Changes in Tissue Composition and Load Response After Transtibial Amputation Indicate Biomechanical Adaptation

Abstract

Despite the potential for biomechanical conditioning with prosthetic use, the soft tissues of residual limbs following lower-limb amputation are vulnerable to damage. Imaging studies revealing morphological changes in these soft tissues have not distinguished between superficial and intramuscular adipose distribution, despite the recognition that intramuscular fat levels indicate reduced tolerance to mechanical loading. Furthermore, it is unclear how these changes may alter tissue tone and stiffness, which are key features in prosthetic socket design. This study was designed to compare the morphology and biomechanical response of limb tissues to mechanical loading in individuals with and without transtibial amputation, using magnetic resonance imaging in combination with tissue structural stiffness. The results revealed higher adipose infiltrating muscle in residual limbs than in intact limbs (residual: median 2.5% (range 0.2-8.9%); contralateral: 1.7% (0.1-5.1%); control: 0.9% (0.4-1.3%)), indicating muscle atrophy and adaptation post-amputation. The intramuscular adipose content correlated negatively with daily socket use, although there was no association with time post-amputation. Residual limbs were significantly stiffer than intact limbs at the patellar tendon site, which plays a key role in load transfer across the limb-prosthesis interface. The tissue changes following amputation have relevance in the clinical understanding of prosthetic socket design variables and soft tissue damage risk in this vulnerable group.

Keywords: Infiltrating adipose; Magnetic resonance imaging; Muscle atrophy; Remodelling; Transtibial amputation.

Figure 1

(a) Measurement sites on the right lower limb, each of area 50 × 50 mm; (b) 3D printed indenter positioned at each measurement site via adhesive fixation ring, enclosed by a pressure cuff with the limb in the supported test position (middle) and MRI test set up prior to imaging (bottom), (c) Timeline of the MRI test protocol.

Figure 2

Image processing steps applied to the axial MRI fat-only slice of the lower limb at the posterior calf measurement site, showing (a) original image, and (b) after binarization and masking. (c) superficial adipose mask (yellow) and muscle-infiltrating adipose mask (red) superimposed over the corresponding opposed-phase image at same slice, and (d) superimposed outlines of limb under uninflated cuff baseline (solid line) and 8 kPa inflated cuff (dashed line) conditions. Example measures are shown for calculating the displacement and gross strain arising from cuff inflation between the posterior calf indenter to the nearest bony prominence, uninflated (d₀) and at 8 kPa inflation (d₈).

Figure 3

Exemplar transverse MRI slices in the calf with superficial adipose (yellow) and adipose infiltrating muscle (red) tissue overlays. Images represent the right control limb of ten participants without amputation (left columns #1-10), and both the control (C) and residual (R) limbs for ten participants with transtibial amputation (right columns, #1A-#10A).

Figure 4

Median, interquartile range (IQR) and range in percentage of tissue constituents of the overall limb, in a 60mm segment distal from the tibial plateau. + indicates outliers; * indicates significance at p ≤ 0.05; ** indicates significance at p ≤ 0.01.

Figure 5

Positive correlations were observed between residual limb and contralateral limb superficial adipose (a) and infiltrating adipose (b). Negative correlation was observed between percentage volume of infiltrating adipose tissue in contralateral limbs and estimated daily socket use (c), though no correlation was seen between contralateral limb infiltrating adipose and time since amputation (d). Number indicates participant ID.

Figure 6

Median, interquartile range (IQR) and range of lower limb soft tissue deformation under 60mmHg pressure cuff loading for all participant groups. * indicates significance at p ≤ 0.05 and ** indicates significance at p ≤ 0.01.

Figure 7

Mean values of tissue structural stiffness at three measurement sites on the right control limb of eight participants without amputation and both contralateral and residual limbs of ten participants with unilateral transtibial amputation. * indicates p ≤ 0.05.