Knee MRI Anatomy

Overview

Magnetic Resonance Imaging (MRI) is the unparalleled gold standard for evaluating internal derangements of the knee. Mastery of knee anatomy requires a precise understanding of the capsular relationships, ligaments, menisci, and fluid-filled bursae, as pathological tears strictly follow these anatomical boundaries.

The Collateral Ligaments

The collateral ligaments provide massive coronal plane stability (preventing valgus and varus stress). Their relationships to the joint capsule are frequently tested and surgically crucial.

• Medial Collateral Ligament (MCL): A broad, flat band that provides valgus stability. Crucially, its deep fibers inextricably blend with and adhere intimately to the medial joint capsule and the peripheral margin of the medial meniscus. Thus, severe MCL tears often coincide with medial meniscal tears.
• Lateral Collateral Ligament (LCL): A distinct, cord-like structure extending from the lateral femoral epicondyle to the fibular head. Unlike the MCL, the LCL is strictly extracapsular and does NOT attach to the lateral meniscus.

The Cruciate Ligaments & Fat Pad

The cruciate ligaments reside in the intercondylar notch, providing sagittal stability. The capsule and synovium possess a unique relationship here.

• Intracapsular but Extrasynovial: The Anterior and Posterior Cruciate Ligaments (ACL/PCL) are physically located inside the fibrous joint capsule (intracapsular). However, the synovial membrane brilliantly reflects anteriorly around them, leaving them bare to the joint fluid. This makes them specifically extrasynovial.
• Hoffa's Fat Pad: The infrapatellar fat pad similarly occupies the anterior compartment of the knee inside the fibrous capsule, but uniquely, it also remains strictly outside the synovial cavity (extrasynovial).

The Bursal Network

The knee is surrounded by numerous bursae to reduce excruciating friction during flexion and extension. The most radiologically prominent is the suprapatellar bursa.

• Suprapatellar Bursa: This large bursa extends superiorly between the distal femur and the quadriceps tendon. It is unique because it acts as a massive functional extension of the synovial cavity and communicates freely with the knee joint space proper in pristine adults. Therefore, a massive knee joint effusion will always pool heavily here.
• Prepatellar Bursa: Located strictly anterior to the patella. Does NOT communicate with the joint. (Inflammation = Housemaid's Knee).
• Popliteal Extravasation: An excessive buildup of synovial fluid can herniate posteriorly through the joint capsule, presenting as a classic Baker's (Popliteal) cyst.

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