Effects of Acetabular Shell Deformation and Liner Thickness on Frictional Torque in Polyethylene Acetabular Bearings

Introduction: Studies have shown that frictional forces in an acetabular component during the gait cycle may be a contributing factor to acetabular shell loosening.1,2 Recent publications have indicated that metal shell deformation can cause secondary forces attributed to insertion technique of press-fit components.3,4 In addition, it is also recognized that larger diameter femoral heads offer the advantages of an enhanced range of motion (ROM) and increased stability. Therefore, larger head diameters can effectively reduce dislocation, which is a major complication after total hip arthroplasty.5 The effect of frictional torque produced with a deformed metal shell with a larger head diameter or a thinner polyethylene insert is unknown. The purpose of this study was to evaluate how changes in polyethylene insert thickness and polyethylene insert inner diameter affect frictional torque within the acetabulum. Materials and Methods: Polyurethane synthetic bone model (Pacific Research Laboratories, Vashon, WA) was designed to represent a worst-case scenario for acetabular shell deformation as defined by Jin et al.3 To create the test model, a 30lb/ft3 density polyurethane foam block was machined to be 2mm (± 0.1mm) less than the diameter of the acetabular shell. A second cavity was then formed to remove supporting material from the superior and inferior acetabular rim that to create a deformed shell. Highly polished cobalt chrome femoral heads and matched inside diameter highly cross-linked UHMWPE acetabular inserts with an appropriately matching sized titanium acetabular shells (i.e. D-smallest, E, F-largest) were utilized (Stryker Orthopaedics, Mahwah, NJ). A mallet was used to impact the acetabular shells with the poly insert in the foam, to simulate impaction into the acetabulum. The following measurements were taken to determine deformation of the acetabular shell and polyethylene insert using a caliper (±0.01 mm): diameter before insertion and diameter after insertion. A femoral head (sizes 36 mm/40mm/44mm) was mounted onto a rotating actuator at an angle of 50° from the superior direction to simulate an average femoral stem neck angle of 130°. The foam block and fixture, that simulate the acetabulum, was mounted onto a multi-axis test frame (MTS Corp, Eden Prairie, MN) with a multi-planer moveable table at an angle to simulate 45° of shell abduction, which is the targeted inclination profile. The insert and head maintained in a bath containing bovine calf serum (Hyclone Labs, Logan, Utah) at room temperature that was diluted to 50% with 40% deionized water and 10% of 7 pH 20mMole ethylenediaminetetraacetic acid. A 890 N load was applied superiorly to the model while the rotating actuator was oscillated 20 cycles to ±20° at 0.5 Hertz. In order to determine the thickness of the polyethylene insert and its effect on the frictional torque, two different sized polyethylene thickness inserts were tested. In addition, two different sized femoral heads were tested to determine the effect of the diameter on frictional torque. All statistical analysis was performed using a student’s paired t-test with a two-tailed distribution. Results: The average amount of shell deformation measured in the 36E (5.9mm poly) group was 0.60 ± 0.04 mm compared to 0.81± 0.02 mm in the 36D group (3.9mm poly). This difference was statistically significant (p=0.0021). The average amount of polyethylene deformation measured in 36E (5.9mm poly) was 0.85± 0.05 mm compared to 0.97±0.06 mm in the 36D (3.9mm poly). This difference was also statistically significant (p=0.035).