An experimental investigation in the dog into the mode of osseous integration of total joint implants

Abstract

An uncemented scaled down version of the Mueller SLS femoral component was inserted into 30 dogs. The material (Protasul 100), geometry, and finish were identical to those designed for clinical use. Half of the prostheses inserted were fitted out with a modular collar to improve stability. Initial stability was achieved through press fit which was made possible by the special modification of the surface area and external configuration of the prosthesis. The function of the prosthesis was assessed through clinical assessment and force plate gait analysis. This was correlated with the degree of osseous integration which was evaluated on the basis of regular radiological examinations, and at post mortem by microradiography of the serially sectioned undecalcified femurs, and histological examination of selected sections under regular and ultraviolet light. The animals were sacrificed at 4, 8, and 12 months. Force plate gait analysis revealed that all animals walked with a normal gait by the 4th post-operative month. Evaluation revealed that 29/30 femoral prostheses became successfully integrated. The prostheses without the collar were less stable and most achieved stability only by incorporation of the distal tip of the prostheses in bone and the remainder in dense fibrous tissue. The prostheses with the collar were stable and achieved without fail full bony integration. The stability was provided by appositional growth of bone which came into direct contact with the implant over the proximal 1/3 and the distal 1/3 without an intervening fibrous membrane. Despite failure of full integration of the collarless prostheses clinically and by force plate analyses the two groups were indistinguishable. Microscopic examination under ultraviolet light of the flourochrome labeled sections revealed that the bone formation and remodelling were almost complete by the 4th months. The stability achieved through press fit is enhanced by the use of a modular collar which increases the success rate of osseous integration.