[Is there a bone-preserving bone remodelling in short-stem prosthesis? DEXA analysis with the Nanos total hip arthroplasty].

BACKGROUND: It has been suggested that the use of a short-stem prosthesis could conserve proximal bone by proximal load transfer. Proximal stress shielding should be reduced, a phenomenon that has been associated with bone resorption around traditional stems. Bone remodelling of a metaphyseal fixed stem (Nanos, Smith & Nephew Int.) was analysed by the dual-energy x-ray absorptiometry. PATIENTS AND
METHOD: This study included 36 patients undergoing the total hip replacement using the Nanos short stem in comparison to 36 patients operated by a traditional long-stemmed femoral stem (Alloclassic). In all cases a threaded cup was inserted. Both groups were not different in regard to the BMI or in regard to the quality of bone (BMI). The average age of the group of patients with the short-stem prosthesis was slightly younger (average 54.2 years [range: 29 to 75]) than the patient group with the long-stem prosthesis (average 61.1 years [range: 39 to 71]). A prospective clinical analysis was done by the Harris hip score (HHS) and the Sutherland score to evaluate the social quality of life. With a minimum follow-up of 12 months in all cases, radiological changes in regard to stem subsidence, periprosthetic osteolysis or linear radiolucencies were analysed. The changes of periprosthetic bone density were examined with DEXA in all patients 3 and 12 months postoperatively.
RESULTS: No patients required reoperation because of loosening or subsidence of the short-stem prosthesis. The HHS improved from a mean of 43.1 (range: 9 to 51) to 96.5 points (range: 79 to 100) in the short-stem group and to 91.3 points (range: 61 to 100) in the group of patients with long-stemmed femoral component. Radiographic follow-up revealed no evidence of component loosening or migration of the short-stem. Along the greater trochanter an osteolysis of the bone structure was found in two cases. A decrease of the proximal periprosthetic bone density (Gruen zone I, -6.4%) and in zone VII (-7.2%) were measured. An increase of the BMD in the lateral inferior region (Gruen zone II, +9.7%) superior to the polished tip of the short stem was observed over a period of one year after implantation. At the polished tip of the prosthesis a significant change of bone density in zone III (+1.03%) and in zone V (+0.7%) could not be observed.
CONCLUSION: The desired proximal load transfer of a short-stemmed implant in the metaphyseal region of the proximal femur could not be reached with this device. On the basis of the excellent clinical results of the patients operated with the Nanos short-stem prosthesis we conclude that the component induces bone ingrowth in the lateral/distal region of the proximal femur. Georg Thieme Verlag KG Stuttgart, New York.