Stress analysis of threaded cups

Abstract

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...

Fixation of trapezial implants in a trapeziometacarpal total joint prosthesis tested in a model of porcine bone

“…Klanke et al [14] used saw bone to test the primary fixation of different designs of acetabular screw implants. Another method of analysis is FEA, where computer simulation calculates stress zones and predicts migration of implants [15,16]. Both methods may be used for uncemented trapezial implants, but are not suitable for cemented implants.…”

Long-term results of the threaded Weill cup in primary total hip arthroplasty

“…Various designs of threaded uncemented cups have been introduced, achieving initial stability by interference of the threads with acetabular bone. A high primary stability which is a prerequisite factor for bone ongrowth and long-term fixation has been described for threaded acetabular components [13,20,22]. The surface treatment of these implants was smooth in most cases.…”

Long-term results of the threaded Mecron cup in primary total hip arthroplasty

“…Uncemented acetabular components were designed to overcome the problems of aseptic loosening, one of the major problems in cemented total hip arthroplasty, especially in young patients, despite improved techniques for the application of cement. Threaded components have been advocated because of their high primary stability through interference of the threads with acetabular bone, which is a prerequisite factor for bone ongrowth and long-term fixation [11,16,18]. Threaded designs lead to rigid primary fixation, but long-term secondary fixation is less liable to occur, as surface treatment of first generation threaded hip cups was commonly smooth.…”