On the morphological relations of the Achilles tendon and plantar fascia via the calcaneus: a cadaveric study

Abstract

Current treatments of plantar fasciitis are based on the premise that the Achilles tendon (AT) and plantar fascia (PF) are mechanically directly linked, which is an area of debate. The aim of this study was to assess the morphological relationship between the AT and PF. Nineteen cadaveric feet were x-ray imaged, serially sectioned and plastinated for digital image analyses. Measurements of the AT and PF thicknesses and cross-sectional areas (CSA) were performed at their calcaneal insertion. The fiber continuity was histologically assessed in representative subsamples. Strong correlations exist between the CSA of the AT and PF at calcaneal insertion and the CSA of PF's insertional length (r = 0.80), and between the CSAs of AT's and PF's insertional lengths. Further correlations were observed between AT and PF thicknesses (r = 0.62). This close morphological relationship could, however, not be confirmed through x-ray nor complete fiber continuity in histology. This study provides evidence for a morphometric relationship between the AT and PF, which suggests the presence of a functional relationship between these two structures following the biological key idea that the structure determines the function. The observed morphological correlations substantiate the existing mechanical link between the AT and PF via the posterior calcaneus and might explain why calf stretches are a successful treatment option for plantar heel pain.

Figure 1

A representative x-ray of the right calcaneus of one specimen is depicted. The midpoint of a line (yellow) drawn from the insertion of the PF into the calcaneus (A) to the insertion of the AT into the calcaneus (B) is where a perpendicular line (blue) was drawn to measure the thickness (C) of the apophysis of the calcaneus.

Figure 2

(a) Representative section of an E12-plastinate in the sagittal plane. Image (b) displaying the calcaneus and the surrounding tissues, is a magnification of the dotted box shown in the image (a). The following six measurements used in this study are labelled: Thickness of Achilles tendon (AT) at 10 mm proximal to calcaneal insertion, the thickness of AT at calcaneal insertion, AT’s insertional length, plantar fascia’s (PF) insertional length, the thickness of PF at calcaneal insertion, and the thickness of PF at 10 mm distal to calcaneal insertion. The white arrows highlight the aligned dense trabeculae in the superficial posterior-inferior calcaneus. Image (c) is a schematic diagram of the calcaneus, with complementary AT and PF cross-sectional areas (CSA) for the distances of the measurements given in a. a anterior; i inferior, p posterior; s, superior.

Figure 3

The cross-sectional areas (a) and thicknesses (b) of the samples are graphically depicted. AT Achilles tendon; PF plantar fascia.

Figure 4

Graph (a) shows the cross-sectional area (CSA) of the Achilles tendon (AT) at calcaneal insertion in relation to the CSA of plantar fascia’s (PF) insertional length (r = 0.80, p ≤ 0.001). Graph (b) shows the CSA of AT’s insertional length in relation to CSA of PF’s insertional length (r = 0.84, p ≤ 0.001).

Figure 5

Image (a) shows a representative section of an E12-plastinate in the sagittal plane in black and white with the area of interest in this study in natural color. The aligned dense trabeculae within the superficial posterior-inferior calcaneus appear to be the continuation of the Achilles tendon (AT) towards the plantar fascia (PF). The location of the histological sections 4b and 4c is indicated with dotted squares. Image (b) shows a silver staining of calcaneal trabecular bone (TB) and the inserting AT. Superficial to the calcaneus, the AT subcutaneous tissue (ST) is depicted. The black arrows point to the collagen bundles of the AT running from superior to inferior in this slice. Image (c) shows a silver stained slice of the foot in the area of the PF. Several bundles of collagens jointly run in an anisotropic fashion (black arrows). Scale bar, 5 mm.

Figure 6

The soft tissue connection between the Achilles tendon (AT) and the plantar fascia (PF) is shown after the calcaneus was removed. The macroscopically most superficial layer of the AT (paratenon of the AT) continues as the calcaneal periosteum to merge with the PF, thereby forming a soft tissue connection between the two. d distal; l lateral; m medial; p  proximal.

Figure 7

Computed tomography images of the calcaneus are depicted. The aligned dense trabeculae (white arrows) within the superficial posterior calcaneus are shown in a sagittal (a), transverse (b) and coronal (c) plane (white arrows). The center of the yellow cross marks the same spatial point within the three different planes. an anterior; i, inferior; l lateral; m medial; p posterior; s superior; red asterisk, calcaneal spur; scale bar: 5 mm.