CORRELATION OF BONY INGROWTH TO THE DISTRIBUTION OF STRESS AND STRAINPARAMETERS SURROUNDING A POROUS-COATED IMPLANT

The ability of shear strains to inhibit bony ingrowth was investigated by use of a transcortical porous-coated cylindrical plug implant in a functionally isolated turkey ulna model in which the mechanical loadi ng environment could be accurately controlled and rigorously defined. The distribution of ingrowth at the bone-implant interface was quantif ied following 8 weeks of in vivo loading consisting of 100 seconds per day of a 20 Hz sinusoidal stimulus sufficient to cause a local peak s train of approximately 100 microstrain in the cortex at the bone-impla nt interface in four turkeys. A nonuniform but repeatable pattern of b ony ingrowth, from 33 +/- 6 to 72 +/- 6% (mean +/- SE), was observed. The mechanical environment in the vicinity of the bone-implant interfa ce was calculated using a three-dimensional elastic orthotropic finite element model. The general stress-strain state of the bone as predict ed by the finite element model was validated in two additional turkeys using four three-element rosette strain gauges, while high resolution moire' interferometry was used to determine the mechanical state of t he region immediately adjacent to the implant itself. Shear strains an d stresses were evaluated at the interface and correlated to the patte rn of bony ingrowth circumscribing the implant interface. Linear regre ssions between ingrowth and both shear strain and shear stress were ne gative, with the values of R = -0.75 and R = -0.75 (p < 0.001), respec tively, indicating significant inhibition of ingrowth where shear comp onents were maximal. These results suggest that the minimization of sh ear stress and strain components is a major determinant in achieving s uccessful ingrowth of bone into a prosthesis.