Rat rotator cuff tendon-to-bone healing properties are adversely affected by hypercholesterolemia.

BACKGROUND: Rotator cuff tendon tears represent a major component of reported orthopaedic injuries. In addition, more than one quarter of U.S. adults either currently have high cholesterol levels or have reduced their previously high cholesterol levels through the use of pharmaceuticals. Our clinical data have already linked hypercholesterolemia to full-thickness rotator cuff tears, and experimental data from our laboratory have shown effects on native tendon properties in multiple species. The objective of this study was to evaluate healing of supraspinatus tendons in our rat rotator cuff injury model. We hypothesized that tendon healing would be inferior in rats receiving a high-cholesterol diet for 6 months compared with those receiving standard chow. METHODS: All animals were subjected to a unilateral supraspinatus detachment and repair surgery, with contralateral limbs serving as within-animal comparative data. Animals continued their respective diet courses, and their supraspinatus tendons were biomechanically or histologically evaluated at 2, 4, and 8 weeks postoperatively. RESULTS: Biomechanical testing revealed a significant reduction in normalized stiffness in hypercholesterolemic rats compared with controls at 4 weeks after injury, whereas histologic analyses showed no significant differences in collagen organization, cellularity, or cell shape between groups. CONCLUSION: On the basis of our findings, hypercholesterolemia may have a detrimental biomechanical effect on tendon healing in our rat rotator cuff injury and repair model. LEVEL OF EVIDENCE: Basic science study, animal model.

Effect of preconditioning and stress relaxation on local collagen fiber re-alignment: inhomogeneous properties of rat supraspinatus tendon.

Repeatedly and consistently measuring the mechanical properties of tendon is important but presents a challenge. Preconditioning can provide tendons with a consistent loading history to make comparisons between groups from mechanical testing experiments. However, the specific mechanisms occurring during preconditioning are unknown. Previous studies have suggested that microstructural changes, such as collagen fiber re-alignment, may be a result of preconditioning. Local collagen fiber re-alignment is quantified throughout tensile mechanical testing using a testing system integrated with a polarized light setup, consisting of a backlight, 90 deg-offset rotating polarizer sheets on each side of the test sample, and a digital camera, in a rat supraspinatus tendon model, and corresponding mechanical properties are measured. Local circular variance values are compared throughout the mechanical test to determine if and where collagen fiber re-alignment occurred. The inhomogeneity of the tendon is examined by comparing local circular variance values, optical moduli and optical transition strain values. Although the largest amount of collagen fiber re-alignment was found during preconditioning, significant re-alignment was also demonstrated in the toe and linear regions of the mechanical test. No significant changes in re-alignment were seen during stress relaxation. The insertion site of the supraspinatus tendon demonstrated a lower linear modulus and a more disorganized collagen fiber distribution throughout all mechanical testing points compared to the tendon midsubstance. This study identified a correlation between collagen fiber re-alignment and preconditioning and suggests that collagen fiber re-alignment may be a potential mechanism of preconditioning and merits further investigation. In particular, the conditions necessary for collagen fibers to re-orient away from the direction of loading and the dependency of collagen reorganization on its initial distribution must be examined.

Recapitulation of the Achilles tendon mechanical properties during neonatal development: a study of differential healing during two stages of development in a mouse model.

During neonatal development, tendons undergo a well-orchestrated process whereby extensive structural and compositional changes occur in synchrony to produce a normal tissue. Conversely, during the repair response to injury, structural and compositional changes occur, but a mechanically inferior tendon is produced. As a result, developmental processes have been postulated as a potential paradigm through which improved adult tissue healing may occur. By examining injury at distinctly different stages of development, vital information can be obtained into the structure-function relationships in tendon. The mouse is an intriguing developmental model due to the availability of assays and genetically altered animals. However, it has not previously been used for mechanical analysis of healing tendon due to the small size and fragile nature of neonatal tendons. The objective of this study was to evaluate the differential healing response in tendon at two distinct stages of development through mechanical, compositional, and structural properties. To accomplish this, a new in vivo surgical model and mechanical analysis method for the neonatal mouse Achilles tendons were developed. We demonstrated that injury during early development has an accelerated healing response when compared to injury during late development. This accelerated healing model can be used in future mechanistic studies to elucidate the method for improved adult tendon healing.

The effects of alpha-helical structure and cyanylated cysteine on each other.

Beta-thiocyanatoalanine, or cyanylated cysteine, is an artificial amino acid that can be introduced at solvent-exposed cysteine residues in proteins via chemical modification. Its facile post-translational synthesis means that it may find broad use in large protein systems as a probe of site-specific structure and dynamics. The C[triple bond]N stretching vibration of this artificial side chain provides an isolated infrared chromophore. To test both the perturbative effect of this side chain on local secondary structure and its sensitivity to structural changes, three variants of a model water-soluble alanine-repeat helix were synthesized containing cyanylated cysteine at different sites. The cyanylated cysteine side chain is shown to destabilize, but not completely disrupt, the helical structure of the folded peptide when substituted for alanine. In addition, the [triple bond]N stretching bandwidth of the artificial side chain is sensitive to the helix-coil structural transition. These model system results indicate that cyanylated cysteine can be placed into protein sequences with a native helical propensity without destroying the helix, and further that the CN probe may be able to report local helix formation events even when it is water-exposed in both the ordered and disordered conformational states. These results indicate that cyanylated cysteine could be a widely useful probe of structure-forming events in proteins with large in vitro structural distributions.