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Hip: J. M. Loughead, D. Chesney, J. P. Holland, and A. W. McCaskie Comparison of offset in Birmingham hip resurfacing and hybrid total hip arthroplasty J Bone Joint Surg Br 2005; 87-B: 163-166 [Abstract] [Full text] [PDF]

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Comparison of offset in Birmingham hip resurfacing and hybrid total hip arthroplasty

Pradeep Moonot, DJ Matthews, MD Cronin   (8 March 2006)

Comparison of offset in Birmingham hip resurfacing and hybrid total hip arthroplasty 8 March 2006
Pradeep Moonot, Clinical research Fellow Epsom & St Helier NHS Trust, DJ Matthews, MD Cronin Send letter to journal: Re: Comparison of offset in Birmingham hip resurfacing and hybrid total hip arthroplasty drmoonot{at}yahoo.co.uk Pradeep Moonot, et al.	Sir, We read this article with interest. In the methods section the authors mention that the measurements on the operated hip were compared with those on the contralateral hip. They assumed that since all the measurements were taken from the same radiograph the magnification was constant. This assumption would only be correct if a ratio of the measurements was used and not the actual measured lengths. The difference between the two offsets (contralateral - operated) is altered by the magnification of the radiograph: the higher the magnification, the larger the calculated difference. As a mean of these figures is used in the formula for the paired t-test, the result can be significantly different as the t-value is equal to the difference between the group means divided by the variability of the groups.1 Hence the p values shown in Table II of the article are incorrect. The overall decrease in offset is actually less than the authors state. This decrease will be proportional to the magnification of the radiographs. Another point of confusion is found in the difference between the text and the tables in the article. In the methods section the authors state that the measurements on the operated hip were compared with those on the contralateral hip, and yet in Table II they claim to be comparing pre-operative and post-operative measurements. P.B. Moonot, Clinical Research Fellow, D.J. Matthews, M.D. Cronin, Epsom & St. Helier NHS Trust, Surrey, UK. 1. http://www.socialresearchmethods.net/kb/stat_t.htm. (Accessed on 08.03.2006)