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 bo ne and then pulled out, using a material testing machine. The maximum force (pullout strength) was selected over other measurements to compare the amo unt of fixation of the two types of pins. All of the pins were retrieved fo r microscopic analysis. A paired t test was performed to analyze the differ ences between the pullout strength of the two types of pins. Setting: Orthopaedic Bioengineering Laboratory, University of Louisville Sc hool of Medicine, Louisville, Kentucky, U.S.A. Specimens: Two young fresh bovine distal femurs, ten two-millimeter-diamete r 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 requi red to remove the two types of pins (p < 0.01) The K-wire mean pullout forc e 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 S R PLLA rods. Discussion: SR-PLLA composites have shown comparable clinical results to th eir metallic counterparts. In this study, the pullout strength of SR-PLLA r ods was compared with that of conventional K-wires. A significant differenc e (p < 0.01) favoring bioabsorbable pullout strength was noted. The bioabso rbable pin surface modification during insertion is an interesting finding that warrants further investigation as a potential source of improved fixat ion. Conclusion: SR-PLLA rods retain their hold in bovine cancellous bone better than K-wires. This finding offers to the orthopaedic surgeon more informat ion about new pin fixation methods.