Journal of Biomechanics 40 (2007) 561–568 The role of repair tension on tendon to bone healing in an animal model of chronic rotator cuff tears Jonathan A. Gimbel, Jonathan P. Van Kleunen, Spencer P. Lake, Gerald R. Williams, Louis J. Soslowsky à McKay Orthopaedic Research Laboratory, University of Pennsylvania, 424 Stemmler Hall, Philadelphia, PA 19104-6081, USA Accepted 15 February 2006 Abstract Rotator cuff tendon tears are one of the most common shoulder injuries. Although surgical repair is typically beneficial, re-tearing of the tendons frequently occurs. It is generally accepted that healing is worse for chronic tears than acute tears, but the reasons for this are unknown. One potential cause may be the large tensions that are sometimes required to repair chronically torn tendons back to bone (i.e., repair tension). Therefore, the objective of this study was to utilize an animal model of chronic rotator cuff repairs to investigate the role of increased repair tension on tendon to bone healing. We hypothesized that an increase in repair tension would be related to detrimental changes to the healing insertion site. To test this hypothesis, the supraspinatus tendon of rats was surgically detached and then repaired immediately or after a delay of 2, 4, or 16 weeks. The repair tension was measured using a tensiometer and the mechanical properties, collagen organization, and protein expression of the healing insertion site were evaluated 4 and/or 16 weeks following repair. We found that the repair tension increased with time following detachment, and was related to a decrease in the failure properties and viscoelastic peak stress and an increase in cross-sectional area and stiffness of the insertion site. Therefore, repair tension should be minimized in the clinical setting. Future studies will include additional animal model studies involving the relationship between tension and muscle properties and a clinical study investigating the role of repair tension on repair failure. r 2006 Elsevier Ltd. All rights reserved. Keywords: Supraspinatus tendon; Rotator cuff; Shoulder; Chronic; Tension; Animal model 1. Introduction There are more than 3 million visits to physicians each year in the United States concerning shoulder-related injuries (American Academy of Orthopaedic Surgeons, 1999–2003). One of the most common injuries are tears to the rotator cuff tendons of the shoulder, which are often associated with pain, instability, and decreased shoulder function (Cofield, 1985). When conservative treatments are not successful in managing this condi- tion, surgery is often performed to repair the torn tendons back to bone. While this is typically beneficial, re-tearing of the tendons following repair frequently occurs partly because of inadequate tendon to bone healing (Harryman et al., 1991). It is generally accepted that healing is worse for chronic tears than for acute tears, but the reasons for this are unclear (Warner and Gerber, 1998). Chronic tears are characterized, in part, by a retracted and stiff musculotendinous unit, and large tensions are some- times required to repair the tendon back to bone (Hersche and Gerber, 1998). Since mechanical forces are thought to play a large role in the maintenance, remodeling, and healing of the tendon to bone insertion site (Woo et al., 1982), one potential cause for poor healing may be the large repair tensions. Evidence for this comes from a clinical study that has shown that an increase in repair tension has a negative effect on surgical outcome (Davidson and Rivenburgh, 2000). ARTICLE IN PRESS www.elsevier.com/locate/jbiomech www.JBiomech.com 0021-9290/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jbiomech.2006.02.010 à Corresponding author. Tel.: +1 215 898 8653; fax: 215 573 2133. E-mail address: soslowsk@mail.med.upenn.edu (L.J. Soslowsky).