Therapeutic Advances in Tendinopathy Quantified Microscopically Using Bonar Score, with a Special Reference to PRP Therapy—A Systematic Review of Experimental Studies

(1) Background: The Bonar scoring system serves in the microscopic evaluation of tendon pathology. However, it can be easily adapted to investigate decreasing degeneration after treatment and quantify the healing progress. We believe that there is an actual need for a connection between clinical observations and tissue alterations arising during the treatment process, to gain superior functional outcomes. Herein, we perform a systematic review of the Bonar score’s application in the histopathological assessment of therapeutic advances in tendinopathy, with special reference to PRP therapy. (2) Methods: A systematic review was performed based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. The searching strategy was conducted across major databases: PubMed, Cochrane Central, ScienceDirect, SciELO, and Web of Science. The risk-of-bias assessment was made using the Cochrane Collaboration’s Risk of Bias Tool and SYRCLE. (3) Results: The searching strategy produced 807 articles, and after selection, 22 studies were included. We collected 21 animal studies (n = 472) and 1 human study (n = 45). Three types of tendons were taken into account: 14 Achilles tendons, 7 supraspinatus tendons (SST), and in 1 case, Achilles and patellar tendons simultaneously. A variety of therapeutic methods were used—from intra-tendinous substance injections to surgical procedures or mechanical stimuli—but platelet-rich plasma (PRP) therapy dominated among them and was present in six studies. Most of the collected studies included an assessment of the tendons’ histopathology based on the classical Bonar score (with four variables and one observer). The staining protocol was based on the hematoxylin and eosin technique. An evaluation of therapeutic effects showed 15 positive results, 6 negative results, and 1 neutral result of treatments. (4) Conclusions: To understand the tendinopathy phenomenon, a link between histopathology and clinical observations in chronic tendon disorders is required due to the possibility of functional outcome improvements. The Bonar scoring system is well established in tendon pathology assessment and could also be adopted to assess therapeutic results in tendon disorders. Studies that included the PRP application showed Bonar-scoring-system-based evidence of superior tendinous tissue healing related to improved clinical results.