Several anatomic features, both static and dynamic, contribute to knee stability. Static stabilizers include the bony articulations, menisci, and ligaments. Dynamic stabilizers include the musculature that crosses the knee joint. The articulation of the tibiofemoral joint is maintained in part by the bony anatomy of the femoral condyles and tibial plateau as well as the menisci, which increase contact area between the tibia and femur. The most significant ligamentous stabilizers are the anterior and posterior cruciate ligaments, the medial and lateral collateral ligaments, and the posteromedial and posterolateral corners. The capsular ligaments of the knee are aponeurotic extensions of the thigh and leg musculature that terminate on the menisci. They function to activate motion of the joint and impart stability as ligament tension is modulated by the attached musculature (22).

The medial aspect of the knee can be conceptualized in terms of three layers and three longitudinal divisions (77,79). The first and most superficial layer is the sartorius and its fascia. Next is the tibial collateral ligament. The third and deepest layer consists of the medial capsular ligament. The gracilis and semitendinosis muscles run between the two superficial layers. The medial aspect of the knee is divided longitudinally into thirds. The anterior third consists of the medial retinacular ligament of the extensor aponeurosis, which has only meniscal and tibial attachments. The middle third contains the medial mid-third capsular ligament and the tibial collateral ligament superficially. The posterior third contains the termination of the semimembranosis tendon, which consists of the posterior oblique ligament and the origin of the oblique popliteal ligament. Each ligament can be divided into meniscofemoral and meniscotibial components in the coronal plane.

The lateral aspect of the knee can also be divided into layers (79). The deepest layer is the lateral capsule which divides into two laminae just posterior to the overlying iliotibial tract. The laminae encompass the lateral collateral, fabellofibular, and arcuate ligaments. The second layer consists of the quadriceps retinaculum and the two patellofemoral ligaments posteriorly. The most superficial layer consists of the iliotibial tract and the superficial portion of the biceps and its expansion. The peroneal nerve lies deep to the iliotibial tract, just posterior to the biceps tendon (40).

In the popliteal fossa, the popliteal artery and vein are separated from the posterior capsule by a layer of fat. The artery is tethered proximally by the adductor hiatus and distally by the soleus arch where it bifurcates into anterior and posterior tibial arteries. Genicular arteries give rise to the collateral circulation around the joint. The close proximity of the popliteal artery to the joint as well as its immobility makes it especially susceptible to injury with dislocations of the knee joint (54). The tibial and common peroneal nerve run superficial to the artery and are less vulnerable to injury when the knee is dislocated.

To assess the structural integrity of the ligamentous structures, the function of each must be well understood. The ACL functions to prevent anterior translation of the tibia relative to the femur, limits rotation of the tibia when the knee is in extension, and limits varus and valgus stress when the lateral collateral ligament (LCL) or MCL are injured (10,83). The PCL is located near the center of rotation of the knee. It functions as the primary static stabilizer of the knee and the primary restraint against posterior translation of the tibia. The MCL and LCL function to prevent valgus and varus stresses respectively when the knee is flexed to 30 degrees. A secondary function of both is to limit anterior or posterior translation and rotation of the tibia (22). The posterolateral corner functions to resist posterolateral rotation as well as posterior tibial translation relative to the femur. The posteromedial corner functions to resist posteromedial tibial translation relative to the femur and valgus stress at the knee. A thorough clinical exam is necessary to evaluate each of these structures in the multiple ligament injured knee (19,20,21).

There are several ways to classify knee dislocations: a) the direction of dislocation of the tibia; b) the presence of an open or closed injury; or c) the amount of energy required for the dislocation (high-energy vs. low-energy). Each classification system provides valuable information on the type of injury, its risk of neurovascular complication, and its risk of infection. Each knee dislocation needs to be described using a combination of these classification systems.

