No abstract

For biologic fixation, press-fit acetabular cups should have initial stability with minimal micromotion and osteoconductive surfaces in contact with bone. Inadequate exposure potentially influences initial stability by increasing the possibility of soft tissue interposition and contamination at the implant-tissue interface. A sawbone model was used to examine how interposed tissue and contamination influence initial cup stability. Seven groups (n ¼ 4) were tested with varying levels of interposed fatty and fibrous tissue placed around the rim of the cup. 54 millimeter in diameter highly porous hemispherical acetabular cups (Stryker, Mahwah NJ) and 54 mm reamed cavities in sawbone blocks were used. Shells were seated and maximum lever out force was recorded for each sample. Cups with fibrous tissue spaced evenly along the rim had a lever out force that was 150% of the control (107 AE 6 vs. 150 AE 12N, p ¼ 0.005), and fatty tissue contamination had a lever out force that was 140% of the control (143 AE 18 vs. 107 AE 6N, p ¼ 0.04). Cups with fibrous tissue placed eccentrically along the rim had a lever out force that was double the control 107 AE 6 N vs. 200 AE 15 N (p ¼ 0.001). Surprisingly, fatty tissue contamination and fibrous tissue interposition at the rim increased initial stability. The eccentrically interposed tissue forced the opposite pole of the cup into the bone, resulting in a more secure press-fit. However, soft tissue interposition decreases implant/bone apposition, and the effect on long term fixation is unknown. Statement of Clinical Significance: Soft tissue interposition between the bone and cup may provide higher initial stability, but its long-term effects are unknown.

Cementing metallic liners into well-fixed acetabular shells facilitates utilizing dualmobility cups in revision total hip arthroplasty without shell replacement. The current biomechanical study investigated the effect of increasing cemented liner (a) inclination; and (b) offset on the cement retention strength measured as the leverout moment at cement failure. Eighteen metallic liner prototypes were cemented into cluster-hole acetabular shells at variable inclinations (0°, 10°, and 20°) and offsets (0 and 10 mm) relative to the enclosing acetabular shell (6 groups; n = 3 constructs per group). The constructs were connected to a material testing frame, and lever-out failure moments were tested through an established protocol. Failure occurred at the liner-cement interface (18/18). There was no correlation between liner inclination and the lever-out failure moment (r = −0.327, P = .185). Liner offset demonstrated a strong negative correlation to mean lever-out failure moments (r = −0.788, P < .001). There was no significant difference between mean lever-out failure moments at variable liner inclinations, regardless of offset (P = .358). Greater liner offset was associated with diminished mean lever-out failure moments (P < .001). Compared with neutral (0°inclination, 0 mm offset), the maximum inclination and offset group had the lowest mean lever-out failure moment (P = .011). Cemented metal-in-metal constructs are significantly affected by the liner positioning. While a correlation between liner inclination and cement retention strength could not be asserted, cement retention strength is significantly diminished by increased liner offset.

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.

No abstract