Influence of Acetabular Shell Position and Component Design on Hip Dynamic Dislocation.

BACKGROUND: Dislocation is a major complication following total hip arthroplasty, with risk factors such as surgical technique, implant positioning, and implant design. Literature has suggested the distance the femoral head must travel before dislocation to be a predictive factor of dislocation where smaller travel distance has increased dislocation risk. The purpose of this study was to compare 3 designs (hemispherical, metal-on-metal, and dual mobility [DM]) in terms of the dynamic dislocation distance and force required to dislocate. METHODS: This dynamic dislocation distance model used a material testing system that defined acetabular component inclination (30°, 45°, and 60°), anteversion angles (0°, 15°, and 30°), and pelvic tilt (5° [standing] and 26° [chair rise]). Testing groups included a hemispherical shell with a modular polyethylene liner and 32-mm head, a metal-on-metal hip resurfacing cup design with a 40-mm CoCr head, and a DM design with a 42-mm outside diameter articulating liner and an inner 28-mm articulating head. RESULTS: The dynamic dislocation distance of the DM hip was greater than that of the other designs for all inclination, anteversion, and pelvic tilt angles tested with the exception of 60° inclination/0° anteversion. At 26° pelvic tilt, it was observed that dislocation distance increased with greater anteversion and decreased with larger inclination. CONCLUSION: Clinical results have shown the DM design may reduce dislocation. These data support those findings and suggest that if instability is a concern preoperatively or intraoperatively, using a DM implant increases the dynamic dislocation distance.

Effects of Seating Load Magnitude on Incremental Cyclic Fretting Corrosion in 5°40' Mixed Alloy Modular Taper Junctions.

BACKGROUND: Mechanically assisted crevice corrosion of modular tapers continues to be a concern in total joint arthroplasties. A surgical factor that may affect taper fretting corrosion during cyclic loading is seating load magnitude. In this study, modular head-neck taper junctions were seated, capturing load-displacement, over a range of axially oriented loads, and electrochemical and micromotion data were captured during short-term incremental cyclic fretting corrosion (ICFC) tests. The hypothesis is low seating loads result in greater motion and fretting corrosion in ICFC tests. The effect of assembly load on pull-off force post-ICFC testing was also evaluated. METHODS: The study employed custom-built test fixtures which measured head-neck micromotion and an electrochemical chamber to monitor electrochemical reactions. Head-neck motion measurements were captured using 2 noncontact differential variable reluctance transducers mounted to the head. Seating experiments ranged from 1000 to 8000 N. RESULTS: Significant differences due to seating loads were reported in seating displacement, ICFC subsidence, and fretting current at 4000 N cyclic load. Seating load decreased but did not eliminate currents. Fretting onset load remained fixed (approximately 1200 N) for tapers seated above 2000 N. Fretting subsidence was negligible for seating loads of 4000 N or higher, and increased subsidence was observed below 4000 N. CONCLUSION: This short-term test method evaluated the acute performance of modular implants which were assembled under various loads and demonstrated the link between seating loads, fretting motions, and electrochemical reactions. While increased seating loads reduced fretting corrosion and taper subsidence, it did not prevent fretting corrosion even at 8 kN seating.

Effects of seating load magnitude and load orientation on seating mechanics in 5°40' mixed-alloy modular taper junctions.

BACKGROUND: Mechanically-assisted crevice corrosion of modular tapers continues to be a concern in total joint replacements. Surgical factors that may affect taper seating mechanics include seating load magnitude and load orientation. Seating mechanics is defined as the seating load versus displacement behavior. In this study, mixed-alloy (CoCrMo/Ti-6Al-4V) modular head-neck 5°40' taper junctions were seated over a range of axially-oriented loads and off-axis orientations, capturing load-displacement during seating. The goals of the study were to assess the effects of seating load magnitude and load orientation on seating mechanics and correlate those findings with the taper pull-off load. METHODS: A testing fixture measured head-neck seating displacement as the load was quasistatically applied. Motion was captured using two non-contact differential variable reluctance transducers which were mounted to the neck targeting the head. Seating experiments ranged from 1000 N to 8000 N. Load orientation ranged from 0° to 20° at 4000 N. RESULTS: Seating load-displacement behavior at different seating loads showed a consistent characteristic behavior. Testing demonstrated increased seating displacement with seating load. Pull-off loads increased with seating load and were approximately 44% of the seating load across the range of seating loads investigated. Seating load orientation up to 20° had no significant effect on seating displacement and taper pull-off load. CONCLUSION: Increased seating load magnitude increased seating displacement, work of seating and pull-off loads in mixed-alloy 5°40' head-neck tapers. Altering load orientation up to 20° off-axis had no significant effect. Direct measurements of seating mechanics provides insights into the locking of taper junctions.

