Digital Theses Archive


Tesi etd-12152017-145002

Type of thesis
Synergetic Partial Hand Prostheses: Design and Fitting
Scientific disciplinary sector
INGEGNERIA - Biorobotics
  • Biomechanics
  • Manufacturing
  • Myoeletric prosthesis
  • Powered Finger
  • Robotics
  • Sensory feedback
  • Synergetic Prehension
Exam session start date
Partial hand amputation is perhaps the most frequent amputation level, worldwide. Despite its incidence, treatments of partial hand amputations have modestly progressed so far, especially if compared to the remarkable advances in other body segments prostheses. This is due many limitations, such as the wide range of different clinical cases which makes difficult to find standardized and scalable solutions; the complexity in replacing the motor and sensory function of a lost digit in the size of a digit; the fact that partial hand fittings can be functionally successful mostly when they restore the ability to oppose the thumb against the fingers and, finally, the lack of modern engineering methods for prostheses manufacturing. <br><br><br>Amputees wearing myoelectric prostheses mostly rely on vision or on other sensory cues (e.g., motor sound) to regulate grasp with significant cognitive effort, especially when either the thumb or all fingers are missing. In contrast, it was recently shown that sensory feedback may reduce phantom limb pain while enhancing closed-loop controllability and embodiment of the prosthesis in amputees. <br>Another basic observation is that the majority of partial hand amputations retain close-to-normal use of either the thumb or of one (or more) of the long fingers. Hence, since the sound part is capable of providing enough force to grasp, the prosthesis should only be able to maintain/hold the grip against it while providing enough grip aperture and speed.<br><br><br><br>Building on these observations, a novel prosthetic finger, integrated with an unobtrusive vibrotactile sensory feedback system was designed. The mechanical transmission is alternative to the clinical available solutions. In particular the work included a new, compact transmission based on a face-gear pair coupled with a bidirectional non-back drivable roller clutch. The face-gear is a high efficiency, low ratio (2:1) reduction stage whereas the roller clutch makes the system non-back drivable without decreasing the efficiency of the overall transmission. In this way, the digit exhibits speed comparable to the commercial available prostheses and, although unable to apply large active forces, it is able to sustain large passive loads. This architecture was based on the synergetic prehension approach proposed by Childress and for that reason the finger was dubbed \textit{S-Finger} (i.e. synergetic finger).<br><br>The digit is equipped with a myoelectric controller and a tactile sensor able to provide users with discrete events sensory feedback. A clinical viability study of the prosthesis involving partial hand amputees was carried out at Inail Centro Protesi (Budrio). The subjects successfully completed several dexterity tests and gave relevant feedback for future developments. <br><br><br>In the last part of the dissertation, a method for the optimization of the finger placement and standardization of the fitting is proposed and investigated. The method is articulated into two steps. In the first stage the model focuses on the opposition capability of a prosthetic index finger and an unpaired thumb in a pinch grasp as function of the distance of the two fingers. The optimal posture is the one that allows the maximum control accuracy of the force along the direction of the grasp. The author argues that the proposed method could be used to predict the quality of the fitting and to standardize the process. The second step is the exploiting of the 3D CAD environment to develop partial hand prostheses fitting. In this environment, the alignment of thumb and prostheses can be assessed in a broader way, with the investigation of more grasp types and the simulation of the actual range of motion of prosthesis index and thumb. As outcome a geometry of the proposed socket is sketched and fitted on the solid model of the residual limb. Albeit preliminary, the method can be employed to assess the result of fitting in advance. More importantly it can pave the way for 3D-printed manufacturing of sockets, bringing innovation in a very handmade process.<br><br>