Female patients with low systemic BMD

Sir—We read with interest the paper of Alm et al. (2009). The authors looked at post-total hip arhtroplasty periprosthetic (THA) bone changes in 39 subjects using DXA to analyse bone mineral density (BMD) in the Gruen zones surrounding the femoral prosthesis component. The purpose of the study was to look for factors such as bone mineral status that might influence femoral periprosthetic bone loss. In Figure 4 of their paper, the authors graphed the change in BMD in Gruen zone 7 at 2 years post-THA expressed as a percentage of baseline BMD at that site (y-axis) against the lowest preoperative T score from lumbar spine, proximal femur or non-dominant distal forearm (x-axis). From the significant positive correlation they obtained between these graphed variables , they concluded that " low preoperative systemic BMD was associated with higher bone loss in Gruen zone 7 ". We would like to introduce a note of caution here because we will show that it is possible to demonstrate a significant correlation between the variables as defined by Alm et al when there is absolutely no influence of systemic BMD on postop-erative BMD change at Gruen zone 7. In order to do this we need to mimic their experimental data with " mock data " for both x and y variables as per Alm's Figure 4. From inspection of that figure, the x-axis T-scores appear to have a mean of around-1.5 and a standard deviation (SD) of 1. From this information, it is possible to produce a set of 39 random numbers normally distributed with this mean and SD using, for example, the " Data Analysis " toolkit in Microsoft Excel. This gives us our x variable. The y variable is comprised of a numerator showing the BMD difference between Gruen zone 7 BMD at 2 years and that at a preoperative baseline. From Alm et al's data in Table 2, there is an average Gruen zone 7 BMD loss of-0.25 g/cm 2 over 2 years. We make the assumption that this is fixed at that value for all patients, i.e. no possibility of variation with systemic bone mineral status. The denominator of the y variable is the baseline Gruen zone 7 BMD expressed in g/cm 2. From our own data on 26 adult females scheduled for THA, we obtain a regression equation for the excellent positive correlation of baseline Gruen zone 7 BMD against baseline …