Description of Posterolateral Knee Injuries

Located on the outside of the knee joint, the posterolateral corner (PLC) of the knee functions to stabilize the knee against direct lateral or external forces. Injuries that occur to this area are often due to a sports impact injury – from sports like – football, soccer skiing and basketball. Although injuries to this area of the knee represent a smaller percentage of cases compared to the anterior cruciate ligament (ACL) or medial collateral ligament (MCL), this injury pattern can create a devastating impact on athletic performance.

The posterolateral corner of the knee is one of the more complex areas to both diagnose and surgically treat when injured.

Symptoms of a PLC injury include:

• Side-to-side instability
• Difficulty twisting, turning and pivoting
• Minimal swelling and pain along the outside of the knee
• A foot drop with a concurrent peroneal nerve injury

Patients gradually note the onset of instability patterns over the ensuing weeks to months after injury. This is problematic because the majority of patients are recommended to undergo surgery within the few 2 to 3 weeks after the initial injury to maximize the chance of a successful outcome.

The main anatomic structures of the posterolateral aspect of the knee are the lateral (fibular) collateral ligament (LCL), the popliteus tendon and the popliteofibular ligament. In addition, the lateral capsule, with its thickening called the anterolateral ligament, and the biceps femoris attachment to the fibular head serve as very important stabilizer. Concurrent with these ligaments, it is very important to assess the function of the common peroneal nerve because the nerve crosses the fibular head very close to these structures. Approximately 15-20% of patients may have a common peroneal nerve injury with ankle numbness or weakness who present with a posterolateral corner injury.

Diagnosis of Posterolateral Knee Injuries

There are many tests that must be synthesized to diagnose a posterolateral corner knee injury. These include:

• Varus Stress Test – in full extension and at 30° of knee flexion
• Dial Test at 30° and 90° of knee flexion
• Posterolateral Drawer Test
• External Rotation Recurvatum Test,
• Reverse Pivot Shift Test
• Assessment for a Varus Thrust Gait

All of these tests must be assessed in patients to determine the type of instability present. About 72% of patients who have a PLC injury will also have a concurrent cruciate ligament injury. Thus, one must not overlook the possibility of a posterolateral injury by focusing only on the injured cruciate ligament, because missing a posterolateral knee injury can cause failure of a reconstructed cruciate ligament.

Dr. LaPrade will examine the patient and use plain x-rays, stress x-rays, and often times, an MRI to utilize and examine the injury and to determine whether or not a posterolateral corner injury exists. In many cases, a fracture of the medial tibial plateau and the fibular head will also be present. Posterior kneeling stress x-rays should also be obtained when there is a concern for concurrent PCL tears to determine the amount of any increase in posterior translation on the injured knee.

Posterolateral knee injuries are classified according to the degree of injury:

• Grade I Injury: small partial tear with minimal instability
• Grade II Injury: partial tear with an endpoint to stressing
• Grade III Injury: complete tear with no good endpoint to stressing

In general, all grade III injuries are recommended for surgery because of the low likelihood of healing. This is primarily due to the unique anatomy of the posterolateral corner – there are two opposing convex bony surfaces present which leads to inherent bony instability. In general, due to the high risk of scaring and retraction of torn structures for injuries of the posterolateral knee, it is recommended that most surgeries be performed within two to three weeks after the injury to allow for early range of motion and to make sure sutures can easily be held into the torn structures with early motion after surgery.

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