Aim
UV light exposure is a recognized causative factor in cutaneous melanoma. However melanomas can emerge in sun-shielded areas 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-analysis 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. 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-analysis. Plantar melanomas were most often found in the hindfoot ER: 50% (95% CI 42%-57%, I2 = 72.06% p = 0.97), followed by the 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 disproportionally 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.