Purpose of the study We would like to describe an arthroscopic sign of injury to the posterolateral corner of the knee.

Methods used and the results The clinical signs of injury to the ligaments of the posterolateral corner of the knee have been well described but we are aware of only one arthroscopic sign of instability of the posterolateral structure having been described in the English literature. The “drive through sign” i.e. 1 cm of lateral opening and exceptional posterior visualisation of the lateral meniscus. This is only present when there is significant posterior cruciate injury. With Grade I and Grade II injuries to the posterolateral corner, a more subtle sign is described below.

During the routine arthoscopic examination of the knee, the intraarticular part of the popliteal tendon can be inspected. The tendon is inserted into the lateral aspect of the lateral femoral condyle just distal and inferior to the lateral epicondyle. It exits the knee lateral to the lateral meniscus through a synovial tunnel that passes posteriorly and inferiorly. A synovial fold passes over the tendon as it descends inferiorly and posteriorly out of the knee. If the posterolateral ligament complex has been damaged, the roof over the tendon is baggy and does not form the tight covering normally observed.

Conclusion Examination of these structures should form part of the normal arthroscopic inspection of the knee joint prior to any therapeutic procedure being performed. As we become familiar with the normal appearance of the synovial roof over the tendon, the lax and expanded appearance over the popliteal tendon becomes easy to diagnose.