Strain Distribution of Intact Rat Rotator Cuff Tendon-to-Bone Attachments and Attachments With Defects

Abstract

This study aimed to experimentally track the tissue-scale strains of the tendon-bone attachment with and without a localized defect. We hypothesized that attachments with a localized defect would develop strain concentrations and would be weaker than intact attachments. Uniaxial tensile tests and digital image correlation were performed on rat infraspinatus tendon-to-bone attachments with defects (defect group) and without defects (intact group). Biomechanical properties were calculated, and tissue-scale strain distributions were quantified for superior and inferior fibrous and calcified regions. At the macroscale, the defect group exhibited reduced stiffness (31.3±3.7 N/mm), reduced ultimate load (24.7±3.8 N), and reduced area under the curve at ultimate stress (3.7±1.5 J/m2) compared to intact attachments (42.4±4.3 N/mm, 39.3±3.7 N, and 5.6±1.4 J/m2, respectively). Transverse strain increased with increasing axial load in the fibrous region of the defect group but did not change for the intact group. Shear strain of the superior fibrous region was significantly higher in the defect group compared to intact group near yield load. This work experimentally identified that attachments may resist failure by distributing strain across the interface and that strain concentrations develop near attachment defects. By establishing the tissue-scale deformation patterns of the attachment, we gained insight into the micromechanical behavior of this interfacial tissue and bolstered our understanding of the deformation mechanisms associated with its ability to resist failure.

Partial-Width Injuries of the Rat Rotator Cuff Heal with Fibrosis

“…The structure of the healthy tendon-bone attachment is ideal for its function to transmit forces generated by muscle to bone (23, 25–27). Although the attachment resists failure, injuries commonly occur near the attachment within rotator cuff tendons (6, 17, 28–30).…”

“…Adult Sprague Dawley rats (N = 8 females, N = 8 males for in vivo healing; ~200-250g) and adult Long Evans rats (N=20 female dams, for ex vivo validation at time zero) were used in accordance with the University of Delaware Institutional Animal Care and Use Committee approval. Rats underwent a surgical procedure under anesthesia (isoflurane carried by 1% oxygen) to model a partial-width rotator cuff injury at the center of the IS attachment (Figure 1A–C) (23). The IS tendon was exposed and the forearm was internally rotated.…”

Contribution of the Endplates to Disc Degeneration

“…The complex morphology and graded material properties minimize stress concentrations during complex loading that includes tension, compression, and shear forces [23, 24]. …”

Short cracks in knee meniscus tissue cause strain concentrations, but do not reduce ultimate stress, in single-cycle uniaxial tension

“…Superficial zone articular cartilage, which is similar to meniscus, has been observed to rupture by fracture mechanics-dominant processes (Chin-Purcell and Lewis 1996;Taylor et al 2012). However, cracks in tendon, which like meniscus contains highly aligned collagen fascicles, tend to become blunt instead of growing (Schechtman and Bader 1997;Von Forell and Bowden 2014b;Von Forell and Bowden 2014a;Andarawis-Puri et al 2009;Locke et al 2017;Szczesny et al 2015). These findings suggest that limit stress analysis is more appropriate for cracked tendon, and by extension possibly also for meniscus.…”

“…Data from mechanical tests of other tissues that are similar to meniscus fibrocartilage, namely superficial zone articular cartilage (Chin-Purcell and Lewis 1996) and temporomandibular joint disc (Koombua et al 2006), indicate that fracture probably occurs in those tissues (Taylor et al 2012). However, cracks in tendon tend to blunt instead of propagating (Schechtman and Bader 1997;Von Forell and Bowden 2014b;Von Forell and Bowden 2014a;Andarawis-Puri et al 2009;Locke et al 2017;Szczesny et al 2015). A definitive test to determine if a cracked specimen failed by fracture is to check if its peak stress was less than that of identically tested crack-free specimens (Taylor et al 2012).…”