Objective- To determine the effect of collagen type I on hydroxyproline content in Achilles tendon in dogs.
Design- Experimental in vivo study.
Animals- A total of 12 adult dogs.
Procedure- Under general anesthesia and aseptic condition the dorsal surface of right Achilles tendon was exposed and after complete separation of connective tissue, it was splitted (striking 10 times) in full thickness in longitudinal fashion of 3cm in length in the mid-tendon area using BP blade no 15 in each one. These animals were divided into two groups of control (untreated) and experiment (treated with collagen type I) having 6 animals each. The hydroxyproline concentration was measured by modified spectrophotometer method by collecting samples from mid splitted area of injured and normal tendons of each animal from treated and untreated tendons on 30 in three dogs in each group and 90 days in remaining animals after surgery. The collected data was analyzed using student t-test at P<0.05% significant level.
Results- There was significant differences between hyroxyproline contents of treated tendon with that of control ones. The normal values of hydroxyproline content in control group was 97.66mg/g of dry matter (DM), whereas it was 42.33mg/g of DM  after one month and 62.06mg/g of DM after end of 3 months. These data were 97.66mg/g of DM for normal tendon and 52mg/g of DM and 83.4mg/g of DM respectively for one and three months in experimental group (P<0.05%). This study suggests using hydroxyproline content as a direct marker of the effect of collagen type I on collagen content in injured tendon. 
Conclusion and Clinical Relevance- The results of this study indicated that addition of collagen type I as a local stimulator on severely injured or extensive lesion of Achilles tendon in dog will be highly useful in enhancing hydroxyproline content in the treated tendon.