A simulator study of adverse wear with metal and cement debris contamination in metal-on-metal hip bearings.

OBJECTIVES: Third-body wear is believed to be one trigger for adverse results with metal-on-metal (MOM) bearings. Impingement and subluxation may release metal particles from MOM replacements. We therefore challenged MOM bearings with relevant debris types of cobalt-chrome alloy (CoCr), titanium alloy (Ti6Al4V) and polymethylmethacrylate bone cement (PMMA). METHODS: Cement flakes (PMMA), CoCr and Ti6Al4V particles (size range 5 µm to 400 µm) were run in a MOM wear simulation. Debris allotments (5 mg) were inserted at ten intervals during the five million cycle (5 Mc) test. RESULTS: In a clean test phase (0 Mc to 0.8 Mc), lubricants retained their yellow colour. Addition of metal particles at 0.8 Mc turned lubricants black within the first hour of the test and remained so for the duration, while PMMA particles did not change the colour of the lubricant. Rates of wear with PMMA, CoCr and Ti6Al4V debris averaged 0.3 mm(3)/Mc, 4.1 mm(3)/Mc and 6.4 mm(3)/Mc, respectively. CONCLUSIONS: Metal particles turned simulator lubricants black with rates of wear of MOM bearings an order of magnitude higher than with control PMMA particles. This appeared to model the findings of black, periarticular joint tissues and high CoCr wear in failed MOM replacements. The amount of wear debris produced during a 500 000-cycle interval of gait was 30 to 50 times greater than the weight of triggering particle allotment, indicating that MOM bearings were extremely sensitive to third-body wear. Cite this article: Bone Joint Res 2015;4:29-37.

What is a "normal" wear pattern for metal-on-metal hip bearings?

In addition to classical run-in and steady-state wear phases, metal-on-metal (MOM) hip bearings have encountered "runaway wear" (RAW) trends in simulator studies. This puzzling behavior has resulted in 2- to 19-fold wear increases compared with other apparently "identical" bearings. MOM bearings have shown five identifiable RAW wear patterns in joint simulators; therefore, additional descriptive terms were used here to indicate various observed patterns, for example, "breakaway wear" (BAW), which was defined as a higher wear trend that recovers to steady-state wear. As these trends commonly occur for MOM, this raises the question of what can be considered "normal" behavior or "abnormal"? In an effort to identify possible causes for this behavior, the current study investigated six Co-Cr bearings, which closely matched with respect to geometrical tolerances. Despite close control of design and test variables, BAW occurred in 30% of the MOM bearings, producing a threefold wear increase above otherwise identical MOM bearings within the same group. The majority of the BAW (85%) occurred on the cup side and was validated by growth of wear scars and concentrations of metal ions. One bearing that showed continuing BAW at 5 Mc revealed a cup that was 50% smoother than other cups whereas its mating head was 50% rougher, thus signifying that highly polished areas were sites of the highest MOM wear. The two BAW bearings with high wear showed the greatest conformity at 5 Mc, in apparent contradistinction to classical lubrication theory.

"Severe" wear challenge to 36 mm mechanically enhanced highly crosslinked polyethylene hip liners.

Our purpose was to compare the wear performance of mechanically enhanced 5Mrad highly crosslinked polyethylene (MEP, ArComXL) hip liners to (control) 3Mrad UHMWPE liners (ArCom) in 36 mm head size. As a more severe synergy of clinically relevant test models, we contrasted wear with custom roughened Co-Cr surfaces (Ra 500 nm) to the standard pristine Co-Cr heads (Ra < 20 nm) using a severe microseparation test mode in our hip simulator. We adopted a previously published model to estimate potential biological activity. On new Co-Cr heads, the MEP liners showed a 47% reduction in volumetric wear a 13% reduction in wear particle size and a 27% reduction in Functional Biological Activity (FBA) compared to our control. On rough Co-Cr heads, the MEP liners showed little advantage in terms of volumetric wear compared with the control. However, the MEP liners overall showed a 38% reduction in FBA compared to the control owing to a larger volume fraction of larger particles. Thus overall the MEP liners appeared to offer advantages in terms of reduced FBA indices.

Femoral neck fracture fixation: a biomechanical study of two cannulated screw placement techniques.

This biomechanical study evaluated two different methods of femoral neck fracture fixation. Ten pairs of embalmed femora were used. A standardized midcervical osteotomy was performed. One side from each pair was fixed with either conventional (central) or calcar (cortical adjacent) screw placement. The femora pairs then were subjected to cyclic and load to failure biomechanical testing. The results demonstrated significant improved stability, load, stiffness, and displacement in all tested parameters for the group with calcar screw fixation. Grossly, the conventional fixation group tended to fail in an excessively varus position, while the calcar fixation group failed with impaction of the head on the calcar, a potentially stable position.

Effect of superior and superolateral relocations of the hip center on hip joint forces. An experimental and analytical analysis.

With the extensive use of uncemented acetabular components in total hip arthroplasty, relocation of the hip center has become increasingly necessary to avoid bulk grafts and to promote contact between the porous prosthetic surface and bone. Compared with the anatomic hip center, superolateral relocation theoretically results in higher hip joint forces and has been shown in cemented acetabular components to result in an increased clinical failure rate. This study experimentally and analytically compared the hip joint forces in normal, superior, and superolateral hip center locations during both single-leg stance and stairclimbing, performing this comparison over a wide range of hip joint applied flexion moments. An advanced loading fixture was designed to allow any applied moment to be set independently of femoral position, incorporating all three major muscle groups active in stairclimbing position: extensors, abductors, and adductors. For all positions and moments tested, it was found that superolateral relocation caused significant increases in the total hip joint force, but did not affect the nonsagittal force component. Also, superior-only hip center relocation did not significantly affect the total joint force magnitudes or directions. The force increase on hip center lateralization can be attributed to a corresponding increase in the adduction moment. Results from the static analytical model developed supported these findings. The results of this study suggest that superolateral hip center relocation should be avoided and that superior-only relocation may be mechanically acceptable within the confines of the osseous anatomy of the acetabulum.