Knee Dislocation
DR. KS Dhillon
Introduction
A dislocation of the knee can be defined as complete congruency loss between the proximal tibial and distal femoral articular surfaces. Bicruciate ligament injuries are also equivalent to knee dislocations with regard to the mechanism of injury, severity of ligamentous injury, and frequency of major arterial injuries (1). A knee dislocation is a potentially devastating injury. It is often a surgical emergency. A knee dislocation requires prompt identification, evaluation, and consultation with a surgeon for definitive treatment. Vascular injury and compartment syndrome are complications that should not be missed in the workup of a knee dislocation (2,3). This is unlike patellar dislocations, which generally do not require immediate surgical or vascular intervention (4,5).
Etiology
High-energy trauma is usually required to produce a knee dislocation. With disruption of the joint, multiple concomitant ligamentous injuries and instability can also be expected. Motor vehicle accidents, high-velocity sports-related injuries, and falls can all cause knee dislocation. Most often posterior and anterior dislocations occur. Medial rotatory, and lateral rotatory dislocations are also possible.
Epidemiology
Knee dislocations are infrequently seen. They are potentially limb-threatening injuries. Associated undiagnosed vascular injuries can lead to limb ischemia which will require amputation (6-8). Knee dislocations represent about 0.001% to 0.013% of all orthopaedic injuries (9-11). The incidence of knee dislocation is however underreported, since almost 50% of knee dislocations reduce spontaneously at the scene, before arrival at the hospital, or are misdiagnosed. The incidence of knee dislocations is higher in men than women, with a ratio of 4:1. Obesity is an independent risk factor for sustaining this injury from an ultra-low energy injury.
Pathophysiology
Anatomy
The knee is a ginglymoid joint with 3 articulations. These include the tibiofemoral, patellofemoral, and tibiofibular. Four major ligaments stabilize the knee joint. These include the posterior cruciate ligament (PCL), anterior cruciate ligament (ACL), medial collateral ligament (MCL), and lateral collateral ligament (LCL). Multiligamentous disruption can occur with a knee dislocation. The normal range of motion (ROM) of the knee is 0 to 140 degrees. There is 8 to 12 degrees of rotation during flexion and extension.
Mechanism of Injury
Knee dislocation can occur following high-energy injuries such as traffic accidents, falls from heights, dashboard injuries, and crush injuries. Low-energy injuries can also result in a knee dislocation such as those seen in athletic injuries. Even ultra-low-energy injuries can result in a knee dislocation in patients who have morbid obesity.
Associated Neurovascular Injuries
The popliteal artery is at the highest risk of injury due to knee dislocation. The popliteal artery stretches across the popliteal space. It gives off several branches in a collateral system around the knee. Upto 40% of patients with a tibiofemoral disruption will sustain an associated vascular injury. These injuries are due to tethering at the popliteal fossa, proximally by the fibrous tunnel at the adductor hiatus, and distally by the fibrous tunnel at the soleus muscle. There is collateral circulation around the knee that is formed by the geniculate arteries that can provide vascular flow and palpable pulses that mask a limb-threatening vascular injury (2). The peroneal nerve is located at the fibular neck. It is injured in more than 20% of patients with knee dislocation (2).
Associated Bony and Soft Tissue Injuries
Sixty percent of the patients with knee dislocations have associated fractures. Multiple soft tissue injuries can also be associated with knee dislocation. These include patellar tendon rupture, periarticular avulsion, and displaced menisci.
History and Physical Examination
Spontaneously Reduced Cases
About 50% of knee dislocations spontaneously reduce before contact with a clinician. Obtaining a thorough problem-oriented history is paramount when evaluating knee dislocations. Inquiries into the mechanism of injury and position of the lower leg immediately after the injury are necessary. When the emergency management services or the patient reports a change in the position of the tibia relative to the femur, the doctor should assume a knee dislocation occurred with subsequent return to normal anatomical position. Subtle signs of trauma, such as abrasions, bruising, ecchymosis, and effusion should be looked for. Hyperextension of the knee to more than 30 degrees when lifting the heel is indicative of gross joint instability and it strongly suggests a knee dislocation.
