SYNOVIAL JOINTS

A joint in the human body is defined as the point where two bones join together. A joint must also transmit forces between participating bones.

Anatomical Characteristics

Diarthrotic joints are also called synovial articulations. Because of their ability to move freely they become more susceptible to trauma. 

Anatomical characteristics of the synovial articulations consist of four features:

1. they have a capsule or ligaments

2. the capsule is lined with a synovial membrane

3. the opposing bone surfaces contain hyaline cartilage

4. there is a joint space (joint cavity) containing a small amount of fluid (synovial fluid) (Figure 6-9).

Joint Capsule

• Bones of the diarthrotic joint are held together by a strong cuff of fibrous tissue known as the capsule, or capsular ligament.

• Parts of the capsule become slack or taut depending on the joint movements.

Ligaments

• Ligaments are sheets or bundles of collagen fibers that form a connection between two bones.

• Ligaments act as protective backups for the joint.

• Primary protection occurs from the dynamic aspect of muscles and their tendons.

Ligaments fall into two categories:

1. Intrinsic ligaments occur where the articular capsule has become thickened in some places.

2. Extrinsic ligaments are separate from the capsular thickening.

A major factor in ligamentous injury is the viscoelastic tissue properties of ligaments and capsules.

1. Constant compression or tension causes ligaments to deteriorate,

2. Chronic inflammation of ligamentous, capsular, and fascial tissue causes a shrinkage of collagen fibers; therefore repeated microtraumas over time make capsules and ligaments highly susceptible to major acute injuries.

Synovial Membrane and Synovial Fluid

Lining the synovial articular capsule is a synovial membrane. 

• Synovial fluid is secreted and absorbed by the synovial membrane.

• Synovial fluid has the consistency of egg white and acts as a joint lubricant.

Synovial fluid has the ability to vary its viscosity.

1. During slow movement, the fluid thickens
2. during fast movement it thins to a greater or lesser extent

Articular Cartilage

• The ends of the bones in a diarthrotic joint are covered by hyaline cartilage, which acts as a cushion for the bone ends. Its general appearance is smooth and pearly.
• Hyaline cartilage acts like a sponge in relation to synovial fluid. As movement occurs, it absorbs and squeezes out the fluid as pressures vary between the joint surfaces.
• Cartilaginous degeneration, producing microtrauma, may occur during the abnormal compressional forces that occur over a period of time.
• Hyaline cartilage has no direct blood supply, receiving its nourishment from the synovial fluid.
   

Synovial Joint Injury Classification

Acute Joint Injuries

• The major injuries that happen to synovial joints are sprains, subluxations, and dislocations.

Sprains

• The sprain, one of the most common and disabling injuries seen in sports, is a traumatic joint twist that results in stretching or total tearing of the stabilizing connective tissues.

• When a joint is forced beyond its normal anatomical limits, there is injury to ligaments and to the articular capsule and synovial membrane.

According to the extent of injury, sprains are graded in three degrees.

1. A grade 1 sprain is characterized by some pain, minimum loss of function, mild point tenderness, little or no swelling, and no abnormal motion when tested.
2. With a grade 2 sprain there is pain, moderate loss of function, swelling, and in some cases slight to moderate instability.
3. A grade 3 (or severe) sprain is extremely painful with major loss of function, severe instability, tenderness, and swelling. A grade 3 sprain may also represent a subluxation that has been reduced spontaneously.

• Effusion of blood and synovial fluid into the joint cavity during a sprain produces joint swelling, local temperature increase, pain or point tenderness, and skin discoloration (ecchymosis).

• Ligaments and capsules heal slowly because of a relatively poor blood supply; however, their nerves are plentiful often producing a great deal of pain when injured.

The joints that are most vulnerable to sprains in sports are the ankles, knees, and shoulders.

   
Subluxations, dislocations, and diastasis

• Dislocations are second to fractures in terms of disabling the athlete.

• The highest incidence of dislocations involves the fingers and, next, the shoulder joint (Figure 6-11).

Dislocations, which result primarily from forces causing the joint to go beyond its normal anatomical limits, are divided into two classes:

1. Subluxations are partial dislocations in which an incomplete separation between two articulating bones occurs.
2. Luxations are complete dislocations, presenting a total disunion of bone apposition between the articulating surfaces.

Several factors are important in recognizing and evaluating dislocations:

1. There is a loss of limb function. The athlete usually complains of having fallen or of having received a severe blow to a particular joint and then suddenly being unable to move that part.

2. Deformity is almost always apparent. Because the deformity can often be obscured by heavy musculature, it is important for the examiner to palpate the injured site to determine the loss of normal body contour. Comparison of the injured side with its normal counterpart often reveals distortions.

3. Swelling and point tenderness are immediately present.

• At times, as with a fracture, x-ray examination is the only absolute diagnostic measure.

• A first-time dislocation should always be considered and treated as a possible fracture.

Bursitis

• Bursae provide protection between tendons and bones, between tendons and ligaments, and between other structures where there is friction.

• Bursae located around synovial joints can become acutely or, over a period of time, chronically inflamed.

• Bursitis in the knee, elbow, and shoulder is common among athletes.