Friction in modern total hip arthroplasty bearings: Effect of material, design, and test methodology.

The purpose of this study was to characterize the effect of a group of variables on frictional torque generated by acetabular components as well as to understand the influence of test model. Three separate test models, which had been previously used in the literature, were used to understand the effect of polyethylene material, bearing design, head size, and material combinations. Each test model differed by the way it simulated rotation of the head, the type of frictional torque value it reported (static vs. dynamic), and the type of motion simulated (oscillating motion vs. continuous motion). It was determined that not only test model may impact product ranking of fictional torque generated but also static frictional torque may be significantly larger than a dynamic frictional torque. In addition to test model differences, it was discovered that the frictional torque values for conventional and highly cross-linked polyethylenes were not statistically significantly different in the more physiologically relevant test models. With respect to bearing design, the frictional torque values for mobile bearing designs were similar to the 28-mm diameter inner bearing rather than the large diameter outer liner. Testing with a more physiologically relevant rotation showed that frictional torque increased with bearing diameter for the metal on polyethylene and ceramic on polyethylene bearings but remained constant for ceramic on ceramic bearings. Finally, ceramic on ceramic bearings produced smaller frictional torque values when compared to metal on polyethylene and ceramic on polyethylene groups.

The effects of acetabular shell deformation and liner thickness on frictional torque in ultrahigh-molecular-weight polyethylene acetabular bearings.

The purposes of this study were to determine if there were differences in the frictional torque generated between spherical acetabular shells and acetabular shells deformed as a result of implantation and to evaluate how changes in polyethylene insert thickness and head diameter affected these frictional torque data. An established bench top model was used for mechanical testing. A total of 70 samples were tested. Acetabular shells were impacted into polyurethane foam that was designed to create spherical or deformed shell models. We found that deformed acetabular shells produced higher frictional torque than spherical shells. Also, larger femoral head sizes produced greater frictional torque than smaller femoral head sizes. For the deformed models, the thicker polyethylene inserts produced greater frictional torque than the thinner polyethylene inserts.

Orthogonal pin construct versus parallel uniplanar pin constructs for pelvic external fixation: a biomechanical assessment of stiffness and strength.

OBJECTIVE: The purpose of this study is to compare the structural stiffness of an orthogonal pelvic external fixator pin construct with 2 different parallel external fixator pin constructs in a simulated bone model. HYPOTHESIS: An orthogonal pelvic external fixator pin construct would be significantly more stiff than a parallel pin construct when loaded in-plane under similar conditions. DESIGN: Thirty synthetic pelvic bone models were configured with orthogonal pins (group 1), parallel iliac crest pins (group 2), or parallel supra-acetabular pins (group 3). Specimens were loaded either in-plane (flexion/extension moment) or out-of-plane (internal/external rotation moment) to assess construct stiffness. SETTING: Orthopaedic industry mechanical testing laboratory (Stryker Orthopedics, Mahwah, NJ). INTERVENTION: Single load cycle to failure with load application modified to assess stiffness both in-plane and out-of-plane with the pin constructs. MAIN OUTCOME MEASURE: Pelvic external fixation pin construct stiffness. RESULTS: Stiffness for in-plane loading was 150.2 +/- 51.2 N/mm for the orthogonal pin construct, 105.0 +/- 46.9 N/mm for the iliac crest pin construct, and 104.7 +/- 20.7 N/mm for the supra-acetabular pin construct. Pairwise comparisons demonstrated that the difference was significant (P < 0.05) between groups 1 and 2 and groups 1 and 3 but not between groups 2 and 3. Stiffness for out-of-plane loading was 49.6 +/- 3.4 N/mm for the orthogonal pin construct, 53.9 +/- 3.5 N/mm for the iliac crest pin construct, and 100.6 +/- 4.3 N/mm for the supra-acetabular pin construct, with significant differences (P < 0.05) between groups 1 and 3 and groups 2 and 3 but not between groups 1 and 2. CONCLUSION: An orthogonal pelvic external fixator pin construct produced a significantly stiffer construct for in-plane loading (flexion/extension moment) compared with either parallel pin construct; however, a parallel supra-acetabular pin construct was stiffer for out-of-plane loading.

A new test for bipolar prosthesis disassociation.

Because of several failures by dissociation of a redesigned bipolar prosthesis, a new, dynamic test was developed. This dynamic cam-out test represents a closer simulation of one possible clinical mechanism of bipolar disassociation. The results of dynamic testing are affected by the bipolar design, particularly the locking mechanism that was the problem with the redesigned prosthesis.