Cases Presenting With Notable Deformity
When a patient presents with an obvious deformity consistent with knee dislocation the diagnosis is more straightforward. A significant joint effusion, ecchymosis, and swelling may be present. Buttonholing of the medial femoral condyle through the medial capsule, known as a "dimple" or "pucker sign," may occur. It indicates an irreducible posterolateral dislocation (5). A closed reduction is contraindicated in such situations due to the risk of skin necrosis.
Physical Examination
A comprehensive physical examination of the affected extremity is required if the patient gives a history suggestive of knee dislocation. Special attention should be paid to the neurovascular stability of the extremity and ligamentous stability of the joint.
Popliteal and distal pulses should be assessed and compared with the contralateral side. A vascular injury should be excluded before and after reduction of the dislocation. Serial examinations are usually necessary. A palpable distal pulse does not exclude the presence of a vascular injury. Limb-threatening vascular ischemia can result in the presence of palpable distal foot pulses. This is because collateral circulation can mask a complete popliteal artery injury (12).
In all patients suspected of knee dislocation, the ankle-brachial index (ABI) should be measured (13). The patient should be monitored with serial examinations when the ABI exceeds 0.9. If the ABI is less than 0.9 further investigation with an arterial duplex ultrasonography or computed tomography angiography is necessary. If arterial injury is confirmed vascular surgical consultation is imperative.
Patients who have diminished or absent pulses should undergo immediate joint reduction. Joint reduction should be followed by reassessment and if the pulses remain undetectable or diminished, surgical exploration should be carried out. Amputation rates of up to 86% have been reported with ischemia of greater than 6 hours. If the pulses return following joint reduction, ABI measurements should be taken. This should be followed by observation and serial examination or angiography.
With vascular injuries concomitant neurological deficits can occur. Sensory and motor function should be assessed and documented. The ligamentous integrity of the 4 major knee stabilizers should also be assessed.
Evaluation
Ankle-Brachial Index
The ankle-brachial index (ABI) is defined as the ratio of lower extremity perfusion via the posterior tibial and dorsalis pedis arteries and upper extremity perfusion via the brachial artery. An ABI of 0.9 or greater is considered normal. An ABI of less than 0.9 indicates a vascular compromise (13). Pulse and perfusion examinations can be conducted. Such examination has limited utility unless there are hard signs of vascular compromise. When there is vascular compromise prompt evaluation by a surgeon is necessary. Normal pulses or ABIs do not necessarily rule out any injury. There are reports of popliteal artery contusion, intimal layer disruption, and delayed thrombus formation in patients with distal perfusion after knee dislocation.
Duplex Ultrasonography
Duplex ultrasonography can evaluate the vascularity at the bedside. Imaging with computed tomography angiography is carried out in patients with decreased ABI, asymmetric pulses, or abnormal duplex ultrasonography. Vascular surgeon consultation is necessary for patients with absent or weak pulses, pale or cool extremities, paresthesias, or paralysis (14).
Imaging Studies
Anterior-posterior (AP) and lateral knee radiographs can confirm the status of the joint and any concomitant fractures. A 45-degree oblique radiograph may be useful in patients with associated fractures. The radiographs can be normal in patients with spontaneously reduced knee dislocations. The following radiographic findings can be seen in knee dislocation:
Joint space asymmetry or irregularity
Associated avulsion fractures such as Segond sign or lateral tibial condyle avulsion fracture
Osteochondral fractures
Defects.
Computed tomography (CT) is needed when fractures are identified in postreduction radiographs. CT better delineates the fracture patterns and the level of extension, into the tibial tubercle, tibial eminence, or tibial plateau. Magnetic resonance imaging (MRI) may be needed to assess soft tissue structures. It is best performed after reduction of the dislocation and before placement of hardware to obtain better-quality images (15).
