Introduction: Immediate postoperative stability of cementless hip stems is one of the key factors for the long-term success of total hip replacement. The ability to discriminate between stable and unstable stems in the laboratory constitutes a desirable tool for the industry, as it would allow the identification of unsuitable stem designs prior to clinical trials. The use of composite femora for stability investigations is wide spread [1,2] even though their use in this application is yet to be validated. This study is aimed at establishing whether Sawbones composite femora are suitable for the assessment of migration and micromotion of a cementless hip stem. The stability of two SL Plus stems (Precision Implants, CH) implanted into Sawbone was compared to that of two SL Plus stems implanted into cadaveric femora. Ethical approval was obtained for the harvest and use of cadaveric material.

Methods: Stability was assessed in terms of micromotion and migration. Micromotion was defined as the recoverable movement of the implant relative to the bone under cyclic loading. Migration was defined as the non-recoverable movement of the implant with respect to the surrounding bone. Movement of the implant with respect to the surrounding bone was monitored at two locations on the lateral side of the stem by means of two custom made transducers based on the concept described by Berzins et al [3]. Each femur was tested in two different sinusoidal loading configurations: single leg stance (SLS-11° of adduction and 7° of flexion) [4] loaded up to 400N and stair climbing (SC-11° of adduction and 32° of flexion) loaded up to 300N. The effect of the abductor muscles was included in the model [5]. Each test consisted of 200 loading cycles applied at 50 Hz. The captured data was post-processed by a MATLAB routine and converted into translations and rotations of the stem with respect to the bone.

Results: The proximal part of the implant was subject to the highest amplitudes of micromotion in both loading configurations independent of the host. During SLS the largest micromotion was measured in the direction of the axis of the femur, this amplitude was in the order of 20 μm for the stems implanted in sawbones and varied between 13 and 39 μm for the stems implanted in cadaveric femora. The migration of the implants was minimal both in SLS and SC for both hosts with values measured in the sawbones model nearly on order of magnitude smaller than the cadaveric. In the case of SLS the prevalent movement consisted of a translation along the axis of the bone, while during SC the rotations became prevalent.

Discussion: This study has demonstrated that Sawbones provide an effective model to establish micromotion with oscillation patterns and orders of magnitiude similar to cadaveric bone. However the migration is much more dependent on the quality of fit and the internal geometry of the femur and therefore more caution should be placed on interpreting migration data from Sawbones models.

Aims: To evaluate the results of a novel modular press-þt acetabular cup in primary total hip arthroplasty (THA) for osteoarthrosis (OA) and inßammatory arthritis (IA). Methods: From February 1996 to June 1999 in 324 patients (249 women, 75 men) 355 THA using a novel cup has been carried out. The shell is non-hemispherical on cross-section and has a hydroxyapatite coating on porous titanium for osseointegration. Diagnosis was: osteoarthrosis (OA) 236, developmental dysplasia (DDH) 21, post-traumatic arthrosis 27, avascular necrosis 6, IA 65 Ð mainly rheumatoid arthritis (RA). Average age at operation was 65.8 years. The patients were studied prospectively using Harris Hip Score (HHS), by measuring any radiolucency around the cup and by looking for signs of migration. Results: Median follow-up was 4.5 years. At follow-up, 21 patients had deceased. Revision for deep infection was carried out in 5 hips (3 low-grade infections). No septic loosening occurred with low-grade infection. Recurrent dislocations required revision of 1 cup and exchange of 2 inserts. Only 1 cup in a RA patient with severe superior bone loss became unstable after a fall 4 months postoperatively. Survival with aseptic loosening of the cup as endpoint was 1 in OA and 0.98 in IA. In all 290 patients with 318 THA in follow-up the cup was functioning well, both clinically and radiographically. HHS increased from 44.5 to 90.3. Conclusion: Press-þt þxation using a modern acetabular component is an excellent treatment option in primary total hip arthroplasty for all diagnostic groups.