An Unstable Knee

A 28 year old man was brought to hospital after a road traffic incident. He described hearing a “pop” as his right knee impacted against the dashboard at the time of collision, and he subsequently experienced extreme pain. On examination in the emergency department, his knee was swollen and tender, and range of movement was severely limited. After initial management in the emergency department, he was admitted under the orthopaedic team. Once the swelling was resolved, his knee was re-examined, and was found to be grossly unstable (fig 1⇓). Fig 1 Clinical photograph of the patient’s right knee ### 1. What sign is shown in the clinical photograph? #### Short answer Posterior sag sign suggesting injury to the posterior cruciate ligament. #### Discussion The posterior cruciate ligament serves as the main restraint to posterior displacement of the tibia relative to the femur. Secondary functions include resisting excessive valgus, varus, and external rotation movements. Disruption of the posterior cruciate ligament can be seen via posterior draw testing or by assessing for posterior sag (fig 2⇓). Normally, the anterior surface of the tibia lies around 1 cm anterior to the femoral surface when the knee is held in 90° of flexion. Any …