Prosthetic fitting : stump-socket interaction in transtibial amputees

Abstract: This thesis is comprised of five different studies, all concerning the interaction between the transtibial amputation stump and the prosthetic socket. The aim was to analyse some of the factors influencing this interaction.CAD CAM (Computer Aided Design Computer Aided Manufacturing) and roentgen were used as measuring instruments. Errors of measurements of one CAD CAM system were evaluated. Volume deterrninations were performed on regular cylinder objects, and on amputation stump models with an irregular shape. For the cylindrical objects there was no statistically significant systematic error, but the volumes of the stump models were significantly overestimated with 2.5%. The random errors were small, with 95% of the random variation within ±1%. A clinical evaluation of a commercial CAD CAM system was performed, primarily from the patient's point of view. The only variable showing a significant difference between CAD CAM made and conventionally made prostheses was the number of terry cloth stockings necessary to compensate for volume differences between the stump and the socket. After operation, amputation stumps may change their volume considerably. For this reason patients should not be provided with expensive definitive prootheses until the post-amputation volume of the stump has stabilized. The volumes of amputation stumps were evaluated with CAD CAM for 160 days after operation in eleven transtibial amputees. The volume changes could be described by a negative power function, (γ = a · x2). From these results, proper time for a definitive prosthesis could be determined - three to four months after amputation. In order to obtain a prosthetic socket with correct volume and shape, an accurate and precise hand cast must be taken. One important question is whether the muscles of the amputation stump should be relaxed or contracted during the casting session. In this thesis is reported a volume difference between a relaxed and contracted amputation stump of approximately 5.8%, and 3.5% when the stump was donned with a silicon liner. During gait the tibia moves inside the amputation stump, and these movements may reflect the success of prosthetic fitting. Tibial displacements in the sagittal plane in a PTB (Patella Tendon Bearing) prosthesis were evaluated with radiographic technique. The average total tibial movement was 7.5 cm, the average movements in the anterio-posterior direction was 2.2 cm, and in the proximo-distal direction 2.8 cm.From the studies of this thesis can be concluded, that there are tibial movements inside a prosthetic socket, large enough to influence the prosthetic fitting; that the examined CAD CAM system had sufficient accuracy and precision for routine clinical practice in prosthetics; that CAD CAM technique had no major advantage compared with traditional technique from the patient's point of view; that the hand cast should be taken on a relaxed amputation stump; and that proper time for definitive prosthetic fitting after transtibial amputation was approximately three to four months after operation.