Synovial Joints Anatomy Summary Questions

Shoulder Surgeries

We already discussed shoulders and their basic design.  They are a ball and socket synovial joint.  They have more mobility than some other synovial joints, but that means less stability.  For this reason, injuries are common.  Repairs can be done arthroscopically for some injuries, but other require you to enter the shoulder by completely opening it up.  I have two surgeries for you.  If you are queasy or do not want to watch the actual surgeries, you may turn your chair around and have your computer at your back during class.  I do not want you playing on your computer or using other tabs.  Just turn around.  

Arthroscopic rotator cuff surgery

Full thickness rotator cuff repair

Synovial Joint Anatomy

Figure 9.8 Synovial Joints Synovial joints allow for smooth movements between the adjacent bones. The joint is surrounded by an articular capsule that defines a joint cavity filled with synovial fluid. The articulating surfaces of the bones are covered by a thin layer of articular cartilage. Ligaments support the joint by holding the bones together and resisting excess or abnormal joint motions.

Structural Features of Synovial Joints

Synovial joints are characterized by the presence of a joint cavity.

• The walls of this space are formed by the articular capsule, a fibrous connective tissue structure that is attached to each bone just outside the area of the bone’s articulating surface.
• The bones of the joint articulate with each other within the joint cavity.
• Friction between the bones at a synovial joint is prevented by the presence of the articular cartilage, a thin layer of hyaline cartilage that covers the entire articulating surface of each bone.
  • However, unlike at a cartilaginous joint, the articular cartilages of each bone are not continuous with each other.
  • Instead, the articular cartilage acts like a Teflon coating over the bone surface, allowing the articulating bones to move smoothly against each other without damaging the underlying bone tissue.
• Lining the inner surface of the articular capsule is a thin synovial membrane.
  • The cells of this membrane secrete synovial fluid (synovia = “a thick fluid”), a thick, slimy fluid that provides lubrication to further reduce friction between the bones of the joint.
  • This fluid also provides nourishment to the articular cartilage, which does not contain blood vessels.
• The ability of the bones to move smoothly against each other within the joint cavity, and the freedom of joint movement this provides, means that each synovial joint is functionally classified as a diarthrosis.

Outside of their articulating surfaces, the bones are connected together by ligaments, which are strong bands of fibrous connective tissue.

• These strengthen and support the joint by anchoring the bones together and preventing their separation.
• Ligaments allow for normal movements at a joint, but limit the range of these motions, thus preventing excessive or abnormal joint movements.
• Ligaments are classified based on their relationship to the fibrous articular capsule.
  • An extrinsic ligament is located outside of the articular capsule, an intrinsic ligament is fused to or incorporated into the wall of the articular capsule, and an intracapsular ligament is located inside of the articular capsule.

At many synovial joints, additional support is provided by the muscles and their tendons that act across the joint.

• A tendon is the dense connective tissue structure that attaches a muscle to bone.
• As forces acting on a joint increase, the body will automatically increase the overall strength of contraction of the muscles crossing that joint, thus allowing the muscle and its tendon to serve as a “dynamic ligament” to resist forces and support the joint.
• This type of indirect support by muscles is very important at the shoulder joint, for example, where the ligaments are relatively weak.

Additional Structures Associated with Synovial Joints

A few synovial joints of the body have a fibrocartilage structure located between the articulating bones.

• This is called an articular disc, which is generally small and oval-shaped, or a meniscus, which is larger and C-shaped.
• These structures can serve several functions, depending on the specific joint.
  • In some places, an articular disc may act to strongly unite the bones of the joint to each other.
    • Examples of this include the articular discs found at the sternoclavicular joint or between the distal ends of the radius and ulna bones.
  • At other synovial joints, the disc can provide shock absorption and cushioning between the bones, which is the function of each meniscus within the knee joint.
  • Finally, an articular disc can serve to smooth the movements between the articulating bones, as seen at the temporomandibular joint.
  • Some synovial joints also have a fat pad, which can serve as a cushion between the bones.

Additional structures located outside of a synovial joint serve to prevent friction between the bones of the joint and the overlying muscle tendons or skin.

• A bursa (plural = bursae) is a thin connective tissue sac filled with lubricating liquid.
  • They are located in regions where skin, ligaments, muscles, or muscle tendons can rub against each other, usually near a body joint.
  • Bursae reduce friction by separating the adjacent structures, preventing them from rubbing directly against each other.
• Bursae are classified by their location.
  • A subcutaneous bursa is located between the skin and an underlying bone.
    • It allows skin to move smoothly over the bone.
    • Examples include the prepatellar bursa located over the kneecap and the olecranon bursa at the tip of the elbow.
  • A submuscular bursa is found between a muscle and an underlying bone, or between adjacent muscles.
    • These prevent rubbing of the muscle during movements.
    • A large submuscular bursa, the trochanteric bursa, is found at the lateral hip, between the greater trochanter of the femur and the overlying gluteus maximus muscle.
  • A subtendinous bursa is found between a tendon and a bone.
    • Examples include the subacromial bursa that protects the tendon of shoulder muscle as it passes under the acromion of the scapula, and the suprapatellar bursa that separates the tendon of the large anterior thigh muscle from the distal femur just above the knee.

Bursae Bursae are fluid-filled sacs that serve to prevent friction between skin, muscle, or tendon and an underlying bone.

• Three major bursae and a fat pad are part of the complex joint that unites the femur and tibia of the leg.

A tendon sheath is similar in structure to a bursa, but smaller.

• It is a connective tissue sac that surrounds a muscle tendon at places where the tendon crosses a joint.
• It contains a lubricating fluid that allows for smooth motions of the tendon during muscle contraction and joint movements.

Bursitis

Bursitis is the inflammation of a bursa near a joint.

• This will cause pain, swelling, or tenderness of the bursa and surrounding area, and may also result in joint stiffness.
• Bursitis is most commonly associated with the bursae found at or near the shoulder, hip, knee, or elbow joints.
  • At the shoulder, subacromial bursitis may occur in the bursa that separates the acromion of the scapula from the tendon of a shoulder muscle as it passes deep to the acromion.
  • In the hip region, trochanteric bursitis can occur in the bursa that overlies the greater trochanter of the femur, just below the lateral side of the hip.
  • Ischial bursitis occurs in the bursa that separates the skin from the ischial tuberosity of the pelvis, the bony structure that is weight bearing when sitting.
  • At the knee, inflammation 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 floor and carpet installers who do not use knee pads.
  • At the elbow, olecranon bursitis is inflammation of the bursa between the skin and olecranon process of the ulna.
    • The 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 muscle overuse, trauma, excessive or prolonged pressure on the skin, rheumatoid arthritis, gout, or infection of the joint.
• Repeated acute episodes of bursitis can result in a chronic condition.
• Treatments for the disorder include antibiotics if the bursitis is caused by an infection, or anti-inflammatory agents, such as nonsteroidal anti-inflammatory drugs (NSAIDs) or corticosteroids if the bursitis is due to trauma or overuse.
• Chronic bursitis may require that fluid be drained, but additional surgery is usually not required.