midline of the body; for example, moving feet apart to stand spread-legged or raising the arm to the side. (Fig. 9.13)

6. Adduction is movement in the frontal plane back to the midline. (Fig. 9.13b)

7. Some joints can be hyperadducted, as in standing with the ankles crossed or crossing the fingers; other joints can be hyperabducted, as in abducting the arm so that it crosses past the head.

8. Elevation is a movement that raises a body part vertically in the frontal plane; depression lowers a body part in the same plane. (Fig. 9.14)

9. Protraction is the anterior movement of a body part in the transverse plane; retraction is the posterior movement. (Fig. 9.15)

10. In circumduction, one end of an appendage remains stationary while the other end makes a circular motion; for example, when an artist draws a circle. (Fig. 9.16)

11. Rotation means a movement in which a bone turns on its long axis; for example, if the elbow is bent, and the forearm is moved side to side, the humerus of the arm rotates. (Fig. 9.17)

12. Supination is the movement that turns the palm to face anteriorily or upward; pronation turns the palm posteriorly or downward. (Fig. 9.18)

13. The head and trunk exhibit certain special movements. (Fig. 9.19)

a. Flexion of the vertebral column produces forward-bending movements, whereas extension straightens the trunk or the neck; hyperextension is employed in looking up to the sky or bending backward.

b. Lateral flexion is tilting the head or trunk to the right or left of the midline.

c. Twisting at the waist or turning of the head is right or left rotation.

14. The mandible exhibits special movements for biting and chewing. (Fig. 9.20)

a. In biting, the mandible is protracted to bring the lower incisors forward; after the bite is taken, the mandible is retracted.

b. To actually take the bite, the mandible is depressed to open the mouth, then elevated to close the incisors.

c. Chewing involved a grinding action with a side to side movement called lateral excursion and medial excursion.

15. The hand and digits also exhibit specialized movements. (Fig. 9.21)

a. Ulnar flexion tilts the hand toward the little finger, and radial flexion tilts it toward the thumb.

b. Flexion of the fingers is curling them; extension is straightening them.

c. Spreading the fingers apart is abduction, and bringing them together so they touch is adduction.

d. The thumb rotates during embryonic development so that it is at almost 90° from the rest of the hand.

i. Flexion of the thumb means bending the joints to that the tip is toward the palm, and extension is straightening the thumb.

ii. Moving the thumb outward so it’s 90° from the index finger is called radial abduction.

iii. Moving the thumb away from the plane of the hand so it points anteriorly, as in picking up a tool, is called palmar abduction.

iv. The closing movement, adduction, brings the thumb back to zero position, touching the base of the index finger.

v. Two terms are unique to the thumb—opposition is moving the thumb to touch the tip of any of the fingers; reposition is to return to zero position.

16. The foot also exhibits some unique movements. (Fig. 9.22)

a. Dorsiflexion is a movement that elevates the toes by pulling the foot upward.

b. Plantar flexion is movement of the foot so the toes point downward.

c. Inversion is a movement that tips the soles medially, somewhat facing each other.

d. Eversion tips the soles laterally, away from each other.

e. Pronation and supination also apply to the feet but refer to a complex combination of movements.

i. Pronation is a combination of dorsiflexion, eversion, and abduction.

ii. Supination is a combination of plantar flexion, inversion, and adduction.

A. The temporomandibular (jaw) joint, or TMJ, is the articulation of the condyle of the mandible with the mandibular fossa of the temporal bone. (p. 305) (Fig. 9.23)

1. The synovial cavity is divided into superior and inferior chambers by an articular disc that permits lateral and medial excursion.

2. Two ligaments support the joint.

a. The lateral ligament prevents posterior displacement of the mandible.

b. The sphenomandibular ligament on the medial side of the joint extends from the sphenoid bone to the ramus of the mandible.

i. A stylomandibular ligament from the styloid process to the angle of the mandible is not part of the TMJ proper.

3. The TMJ may be dislocated by a deep yawn or other strenuous depression that causes the condyle to pop out of the fossa; it can be relocated by pressing down on the molars while pushing the jaw backward.

B. The glenohumeral (humeroscapular) joint, or shoulder joint, is where the hemispherical head of the humerus articulates with the glenoid cavity of the scapula. (pp. 305–307) (Figure 9.24)

1. The relatively loose shoulder joint capsule and shallow glenoid cavity sacrifice stability to maximize mobility.

2. The glenoid labrum, a fibrocartilage ring around the cavity, makes it somewhat keeper than it looks in dried skeletons.

3. The shoulder is stabilized mainly by the biceps brachii muscle on the anterior side of the arm.

a. A tendon arising from the long head of the biceps brachii passes through the intertubercular groove of the humerus and inserts on the superior margin of the glenoid cavity, acts as a taut strap.

