Author's reply.

Sir,

We thank the authors1 for the interest shown in our paper.2 We would like to address the issues raised.

No selection criteria were used as far as choice of head diameter is considered in this study. Patients were randomized to receive either 36 or 28 mm head diameters

As this study was not an age or sex matched controlled study and the research objective was to assess the role of use of 36 mm head diameter in reducing the rate of dislocation in total hip replacement (THR), there were some demographic differences in the two groups as far as female sex distribution is concerned. However, if we look at the difference in number of total dislocations of male and female, it was 7 versus 4 (M: F) and the number of total dislocations itself was small (11 out of 317 hips) in this series, we were not in a position to comment on role of female sex as a risk factor to the increased rate of dislocation.

Even if we look at the literature, no consensus on the contribution of sex as an independent risk factor has been reached3

We do agree with authors that component malposition itself is an independent risk factor for dislocation.45 A “safe zone” had been defined previously as 45°±10° for inclination and 15°±10° for anteversion.6 In a study with a larger population without standardized measurement, almost 50% of dislocated in total hip arthroplasty (THA) had high anteversion or inclination,7 but other studies could not confirm these results.8 These inconsistencies may be caused by different and imprecise radiographic measurement methods.

In the control match study, matched with the dislocated patients for age, preoperative diagnosis, operative approach, and surgical site by Leichtle et al.,3 most acetabular cups were within the “safe zone”, without any significant difference between patients with or without dislocation. Therefore, no acetabular cup position gave full protection against dislocation. Although a well positioned acetabular cup does not guarantee a stable THA, cup position is a key issue in dislocation. This suggests that there may be a wide range of acceptable implant positions and that multiple causes may contribute to dislocation.

In our own study,1 we looked for acetabular component malposition only in the cases where dislocation occurred, that is, 1 in group A and 10 in group B. We suggest that this number may be higher in group A but may be with use of 36 mm head diameter, the dislocation rate was less in this group

A recent article by Malkani et al.9 (2010) did multivariate cox regression analysis and found that decrease in rate of dislocation with increase in use of head size of 32 mm or more. In another recent study by Dudda et al.10 (2010) using multivariate regression model found that ‘Larger head sizes were associated with significantly lower risk of dislocation (OR = 0.84, P = 0.02)’. Another interesting finding in their study was “compared to ideal cup positioning, hips with acceptable (OR = 1.4, P = 0.69) and malpositioned (OR = 1.49, P = 0.34) cup positioning were not at significantly higher risk of dislocation”.