Investigating the mechanisms of soft tissue damage at the residual limb-socket interface

The residual limb tissues of an individual with below knee amputation form a critical loaded interface with the prosthetic limb. In the early stages of rehabilitation, residual limb tissues have not been conditioned to support loading and are vulnerable to damage. This impacts upon quality of life and can lead to rejection of the prosthesis. Bioengineers have established an array of measurements to understand the pathogenesis of soft tissue damage and assess multiple aspects of tissue tolerance during loading. However, to date, there is a scarcity of literature utilising these techniques to evaluate the residual limb-socket interface, resulting in a lack of evidence-based practice to prevent socket sores. A protocol for applying representative mechanical loading on lower limb tissues was developed with a cohort of volunteers without amputation. This involved incremental pressure application through a pneumatic cuff and an array of measurements before, during and after this loaded period to characterise the response of the underlying skin and soft tissues. The protocol was then applied to a cohort of participants with unilateral transtibial amputation. In order to evaluate intrinsic factors (soft tissue composition), Magnetic Resonance Imaging was used to visualise tissue composition and gross soft tissue deformation and a MyotonPROTM device was used to estimate tissue stiffness. Transcutaneous oxygen and carbon dioxide tensions were measured, and inflammatory biomarkers were collected at sites relevant to prosthetic load transfer, each of which reflected compromise to the skin tissues. MRI revealed increased adipose infiltrating muscle tissue in residual limbs (median 2.5 %, range 0.2 - 8.9 %) compared to intact limbs (median ≤ 1.7 %, range 0.1 - 5.1 %), indicating muscle atrophy post-amputation. This effect was reduced significantly in the contralateral limbs of those individuals with greater socket use (r = -0.88, p <0.01), indicative of adaptation post-activity. During prescribed loading, cuff pressure at the highest inflation of 60 mmHg resulted in mean interface pressures ranging from 66.2 - 83.6 mmHg. In the majority of cases, residual limbs displayed less compressive strain when loaded compared to intact limbs, the differences being statistically significant at a number of tested sites (median strains -6 to 2 % vs. 4 to 13 %, respectively). Cuff loading was observed to produce a transient compromise to tissue viability, reflected in a reduction in transcutaneous oxygen tension and an upregulation of inflammatory biomarkers, suggesting a degree of local ischaemia and inflammation, respectively. In most cases, reduced ischaemia and inflammatory biomarker upregulation was observed in residual limbs compared to intact limbs, suggesting enhanced tolerance to loading. Nonetheless there was considerable variation within the heterogeneous cohort of participants with amputation. These studies represent a first-of-kind evaluation of residual limb tissue tolerance to representative prosthetic loads involving tissue characterisation and physiological monitoring. This experimental approach could be implemented to identify individual susceptibility to tissue damage which, in turn, could help inform appropriate rehabilitation programmes to maintain health of tissue during prosthetic rehabilitation. Furthermore, development of these techniques into real-time portable measurements would support both prosthetic users and prosthetists to assess in-socket tissue health, leading to more informed management of residual limb tissues