The three main structures that provide stability to the outside of the knee are the lateral collateral ligament (LCL), the popliteofibular ligament (PFL) and the popliteus.
The LCL arises from the bony prominence on the outside of the distal femur known as the lateral epicondyle. It is a long, thin rope-like ligament that runs down to insert on the anterolateral aspect of the fibular head.
The popliteus muscle arises from the back of the tibia and heads upwards and outwards and curves around the distal femur where it lies in a bony groove, the popliteal sulcus. It inserts at the front of this sulcus, almost 2cm anterior and distal to the lateral epicondyle.
The popliteus muscle internally rotates the tibia on the femur (or externally rotates the femur on the tibia if the foot is fixed) and thus unlocks the screw home mechanism of the knee. The screw home mechanism refers to the external rotation of the tibia as the knee moves into full extension. This rotation tensions both cruciate ligaments and is responsible for the stability of the joint in extension.
The PFL runs from the musculotendinous portion of the popliteus muscle and inserts onto the posteromedial aspect of the fibular head.
Both the popliteus and PFL, assisted by the PCL help to resist external rotation of the tibia and this is the basis for the dial tests that are used to clinically examine their integrity. Note that this aspect of knee stability and anatomy is quite complex and difficult to fully understand.
Much less commonly than with ACL injuries, the ligament on the outside of the knee, the lateral collateral ligament or LCL can be injured. Other thickenings of the capsule of the knee joint and the biceps portion of the hamstrings and the popliteus muscle on the outside and towards the back of the knee are often injured in association with this and they are collectively known as the postero-lateral corner.
The LCL is responsible for side to side stability of the knee. Complete injuries to the LCL often do not heal well with conservative treatment in comparison to the medial collateral ligament or MCL. If the injury is discovered acutely (within a couple of weeks of the tear), it is sometimes possible to repair the structures or certainly to combine a repair with a reconstruction. After this, a reconstruction is required whereby a new lateral ligament is fashioned, usually using the smaller hamstrings on the inside of the knee as a tendon graft.
The procedure is performed as an open procedure with an incision down along the outside of the knee. The landmarks for the new ligament are carefully identified as is the common peroneal nerve in order to protect it from injury. A hole is drilled in both the femur (thigh) bone at the lateral epicondyle and the head of the fibula and the graft pulled through and secured with screws on either side to reconstruct the LCL. If the postero-lateral corner also needs to be reconstructed, a second hole is drilled in the femur at the insertion of the popliteus muscle and one end of the second graft secured here. A hole is then drilled through the tibia and both grafts pulled through and secured. The portion of the LCL graft as it passes from the back of the fibula to the back of the tibia forms the PFL.
A brace is required after surgery for a minimum period of 6 weeks.
Compiled October 2015
Review date: March 2016
Review date: March 2016