Indirect measures

Several recent papers describe directly measured loading of prosthetic components. There are distinct benefits to such devices over conventional inverse dynamic analysis, and prosthetic studies have unique advantages with this technique. The approach is to use strain gauge pairs to collect values with varying relationships to planar forces and moments. Through careful alignment and calibration, direct assessment of the planar values for force and moment can be made at a point close to the prosthetic interface. Inverse dynamics relies on several assumptions relating to rigidity of components and joint dynamics that can have limited validity in prosthetics users, in additional to the usual limitations of three dimensional gait analysis in amputees (Kent and Franklyn-Miller 2011).

Berme et al. (1975) described an instrumented method for measuring prosthetic forces, moments and shear forces at a point on the prosthesis as being developed in Strathclyde in the late 1960s. Their device is described using two levels of strain gauges to measure M/L (medial-lateral) and A/P (anterior-posterior) moments, and axial load and torque. Their design uses a tube/flange design: bending moment and axial gauges were bonded to the surface at least 15mm from either flange.

Sanders et al. (1997) identified three substantial uses for a six-directional transducer: evaluating and designing components, use in finite element analysis and as a prosthetic fitting tool. Each kind of investigation is benefitted by a thorough assessment of the forces and moments present during different actions. This group developed a prosthetic-specific load cell: this consisted of two rings connected by three beams.

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During testing, moment values were found to be more reliable than force values, with axial force being underestimated. The system was initially tested clinically with a single subject. Issues with the design that were identified include the possibility of strain gauge

misalignment and hence increased cross-talk. Additional development to cope with more strenuous activity was also suggested.

The thesis by Boone (2005) describes efforts in measuring socket reaction moments. The device described measures axial forces and sagittal and coronal moments. This system is not able to measure transverse moments, and these were neglected as the preliminary results indicated these were ~100 times smaller than the other components. These

moments as measured near the base of the socket about an origin that is on the axis of the prosthetic shank, and collinear with the centre of the angular measurement device. Overall error was less than 3% for moments in the sagittal and coronal planes.

A study by Frossard et al. (2003) describes a wireless force and moment sensor used with a transfemoral amputee during activities of daily living. The authors identify the issues with inverse dynamics – namely the limits imposed by a lack of information about the inertial aspects of the residuum and the prosthesis and the compromises in modelling the system using traditional models. The difficulties in measuring conditions other than flat walking were also considered. Furthermore, they concluded that direct kinetic devices are unsuitable for all subjects due to the length of the instrumentation required.

The system was tested on a single transfemoral amputee, measured walking on flat, sloped ground and stairs. The authors reported that there were several applications for this design, specifically in the design and testing of prosthesis components.

Dumas et al. (2009) published a study of inverse dynamic modelling in the measurement of forces and moments with direct measurement. Maximum RMS errors were 56N and 5 Nm, and described as reasonably small. The authors state that the study demonstrated the typical errors seen in 3D measurement of prosthetic components. Differences in the modelled joint geometry may also be responsible for some errors. Improved classification of results was completed by the same team using the same set-up (Frossard and Stevenson 2011). Activity was divided into directional locomotion, localised locomotion, stationary loading and inactivity.

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The same transducer was used in a study of transtibial amputees (Neumann et al. 2012). The device was mounted rigidly between the pylon and the socket, and in a test of the resultant force was within two percent of the magnitude as measured by a force platform. The authors concluded that transducer obtained patterns were able to detect differences in components, activities whilst eliminating many of the characteristic deficiencies of gait lab studies. The same team used this system to examine the loading due to transverse plane moments on the residual limb during flat and curved path walking (Neumann et al. 2013b). Curved path walking is difficult to assess using gait laboratories and inverse dynamics due to the problems of placing and hitting measuring equipment.

Socket moment impulse was used as an outcome measure in a study of alignment

alterations, defined as the area under the socket reaction moment curve during stance, as measured using a Smart Pyramid in 10 transtibial amputees (Kobayashi et al. 2014a). The authors discussed the use of moment impulse in analysis of alignment: that the goal is not to minimise but to normalise this outcome measure. For example, they measured lower extension moment impulse with a high degree of anterior misalignment than in the

nominal position. They suggest that acceptable limits may exist for assessment of adequate alignment.

The authors completed a further study on the effect of alignment changes, this time in ESR prosthetic feet (Kobayashi et al. 2014b).This study also used a Smart Pyramid. Footsteps from each trial were normalised to body mass and averaged for 25 different component configurations. Additional reports (Kobayashi et al. 2014c) measured the effect of random perturbation in both sagittal and coronal planes, using the same device and processing.

Socket reaction moments were used in a study to perform dynamic alignment of transtibial prostheses (Kobayashi et al. 2015) – direct kinetics were measured in a range of angular and translational changes in socket position, and found to be sensitive to these differences. However, the difficulty in interpreting kinetic changes when these were linked to the alteration in gait kinematics was highlighted: as were the issues in supplying an acceptable alignment when a range of these values may exist.

A new load cell, the ‘iPecs’ was validated in a 2014 study (Koehler et al. 2014). The system was tested on a single transtibial subject, and compared to an inverse-dynamics model.

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Mean RMS errors were 3.4% for force measurements and 5.2% for moments at full scale output. The authors concluded system was a comparable alternative to force platform measurement and that direct measurements may produce lower errors at proximal joints than inverse dynamics.

The results have direct relevance to transtibial socket mechanics, and demonstrate systemic changes to within-socket biomechanics from changes in prosthetic component alignment. The results published also indicate the advantages of this measurement over inverse dynamics, in that longer collection sessions, with a wider range of test conditions and more directly relatable results are possible. However, studies by this group failed to present ground reaction force measurements, thus understanding the impact on direct kinetics is difficult to evaluate.

The majority of studies reported socket moments about the geometric centre of the socket. Although these measures seem sensitive to alterations in device configuration, the ability of such techniques to provide understanding of the conditions within the stump socket

interface is limited. Pressure values vary between locations on the stump, and so the measurement of these changes in socket loading may be of limited use in quantifying the quality of socket fit as opposed to changes in broader aspects of the device.