Mechanism

The available literature on knee dislocations includes several retrospective studies with very few patients owing to the relatively rare nature of the injury. Incidence in males out number females almost 2.5:1.0; these injuries usually result from a high-energy mechanism, the most common being motor vehicle collision (up to 50% of reported cases). The other 2 most common mechanisms include sports injuries (up to 33%) and simple falls (up to 12%). Patients in the high-energy group are often polytrauma patients with associated fractures and ipsilateral joint dislocations. A fourth subset, designated ultralow energy, has been recently described and some patterns elucidated.

Classification

As with all traumatic injuries, the first description of a knee dislocation includes whether the injury is closed versus open and the time from injury to presentation. It is important for the examiner to determine if the dislocation is partial (subluxed), spontaneously reduced, or complete. The classification systems used in the past for knee dislocations are summarized in Table 1 . Kennedy described knee dislocations based on the direction of tibial translation relative to a stationary femur ( Fig. 1 ). This system enables effective communication if the knee remains dislocated. The major limitation with this system is the variability in injured ligaments when only accounting for the dislocation direction. McCoy and colleagues and Shelbourne created classification systems, and each used the energy of the injury mechanism. High-, low-, and ultralow energy mechanisms were described; higher-energy injuries have a higher incidence of vascular injury. Taking a thorough history is always important with these injuries. Palmer classified knee dislocations based on the time since the injury, defining the 3-week mark as an important date. Before 3 weeks, the joint capsule has not healed and surgical intervention for ligament repair was not advised. Boisgard and colleagues created a classification system that included all bicruciate ligament injuries but also included knees that did not dislocate (see Table 1 ).

( A – D ) ( A , B ) Anteroposterior (AP) and lateral radiographs of knee before reduction attempt demonstrating frank joint incongruity and medial dislocation. ( B , C ) AP and lateral radiographs of knee before reduction attempt demonstrating frank joint incongruity and posterolateral knee dislocation.

Schenk developed a classification system that is based on the anatomic structures injured. This system was modified by Wascher and then Yu and is now the most widely used and accepted classification available. It accounts for injured ligaments, vascular or neurologic injury, and also whether an associated fracture is present. Schenk’s classification is strictly for knee dislocations and does not include knees with bicruciate injuries that did not dislocate ( Table 2 ).

Schenk’s classification was later modified to include 3 letter designations. Dislocations with an associated fracture were designated V , those with associated arterial injury designated C , and those with associated nerve injury designated N ( Table 3 ).

Acute Assessment

Cases of suspected knee dislocation in the acute setting are often the result of high-energy mechanisms. Initial evaluation includes the primary survey according to the Advanced Trauma and Life Support protocol before the secondary survey, which includes prompt but careful evaluation of the neurologic and vascular status of the affected limb.

The diagnosis is relatively straightforward in patients with an unreduced knee dislocation ( Fig. 2 ). Proceeding in a stepwise pattern is recommended. In patients with a spontaneously reduced knee dislocation, identifying those at risk for vascular or soft tissue compromise is much more difficult. Subtle signs of bruising or swelling surrounding the knee may suggest capsular disruption. It is for this reason that significant hemarthrosis is often not present.

( A , B ) These clinical photographs depict a gross deformity secondary to knee dislocation (which is also an open injury in this case).

The incidence of open knee dislocations varies between sources from 15% to more than 35%. As with all open injuries, the complication rates increase; long-term outcomes and satisfaction rates are poor.

Vascular Examination

The incidence of vascular injury in knee dislocations ranges in the literature from less than 5% up to 65%, depending on the mechanisms of injury. A review of larger numbers of patients shows the overall incidence is 20%. Historically, high-energy mechanisms resulting in a hyperextension moment were thought to be more likely to cause vascular compromise. A more recent review of available literature did not demonstrate an association between direction of dislocation and vascular insult. The popliteal artery has anatomic features that put it at risk during any high-energy mechanism. It has a fibrous tethering on either side of the knee, proximally at the adductor hiatus and distally at the soleus arch. It lays in close proximity to the posterior knee joint capsule and is protected by a very thin layer of fat. Leg compartment syndrome is a known complication. The anatomic proximity of the trifurcation of the popliteal artery to the knee attributes to this risk.

Establishing a well-perfused limb is essential in all cases of suspected knee dislocation. The mechanisms for doing so, however, are controversial. A standard examination includes palpating dorsalis pedis and posterior tibial pulses bilaterally and assessing for any asymmetry. Vasospasm is common with tension injuries on arteries, and this can be a pitfall for thorough evaluation. There is literature to suggest that, in the absence of any asymmetry, further assessment is not necessary. Other investigators think obtaining bilateral ankle-brachial-indices (ABI) evaluations in the initial assessment is critical. Using a cutoff of less than 0.9, the sensitivity of ABI in detecting vascular injury requiring surgical intervention approaches 100%.

