Articular capsule of knee

The articular capsule of the knee (Joint capsule) is a dense fibrous connective tissue envelope that completely encloses the knee joint, sealing the joint space and containing synovial fluid within it. [1] It is the largest articular capsule in the body, corresponding to the knee's status as the largest synovial joint.

General Structure

The knee joint capsule consists of two functional layers:

• Fibrous layer (outer): A tough, dense connective tissue sleeve that attaches to the bones surrounding the joint. It varies in thickness depending on the mechanical stresses at each location and is locally thickened to form capsular ligaments. It may also incorporate tendons (e.g., the semimembranosus tendon posteriorly).

• Synovial membrane (inner): Lines the inner surface of the capsule and produces synovial fluid, which lubricates the joint and nourishes the avascular articular cartilage.

Layered Organization

The capsule and its supporting structures are classically described in a three layer model on both the medial and lateral sides:

• Medial side: Layer 1 is the deep crural fascia; Layer 2 contains the superficial medial collateral ligament (MCL); Layer 3 is the true capsule, including the deep MCL, meniscofemoral and meniscotibial ligaments, and the patellomeniscal ligament.

• Lateral side: The deepest layer (capsule) divides into two laminae posteriorly, encompassing the lateral collateral ligament, fabellofibular ligament, and arcuate ligament, with anatomic variations in which of these reinforcing ligaments are present.

Attachments

The capsular attachment to bone varies in width by location. On the lateral tibial side, the attachment is widest (~8.6 mm) at the posterior border of Gerdy's tubercle and tapers posterolaterally to a linear attachment at the fibular head. Histologically, these attachment zones contain fibrocartilage rich in type II collagen, which changes with aging and disease.