Abstract
Introduction
Frictional torque is generated at the hip joint during normal gait loading and motion [1]. This study investigated the effect of shell deformation due to press-fit on frictional torque generated at the articulating surfaces of cementless acetabular shells that incorporated fixed and dual mobility bearing designs.
Materials and Methods
Figure 1 lists the study groups (minimum of n = 5). All groups were tested with a 50 mm Trident PSL shell (Stryker Orthopaedics, NJ) and a Ti6Al4V trunnion. Metal-on-Metal specimens were custom designed and manufactured, and are not approved for clinical use. The remaining groups consisted of commercially available products (Stryker Orthopaedics, NJ).
All groups were tested with the shells in deformed and undeformed states.
Deformed Setup: A two-point relief configuration was created in a polyurethane foam block (Figure 2) with a density of 30 lb/ft3 to replicate shell deformation due to press-fit [2]. The blocks were machined to replicate the press-fit prescribed in the shell's surgical protocol. Each shell was assembled into the foam block by applying an axial force at 5 mm/min until it was completely seated.
Undeformed Setup: Each shell was assembled in a stainless steel block with a hemispherical cavity that resulted in a line-to-line fit with the shell OD.
Frictional torque was measured using a physiologically relevant test model [3]. In this model, the specimen block was placed in a fixture to simulate 50° abduction and 130° neck angle (Figure 2). A 2450N side load was applied and the femoral head underwent angular displacement of ± 20° for 100 cycles at 0.75 Hz. The articulating surfaces were lubricated with 25% Alpha Calf Fraction Serum.
Peak torque was observed towards the end or the beginning of each cycle where the velocity of the femoral head approaches 0 and the head changes direction. This torque is referred as maximum static frictional torque. Specimen groups were statistically compared with a single-factor ANOVA test and a Tukey post-hoc test at 95% confidence level. Paired t-tests were performed to compare individual groups in deformed and undeformed states.
Results
Figure 3 contains the results. In both deformed and undeformed states, the MoM group exhibited the highest frictional torque whereas the MDM group had the lowest frictional torque. In both states, the difference in frictional torque between MoM vs. 28 mm–Fixed Bearing and MoM vs. MDM was statistically significant whereas the difference between 28 mm-Fixed Bearing and MDM was not statistically significant. Shell deformation due to press-fit did not have a significant effect on any of the groups.
Discussion
This study evaluated the effect of press-fit on the frictional torque generated in various cementless acetabular systems using a physiologically relevant in-vitro test model. Results from this test suggest a trend towards lower frictional torque for dual mobility bearings, which is worthy of further investigation.
