The Treatment of Combined Posterolateral Knee Injuries and Anterior Cruciate Ligament Tears

Introduction

Posterolateral knee injuries have been noted to be some of the most difficult knee injuries to diagnose and treat due to the multiple different structures, the intricate anatomical relationships, and the fact that there is no one specific clinical test to diagnose these injuries. In addition, the majority of posterolateral knee injuries occur in combination with cruciate ligament injuries and can often be overlooked. Thus it is especially important to always assess for the presence of posterolateral knee injuries whenever a patient presents with an anterior cruciate ligament (ACL) tear.

This chapter will review the surgically relevant anatomy of the posterolateral corner of the knee, the clinical and radiographic diagnosis of these injuries, and the treatment of both acute and chronic combined posterolateral and ACL knee injuries.

Anatomy of the Posterolateral Corner of the Knee

While there have been different defined components of the structures of the posterolateral aspect of the knee, the most important structures are the fibular (lateral) collateral ligament (FCL), popliteus tendon, and popliteofibular ligament (PFL; Fig. 104.1 ). These structures have been defined by biomechanical testing to be the most important to prevent abnormal increases in varus translation, external rotation, and combined coupled posterolateral rotation.

The FCL is the primary stabilizer to varus translation of the knee. It attaches in a small depression just proximal and posterior to the lateral epicondyle and courses distally under the superficial layer of the iliotibial band and the lateral aponeurosis of the long head of the biceps femoris muscle to attach to the lateral aspect of the fibular head. The average length of the FCL has been measured to be 71 mm. Along its more distal quarter, the FCL is located within the confines of the biceps bursa. One of the most important clinical pearls is to identify the FCL by making a horizontal incision in the anterior arm of the long head of the biceps femoris muscle, approximately 1.5 cm proximal to the fibular head, to gain access. Injuries along its distal attachment can be identified here, and a traction stitch placed into the FCL at this location allows for assessment of its integrity and also allows one to follow it proximally to either its avulsed location or its proximal attachment on the femur.

The popliteus tendon is an important primary stabilizer to external rotation and coupled posterolateral rotation of the knee at higher flexion angles. While a true tendon, the popliteus tendon can be considered as the fifth ligament of the knee because it does have some static function, due to it attachment to the fibular styloid, which has been defined by biomechanical testing. Its attachment site is at the anterior fifth of the popliteus sulcus. An important clinical point to recognize is that the distance between the FCL and popliteus tendon femoral attachment site averages 18.5 mm. This anatomical relationship is important to recognize because one graft cannot reconstruct both of these structures due to their differing locations and tensioning patterns with knee flexion, and measuring between the two presumed attachment sites to verify this distance can assist one in finding proper tunnel placement prior to reaming tunnels or for repairing a structure.

The PFL has also been shown to be an important stabilizer to external rotation and a secondary stabilizer to varus opening of the knee. It is a stout structure that connects the popliteus tendon to the posteromedial aspect of the fibular head. The PFL originates at the musculotendinous junction of the popliteus muscle. It courses slightly distal and lateral to attach to the posteromedial aspect of the fibular styloid. It has two divisions, which are called the anterior and posterior divisions. The posterior division is larger and is the division most commonly reconstructed with current techniques.

Anatomy of the Posterolateral Corner of the Knee

While there have been different defined components of the structures of the posterolateral aspect of the knee, the most important structures are the fibular (lateral) collateral ligament (FCL), popliteus tendon, and popliteofibular ligament (PFL; Fig. 104.1 ). These structures have been defined by biomechanical testing to be the most important to prevent abnormal increases in varus translation, external rotation, and combined coupled posterolateral rotation.

The FCL is the primary stabilizer to varus translation of the knee. It attaches in a small depression just proximal and posterior to the lateral epicondyle and courses distally under the superficial layer of the iliotibial band and the lateral aponeurosis of the long head of the biceps femoris muscle to attach to the lateral aspect of the fibular head. The average length of the FCL has been measured to be 71 mm. Along its more distal quarter, the FCL is located within the confines of the biceps bursa. One of the most important clinical pearls is to identify the FCL by making a horizontal incision in the anterior arm of the long head of the biceps femoris muscle, approximately 1.5 cm proximal to the fibular head, to gain access. Injuries along its distal attachment can be identified here, and a traction stitch placed into the FCL at this location allows for assessment of its integrity and also allows one to follow it proximally to either its avulsed location or its proximal attachment on the femur.

