AJR 2004; 183:352-354
© American Roentgen Ray Society
Fabellar Snapping as a Cause of Knee Pain After Total Knee Replacement: Assessment Using Dynamic Sonography
Ayal Segal1,
Theodore T. Miller2 and
Eugene S. Krauss1
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.
Introduction
The fabella is subject to injury and abnormalities, such as fracture
[1], chondromalacia
[2], osteoarthritis
[3], and dislocation
[4]. We present a case of
posterolateral knee pain after total knee replacement caused by snapping of
the fabella over the prosthetic lateral femoral condyle during
flexion–extension that was diagnosed on dynamic sonography.
Case Report
A 53-year-old woman underwent primary total knee replacement for
degenerative arthritis of her left knee. The early postoperative period was
uneventful, but 8 weeks after the surgery, the patient began to complain of
pain in the posterior and distal thigh and popliteal region during physical
therapy. When the range of motion in her knee and her activity level
increased, she experienced pain at the posterolateral aspect of the popliteal
fossa, with intermittent swelling and a grinding sensation. Physical
examination revealed mild soft-tissue swelling and intermittent crepitus over
the posterolateral aspect of the knee at flexion and extension.
Anteroposterior and lateral radiographs obtained at the time showed
well-aligned components of the knee replacement and a normal-appearing fabella
that was slightly smaller than 1 cm (Fig.
1A).
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Fig. 1A. —53-year-old woman with snapping fabella after total knee
replacement. Lateral radiograph obtained after total knee arthroplasty shows
anatomic alignment of components and fabella (arrow).
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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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Fig. 1B. —53-year-old woman with snapping fabella after total knee
replacement. Longitudinal sonogram of posterolateral aspect of extended left
knee shows fabella (small straight arrow), posterior aspect of
prosthetic femoral condyle (large straight arrows), and echogenic
surface of tibial polyethelene (slanted arrows).
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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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Fig. 1C. —53-year-old woman with snapping fabella after total knee
replacement. Lateral spot radiograph obtained during fluoroscopically guided
injection of anesthetic shows needle tip along anterior surface of
fabella.
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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°.
Discussion
The fabella is a sesamoid bone in the musculotendinous junction of the
lateral head of the gastrocnemius muscle
[5]. The fabella usually
ossifies at the age of 12–15 years, is present in 10–30% of
individuals, and is bilateral in 80% of cases
[5]. The anterior surface of
the fabella is covered with hyaline cartilage and articulates with the lateral
femoral condyle when the knee is extended
[3]. Anatomically, the size of
the ossified fabella can range from a pinpoint to 2.2 cm with a mean average
size in adults of 1 cm, as observed by Pancoast
[6] in a series of knee
radiographs.
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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Introduction
Case Report
