The Achilles tendon watershed zone (2-6 cm from the calcaneal insertion) is the least vascularised region of the tendon, supplied only by the paratenon vessels and lacking intrinsic vasculature. This relative avascularity makes the zone susceptible to degenerative tendinopathy from accumulated micro-damage without adequate healing response. Non-insertional Achilles tendinopathy and the vast majority of acute Achilles tendon ruptures occur in this zone.
Maximum tensile load transmission zone; the most common site of Achilles tendon rupture and mid-portion tendinopathy
Mid-portion Achilles tendinopathy is diagnosed clinically by the arc sign (the tender area moves with the tendon in dorsiflexion-plantarflexion, distinguishing tendon from paratenon pathology) and the Royal London Hospital test. Eccentric calf loading (Alfredson protocol — 3 × 15 repetitions twice daily for 12 weeks) produces 85% good-excellent outcomes. PRP injection added to eccentric loading shows additive benefit in RCTs.
Hypovascular watershed zone degeneration producing Achilles mid-substance pain managed with eccentric calf loading (Alfredson protocol) and PRP injection.
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