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.