Introduction

A knee dislocation injury is a rare but potentially devastating injury. The definition of knee dislocation includes the grossly unstable knee, with a minimum of two of the four major knee ligaments injured, regardless of a reduced joint line. Some authors suggest that any combined anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) injuries be considered a knee dislocation, although knee dislocations have been described without cruciate injury. The injury is commonly attributed to high-velocity motor vehicle accidents and low-velocity sports injuries, with the rate of knee dislocation reported to be 0.001%–0.013% of all knee injuries. This may represent an underestimation of this devastating injury because some knee dislocations spontaneously reduce before the patient receives a physical examination, and the patient may suffer other physical injuries that require medical attention.

Commonly, a knee dislocation involves injury to the ACL, PCL, and either the medial collateral ligament (MCL) or the lateral-side structures of the knee. Of knee dislocations, associated medial-side tears represent approximately 90% of all the injuries, whereas lateral-sided injuries represent approximately 10% of the knee dislocation injuries.

We see almost 10 times more knee dislocations involving the medial side than we do involving the lateral side. Nonoperative treatment of knee dislocations involving the lateral side usually results in a grossly unstable knee and causes severe functional disability for the patient. Because of these occurrences, acute reconstruction of all injured structures with all knee dislocations has been advocated; this recommendation has included knee dislocations involving the medial side. This approach has resulted in many stable but stiff knees after surgery.

The morbidity associated with acute surgery for knee dislocations caused us to alter our treatment approach for knee dislocations to consider the healing potential of each torn structure. Although a knee dislocation involving the lateral side is an injury that requires surgery at least semiacutely, a knee dislocation involving the medial side is not an injury that requires immediate surgery and may not require surgery at all.

We will review our treatment approach to dislocated knees involving the ACL, PCL, and either the MCL injury or lateral-side structures. This approach was derived from an understanding of the injuries to the individual ligaments and their potential to heal, the natural history of the injury, and the effects of the injury in combination.

Ligament Healing

Posterior Cruciate Ligament

In contrast to the ACL, PCL injuries have the potential for intrinsic healing ( Fig. 99.1 ). Evaluation with MRI of acute PCL injuries has been found to be 99%–100% sensitive and specific in documenting acute PCL tears. In contrast, MRI evaluation of chronic PCL laxity are less accurate than that of acute injury because the PCL appears healed even when the patient has laxity. Shelbourne et al. evaluated 40 patients who had acute PCL injuries with MRI at the time of the acute injury and again at a mean of 3.2 years after injury. Twenty-three patients had isolated PCL tears, and 17 patients had combined PCL and additional ligament injury. The healing of partial and complete tears was graded with MRI. The results showed that 37 of 40 PCLs healed with continuity. All partial tears and most complete (19 of 22) PCL tears regained continuity. Twelve of 12 combined PCL/MCL injuries healed. In two patients with acute ACL, PCL, and MCL injuries, the MCL and PCL healed without treatment. Location, severity, and associated ligament injuries did not affect healing. The healed PCL demonstrated abnormal morphology in 25 of the 37 cases on follow-up. In a recent follow-up study at a mean of 4.6 years after knee dislocations to the lateral side, the PCL in 16 of 16 patients appeared healed on the MRI and no patient had more than 1+ laxity upon examination (Shelbourne et al.). Tewes et al. evaluated follow-up MRIs on 13 patients with high-grade PCL injury an average of 20 months postinjury. Their results showed 10 of 13 patients (77%) had regained MRI continuity of the PCL, although with an abnormal appearance. They could not correlate functional or clinical status with degree of clinical laxity. The time to obtain healing after acute PCL injury is yet unknown. However, Shelbourne et al. described a firm endpoint, and a painless posterior drawer at follow-up examination of acute PCL injuries approximately 2 weeks postinjury.

A, Magnetic resonance imaging (MRI) of an acute posterior cruciate ligament injury. B, Follow-up MRI image at 3 months after injury shows the posterior cruciate ligament is in continuity, which may be read by the radiologist as a normal posterior cruciate ligament.