Effect of the support systems' compliance on total hip modular taper seating stability.

Assembly of a femoral head onto the stem remains non-standardized. The literature shows altering mechanical conditions during seating affects taper strength and lower assembly load may increase fretting corrosion during cyclic tests. This suggests overall performance may be affected by head assembly method. The purpose of this test was to perform bench-top studies to determine influence of peak force magnitude, load rate, and compliance of the system's support structure on initial stability of the taper. Custom manufactured CoCrMo femoral heads and Ti-6Al-4V taper analog samples were assembled with varying peak force magnitudes (2-10.1 kN), load rates (quasi-static vs impaction), and system compliance (rigid vs compliant). A clinically-relevant system compliance design was based off of force data collected during a cadaver impaction study. Tensile loads were then applied to disassemble the taper and quantify initial taper stability. Results indicated that taper stability (assessed by disassembly forces) increased linearly with assembly force and load rate did not have a significant effect on taper stability. When considering system compliance, a 42%-50% larger input energy, dependent on assembly force, was required in the compliant group to achieve a comparable impaction force to the rigid group. Even when this impaction force was achieved, the correlation between the coefficient, defined as distraction force divided by assembly load, was significantly reduced for the compliant test group. The compliant setup was intended to simulate a surgical scenario where patient and surgical factors may influence the resulting compliance. Based on results, surgical procedure and patient variables may have a significant effect on initial taper stability.

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.

Teriparatide therapy and beta-tricalcium phosphate enhance scaffold reconstruction of mouse femoral defects.

To investigate the efficacy of endocrine parathyroid hormone treatment on tissue-engineered bone regeneration, massive femoral defects in C57Bl/6 mice were reconstructed with either 100:0 or 85:15 poly-lactic acid (PLA)/beta-tricalcium phosphate (ß-TCP) scaffolds (hereafter PLA or PLA/ßTCP, respectively), which were fabricated with low porosity (<30%) to improve their structural rigidity. Experimental mice were treated starting at 1 week postop with daily subcutaneous injections of 40 µg/kg teriparatide until sacrifice at 9 weeks, whereas control mice underwent the same procedure but were injected with sterile saline. Bone regeneration was assessed longitudinally using planar X-ray and quantitative microcomputed tomography, and the reconstructed femurs were evaluated at 9 weeks either histologically or biomechanically to determine their torsional strength and rigidity. Teriparatide treatment increased bone volume and bone mineral content significantly at 6 weeks and led to enhanced trabeculated bone callus formation that appeared to surround and integrate with the scaffold, thereby establishing union by bridging bone regeneration across the segmental defect in 30% of the reconstructed femurs, regardless of the scaffold type. However, the bone volume and mineral content in the PLA reconstructed femurs treated with teriparatide was reduced at 9 weeks to control levels, but remained significantly increased in the PLA/ßTCP scaffolds. Further, bridged teriparatide-treated femurs demonstrated a prototypical brittle bone torsion behavior, and were significantly stronger and stiffer than control specimens or treated specimens that failed to form bridging bone union. Taken together, these observations suggest that intermittent, systemic parathyroid hormone treatment can enhance bone regeneration in scaffold-reconstructed femoral defects, which can be further enhanced by mineralized (ßTCP) particles within the scaffold.

Evaluation of dense polylactic acid/beta-tricalcium phosphate scaffolds for bone tissue engineering.

Advances in biomaterial fabrication have introduced numerous innovations in designing scaffolds for bone tissue engineering. Often, the focus has been on fabricating scaffolds with high and interconnected porosity that would allow for cellular seeding and tissue ingrowth. However, such scaffolds typically lack the mechanical strength to sustain in vivo ambulatory stresses in models of load bearing cortical bone reconstruction. In this study, we investigated the microstructural and mechanical properties of dense PLA and PLA/beta-TCP (85:15) scaffolds fabricated using a rapid volume expansion phase separation technique, which embeds uncoated beta-TCP particles within the porous polymer. PLA scaffolds had a volumetric porosity in the range of 30 to 40%. With the embedding of beta-TCP mineral particles, the porosity of the scaffolds was reduced in half, whereas the ultimate compressive and torsional strength were significantly increased. We also investigated the properties of the scaffolds as delivery vehicles for growth factors in vitro and in vivo. The low-surface porosity resulted in sub optimal retention efficiency of the growth factors, and burst release kinetics reflecting surface coating rather than volumetric entrapment, regardless of the scaffold used. When loaded with BMP2 and VEGF and implanted in the quadriceps muscle, PLA/beta-TCP scaffolds did not induce ectopic mineralization but induced a significant 1.8-fold increase in neo vessel formation. In conclusion, dense PLA/beta-TCP scaffolds can be engineered with enhanced mechanical properties and potentially be exploited for localized therapeutic factor delivery.