Patella The patella is the largest sesamoid bone of the body. A sesamoid bone is a bone that forms within a tendon. The patella occupies the posterior half of the quadriceps tendon just anterior

Fibrous joints are synarthrotic joints that bind bone to bone with collagenous connective tissue. The amount of connective

tissue binding the neighboring bones can vary considerably. Examples of fi brous joints are depicted on this and the facing page. Gomphoses and sutures (the four different suture types are shown on the opposite page) have a very thin membrane of collagenous connective tissue anchoring neighboring bony structures to one another. On the other hand, the syndesmoses between the tibia and fi bula — both the interosseous membrane and the tibiofi bular ligaments at the distal end — have considerably more binding connective tissue. There is also an example of another syndesmosis, the interspinous ligament, in the next section.

7

6

8

2

4

3

1

5

Dento-alveolar syndesmosis or gomphosis

Sagittal section of tooth in mandible

9

Crural skeleton – tibia and fibula

Anterior view

124

Diarthroses differ from synarthroses in one major way: instead of connecting neighboring bones by a solid mass

of connectve tissue, the bony connection consists of a double-layered connective tissue capsule that surrounds a lubricated cavity between the bones. Within the capsule the ends of neighboring bony surfaces are covered by a smooth layer of hyaline cartilage. As a result of this design there is typically a much greater range of motion present in synovial joints, and they form the joints of the skeleton that are responsible for the major movements of the body. The outer layer of the capsule, the fi brous membrane, is continuous with the periosteum on the adjoining bones, while the inner layer of the capsule, the synovial membrane, attaches from the border of the articular cartilage on one bone to the border of the articular cartilage on the other bone. Additionally, the synovial membrane secretes synovial fl uid, a lubricant that reduces friction between the mobile cartilage-covered articular surfaces of the bones. The section through a fi nger joint below and the dissections of the knee joint on the opposite page illustrate the basic features of a synovial joint. The pages that follow depict the major synovial joints of the skeleton. One other key feature among synovial joints that is responsible for their varied range of motion is the shape of the adjoining bone surfaces. It is this feature that anatomists use to describe the different types of synovial joints.

4

2

Proximal interphalangeal joint showing design of synovial joint

Frontal section, anterior view

128

There are seven types of synovial joints in the body. Each of the different synovial joints has the basic structural features common to all synovial joints but is

further classified based on the shape of and motion that occurs at the articular surfaces of the joint. The different types of synovial joint are depicted below and on the opposite page. Note the shapes of the reciprocal surfaces as you study these photos.

Hinge joint example

Humero-ulnar joint of elbow

130

The complex temporomandibular joint differs from other synovial joints by having an articular disc that usually separates the joint into two separate

synovial capsules, one above and one below the disc. The articular surfaces have a covering of dense fi brocartilage rather than the typical hyaline cartilage of most synovial joints. With its associated ligaments this joint structure accounts for the complex series of movements that are essential during the activities of eating and speech. Each temporomandibular joint is a condylar joint and both joints together form a bicondylar joint. The fi brous membrane of the articular capsule spans from temporal bone to mandible only on the lateral side. Anteriorly, medially, and posteriorly the fi bers attach from mandible and temporal bone to the articular disc. Extrinsic ligaments that help stabilize the joint are the lateral temporomandibular ligament, sphenomandibular ligament, and stylomandibular ligament.

5

4

2

10

11

Section of right temporomandibular joint

Lateral view of sagittal section

132

The glenohumeral or shoulder joint is a ball and socket joint and is the most mobile joint in the body. The tremendous range of motion at this joint is the result of few external ligaments that present little

limitation to movement, and shallow, ovoid articular surfaces that make movements in all planes of space possible. In fact, surrounding muscles and tendons play a more signifi cant role in joint support than do the joint structures. The capsular ligament is extremely lax, providing limited support to the joint. Blending with the capsule are the tendons of four muscles. Together the capsule and tendons form the rotator cuff, which is the major support structure of the joint.

