Effect of Simulated Early Weight Bearing on Micromotion and Pullout Strength of Uncemented Distal Femoral Stems

Effect of Simulated Early Weight Bearing on Micromotion and Pullout Strength of Uncemented Distal Femoral Stems Jennifer S. Barr, MD; JeDeDiah K. White, BS; Stephanie e. W. punt, BS; erneSt u. ConraD iii, MD; ranDal p. Ching, phD The effect of simulated early weight bearing on both micromotion and pullout strength of uncemented distal femoral stems was evaluated in this study. The effect of stem endosteal contact and bone quality on implant pullout strength was also analyzed. A randomized matched-pair study was performed using 8 bilateral pairs of fresh human cadaveric femoral specimens. Each specimen pair was dual-energy x-ray absorptiometry scanned, uniformly implanted, fluoroscopically imaged, and randomly assigned to the cycled or uncycled group. The cycled group received 5000 cycles of axial compressive loading (to 700 N) and the contralateral side was not cycled. Micromotion was monitored during cycling and compared with a failure threshold (150 μm), and all implants underwent direct axial distraction (pullout) testing. During cycling, minimal micromotion was observed with an asymptotic decrease in differential motion between the first and last 50 cycles. Both cycled and uncycled groups demonstrated no statistical difference in average pullout force (4888±2124 N vs 4367±1154 N; P=.43). The percentage of cortical contact for each implant was determined from panoramic fluoroscopy images using digital image analysis software. Contact area for the distal third of the stem showed the highest correlation with pullout force and with predicting pullout force. Bone quality did not correlate with pullout force (r2=0.367) or stem contact area (r2=0.394). In sum, press-fit uncemented femoral stems did not loosen or demonstrate decreased pullout strength with early weight bearing simulated by cyclical axial compressive loading. [Orthopedics. 2015; 38(5):e417-e422.] The authors are from the Department of Orthopedic Surgery and Rehabilitation (JSB), University of Mississippi Medical Center, Jackson, Mississippi; the Department of Orthopedics and Sports Medicine (JKW, SEWP) and the Department of Mechanical Engineering (RPC), University of Washington, Seattle, Washington; and the Department of Orthopedics and Sports Medicine (EUC), Seattle Children’s Hospital, Seattle, Washington. The authors have no relevant financial relationships to disclose. Correspondence should be addressed to: Randal P. Ching, PhD, Department of Mechanical Engineering, University of Washington, 205 NE Northlake Way, Ste 100, Seattle, WA 98105 (rc@uw.edu). Received: February 5, 2014; Accepted: July 28, 2014. doi: 10.3928/01477447-20150504-60