The bone/cement interface in total hip arthroplasty: An in vitro study of bone surface preparation, fatigue and interface mechanics

Loosening of the cement/bone interface is a primary cause of failure in total hip arthroplasty (THA), i.e., total hip replacement (THR). In this investigation the influence of the bone surface topography on fatigue of the cement/bone interface was studied using a hybrid approach incorporating both experimental and numerical methods. New parameters were also developed for characterizing the cement/bone interface including the volume of cement interdigitation (Vi), total volume of interdigitation ( Vi,total) and cement to bone volume ratio (VR c/b). In the experimental program, model interface specimens were developed with unique bone surface preparation. The specimens were subjected to cyclic shear loads, which were scaled from the shear strength identified from monotonic tests. The fatigue response was quantified in terms of displacement between the prosthesis and bone with use of Digital Image Correlation (DIC) and the loosening response was characterized in terms of three regions comprised of an initial migration (Region I), steady state loosening (Region II) and unstable loosening to failure (Region III). Region I was characterized by the Initial Migration Amplitude (IMA) whereas Region II was described using a power law that was defined in terms of a loosening exponent (b) and coefficient (A). The fatigue response was further evaluated using finite element models of the interface specimens where the numerical response was benchmarked with results of experiments.;Experimental results showed that the ultimate shear strength of the cement/bone interface increased with average surface roughness (Ra), Vi as well as surface profile slope (Delta q). The fatigue response of the cement/bone interface was also significantly influenced by the bone topography. The apparent fatigue strength of the cement/bone interfaces ranged from 0.8 to 4.35 MPa and the ratio of the apparent fatigue strength to ultimate shear strength was approximately 0.2. Regardless of the bone surface topography, the fatigue strength of the cement/bone interface can be estimated from the results of monotonic experiments.;Evaluation of the fatigue responses in terms of loosening showed that while both IMA and b were a function of surface topography, they were affected differently. The IMA was influenced most by R a, Vi and Deltaq and decreased with an increase in each of these three parameters. The b was influenced most by surface asymmetry and a strong correlation was found with VRc/b; the rate of steady state loosening decreased with decreasing VRc/b. Properties of the optimal surface topography resulting from surgical preparation that are required to minimize the evolution of micromotion with fatigue loading were identified as a Gaussian surface, large height amplitude resulting in high Ra, Vi and Vi,total, relative high Deltaq as well as low VRc/b near 1. Results of this investigation provide the foundation of knowledge for guiding surgical preparation of bone and the development of new surgical tools for maximizing the success of cemented total joint replacements.