Anatomical Study Of Sites And Footprints Of Tibial Posterior Tendon Attachment

Background: The purpose of this study was to clarify variations in the tibialis posterior tendon (TPT) attachment site using a large sample and to quantify the contribution of each attachment site by examining the surface area of the attachment region. Methods: We examined 100 feet from 50 Japanese cadavers. The TPT of attachment to the the navicular bone (NB), medial cuneiform bone [1], and lateral cuneiform bone (LCB) was set as the main attachment site (Type I). Type II was de�ned as one extra attachment site in addition to those of Type I, meaning attachment to a total of four sites. Classi�cation up to Type VII was seen according to the number of additional attachment sites. Furthermore, surface area was measured using a 3-dimensional scanner to make the foot sample three-dimensional. Surface area was then calculated using 3-dimensional software. Results: Attachment to the NB, MCB, and LCB was present in all specimens. Attachment to the intermediate cuneiform bone was seen in 25 feet (25%), and to the cuboid bone in 66%. Among 1st–5th metatarsal bones, 4th metatarsal bone was the most attached, at 95%. The TPT attachment to the NB, MCB, and LCB were comprising 75.1% of total attachment surface area. The ratio of the NB, MCB and LCB in each type was about 90% in Types II and III, and 70–80% in Types IV–VII. Conclusion: Attachment of the TPT to the NB, MCB, and LCB may thus contribute to stability of the foot arch.


Introduction
The tibialis posterior (TP) plays a very important role during gait as the primary dynamic stabilizer of the medial longitudinal arch [2]. A typical disorder of the tibialis posterior tendon (TPT) is TPT dysfunction (TPTD), which occurs when the TPT becomes in amed or torn [3]. Previously, although TPTD was thought to be secondary to an in ammatory process resulting in acute and chronic tendinitis, more recent histopathological evidence has revealed this pathology as a degenerative tendinosis with a nonspeci c reparative response to tissue injury [4]. TPTD is the prevailing cause of adult acquired atfoot deformity (AAFD), and it is characterized by collapse of the medial longitudinal arch [5]. An anatomical understanding of the TPT is therefore important not only for understanding the movement and stability of the foot and ankle joints, but also for the prevention of disorders. However, although the anatomical features of TPT have been investigated in various previous studies, no consensus has been obtained regarding the sites of TPT attachment [5][6][7][8][9][10][11][12].
Regarding the sites of TPT attachment, Musial [12] described a permanent, main TPT insertion onto the navicular bone (NB), medial cuneiform bone [1], intermediate cuneiform bone (ICB), lateral cuneiform bone (LCB), and 2nd-3rd metatarsal bones . However, among the 122 feet examined, different sites of attachment were found for the accessory bundles, including the cuboid bone [1] (116 feet), 4th metatarsal bone (4MB) (106 feet), exor hallucis brevis (104 feet), calcaneus and CB (36 feet), peroneus longus muscle (20 feet), and 5th metatarsal bone (5MB) (four feet). In addition, Bloome et al. [6] classi ed TPT into three bers, anterior, middle, and posterior bundles in an investigation of 11 fresh frozen cadaver feet, and examined the sites of attachment for each bundles. The anterior bund was reported to attach to the navicular tuberosity, inferior capsule of the naviculocuneiform joint, and the inferior surface of the MCB. The middle bund attached to the ICB, LCB, CB, 2nd-5th metatarsal bones, exor hallucis brevis, and peroneus longus tendon. The posterior bund attached to the sustentaculum tali and spring ligament. Furthermore, in recently, Olewnik [7] investigated the site of TPT attachment in 80 xed cadaver feet, de ning attachment to the NB and MCB as Type I, and categorizing Types II-IV and subtypes A-C according to additional sites of attachment. Willegger et al. [5] showed that the major site of TPT attachment in 41 xed cadaveric feet was the NB. Branching and attachment to up to eight bones was reported among any of the MCB, ICB, LCB, CB, calcaneus, and 1st-5th metatarsal bones.
Possible obstacles to development of such a consensus include ethnic differences, differences in laterality, and differences in numbers of samples [5]. While previous studies [5][6][7]12] have considered that the NB, MCB, and LCB are the main sites of TPT attachment, the degree of contribution of these attachment sites has not been quanti ed, and whether they actually contribute signi cantly to the actions of the TPT remains unknown. Main attachment sites thus need to be clari ed by calculating and comparing the surface areas of individual attachment sites.
The purpose of this study was to clarify variations in TPT attachment site using a large sample and to quantify the contribution of each attachment site by examining the surface area of the attachment region.

Cadavers
We examined 100 feet from 50 Japanese cadavers (mean age at death, 80 ± 11 years; 56 sides from 28 men, 44 sides from 22 women; 50 right sides, 50 left sides) that had been switched to alcohol after placement in 10% formalin. No legs showed any sign of previous major surgery around the ankle. This study was approved by the ethics committee at our institution.

