Morphological variability of the fibularis longus tendon in human fetuses

The morphological variability of the fibularis longus tendon (FLT) in adults is well understood. However, no comprehensive classification exists in human fetuses. The goal of this study was to prepare the first comprehensive classification of the fibularis longus tendon based on its insertion in human fetuses.

Forty-seven spontaneously-aborted human fetuses were examined: 38 male, 56 female, a total of 94 lower limbs (Central European population). Age ranged from18-38 weeks of gestation at death.

The classification comprised three types of FLT. The most common type was Type I (49%), characterized by the single distal attachment. This type was divided into two subtypes (A-B): A - the tendon inserts to the lateral tubercle of the base of the 1st metatarsal bone, B - the tendon inserts to the head of the 1st metatarsal bone. The second most type was Type II, characterized by a bifurcated distal attachment (24.5%). This type was divided into three subtypes (A-C): A - the main tendon inserts to the lateral tubercle of the base of the 1st metatarsal bone and the accessory band inserts to the medial cuneiform bone; B - the strong, main tendon inserts to both the base of the 1st metatarsal bone and medial cuneiform bone, including the first metatarsal-cuneiform joint, and the accessory bands inserts to the fourth interosseus dorsalis muscle; C - the main tendon inserts to the lateral tubercle of the base of the 1st metatarsal bone and the accessory band inserts to the first interosseus dorsalis muscle. The rarest type was Type III, characterized by a trifurcated distal attachment: the main tendon inserts to the lateral tubercle of the base of the 1st metatarsal bone and the first accessory band inserts to the medial cuneiform bone and the second accessory bands inserts to the first interosseus dorsalis muscle. The anterior frenular ligament was observed in 16% of all cases, and posterior frenular ligament in 6.4%.

The FLT displays high morphological variability. The proposed classification consists of three main types, with Type I and Type II divided into sub-types; it also provides additional data regarding its accessory tendon bands.