Anatomical abnormalities of the extensor mechanism are uncommon, and those that are encountered are usually minor with little clinical significance. However, as profoundly illustrated by this case, significant anomalies can and do arise and are unlikely to conform to conventional diagnosis. Their mechanical and clinical effects can be complex, and management requires ingenuity and a sound understanding of the delicate biomechanics of the extensor mechanism of the knee.
Kasten et al. describe a number of cases of patellar tendon aplasia and their attempts at reconstruction, whereas Loizides et al. describe a case of a crossed-doubled patellar tendon incidentally found during a knee MRI scan12,14. Tyler et al. have described a number of other interesting and uncommon variants of the extensor mechanism with a focus toward diagnostic imaging13. Cases of developmental patella baja have also been described in association with a number of clinical syndromes including Osgood-Schlatter disease, Sinding-Larsen-Johansson syndrome, and patellar tendinitis3,5,6.
Only a single other case report involving an intramedullary insertion of the patellar tendon has been described. Beall et al. discuss the radiological findings of such an anomaly in an 8-year-old girl. Interestingly, many similarities can be drawn between the 2 cases, with an almost identical clinical presentation. Both patients were found to have a significant patellar baja and genu recurvatum deformity with symptoms of instability and patellofemoral pain16.
The major biomechanical effects of the anatomical anomaly are derived from the lengthened patellar tendon and its altered insertion, distal and posterior to the extension-flexion axis of the knee. We hypothesize that both these effects will increase the lever arm of the extensor mechanism and increase the excursion of the knee joint, resulting in hyperextension and subsequent genu recurvatum—however, further mechanical analysis is required (Fig. 11).
The associated patella baja will reduce the Q-angle of the extensor mechanism and subsequently alter the transmission of forces across the knee joint (Fig. 12). Increased compressive forces across the articulation will likely result in abnormal pressure on the articular cartilage, especially nearing complete extension, which in time may contribute to arthritis and wear19.
Our reconstructive approach was aimed at modifying the unusual anatomy to restore the appropriate functional biomechanics of the extensor mechanism, and in many ways mimics and achieves the same outcomes as a tibial tubercle transfer and proximalization procedure20. By relocating the patellar tendon insertion, we have restored patella height, “normal” joint biomechanics, and patellar tracking, subsequently reducing the forces transmitted across the patellofemoral articulation2,21-23.
Given its scarcity, we feel that this case is not only fascinating to those with an interest in musculoskeletal pathology, but provides further insights into the anatomical and biomechanical complexity of the extensor mechanism and the effects such anatomical variants may have. Furthermore, we hope that this case will help to provide a frame of reference, and some guidance, to a possible management approach when faced with such unusual and fascinating anomalies.
Note: The authors acknowledge Ayeesha McComb for her work in the design and generation of the illustrations for this article and Dr John Read for his assistance with radiological interpretation and reporting.
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