Design of splints based on the NiTi

Background

The proximal interphalange joint (PIP) is fundamental for the functional nature of the hand. The contracture in flexion of the PIP, secondary to traumatisms or illnesses leads to an important functional loss. The use of correcting splints is the common procedure for treating this problem. Its functioning is based on the application of a small load and a prolonged stress which can be dynamic, static progressive or static serial.
It is important that the therapist has a splint available which can release a constant and sufficient force to correct the contracture in flexion. Nowadays NiTi is commonly used in bio-engineering, due to its superelastical characteristics. The experience of the authors in the design of other devices based on the NiTi alloy, makes it possible to carry out a new design in this work - the production of a finger splint for the treatment of the contracture in flexion of the PIP joint.

Methods

Commercial orthosis have been characterized using a universal INSTRON 5565 machine. A computational simulation of the proposed design has been conducted, reproducing its performance and using a model "ad hoc" for the NiTi material. Once the parameters have been adjusted, the design is validated using the same type of test as those carried out on commercial orthosis.

Results and Discussion

For commercial splint the recovering force falls to excessively low values as the angle increases. Angle curves for different lengths and thicknesses of the proposed design have been obtained, with a practically constant recovering force value over a wide range of angles that vary between 30° and 150° in every case. Then the whole treatment is possible with only one splint, and without the need of progressive replacements as the joint recovers.

Conclusions

A new model of splint based on NiTi alloy has been designed, simulated and tested comparing its behaviour with two of the most regularly used splints. Its uses is recommended instead of other dynamic orthosis used in orthopaedics for the PIP joint. Besides, its extremely simple design, makes its manufacture and use on the part of the specialist easier.

Background

The proximal interphalange joint (PIP) is fundamental for the functional nature of the hand. It is considered to be the functional epicentre, since 85% of total encompassment when an object is grasped depends on this joint [1]. The contracture in flexion of the PIP, secondary to traumatisms or illnesses leads to an important functional loss.
The use of correcting splints is the common procedure for treating this problem. Its functioning is based on the application of a small load and a prolonged stress which can be dynamic, static progressive or static serial [2]. Despite the force applied being small and progressive the neighbouring joints should be evaluated before its use in patients with systematic illnesses, as the splints increase the stress on the joints and can cause finger edema [3]. This progressive application of forces on the PIP joint stimulates the histic changes, which enable the elongation of the capsuloligamentous structures until the correction of the deformity is achieved [4].
The straightening forces developed by static splints were analysed by Wu [5] and those of the dynamic splints by Fess [6]. Both systems base the biomechanical action on the application of three parallel forces. Later analysis [3] consider that the force released by both systems is similar, and present the pressure exerted on the back of the damaged joint (PIP), greater in the static systems, as the main inconvenience of both [3].
The materials used in both types of splints are to a great extent thermoplastics, which in the short term suffer a change in resistance [7]; New materials such as neoprene have been proposed by other authors [8], although with this proposed model the inconvenience of covering all of the finger arises, something which can generate the edema and be counterproductive.
Considering the effectiveness of the two systems, good results have been published for both [9-12]. However it is fundamental to know the biomechanics of each system in order to produce personalised devices adapted to the characteristics of each patient [3].
It is important that the therapist has a splint available which can release a constant and sufficient force to correct the contracture in flexion. Nowadays NiTi is commonly used in bio-engineering, due to its superelastical characteristics and its shape memory [13]. The experience of the authors in the design of other devices based on the NiTi alloy [14-17], makes it possible to carry out the proposed design in this work - the production of a finger splint for the treatment of the contracture in flexion of the PIP joint.
This paper describes the characteristics of the splint designed, comparing its biomechanical behaviour with that of commercial dynamic splints regularly used to treat the stiffness of the PIP joint.

Methods

The first step consisted in characterising the biomechanical properties of the splints that were to be used as reference. Two of the most regularly used splints have been chosen: the LMB Spring Finger Extension Splint (splint 1) and the LMB Spring-Coil Finger Extension Splint (splint 2).
We are dealing with two simple designs which basically consist of a torsion spring with two angled arms which make it possible to fix and lock onto the finger (Figs. 1a and 1b). The spring restoration torque is the origin of the forces applied to the extremes of the splint, which are balanced with the reaction in the central section (Fig. 1c).