One common way is to describe the direction of displacement of the tibia in relation to the distal femur. This gives a combination of possible dislocations including anterior, posterior, medial, lateral, and rotational. Rotational dislocations include anteromedial, anterolateral, posteromedial, and posterolateral. Anterior dislocations are thought to be the most common, representing 31% to 70% of all knee dislocations (25,33). Posterior dislocations are the next most frequent at 25% (25,33). Rotational dislocations occur in 3% to 5% (25,33). Many knee dislocations spontaneously reduce. A high index of suspicion must exist if multiple ligaments are injured that a knee dislocation is likely to have occurred.

Open dislocations occur in 19% to 35% of all knee dislocations and have a poorer prognosis (46,69). Open dislocations also limit surgical management because it may make arthroscopic surgery difficult.

Another method to classify knee dislocations is to differentiate between high- and low-energy mechanisms. Low-energy mechanisms include nonmotorized sports injuries. These injuries have a lower incidence of neurovascular injury (67). Higher energy mechanisms include motor vehicle collisions and falls from height.

A classification system has been created by Schenck (63) that describes the injury pattern, as well as any associated neurovascular injury (Table 37-1). This classification system may be helpful in planning surgical treatment and for predicting functional outcome.

Regardless of the mechanism or position of the tibia, prompt reduction is needed. If after reduction, there is an asymmetrical vascular exam between the two lower extremities urgent vascular studies should be obtained.

The typical mechanism of injury is a violent force on the proximal tibia or knee. The direction that the force is applied will determine the ultimate position of the dislocation as well as the ligaments injured. Hyperextension results in anterior dislocation. Kennedy (42) has described the sequence of events in a cadaveric model. The posterior capsule fails first. The ACL and PCL as well as the popliteal artery then fail at approximately 50 degrees of hyperextension (29,43). Kennedy’s is the only published experimental study that has investigated mechanism of injury for knee dislocations.

A posteriorly directed force on the proximal tibia, typical of a dashboard injury, is thought to result in a posterior knee dislocation. Varus and valgus stresses result in lateral and medial dislocations, respectively. A combination of forces in the anterior-posterior plane and in the medial-lateral plane will likely produce a rotational type dislocation.

The posterolateral dislocation is thought to arise from flexed nonweight bearing knee with a rapid abduction and internal rotation moment (57).

Several anatomic structures are at risk for injury in the dislocated knee. Patients can present with varying combinations of ligamentous involvement. All four major knee ligaments as well as the posteromedial and posterolateral corners can be compromised. Additionally, vascular and neurologic injuries are common. Furthermore, bony avulsion injuries, fractures of the distal femur or tibial plateau, and ipsilateral tibial or femoral shaft fractures are often seen with concomitant knee dislocations.

Numerous reports exist in the literature citing knee dislocations in which less than three of the major knee ligaments were torn (9,13,43,47,68,69); however, this appears to be the exception rather than the rule. If a dislocation occurs solely in the sagittal plane, it would not be uncommon to find macroscopic continuity of both collateral ligaments, but several authors propose that frank dislocations invariably result in rupture of at least three of the four major knee ligaments. Sisto and Warren (69) found that all knees in their series treated operatively had three or more ligaments compromised. In a series by Frassica et al. (25), all patients were found to have disruption of the ACL, PCL, and MCL at time of surgery. In Fanelli’s study, 19 of 20 patients had bicruciate disruption coupled with injury to a variable third component, either the MCL or the posterolateral corner (20). A meticulous ligamentous examination
is essential in order to fully evaluate the extent of the injury.

Cruciate injuries in knee dislocations typically fall into one of two patterns: either bony avulsions or midsubstance tears. These often exist in multiple combinations with other soft tissue and ligamentous injuries. Bony avulsion or peel-off injuries are often amenable to surgical reattachment and avoid need for reconstruction of the involved cruciate. Avulsion injuries are more likely to occur in high-energy knee dislocations. Midsubstance tears of the cruciates occur commonly with knee dislocations. Results of primary repair of midsubstance tears are far inferior to that of surgical reconstruction.

