[Medium-term results of the Mayo™ short-stem hip prosthesis after avascular necrosis of the femoral head].

AIM: Short-stemmed prostheses are increasingly regarded as implants of first choice in coxarthrosis, especially in young patients. Despite promising short-term results, long-term follow-up studies are still lacking. Short-stemmed femoral implants are characterised by a metaphyseal osseointegration and strain distribution. Therefore a reduced stress shielding of the proximal femur is hypothesized and in some studies already proven. There is histological evidence that osteonecrosis (ON) of the femoral head may involve not only the intracapital region but also the femoral neck and metaphyseal area. This could lead to a higher rate of aseptic loosening of short-stemmed implants. The aim of this retrospective study was to analyze the midterm results of the Mayo™ short-stem prosthesis after ON with particular attention on osseointegration.
METHOD: From 2002-2004, in 21 patients (2 females, 19 males; mean age 45 years; mean BMI = 27) with secondary coxarthrosis after ON implantation of 26 Mayo™ Conservative Hips was performed. Postoperatively, all patients were mobilised with full weight-bearing. Using the specially developed Wristing® software, longitudinal stem migration and varus-valgus femoral stem alignment were examined digitally in anteroposterior X-rays taken immediately after surgery and in standing AP radiographs after 8.2 months and on average after 7.9 years (16 patients). The incidence of periprosthetic radiolucent lines was captured in the anteroposterior X-rays and assigned to the Gruen zones and a DEXA scan was performed. The X-rays of a matched control group with implantation of a Mayo™ short-stem prosthesis in primary coxarthrosis were analyzed by the same method. In all patients the Harris hip score (HHS) was obtained pre- and postoperatively.
RESULTS: There was no significant migration or valgus tilt of the Mayo™ prosthesis in the study and control groups during postoperative follow-up (paired t-test, p = 0.13 and 0.69, respectively). In six of 26 Mayo™-Stems 12 radiolucent lines (RL) of the Mayo™ prosthesis were observed. The control group showed at ten of 30 Mayo™ stems 17 radiolucent lines. The difference between the groups was not statistically different (chi-square test for the total number of RL: χ² = 0.001, p = 1.0 and χ² = 0.06, p = 0.79 for the number of Mayo™ stems with RL). The DEXA scan showed a slightly higher bone mineral density (BMD) in Gruen zones 3 and 5 compared with a control group: study group. In the study group the postoperative HHS was 93.5 (SD 5.6) compared to 94.2 (SD 6.9) in the control group (t-test, p = 0.63).
CONCLUSION: In the mid-term course no increased migration or tilt could be proven for Mayo™ short-stem THA in patients with osteonecrosis of the femoral head. Due to the absence of differences in the occurrence of radiolucent lines and the same results in the DEXA scan an unimpaired osseointegration of the Mayo™ stem is assumed. Therefore it is concluded that the Mayo™ Conservative Hip can be regarded as an alternative for operative treatment of ON of the femoral head. © Georg Thieme Verlag KG Stuttgart · New York.