Effect of asymmetrical configuration of pins in the TSF external fixator used for tibial lengthening in a pediatric population.

Background: The Taylor Spatial Frame (TSF) is a relatively new circular external fixator linked with a computer program used in limb reconstruction surgery. It is routinely used in our unit (Royal Manchester Children's Hospital) for pediatric tibial lengthening.

Methods: The case notes and radiographs of 17 children who underwent 18 tibial lengthenings were reviewed. The TSF was attached to the tibia using a hybrid fixation method in the safe zone with two 1.8 mm tensioned smooth wires and 2 anteriorly placed hydroxyapatite-coated pins. Mechanical compression testing of our TSF construct was performed and compared with a double-ring Ilizarov frame construct fixed with only tensioned wires.

Results: Out of the 18 tibiae lengthened 14 (77.8%) had a significant anterior angulation (>10 degrees) at the end of the distraction period that was corrected with a residual TSF program, with no known long-term consequences. The mechanical tests showed that the Ilizarov frame showed a more symmetrical distribution of loading in compression when compared with the TSF. With increasing load the TSF had the tendency to angulate anteriorly, that is, the apex of the angulation was pointing toward the 2 pins.

Conclusions: The problem of anterior angulation observed in our clinical and mechanical studies is due to instability of the fixation of the proximal part of the frame. Surgical techniques that can improve the stability of the proximal fixation and promote symmetrical loading are discussed.