1 Articular cartilage

2 Synovial membrane

3 Fibrous membrane

4 Glenoid labrum

5 Acromioclavicular ligament

6 Clavicle

7 Humerus

8 Glenoid of scapula

9 Acromion of scapula

10Supraspinatus muscle

11Subscapularis muscle

12Deltoid muscle

13Tendon of long head of biceps brachii

14Skin

15Subcutaneous layer

11

2

4 3

15

Section of left glenohumeral joint

Anterior view of frontal section

133

The elbow joint is a complex joint comprised of multiple articular surfaces within one articular capsule. The elbow joint can be subdivided into three distinct articular interfaces — the humero-ulnar joint (hinge), the humeroradial joint (combined hinge and pivot), and

the proximal radioulnar joint (pivot). Two distinct pairs of movements occur as a result of the articulations within the elbow joint — the hinged movements of fl exion and extension, and the rotational movements of pronation and supination. Unlike the shoulder joint, the joints fo the elbow have strong extrinsic ligaments that limit movemnts and stabilize the articulating bones. The fi brous capsule is thin anteriorly and posteriorly, allowing for free range of motion during fl exion and extension. On either side the capsule is reinforced by strong extrinsic ligaments, the ulnar collateral and radial collateral ligaments. Wrapping from the back of the ulna at the base of the olecranon to the front of the ulna at the lateral surface of the coronoid process is the semicircular anular ligament. With the radial notch of the ulna this ligament forms a fi bro-osseous ring for the pivoting action of the radial head.

10Triceps brachii muscle

11Brachioradialis muscle

9

Bones of elbow joint

Anterior view

Section of pronated left elbow joint

Medial view of sagittal section

134

The knee joint is a combined bicondylar and saddle joint. The relationships between the femur and the tibia provide no interlocking joint mechanisms or stability between the neighboring bones, and from this perspective the knee joint is completely unstable. The strength

of the knee joint is dependent on strong ligaments and surrounding muscles. Although its primary motions are of a hinge nature, it is a complex joint with subtle rotational and sliding movements also. The major stabilizers of the joint are four strong ligaments. Two collateral ligaments support the joint on either side, while two cruciate ligaments criss-cross through the middle of the joint. The tibial or medial collateral ligament is a strong, fl at band that stretches from the femoral epicondyle to the tibial condyle. Posteriorly it fi rmly attaches to the joint capsule and the medial meniscus, while anteriorly bursae separate it from these structures. The fi bular or lateral collateral ligament is a strong cord that runs from the lateral femoral

6

19

24

Section of right knee joint

Lateral view of sagittal section

136

A synovial bursa is a small sac-like structure interposed between structures that generate significant amounts of friction.

Bursae have a similar design to the articular capsule of a synovial joint. These small bags have an outer fi brous membrane of dense irregular collagenous connective tissue and an inner lining of synovial membrane. The synovial membrane produces a small amount of synovia as a lubricant inside the sac. The fi brous membrane binds to surrounding tissues, allowing the juxtaposed walls of synovial membrane to rub together in a frictionless manner. Many bursae arise as outgrowths of synovial joint cavities. In some cases these pinch off from the joint forming sacs that are independent from the joint, while other bursal sacs retain their connections with the joint cavity. A synovial sheath is a modifi ed bursa that wraps around a tendon to protect it from friction on all sides. In the tight confines of the wrist, ankle, and digits, tendons often pass beneath fi brous bands called retinacula. The retinaculum is a connective tissue band that crosses over the tendons and keeps them from being displaced upward when the muscle shortens and bends the joints. Because the retinaculum and bone create a fi broosseous tunnel around the tendon, considerable friction can occur on all surfaces of the tendon at these locations. As the tendon moves through the tunnel, the juxtaposed synovial membranes smoothly glide over each other with minimal friction.

2

7

3

6

5

Synovial bursae around the knee joint

Medial view of sagittal section

88

4

9

10

Tendon sheath of fingers

Anterior view, pin inserted into tendon sheath

138