Over a period of eight years, we implanted a total of 76 cementless hip prostheses in patients with rheumatoid arthritis. The clinical results of 47 patients (70 hips) increased from a mean Harris Hip Score of 33 to 85 after an average of 49 months (range 1-11 years). One threaded cup has had to be revised because of loosening, and one stem because of femoral fracture. At the latest follow-up, 88% of Hofer-Imhof threaded cups had complete bone ingrowth (Type 0); 10% had near-complete bone ingrowth with minimal radiolucency in one third of the bone contact area (Type 1), and 2% had radiolucency in two thirds of the bone contact area (Type 2). Hemispherical push-in cups showed significantly more radiolucency around the cup. For the stems (Uni, Zweymüller SL), 83% showed no radiolucency (Type 0); 17% had radiolucency only very proximally (Type 1). Minor remodelling (Type 1) occurred in 60% of the femoral shafts; 30% had moderate femoral density loss (Type 2), and 10% had severe bone loss and cortical thinning (Type 3). There was no correlation between marked shaft atrophy and clinical symptoms. With regard to radiolucency and remodelling, there was no significant difference between the two types of stem used. Cementless hip arthroplasty using titanium implants has an excellent outcome in the medium term.

Navigation of the acetabular cup in total hip replacement (THR) is used to improve the reproducibility of acetabular component positioning. When the palpation of anatomic landmarks, which is necessary to determine the pelvic coordinate system, is performed epicutaneously, the question as to how uneven soft tissue distribution can influence navigation accuracy arises. To obtain data, the questionable soft tissue thickness was measured in 72 patients scheduled for THR. In addition, distances between the landmarks were recorded. On the basis of this information, we were able to calculate the expected misinterpretation of the anteversion given by a navigation system for each patient. The calculations suggest that a navigation system would have underestimated the anteversion on average by 2.8 degrees +/- 1.8 degrees. The median of anteversion misinterpretation was 2.4 degrees and its 95% confidence interval was calculated to be 2.2 degrees -3.0 degrees. No correlation with substantial significance between anteversion misinterpretation and the patients' biometrical data could be found. According to the current knowledge, acetabular cups in THR should be positioned within a range of 30 degrees -50 degrees of inclination and 10 degrees -30 degrees of anteversion. In comparison with these permitted +/- 10 degrees windows, the amount of misinterpretation that was found due to uneven soft tissue distribution seems to be acceptable.

Permanent secondary stability of the uncemented femoral stem of hip prostheses can only be achieved once primary rotational and axial stability has been ensured. Rotational stability means that the stem is resistant to the articular forces that induce rotation around the implant's longitudinal axis. The 10-year survival rates of two uncemented conical shaped stems with decisively different proximal shapes were significantly different (Schenker SK 63.9%, Zweymüller SL 97.0%). The shapes were analyzed biomechanically by testing rotational stability in a silicone model. A reduction in the mediolateral height of the metaphyseal part by one-third led to a 20% smaller angle of ascent (P < 0.01), which indicated lower resistance. The proximally broader (mediolateral) prosthesis therefore proved to have better rotational stability. The study confirms that aseptic stem loosening is attributable to the design of the proximal part of the prosthesis.

We have reviewed 142 Miller-Galante I (MG I) total knee arthroplasties (TKAs) with a follow-up of 56 months, and compared these with the outcome of 219 Miller-Galante II (MG II) TKAs with a follow-up of 36 months. In the MG II TKAs we found markedly lower revision rates, higher postoperative Hospital for Special Surgery (HSS) scores, less retropatellar pain and better patellar centring without patellar resurfacing. The higher revision rate in MG I TKAs was mainly due to the need to revise the metal-backed patellae.

Using finite element analysis we have studied the pelvic bony socket and compared it with radiological imaging using threaded acetabular cups of three different shapes (parabolic, conical, hemispherical). The twodimensional model depicted a planar section through a left pelvic hemisphere. In all three cups the stress in the bony socket increased from lateral towards medial. Compressive stress was found on the superior and inferior parts of the cup, but mainly on the superior aspect, seen radiologically as new trabecular bone formation. The maximum compressive stresses were seen in the cranial curvature of the conical cup, with less in the parabolic form and least in the hemispheric form. The tensile stress at the bottom of the socket increased from the hemispheric to the conical shape. Radiological rarefaction gave an indication of lower stress. There was lower compressive stress between the teeth of the threads. This FE model uses computer simulation to predict bony changes with different designs of implant. The ability to simulate biological conditions is a valuable addition to the testing of mechanical strength.Résumé Par éléments finis nous avons étudié le cotyle osseux et l'avons comparé avec les images radiologiques faites avec des cupules vissées de trois formes différen-tes (parabolique, conique, hémisphérique). Le modèle à deux dimensions a représenté une section plane à travers une hémisphére pelvienne gauche. Pour les trois cupules la contrainte dans l'os augmente de dehors en dedans réalisant un toit osseux en pince. La contrainte de compression existe sur les parties supérieure et inférieure de la cupule, mais d'une manière prédominante sur le côté supérieur, radiologiquement représentée par une nouvelle formation osseuse trabeculaire. Les contraintes de compression maximales étaient présentes dans la courbure polaire crâniale des cupules coniques, avec une tendance décroissante de la forme parabolique à la forme hémisphérique. Des contraintes en tension ont été démontrées au fond du cotyle osseux, en augmentant de la cupule hémisphérique à la cupule conique. La raréfac-tion osseuse radiologiquement visible a représenté la contrainte calculée et moindre de contrainte de compression entre les filets. Ce modèle utilise la simulation informatique pour prédire les changements de l'os selon les différents dessins des implants. La possibilité de simuler des conditions biologiques complète utilement les essais de résistance mécanique. IntroductionUncemented hip arthroplasty using threaded cups [3,4,8] is a procedure with good medium and long-term results that can be equivalent to those with cemented techniques. Uncemented cup implantation is carried out with a press-fit technique. As soon as the cup is implanted, "pre-stress conditions" are produced [2], and this will occur both when a threaded cup is screwed in and when a slightly oversized unthreaded hemispherical cup is implanted. With both techniques, preparation of the bony socket causes loss of supportive bone, which has to be provided biolo...