In addition to cruciate rupture, injury to the soft tissues, collateral ligaments, joint capsule, and supporting tendinous structures are common in knee dislocations. These structures also frequently require operative attention. Again, a careful physical and surgical evaluation is crucial to identify injury to the MCL, LCL, menisci, and tendons of the iliotibial band, the biceps femoris, the popliteus, and the quadriceps mechanism. All open injuries require thorough debridement, pulsatile irrigation, and bony stabilization.

The incidence of vascular compromise in knee dislocation has historically been estimated to be about 32% (33). Other studies have documented rates anywhere from 16% to 64% (37,46). The more recent literature confirms the significant incidence of arterial injury (2,25,66,70) reaffirming the need for a complete vascular evaluation. The popliteal artery has very limited mobility secondary to the tethered nature of the vessel at the adductor hiatus and the entrance through the gastrocnemius-solues arch. This tethering makes it extremely vulnerable to injury in cases of blunt trauma to the knee. Two major types of injury mechanisms are described. One involves a stretching of the artery, often seen with hyperextension, which results in extensive intimal damage. This is more common in anterior dislocations. Posterior dislocations, however, typically result in a direct contusion of the vessel by the posterior aspect of the tibial plateau and are more likely to produce complete rupture of the artery (33). As the popliteal artery is an end-artery to the leg, with minimal collateral circulation provided by the geniculate system, any compromise to the point of prolonged obstruction often leads to ischemia and eventual amputation. Furthermore, the popliteal vein is responsible for the majority of the venous outflow from the knee. Injury to this structure also compromises the viability of the lower limb.

It is important to note that in the case of frank knee dislocations and suspected dislocations, the presence of pulses does not rule out an arterial injury. Serial vascular examinations are essential as intimal flaps may often present as delayed thrombus formation. Additionally, the absence of pulses implies an arterial injury and cannot be attributed to vascular spasm. Failure to recognize an arterial injury can lead to disastrous outcomes.

Nerve injury is also quite common following dislocation of the knee. A reasonable estimate of the documented incidence is anywhere from 20% to 30% (39,43,47,69,70,74). The majority of nerve injuries involve the peroneal nerve, but reports of tibial nerve compromise have been reported (81). Nerve palsies have been described in all types of knee dislocations, but a common theme is association with injury to the lateral ligamentous complex. The peroneal and tibial nerves are not as tightly tethered as the popliteal artery and are thus less prone to injury. A stretch neuropraxia is the most common mechanism of injury, which often extends well proximal to the fibula. Occasionally, complete nerve transection occurs. Recovery of nerve function is unpredictable with most series reporting no recovery in more than 50% of injuries, though Sisto and Warren (69) reported spontaneous complete recovery in two of two patients with an incomplete peroneal nerve palsy (46,70,72,73). Nerve injury must be differentiated from stocking paresthesias that may be indicative of a developing compartment syndrome as opposed to a simple neuropraxia.

Osseous integrity is also often times compromised in knee dislocations. Reports have indicated that the incidence of bony injury may be as high as 60% (47). Avulsion fractures of ligamentous and tendinous attachments are frequently seen in knee dislocations (Segond fractures, fibular head avulsion fractures, cruciate avulsions) but should be considered ligamentous injuries, unlike major fractures as are seen in true fracture-dislocations of the knee.

Moore (49) coined the term “fracture-dislocation” to distinguish between tibial plateau fractures and purely ligamentous knee dislocations because of different treatment protocols. Tibial plateau fractures require bony stabilization, whereas pure knee dislocations necessitate ligamentous reconstruction. Fracture-dislocations of the knee are a combination of the two, adding an element of complexity to their treatment. Similar to pure knee dislocations, these are typically high-energy injuries that can result in marked joint instability and are associated with a high risk of soft-tissue and neurovascular compromise. It is also important to differentiate between these three entities as outcomes correlate directly with the underlying injury. Tibial plateau fractures have the best prognosis and pure dislocations have the worst prognosis with fracture-dislocations lying somewhere in between.