The tibialis posterior tendon footprint: an anatomical dissection study

The tibialis posterior tendon (TPT) elevates the medial arch and inverts, adducts, and plantar flexes the foot [1‐3]. During the stance phase of gait the tibialis posterior is the main dynamic stabilizer of the foot. As a strong contributor to the midtarsal joint locking mechanism the TPT inverts the hindfoot, creating a rigid midfoot allowing the gastrocnemius-soleus complex to transmit plantar flexion forces to the metatarsal heads [4]. Posterior tibial tendon dysfunction (PTTD) is the prevailing cause of adult acquired flatfoot deformity (AAFD) which is characterized by a collapse of the medial longitudinal arch. Loss of tibialis posterior function enables hindfoot eversion, “unlocking” of the midtarsal joints and causing plantar flexion at the talonavicular joint as well as forefoot abduction [5]. TPT degeneration and elongation is associated with age-related AAFD, but multiple etiologies including traumatic TPT rupture have been identified [6‐8]. Although controversy exists about the condition and its treatment, it is clear that the complex course and function of the TPT plays a detrimental role in the pathoanatomy of AAFD.

The anatomy of the TPT has been extensively investigated regarding its excursion, vascularity and tendon sheaths, but the insertional anatomy was disregarded by most of the studies so far [9‐17]. The tibialis posterior muscle arises from the interosseous membrane and adjacent surfaces of the proximal tibia and fibula. The myotendinous junction appears in the distal third of the leg. The TPT courses behind the medial malleolus at a relatively acute angle and further passes posterior to the axis of the tibiotalar joint and medial to the axis of the subtalar joint [3, 4]. From an anatomical point of view, the main insertion of the TPT is at the navicular bone, but several additional insertions in the hindfoot and midfoot have been described [11, 14, 17]. Nevertheless, no study so far has reported the dimensional characteristics of the tendon footprint and the detailed bony insertional anatomy. More detailed knowledge about the normal anatomy of this tendon may aid to better understand its function and improve surgical reconstruction techniques by providing references for tendon reconstruction or tenodesis location.

The purpose of this study was to provide quantitative and qualitative information regarding the anatomical bony insertions of the tibialis posterior tendon and to quantify the prevalence of variations.

Forty-one (41) adult formalin-fixed lower leg specimens were included in this study. The specimens were obtained from voluntary body donors who consented during life to donate their body for research and teaching purpose to the Center for Anatomy and Cell Biology, Division of Anatomy, Medical University of Vienna. The study has been approved by the local ethics committee (EK 1555/2015) prior to conducting the study. Specimen age ranged from 67 to 101 years (mean age 85.2 years). Twenty-six female and 15 male donor limbs were dissected including 20 left and 21 right lower legs. Inclusion criteria comprised specimens who were of sufficient quality without any evidence of surgical intervention in the area examined to allow for the complete identification of the TPT attachment. The skin, subcutaneous tissue and the muscles were removed with a scalpel. Care was taken to not injure the tibialis posterior muscle and its tendon. Each course of the TPT was documented by photograph according to a standardized protocol with a reference scale. A full-length photograph of the lower limb was taken from a medial view. Afterwards the foot was held in inversion in order to document the course of the TPT attaching at the medial and plantar aspect of the foot. Dissection was further continued and the main tendon and additional tendon slips were followed from proximal to distal exposing the bony attachments. The TPT was carefully dissected and then removed at the osseous insertions. The bony footprints were marked with ink and documented by photograph with a scale in a standardized manner [18, 19]. Photographs were taken from a plantar and medial view. The precise evaluation of the dimensions and shapes of the main navicular footprint was carried out after disarticulation of the bones followed by photography of the TPT insertion with a reference ruler. Qualitative analysis included the detection of variations and frequencies of bony TPT insertions in each disarticulated bone. Thereafter the photographs were digitally evaluated performing quantitative measurements of the footprint dimensions. The footprint length and width (in millimeters) were recorded, and areas of insertion (AOI, mm²) were calculated. All photographs were digitally measured by use of Image J (http://​rsb.​info.​nih.​gov/​ij/​) software. Image J is a Java-based image processing program developed at the National Institutes of Health (NIH) and is available for Microsoft Windows, the classic Mac OS, macOS, Linux, and the Sharp Zaurus PDA. The source code for Image J is freely available [20].

