Clinical History: A 35 year-old man sustained a hyperextension injury to his right knee while playing football. (1a,1b) Sagittal proton-density and (1c) coronal fat-suppressed T2-weighted images from the MR examination are provided. What are the findings? What is your diagnosis?
Multi-ligamentous and tendon injury suggesting knee dislocation.
Additional fat-suppressed T2-weighted coronal images from the above case:
In the medical literature, the term “knee dislocation” is commonly used to describe a knee either with multiple injured ligaments or with multi-directional instability, findings that can result from either knee subluxation or a complete knee dislocation1. Although knee dislocations are rare, their true prevalence is unknown because of missed diagnoses or misdiagnosis. The prevalence has been reported to be as high as 0.11% in one series that reviewed over 17,000 consecutive knee MRI examinations2. Most commonly knee dislocations are associated with high-velocity trauma related to motor vehicle accidents1; however, in some reports low-velocity injuries from sports activities have been the predominate cause of knee dislocation2,3.
Evaluation of the remainder of the images in this case also demonstrated disruption of the peroneal nerve (surgically proven) and intact popliteal vessels. Given the involvement of all four major ligamentous complexes, the findings are consistent with a recent knee dislocation.
The most commonly utilized classification system to describe knee dislocations is based on the direction of displacement of the tibia relative to the femur: anterior (7a,8a), posterior, lateral, medial, or rotatory4. In most reported series, anterior dislocations are the most common followed by posterior dislocations1,4,5. Anterior knee dislocations are most frequently due to hyperextension, which first tears the posterior joint capsule and then the anterior and posterior cruciate ligaments 4,6. Posterior knee dislocations require considerably more force compared with anterior dislocations, can occur from crushing injuries to the leg, and most commonly occur with disruption of both cruciate ligaments4. Lateral and medial knee dislocations are far less common than anterior and posterior dislocations, can be seen with motor vehicle accidents4, and are invariably associated with damage to the collateral ligaments7. Rotatory knee dislocations are historically considered one of the more rare forms5 with numerous subdivisions1.
The MR findings associated with knee dislocations are often profound with high-grade injuries of multiple ligaments and other supporting structures (9a,10a). The specific pattern of ligamentous disruption is influenced but not dictated by the direction of dislocation; however, the diagnosis of a knee dislocation and certainly its direction may not be evident at the time of examination8. Therefore, the radiologist must remain alert for this entity and based on the findings suggest the diagnosis. Involvement of both cruciate ligaments and one or both of the collateral ligamentous complexes is common9. Bicruciate ligament injury alone is very strong evidence for a prior knee dislocation10. Less commonly, knee dislocations have been described with sparing of the PCL11,12 and ACL6. Based on previous descriptions of prior knee dislocations, the following principles can be outlined13, as summarized in Table 1.
Traumatic knee dislocations are devastating injuries and even cases with spontaneous or easy reduction demonstrate the same frequency and extent of complications such as vascular injury14 (11a,12a). Due to the relatively fixed position of the popliteal artery between its origin at the hiatus of the adductor magnus muscle and its termination as it passes beneath the tendinous arch of the soleus muscle, this artery is prone to injury, a complication that is seen in approximately one-third of knee dislocations4,5,15. If this vascular injury is not emergently corrected, amputation is often required with reported rates as high as 86% at 8 hours5. The peroneal nerve is also prone to injury, a complication that occurs in approximately one-third of patients (13a,14a)16. Injury to the popliteus tendon denotes a more severe mechanism of injury and may be predictive of injury to the peroneal nerve9.
Years ago, non-operative treatment with prolonged immobilization was the standard treatment for knee dislocation17. With modern surgical techniques, early repair often utilizing multi-ligamentous reconstruction and aggressive physical therapy has resulted in more favorable outcomes17. When present, vascular or neural injuries often require the most expeditious treatment, after which ligamentous injuries may be addressed. Prior to surgery MRI allows accurate characterization of the extent of injuries to supporting structures of the knee, as well as identification of meniscal tears which may require partial meniscectomy or repair. MR thus provides a valuable roadmap towards determining the appropriate surgical approach.
Knee dislocation commonly involves injuries to many of the soft-tissue stabilizing structures of the knee. Reduction may occur prior to imaging and the radiologist must remain vigilant to this diagnosis and to the potential for devastating associated complications that may occur with this entity.
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