Knee Dislocations

 

Written by: Rozalyn Hesse, MD; Edited by: Timothy Khowong, MD


Case and Introduction

You are working at an ER in rural Vermont. A 35-year-old female is brought in by EMS from a nearby ski-resort for knee pain after falling. She is endorsing severe right knee pain. On exam she has an isolated knee injury with gross deformity. You can palpate distal pulses, but they appear slightly decreased compared to the uninjured side. As you prepare to reduce the injury you also obtain a stat XR which demonstrates the following injury…

Knee Dislocations are limb threatening orthopedic injuries. They have a high association with significant vascular injury and can lead to ischemia and loss of limb if not swiftly treated. They are typically a result of high mechanism injuries such as MVAs or a sports accident. 

 

Anatomy of knee

The popliteal artery runs posterior to the knee joint and gives multiple collaterals that feed around the knee. The knee joint itself is stabilized by four major ligaments: the anterior cruciate, posterior cruciate, medial collateral, and lateral collateral ligaments. Knee dislocations occur when all or a majority of these ligaments are torn, allowing free and unstable movement of the tibia and femur in relation to each other. 


History and Physical

Obtain a thorough history and physical for these patients. An active knee dislocation has obvious physical features of gross deformities, however some patients actually have spontaneous relocation of the knee before the arrival to the Emergency Department, therefore have a high index of suspicion for patients reporting a high mechanism injury with severe knee pain and description of an abnormal tibial location. On exam the knee joint would have a high degree of laxity. Utilize the Lever or Lachman tests and varus and valgus stress to evaluate ligament stability. The joint should demonstrate hyperextension when testing ROM. Evaluate for other signs of knee injury such as swelling, effusion, and ecchymosis. 

Palpate the popliteal artery and distal pulses including the dorsalis pedis and posterior tibialis. The presence of distal pulses does not rule out vascular injury. Take note if the limb is demonstrating any of the hard signs of vascular injury: Absence of pulse, pale/dusky leg, paresthesias/paralysis, rapidly expanding hematoma, pulsatile bleed, or bruit. 

Additionally, one should evaluate for possible peroneal nerve injury. The common peroneal nerve courses around the fibular head and provides sensation to the lateral lower leg and dorsal aspect of the foot. Injuries to the nerve may present as paresthesias and foot drop. 

 

Classifications

There are 5 types of knee dislocations. They are classified based on the direction of tibia displacement:

  • Anterior is the most common and is due to hyperextension injuries. Arterial injuries occur due to traction, resulting in intimal tears. There are a high degree of peroneal nerve injuries with this pattern. 

  • Posterior is the second most common and is due to axial load on a flexed knee, like what occurs in a dashboard injury. These have the highest rate of vascular injury and can cause complete tear of popliteal artery.

  • Lateral and Medial occur from a varus or valgus force or twisting injury such as in sports.

  • Rotational is rare and is irreducible. 

 

Evaluation and Management

Knee dislocations are orthopedic injuries and need to be treated promptly. Irreparable limb ischemia can occur within 6-8 hours time. XRs are not necessary as the first step of evaluation, especially if the knee is obviously dislocated and there is neurovascular compromise. The first step in this case should be immediate reduction, which will be discussed in the next section. In addition to checking for pulses or pulse deficit, vascular compromise should also be evaluated using an Ankle Brachial Index or (ABI). This is the measurement in ratio of lower extremity perfusion and upper extremity perfusion. A value of less than 0.9 is concerning for vascular compromise. Vascular surgery should be consulted and a CTA angiogram of extremity should be obtained for further evaluation. 

Absent pulses or hard signs of vascular compromise requires a stat vascular surgery consult for emergent surgical exploration. Imaging such as CTA can delay treatment.

 

Reduction Technique

The first step in reduction is to give longitudinal traction to the leg by pulling below the knee. Many times this alone is successful in reducing the joint. In an anterior dislocation you will want to simultaneously lift the distal femur. Conversely in a posterior dislocation you will lift the proximal tibia. After reduction you must recheck pulses and neurological function. Place the leg in a long posterior splint with the leg in slight flexion at 15 degrees or in a knee immobilizer.

 

Summary

  • Limb threatening injury

  • Often result of MVAs and sports injuries

  • Classification is based on relation of tibia with distal femur 

  • Evaluation of pulses/perfusion and hard signs of ischemia is paramount, any positive findings requires stat vascular surgery consult

  • If knee dislocation is obvious on evaluation, proceed straight to reduction, remembering to recheck pulses and place in long leg splint 












Reterences

Frank, R., Suk, M., Lane, C., Team, O., Patel, S., & Stuart, M. (n.d.). Knee dislocation. Orthobullets. Retrieved November 30, 2022, from https://www.orthobullets.com/trauma/1043/knee-dislocation

Mohseni, M., & Simon, L. V. (2018, July 18). Knee Dislocation. National Library of Medicine. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK470595/

Singh, M. (2018, September 3). Knee dislocation: Pearls and pitfalls. emDOCs.net - Emergency Medicine Education. Retrieved November 30, 2022, from http://www.emdocs.net/knee-dislocation-pearls-and-pitfalls/

Wu, G. (n.d.). True knee + patellar dislocations. Core EM. Retrieved November 30, 2022, from https://coreem.net/core/true-knee-patellar-dislocations/ 












 
Booth EM