Knee MRI Anatomy & Positioning

Knee MRI Anatomy & Positioning

Musculoskeletal Anatomy of the Knee Joint

The knee encompasses three distinct functional articulations enclosed within a single expansive synovial cavity: the medial femorotibial joint, the lateral femorotibial joint, and the patellofemoral joint.

Osseous Articulations: Distal Femur, Proximal Tibia & Patella

• Distal Femur: Features two large, asymmetrical convex osseous prominences—the medial femoral condyle and the lateral femoral condyle. The medial condyle is larger, more curved, and extends further distally than the lateral condyle. Posteriorly, the condyles are separated by the deep intercondylar notch (fossa), which houses the cruciate ligaments. Anteriorly, they coalesce into the trochlear groove (sulcus), a smooth cartilage-lined depression that articulates with the patella.
• Proximal Tibia: Expands into the medial tibial plateau and lateral tibial plateau. The medial plateau is larger, oval, and slightly concave, whereas the lateral plateau is circular, sits slightly higher, and is convex in the sagittal plane. Between the plateaus lies the intercondylar eminence, topped by the medial and lateral intercondylar tubercles (tibial spines), which serve as osseous landmarks for ligamentous attachments. Anteriorly, the tibial tuberosity anchors the patellar tendon; laterally, Gerdy's tubercle anchors the distal iliotibial band (ITB).
• Patella: The largest sesamoid bone in the human body, embedded within the confluent extensor mechanism. Its posterior articular surface is covered by the thickest hyaline cartilage in the human body (up to 5 to 6 mm) and is divided by a vertical median ridge into a larger lateral facet and a smaller medial facet, with a small non-articulating "odd facet" along the extreme medial border.

Cruciate Ligaments (ACL & PCL)

The cruciate ligaments are situated within the central intercondylar notch of the joint capsule. They are intra-articular but extrasynovial (invested by a reflected synovial sleeve):

Cruciate Ligament Architecture:
- Anterior Cruciate Ligament (ACL): Posteromedial Lateral Femoral Condyle ➔ Anterior Tibia
  ➔ Primary restraint to anterior tibial translation
- Posterior Cruciate Ligament (PCL): Anterolateral Medial Femoral Condyle ➔ Posterior Tibia
  ➔ Primary restraint to posterior tibial translation

1. Anterior Cruciate Ligament (ACL): Originates from the posteromedial aspect of the lateral femoral condyle, courses obliquely anteromedially and inferiorly, and inserts onto the anterior intercondylar area of the tibial plateau anterior to the intercondylar eminence. It comprises two functional bundles: the anteromedial (AM) bundle (taut in flexion) and the posterolateral (PL) bundle (taut in extension). The ACL is the primary static restraint preventing anterior translation of the tibia relative to the femur. On normal sagittal MRI, it appears as a taut, straight, uniformly hypointense band, frequently showing subtle linear striations near its distal tibial insertion.
2. Posterior Cruciate Ligament (PCL): Originates from the anterolateral aspect of the medial femoral condyle and inserts onto the posterior intercondylar fossa of the proximal tibia, approximately 1 cm below the joint line. It is significantly thicker, stronger, and denser than the ACL, appearing on sagittal MRI as a well-defined, smooth, homogeneous low-signal crescent or "hockey stick." It functions as the primary static restraint preventing posterior translation of the tibia relative to the femur.

Collateral Ligaments & the Posterolateral Corner

• Medial Collateral Ligament (MCL): The primary restraint resisting valgus stress (forces angling the knee inward). It consists of two distinct layers:
  • Superficial MCL (Tibial Collateral Ligament): A broad, flat fibrous band originating from the medial femoral epicondyle and inserting broadly onto the proximal medial tibia 4 to 5 cm distal to the joint line, deep to the pes anserinus tendons.
  • Deep MCL: Comprises localized capsular thickenings firmly anchored to the mid-body of the medial meniscus: the meniscofemoral ligament superiorly and the meniscotibial (coronary) ligament inferiorly.
• Lateral Collateral Ligament (LCL / Fibular Collateral Ligament): A discrete, cord-like extracapsular ligament originating from the lateral femoral epicondyle and descending posteroinferiorly to insert onto the head of the fibula, forming a conjoint insertion with the biceps femoris tendon. It is the primary restraint resisting varus stress (forces angling the knee outward).
• Posterolateral Corner (PLC): A critical stabilizing complex preventing excessive varus, external rotation, and posterior translation. It comprises the LCL, the popliteus tendon, the popliteofibular ligament (spanning from the popliteus tendon to the fibular styloid), the fabellofibular ligament, and the arcuate ligament. Missed PLC tears lead to catastrophic failure of ACL graft reconstructions.

Meniscal Anatomy: Medial vs. Lateral Menisci

The menisci are two crescent-shaped, wedge-shaped fibrocartilaginous structures interposed between the femoral condyles and tibial plateaus. They function as shock absorbers, transmit 50% to 70% of axial compressive load across the joint, enhance joint congruity, and lubricate the articular surfaces:

• Medial Meniscus: C-shaped (semi-lunar), covering approximately 60% of the medial tibial plateau. Its posterior horn is significantly larger (two to three times wider) than its anterior horn. Because its outer periphery is firmly attached to the joint capsule and the deep MCL along its entire length, the medial meniscus is relatively immobile, making its mobility different from that of the lateral meniscus.
• Lateral Meniscus: More circular (O-shaped), covering up to 80% of the lateral tibial plateau. Its anterior and posterior horns are roughly equal in size on sagittal slices. Posterolaterally, the lateral meniscus is interrupted by the popliteal hiatus, where the popliteus tendon passes through the joint without direct capsular attachment to the meniscus. This lack of peripheral anchoring makes the lateral meniscus far more mobile and compliant under load.

Extensor Mechanism & Popliteal Fossa

• Extensor Mechanism: Comprises the quadriceps femoris tendon (a trilaminar tendon formed by the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius inserting on the superior patellar pole), the patella, and the patellar tendon (ligament) (extending from the inferior patellar pole to the tibial tuberosity). The patella is dynamically and statically stabilized by the medial and lateral patellar retinacula, most notably the medial patellofemoral ligament (MPFL), which tears during acute lateral patellar dislocation.
• Popliteal Fossa: A diamond-shaped space behind the knee bounded by the biceps femoris superolaterally, semimembranosus and semitendinosus superomedially, and the gastrocnemius heads inferomedially and inferolaterally. Deep within the fossa lies the popliteal neurovascular bundle:
  • Popliteal Artery: Deepest structure, resting directly against the posterior joint capsule.
  • Popliteal Vein: Intermediate structure.
  • Tibial Nerve: Most superficial component of the bundle.
• Baker (Popliteal) Cyst: A fluid distension of the gastrocnemio-semimembranosus bursa located in the posteromedial popliteal fossa. A true Baker cyst always arises between the medial head of the gastrocnemius muscle and the semimembranosus tendon, communicating with the posterior knee joint through a narrow connecting "neck" or stalk.