INITIAL STABILITY OF PRESS-FIT ACETABULAR COMPONENTS UNDER RIM LOADING: A BIOMECHANICAL STUDY

Introduction Total hip arthroplasty has seen a transition from cemented acetabular components to press-fit porous coated components. Plasma sprayed titanium implants are often press-fit with 1mm under-reaming of the acetabulum; however, as porous coating technologies evolve, the amount of under-reaming required for initial stability may be reduced. This reduction may improve implant seating due to lowered insertion loads, and reduce the risk of intraoperative fracture. The purpose of this study was to investigate the initial fixation provided by a high porosity coating (P2, DJO Surgical), and a plasma sprayed titanium coating under rim loading with line-to-line and 1mm press-fit surgical preparation. Methods Five, 52mm high porosity acetabular cups (60% average porosity) and five 52mm plasma sprayed titanium coated cups were inserted into low density (0.24g/cc) biomechanical test foam (Pacific Research Laboratories). Foam test material was cut into uniform 90×90×40mm blocks. Reaming was performed using standard instrumentation mounted on a vertical mill. Cups were first inserted into foam blocks prepared with line-to-line (52mm) reaming. Following mechanical testing, cups were removed from the foam, cleaned, and inserted into foam blocks prepared with 1mm under reaming (51mm). In total 4 test conditions were evaluated: Group A: P2 + line-to-line Group B: Plasma sprayed + line-to-line, Group C: P2 + 1mm under-reaming Group D: Plasma sprayed + 1mm-under reaming Acetabular cup impaction was carried out using a single axis servohydraulic test machine (Instron 8500). Cups were inserted at 1mm/s to a load of 5kN. Insertion load was calculated as a 0.1mm offset from the linear portion of the force/displacement curve; insertion energy was the area under the curve. Tangential rim loading was applied at 0.0254mm/s by a conical indenter to the implant rim. Load data were recorded at 1kHz. Cup displacement was recorded by a 3D, marker-based tracking system at 15Hz (DMAS, Spicatek). Six markers were attached to a disk secured in the acetabular cup (Figure 1). Yield failure was defined as 0.331o of angular displacement (150µm of relative displacement). Angular displacement was derived by calculating the normal vector of a best-fit plane based on marker centroids. Results Under-reamed groups (C,D) showed statistically higher insertion loads and insertion energies than line-to-line groups (A,B), with group C requiring the highest insertion load. Despite greater ease of insertion, groups A and B attained comparable yield loads with group A statistically outperforming D. Group C attained the highest ultimate failure loads, outperforming A and D (Figure 2). Discussion Implants with high porosity coating and line-to-line preparation required less effort for full seating and maintained yield and ultimate performance which exceeded, or was comparable to, plasma sprayed titanium coated implants in either under-reamed or line-to-line preparation. Limitations of this study include the use of a mill for foam block preparation and automated implant insertion. Initial results in three matched cadaveric acetabular pairs with line-to-line preparation indicate that the advantages of high porosity coating may be preserved in human tissue with average yield failure and ultimate failure load improvements of 108% and 73% respectively (Figure 3). Study is ongoing.