|Plain lateral X-ray of the left knee showing a posterior knee dislocation|
|Symptoms||Knee pain, knee deformity|
|Complications||Injury to the artery behind the knee, compartment syndrome|
|Types||Anterior, posterior, lateral, medial, rotatory|
|Diagnostic method||Based on history of the injury and physical examination, supported by medical imaging|
|Differential diagnosis||Femur fracture, tibial fracture, patellar dislocation, ACL tear|
|Treatment||Reduction, splinting, surgery|
|Prognosis||10% risk of amputation|
|Frequency||1 per 100,000 per year|
A knee dislocation is a knee injury in which there is a complete disruption of the joint between the tibia and the femur. Symptoms include knee pain and instability of the knee. Complications may include injury to an artery around the knee, most commonly the artery behind the knee, or compartment syndrome.
About half of cases are the result of major trauma and about half occur as a result of minor trauma. In about half of cases the joint reduces itself before a person arrives at the hospital. Typically there is a break of the anterior cruciate ligament, posterior cruciate ligament, and either the medial collateral ligament or lateral collateral ligament. If the ankle–brachial pressure index is less than 0.9, CT angiography is recommended to detect blood vessel injury. Otherwise repeated physical exams may be sufficient.
If the joint remains dislocated, reduction and splinting is indicated; this is typically carried out under procedural sedation. In those with signs of arterial injury, immediate surgery is generally carried out. Multiple surgeries may be required. In just over 10% of cases, an amputation of part of the leg is required.
Knee dislocations are rare, occurring in about 1 per 100,000 people per year. Males are more often affected than females. Younger adults are most often affected. Descriptions of this injury date back to at least 20 BC by Meges of Sidon.
Signs and symptoms
Complications may include injury to the artery behind the knee in about 20% of cases or compartment syndrome. Damage to the common peroneal nerve or tibial nerve may also occur. Nerve problems if they occur often never fully heal.
About half are the result of major trauma and about half occur as a result of minor trauma. Major trauma may include mechanisms like falls from a significant height, motor vehicle collisions, or a pedestrian being hit by a motor vehicle. Cases due to major trauma often have other injuries.
As the injury may reduce on its own before a person arrives at the hospital, the diagnosis may be missed. Diagnosis may be suspected based on the history of the injury and a physical examination. This may include anterior drawer test, valgus stress test, varus stress test, and posterior sag test. An accurate physical exam can be difficult due to pain.
Plain X-rays, CT scan, ultrasonography, or MRI may help with the diagnosis. Findings on X-ray that may be useful among those who have already reduced include a variable joint space, subluxation of the joint, or a Segond fracture.
If the ankle–brachial pressure index (ABI) is less than 0.9, CT angiography is recommended. Standard angiography may also be used. If the ABI is greater than 0.9 repeated physical exams over the next 24 hours to verify good blood flow may be sufficient. The ABI is calculated by taking the systolic blood pressure at the ankle and dividing it by the systolic blood pressure in the arm.
CT angiogram 3D reconstruction, posterior view showing a normal artery on the left, and occlusion to right popliteal artery as a result of a knee dislocation
They may be divided into five types: anterior, posterior, lateral, medial, and rotatory. This classification is based on the movement of the tibia with respect to the femur. Anterior dislocations are the most common, followed by posterior dislocations. They may also be classified based on what ligaments are damaged.
Initial management is often based on Advanced Trauma Life Support. If the joint remains dislocated reduction and splinting is indicated. Reduction can often be done with simple traction after the person has received procedural sedation. If the joint cannot be reduced in the emergency department emergency surgery is recommended.
In those with signs of arterial injury immediate surgery is generally carried out. If the joint does not stay reduced external fixation may be needed. If the nerves and artery are intact the ligaments may be repaired after a few days. Multiple surgeries may be required. In just over 10% of cases an amputation of part of the leg is required.
Knee dislocations are rare: they represent about 1 in 5,000 orthopedic injuries, and about 1 knee dislocation occurs annually per 100,000 people. Males are more often affected than females, and young adults are most often affected.
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