Photothermal treatment of cutaneous lesions
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis reviews the understanding of the processes involved in the laser treatment of cutaneous blemishes. The current treatment protocol for the treatment of vascular lesions - double scanning with transient blanching used at St George's Hospital is shown to give excellent results. The protocol takes advantage of the precise control provided by the SCANALL automatic scanner and the 5 W, 578 nm output of the copper vapour laser. The clinical endpoint - transient blanching - is shown to be due to a temporary halting of blood flow (probably by vasoconstriction) rather than coagulation necrosis of overlying tissues. Various models of the laser treatment of vascular lesions are presented and examined. A histological study of the double scanning, transient blanching protocol shows that tissue damage is confined to vascular and perivascular tissue. Cosmetic lightening is due to a reduction in both the number and size of the vessels in the upper dermis. The protocol is also investigated by interview and postal survey. The incidence of adverse effects is small. For example, there are only two 1 cm² adverse skin texture changes in 64000cm² of treated area. Patients receiving treatment for telangiectasia and spider naevus are satisfied with the outcome after one or two treatments, but many with port-wine stain cease treatment after four sessions when government funding runs out. Patient perception of the success is compared with the surgeon's perception. Patients often needed to be reminded of the size and severity of their original lesion with a photograph. The thesis reports on a parallel investigation of the use of millisecond scale pulses of white light for the treatment of tattoos. A xenon flash-lamp system is designed, constructed, and used in a clinical trial. This includes building pulse forming networks to produce rectangular current pulses of differing lengths. During the clinical trial the system produced a strong inflammatory response in the skin adjacent to the pigment, and lightening of the tattoo. Modelling, histology and other literature studies lead to the conclusion that the pulse length is too long to cause the explosive rupture of pigment-containing cells observed after Q-switched laser treatment, and too short to cause sufficient necrosis and phagocytosis of the pigment-containing cells for it to be useful clinically. The thesis also describes the construction of a device to measure muscle tension during tendon transfer surgery. The device uses diffraction to measure the separation of the fundamental unit of muscle tissue - the sarcomere.