The most important finding of this study is that 63% of navicular TPT insertions were located at the plantar aspect. As the tendon dived deeper to the plantar side of the foot it spread into several branches which inserted at up to 8 distinct tarsometatarsal footprints. The second most common additional footprint was the plantar lateral cuneiform (93% of dissected feet). However, due to the complex course of the tendon slips and its high variability among the dissected specimens, we did not find it reasonable to classify different types of TPT insertions [11, 17]. Sarrafian described an anterior, middle, and posterior component of the TPT [17]. Other studies described a variable insertion pattern and supplementary attachments to the flexor hallucis brevis muscle, abductor hallucis muscle, peroneus longus tendon and to the spring ligament [11, 21]. Our study focused on the osseous footprints since these attachments represent the main anchorage of the tibialis posterior muscle.

From a biomechanical point of view, the course of TPT lies posterior to the axis of the tibiotalar joint and medial to the axis of the subtalar joint, allowing the muscle to act as a plantar flexor and invertor of the foot. During normal gait, the tibialis posterior acts to invert the hindfoot, causing the midtarsal joints to lock. In AAFD the valgus deformity of the hindfoot results from a collapse of the medial supporting TPT leading to an increased eversion of the calcaneus due to the position of the Achilles tendon lateral to the axis of the subtalar joint [2, 22]. According to this theory and applying tendon transfer principles, it has been postulated that the flexor digitorum longus (FDL) tendon, should be placed as far as possible from the subtalar joint axis (at the medial aspect of the navicular bone) in order to maximize leverage in FDL tendon transfer [4]. Nevertheless, following the anatomical course of the TPT, the first fulcrum of the TPT is the medial malleolar groove and the second fulcrum is the navicular tuberosity. Our study showed that the TPT primary inserts at the plantar aspect of the navicular bone and further spreads deeply, distally and laterally to additional bony insertions. With this anatomical knowledge we claim that the navicular tuberosity acts as an additional pivot point of the TPT to ease inversion of the foot. The course of the TPT with its attachment at the plantar aspect of the navicular bone works as a buttress for the medial longitudinal arch and the complex insertion at the plantar aspect of the foot provides a firm grip [23]. In stage II PTTD the flexor digitorum longus transfer replaces or supplements a pathologic TPT with the FDL. As both tendons are directly adjacent to each other posterior to the medial malleolus, they have the same line of pull. Based on the findings of this anatomical footprint study we advocate a plantar location of FDL tendon transfer in flexible AAFD in order to restore the anatomical lever and insertion of the TPT. If the FDL tendon is transferred too medially the direction of pull of the tendon alters and makes the FDL more an adductor of the foot rather than a supinator [24].

This principle is also applicable for suture anchor placement in the Modified-Kidner procedure. If the surgeon pursues an anatomical reconstruction, the anchor should be placed from plantar and perpendicular to the effective lever arm of the TPT from plantar medial to dorsal lateral [25].

This study comprises some inherent limitations. A comparison of left and right specimens was omitted due to the unpaired specimen study design. The described anatomical insertion of the TPT may vary according to the geographical origin, ethnicity, and the number of examined specimens. Nevertheless, the sample size of 41dissected feet constitutes a good sample size. Another potential drawback might be that the occurrence of an os tibiale externum was not analyzed. With a prevalence of 10 to 14% in normal feet the os tibiale externum is one of the most common accessory bones of the foot [26]. Therefore a huge amount of specimens would be necessary to analyze differences of TPT insertion in feet with normal navicular bones compared to type I-III accessory navicular bones. Additionally, pathological foot alignment (i.e. cavus or flatfoot) could be associated with anatomical variations of the TPT insertion. Evaluation of foot deformities was not part of the study protocol.

In conclusion, this study adds to current knowledge on the anatomical insertion of the tibialis posterior tendon. The main footprint was located plantar at the navicular bone and several tendon slips extended up to 8 bony footprints located at the hind- and midfoot. The present data can be used as reference for anatomical TPT reconstructions or subsequently assist in surgical preparation of flexor digitorum longus transfer.