In this study, TPT was classified into seven types and four subtypes (A–D) according to the number of attachment sites. A previous study [7] reported four types and three subtypes (A–C), differing from the present. In addition, no significant difference by sex or laterality was seen between types, and we used 100 samples, representing a larger cohort than previous studies [5–7, 12]. The results of this study may thus have yielded different classifications from the previous studies because of differences in the numbers of samples, not factors of sex or laterality.
In this study, attachment to the NB, MCB, and LCB, collectively representing the main site of attachment, was observed in all specimens (100%). Furthermore, the surface area of these main attachment sites (NB, MCB and LCB) accounted for 75.1% of the total, and even among the different types, consistently accounted for more than 70% of the total regardless of the number of additional attachment sites. According to previous studies [5–7, 12], the main attachment sites of the TPT were still the NB, MCB, and LCB, but only the proportion of attachment sites was examined, and how much of the TPT was attached to each site was not examined. The quantitative results obtained in the present study confirmed the NB, MCB, and LCB as the main sites of TPT attachment, broadly supporting the findings of previous studies.
The TPT elevates the medial arch and inverts, adducts, and plantar-flexes the foot [14]. TPTD is the prevailing cause of AAFD, which is characterized by a collapse of the medial longitudinal arch [3, 15]. Swanton et al. [9] reported an extension onto the MCB from the anterior band, naming this as the “navicular cuneiform ligament”. This forms a static restraint between two bony insertions (NB and MCB) and increases the lever arm of the TPT. Gwani et al. [16] also clarified three relationships between the medial and lateral longitudinal arches and the lateral arch. Deformation of the medial longitudinal arch reportedly affects other arches. In addition, the lateral arch has been reported to comprise three cuneiform bones and the CB [8]. From those previous studies and the results of this study, attachment to the NB and MCB was considered related to the function of the medial longitudinal arch, while attachment to the LCB appears related to the function of the lateral arch.
Some limitations need to be considered when interpreting the findings from this study. First, since only Japanese cadavers were used, potential differences between different ethnicities were not examined. The existence of ethnic differences in foot muscles has been suggested in several papers [5, 17]. Caucasian individuals were found to be nearly three times more likely to show tendinopathic findings when compared to African-American individuals according to a study using ultrasound [18]. Future studies will therefore need to consider comparisons between different ethnicities to clarify the potential for variations in TPT attachment sites. Second, in type classifications for the TPT, only attachments to bone were considered. Previous studies have reported attachments to the abductor hallucis, flexor hallucis brevis, peroneus longus tendon, spring ligament, and plantar calcaneocuboid ligament [6, 7, 12]. Type classification thus needs to be performed with consideration of not only attachment to bone, but also attachment to muscles and ligaments.
In conclusion, the results of this study confirmed TPT attachments to the NB, MCB, and LCB in all specimens, and the surface area of these attachment sites occupied 75.1% of the total attachments to bone. Attachment of the TPT to the NB, MCB, and LCB may thus provide the primary contribution to the stability of the foot arch. In the future, comparisons between races and classification in consideration of attachments to muscles and ligaments will be needed in addition to attachments to bone.