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Case Report |
1 Department of Orthopedic Surgery, North Shore University Hospital–Glen
Cove, Glen Cove, NY 11542.
2 Department of Radiology, North Shore University Hospital, 825 Northern Blvd.,
Great Neck, NY 11021.
Received October 24, 2003;
accepted after revision November 12, 2003.
Address correspondence to T. T. Miller.
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The patient was referred for sonographic evaluation because a dissecting Baker's cyst was suspected as a possible cause of her symptoms. Sonography was performed with an HDI 3000 unit (ATL) using a 12-5–MHz linear transducer, but no such cyst was present. However, during the sonographic examination, the patient reported to the radiologist that her pain was posterolateral and that she could localize the exact area. Sonography of that region revealed a normal-appearing fabella (Fig. 1B).
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The patient then mentioned that her pain occurred only during certain motions such as walking up and down stairs or getting up and sitting down. She was then asked to reproduce her symptoms, and dynamic sonography during those motions showed sudden snapping of the fabella over the posterior aspect of the prosthetic lateral femoral condyle as the patient flexed and extended the knee. The image of the sudden snap corresponded to both a palpable snap felt by the radiologist through the sonographic transducer and a painful snap reported by the patient. Confirmation of fabella-snapping as the cause of her pain was obtained by a fluoroscopically guided injection of 0.5% bupivacaine anesthetic (Sensorcaine-MPF, AstraZeneca) anterior to the fabellar articular surface, which temporarily relieved her symptoms (Fig. 1C).
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The patient underwent fabellectomy under regional anesthesia, 9 months after the initial arthroplasty. The fabella could be felt by the surgeon to snap during flexion and extension. A posterolateral incision was made to excise the fabella, and direct visualization through the capsular defect confirmed that the snapping occurred over the lateral ledge of the prosthetic femoral condyle corresponding to that seen on sonography. At follow-up examination 3 months later, all the patient's symptoms had resolved, and her knee range of motion was 0–120°.
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Several pathologic conditions of the fabella have been reported, the most common of which is the "fabellar syndrome" characterized by posterolateral knee pain, usually exacerbated in extension, and local tenderness over the region of the fabella, due to chondromalacia fabellae [2]. The condition usually occurs during adolescence and is not related to previous knee surgery. Other conditions such as fracture or dislocation are rarer.
Jaffe et al. [7] were the first to report fabellar impingement after total knee replacement. In their patient, a large fabella impinged on the posterior aspect of the polyethylene portion of the tibial component with painful snapping over the prosthesis during flexion. Postoperative lateral radiographs showed notching of the inferior aspect of the fabella not found on the preoperative radiographs, and the intermittent abnormal contact of the fabella with the tibial component was documented on cineradiography. The patient's pain was relieved after excision of the fabella. Larson and Becker [8] reported a patient with posterolateral knee pain, swelling, and catching of the fabella on the femoral component after a total knee replacement. Diagnosis was made using lateral radiographs, and the symptoms resolved after fabellectomy. Larson and Becker also reported their experience that the fabella may impinge on either the tibial polyethylene component or the femoral component.
The cause of fabellar impingement after total knee replacement is probably multifactorial. It could be related to the large size (> 1 cm) of the fabella, as hypothesized by Jaffe [7] and Larson and Becker [8], or to anatomic variation in the location of the fabella in the lateral head of the gastrocnemius muscle that may affect the relationship of the fabella to the prosthetic components. Other causes of postoperative fabellar impingement include incorrect positioning of the prosthetic components, a mismatch in sizes of the prosthetic and native femoral condyles, or ligament instability. As in the native knee, fracture or dislocation of the fabella and compression of the peroneal nerve are rarer causes of posterolateral pain.
Although the previous cases of fabellar impingement and catching were diagnosed radiographically, the dynamic capability of sonography is well suited to the evaluation of the snapping fabella, allowing visualization of the abnormally moving fabella and correlation of that movement with the patient's symptoms. Thus, although a snapping fabella was not an initial clinical consideration in our case, dynamic sonography revealed it to be the cause of our patient's pain.
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