Chronic rotator cuff injury and repair model in sheep.
BACKGROUND
Most rotator cuff surgery is performed on chronic tears. As there is no animal model in which to examine the physiology of muscle and tendon injury and repair in this setting, we developed a chronic rotator cuff injury model in sheep.


METHODS
The infraspinatus tendon was released in thirty-six female sheep. Biopsy specimens were obtained from the muscle and were analyzed for fat content. The force generated by the muscle with supramaximal stimulation was recorded intraoperatively. A control group (twelve sheep) underwent an immediate tendon repair. In the remaining twenty-four sheep, the tendon was wrapped in a dura substitute to prevent scarring and was repaired at six weeks (eight sheep) and eighteen weeks (sixteen sheep) after release. In the immediate repair group, four animals were killed at six weeks; four, at twelve weeks; and four, at twenty weeks. In the six-week delayed repair group, four animals were killed at twelve weeks and four were killed at twenty weeks after the repair. In the eighteen-week delayed repair group, eight animals were killed at twelve weeks; four, at twenty weeks; and four, at thirty weeks after the repair. Muscle biopsies and testing were repeated prior to killing of the animals.


RESULTS
The average force of muscle contraction decreased 3.6 lb (1.6 kg) by six weeks after the injury and 3.9 lb (1.8 kg) by eighteen weeks after the injury. After the repair, the force of contraction in the six-week group improved by 0.8 lb (0.4 kg) at twelve weeks postoperatively and by 1.3 lb (0.6 kg) at twenty weeks postoperatively. In contrast, no improvement occurred in the eighteen-week group until thirty weeks after the repair, at which time a 0.9-lb (0.4-kg) improvement was noted. There was a twelvefold increase in intramuscular fat concentration; this lipid infiltration was partially reversed after the tendon repair. Isolated tendon samples demonstrated an increase in the modulus of elasticity after chronic detachment that partially corrected after the tendon repair in the earlier (six-week) repair group.


CONCLUSIONS
We found that earlier repair of the tendon results in a more rapid recovery of both muscle function and tendon elasticity compared with a more delayed repair. We concluded that there may be a "point of no return" in rotator cuff injury after which the elasticity of the muscle-tendon unit does not return to normal.
Coleman
Struan H
SH
Department of Sports Medicine and Shoulder Service, The Hospital for Special Surgery, 535 East 70th Street, New York, NY 10021, USA. colemans@hss.edu
Fealy
Stephen
S
Ehteshami
John R
JR
MacGillivray
John D
JD
Altchek
David W
DW
Warren
Russell F
RF
Turner
A Simon
AS
D000818:Animals; D001696:Biomechanical Phenomena; D001707:Biopsy, Needle; D002908:Chronic Disease; D004195:Disease Models, Animal; D005260:Female; D007150:Immunohistochemistry; D019637:Orthopedic Procedures / Q000379:methods*; D011183:Postoperative Complications; D011336:Probability; D011897:Random Allocation; D012016:Reference Values; D017006:Rotator Cuff / Q000473:pathology / Q000601:surgery*; D000070636:Rotator Cuff Injuries; D012680:Sensitivity and Specificity; D034561:Sheep, Domestic; D013708:Tendon Injuries / Q000473:pathology* / Q000601:surgery*; D013718:Tensile Strength; D013997:Time Factors