Early Cementing Does Not Increase Toughness of Grit Blasted Interfaces

Introduction: Grit blasted stem-cement interfaces have been advocated as a means of increasing the bond strength of the stem-cement interface in cemented total hip arthroplasty. Recent work has suggested that early cementing of rough stem surfaces can increase the bond strength of the interface when compared to the same interface cemented using very doughy cement [1]. Based on these findings, suggestions have been made to encourage early cementing of stems with a rough or textured surface. However, these studies were performed using ideal test specimen geometries and testing was performed in air at room temperature. Tests of stem-cement interfaces have shown that exposure to a physiological environment at 37 degC can substantially reduce the fracture toughness of the stem-cement interface when compared to testing dry at room temperature [2]. The purpose of this study was to determine if the fracture toughness of grit blasted stem-cement interfaces are affected by the time of cementation using a realistic model of the cemented stem construct. We hypothesized that the fracture toughness of constructs cemented early would be stronger than constructs cemented late. Methods: A new stem/mould construct was developed to achieve two main goals: 1) provide boundary conditions similar to the in vivo condition and 2) produce specimens for clamped cantilever beam fracture toughness tests (Fig 1). The CoCr ‘stems’ consisted of a central square bar to which two half-rounds were bonded (see inset Fig1). The stems were grit blasted (Ra = 5.7 μm) over a 75mm length using 16grit aluminum oxide. An aluminum mould was created to simulate the proximal femur with a consistent 6.3mm mantle thickness. One hundred NF10-32 screws were used to create a rough trabecular surface in the mould. PMMA cement was vacuum mixed for 1 minute followed by filling of the mould in a retrograde fashion at either 2 minutes (early) or 6 minutes (late) from start of mix. Stem and cement temperature was 21 deg C while mould was warmed to 37 deg C before cementation. The mantle construct was removed from the mould after 24 hours and then stored for two weeks in saline at 37 deg C. Cement in the half-round regions was then carefully machined, the half rounds were removed, resulting in a clamped cantilever beam specimen.