(D-143) Comparison of prosthetic knee joint types in relation to slip risk

Unilateral transfemoral amputees are at an increased risk of falling due to a loss of balance [1,2,3]. Slips on level-ground are one of the most common reasons for reported falls in the transfemoral amputee (TFA) population [1,3]. Many gait and balance assessment studies, with TFA participants, have analyzed the biomechanical factors that are linked to fall risk [2]. While there exists a body of research assessing fall risk for TFA subjects in a laboratory setting [2], only a few studies have focused on slip perturbations and slip risk assessments [4,5,6,7].

Microprocessor-controlled knee (MPK) joints in comparison to passive mechanical or non-microprocessor-controlled knee (NMPK) joints have been shown to improve gait and balance, as seen in Kaufman et al. [8]. These improved results in stability show a reduced risk of falling for MPK joint users. The primary aim of this project is to examine known biomechanical variables associated with a greater risk of slipping during level-ground walking for TFA participants using both MPK and NMPK joints. Specifically, the required coefficient of friction (RCOF) at heel contact of the prosthetic limb will be analyzed. RCOF has been found to be an important variable for predicting slipping events with Beschorner et al. relating high peak RCOF values to a higher likelihood of slipping [9]. The results of this research may lead to a further understanding of how advanced technology in the field of prosthetics can be used to improve fall prevention for lower limb prosthesis users.

This report analyzes data collected by the Human Movement and Balance Laboratory at the University of Pittsburgh for a previous prosthetic knee joint gait analysis study. Six older-adult males with transfemoral amputations participated in the study (mean age 66.7 ± 6.7 years) which consisted of two laboratory gait assessments. Subjects completed the first gait assessment with the prosthetic knee joint type that they were using regularly prior to the visit (MPK or NMPK) since they were already acclimated to it. Within a one-week period of the first gait assessment, subjects returned to the laboratory to exchange their prosthetic knee joint type to the opposing knee joint type by a certified prosthetist. Following the exchange, an eight-week acclimation period was given before the second gait assessment was conducted in the laboratory.

A 14-camera Vicon T40S motion analysis system (Vicon, Oxford, UK) with a sampling rate of 120 Hz was used to record the movement of the subject as they walked across the 5.5 m long walkway. Two Bertec force plates (Bertec, Columbus, OH) were embedded within the walkway for the purpose of recording ground reaction forces at a sampling rate of 1080 Hz. Participants completed three to ten level-ground walking trials at a comfortable pace, attached to a safety harness to prevent any fall-related injuries at each visit.

The mean (standard deviation) RCOF for the MPK joint trials was 0.161 (0.051) compared to 0.192 (0.061) for the NMPK joint trials. Statistical analyses were performed in JMP (Version 16.0, SAS Institute Inc., Cary, NC). Significance levels were set at 0.05.

This research supports the literature in finding a greater potential slip likelihood for lower limb prosthesis users compared to able-bodied walkers. Previous studies have examined RCOF results for amputee participants and found notably high values when compared to controls [10,11]. Normal values for able-bodied subjects range from 0.17 to 0.20 as seen in Redfern et al. [12]. While the RCOF results from this study fall within the normal range, it is important to note that participants walked at a mean gait speed of 0.70 m/s during these walking trials. Normal gait speeds on level-ground range from 0.97 m/s to 1.51 m/s [12]. Considering the effect that slower gait speeds have on lowering RCOF and slip potential, the results from this research support the literature with abnormally high RCOF results found for amputee subjects. Durá et al. reported a mean RCOF value of 0.44 for amputee subjects walking on wet, level flooring [10]. If a liquid contaminant were used in this study, we would expect to see similar results to Durá et al. [10].

Further research is needed with a larger, more diverse sample size and a greater acclimation period to confirm these findings in relation to the general lower limb prosthesis user population. Additional known biomechanical parameters associated with slip-and-fall risk need to be investigated to further assess any differences in slip-and-fall likelihood between MPK and NMPK joints.  

Funding was provided by the Swanson School of Engineering and the Office of the Provost at the University of Pittsburgh.

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