Dense type I collagen with poor blood supply and slow healing
Expansion
Structure: densely packed parallel type I collagen with tenocytes, arranged hierarchically into fibrils, fibres and fascicles, with a crimped resting configuration that straightens under load (the toe region of the stress-strain curve).
Blood supply is poor, and there are recognised watershed zones: the Achilles tendon 2 to 6 cm above its insertion, and the supraspinatus near its insertion. These are precisely the sites of degeneration and rupture.
Healing is slow, taking months, and produces disorganised type III collagen initially, remodelling to type I over a year or more. Strength never fully returns to baseline.
Tendinopathy is degenerative, with collagen disorganisation, neovascularisation and mucoid change, and typically little inflammation. This is why corticosteroid injection gives short-term relief but may worsen long-term outcome and increase rupture risk.
Eccentric loading programmes are the best evidenced treatment, promoting collagen remodelling.
Risk factors for rupture include fluoroquinolone antibiotics, corticosteroids, diabetes and a sudden increase in training load.