Mechanical loading of bone-anchored implants during functional performance tests in service members with transfemoral limb loss

Introduction

For individuals with limb loss, bone-anchored implants create a direct structural and functional connection to a terminal prosthesis. Here, we characterized the mechanical loads distal to the abutment during several functional performance tests in Service members with transfemoral (TF) limb loss, to expand on prior work evaluating more steady-state ambulation on level ground or slopes/stairs.

Methods

Two males with unilateral TF limb loss and two males with bilateral TF limb loss participated after two-stage osseointegration (24 and 12 months, respectively). Tri-directional forces and moments were wirelessly recorded through a sensor, fit distal to the abutment, during six functional tests: Timed Up and Go (TUG), Four Square Step Test (FSST), Six Minute Walk Test (6MWT), Edgren Side-Step Test (SST), T-Test (TTEST), and Illinois Agility Test (IAT). Additionally, participants performed a straight-line gait evaluation on a 15 m level walkway at a self-selected speed (0.93-1.24 m/s). Peak values for each component of force and moment were extracted from all six functional tests; percent differences compared each peak with respect to the corresponding mean peak in straight-line walking.

Results

Peak mechanical loads were largest during non-steady state components of the functional tests (e.g., side-stepping during SST or TTEST, standing up from the ground during IAT). Relative to walking, peak forces during functional tests were larger by up to 143% (anterior-posterior), 181% (medial-lateral), and 110% (axial); peak moments were larger by up to 108% (flexion-extension), 50% (ab/adduction), and 211% (internal/external rotation).

Conclusions

A more comprehensive understanding of the mechanical loads applied to bone-anchored implants during a variety of activities is critical to maximize implant survivability and long-term outcomes, particularly for Service members who are generally young at time of injury and return to active lifestyles.

The effects of total ankle arthroplasty on postural stability and loading symmetry in quiet stance

Ankle osteoarthritis is a debilitating condition affecting about 1% of the population with approximately 50,000 new instances annually. One treatment is total ankle arthroplasty (TAA), however, its effects on balance are not well understood. This study analyzed balance over a two-year period following TAA. 408 subjects (177 left, 231 right ankles) diagnosed with end-stage ankle osteoarthritis performed quiet standing trials while center of pressure (COP) data were collected. Data were compared across three time points (pre-op, 1-year, and 2-years post-op) and between surgical and non-surgical limbs using a linear mixed model with significance set at P = 0.05. COP excursions in the feet-together condition were not significantly different between limbs after 2 years in anteroposterior or mediolateral directions (P = 0.06, 0.08) after being significantly different between limbs in the anteroposterior (P = 0.014) and mediolateral direction (P < 0.001) pre-op. The vertical ground reaction force significantly decreased across time in the non-surgical limb, while reciprocally increasing in the surgical limb (P < 0.001). After 2 years, no significant difference in vertical ground reaction force between limbs existed (P = 0.20). Limb asymmetry indices decreased at each time point in both conditions (all P < 0.001) and were not significantly different from zero after 2 years in the feet-together condition (P = 0.290). In conclusion, surgical limb balance improved compared to pre-op, resulting in increased symmetry between limbs after 2 years. Vertical ground reaction forces on both limbs converge and limb asymmetry indices approach zero two years post-op. Differences in the COP excursion-loading symmetry relationship between limbs could be useful for identifying instability in other pathologies.

Center of pressure profiles in unilateral compared to bilateral end-stage ankle osteoarthritis patients

Ankle osteoarthritis (OA) compromises ankle joint stability in regards to static balance. Unilateral and bilateral osteoarthritis patients often exhibit different limb-loading patterns during static balance tasks. Symmetrical loading has been posited to improve balance performance. Therefore, the purpose of this study was to quantify balance performance in both ankle osteoarthritis patient groups. Twenty-two unilateral and twenty-one bilateral ankle osteoarthritis patients performed three 10-s quiet standing trials with their feet together. Ground reaction force data were collected from force platforms with one under each foot. Center of pressure excursion in the anteroposterior and mediolateral directions as well as the resultant center of pressure were calculated using custom Matlab software. A 2 × 2 repeated measures ANOVA with Cohen's d were used to analyze the differences between groups (unilateral vs. bilateral) and between limbs (affected vs. unaffected) (α = 0.05). No significant differences were found between limbs or groups in either the anteroposterior or mediolateral direction for any measured variable. Though not statistically different, moderate to large effect sizes were observed for mean resultant distance between unilateral and bilateral (d = 0.096, d: 1.0) as well as anteroposterior excursion (p = 0.077, d: 1.731) and mean velocity (p = 0.084, d: 1.50) between affected and unaffected limbs. These large effect sizes suggested clinically relevant differences may exist, particularly in the anteroposterior direction. These findings may suggest that center of pressure is a better measure of postural strategy while center of mass measures may be more representative of postural steadiness. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2749-2754, 2017.