Analysis of strategies to increase external fixator stiffness: is double stacking worth the cost?

We compared the mechanical benefits and costs of 3 strategies that are commonly used to increase knee-spanning external fixator stiffness (resistance to deformation): double stacking, cross-linking, and use of an oblique pin. At our academic trauma centre and biomechanical testing laboratory, we used ultra-high-molecular-weight polyethylene bone models and commercially available external fixator components to simulate knee-spanning external fixation. The models were tested in anterior-posterior bending, medial-lateral bending, axial compression, and torsion. We recorded the construct stiffness for each strategy in all loading modes and assessed a secondary outcome of cost per 10% increase in stiffness. Double stacking significantly increased construct stiffness under anterior-posterior bending (109%), medial-lateral bending (22%), axial compression (150%), and torsion (41%) (p<0.05). Use of an oblique pin significantly increased stiffness under torsion (25%) (p<0.006). Cross-linking significantly increased stiffness only under torsion (29%) (p<0.002). Double stacking increased costs by 84%, cross-linking by 28%, and use of an oblique pin by 15% relative to a standard fixator. All 3 strategies increased stiffness under torsion to varying degrees, but only double stacking increased stiffness in all 4 testing modalities (p<0.05). Double stacking is most effective in increasing resistance to bending, particularly under anterior-posterior bending and axial compression, but requires a relatively high cost increase. Clinicians can use these data to help guide the most cost-effective strategy to increase construct stiffness based on the plane in which stiffness is needed.