Effect of overuse-induced tendinopathy on tendon healing in a rat supraspinatus repair model

Abstract

Supraspinatus tears often result in the setting of chronic tendinopathy. However, the typical repair model utilizes an acute injury. In recognition of that distinction, our laboratory developed an overuse animal model; however it is unclear whether induced overuse is necessary in the repair model. We studied the repair properties of overuse-induced tendons compared to normal tendons. We hypothesized that histological and mechanical properties would not be altered between the overuse-induced and normal tendons 1 and 4 weeks after repair. Thirty-one adult male Sprague-Dawley rats were subjected to either overuse or cage activity for 4 weeks prior to bilateral supraspinatus tendon repair surgery. Rats were sacrificed at 1 and 4 weeks post-surgery and evaluated for histology and mechanics. Results at 1 week showed no clear histologic changes, but increased inflammatory protein expression in overuse tendons. At 4 weeks, percent relaxation was slightly increased in the overuse group. No other alterations in mechanics or histology were observed. Our results suggest that the effects of the surgical injury overshadow the changes evoked by overuse. Because clinically relevant mechanical parameters were not altered in the overuse group, we conclude that when examining tendons 4 weeks after repair in the classic rat supraspinatus model, inducing overuse prior to surgery is likely to be unnecessary.

Keywords: animal model; overuse; supraspinatus tendon repair.

Figure 1

Tendon mechanical properties. (A) No difference was found in cross sectional area. (B) Percent relaxation was significantly increased (5% increase) in the overuse group. (C) No difference was found in maximum load. (D) No difference was found in maximum stress. Data represented as mean ± SD.

Figure 2

Tendon mechanical propertie. (A) No difference was found in stiffness. (B) No difference was found in modulus. Data represented as mean ± SD.

Figure 3

Histology grading. Grading scale is independent for each region. No differences were noted in injury or midsubstance at both 1 and 4 weeks. Data represented as median ± interquartile range.

Figure 4

Circular standard deviation. No differences were noted at the injury site and the midsubstance at both 1 and 4 weeks. Data represented as mean ± SD.

Figure 5

Immunohistochemistry grading. Grading scale is independent for each region. (A) IL-1β was significantly increased in the overuse group at the injury site (p=0.03) and the midsubstance (p=0.03). (B) No differences were noted in TNF-α. (C) CD45 was significantly increased in the overuse group at the midsubstance (p=0.03). No differences were notes at the injury site. Data represented as median ± interquartile range.