The popliteus tendon is an important primary stabilizer to external rotation and coupled posterolateral rotation of the knee at higher flexion angles. While a true tendon, the popliteus tendon can be considered as the fifth ligament of the knee because it does have some static function, due to it attachment to the fibular styloid, which has been defined by biomechanical testing. Its attachment site is at the anterior fifth of the popliteus sulcus. An important clinical point to recognize is that the distance between the FCL and popliteus tendon femoral attachment site averages 18.5 mm. This anatomical relationship is important to recognize because one graft cannot reconstruct both of these structures due to their differing locations and tensioning patterns with knee flexion, and measuring between the two presumed attachment sites to verify this distance can assist one in finding proper tunnel placement prior to reaming tunnels or for repairing a structure.

The PFL has also been shown to be an important stabilizer to external rotation and a secondary stabilizer to varus opening of the knee. It is a stout structure that connects the popliteus tendon to the posteromedial aspect of the fibular head. The PFL originates at the musculotendinous junction of the popliteus muscle. It courses slightly distal and lateral to attach to the posteromedial aspect of the fibular styloid. It has two divisions, which are called the anterior and posterior divisions. The posterior division is larger and is the division most commonly reconstructed with current techniques.

Diagnosis of Posterolateral Knee Injuries

Clinical Examination of Posterolateral Knee Injuries

The posterolateral structures of the knee are responsible for providing stability to varus translation, external rotation, and coupled posterolateral rotation. They also have important secondary stabilizer roles to providing anterior and posterior translation stability to the knee in the face of cruciate ligament tears.

In the examination for varus laxity, the knee is flexed to between 20 and 30 degrees, with the thigh resting on the examination table to assist with patient relaxation, and a varus translation force is applied to the knee. A varus force should be applied to the knee through the foot and ankle to allow for any potential rotatory component to occur, and not through the distal leg. It is especially important in the assessment of posterolateral knee injuries to also concurrently examine the contralateral limb. There can be a wide variance in the amount of physiologic opening between patients, and the contralateral knee should always be examined (if it is normal) to compare it with the injured side. The dial, or external rotation, test assesses for the amount of increased external rotation of the knee at both 30 and 90 degrees of knee flexion, and is compared with the contralateral side. For the dial test, which can be performed both prone and supine, the examiner rotates both feet and ankles simultaneously and assesses for the amount of increased external rotation at 30 degrees of knee flexion. If there is approximately 15 degrees of increased external rotation on the injured side, it is reported that there is a concurrent posterolateral corner knee injury present. The examiner then flexes the knees to 90 degrees, and the amount of increased external rotation is also evaluated in this position. For isolated posterolateral corner knee injuries, it has been reported that the amount of increased external rotation should be approximately 5 degrees, compared with the contralateral side at 90 degrees of knee flexion.

In addition to the clinical tests assessing the primary stability of the posterolateral structures, one should also assess for any increase in translation on both the Lachman and posterior drawer tests. Patients who have a significant increase in anterior translation on the Lachman test or a gross pivot-shift examination should be carefully assessed for any concurrent lack of secondary restraints. The most commonly injured secondary restraints to look for in this situation would be a combined posterolateral corner injury, a deficient posterior horn of the medial meniscus, or a root tear of the posterior horn of the medial meniscus. Therefore when one does find a significant increase in anterior translation on the Lachman test, one should be suspicious for a possible combined posterolateral corner knee injury.

Likewise, patients who have significant translation changes on their posterior drawer tests should also be carefully assessed for combined PCL and posterolateral corner knee injuries. The most accurate means to assess this is with posterior knee stress radiographs.

Radiographic Diagnosis of Posterolateral Knee Injuries

Stress radiographs can also be very useful to diagnose posterolateral corner knee injuries. These include varus stress x-rays as well as PCL stress x-rays. It has been reported that isolated FCL tears have an increase in lateral compartment gapping of 2.7 mm, whereas complete posterolateral corner injuries have gapping of 4 mm or greater compared with the contralateral side. While we have found these not to be as useful in the acute situation due to problems with patient guarding, they can be very useful to assess for chronic posterolateral corner knee injuries, where it may be very difficult to determine if a patient has increased gapping, medial compartment pseudolaxity, or if there is a possible component of a medial compartment injury concurrent with a posterolateral corner knee injury.

Treatment of Acute Combined Posterolateral Corner Knee Injuries and Anterior Cruciate Ligament Tears

The usual approach for these combined injuries is to perform the surgical approach to the posterolateral corner structures first. We use a leg holder and make sure that the knee can be flexed to approximately 120 degrees of flexion ( Fig. 104.2 ). A standard lateral hockey stick incision is made, which is centered over the posterior border of the iliotibial band and continues distally over Gerdy tubercle to extend slightly over the anterior compartment of the leg. If the biceps femoris muscle has avulsed off the fibular head, it is important to identify it proximally and follow it distally, because we have found the common peroneal nerve to often be out of its normal tissue plane, sometimes displaced over the fibular head, when this avulsion pattern occurs.