Impact of trochanteric heterotopic ossification on measurement of femoral bone density following cemented total hip replacement.

During a study of bone mineral density changes around cemented femoral implants, we recognized heterotopic ossification occurring regularly in a position anterior to the greater trochanter and proximal femur. The aim of this study was to describe the incidence, distribution, and effect of this ossification on periprosthetic DXA scans following primary cemented total hip replacement. One hundred eleven patients underwent postoperative DXA examinations measuring changes in bone mineral density with heterotopic ossification identified and localized on standard radiographs with confirmation using DXA subtraction imaging. Male gender and age within the male group were significantly associated with occurrence of heterotopic ossification (p = 0.003 and 0.046, respectively). Femoral stem type, weight, and body mass index had no significant effect (p = 0.525, 0.372, and 0.243, respectively). Examining the Gruen zones in all patients suggested a median (plus interquartile range) zone 1 density drop of 4% (-12% to +7%). When separated and analyzed for the effect of heterotopic ossification, the 45 patients with heterotopic ossification showed a 2-year density gain of +6% (-5% to +15%), whereas those without heterotopic ossification showed a loss of 8% (-14% to 0%), a significant difference (p < 0.001). Zone 2 also showed a significant difference (p = 0.048). We therefore recommend that affected zones should be identified and excluded from analysis at all time points. Without this precaution, researchers risk underestimating periprosthetic bone loss in their studies and reporting misleading conclusions.