Methods
The procedure for dissecting the TPT is described below. Isolated specimens of the leg were created by transection about10 cm above the ankle. The skin and subcutaneous tissue were removed and the peroneus longus and the peroneus brevis muscles and TPT were carefully dissected out and inspected. The plantar aponeurosis was carefully dissected from the exor digitorum brevis and removed. The rst, second, and third plantar layers were then dissected and excised. Upon reaching the fourth layer, to clarify sites of attachment to the TPT, the exor halluces brevis, exor digiti minimi brevis, adductor hallucis, opponens digiti minimi, rst-fourth dorsal interossei, rst-third plantar interossei, part of peroneus longus, the lateral Lisfranc ligament, and the metatarsal plantar ligament were dissected and excised ( Fig. 1A). Based on a previous study [7], the regions of attachment to the NB, MCB, and LCB were set collectively as the main attachment site (Type I). The attachment seen in Type I with addition of one additional site of attachment was de ned as Type II, which thus showed attachment at a total of four sites. In the same manner, classi cation up to Type VII was made according to the number of additional attachment sites. Furthermore, the attachment site area was identi ed by peeling away any adherent tissue, then coloring the attachment site with a pencil (Fig. 1B). The surface area was then measured using a 3-dimensional (3D) scanner (EinScan Pro HD; SHINING 3D, Hangzhou, China) to make the foot sample 3D (Fig. 1C). The resulting data were read into Geomagic Freeform 2021 design software (3D SYSTEMS), and the boundary of the attachment site was drawn as a curve with a pen-type device (Touch; 3D SYSTEMS) (Fig. 1D). Surface area was then calculated using Rhinoceros7 3D software (McNeel) (Fig.   1E). Each attachment site was measured twice to allow calculation of the mean value and standard deviation. The ratio of each adhered area was calculated so that the sum of adhered areas for each sample as 100%. All measurements were made by the same physical therapist (I.U.).
The reliability of surface area measurement by 3D scanner was calculated using the intraclass correlation coe cient (ICC) (1,2) for 10 of 100 feet, yielding an ICC of 0.993. In this study, measurement of surface area showed almost perfect reliability, consistent with previous results [13].

Statistical analysis
Statistical analyses were performed using SPSS version 24.0 (SPSS Japan, Tokyo, Japan). Pearson's chisquared test was used to compare differences in insertion location between sex and laterality. The level of signi cance was set at 5%.

Results
Sites and ratios of TPT attachment (Table 1) Attachment to the NB, MCB, and LCB was present in all specimens (100%). Attachment to the ICB was seen in 25 feet (25%), and to the CB in 66 feet (66%). Among 1-5MBs, 4MB was the most commonly attached, at 95 feet (95%).

Type Classi cation For Site Of Tpt Attachment
With NB, MCB, and LCB considered together as the most basic attachment site (Type I), 7 types were classi ed according to the number of additional attachment sites, with subtypes A-D de ned according to the types of additional attachment sites (Fig. 2, Table 2). Of the seven types, Type IV was the most common, seen in 29 feet (29%), while Type VII was the least common, in 2 feet (2%). No cases of Type I were identi ed ( Table 2). In addition, Types II and III could be classi ed into three subtypes (A-C), Types IV and V could be classi ed into four subtypes (A-D), and Type VI could be classi ed into two subtypes (A and B).

Differences In Laterality And Sex By Type
No signi cant differences were observed between types (Table 3).

Surface Area Of Site Of Tibialis Posterior Tendon Attachment
Surface areas were 120 ± 69.9 mm 2 (25.7%) for NB, 182.4 ± 50.0 mm 2 (39.0%) for MCB, 48.6 ± 24.7 mm 2 (10.4%) for LCB, and the three main attachment sites provided a mean of 75.1% of the total attachment ( Table 4). The ratio of the three main attachment sites (NB, MCB and LCB) in each type was about 90% in Types II and III, and 70-80% in Types IV-VII (Table 5).

Discussion
In this study, TPT was classi ed 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 signi cant difference by sex or laterality was seen between types, and we used 100 samples, representing a larger cohort than previous studies [5][6][7]12]. The results of this study may thus have yielded different classi cations 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][6][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 con rmed the NB, MCB, and LCB as the main sites of TPT attachment, broadly supporting the ndings of previous studies.
The TPT elevates the medial arch and inverts, adducts, and plantar-exes 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 clari ed 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 ndings 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 ndings 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 classi cations for the TPT, only attachments to bone were considered. Previous studies have reported attachments to the abductor hallucis, exor hallucis brevis, peroneus longus tendon, spring ligament, and plantar calcaneocuboid ligament [6,7,12]. Type classi cation 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 con rmed 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 classi cation in consideration of attachments to muscles and ligaments will be needed in addition to attachments to bone.

Declarations
Ethical approval and consent to participate All methods were carried out in accordance with the 1964 Declaration of Helsinki, and all cadavers were legally donated for research purposes to the Dental Sciences at Niigata, Japan. This study was approved by the ethics committee of the Niigata University of Health and Welfare. Informed consent was obtained from the families of all subjects.

Consent for publication
Not Applicable Availability of data and materials The datasets generated and/or analysed during the current study are not publicly available due to limitations of ethical approval involving the patient data and anonymity but are available from the corresponding author on reasonable request. C: The surface area was measured using a 3D scanner to make the foot sample 3-dimensional.
D: Enlarged view of the medial cuneiform bone. A curve is drawn as the boundary of the site of attachment of the navicular bone with a pen-type device.
E: Surface area was calculated using Rhinoceros 3D software.