Pullout strengths of self-reinforced poly-L-lactide (SR-PLLA) rods versus Kirschner wires in bovine femur

OBJECTIVE: To determine the relative amount of fixation of self-reinforced poly-L-lactide (SR-PLLA) rods and Kirschner wires in bovine cancellous bone by comparing their pullout strength DESIGN: An in vitro laboratory study was performed using bovine femurs. Ten two-millimeter-diameter pins of each type were inserted into cancellous bone and then pulled out, using a material testing machine. The maximum force (pullout strength) was selected over other measurements to compare the amount of fixation of the two types of pins. All of the pins were retrieved for microscopic analysis. A paired t test was performed to analyze the differences between the pullout strength of the two types of pins. SETTING: Orthopaedic Bioengineering Laboratory, University of Louisville School of Medicine, Louisville, Kentucky, U.S.A. SPECIMENS: Two young fresh bovine distal femurs, ten two-millimeter-diameter Kirschner wires, ten two-millimeter-diameter bioabsorbable SR-PLLA rods MAIN OUTCOME MEASUREMENTS: Pullout strength in Newtons, and microscopic pin surface aspect after insertion. RESULTS: Significant differences were noted between the maximum force required to remove the two types of pins (p < 0.01) The K-wire mean pullout force was 37.7 N (SD 13.6), and the SR-PLLA rod mean pullout force was 53.6 N (SD 19.3). Microscopic analysis indicated surface modification only on the SR PLLA rods. DISCUSSION: SR-PLLA composites have shown comparable clinical results to their metallic counterparts. In this study, the pullout strength of SR-PLLA rods was compared with that of conventional K-wires. A significant difference (p < 0.01) favoring bioabsorbable pullout strength was noted. The bioabsorbable pin surface modification during insertion is an interesting finding that warrants further investigation as a potential source of improved fixation. CONCLUSION: SR-PLLA rods retain their hold in bovine cancellous bone better than K-wires. This finding offers to the orthopaedic surgeon more information about new pin fixation methods.