Management
Nonoperative Management
Dislocation of the knee is an orthopedic emergency that requires acute closed reduction followed by evaluation of the vascular status (1,16). Hospital admission for vascular observation is an option only for patients with clearly strong distal pulses, normal ABI, and normal duplex ultrasonography. In other patients, vascular surgery consultation and CT angiography are required to rule out popliteal artery injury. For closed reduction of an anterior dislocation, axial traction is applied and anterior translation of the femur is done. For a posterior dislocation, axial traction is applied, and anterior translation of the tibia is done. In patients with medial, lateral, or rotatory dislocations, the technique comprises axial traction and manipulation opposite to the deformity present. After close reduction, the knee is splinted with 20 to 30 degrees of flexion.
Operative Management
Immobilization can be the definitive treatment following a successful acute closed reduction in the absence of vascular injury. Nonoperative treatment as a definitive treatment produces inferior outcomes. Prolonged immobilization can be complicated by an unstable knee with limitation of movements (17).
Open reduction
An open reduction is indicated in knee dislocations that are recalcitrant to closed reduction and in those that present late (18). An open reduction is also indicated in posterolateral dislocations and open fracture dislocations. Obesity can pose a challenge to the closed reduction of dislocated knees. In such situations, an open reduction could be an alternative option. Open reduction is also indicated in patients with associated vascular injury.
Open reduction is carried out via a midline incision with a medial parapatellar approach. If the medial femoral condyle has buttonholed through the medial capsule, the condyle has to be reduced and the medial capsule repaired. Associated soft tissue injuries, such as meniscal tears, patella tendon rupture, or periarticular ligament avulsions, may require an acute repair. Concomitant bony injuries are treated by internal fixation or external fixation followed by planned delayed definitive management. External fixation is also indicated in patients with vascular repair and in cases complicated by obesity. Other indications for external fixation include open fracture–dislocation, polytrauma, and compartment syndrome (19).
Early ligament repair or reconstruction
Arthroscopic management is not an option for early ligament repair or reconstruction, especially in patients with large capsular injury because there is a risk of fluid extravasation and compartment syndrome. Open reconstruction is usually indicated for the posteromedial and posterolateral corners, including the collaterals since they are subcutaneous and close to neurovascular structures. Meniscal, cartilaginous, and capsular injuries, can also be addressed acutely. Unstable knees are managed with ligament repair or reconstruction. There are improved outcomes if surgery is done within 3 weeks of the injury (20). A knee immobilizer should be kept until definitive management of the ligamentous injuries. Acute and staged reconstructive procedures have similar outcomes.
Differential Diagnosis
The differential diagnosis for knee dislocation includes the following:
Medial collateral ligament injury
Meniscus injuries
Patellar injury and dislocation
Anterior cruciate ligament injury
Femoral shaft fractures
Knee fractures
Patellofemoral joint syndromes
Tibia and fibula fractures.
Staging
There are 2 commonly used classification systems for knee dislocation i.e. the Kennedy Classification, which relies mainly on the direction of tibia displacement, and the Schenck Classification, which considers concomitant knee ligament injuries.
Kennedy Classification
Anterior dislocation
This occurs due to a hyperextension injury. It is the most common knee dislocation pattern and accounts for 30% to 50% of dislocation injuries. Anterior dislocations have the highest rate of peroneal nerve injury. Usually, the PCL is also injured. When there is concomitant vascular injury, it is usually an arterial intimal tear from traction.
Posterior dislocation
This is the second most common knee dislocation pattern. It comprises 30% to 40% of knee dislocations. It is caused by axial loading to a flexed knee, as occurs in a dashboard injury. Posterior dislocation has the highest rate of vascular injury. It is most commonly characterized by a complete popliteal artery tear.
Lateral dislocation
The mechanism of injury in lateral knee dislocations is either varus or valgus force. There usually is concomitant PCL and ACL injury. Lateral dislocations occur in about 13% of knee dislocation injuries.
