Design and development of a medical device for treatment of proximal interphalangeal joint contracture

The proximal Interphalangeal (PIP) joint is vital for hand function. It is frequently affected by contractures as a result of injury or disease. Dynamic external fixators are one of the useful treatment methods of these contractures, enabling the ability to exercise adjacent joints during healing. The aim of this study was a development of the Compass hinge device to present a new design device for the digit PIP joint. A failed Compass hinge external fixator has been analysed. The device consists of polymer parts manufactured from polyetherimide (PEI). Finite element analysis (FEA) was used to investigate the principal stresses in the device under different loading conditions. Scanning electron microscopy (SEM) was used to investigate the fracture surfaces. The FEA showed that the maximum principal stress was greater than the fatigue strength of PEI. The SEM fractographs confirm that failure was by brittle fatigue. A new finger fixator named the PIP joint protractor hinge device comprises 17 parts which are assembled together. The proposed device materials consist of Poly-ether -ether-ketone (PEEK) and stainless steel 316LS. The design was subjected to FEA and a working model was manufactured and subjected to cyclic mechanical testing. The FEA showed that the maximum stress was 242.9 MPa and this was less than the yield strength and the fatigue endurance limits for the selected materials. Mechanical testing showed that testing reached run-out of 170,000 cycles with no cracks or damage visible in the device parts.