Page 247 - Clinical Anatomy

ECA4 7/18/06 6:47 PM Page 232

232 The lower limb

yourself that some degree of rotation of the knee is possible when this joint
is in the flexed position. In full extension, i.e. in the standing position, the
knee is quite rigid because the medial condyle of the tibia, being rather
larger than the lateral condyle, rides forward on the medial femoral
condyle, thus ‘screwing’ the joint firmly together. The first step in flexion of
the fully extended knee is ‘unscrewing’ or internal rotation. This is brought
about by popliteus, which arises from the lateral side of the lateral condyle of
the femur, emerges from the joint capsule posteriorly and is inserted into
the back of the upper end of the tibia.
The principal muscles acting on the knee are:
•◊◊extensor—quadriceps femoris;
•◊◊flexors—hamstrings assisted by gracilis, gastrocnemius and sartorius;
•◊◊medial rotator—popliteus (‘unscrews the knee’).

Clinical features

1◊◊The stability of the knee depends upon the strength of its surround-
ing muscles and of its ligaments. Of the two, the muscles are by far the
more important. Providing quadriceps femoris is powerfully developed,
the knee will function satisfactorily even in the face of considerable liga-
mentous damage. Conversely, the most skilful surgical repair of torn liga-
ments is doomed to failure unless the muscles are functioning strongly;
without their support, reconstructed ligaments will merely stretch once
more.
2◊◊When considering soft tissue injuries of the knee joint, think of three Cs
that may be damaged —the Collateral ligaments, the Cruciates and the
Cartilages.
The collateral ligaments are taut in full extension of the knee and are,
therefore, only liable to injury in this position. The medial ligament may be
partly or completely torn when a violent abduction strain is applied,
whereas an adduction force may damage the lateral ligament. If one or
other collateral ligament is completely torn, the extended knee can be
rocked away from the affected side.
The cruciate ligaments may both be torn (along with the collateral liga-
ments) in severe abduction or adduction injuries. The anterior cruciate,
which is taut in extension, may be torn by violent hyperextension of the
knee or in anterior dislocation of the tibia on the femur. Since it resists rota-
tion, it may also be torn in a violent twisting injury to the knee. The poste-
rior cruciate tears in a posterior dislocation (Fig. 170).
If both the cruciate ligaments are torn, unnatural anteroposterior mobil-
ity of the knee can be demonstrated.
If there is only increased forward mobility, the anterior cruciate liga-
ment has been divided or is lax. Increased backward mobility implies a
lesion of the posterior cruciate.
The semilunar cartilages can only tear when the knee is flexed and is thus
able to rotate. If you place a finger on either side of the ligamentum patellae
on the joint line and then rotate your flexed knee first internally and then