The knee is a complex joint that is commonly injured. The accurate diagnosis of knee injuries requires a rather detailed knowledge of anatomy.
The knee is composed of three articulations: the medial and lateral condylar joints and the patellofemoral joint. The knee is capable of a wide range of motion including flexion, extension, internal and external rotation, abduction, and adduction. In full extension, no rotary motion is permitted, as the ligamentous structures are taut. This tightening with extension is referred to as “the screwing home mechanism.” Beyond 20 degree flexion, the supporting ligaments are relaxed and axial rotation is permitted.1 At 90 degree flexion, there is a maximum of laxity allowing up to 40 degree of rotation.
The surface anatomy including the major muscles surrounding the knee can be easily visualized and palpated. With the knee extended, the large dominant vastus medialis and the smaller vastus lateralis can be visualized and palpated. The larger medialis pulls the patella medially during extension, thus preventing lateral subluxation or dislocation. The sartorius, gracilis, and semitendinosus are palpable medially along their common insertion on the tibia referred to as the pes anserinus. Laterally, the iliotibial tract and the tendon of the biceps femoris can be palpated (Fig. 20–1A).
Anatomy of the knee. A. Anterior view. B. Medial view. C. Posterior view. The semimembranosus tendon sends extensions to the medial meniscus and to the posterior aspect of the capsule. D. Lateral view.
The bony anatomy of the knee can also be palpated. The patella and patellar tendon are palpated along the anterior surface of the knee. Medially, the medial tibial plateau and medial femoral condyle are noted. The adductor tubercle extends posteriorly from the medial femoral condyle and can be palpated. The joint line can be readily located by noting the natural depression just medial and lateral to the patellar tendon with the knee in flexion. These indentations overlie the articular surfaces.
The patellar tendon inserts on the anterior tibial tubercle, which is easily palpable. The lateral tibial plateau is located just lateral to the tubercle. Posterior and lateral to the plateau is the fibular head, palpable just inferior to the lateral femoral condyle.
The medial meniscus is palpable along the medial joint line as the knee is internally rotated and gently extended. The lateral meniscus is not palpable although injury to this structure reliably produces joint line tenderness. The menisci of the knee migrate anteriorly with extension. The medial meniscus is less mobile because of its attachment to the medial collateral ligament. With flexion, there is posterior migration of both menisci, secondary to the pull of the (medial) semimembranosus and the (lateral) popliteus.
The supporting structures surrounding the knee can be divided into two groups, static (ligaments) and dynamic (muscles) stabilizers. The static stabilizers can be further divided into medial, lateral, and posterior compartments.
The medial compartment static stabilizer is the medial collateral ligament (Fig. 20–1B). This capsular structure, also known as the tibial collateral ligament, is the primary medial stabilizer against a valgus or rotary stress. It inserts on the medial femoral and tibial condyles. A deep portion of the ligament inserts on the medial meniscus. The medial collateral ligament can also be divided into anterior, middle, and posterior components. The posterior component merges with the oblique popliteal ligament.2,3 The semimembranosus tendon inserts on the oblique popliteal ligament adding stability and posterior mobility to the ligament as well as the medial meniscus during flexion (Fig. 20–1C).
The medial collateral ligament is the most commonly injured ligament of the knee. This ligament normally glides anteriorly during extension and posteriorly during flexion and is taut only in extension.2 The ligament's normal function is to limit forward glide of the tibia on the femur and to limit rotation and abduction. The collaterals are twice as effective at inhibiting rotational laxity when compared with the cruciate ligaments.
The lateral compartment static stabilizer is the lateral collateral ligament (Fig. 20–1D). This band-shaped ligament extends from the lateral femoral epicondyle to the fibular head. The ligament is extracapsular and does not insert on the lateral meniscus. This ligament offers little stability and is uncommonly injured. The lateral collateral ligament can be palpated laterally with the patient sitting cross-legged and the knee in 90 degree flexion.
The posterior compartment static stabilizer is the posterior capsule, which in reality is a continuation of the medial capsular ligament. The posterior capsular ligament is taut in extension and is the first line of defense against anteromedial or anterolateral rotary instability.3
There are two noncapsular static stabilizers of the knee: the anterior and posterior cruciate ligaments. The cruciate ligaments extend from the area of the intercondylar fossa of the femur to the tibial intercondylar eminence. The ligaments cross over each other forming an “X” on lateral inspection (Fig. 20–2). The ligaments are named on the basis of their tibial attachment.
The ligamentous and meniscal structures of the knee.
The anterior cruciate prevents anterior displacement of the tibia, excessive lateral mobility in flexion and extension, and controls tibial rotation. Some authors believe the ligament serves to prevent hyperextension and acts as a rotational guide in the screwing home (extension) mechanism.3 Anterior cruciate injuries are rarely isolated and typically are associated with medial collateral tears. ...