The Frank Stinchfield Award: Assembly and Dissociation Forces Differ Between Commonly Used Dual Mobility Implants: A Biomechanical Study.

BACKGROUND: Intra-prosthetic dissociation (IPD) is a complication unique to dual mobility (DM) implants where the outer polyethylene head dissociates from the inner femoral head. Increasing reports of IPD at the time of closed reduction of large head DM dislocations prompted this biomechanical study evaluating the assembly and dissociation forces of DM heads.

METHODS: We tested 17 polyethylene DM heads from five vendors. Of the heads, twelve were highly cross-linked polyethylene (4 vendors), and five were infused with vitamin E (2 vendors). Heads were between 46 and 47 mm in diameter, accepting a 28 mm inner ceramic head. Implants were assembled and disassembled using a servo-hydraulic machine that recorded the forces and torques applied during testing. Dissociation was tested via both axial pull-out and lever-out techniques, where lever-out simulated stem-on-acetabular component impingement.

RESULTS: The initial maximum assembly force was significantly different between all vendors (P < 0.01) and decreased for all implants with subsequent assembly. Vendor 4-E heads required the highest assembly force (1,831.9 ± 81.95 N), followed by Vendor 3, Vendor 5, Vendor 1-E, Vendor 2, and Vendor 1. Vendor 4-E implants showed the greatest dissociation resistance in both pull-out (2,059.89 N, n = 1) and lever-out (38.95 ± 2.79 Nm) tests. Vendor 1-E implants with vitamin E required higher assembly force, dissociation force, and energy than Vendor 1 heads without vitamin E.

CONCLUSION: There were notable differences in dual mobility assembly and dissociation forces between implants. Diminishing force was required for assembly with each additional trial across vendors. Vendor 4-E DM heads required the highest assembly and dissociation forces. Vitamin E appeared to increase the assembly and dissociation forces. Based on these results, DM polyethylene heads should not be reimplanted after dissociation, and there may be a role for establishing a minimum dissociation energy standard to minimize IPD risk.