The Bonar score revisited: Region of evaluation significantly influences the standardized assessment of tendon degeneration

Objectives: Tendinopathy is a common, costly condition affecting both sporting and sedentary populations. Research into tendinopathy frequently involves the evaluation of tendinosis, a pathology characterized by a lack of inflammatory cells, collagen disruption, neovascularisation, altered cell numbers and morphology and increased glycosaminoglycans. Evaluation of these characteristics can be undertaken using the Bonar histopathology score, but the characteristics are heterogeneous throughout tendon specimens with no standardized method of determining the area to be evaluated. The objective of this study was to assess whether the Bonar score varies depending on the criteria used to define the area of evaluation. Design: Case series. Methods: Two independent assessors, with a third to resolve disputes, evaluated 103 areas from 35 tendon specimens using the Bonar score. Specimens were scored once each in the area of worst collagen disruption, degree of vascularization, and cell morphological changes. The inter-tester reliability of the updated Bonar scale was good (r²=0.71). Results: The Bonar score was highest in the areas of worst cell morphological (CM) changes, followed by collagen disruption (CD) and lowest for the area of most extensive vascular proliferation (VS) (regression: CD vs. CM, p= 0.008, CM vs. VS, p < 0.001, CD vs. VS, p= 0.013). Suggested modifications to the Bonar score include the addition of a cellularity domain, specific definitions of hypo- and hypercellularity, and changes to the vascularity score to include pathological avascularity. Conclusions: The updated Bonar score includes a standardized method of selecting the area of evaluation, which should provide increased reliability when assessing the extent of tendon degeneration.