Comparison of Three Circular Frames in Lower Limb Deformity Correction: A Biomechanical Study

Abstract

Background: The use of circular frames in correcting lower limb deformity is well-established and has evolved dramatically over the years. Three new frames have been introduced recently, and this study is set to compare them in terms of accuracy and efficiency in correcting a similar long bone deformity. These frames are the Taylor Spatial Frame (TSF; Smith & Nephew, London, United Kingdom), the Truelok Hexapod System (TL-HEX; Orthofix, Lewisville, Texas), and Orthex (OrthoPediatrics, Warsaw, Indiana).

Methods: This is a biomechanical study comparing the above three types of circular frames to correct similar deformities in Sawbones models. The deformities that are compared were: (1) 30° valgus deformity of the distal femur; (2) 30° varus deformity of the proximal tibia. Each frame was applied to the deformed bone in the standard way that we apply to normal bone. X-rays were taken before and after the deformity correction. The frames’ software was used to estimate the deformities. The variations between the software’s estimations and the known bone deformities were compared. Residual deformity after initial correction and the number of re-programmings was compared among these three frames. The least residual deformity and re-programming is the favorable outcome.

Results: All the Sawbones models had a 30° actual coronal angulation. The Orthex software estimated the deformity at around 25.35° (SD 4.6), TSF 25.6° (SD 2), and TL-HEX 29.87° (SD 2.1). One-way analysis of variance (ANOVA) showed a significant difference in the findings (P-value 0.014). Accuracy was measured by comparing the residual deformity in angulation in the coronal plane after the first and second correction. The Orthex median residual deformity was 1°, TSF was 2.5°, and TL-HEX was 3° with a range of less than 5° for all of them. The independent samples Kruskal-Wallis test shows that there is no significant difference between the three groups (P=0.549). The frequency of strut changes required throughout the correction was not significant among the three frames using the Fisher exact test (P=0.336). TSF struts are not designed to be readjusted.

Conclusion: The three frames were comparable in terms of accurate correction of the two deformities, strut changes, and strut adjustments. The TL-HEX frame software was superior to other frames in terms of analyzing the deformity but the difference, although statistically significant clinically, was not.