The Effects of Some Clinical Treatments on the Mechanics of Skin

Abstract

lntrod uct ionMeasurements of the mechanical properties of skin serve as a useful clinical guide to the state of the skin in health and disease. Monitoring these mechanical properties during treatments which are known or suspected to cause structural changes can indicate the treatment rCgime giving a maximum skin sparing effect. For instance, in the treatment of basal cell epitheliomas, the most common skin tumours, by radiotherapy a variety of treatment rtgimes are employed all of which obtain a cure rate over 90 per cent but have different long term effects on the skin. Apparatus ExtensometerAn extensometer that stretches the skin uniaxially at a constant rate and gives a simultaneous measurement of the force developed has been constructed for use on patients in uivo (Gunner et a1 1979a). The extensometer ( Fig. 1) has two flat rectangular tabs, attached to the ends of strain-gauged arms, that are stuck to the skin surface with double-sided adhesive tape. A motor driven lead screw incorporated in the apparatus drives one arm away from the other at a constant speed of 0.35 mm/s, the strain-gauged arms providing the force measurement. The distance between the tabs is measured by a 5 l R A l N GUACES \ L E A D S C R E W A N D C A R R I € R linear variable differential transformer (LVDT) transducer fixed to the body of the extensometer. The core of the transducer is attached to the moving arm. Signal conditioning for the strain-gauges and the LVDT transducer provides outputs that are fed to two channels of a UV recorder. Recoil apparatusInformation on the ability of the skin to recover after stretching is also of clinical interest. To measure this property in vivo, a recoil apparatus (Fig. 2) has been constructed (Gunner et a1 1979b). This has two flat rectangular tabs which are stuck to the skin surface with double-sided adhesive tape. One tab slides freely in a slot in the body of the apparatus, friction being minimized by a PTFE to perspex contact lubricated with graphite, while the other remains fixed to the body of the apparatus. The distance between the tabs is measured by an LVDT transducer. In use the tabs are pulled apart almost instantaneously, stretching the skin, and then released to obtain the recoil characteristic. Ultrasonic apparatus to determine skin thicknessIn order to obtain absolute values of mechanical parameters such as stress and elasticity the skin thickness must be determined; an ultrasonic pulse-echo system is