The musculoskeletal system relies on structures that minimize friction for smooth, pain-free movement. When muscles contract, tendons glide across surfaces, and soft tissues slide over bony prominences, creating shear forces that must be managed. The bursa and the tendon sheath are two specialized structures that perform this function by containing synovial fluid. Although both reduce wear and tear, their distinct anatomical configurations and locations determine their specific roles.
The Bursa: Function and Location
The bursa acts as a mobile, fluid-filled cushion or shock absorber for underlying tissues. These small sacs are strategically positioned in areas subjected to direct pressure or high friction forces, dissipating the force created between bone and overlying soft tissues (skin, muscle, or tendon).
Bursae are commonly found near major joints where tissues move repeatedly. Examples include the olecranon bursa at the elbow, the prepatellar bursa overlying the kneecap, and the trochanteric bursa near the hip. By creating a slippery interface, bursae ensure that movement does not cause abrasive damage to the structures beneath. There are approximately 160 bursae located throughout the human body, providing cushioning and facilitating motion.
The Tendon Sheath: Function and Location
The tendon sheath functions as a lubricated sleeve, ensuring that long tendons glide effortlessly through confined spaces. This structure is necessary where a tendon changes direction or travels through a tight channel formed by bone and connective tissue. Its main purpose is to protect the tendon from rubbing against surrounding hard structures.
Sheaths are found in areas requiring precise, extensive tendon movement within a constrained pathway. Examples include the flexor tendons of the fingers, the tendons crossing the wrist, and those around the ankle. The fluid-filled sheath provides constant lubrication necessary for the repetitive, high-tension movements performed by these tendons.
Structural Differences and Composition
The fundamental difference lies in their shape and interaction with the tissues they protect. A bursa is a simple, closed sac or pouch, similar to a flattened balloon filled with fluid. It is interposed between two moving surfaces, acting as a buffer that absorbs and disperses pressure.
In contrast, a tendon sheath is a tubular structure that completely wraps around the length of a moving tendon, forming a double-layered envelope. It is composed of an outer parietal layer and an inner visceral layer adhering to the tendon’s surface. The space between these layers is filled with synovial fluid, allowing the tendon to slide smoothly within its casing.
Both structures contain synovial fluid, produced by the synovial membrane lining their interior surfaces. This viscous fluid acts as a lubricant, reducing friction. In the bursa, the fluid cushions and separates tissue layers, while in the tendon sheath, it facilitates maximum gliding and movement of the enclosed tendon.
Common Inflammatory Conditions
Inflammation of these structures leads to distinct clinical conditions reflecting their anatomical differences. Inflammation of a bursa is called bursitis, where the fluid-filled sac becomes swollen and irritated. Symptoms often involve localized tenderness and swelling that is easily palpable, even when the joint is at rest.
Inflammation of a tendon sheath is known as tenosynovitis, affecting the lubricating sleeve surrounding the tendon. The primary symptom is pain and difficulty during active motion of the affected tendon. A distinctive symptom of tenosynovitis can be crepitus, a grating sensation that occurs as the inflamed tendon struggles to glide within its constricted sheath. Treatment approaches differ: bursitis responds well to rest and pressure avoidance, while tenosynovitis requires limiting the specific movement causing friction within the sheath.