Chapter 8



Chapter 8

Joints

Joints are classified by structure and function.

Structural Classification involves the material binding the joint together and whether a joint cavity is present. There are three types of joints:

• Fibrous- typically immovable

• Cartilaginous- slightly movable

• Synovial- free movement

Functional Classification is based on the amount of movement in the joint. There are three types of movement:

• Synarthroses-immovable

• Amphiathroses-slight movement

• Diathroses-free movement

I. Fibrous Joints

Bones are joined by bands of dense fibrous connective tissue. There is no joint cavity. There are three types of fibrous joints:

• Sutures

• Syndesmoses

• Gomphoses

1. Sutures- found connecting the plates of the skull. Examples include the coronal, lamboid and squamous sutures of the skull. As the plates fuse in the adult, the sutures are called synostoses and result in immovable joints.

2. Syndesmoses- bones connected by ligaments or other connective tissue. Movement is limited and depends on the length of the connecting fibers. Examples include the joint at the distal end of the tibia and fibula.

3. Gomphoses- This is a peg and socket joint. The tooth in its boney alveolar socket is the only example. The periodontal ligament connects the tooth to the socket.

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II. Cartilaginous Joints

These are joints where the articulating bones are united by cartilage. There is no joint cavity and mobility is limited. There are 2 types:

1. Synchondroses- These are joints where hyaline cartilage unites the bones. The most common example is the epiphyseal plates of the long bones. These eventually form synostoses. The costal cartilage and first rib is a second example of a cartilaginous joint.

2. Symphyses- Articular surfaces of bones are covered with hyaline cartilage which fuses fibroelastic cartilage. These joints are amphiarthrotic and have strength and flexibility. Examples include intervertebral joints and the pubic symphyses.

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III. Synovial Joints

The articulating bones are separated by a fluid containing cavity. These joints are diarthrotic and provide the greatest movement. They have 6 distinguishing features.

1. Articular cartilage that is approximately 1 mm thick and composed of hyaline cartilage.

2. Synovial cavity that is a potential space containing a small amount of liquid.

3. Articular capsule that is made of 2 layers. A dense outer fibrous capsule of dense irregular connective tissue that is continuous with the periosteum of the bone and a synovial membrane composed of loose connective tissue.

4. Synovial fluid occupies the free space of the joint capsule. Its contents are derived from the blood. This provides a slippery surface that reduces friction.

5. Reinforcing ligaments provide additional structure to the joints and are of two types, intrinsic ligaments which are part of the fibrous capsule and extrinsic ligaments that are distinct and separate from the joint capsule.

6. Nerve and blood vessels supply nutrients and provide sensory information. Sensory nerves can indicate pain but are also important in monitoring the stretching of a joint and adjusting muscle tone.

Associated Structures

In addition to these 6 characteristics, other structures commonly associated with synovial joints include fat pads and articular discs of Fibrocartilage such as the menisci in the knee. Also found in synovial joints are bursae and tendon sheaths. They work like “ball bearings” and reduce friction where ligaments, tendons and muscles rub against each other. A tendon sheath is an elongated bursa that wraps completely around a tendon. One example is the tendons running together in the wrists (carpal tunnel).

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Stabilizing Factors

Three factors, shape of the articular surfaces, ligaments and muscle tone all provide additional stability for the synovial joint. Of these, muscle tone is the most important. Tendons are kept taught, keeping the joints together. This is important for joints like the knee, shoulder and foot. Ligaments provide bracing and help prevent excessive motion. They snap if their length is exceeded by 6%. Articular surfaces help to direct motion but provide little stability.

IV. Movement of Synovial Joints

All skeletal muscles attach to bone or connective tissue by at least 2 points.

• The Origin is the muscle attachment to the less movable bone

• The Insertion is the muscle attachment to the movable bone.

Motion occurs when a muscle contracts and moves towards its origin. Motion can occur along transverse, frontal and sagittal plans.

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Range of Motion of a joint is classified as follows:

• Nonaxial movement- slipping

• Uniaxial movement- 1 plane

• Biaxial movement- 2 planes

• Multiaxial movement- many planes

These motions are seen in 3 general types of movement:

• Gliding

• Angular

• Rotational

1. Gliding motion is the simplest and consists of one bone sliding over another. This is seen with the intercarpal and intertarsal joints and between vertebrae.

2. Angular movements increase or decrease the angle between 2 bones. Movements include:

• Flexion

• Extension

• Hyperextension

• Abduction

• Adduction

• Circumduction

a) Flexion is the bending along a sagittal plane; this decreases the angle of the joint and brings the bones closer together.

b) Extension is the straightening of bones resulting in the increase angle between bones.

c) Hyperextension is the bending of a joint beyond anatomical position.

d) Abduction is moving away from the midline along the frontal plane.

e) Adduction is moving toward the midline.

f) Circumduction is moving the limb to describe a cone, for example the motion made by a pitcher. This motion involves the motions described above.

