ABSTRACr The incidence of sarcoidosis in a clearly defined island community has been determined in a seven year study. Special efforts were made to identify all cases, including those presenting to their general practitioners with only minor symptoms. A mean incidence of 14.7 per 100 000 per annum has been found in the years 1977-83 compared with an annual incidence of 3.5 per 100 000 in the preceding 15 years, when no specific attempts were made to identify sufferers from sarcoidosis. The lower figure is similar to the reported incidence in the United Kingdom and the difference is considered to be due to increased detection of cases during the study period. These findings suggest that a large proportion of cases of sarcoidosis in a general population may not be recorded in a registration system.

SummaryThe authors present a case of a patient with sickle-β thalassaemia (Sβ 0 Thal) who had bilateral psoas abscesses on a background of splenectomy in early childhood. The patient also turned out to have vertebral osteomyelitis and hydronephrosis on the side of the larger abscess. The only organism recovered from the patient was a Bacteroides species. The patient was managed with percutaneous drainage and intravenous antibiotics and made a full recovery. BACKGROUND

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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

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