Which femoral neck for a dual mobility cup? A biomechanical evaluation.

PURPOSE: This study aimed to evaluate polyethylene (PE) damage and wear lesions to the chamfer of mobile components under mobile and fixed femoral neck impingement at the third articulation, and to determine which femoral neck characteristics should be considered with a dual mobility cup to limit those lesions. METHODS: Two femoral neck geometries (cylindrical and quadrangular) with two surface finishing roughness (rough and polished), and two head-to-neck ratios (28- and 22.2-mm diameter femoral heads) were evaluated in a hip simulator testing. For each characteristic, six femoral necks were tested with six dual mobility cups under fixed and mobile femoral neck impingement conditions. Chamfer PE damage and volumetric wear were evaluated and compared for each femoral neck characteristic and impingement condition. RESULTS: Under mobile impingement condition, femoral neck characteristics did not significantly affect PE damage and wear lesions to the chamfer (p = 0.283 to 0.810). However, under fixed impingement condition, significantly higher PE damage and wear lesions to the chamfer were produced by the quadrangular geometry compared to the cylindrical geometry (p = 0.004 to 0.025). In addition, with the quadrangular geometry, rough surface finishing was demonstrated to increase volumetric wear of the chamfer (p = 0.009). No significant influence of head-to-neck ratio was observed on PE damage and wear lesions to the chamfer (p = 0.244 to 0.714). DISCUSSION: This biomechanical study emphasized that femoral neck characteristics are critical with dual mobility cup and tend to favor a cylindrical geometry particularly whether fixed impingement at the third articulation occurs.

Effects of corrosion and cleaning method on taper dimensions: an in vitro investigation.

PURPOSE: Taper corrosion related revisions have recently been reported in the orthopaedic literature. Cleaning procedure of the trunnions during hip revision is not standardised. The purpose of this bench top investigation was to understand the alterations in the trunnion dimensions and surface roughness characteristics as a result of corrosion product build-up. METHODS: 8 titanium alloy trunnions and CoCr femoral heads assemblies were cyclically tested in a mechanical simulator. Following disassembly of the tested constructs, the trunnions were cleaned using 2 methods. The trunnion dimensions were measured using coordinate measuring machine, and surface roughness was measured using white light interferometry. The trunnions were reassembled with ceramic femoral heads and titanium sleeves following cleaning. Head/sleeve pull-off testing was conducted to understand the effects of cleaning methods on the pull-off strength. RESULTS: Grade 4 corrosion was observed on all trunnions after mechanical testing. The aggressive cleaning methods had a larger impact on the surface roughness when compared to the light cleaning method. The aggressive cleaning method also decreased the taper cone angle. The pull-off strength was not affected by the cleaning method and the pull-off values were approximately 50% of the assembly loads. CONCLUSIONS: The study suggests that trunnion cleaning method may alter the surface roughness and taper cone angle of the existing trunnion. However, the effects of these changes on the pull-off strength did not reach statistical significance. Complex corrosion testing under cyclic loading conditions are warranted to understand the long-term effects of these changes.

Influence of Assembly Force and Distraction on the Femoral Head-Taper Junction.

BACKGROUND: This study investigates if the placement of femoral heads (trials and actual implants) using varying impaction forces causes physical compromise to the trunnion. METHODS: Trunnion and head taper wear patterns were evaluated after impaction and removal of new femoral stem trunnions and ceramic heads at various impaction loads (2 kN, 4 kN, or 6 kN, n = 6/group). In addition, trunnion wear patterns were measured after plastic trials were hand-placed on new trunnions and underwent range of motion testing in a Hip Simulator (n = 5). RESULTS: There was no significant difference in trunnion or head surface deviation, taper angle, or surface roughness in any groups preimpaction and postimpaction and removal. There was no significant surface trunnion damage from assembly and range of motion testing of the plastic femoral head trial. CONCLUSIONS: The use of femoral head trials and the concurrent impaction and removal of a new femoral head were not associated with significant trunnion surface damage for the impaction loads observed in this study.

Impaction Force Influences Taper-Trunnion Stability in Total Hip Arthroplasty.

BACKGROUND: This study investigated the influence of femoral head impaction force, number of head strikes, the energy sequence of head strikes, and head offset on the strength of the taper-trunnion junction. METHODS: Thirty titanium-alloy trunnions were mated with 36-mm zero-offset cobalt-chromium femoral heads of corresponding taper angle. A drop tower impacted the head with 2.5J or 8.25J, resulting in 6 kN or 14 kN impaction force, respectively, in a single strike or combinations of 6 kN + 14 kN or 14 kN + 14 kN. In addition, ten 36-mm heads with -5 and +5 offset were impacted with sequential 14 kN + 14 kN strikes. Heads were subsequently disassembled using a screw-driven mechanical testing frame, and peak distraction force was recorded. RESULTS: Femoral head pull-off force was 45% the strike force, and heads struck with a single 14 kN impact showed a pull-off force twice that of the 6 kN group. Two head strikes with the same force did not improve pull-off force for either 6 kN (P = .90) or 14 kN (P = .90). If the forces of the 2 impactions varied, but either impact measured 14 kN, a 51% higher pull-off force was found compared to impactions of either 6 kN or 6 kN + 6 kN. Femoral head offset did not significantly change the pull-off force among -5, 0, and +5 heads (P = .37). CONCLUSION: Femoral head impaction force influenced femoral head trunnion-taper stability, whereas offset did not affect pull-off force. Multiple head strikes did not add additional stability, as long as a single strike achieved 14 kN force at the mallet-head impactor interface. Insufficient impaction force may lead to inadequate engagement of the trunnion-taper junction.

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.

Metal-on-metal bearings: the problem is edge-loading wear.

Metal-on-metal bearings are promoted as a low wear bearing alternative to traditional hip replacement bearings. While most in vitro studies support this, recent clinical reviews have found a significant number of early revisions in some designs of metal-on-metal bearings related to wear. Metal-on-metal bearings exhibit a bi-phasic wear pattern with high initial wear that generally settles down to low steady state wear. Previous publications from the authors have found that steady state wear occurs due to the formation of a critical conforming contact area. This contact area was found to be surprisingly constant regardless of bearing size, clearance, or even contact mode. The authors hypothesized that steady state wear may never be reached if formation of this critical conforming contact area is disrupted. Several hip simulator tests were performed to assess the wear performance of generic metal-on-metal samples at various angles of inclination. Three-dimensional modeling was performed on the generic bearing design as well as typical resurfacing and hemispherical bearing designs including various sizes and clearance ranges. Simulator results support the hypotheses, and wear rates were linear or accelerating when the critical contact area size could not be achieved due to its proximity to the rim of the bearing. Modeling studies show a correlation between bearing size and design and the maximum inclination angle allowed to reach steady state conditions. Smaller bearing size and shallower cup designs were found to reduce the maximum safe inclination angle and this corresponds to clinical observation of increased failure rates in these bearings. This simple method for assessing runaway wear risk can be utilized in the design of more robust and forgiving metal-on-metal bearings.