Design of monolimb using finite element modelling and statistics-based Taguchi method.

Background: Monolimb is a transtibial prosthesis having the socket and shank molded into one piece of thermoplastic material. If properly designed, the shank of a monolimb can have a controlled deflection during walking which simulates the ankle joint motions to some extent. However, there is no clear guidance for the design of monolimb considering the dilemma between shank flexibility and structural integrity.

Methods: Finite element analysis was used to simulate structural tests based on ISO10328 on monolimbs of different configurations. Statistics-based Taguchi method was employed to identify the significance of each design factor in controlling the deformation and stress within monolimbs. The design factors considered were the thickness of the thermoplastics, anteroposterior and medialateral dimensions of the elliptical shank, and depth of the posterior seam line. By progressively fine-tuning the design factors, the monolimb configuration was optimized giving offering appropriate flexibilities of the shank and would not structurally fail in normal uses. Experimental structural test was used to validate the finite element model.

Findings: Anteroposterior dimension of the shank was shown to be the most important design factor determining the peak von Mises stress values, deformation and dorsiflexion angles of monolimbs. Depth of seam line appears much less important than the other three factors. A monolimb fulfilling the design requirements was suggested. Experimental test results reasonably matched with the finite element results.

Interpretation: Finite element analysis and Taguchi method was shown to be an effective method in optimizing the structural design of prostheses. Further prosthetic design can be facilitated based on the degree of importance of the design factors on the structural behavior of the prosthesis. Gait analysis of amputees using the suggested monolimb design is needed in the future.