The structural continuity between the Achilles tendon and the plantar fascia
An anatomical study provides structural evidence for a morphological connection between the Achilles tendon and plantar fascia via the highly aligned calcaneal trabeculae of the posterior calcaneus.
Macroscopic observations during the separation of the Achilles tendon-calcaneus-plantar fascia complex (AT-calcaneus-PF complex). (A) The AT-calcaneus-PF complex can be observed looking from proximal to distal after the AT was separated from its weak attachment on the calcaneus and reverted backwards. (B) The calcaneal tuberosity becomes visible if the separation of the AT from the calcaneus is continued. The white arrow points at the median fibrous septum originating from the calcaneus and contributing to the calcaneal periosteum. (C) The macroscopic transition of ligamentous structures between the AT and the PF is shown after the calcaneus was removed (view from medial to lateral onto a sagittal plane after the AT-calcaneus-PF complex had been longitudinally divided into two parts).
AT, Achilles tendon; black arrows, transverse insertion line; CAL, calcaneus; CP, calcaneal periosteum; PF, plantar fascia; RCB: retrocalcaneal bursa, white asterisk, most proximal point of the calcaneal tuberosity; d, distal; m, medial; l, lateral; p, proximal. From Zwirner et al. 2020 (Creative Commons)
Plantar fasciitis is a debilitating heel pain condition that affects one percent of the adult population. There has been a hypothesis that Achilles tendon and the plantar fascia is a one connected functional unit. A shortened or tight Achilles tendon, is an established risk factor for plantar fasciitis. Stretching of plantar fascia and Achilles tendon have been established as an effective therapy. High-load strength training of repetitive heel rises out of a dorsiflexed position, was reported to be useful for plantar fasciitis pain.
A functional relationship of plantar fascia and Achilles tendon had been proposed. In bodywork, some proposed that calcaneus behaves like the patellar for the knee. Plantar fascia is considered as a tendon or extended Achilles tendon that embalms the periosteum of the calcaneus as a “movable plate” in an open chain movement.
Highly regular aligned trabeculae are found in the superficial posterior and inferior calcaneus appearing to connect the Achilles tendon to the plantar fascia in a bridge-like manner. A question was asked whether the two are structurally different tissues, inserting separately into the calcaneus from two ends, or if both are one continuing collagenous structure. The continuity of collagen fibres between the Achilles tendon and plantar fascia is being debated.
A new study investigated the Achilles tendon-calcaneus-plantar fascia complex by using histology and plastination. The complex was biomechanically mapped based on 13 sub-regions using 76 samples.
The study found regular calcaneal trabeculae were surrounded by tendon-like collagen fibre bundles and adipocytes. The orientation of calcaneal trabeculae similar to the direction of the plantar fascia collagen fibre bundles. Highly regular interindividual trabecular arrangements in the superficial posterior and inferior calcaneus, form an osseous bridge between Achilles tendon and plantar fascia.
This complex is one united tendon of the gastrocnemius and soleus muscle with Achilles tendon represents a proximal part, the calcaneal trabeculae including the calcaneal periosteum the intermediate part and the plantar fascia is the distal part.
The study also mapped the load-deformation characteristics of the entire complex. The biomechanical analysis describes the applicable maximum stress a material can withstand before breaking when stretched. Low elastic moduli were measured in the decalcified calcaneal samples and the lateral calcaneal periosteum sample. High elastic moduli were found only in the plantar fascia.
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