3. Rotation is the turning of the bone along its axis. This is seen with the first and second vertebrae and the hip and shoulder joints.

4. Special movements are defined by specific joints.

a) Supination and pronation are seen with the movement of the radius over the ulna. Supination is the radius and ulna are parallel and the palms are facing forward. Pronation the radius and ulna form an X and the palms face posterior. (Think 3 P’s-pronation palms posterior). This is the position the hands take when dribbling a basketball.

b) Dorsi flexion and plantar flexion is seen with the bending of the foot up and down. Bending the foot towards the tibia is dorsi flexion and pointing the toes away is plantar flexion. This latter motion is seen with ballet dancers.

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c) Inversion/eversion is medial and lateral movement of the ankle. Overextension or inversion is commonly seen with the sprained ankle.

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d) Protraction and Retraction are the anterior and posterior movement of the jaw. Protraction is the jutting out of the jaw.

e) Elevation and depression are the movement of the jaw in an inferior superior direction. (Opening and closing)

f) Opposition is seen with touching of the thumb to the tips of the other phalanges.

V. Types of Synovial Joints

There are six major categories of synovial joints.

1. Plane joints- short, .nonaxial motion also known as gliding joints. Examples are the carpals and tarsals.

2. Hinge joints- motion is a single plane. A cylindrical end of 1 bone conforms to a trough of another. Typical examples include the elbow and phalanges.

3. Pivot joints- The rounded end of one conforms to a sleeve of another, the dens of the axis and atlas of the cervical vertebrae.

4. Condyloid joints- both articulating surfaces are oval and this allows for angular motion. The oval articular surface of one bone fits into the depression of the adjoining bones; examples include phalangeal joints (knuckles) and radial carpal (wrist).

5. Saddle joints- both joints have concave and convex areas and are shaped like saddle. This motion is seen with the twiddling of the thumbs and involves the carpometacarpal joint of the thumb.

6. Ball and socket- Multiaxial and are the most freely moving of the synovial joints. These joints have a spherical head fitting into a cup like socket.

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VI. Selected Synovial Joints

1. Knee Joint

This is considered the most complex joint in the human body. It is actually considered three joints working together.

These are:

• An intermediate joint between the patella and distal end of the femur (femoropatellar joint). This is a plane joint.

• A lateral and medial tibiofemoral joints between the femoral condyles and the menisci below. The menisci help prevent lateral motion and attach to the outer margins of the joint capsule on the tibia. They are easily torn. These are an example of a hinge joint.

The knee is unique in that it is not completely enclosed by a capsule. The articular capsule is found only on the lateral and posterior surfaces. The anterior surface is covered by three ligaments going from the patella to the tibia. They are:

• The patella ligament

• The medial and lateral patellar retinacula ligaments. They merge with the articular capsule on each side.

The synovial cavity of the knee has a complicated shape and over one dozen associated bursae. Some are easily injured such as the subcutaneous prepatellar bursa which lies just over the patella.

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Although the patellar, medial and lateral patellar retinacula cover the anterior surface of the knee joint they do not provide extensive support. Two types of ligaments are responsible for the stability and strength the knee joint. They consist of capsular and extracapsular ligaments. They work to prevent hyperextension of the knee. These ligaments are:

• The fibular and tibial(medial and lateral) collateral ligaments are extracapsular ligaments and prevent lateral and medial rotation when the knee is extended.

• The oblique popliteal ligament is an extracapsular ligament that is an extension of the semimembranous muscle and helps to stabilize the posterior portion of the knee.

• The arcuate popliteal ligament also reinforces the posterior aspect of the knee.

The oblique popliteal and arcuate popliteal ligaments actually form an X on the posterior aspect of the knee.

The intracapsular ligaments are the cruciate ligaments. The anterior and posterior cruciate ligaments cross each other forming an X in the notch between the femoral condyles. They prevent anterior and posterior displacement.

The anterior cruciate ligament attaches to the anterior intercondylar area and attaches on the medial side of the lateral condyle of the femur. It is lax when the knee is flexed and taut when it is extended.

The posterior cruciate ligament is stronger and is attached to the posterior intercondylar area of the tibia and attaches to the lateral side of the medial condyle on the femur.

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The knees are the most susceptible joint for sports injury because of the reliance on nonarticular factors for joint stability. Although the knee can absorb vertical forces equal to seven times the body, it is vulnerable to lateral blows. Common knee injuries involve the 3 C’s, collateral ligaments, cruciate ligaments and cartilage (menisci). Lateral blows are the most dangerous, tearing the tibial collateral ligament and the medial meniscus and the anterior cruciate ligament.

