Purpose To ascertain whether there are differences in the histopathological appearance of tendinopathic Achilles and patellar tendons.

Methods In males, we studied biopsies from tendinopathic Achilles (n = 28; average age 34.1 years) and patellar tendons (n = 28; average age 32.1), and Achilles tendons (n = 21; average age 61.8 years) from deceased patients with no known tendon pathology, and patellar tendons (n = 15; average age 28.3) from patients undergoing anterior cruciate ligament reconstruction. Haematoxylineosin stained slides were interpreted using a semi-quantitative grading scale (0: normal to 3: maximally abnormal) for fibre structure; fibre arrangement; rounding of the nuclei; regional variations in cellularity; increased vascularity; decreased collagen stainability; hyalinisation. All slides were assessed blindly twice, the agreement between two readings ranging from 0.170 to 0.750 (Kappa statistics).

Results The highest mean score of tendinopathic Achilles tendons was not significantly different from that of tendinopathic patellar tendons (11.6 ± 5 and 10.4 ± 3, respectively). The ability to differentiate between an Achilles tendon and a patellar tendon was low.

Conclusions Tendinopathic Achilles and patellar tendons show a similar histological picture. It was not possible to identify whether a specimen had been harvested from an Achilles or a patellar tendon on the basis of histological examination. The general pattern of degeneration was common to both tendinopathic Achilles and patellar tendons. A common, as yet unidentified, etio-pathological mechanism may have acted on both these tendon populations.