Ligament Healing

Posterior Cruciate Ligament

In contrast to the ACL, PCL injuries have the potential for intrinsic healing ( Fig. 99.1 ). Evaluation with MRI of acute PCL injuries has been found to be 99%–100% sensitive and specific in documenting acute PCL tears. In contrast, MRI evaluation of chronic PCL laxity are less accurate than that of acute injury because the PCL appears healed even when the patient has laxity. Shelbourne et al. evaluated 40 patients who had acute PCL injuries with MRI at the time of the acute injury and again at a mean of 3.2 years after injury. Twenty-three patients had isolated PCL tears, and 17 patients had combined PCL and additional ligament injury. The healing of partial and complete tears was graded with MRI. The results showed that 37 of 40 PCLs healed with continuity. All partial tears and most complete (19 of 22) PCL tears regained continuity. Twelve of 12 combined PCL/MCL injuries healed. In two patients with acute ACL, PCL, and MCL injuries, the MCL and PCL healed without treatment. Location, severity, and associated ligament injuries did not affect healing. The healed PCL demonstrated abnormal morphology in 25 of the 37 cases on follow-up. In a recent follow-up study at a mean of 4.6 years after knee dislocations to the lateral side, the PCL in 16 of 16 patients appeared healed on the MRI and no patient had more than 1+ laxity upon examination (Shelbourne et al.). Tewes et al. evaluated follow-up MRIs on 13 patients with high-grade PCL injury an average of 20 months postinjury. Their results showed 10 of 13 patients (77%) had regained MRI continuity of the PCL, although with an abnormal appearance. They could not correlate functional or clinical status with degree of clinical laxity. The time to obtain healing after acute PCL injury is yet unknown. However, Shelbourne et al. described a firm endpoint, and a painless posterior drawer at follow-up examination of acute PCL injuries approximately 2 weeks postinjury.

A, Magnetic resonance imaging (MRI) of an acute posterior cruciate ligament injury. B, Follow-up MRI image at 3 months after injury shows the posterior cruciate ligament is in continuity, which may be read by the radiologist as a normal posterior cruciate ligament.

Clinical Examination

Listening carefully to the patient explain how the injury occurred and the position of the limb at the time of the injury, combined with a thorough physical examination, should allow the physician to arrive at a diagnosis. Evaluation of the uninjured extremity will establish a baseline and gain the patient confidence that the examination will not be painful. Initial evaluation may be difficult because the patient will probably have pain, swelling, muscle spasm, and limited knee motion, and the patient will be apprehensive. The physician should have a high index of suspicion based on the history of the patient’s injury, especially with a multiligamentous knee injury, because 50% of knee dislocations will reduce before evaluation, and capsular tears may prevent the appearance of significant effusion. The complications that arise from not recognizing associated injuries can be devastating. Close follow-up and reexamination are helpful. In addition, imaging studies and vascular surgery consultation may be needed.

Clinical assessment of the ACL can be done using the Lachman test. A positive Lachman test, done properly, is diagnostic of ACL disruption because the ACL prevents contribution from secondary stabilizers to anterior stability.

The PCL is the primary restraint to posterior laxity in the knee. To determine PCL deficiency, the involved knee should be compared with the noninvolved extremity to determine the proper relationship of the tibia to the femoral condyles. When the PCLs are intact, the anteromedial proximal tibia usually rests 1 cm anterior to the distal femoral condyles, with 90 degrees of knee flexion. In patients with PCL deficiency, the anteromedial tibia will “sag” posteriorly in relationship to the femoral condyles.

The most sensitive test for evaluating the PCL is the posterior drawer test at 90 degrees of flexion. Rubinstein et al. found the posterior drawer test in conjunction with palpating anterior tibial step-off to be 96% accurate, 90% sensitive, and 99% specific, with an interobserver agreement of grade to be 81% in diagnosing PCL insufficiency. The posterior drawer test with internal tibial rotation can also provide assessment of medial structures. Posterior tibial translation with posterior drawer testing should decrease with internal rotation of the tibia as the medial capsular structures tighten. In combined PCL/medial-sided injury, this reduction in the posterior laxity is lost.

