Melanomas and stress patterns on the plantar surface of the foot: A systematic review and meta-analysis

Abstract

Background: UV light exposure is a recognized causative factor in cutaneous melanoma. However melanomas can emerge in sun-shielded areas of the skin such as the plantar surfaces, and the pathogenesis of these melanomas are largely unknown. One hypothesis is that repetitive mechanical stress leads to the formation of melanoma on the plantar and subungual surfaces. We performed a systematic review and meta-analy-sis to investigate the link between shear stress and plantar melanoma. Methods: Five electronic databases were systematically searched from their inception to June 2019, in order to identify all studies which explored stress patterns of the foot and melanoma. The data was extracted by two reviewers, and the study quality assessed through the appraisal tool for Cross-Sectional Studies (AXIS), the Joanna Briggs Institute (JBI) 2017 Critical Appraisal Checklist for Case Series, and the Newcastle-Ottawa Scale for cohort studies. Studies which included images of the melanoma sites on the foot were digitally merged to create "foot maps" of the pooled plantar melanoma locations. We used a random-effects model for data analysis based on event rates (ER) and 95% confidence interval (CI). Results: Seventeen studies with a total of 1,200 patients were included in the meta-anal-ysis. Plantarmelanomas were most often found inthe hindfoot ER: 50% (95%CI 42%-57%, I2 = 72.06% p=0.97), followed bythe forefoot ER: 27% (95%CI 23%-31%, I2 = 0.00% p<0.001) and midfoot ER: 20% (95%CI 15%-26%, I2 = 59.58% p<0.001). The digitally merged foot maps corresponded with mechanical stress points on the plantar surface of the foot. With the subungual melanomas, melanomas disproportion-ally affected the nail of the hallux even after accounting for surface area ER: 77% (95% CI 68%-84%, I2 = 65.22% p<0.001). Conclusions: Melanomas of the plantar foot disproportionally affected the hind foot and weight bearing areas, whilst subungual melanomas had a predilection towards the hallux. These findings suggest mechanical stress may increase the risk of melanomas in the plantar and subungual areas.