4. Four additional muscles help stabilize this joint: the supraspinatus, infraspinatus, teres minor, and subscapularis; their tendons form the rotator cuff. (Fig. 10.24)

5. Five principal ligaments also support the shoulder joint.

a. Three of these are collectively called the glenohumeral ligaments; they are weak and sometimes absent. (Fig. 9.23b)

b. The other two are the coracohumeral ligament and the transverse humeral ligament. (Fig. 9.23b)

6. Four bursae occur at the shoulder: the subdeltoid, subacromial, subcoracoid, and subscapular. (Fig. 9.23b)

Insight 9.3 Shoulder Dislocation

C. The elbow joint is a hinge joint composed of two articulations: the humeroulnar joint and the humeroradial joint; these are enclosed in a single joint capsule. (pp. 307–308) (Fig. 9.25)

1. On the posterior side of the elbow is the olecranon bursa, which eases the movement of tendons over the joint.

2. Side-to-side motions are restricted by the radial (lateral) collateral ligament and the ulnar (medial) collateral ligament.

3. A third joint, the proximal radioulnar joint, is the point where the edge of the disclike head of the radius fits into the radial notch of the ulna, held in place by the anular ligament; this joint is not part of the hinge joint but allows rotation of the forearm.

D. The coxal (hip) joint is the point where the head of the femur inserts into the acetabulum of the hip bone. (pp. 308–310) (Fig. 9.26)

1. This joint is much more stable than the shoulder joint.

2. The acetabular labrum, a horseshoe-shaped ring of fibrocartilage, makes the socket somewhat deeper than seen in a dried skeleton.

3. Some infants suffer congenital dislocation of the hip joints that can be treated with traction. (Fig. 9.27)

4. Ligaments that support the coxal joint include the iliofemoral and pubofemoral ligaments on the anterior side, and the ischiofemoral ligament on the posterior side.

5. The head of the femur has a pit called the fovea capitis where the round ligament, or ligamentum teres, arises and attaches to the lower margin of the acetabulum.

a. This ligament probably does not play a significant role in holding the femur in its socket, but contains an artery supplying blood to the head of the femur.

6. The transverse acetabular ligament bridges a gap in the inferior margin of the acetabular labrum.

E. The tibiofemoral (knee) joint is the largest and most complex diarthrosis of the body. (pp. 310–312) (Fig. 9.28, 9.29)

1. The knee is primarily a hinge joint but when flexed allows slight rotation and gliding.

2. The patella and patellar ligament also articulate with the femur to form a gliding patellofermoral joint.

3. The joint capsule encloses only the lateral and posterior aspects, not the anterior, which is covered by the patellar ligament and the lateral and medial patellar retinacula; these are extensions of the tendon of the quadriceps femoris muscle.

4. The knee is stabilized mainly by the quadriceps tendon in front and the tendon of the semimembranosus muscle on the rear of the thigh.

5. The joint cavity contains two C-shaped cartilages, the lateral meniscus and the medial meniscus, joined by a transverse ligament.

6. The posterior popliteal region is supported by several extracapsular ligaments and two intracapsular ligaments.

a. The extracapsular ligaments include the fibular (lateral) collateral ligament and the tibial (medial) collateral ligament.

b. the two intracapsular ligaments deep with the joint cross each other in the form of an X and are called the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL). (Fig. 9.28a)

i. Anterior and posterior refers to their attachment on the tibia, not on the femur.

ii. When the knee is extended, the ACL is pulled tight and prevents hyperextension; the PCL prevents the tibia from being displaced backward.

iii. The ACL is one of the most common sites of knee injury.

7. The ability to “lock” the knees and stand erect, without tiring the extensor muscles, is important in human bipedalism.

a. When the knee is extended to the fullest degree allowed by the ACL, the femur rotates medially on the tibia to lock the knee; in this state, all the major knee ligaments are twisted and taut.

b. To unlock the knee, the popliteus muscle rotates the femur laterally and untwists the ligaments.

8. The knee joint has at least 13 bursae.

a. Four bursae are anterior: the superficial infrapatellar, the suprapatellar, the prepatellar, and the deep infrapatellar.

b. Located in the popliteal region are the popliteal bursa and semimembranosus bursa.

c. At least seven more bursae are found on the lateral and medial sides of the knee joint.

F. The talocrural (ankle) joint includes two articulartions: a medial joint between tibia and talus, and a lateral joint between fibula and talus, both enclosed in a single joint capsule. (p. 312–314) (Fig. 9.31)

1. The malleoli of the tibia and fibula overhand the talus and prevent most side-to-side motion.

2. The ankle includes three groupings of ligaments.

a. The anterior and posterior tibiofibular ligaments bind the tibia to the fibula.

b. The multipart medial (deltoid) ligament binds the tibia to the foot on the medial side.

c. The mutipart lateral collateral ligament binds the fibula to the foot on the lateral side.

3. The calcaneal (Achilles) tendon extends from the calf muscles to the calcaneus.

a. This tendon plantarflexes the foot and limits dorsiflexion.

4. Plantar flexion is limited by extensor tendons on the anterior side of the ankle and the anterior part of the joint capsule

5. Sprains (torn ligaments and tendons) are common at the ankle and result commonly from extreme and sudden inversion or eversion; sprains and other joint disorders are described in Table 9.1.

Insight 9.5 Arthritis and Artificial Joints (Fig. 9.32, 9.33)
Cross References

Additional information on topics mentioned in Chapter 9 can be found in the chapters listed below.

Chapter 8: Parts of the scapula

Chapter 10: Tendons

Chapter 10: Muscles that act on joints and the movements they produce