Introduction: Changes of the proximal femur like oste-olysis, stress shielding and osteopenia are frequently observed after total hip arthroplasty (THA). Such find-ings might be considered as risk factors for aseptic loosening and later revision. Cortical thinning is observed of healthy femora too and it is questioned whether the effect of the implant can be discriminated from age-related changes.

Aim of our study was to analyze cortical bone changes in prosthetic hips with time and compare those changes with the contra lateral non operated femur.

Materials and Methods:From 1984–87 165 hips were operated with a cemented Muller straight stem. Regular clinical and radiological follow up was scheduled after 1, 2, 5, 10, 15 years. We included only patients operated for osteoarthritis without revision and complete follow-up of more than 10 years. 37 THA hips in 35 patients remained for inclusion in the study. The mean follow-up was 16±4,6 years. Thickness of cortices was measured medially and laterally in 6 locations according to the 2nd to 6th Gruen zones and mean cortical thinning was calculated. The measurements were taken on standardized anterior-posterior x-rays of the pelvis. All measurements were analyzed with Image Access 4 Software calibrated with the reference to 32 mm femoral head.

10 patients were not operated on the contralateral hip and were measured in standardized manner in the same locations as in THA femurs.

Results: All included patients had pain free hips and did not require revision surgery at the last follow-up. Mean cortical thinning was 0,17±0,15 mm/year and it was mostly expressed in mid part of the stem (Gruen 2 and 6 zones). Most thinning occurred within the first 5 years (0,32±0,34 mm/year), later thinning was slower (0.09±0,37 mm/year). For the group with non operated contralateral hip mean thickness loss in THA hip was 0,2±0,17 mm/year and there was thickness loss of the contralateral femur too (0,03±0,12 mm/year), being much less as compared to the operated side (p< 0.001).

Conclusions: Loss of cortical thickness in THA hip with the Muller straight stem is frequently observed in long term and is not associated to expression of clinical symptoms and subsequent revision surgery. The effect is pronounced in the first postoperative years, mainly being explained by stress shielding. Additionally there is cortical thinning due to ageing, being much less than the influence of the implant. Thinning of the cortical bone must not be interpreted a sign of aseptic loosening.