Prosthetic forefoot and heel stiffness across consecutive foot stiffness categories and sizes

Abstract

Prosthetic foot stiffness plays a key role in the functional mobility of lower limb prosthesis users. However, limited objective data exists to guide selection of the optimal prosthetic foot stiffness category for a given individual. Clinicians often must rely solely on manufacturer recommendations, which are typically based on the intended user’s weight and general activity level. Availability of comparable forefoot and heel stiffness data would allow for a better understanding of differences between different commercial prosthetic feet, and also between feet of different stiffness categories and foot sizes. Therefore, this study compared forefoot and heel linear stiffness properties across manufacturer-designated stiffness categories and foot sizes. Mechanical testing was completed for five types of commercial prosthetic feet across a range of stiffness categories and three foot-sizes. Data were collected for 56 prosthetic feet, in total. Testing at two discrete angles was conducted to isolate loading of the heel and forefoot components, respectively. Each prosthetic foot was loaded for six cycles while force and displacement data were collected. Forefoot and heel measured stiffness were both significantly associated with stiffness category (p = .001). There was no evidence that the relationships between stiffness category and measured stiffness differed by foot size (stiffness category by size interaction p = .80). However, there were inconsistencies between the expected and measured stiffness changes across stiffness categories (i.e., magnitude of stiffness changes varied substantially between consecutive stiffness categories of the same feet). While statistical results support that, on average, measured stiffness is positively correlated with stiffness category, force-displacement data suggest substantial variation in measured stiffness across consecutive categories. Published objective mechanical property data for commercial prosthetic feet would likely therefore be helpful to clinicians during prescription.

Low-profile prosthetic foot stiffness category and size, and shoes affect axial and torsional stiffness and hysteresis

“…Then, we fit linear and quadratic least-squares curves to the normal force-displacement and torque-angle data from the loading phases of the last three cycles for each prosthetic foot at the heel, midfoot, and forefoot. So that our results are comparable to previous studies that characterized Vari-flex prosthetic feet (the higher profile version of the LP Vari-flex prosthesis) ( 11 , 20 ), we calculated average axial and torsional stiffness values from the discrete value of the slope of the force-displacement and torque-angle curve from a minimum value of 50 N to 1.0 × body weight (BW) for the average body mass recommended for the moderate impact level ( Table 1 ). We averaged that value for the last three test cycles for each foot and test condition.…”

“…In general, we found that the heel and forefoot axial stiffness values of the LP Vari-flex prosthetic foot for a given stiffness category are stiffer than the higher profile Vari-flex prosthetic foot model. For example, the average axial stiffness value of the heel for a size 27 LP Vari-flex without a shoe ranges from 49.1 to 58.7 kN/m for categories 5–7, whereas the average axial stiffness value of the heel for a size 27 Vari-flex prosthetic foot without a shoe ranges from 37.5 to 45.4 kN/m in Turner et al ( 20 ) and 36.4 to 47.1 kN/m in Ruxin et al ( 11 ) for categories 5–7. Moreover, the average axial stiffness value of the forefoot for a size 27 LP Vari-flex without a shoe ranges from 36.2 to 47.0 kN/m for categories 5–7, whereas the average axial stiffness value of the forefoot for a size 27 Vari-flex prosthetic foot without shoes for categories 5–7 ranges from 29.1 to 38.5 kN/m in Turner et al ( 20 ) and 28.6 to 40.0 kN/m in Ruxin et al ( 11 ).…”

“…Prosthetists can compare our objective stiffness values to values reported for other prosthetic feet ( 11 ) rather than using manufacturer-defined categories that can be inconsistent or subjective. For example, the recommended stiffness category of the Vari-flex prosthesis for a 70 kg person with a moderate impact level is a category 4, which has an average stiffness of 35.7 kN/m at the heel and 26.7 kN/m at the forefoot for the size 27 prosthesis ( 20 ). If a prosthetist wants to prescribe an LP Vari-flex prosthesis of the same size with similar characteristics as the Vari-flex prosthesis, they could use the equations from Table 2 (average stiffness = intercept + B 1 × stiffness category + B 2 × size + B 3 × shoe/no shoe) to determine which category LP Vari-flex prosthesis has the same heel and forefoot stiffness as the Vari-flex prosthesis.…”

A passive dorsiflexing ankle prosthesis to increase minimum foot clearance during swing

“…The PDAP mechanism is paired with a custom-compliant low-profile foot designed using the methods described in Bartlett et al ( 2022b ). The foot was selected to have a comparable stiffness to prosthetic feet prescribed at the K3 activity level (Turner et al, 2022 ). The device (including compliant foot) has a mass of 620 g and a build height of 8.5 cm, which is comparable to many commercially available ESR feet (Bartlett et al, 2022b ).…”

Characterizing the mechanical properties of low-profile prosthetic feet

“…These values will inform dynamic function and can be implemented in future prosthetic design. Previous studies have characterized the mechanical properties of passive-elastic prosthetic feet (9)(10)(11)(12)(13)(14)(15)(16)(17)(18) and found that they are well described by linear (11,14) or curvilinear (9,13,(15)(16)(17) force-displacement and torque-angle profiles, which are used to calculate axial (kN/m) and torsional (kN*m/rad) stiffness values. These studies provide axial stiffness values for forces applied to a prosthetic foot heel, midfoot, and forefoot.…”