The concept of a routine angiogram for all suspected knee dislocations has been the focus of numerous studies and the center of significant debate ( Fig. 3 ). Multiple investigators contend that every knee dislocation should undergo angiogram regardless of physical examination. These investigators indicate that vascular injuries are often subintimal, and physical examination can often be unreliable. Other investigators contend that angiography should be reserved for those that have signs of inadequate circulation in the effected extremity. Currently, angiography (routine computed tomographic angiography [CTA] or magnetic resonance angiography [MRA]) is recommended for patients demonstrating insufficient perfusion or any asymmetry in physical examination, though practice varies between institutions and clinicians.

( A ) Routine angiogram showing normal 3-vessel runoff after passing through popliteal hiatus. ( B ) Here is demonstrated a normal computed tomographic angiography reconstruction after knee dislocation. The arterial reconstruction is continuous from femoral artery through the trifurcation of the popliteal artery distal to the knee. ( C ) This clinical photograph shows a patient after a knee dislocation with an asymmetric vascular examination. A cold, mottled foot that appears pale in comparison with the uninjured leg is a clear sign evaluation with angiogram emergently in the operating room is essential. ( D ) This image is a routine angiogram demonstrating disruption at the distal popliteal artery with extravasation of contrast in a patient with a knee dislocation.

One of the largest available studies cites an incidence of popliteal artery injury requiring surgical intervention to be 13%. This rate is lower than previously reported, and the study recommends a more judicious use of angiography. The study failed to identify any commonalities among the patients sustaining vascular injury.

Green and Allen described the importance of timely identification of vascular injury. Of the patients who were identified with vascular compromise, those treated surgically within 8 hours had a significantly lower amputation rate (11%) than those treated after 8 hours (86%).

Neurologic Examination

The physical examination should include a detailed neurologic examination including sensation in the tibial, deep peroneal, and superficial peroneal distributions to light touch, pinprick, and temperature if available. Motor examination including the flexor and extensor hallucis longus, tibialis anterior, and gastrocnemius is important to establish the baseline function.

The incidence of nerve injury associated with knee dislocation ranges from 4.5% to 40.0%. Most commonly, the common peroneal is the injured nerve, though isolated tibial nerve palsy has been reported. The nerve is at risk for injury as well, similar to the artery, due to its anatomic constraints both proximally and distally ( Fig. 4 ). The fibular neck tethers the nerve proximally, and the fibrous arches of the intermuscular septum form the distal tether. Kadiyala and colleagues demonstrated the precarious blood supply of the common peroneal nerve caused by its lack of intraneural vessels in the region of the fibular neck. Of those that do have neurologic deficit, the recovery is unpredictable. In one series, 21% had full neurologic recovery and 29% only partial recovery. The remaining 50% had no useful motor recovery.

( A , B ) Here is an intraoperative photograph showing near-complete disruption of the common peroneal nerve at the popliteal hiatus after knee dislocation. The common peroneal nerve is located at the end of the forceps.

Contrary to intuition, the reported incidence of nerve injury in ultralow-energy knee dislocations is higher (44.4%) than the incidence in the higher-energy trauma patients. These patients are often obese; though the operative times are longer and the procedures more difficult, knee range of motion in those operated on was significantly better (average 91.4°) than those treated nonoperatively (average 53.6°).

Immediate

After confirmation of limb perfusion and before physical examination of ligamentous integrity, standard views of the knee are obtained immediately after reduction. This timing enables the clinician to confirm adequate reduction, evaluate for any fracture, and assess overall knee alignment that may suggest a grossly unstable knee.

Associated fracture has a reported incidence ranging from 10% to 20%. These fractures are often in locations where the injured ligaments originate or insert. Fibular head (arcuate fracture), tibial spine, and lateral tibial condyle (Segond fracture) avulsions are common ( Fig. 5 ). These fractures are often treated by ligament reconstruction but occasionally, if the fragment is large enough, involve internal fixation of the fracture fragments. Routine anteroposterior, oblique, and lateral views of the knee along with full-length tibia and full-length femur films, including the joint above and the joint below, should be obtained.

Subtle signs of knee dislocation. ( A ) Here is an anteroposterior radiograph of the knee demonstrating medial joint space widening, which can often be the only finding suggestive of more severe injury. ( B ) Anteroposterior (AP) radiograph of the knee depicting the classic arcuate fragment ( arrow ). This is an avulsion of the fibular head, which suggests a lateral ligament complex injury. ( C ) AP radiograph of knee dislocation with Segond fracture. Notice the small avulsion off of lateral tibia, which is often a subtle sign of a more severe injury and occurs in high association with ACL rupture.

Secondary

After the limb is reduced, vascular injury is ruled out and grossly unstable knees are stabilized; advanced imaging is appropriate. Computed tomography (CT), MRI, or both may be appropriate. CT is used to better understand the personality of any fracture, whereas MRI can elucidate soft tissue and ligament injuries not appreciated on physical examination. Both aid in operative planning.