Migration of Subtalar Implants in Paediatric Flatfoot Correction: A Pilot Study in Synthetic Bone Models

Subtalar implant migration as a complication following subtalar arthroeresis has been described in the scientific literature. However, clinical studies do not allow for unequivocally determining the underlying causes. The aim of the study is to determine the risk of migration of two geometric types of subtalar implants. Biomechanical tests were carried out on a synthetic bone model with a soft tissue substitute, which allowed for reduction of variability of results caused by biological differences.

A foot model mirroring natural anatomy was made from synthetic bone and a silicone soft tissue substitute with the same hardness as that of the soft tissues of the foot. Two types of 11 mm Ti6Al4V titanium alloy implants were studied, namely, a rectangular subtalar screw and a cylindrical subtalar screw, a type commonly used in flatfoot reconstruction surgery in children. The screws were placed in the sinus tarsi and subjected to cyclic loading (up to 1,000,000 cycles at a frequency of 5 Hz, with a maximum load of 500 N). Comparative pull-out force tests were performed immediately following implantation and after the dynamic loading test. Wyniki. Following the dynamic loading test, all 12 samples were qualified for the pull-out force test. Cylindrical screws demonstrated higher pull-out force values both for the samples tested immediately following implantation and for those that underwent dynamic loading. Implants of the same shape did not show statistically significant differences in the Mann-Whitney U test (p >0.05). Wniosek. The synthetic research model produces reproducible results in the assessment of risk of implant migration. Long-term loading does not significantly affect the risk of implant migration.