Background: Construct stability is a necessary characteristic of external fixators. Many commonly used fixators are constructed as symmetric one-plane frames. We postulate that asymmetric two-plane triangular constructs provide enhanced stability with simplicity and freedom in pin placement. We hypothesized that results of finite element analysis would determine optimal geometric configuration, and findings of mechanical testing would confirm the improved stability of two-plane triangular constructs.
Methods: Finite element modeling was used to analyze configurations for 16 triangular designs compared to a single-rod (SR) uniplanar frame. Variables included pin axial angulation (0°, 22.5°, 45°, and 90°), connectivity of the rod-to-pin couplers, and intrafragmentary pin spacing (75mm or 100mm). Construct stiffness and interfragmentary displacement were analyzed for model selection. In a subsequent experimental test, nine synthetic composite tibiae were displaced to a maximum of 4 mm, comparing compressive load and axial stiffness of triangular construct with those of SR and pin-clamp (PC) uniplanar frames.
Results: Computational modeling showed that greater pin spacing results in increased stiffness (P < 0.001), but increased interfragmentary displacement (P =0.01). 22.5° and 45° constructs were significantly stiffer than 0° constructs (P = 0.03 and P = 0.01, respectively). Displacement was significantly less in 22.5°, 45°, and 90° than 0° constructs (P = 0.01). Experimentally, the 22.5° triangular multiplanar constructs were significantly stiffer with higher compressive loads than uniplanar constructs (P < 0.001).
Conclusions: A two-plane triangular frame may be a more stable construct than the two symmetric uniplanar constructs tested. This configuration allows for greater adjustability than SR constructs, requiring no specialized devices as do PC constructs, while allowing simplicity and freedom of pin placement.
Salas, Christina; Daniel Hoopes; Mahmoud M. Reda Taha; and Thomas A. DeCoster. "External Fixation for Treating Tibial Shaft Fractures Using a Triangular Two-Planar Frame: A Computational and Biomechanical Study." UNM Orthopaedic Research Journal 6, 1 (2017). https://digitalrepository.unm.edu/unm_jor/vol6/iss1/25