Introduction Recent studies have highlighted the plantaris tendon and its possible involvement in midportion Achilles tendinopathy. In several patients an enlarged/thickened plantaris tendon has been found to lay in close relationship to the Achilles tendon.1 Excision has shown to lead to good clinical outcome.4 Histological investigations reported of tendinosis-like tissue changes occurring in these tendons.6 It has been speculated that there may be an interference between the Achilles and plantaris tendon contributing to tendon pain.3
The aim of this study was to further analyse the role of the plantaris tendon by characterising the innervation patterns inside the tendon and in the peritendinous connective tissue between the Achilles and plantaris tendon.
Methods 31 patients (22 men, mean age: 47,1; 9 women, mean age: 55,1) with US/CD verified midportion Achilles tendinopathy were included. All patients underwent mini-invasive scraping procedure with plantaris tendon excision.1
36 plantaris tendons with attached peritendinous connective tissue were then cryosectioned and further processed for immunohistochemistry.6 Antibodies for detecting nerve structures (directed against PGP9.5) were used.
Results All 36 samples exhibited PGP9.5 positive areas in the peritendinous loose connective tissue mostly located in close vicinity to blood vessels (Figure1).
In 16 out of 36 tendons reactions were also found in the tendon proper mostly located in loose connective tissue spaces. In 4 tendons, reactions were seen in between the collagen fibres, interpreted as possibly being related to sprouting nerve fibres.
Discussion Most of the innervation occurred in the peritendinous connective tissue. This indicates the importance of this tissue for pain perception. The observation of nerves in loose connective tissue zones inside the tendon goes hand in hand with studies on healthy Achilles tendons and is thus not necessarily related to tendinosis-like changes.2 Sprouting fibres, features of tendinosis tendons,5 were observed only in a small number of patients. The characterisation of the nerves is currently on-going.
These findings indicate that some plantaris tendons themselves show signs of abnormal innervation patterns. The main innervation is however located in the peritendinous connective tissue. This reinforces the importance of this tissue in midportion Achilles tendinopathy. The interaction between the plantaris and Achilles tendon might in turn trigger the pain. The way of interference however has to be analysed in upcoming studies.
References 1 Alfredson. Br J Sports Med. 201145:1023–5
2 Bjur, et al. Cell Tiss Res. 2005;320:201–6
3 Cook, et al. Br J Sports Med. 2012;46:163–8
4 Ruergård, et al. PST. 2014;1:21–24
5 Schubert, et al. Ann Rheum Dis. 2005;64:1083–6
6 Spang, et al. Histol Histopathol. 2013;28:623–32
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