Updated: Oct 13, 2021
33-year-old male in a high-speed motor vehicle collision. What percent of these injuries are complicated by nerve damage? • Xray of the Week
Figure 1. X-ray of knee dislocation.
Knee dislocations, also known as tibiofemoral joint dislocations, are defined as a complete disruption of the tibiofemoral articulation [1,2]. It is a multi-ligamentous injury with most injury patterns consisting of bi-cruciate tears in combination with either medial collateral ligament (MCL) or lateral collateral ligament (LCL) injury [2-4].
Knee dislocations account for less than 0.5% of joint dislocations [1,2]. However, the true frequency of knee dislocations is unknown due to spontaneous reductions following dislocation. High energy trauma is the most common cause of knee dislocations. Motor vehicle collisions (MVC's), sports, falls, and industrial accidents are the most common reported mechanisms of injury [1-3]. MVC's account for more than 50% of the reported mechanisms of injury .
Knee dislocations can be clinically obvious if the knee is in an abnormal position and the patient has a history of acute trauma. However, in cases where the dislocation is spontaneously reduced prior to medical evaluation, the diagnosis should be assumed whenever gross instability of the knee is detected following acute high energy trauma . Advanced imaging, such as magnetic resonance imaging (MRI), can confirm the diagnosis by showing bi-cruciate ligament injury or the complete tear of three or four of the major ligaments (posterior cruciate ligament, anterior cruciate ligament, MCL, and LCL) that stabilize the knee .
Table 1. The Kennedy knee dislocation classification system [4,6].
Kennedy proposed the first classification scheme in 1963 that was based on the position of the displaced tibia in relation to the femur (Table 1.) . Although this classification system is well established and easy to use, many cases cannot be classified using Kennedy’s system because up to 50% of traumatic knee dislocations are reduced at the time of initial medical evaluation . Several alternative systems have been proposed for classifying knee dislocations [7,8], but none of the systems are ideally predictive of treatment and prognosis .
The initial treatment of a knee dislocation is immediate reduction and assessment of the patient’s neurovascular status [2,4]. Closed reduction is preferred, but posterolateral knee dislocations may require open reduction [2,4]. Next, anatomic reduction is confirmed using anteroposterior and lateral radiography, and a thorough exam is performed to look for ligamentous injury . Conservative versus surgical treatment of the ligamentous injuries is dependent on which ligaments were injured, the severity of injury to those ligaments, and the patient’s activity level . Lastly, the patient’s injured leg is placed in an immobilizer at 15 to 20 degrees of flexion .
Knee dislocations are surgical emergencies since patients can develop neurovascular compromise when diagnosis and treatment are delayed. The popliteal artery is susceptible to injury when the tibiofemoral joint disarticulates because the popliteal artery is tethered proximally at the adductor hiatus and distally behind the fibrous arch of the soleus muscle, and it has limited ability to accommodate the acute increase in distance across the popliteal fossa that occurs during dislocation [1-5]. Eighty-six percent of these injuries require amputation if vascular repair is undertaken after eight hours, and 11% require amputation if vascular repair is undertaken before eight hours [1,2,4,5]. The incidence of vascular injury ranges from 7 to 64% amongst various reports, and about one-third of knee dislocations are complicated by peroneal nerve injury [1-4].
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Boisgard S, Versier G, Descamps S, Lustig S, Trojani C, Rosset P, Saragaglia D, Neyret P. Bicruciate ligament lesions and dislocation of the knee: mechanisms and classification. Orthopedics Traumatology Surgery Research. 2009; 95:627–631. English translation of original French: https://core.ac.uk/download/pdf/82436597.pdf
Corey Brown is a medical student at Meharry Medical College in Nashville, TN. He is vice-president of his school’s radiology interest group and a member of Rad Boot Camp. Prior to medical school, he attended Queens University of Charlotte and the University of Toronto. He graduated with degrees in biochemistry and biomedical engineering. As a graduate student, Corey volunteered at Milestone Christian Ministries and worked with Maple Leaf Sports and Entertainment as a Soccer Senior Sport Lead Coach. He enjoys barbering and watching sports in his free time.
Follow Corey Brown on Twitter @coreybrwn
Kevin M. Rice, MD is the president of Global Radiology CME
Dr. Rice is a radiologist with Renaissance Imaging Medical Associates and is currently the Vice Chief of Staff at Valley Presbyterian Hospital in Los Angeles, California. Dr. Rice has made several media appearances as part of his ongoing commitment to public education. Dr. Rice's passion for state of the art radiology and teaching includes acting as a guest lecturer at UCLA. In 2015, Dr. Rice and Natalie Rice founded Global Radiology CME to provide innovative radiology education at exciting international destinations, with the world's foremost authorities in their field. In 2016, Dr. Rice was nominated and became a semifinalist for a "Minnie" Award for the Most Effective Radiology Educator.
Follow Dr. Rice on Twitter @KevinRiceMD