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2. Shoulder (Glenohumeral) Joint

This is an example of a ball and socket joint. The large head of the humerus fits into the glenoid cavity of the scapula. The cavity is extended by a fibrocartilage ring called the glenoid labrum.

The articular capsule runs from the margins of the glenoid cavity to the anatomical neck of the humerus. It is loose and allow for a free range of motion seen with this joint.

Connective tissue support comes from three groups of ligaments.

A) Coracohumeral ligament provides the only strong support of the upper limb. It runs from the coracoid process to the greater tubercle of the humerus.

B) Three Glenohumeral ligaments strengthen the front of the capsule. These ligaments are weak.

C) The Rotator Cuff is formed from four tendons and muscles that encircle the joint. The muscles include the subscapularis, supraspinatus infraspinatus and tees minor.

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Injuries

Rotator cuff injuries are common when the arm is severely circumducted as occurs with pitchers. Dislocations of the shoulder typically occur on the anterior and inferior aspects of the shoulder.

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3. Elbow Joint

The radius rotates on the capitulum of the humerus. It allows for pronation and supination. The articular capsule extends down to the anular ligament of the radius. Side to side movement is prevented by the ulnar collateral ligament and radial collateral ligament.

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Dislocations to the elbow are uncommon due to the tight fitting of the ulna to the humerus. Inflammation of the tendons and ligaments are more common. The most notable being the so called “tennis elbow”

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Tennis elbow is an inflammation, soreness, or pain on the outside (lateral) side of the upper arm near the elbow. There may be a partial tear of the tendon fibers, which connect muscle to bone, at or near their point of origin on the outside of the elbow. It typically involves the lateral epicondyle. It is brought about by the repeated twisting (pronation and supination) of the wrist.

4. Hip (Coxal Joint)

This is a ball and socket joint whose movement is limited by strong ligaments. It is formed from the spherical head of the femur and the deeply cupped acetabulum in the pelvis. This cup is enhanced by a fibrous ring called the acetabulum labrum. Additional support is offered by the iliofemoral ligament. This is a triangular shaped ligament that runs from the surface of the ischium to the greater trochanter of the femur. The pubofemoral ligament runs from the pubis to the lesser trochanter of the femur. A third ligament, the ischiofemoral ligament runs from the ischium to the greater trochanter. Together they form a spiral which screws the femur into the acetabulum when a person stands. The ligamentum teres runs from the head of the femur to the lower lip from the acetabulum.

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Common injuries to the hip joint include fractures and dislocations. Hip fractures typically involve the neck of the femur and are the result of underlying disease such as osteoporosis.

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Motor vehicle accidents are the most common cause of hip dislocations. (Wearing a seatbelt can greatly reduce your risk.) Falls from a height (such as a fall from a ladder) or industrial accidents can also generate enough force to dislocate a hip.

5. Temporomandibular Joint (Jaw)

The mandibular condyle is an egg shape and fits into a complex articulation surface in the temporal bone of the skull. Two distinct movements can occur with the jaw, a hinge like movement and the second is a lateral movement. A lateral ligament attaches the ramus of the mandible to the zygomatic arch of the temporal bone.

Because of the hollow socket, jaw dislocations are the most common injuries seen. A deep yawn can dislocate the jaw.

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6. Injuries to the Joints

a) Cartilage tears occur most commonly on the meniscus of the knee. These are usually a combination of compression and shear stress. Lose pieces of cartilage can cause the joint to lock.

b) Sprains are caused by the stretching of the ligament that can lead to tears or complete rupture. Common sites include the knee and ankle. Ligaments will heal, but it is a slow process. If the ligaments are severely damaged, a surgical graft may need to be done.

c) Dislocations are caused by the bones being moved out of alignment. Subluxation is a partial dislocation.

There are many inflammatory and degenerative conditions which may affect the joints.

a) Bursitis and Tendonitis are inflammations caused by over use. A common example is housemaid’s knee where the prepatellar bursa is inflamed; another one is student’s elbow.

b) Arthritis is a common term for over 100 conditions which describe degenerative processes found in the joints. Major examples include osteoarthritis, rheumatoid arthritis and gout.

Osteoarthritis is associated with wear and tear or overuse. It is commonly associated with old age and is considered irreversible. The hips, knees spine and knuckles are common sites for this condition. Treatment, if possible is through joint replacement.

Rheumatoid arthritis can occur at any age and like osteoarthritis affects women more than men. It is considered an autoimmune disease and begins with inflammation of the synovial joint.

Gout is brought about by uric acid accumulation in the joints. It usually affects only one joint. It is more common in men. It may have a genetic component.

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