Medial dislocation
The mechanism of injury is the same as for lateral dislocation. It is caused by a varus or valgus force. Both the PLC and PCL are usually injured. Medial dislocations are not common and account for only 3% of knee dislocations.
Rotational or rotary dislocation
Posterolateral dislocation is most common, with buttonholing of the femoral condyle through the capsule (21). This is usually an irreducible dislocation
Schenck Classification
It is based on a pattern of multiligamentous injury of knee dislocation (KD).
KD I: Multiligamentous injury with involvement of either the ACL or PCL.
KD II: Injury to 2 ligaments: the ACL and PCL only.
KD III: Injury to 3 ligaments: the ACL and PCL, in addition to either the PMC or PLC.
KD IIIM: Involves the ACL, PCL, and MCL
KD IIIL: Involves the ACL, PCL, and LCL.
KD IV: Injury to 4 ligaments, including the ACL, PCL, PMC, and PLC. KD IV injuries have the highest rate of concomitant vascular injury (5% to 15%).
KD V: A multiligamentous injury with a periarticular fracture.[22]
Prognosis
Knee dislocation is a serious injury in which the knee rarely returns to its pre-injury state. The need for surgical intervention is common.
Complications
Complications are common in traumatic knee dislocations and they vary in incidence. The most common complication is arthrofibrosis. The most serious complication is vascular injury. Other commonly encountered complications include injuries to the peroneal nerve and nearby vascular structures.
Arthrofibrosis or stiffness of the knee is the most common complication of knee dislocation. It occurs in up to 38% of the patients. Prolonged immobilization of the affected joint is a risk factor for arthrofibrosis. Early mobilization of the joint is recommended as a preventative measure. Manipulation of the joint under anesthesia and arthroscopic lysis of adhesion are the treatment options.
Some degree of instability is present in upto 37% of patients with knee dislocation. Redislocation however is uncommon. Management will vary from bracing to revision reconstruction.
Peroneal nerve injuries occur in 10% to 40% of patients. Partial recovery occurs in up to 50% of the patients (23). Men, patients with obesity, and those with associated fibular fractures are at an increased risk for peroneal nerve injuries. Patients with peroneal nerve injury are treated with an ankle-foot-orthosis (AFO) to prevent equinus contracture. In acute injuries, neurolysis or exploration of the nerve at the time of the reconstruction can be carried out. In chronic cases, the treatment includes nerve repair, reconstruction, or tendon transfer. Usually, a transfer of the tibialis posterior tendon to the foot is done.
Vascular injuries are reported in 5% to 15% of knee dislocations. About 50% of all vascular injuries occur with anterior or posterior dislocations (24). The incidence of vascular injuries is highest with KD IV dislocations. Vascular injuries are managed with urgent vascular repair and prophylactic fasciotomies.
Postoperative and Rehabilitation Care
Rehabilitation after knee dislocation is highly demanding. There are no definitive protocols for rehabilitation. The rehabilitative principles, however, remain the same. A delay of 1 to 3 weeks between the injury and operative treatment allows the inflammatory reaction to subside. Straight leg raise exercises should be performed to avoid quadriceps muscle wasting (25). Early postoperative rehabilitation aims to protect the operative repair, especially if the PCL was reconstructed. Some protocols recommend the use of a limited extension brace (26-28). Delayed postoperative rehabilitation is injury, patient, and repair-specific.
Conclusion
Knee dislocation is a relatively uncommon injury seen in the emergency department. The dislocation can be associated with a neurovascular injury that can lead to the loss of a limb. Patients with a knee dislocation are best treated by an interprofessional team. The staff treating the patient must be fully aware that a dislocated knee can disrupt the vascular supply to the distal leg. Immediate admission and a vascular surgery consult are required if there is a loss of pulses in the leg. Orthopedic consultation is necessary in almost all cases.
The outcomes are generally good for most patients with knee dislocations who obtain prompt management. A delay in treatment leads to a chronically unstable and painful knee (29,30). All patients will require physical rehabilitation following a knee dislocation injury.
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