Assessment of the canine model of rotator cuff injury and repair

Abstract

Animal shoulder models are used to systematically investigate the factors influencing rotator cuff injury and repair. Each model has advantages and disadvantages that must be considered in the context of the specific research questions being asked. This study evaluated the utility of the canine model for studies of acute, full-thickness rotator cuff tendon injury and repair. We found that time-zero failure load is dependent on the suture type and configuration used for repair. Acute, full-width tendon repairs fail anatomically within the first days after surgery in the canine model, regardless of suture type, suture configuration, or postoperative protocol. Robust scar tissue forms in the gap between the failed tendon end and the humerus, which can be visually, mechanically, and histologically misconstrued as tendon if an objective test of repair connectivity is not performed. We conclude that a full-width injury and repair model in the canine will provide a rigorous test of whether a new repair strategy or postoperative protocol, such as casting or temporary muscle paralysis, can maintain repair integrity in a high-load environment. Alternatively, a partial-width tendon injury model allows loads to be shared between the tendon repair and the remaining intact portion of the infraspinatus tendon and prohibits complete tendon retraction. Thus a partial-width injury in the canine may model the mechanical environment of many single tendon tears in the human injury condition and warrants further investigation.

Figure 1

(A) To objectively assess repair connectivity, four small tantalum beads were implanted into the shoulder complex at the time of tendon repair: two embedded in the humerus (HUM) at the location of the black arrows and two on the surface of the infraspinatus tendon (INF-T) visualized at the white arrows. (B) Intra-operative fluoroscopy at the time of repair shows the tendon and bone beads as well as the calibration sphere (large black circle, OD = 9.5mm), (C) Follow-up fluoroscopy at 5 days post-op, demonstrates anatomic failure of the repair and approximately 2 cm retraction of the tendon stump.

Figure 2

Acute infraspinatus repairs with simple sutures at sacrifice (12 weeks). A well integrated, connective tissue was observed spanning infraspinatus muscle (INF-M) to humerus (HUM) in all cases. Examples of a repair considered to have (A) remained intact or (B) failed during the post-operative period, based on the anatomic position of the lateral edge of the infraspinatus muscle (hashed arrows) relative to the acromion (ACR), are shown.

Figure 3

Failure loads for native infraspinatus tendon insertion compared to acute tendon injury and repair with simple sutures at 12 weeks (n = 8 in each group). The failure loads of the repaired tendons were significantly less than normal at 12 weeks (*, p<0.05).

Figure 4

Robust, hypertrophic scar tissue formed in the 2–3 cm gap between the failed tendon stump (TS) and the humerus (HUM). Gap scar (GS) tissue at (A) three and (B) six weeks could be subjectively isolated as a tendon-like structure. The white arrow shows that one of the tantalum beads that had been placed on the tendon at surgery could be grossly visualized at dissection. (C) Histologically at six weeks, the isolated gap scar tissue demonstrates spindle-shaped nuclei and crimped, well-organized and oriented collagen (Hematoxylin-Eosin staining, 20X).

Figure 5

(A) The canine infraspinatus tendon (INF-T) connects the infraspinatus muscle (INF-M) to the humerus (HUM) and is a distinct extra-articular structure. (B) The canine scapula lacks a coracoid process. The canine acromion (ACR) is vestigial and does not cover the rotator cuff.