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La prothèse P.R.A.

The PRA cementless hip prosthesis. Preliminary results

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Résumé

Les échecs des implants fémoraux relèvent en très grande majorité de causes mécaniques. La conception de l'implant fémoral PRA exploite l'analyse des échecs observés et publiés, les études biomécaniques, les données histologiques pour satisfaire aux trois impératifs de la stabilité: Stabilité Primaire liée à la forme et aux reliefs de l'implant, Stabilité Secondaire liée à l'état et à l'étendue de sa surface, Stabilité Définitive, garantie par le transfert harmonieux des contraintes entre la tige prothétique et l'os. Implant en alliage de titane (Ti6Al4V), réalisant un bon remplissage et une surface développée maximale de contact os-implant dans la région métaphysaire, la tige PRA est progressivement réduite dans sa portion diaphysaire. Le revêtement de la portion métaphysaire la plus significative sur le plan mécanique par l'hydroxyapatite de calcium entraîne sa fixation rapide par adaptation osseuse sans interposition fibreuse, et le développement précoce d'un pontage trabéculaire osseux entre l'implant et la corticale. La qualité des résultats cliniques et radiologiques obtenus au cours des trois premières années d'implantation confirme le bien-fondé des principes biomécaniques et biologiques retenus dans sa conception.

Summary

Careful analysis of total hip arthroplasty failures including data from biomechanical studies of normal and prosthetic hips, and histological investigation, leads to the conclusion that femoral stem failure is mainly due to inadequate transfer of load from the implant to cortical bone. The long term success of femoral stem implantation requires stable fixation. Primary stability relies on the shape and contour of the implant, - secondary stability relies on the area and type of the surface allowing bony ingrowth into the prosthesis without fibrous interposition and allowing the development of early bony trabecular bridging between implant and cortex. Long term stability is related to the mechanical properties of the metal and shape of the implant allowing optimal transfer of loads between bone and implant. The PRA femoral stem fulfils these requirements: its large proximal part fills the upper femur, avoiding distal migration by its shape and providing excellent resistance to rotational and torsional forces due to its trapezoidal cross-section. Longitudinal grooves and Hydroxyapatite coating of this large surface area allows rapid bony ingrowth. The progressive tapering of the distal part which is not coated, decreases its mechanical influence, so that most of the load transfer occurs at the proximal half of the stem. The excellent clinical and radiological results of the 76 implants performed between October 1988 and January 1992 support the theoretical advantages.

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Petit, R. La prothèse P.R.A.. Orthop Traumatol 2, 239–246 (1992). https://doi.org/10.1007/BF01742439

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