Effect of Sagittal Foot Alignment on Centre of Pressure Progression Velocity in Transtibial Prosthesis Users
The performance of dynamic prosthetic foot devices is influenced by both the functional design and alignment of the device in the prosthetic system. Although this is recognized, studies of the interaction of these factors are limited. One measure of performance in unilateral transtibial amputee gait is the progression of the centre of pressure (COP) throughout stance. Recently, Klenow et al., developed a method of quantitatively identifying the “dead spot” phenomenon (DSP), described as an interruption of the forward progression of prosthetic foot COP during stance in amputee gait, as measured by COP progression velocity. The aim of this study was to isolate the effects of sagittal alignment changes on three DSP measures in the rearfoot and forefoot. These were: 1) DSP Time – the duration of the interruption in forward progression, 2) DSP Magnitude – the minimum forward velocity during the interruption and 3) DSP Progression – the displacement of the COP during the interruption. A study was designed in which a cohort of patients representing a range of prosthesis users were assessed in a clinical setting at the time of alignment of their prosthetic system. A custom portable gait analysis system was fabricated and commissioned to analyse over-ground walking trials of eight unilateral transtibial amputees fitted with prescribed prosthetic devices. Rearfoot and forefoot DSP of the intact and affected limbs were analysed during pre-alignment and post-alignment conditions using the three DSP measures. DSP Progression was determined to be a clinically appropriate measure, capturing the effect of clinical alignment in the sagittal plane with the presence of observed DSP. Observed effects of alignment corresponded with clinical goals in forward progression and limb symmetry. Based on the analysis of aligned systems completed by the partnered prosthetist, recommendations of clinically significant values were determined to be 4mm of rearfoot DSP Progression and 3.5mm of asymmetry in rearfoot DSP Progression. Forefoot DSP Progression was observed to increase on average of 4mm in the intact limb. The recommended DSP values can be used as a clinician tool to confirm alignment changes and to help develop prosthetic foot prescription guidelines related to DSP.