Knee dislocation

Knee dislocation
Plain lateral X-ray of the left knee showing a posterior knee dislocation[1]
Specialty	Orthopedic surgery
Symptoms	Knee pain, knee deformity[2]
Complications	Injury to the artery behind the knee, compartment syndrome[3][4]
Types	Anterior, posterior, lateral, medial, rotatory[4]
Causes	Trauma[3]
Diagnostic method	Based on history of the injury and physical examination, supported by medical imaging[5][2]
Differential diagnosis	Femur fracture, tibial fracture, patellar dislocation, ACL tear[6]
Treatment	Reduction, splinting, surgery[4]
Prognosis	10% risk of amputation[4]
Frequency	1 per 100,000 per year[3]

A knee dislocation is an injury in which there is disruption of the knee joint between the tibia and the femur.^[3][4] Symptoms include pain and instability of the knee.^[2] Complications may include injury to an artery, most commonly the popliteal artery behind the knee, or compartment syndrome.^[3][4][7]

About half of cases are the result of major trauma and about half as a result of minor trauma.^[3] About 50% of the time, the joint spontaneously reduces before arrival at hospital.^[3] Typically there is a tear of the anterior cruciate ligament, posterior cruciate ligament, and either the medial collateral ligament or lateral collateral ligament.^[3] If the ankle–brachial pressure index is less than 0.9, CT angiography is recommended to detect blood vessel injury.^[3] Otherwise repeated physical exams may be sufficient.^[2] More recently, the FAST-D protocol, assessing the posterior tibial and dorsalis pedis arteries for a 'tri-phasic wave pattern' with ultrasound, has been shown to be reliable in ruling out significant arterial injury.^[8]

If the joint remains dislocated, reduction and splinting is indicated;^[4] this is typically carried out under procedural sedation.^[2] If signs of arterial injury are present, immediate surgery is generally recommended.^[3] Multiple surgeries may be required.^[4] In just over 10% of cases, an amputation of part of the leg is required.^[4]

Knee dislocations are rare, occurring in about 1 per 100,000 people per year.^[3] Males are more often affected than females.^[2] Younger adults are most often affected.^[2] Descriptions of this injury date back to at least 20 BC by Meges of Sidon.^[9]

Symptoms include knee pain.^[2] The joint may also have lost its normal shape and contour.^[2] A joint effusion may, or may not, be present.^[2]

Complications may include injury to the artery behind the knee (popliteal artery) in about 20% of cases or compartment syndrome.^[3][4] Damage to the common peroneal nerve or tibial nerve may also occur.^[2] Nerve problems, if they occur, often persist to a variable degree.^[11]

About half are the result of major trauma, the other half as a result of minor trauma.^[3] Major trauma may include mechanisms such as falls from a significant height, motor vehicle collisions, or a pedestrian being hit by a motor vehicle.^[2] Cases due to major trauma often have other injuries.^[5]

Minor trauma may include tripping while walking or while playing sports.^[2] Risk factors include obesity.^[2]

The condition may also occur in a number of genetic disorders such as Ellis–van Creveld syndrome, Larsen syndrome, and Ehlers–Danlos syndrome.^[12]

As the injury may have self-reduced before arrival at hospital, the diagnosis may not be readily apparent.^[2] Diagnosis may be suspected based on the history of the injury and physical examination^[5] which may include anterior drawer test, valgus stress test, varus stress test, and posterior sag test.^[5] An accurate physical exam can be difficult due to pain.^[5]

Plain X-rays, CT scan, ultrasonography, or MRI may help with the diagnosis.^[2][11] 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.^[5]

If the ankle–brachial pressure index (ABI) is less than 0.9, CT angiography is recommended.^[3] Standard angiography may also be used.^[2] If the ABI is greater than 0.9 repeated physical exams over the next 24 hours to verify good blood flow may be sufficient.^[2][11] The ABI is calculated by taking the systolic blood pressure at the ankle and dividing it by the systolic blood pressure in the arm.^[2] More recently, the FAST-D protocol, using ultrasound to assess the posterior tibial and dorsalis pedis arteries for a 'tri-phasic wave pattern', has been shown to be reliable in ruling out significant arterial injury.^[8]

They may be divided into five types: anterior, posterior, lateral, medial, and rotatory.^[4] This classification is based on the movement of the tibia with respect to the femur.^[11] Anterior dislocations, followed by posterior, are the most common.^[2] They may also be classified on the basis of which ligaments are injured.^[2]

Initial management is often based on Advanced Trauma Life Support.^[5] If the joint remains dislocated reduction and splinting is indicated.^[4] Reduction can often be done with simple traction after the person has received procedural sedation.^[11] If the joint cannot be reduced in the emergency department, then emergency surgery is recommended.^[2]

In those with signs of arterial injury, immediate surgery is generally carried out.^[3] If the joint does not stay reduced external fixation may be needed.^[2] If the nerves and artery are intact the ligaments may be repaired after a few days.^[11] Multiple surgeries may be required.^[4] In just over 10% of cases an amputation of part of the leg is required.^[4]

Knee dislocations are rare: they represent about 1 in 5,000 orthopedic injuries,^[5] and about 1 knee dislocation occurs annually per 100,000 people.^[3] Males are more often affected than females, and young adults the most often.^[2]