FACTORS INFLUENCING STABILITY AT THE INTERFACE BETWEEN A POROUS SURFACE AND CANCELLOUS BONE - A FINITE-ELEMENT ANALYSIS OF A CANINE IN-VIVOMICROMOTION EXPERIMENT

Several factors contribute to the success of stable bony ingrowth into the porous coated surfaces of orthopaedic implants used in hip arthro plasty. Despite having good bony apposition, bony ingrowth might not o ccur if the relative motion between bone and implant is large. Therefo re, determining the Limiting micromotion value that inhibits stable bo ny ingrowth is important. From a previous canine in vivo micromotion s tudy performed at our laboratory, this Limiting value was found to be 20 mu m. Initially, cementless orthopaedic implants are stabilized onl y by frictional forces at the bone-implant interface. Therefore, other parameters such as the coefficient of friction and the compressive fo rce normal to the interface should be considered as important factors which stabilize the interface along with micromotion. The purpose of t his analytical study was to elucidate how the stability at the bone-im plant interface is influenced by various factors, namely, motion of th e implant, the coefficient of friction, the degree of press fit, and t he modulus of the surrounding cancellous bone in determining the stabi lity of the bone-implant interface. Nonlinear and Linear finite elemen t models which simulated the immediate postsurgical condition and the end point of the canine in vivo micromotion experiment, respectively, were used to this end. From the results of the finite element models i t was possible to identify the displacement magnitude for which the im plant slipped relative to the bone as the motion of the implant was in creased incrementally. This was done for combinations of the coefficie nt of friction, press fit, and Young's modulus of cancellous bone. Thi s was used as an indicator of the limiting implant motion value beyond which bony ingrowth will be inhibited. The stress distribution in the surrounding cancellous bone bed was also obtained from the results of the finite element analyses for different press-fit conditions. The r esults of the study indicated that under slight press-fit conditions, the implant slipped relative to bone for implant motions as low as 20 mu m. For higher degrees of press fit and reasonable values for the co efficient of friction, no slip occurred for implant motions as much as 100 mu m. Although higher degrees of press fit were theoretically con ducive to better implant stability, the concomitant high stresses in t he adjacent cancellous bone will tend to compromise the integrity of t he press fit. This was also evident when the results of an analytical model with a lower degree of press fit correlated well with those of t he canine in vivo experiment in which a higher press fit was used, sug gesting a possibility of achieving a less than desired press fit durin g the process of implantation. Through this study the importance of fa ctors other than implant motion was emphasized. The results of the stu dy suggest that the limiting value of implant motion that inhibits bon e ingrowth might vary with the degree of press fit for reasonable coef ficients of friction. (C) 1997 John Wiley & Sons, Inc.