Role of Mechanical Loading in the Aetiology of Deep Tissue Injury

Abstract

Prolonged mechanical loading of soft tissues overlying bony prominences, as experienced by bed-bound and wheelchair-bound individuals, can lead to damage in the form of pressure ulcers (PU). One type of PU is initiated in muscle tissues and progresses outward towards the skin layers. The treatment of this condition, termed Deep Tissue Injury (DTI), is necessarily complex and leads to a variable prognosis. The aetiology of DTI involves a number of factors, each triggered by mechanical loading, involves cell and tissue deformation, ischaemia, ischaemic-reperfusion injury and impaired interstitial and lymphatic flows. The present work examines the relative contributions of these factors, by applying different mechanical loading regimens to a well-established animal model [1]. A series of experiments were conducted, each of which involved indentation of the tibialis anterior muscle of the left hind limb of Brown Norway rats. The integrity of this muscle was monitored during both loading and unloading periods with T-2 weighted Magnetic Resonance Imaging (MRI) and muscle perfusion as assessed by dynamic- contrast enhanced MRI. In addition, dedicated Finite Element Models were developed to estimate the local mechanical conditions within the muscle. Experiments involved both continuous and intermittent tissue deformation for 10 minutes and 2 hours, and continuous deformation for 4 hours, followed by an unloading period of 2 hours. Results indicated that, provided a specific damage threshold was exceeded, damage could be evident after 10 minutes of deformation and increased with time. In addition on removal of the mechanical deformation, reperfusion was not evident in all areas of the muscle. These findings suggest that the relative importance of the initiating factors for the aetiology of DTI are temporally-dependent, with deformation dominating at shorter loading periods. The results must be considered in the light of a prevention strategy for DTI and highlight the need to minimise internal tissue strains in those individuals in which prolonged loading is inevitable.