Knee dislocations are uncommon. A knee dislocation is defined as complete displacement of the tibia with respect to the femur, with disruption of three or more of the stabilizing ligaments. [1, 2] Small avulsion fractures from the ligaments and capsular insertions may be present.
An image depicting a knee dislocation can be seen below.
Knee anatomy relevant to dislocations is related to the 4 main ligament and neurovascular structures. The anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), and posterolateral corner (lateral collateral ligament [LCL], arcuate complex, popliteus, and biceps femoris) together with the joint capsule are responsible for knee stability. 
Knee dislocation requires injury to at least 3 of the 4 main ligaments. The popliteal artery is relatively fixed proximally as it exits a fibrous tunnel at the level of the adductor hiatus, enters the popliteal space, and then is again tethered distally under the soleus. When the knee dislocates, the popliteal artery is stretched and vulnerable to injury. Popliteal artery injury occurs in up to 53% of patients with knee dislocations. The peroneal nerve is tethered as it winds around the fibular neck. With knee dislocation, the peroneal nerve is at risk. Peroneal nerve injury may occur in up to 23% of patients with knee dislocations. Nearly one half of the patients with peroneal nerve injuries have a permanent deficit. 
Fractures about the knee are fairly common in knee dislocations. These can be severe periarticular fractures, commonly tibial plateau fractures or ligamentous and tendonous avulsion fractures.  Few data exist on the true incidence of these fractures, as many reports do not mention them. One unpublished study noted a 35% (8 of 23 cases) incidence of fractures associated with high-velocity knee dislocations (Owens, unpublished data, 2003). The presence of the fracture may alter management and require supplemental bony fixation or may allow ligamentous repair versus reconstruction.
Multiple ligament injuries are required for knee dislocation. Generally, both cruciates and one or both collateral ligaments are injured. However, knee dislocations have been described with one of the cruciates intact. It is important to evaluate the competence of each ligament and to consider the possibility of a knee dislocation in knees with 3 or more ligaments torn. Vigilance is required because of the high incidence of neurovascular injuries associated with knee dislocation (vascular injuries 5-79%, nerve injuries 16-40%).
The 5 types of knee dislocations, based on the direction of tibial displacement, are anterior, posterior, medial, lateral, and rotational.  An anterior knee dislocation usually results from a hyperextension injury to the knee that initially tears the posterior structures and drives the distal femur posterior to the proximal tibia. A posterior knee dislocation usually results from a direct blow to the proximal tibia that displaces the tibia posterior to the distal femur. Valgus forces cause medial dislocations. Varus forces cause lateral dislocations of the knee.
Rotational or rotatory dislocations are the result of indirect rotational forces, usually caused by the body rotating in the opposite direction of a planted foot. Rotatory dislocations can be of 4 different types, named for the direction of the displaced tibial plateau. For example, posterolateral rotatory dislocation describes a posterior position of the lateral tibial plateau and is the most common rotatory dislocation reported.
Knee dislocations can also be classified as open or closed and as reducible or irreducible.
Most knee dislocations are the result of high-energy injuries, such as motor vehicle or industrial accidents. They also can occur with low-energy injuries, such as those that occur in sports. The reported mechanisms of injury are variable, but the most common are motor vehicle accidents (50-60%), followed by falls (30%), industrial-related accidents (3-30%), and sports-related injuries (7-20%).
The Mayo Clinic recorded 14 knee dislocations during an interval of 2 million admissions.  The largest reported series of knee dislocations is from Los Angeles County Hospital, where 53 knee dislocations were reported over a 10-year period. The true incidence of knee dislocations is higher than reported because as many as 50% of knee dislocations spontaneously reduce before patients present to the emergency department.
Multiple outcome studies after surgical intervention for knee dislocation universally report that patients rarely claim that their knee function is normal. [7, 8] Wascher reported results after ACL reconstruction, PCL reconstruction, or both in 13 patients with knee dislocations, and results after a mean of 38 months follow-up care.  One patient claimed his knee felt normal, 6 patients returned to unrestricted sports activities, and 4 returned to modified sports.
Shapiro reported the outcome after allograft reconstruction of the ACL and PCL after traumatic knee dislocation.  Seven patients had an average of 51 months follow-up care postoperatively. Only one patient had significant pain, 3 patients had occasional or rare sensations of knee instability, and all 7 were able to return to work or school. Four patients required knee manipulation at an average of 16.8 weeks postoperatively for knee arthrofibrosis. The functional grading was excellent in 3, good in 3, and fair in 1 patient.
Yeh reported the outcome after arthroscopic reconstruction of the PCL with open repair of collateral ligaments and capsule.  Twenty-three patients had a mean follow-up of 27 months. At latest follow-up visit, the mean knee extension was 1° and knee flexion was 129.6°.
Emergency vascular surgery is indicated for dysvascular limbs (see Postreduction assessment, Medical therapy). For indications for surgical repair of ligament avulsions, see Surgical options, Surgical therapy.
Nonsurgical management is recommended in patients who have low functional demands or cannot cooperate with postoperative rehabilitation, such as those with significant closed head injuries (see Nonsurgical management, Medical therapy).
Knee arthroscopy is contraindicated within 2 weeks of knee dislocations because capsular tears cause fluid extravasations into the leg that may result in compartment syndrome (see Surgical therapy).