DOI: 10.1148/radiol.2442061498
(Radiology 2007;244:625-626.)
© RSNA, 2007
Cam and Pincer Impingements Rarely Occur in Isolation
Kwok-Man Tong, MD,
Tu-Sheng Lee, MD and
Yu-Min Lin, MD
Department of Orthopaedics, Taichung Veterans General Hospital, 160, Section 3, Chung-Kang Rd, Taichung, Taiwan 407, Republic of China
e-mail: ymlin{at}vghtc.gov.tw
Editor:
We read, with interest, the well-written article in the September 2006 issue of Radiology entitled "Cam and Pincer Femoroacetabular Impingement: Characteristic MR Arthrographic Findings in 50 Patients" by Dr Pfirrmann and colleagues (1). With surgical diagnosis as the reference standard, hips in 33 patients were classified as having cam femoroacetabular impingement (FAI) and hips in 17 patients were classified as having pincer FAI. To our knowledge, cam and pincer impingements rarely occur in isolation. Most impingements are a combination of these two basic mechanisms and are classified as mixed cam-pincer impingement (2–4). What intrigues us is that a combination of two mechanisms was not observed in the series by Dr Pfirrmann and colleagues. Were the patients with mixed cam-pincer impingement excluded from the current study? Are there other possible explanations for this discrepancy?
In table 4, one column was missing. Table 4 did not show the status of ossicles in patients with pincer FAI, and readers would assume these patients had no ossicles around their acetabular rims, although numbers fit. We also repeated the Wilcoxon rank sum test for tables 2–4 and found that the authors reported the asymptotic P values instead of the exact ones. With the asymptotic method, P value proofreading errors were noted in tables 2 and 3. In table 2, the P values for superior and posterosuperior positions should be .015 and .426, instead of .018 and .455 as printed, respectively. In table 3, the P value for anterosuperior position should be .236 instead of .336 as printed. With a P value of less than .05 considered to indicate a statistic difference, discrepancies between the asymptotic and exact methods of the Wilcoxon rank sum test were noted in the P values of the posterior position (.059 [asymptotic] vs .029 [exact]) in table 2 and of the posterior (.047 [asymptotic] vs .111 [exact]) and posteroinferior (.047 [asymptotic] vs .111 [exact]) positions in table 3. In our view, the exact Wilcoxon rank sum test is preferred in the authors' situation.
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References
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- Pfirrmann CW, Mengiardi B, Dora C, Kalberer F, Zanetti M, Hodler J. Cam and pincer femoroacetabular impingement: characteristic MR arthrographic findings in 50 patients. Radiology 2006;240:778–785.[Abstract/Free Full Text]
- Beck M, Kalhor M, Leunig M, Ganz R. Hip morphology influences the pattern of damage to the acetabular cartilage: femoroacetabular impingement as a cause of early osteoarthritis of the hip. J Bone Joint Surg Br 2005;87:1012–1018.[CrossRef][Medline]
- Peters CL, Erickson JA. Treatment of femoro-acetabular impingement with surgical dislocation and debridement in young adults. J Bone Joint Surg Am 2006;88:1735–1741.[Abstract/Free Full Text]
- Murphy S, Tannast M, Kim YJ, Buly R, Millis MB. Debridement of the adult hip for femoroacetabular impingement: indications and preliminary clinical results. Clin Orthop Relat Res 2004;429:178–181.[CrossRef][Medline]
Response
Christian W. A. Pfirrmann, MD
Department of Radiology, Orthopedic University Hospital Balgrist, Forchstrasse 340, Zurich CH-8008, Switzerland
e-mail: christian{at}pfirrmann.ch
Cam and pincer FAI may be present in combination. Acetabular retroversion and a nonspherical shape of the femoral head at the femoral head-neck junction and reduced depth of the femoral waist may be observed in combination. However, even in cases with combination of cam and pincer impingement, a predominant form is usually present. "Rare" is probably not the correct term and depends on the definition of the term "rare." In the United States, the National Institutes of Health defines a rare disease to have a prevalence of fewer than 200 000 affected individuals in the United States (1); the European Union uses the definition of a prevalence of less than five per 10 000 in the community (2). In this retrospective article, the final surgical diagnosis was used as the reference standard. This final surgical diagnosis reflects the surgical procedure that was performed and which deformity was surgically addressed. In some of our cases, a combination of the two impingement types may have been present; however, this has not been addressed either in the final diagnosis or in the surgical procedure.
There is no column missing in table 4 (3). There were no ossicles present in the pincer FAI group.
I am very glad to see that Dr Tong and colleagues have the time to calculate statistics on data other than their own. We have cross-checked our values. In table 3, the P value for anterosuperior position should be .236 instead of .336 as printed. All other values match the output of our statistics software. The type of test was suggested by our statistical consultant.
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References
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- Office of Rare Diseases homepage. National Institutes of Health Web site. http://rarediseases.info.nih.gov/asp/diseases/diseases.asp. Accessed November 7, 2006.
- Useful information on rare disease from an EU perspective. European Commission, Health & Consumer Protection Directorate-General Web site. http://ec.europa.eu/health/ph_information/documents/ev20040705_rd05en.pdf. Accessed November 7, 2006.
- Pfirrmann CW, Mengiardi B, Dora C, Kalberer F, Zanetti M, Hodler J. Cam and pincer femoroacetabular impingement: characteristic MR arthrographic findings in 50 patients. Radiology 2006;240:778–785.[Abstract/Free Full Text]
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