Current management of foot pain requires foot orthoses (FOs) with various design features (eg, wedging, height) and specific mechanical properties (eg, hardness, volume). Development of additive manufacturing (three-dimensional [3-D] printing) raises the question of applying its technology to FO manufacturing. Recent studies have demonstrated the physical benefits of FO parts with specific mechanical properties, but none have investigated the relationship between honeycomb architecture (HcA) infilling density and Shore A hardness of thermoplastic polyurethane (TPU) used to make FOs, which is the aim of this study.
Sixteen different FO samples were made with a 3-D printer using TPU (97 Shore A), with HcA infilling density ranging from 10 to 40. The mean of two Shore A hardness measurements was used in regression analysis.
Interdurometer reproducibility was excellent (intraclass correlation coefficient, 0.91; 95% confidence interval [CI], 0.64–0.98; P < .001) and interprinter reproducibility was excellent/good (intraclass correlation coefficient, 0.84; 95% CI, 0.43–0.96; P < .001). Linear regression showed a positive significant relationship between Shore A hardness and HcA infilling density (R2 = 0.955; P < .001). Concordance between evaluator and durometer was 86.7%.
This study revealed a strong relationship between Shore A hardness and HcA infilling density of TPU parts produced by 3-D printing and highlighted excellent concordance. These results are clinically relevant because 3-D printing can cover Shore A hardness values ranging from 40 to 70, representing most FO production needs. These results could provide important data for 3-D manufacturing of FOs to match the population needs.