Associations Between Skeletal Alignment and Biomechanical Symmetry Before and After Transfemoral Bone-anchored Limb Implantation
Authors: Gaffney Brecca M. M. , Gimarc David , Thomsen-Freitas Peter B. , Pattee Jack , Wong Cassie , Milius Daniel W. , Melton Danielle H. , Christiansen Cory L. , Stoneback Jason W. 
Published: January 2025

Background
Patients with transfemoral amputation experience socket-related problems and musculoskeletal overuse injuries, both of which are exacerbated by asymmetric joint loading and alignment. Bone-anchored limbs are a promising alternative to treat chronic socket-related problems by directly attaching the prosthesis to the residual limb through an osseointegrated implant; however, it remains unknown how changes in alignment facilitated through a bone-anchored limb relate to loading asymmetry.

Questions/purposes
What is the association between femur-pelvis alignment and hip loading asymmetry during walking before and 12 months after transfemoral bone-anchored limb implantation?

Methods
Between 2019 and 2022, we performed 66 bone-anchored limb implantation surgeries on 63 individuals with chronic socket-related problems. Of those, we considered those with unilateral transfemoral amputation as potentially eligible for this study. Based on that, 67% (42 of 63) were eligible, a further 55% (23 of 42) were excluded because they had incomplete datasets either at baseline (such as an inability to ambulate with a socket prosthesis) or did not complete the 12-month follow-up data collection. This resulted in 19 participants being included in this retrospective longitudinal analysis (9 males and 10 females, mean ± age 51 ± 11 years, mean BMI 28 ± 4 kg/m2). As part of standard clinical care, hip-to-ankle radiographs and motion capture data during overground walking were collected at two timepoints: 2 days before (preimplantation) and 12 months after bone-anchored limb implantation (postimplantation). Femur-pelvis skeletal alignment was measured from the radiographs (femoral abduction angle, residual femur length ratio, and pelvic obliquity). Symmetry indices of hip internal hip moment impulses (flexion/extension, abduction/adduction, internal/external rotation) were calculated from the motion capture data. Differences in alignment and internal joint moment impulse symmetry indices were compared across timepoints using paired t-tests with self-selecting walking speed as a covariate. Associations between skeletal alignment and hip moment impulse symmetry indices were computed at both timepoints using Spearman rank correlation with 5000 bootstrapped resamples.

Results
Twelve months after bone-anchored limb implantation, a comparison of preimplantation and postimplantation measurements showed reductions in the femoral abduction angle (-8° ± 10° versus 3° ± 4°, mean difference 11° [95% confidence interval (CI) 7° to 16°]; p < 0.001) and the residual femur length ratio (57% ± 15% versus 48% ± 11%, mean difference -9% [95% CI -12% to -5%]; p < 0.001). Additionally, a comparison of preimplantation and postimplantation calculations showed that the internal hip moment symmetry was improved in the sagittal and frontal planes (flexion symmetry index: 30 ± 23 versus 11 ± 19, mean symmetry index difference -19 [95% CI -31 to -6]; p = 0.03; extension symmetry index: 114 ± 70 versus 95 ± 63, mean symmetry index difference -19 [95% CI -42 to 4]; p = 0.03; abduction symmetry index: -54 ± 55 versus -41 ± 45, mean symmetry index difference 13 [95% CI -15 to 40]; p = 0.03). A larger length ratio of the residual limb relative to the intact limb was moderately associated with hip moment impulse symmetry in all three anatomical planes of motions both before and 12 months after transfemoral bone-anchored limb implantation, with strong associations observed between postimplantation hip extension and external rotation moment impulse symmetry (extension: ρ = -0.50 [95% CI -0.72 to -0.07]; p = 0.03; internal rotation: ρ = 0.64 [95% CI 0.25 to 0.85]; p = 0.004).

Conclusion
The associations between residual femur length and hip loading symmetry in patients with transfemoral bone-anchored limbs suggest that those with shorter residual limbs will demonstrate more asymmetric joint loading when using a bone-anchored limb. Thus, these findings could potentially be used to better inform targeted interventions based on residual limb morphology, including continued gait training in rehabilitation to promote joint loading symmetry and surgical considerations surrounding limb length changes in those with shorter limbs. Future studies might also examine joint loading symmetry during other activities of daily living after bone-anchored limb implantation to further expand knowledge of how residual limb anthropometry is associated musculoskeletal health after bone-anchored limb implantation.

Level of Evidence
Level III, therapeutic study.