Biomechanical assessment and electromagnetic intervention for diabetic ulcers

Abstract

Diabetic foot ulceration is a major risk factor for lower limb amputation. Novel assessments and treatments are therefore demanded for better management of diabetic foot ulcers in clinical settings. The present thesis hypothesizes that the novel biomechanical assessment can validly and reliably reflect the tensile properties and underlying collagen of diabetic ulcers whereas the pulsed electromagnetic field intervention can effectively exert changes in diabetic wounds in cellular, molecular and biomechanical aspects. An optical coherence tomography-based air-jet indentation system was recently developed to objectively and quantitatively measure the indentation stiffness of the biological tissues in a non-invasive and non-contact manner. In the first study of the present thesis, a parallel comparison was made between the measurement of indentation stiffness by the indentation system and various tensile properties by conventional ex vivo tensile testing over 3 weeks in a biopsy punched wound model at the hindlimbs of streptozotocin-induced diabetic Sprague-Dawley rats. The relationship between the indentation stiffness and collagen visualized by picro-sirius red staining was also explored. The present study showed that the indentation stiffness was significantly negatively correlated to the tensile properties and the abundance and alignment of collagen fibres in different phases of diabetic wound healing. Previous studies demonstrated that pulsed electromagnetic field (PEMF) could accelerate wound closure, increase myofibroblast population and promote biomechanical strength in diabetic wounds. In the second study of the present thesis, we investigated whether the increased myofibroblast population induced by PEMF was associated with enhanced collagen deposition and alignment stained by picro-sirius red in square wounds inflicted at the back of diabetic rats. A significant increase in type I collagen fibres was observed in the PEMF group on day 7, but not day 10 or 14 compared to the control group. The abundance of type I collagen fibre was significantly positively correlated with the myofibroblast population on day 7. No significant between-group differences were found in collagen fibril alignment and collagen fibre orientation at any measured time points. In the third study of the present thesis, we further examined whether PEMF delivered at different intensities might improve the tensile properties of diabetic wound healing using the biopsy punched wound model at the hindlimbs of diabetic rats. The present findings demonstrated that the PEMF delivered at 10mT could increase the energy absorption capacity of diabetic wounds in the early healing phase. PEMF, however, seemed to reduce the maximum stress and Young's modulus in the remodelling phase. As chronic diabetic foot ulcers are frequently infected by Pseudomonas (P.) aeruginosa resistant to conventional antibiotic treatments, novel regimens are needed for controlling bacterial infections in chronic diabetic foot ulcers. In the fourth study of the present thesis, the effects of 5-mT PEMF (sham vs. 20-Hz vs. 72-Hz) in combination with 0 to 1 minimum inhibitory concentration (MIC) of gentamicin on the growth of P. aeruginosa was evaluated in terms of bacterial count at the baseline and after incubation. P. aeruginosa incubated with different MICs of gentamicin was exposed to sham or active PEMF for 10 hours, and then further incubated without PEMF for 14 hours (i.e. 24-hour incubation totally). Twenty-Hz PEMF significantly inhibited the growth of P. aeruginosa under 1-MIC gentamicin. However, under sub-MIC levels of gentamicin, both 20-Hz and 72-Hz PEMF appeared to promote bacterial growth.