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MRI Web Clinic - July 2004
Lateral Patellar Dislocation
by Mark H. Awh, M.D.
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Clinical History: A 23 year-old female presents with medial knee pain following a twisting injury. (A) A single fat-suppressed T2-weighted coronal image is provided. What is the diagnosis?
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A
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Answer
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Lateral Patellar Dislocation
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Discussion
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Although you may feel that being asked to make the specific diagnosis of lateral patellar dislocation from a single image is unrealistic, the coronal view provided in fact reveals a classic and highly characteristic appearance, allowing the diagnosis to be made with confidence.
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A
In this case, it is the bone bruise within the anterolateral aspect of the lateral femoral condyle (arrow) that is the key to the diagnosis.
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The marrow edema pattern above is virtually pathognomonic of a lateral patellar dislocation, as little else, with the exception of a direct blow to the knee at this site, will cause such an appearance. Given the lack of history of direct trauma, a reliable diagnosis of lateral patellar dislocation is thus obtained.
The pattern of bone bruising seen with a lateral patellar dislocation is easy to understand if one considers the mechanical event that occurs with this injury. The injury typically occurs from a twisting event with the knee in flexion, with the patella undergoing a transient, violent lateral displacement in which the medial patella impacts against the anterolateral aspect of the lateral femoral condyle (Figure B), therefore causing the characteristic bone bruise within the anterolateral aspect of the lateral femoral condyle and a frequent osteocartilaginous injury of the inferomedial patella.
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B
An axial representation of a lateral patellar dislocation illustrates the dramatic transient lateral shift of the patella that occurs as the medial patella impacts upon the anterolateral aspect of the lateral femoral condyle.
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Because of the transient and brief nature of lateral patellar dislocations, the diagnosis is frequently unrecognized by both patients and clinicians.1 Pain is often described as medial because of soft tissue injuries that occur to the medial retinaculum and/or the medial patellofemoral ligament (MPFL). Evaluation of both of these soft tissue structures is critical when one examines an MRI following lateral patellar dislocation, as the extent of soft-tissue injury influences the use of operative repair. On MR images, as seen in (B), medial retinacular injuries are frequently identified by the presence of edema, soft tissue thickening, and or discontinuity at the patellar attachment.
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C
An axial proton density-weighted image with fat suppression reveals complete disruption of the medial retinaculum at its patellar attachment (arrow) in another patient who recently suffered a lateral patellar dislocation.
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The MPFL, considered by some authors the most important restraint to lateral patellar dislocation, extends from the mid to upper pole of the patella to its attachment near the adductor tubercle, along the medial aspect of the distal femoral metaphysis. The MPFL is almost always injured with lateral patellar dislocations,2 and when disrupted, usually tears at the femoral attachment (D).
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D
A proton density-weighted axial image with fat-suppression obtained in a patient status-post lateral patellar dislocation reveals disruption of the MPFL (arrow) at its attachment near the adductor tubercle of the distal femur.
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In addition to identifying the previous lateral patellar dislocation and the associated retinacular or MPFL injuries, a number of other important findings should be searched for in affected patients. Patellofemoral anatomy should be scrutinized, as certain configurations, such as a congenitally shallow trochlear groove (E),3 significantly increase a patient's likelihood for recurrent dislocations.
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E
The femoral trochlear groove has a shallow and in fact almost convex contour (arrow) in this patient who recently sustained a lateral patellar dislocation.
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Cartilage injuries, particularly at the inferomedial patella, are common following lateral patellar dislocation, and should be carefully searched for. In patients with large defects, the search for the associated loose body must also be performed (F,G).
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F
A large cartilage defect (arrow) is readily apparent in this patient following lateral patellar dislocation. A congenitally shallow trochlear groove (arrowhead) is also noted.
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G
The large displaced cartilage fragment (arrow) is identified within the suprapatellar bursa on a higher axial slice.
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Conclusion
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Lateral patellar dislocation is a common clinical entity with a characteristic MR appearance. Because the diagnosis of lateral patellar dislocation is often unsuspected, MR provides valuable diagnostic information in such cases. Additionally, MR's ability to delineate the extent of injury and predisposing factors is important in patient care and surgical planning.
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References
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1 Lance E, Deutsch AL, Mink JH. Prior lateral patellar dislocation: MR imaging findings. Radiology 1993; 189:905-907.
2 Sanders TG, Morrison WB, Singleton BA, Miller MD, Cornum KG. Medial patellofemoral ligament injury following acute transient dislocation of the patella: MR findings with surgical correlation in 14 patients. J Comput Assist Tomogr 2001; 25:957-962.
3 Carrillon Y, Abidi H, Dejour D, et al. Patellar instability: Assessment on MR images by measuring the lateral trochlear inclination-initial experience. Radiology 2000; 216: 582-585.
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