To The Editor:

I read with interest the article “ The prevalence of Acetabular retroversion among various disorders of the hip”(1) in which the authors conclude, “retroversion in DDH is more common than previously thought”.

First, I would like to emphasize that frontal plane analysis of the hip as it is used by all traditional methods of analysis and classification for hip dysphasia is often inadequate even when supplemented with special oblique views. In addition to plain films, a CT scan provides the transverse plane anatomy of the acetabulum. I routinely evaluate hips with developmental dysplasia or dislocation in patients who are candidates for THA with plain AP x-rays and three dimensional CT of the pelvis to evaluate retro or excessive anteversion of the acetabulum.

In my experience and in most of the cases found in the literature, retroversion of the acetabulum was present after pelvic osteotomy in younger patients or after conservative treatment with plaster or traction. Mechanical studies and clinical experience have demonstrated that with the performance of a pelvic osteotomy (Salter, Chiari, triple or Bernese) anteroloteral coverage is gained at the expense of posterior coverage. Interestingly, hips, which had undergone additional femoral derotational osteotomies, were associated with significantly more pronounced acetabular retroversion. I suppose that in most cases true retroversion of the acetabulum in DDH is iatrogenic. That is probably the reason why in type III hips (high dislocation) in which patients were usually left untreated retroversion of the acetabulum is absent(2).

Second, the retroversion of the acetabulum in Hips with positive “cross – over” sigh is limited to the upper ¼ of the acetabulum the rest remaining within normal range of anteversion. There are also cases with an osteophyte covering the upper anterior part of the femoral head as an extension of the anterior wall that can give the false impression of retroversion. If the osteophyte is ignored the remaining anteversion is usually within normal range.

I consider true retroversion the one that covers at least ¾ of the acetabulum and is due to posterior wall insufficiency or retroversion of the hemipelvis and not because of the extended anterior osteophyte. Meeting this criteria, retroversion was found mostly in cases with previous pelvic osteotomy.

Furthermore, the anterior overcoverage of the femoral head with an osteophyte and the subsequent “retroversion” is possibly the result of osteoarthritis and not the cause of the disease.

The author(s) of this letter to the editor did not receive payment or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the author(s) are affiliated or associated.

References:

1. Ezoe M, Naito M, Inoue T. The prevalence of acetabular Retroversion among various diseases of the hip. J Bone Joint Surg. Am. 2006; 88:372-9.

2. Vlamis J.A. Three dimensional classification of D.D.H in Adults. SICOT, SIROT third annual international conference, 2004:285

EDITOR'S NOTE, 20 April 2006: The corresponding author was invited to respond to this letter, but to date, has not done so.

To The Editor:

With great interest we read Dr. Ezoe, et al, article on the prevalence of acetabular retroversion among various disorders of the hip.

The key point in measuring acetabular anteversion is its differentiation from inclination. In their study on the effect of pelvic tilt on acetabular retroversion, Siebenrock, et al, (1) have shown a great variability of the presence or absence of retroversion signs on 86 radiographs within normal distribution of a healthy population. Within the range of measured distances between the symphysis and the sacrococcygeal joint in these x-rays, positive and negative retroversion signs in all tested acetabulums of four cadaver pelves were simulated. There was a linear correlation between this distance and the pelvic inclination angle.

You included patients in your study whose distance was within the interquartile range of the measurements by Siebenrock, et al, (1) presuming the wide variation of presence of retroversion signs is not present within this 15mm range (corresponding to a range of the pelvic tilt of ~8° for men and ~3° for women; see Siebenrock, et al, - Fig 4). (1) This assumption is based on measurements of just two pelves for each gender, far below a statistical significant number. In addition to the pelvic tilt position, Siebenrock, et al, (1) have shown a 6°rotation around a vertical axis, corresponding to a 16mm deviation of the middle of the sacrococcygeal joint from the midline through the symphysis, leading to appearance of a cross-over and posterior wall sign in the ipsilateral acetabulum of all four pelves from cadavers. Therefore, the question whether you reported on pelvic tilt rather than acetabular version of your subgroups still remains open. Likewise, the projection of the obturator foramen in your Figures 1 and 2, presenting an anteverted (Figure 1) and a retroverted acetabulum (Figure 2), differs significantly.

We therefore strongly recommend comparing mean values of measured distances between 1) the symphysis and the sacrococcygeal joint and 2) the middle of the sacrococcygeal joint and the midline through the symphysis statistically with a Student t-Test or Mann-Whitney Test respectively in order to present that comparison of your subgroups was valid.

Reference:

1. Effect of pelvic tilt on acetabular retroversion: a study of pelves from cadavers. Siebenrock KA, Kalbermatten DF, Ganz R. Clin Orthop 2003; 407: 241-248.