A device to measure motion between the distal end of a residual limb and a prosthetic socket was developed and evaluated. The resolution of the LSM device was greater than the measured displacements indicating that the system could provide accurate measurements of the motion between the limb and the socket. In addition, the device was inexpensive, simple to manufacture, and the data could be reduced using readily available software.
Movement of the distal portion of the residual limb relative to the prosthetic socket was less than 5 mm in either direction regardless of suspension design. This motion is very small compared with ∼40 mm of SI motion recorded during gait.10 In addition, the motion between the residuum and the prosthesis generally increased and decreased with pedal forces, suggesting that movement of the distal limb within the socket was related to load, for example, task mechanics. The smaller displacements measured with the LSM device during cycling compared with walking is likely due to the differences in the task. Forces parallel with the tibia during cycling are ∼30% body weight15 versus ∼104% during normal gait.16 The pin suspension displayed a trend toward less SI motion that seems to support, in part, our second hypothesis, but more data are needed to more fully explain these results.
The effect of motion between the distal end of the residual limb and the prosthetic socket will have a minimal effect on the calculation of joint moments during cycling. The relatively small amount of movement recorded is similar to the accuracy of a typical multicamera motion capture system.17 In addition, an uncertainty and sensitivity analysis that demonstrated the uncertainty in the calculation of joint moments is related mostly to pedal forces as long as error in the calculation of joint centers is less than 13 mm.18
The added mass of the LSM device did increase the moment of inertia but did not appreciably change the center of mass compared with a prosthesis without an LSM device. The mechanical properties of the prosthesis and LSM device were still less than the subject's intact limb. The potential interaction effect of inertia on the measured motion seems to be minimal, but this cannot be determined with the limited data collected.
This device was designed to measure gross movement of the residual limb within the prosthetic socket, while radiographic-based methods are able to measure the displacement of the tibia relative to the socket.7,8 The movement of the skin/liner relative to the tibia was shown to be ∼2.5 mm.8 This movement of the skin relative to the tibia would, in effect, increase the displacements measured by the LSM device to ∼6 mm. However, this increase in displacement would still have a minimal influence on the calculation of joint moments.18
The position of the knee joint relative to the prosthetic socket was not measured with the LSM device. Anterior/posterior translational movement of the knee relative to the prosthetic socket was noticed visually during data collection and is a common compliant among cyclists with amputation.19 Therefore, it may be more appropriate to concentrate on knee motion relative to the prosthetic socket for future work and treat the intersection of the distal end of the residual limb and the prosthetic socket as a pseudojoint. This would allow for the calculation of angular movement between the residuum and the prosthetic socket using a motion capture system by placing a marker at this point on the socket and another marker at the knee center. However, the lateral superior wall of the prosthetic socket may interfere with placement of the marker on the knee center. This portion of the prosthetic socket may be removed to allow space for a knee center marker during cycling research, because this portion of the prosthetic socket is not necessary for cycling. The lateral superior border of a prosthetic socket is designed to resistance movement in the frontal plane during gait,20 yet the forces and movement in the frontal plane during cycling are much lower than gait.21
The LSM device could measure limb pistoning with high resolution in two dimensions and produced consistent results during a dynamic task. The device could measure differences between two different prosthetic suspension systems, but these differences were very small. The relatively small amount of movement measured during cycling is within the typical error allowed for motion capture systems, thus would not increase error associated with joint moment calculation. Therefore, it seems reasonable to assume that the intersection of the distal residuum and the inferior portion of the prosthetic socket could be treated as a pseudojoint. Future research may then address angular movement between the residuum and the prosthesis using motion capture and reflective markers on the prosthesis and the knee center. The LSM device may also be used to measure the displacement of the distal end of the residuum during gait.
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