In the combined ACL- and PCL-deficient knee, the tibia will be subluxated posteriorly so quantifying the contribution of each ligament to anterior translation will be more difficult. It is important to compare and examine the noninvolved extremity and determine the proper relationship of the tibia to the femoral condyles. The pivot-shift test and flexion-rotation drawer test augment evaluation of ACL insufficiency but may be of no use in an injury involving the ACL, PCL, and MCL because these tests rely on the medial structures being intact for the knee to pivot.

It is difficult to perform ligamentous testing on a patient with an acute knee dislocation. In particular, PCL laxity is difficult to determine because the patient may not be able to bend his or her knee to 90 degrees of flexion. Although MRI is helpful in determining the status of the PCL, treatment should not be determined based on the findings of the MRI. It is important to remember that complete grade 3 PCL injuries can heal with continuity and little or no laxity when left in situ. Predictable healing of the torn PCL is more important than any laxity in the healed PCL. The fact that the PCL will heal with continuity is important to our treatment philosophy for knee dislocation injuries.

An MCL injury is diagnosed and graded by physical examination. Palpation along the ligament will localize the site of the injury, which is critical to determining the treatment and rehabilitation process. The MCL is the primary medial restraint to valgus stress at 30 degrees of knee flexion. Valgus stress testing is performed at 30 degrees of flexion to isolate the MCL, then again at 0 degree of flexion to assess the contribution of capsular structures and cruciate ligaments. In greater degrees of knee extension the ACL, PCL, posterior capsule, and posterior oblique ligaments assume a greater responsibility in preventing medial joint opening. Grading of MCL injury is based on tenderness, laxity, and the presence of a firm endpoint. A grade 1 injury has tenderness, no laxity with valgus stress testing at 30 degrees of knee flexion, and a firm endpoint. A grade 2 injury is similar but reveals some medial laxity and the presence of a firm endpoint. A grade 3 injury represents a complete disruption of the MCL with no palpable endpoint on valgus stress testing.

A lateral-side knee injury usually appears differently than an isolated ACL injury. The knee has a mild effusion, but the lateral side of the leg appears swollen with ecchymosis from the lateral capsule avulsion that allows the hemarthrosis to dissipate into the lateral leg ( Fig. 99.2A–B ). Lateral stability is evaluated with varus stress applied to the knee at 0 and 30 degrees of flexion. Grade 1 laxity involves tenderness over the lateral structures but no laxity and a good endpoint. Grade 2 lateral laxity involves tenderness and increased laxity with varus stress, but a good endpoint is felt. Grade 3 laxity involves tenderness and increased laxity with varus stress, and no endpoint is felt.

Knee injury to anterior cruciate ligament, posterior cruciate ligament, and lateral side. A, The knee has a mild effusion with increased lateral-side swelling. B, The lateral side shows ecchymosis from the lateral capsule, allowing the hemarthrosis to dissipate into the lateral side of the leg.

Associated Neurovascular Injury

A study of an insurance database of knee dislocations between 2004 and 2009 reported the rate of vascular injuries to be 3.3%. Shelbourne et al. in a series of low-velocity sports injuries found a vascular injury rate of 4.8% (1 of 21). Peroneal nerve injuries have been reported in 14%–35% of knee dislocations. Most, if not all, are associated with lateral-sided injury. In a series of low-velocity sports injuries 4 of 21 (19%) patients presented with peroneal nerve injury. All were associated with lateral-sided injury. It should be emphasized, if the lateral side is injured, closely evaluate the peroneal nerve. Conversely, if the peroneal nerve is injured, careful evaluation of lateral-side structures is advised.

Imaging

Radiographic examination of the injured extremity is imperative to rule out associated fracture or joint subluxation. Initial views should include posteroanterior, lateral, and Merchant views. In the delayed setting, a flexed 45-degree weight-bearing view will give more accurate assessment of tibiofemoral joint space. In addition, a PCL avulsion fracture may be detected.

MRI may provide important information about the injured soft tissues and their site of disruption but can overestimate severity of the injury. An MRI can also be used to evaluate, radiographically, the healing of the PCL. Although healing of the PCL on MRI correlates with clinical healing, there may be residual clinical laxity.