Background: In adults, the fibularis tertius (FT) demonstrates great morphological variation. The present study classifies the types of FT insertion in human foetuses and compares their prevalence to the prevailing classification among adults.
Materials and methods: Fifty spontaneously-aborted human foetuses (19 male, 31 female, 100 upper limbs in total) aged 18–38 weeks of gestation at death were examined. The foetuses were obtained from spontaneous abortion after parental consent. The study was performed in accordance with the legal procedures in force in Poland and with the Body Donation Programme for both adults and foetuses.
Results: The most common type of FT found was type VI (32%), characterised by a bifurcated distal attachment: a main tendon inserting onto the base of the fourth metatarsal bone, and accessory bands inserting onto the fourth interosseous space. Five other types were observed: type IV (20%), with a single tendon inserting distally to the fascia covering the fourth interosseous space; type I (18%), with a single tendon inserting distally onto the shaft of the fifth metatarsal bone; type V (14%), with a bifurcated arrangement comprising a main tendon characterised by a very wide insertion onto the base of the fifth metatarsal bone and an accessory band inserting onto the base of the fourth metatarsal bone; and type III (12%) with a single tendon inserting distally onto the shaft of the fourth metatarsal bone and fascia covering the fourth interosseous space. Finally, type II (4%) was characterised by a single tendon inserting onto the base of the fifth metatarsal bone via a very wide distal insertion.
Conclusions: The FT demonstrates high morphological variability, with the most common configuration found in adults — a single insertion onto metatarsal 5 — being actually uncommonly found in foetuses.