Objectives

The aim of this study was to investigate the structural integrity of torn and non-torn human acetabular labral tissue.

Introduction

The pathophysiology of high failure rates following rotator cuff tendon repairs, particularly massive tears, is not fully understood. Collagen structural changes have been shown to alter tendon thermal and mechanical properties. Thermal changes in small biopsies, detected by differential scanning calorimetry (DSC) can help to quantify collagen structural differences in torn rotator cuff tendons. This study aimed to form a quantitative rather than qualitative assessment, of whether differences in collagen structure and integrity existed between small biopsies of normal, small and massive rotator cuff tears using DSC.

Background

High re-rupture rates following repairs of rotator cuff tears (RCTs) have resulted in the increased use of repair grafts to act as temporary scaffolds to support tendon healing. It has been estimated that thousands of extracellular matrix repair grafts are used annually to augment surgical repair of rotator cuff tears. The only mechanical assessment of the suitability of these grafts for rotator cuff repair has been made using tensile testing only, and compared grafts to canine infraspinatus. As the shoulder and rotator cuff tendons are exposed to shearing as well as uniaxial loading, we compared the response of repair grafts and human rotator cuff tendons to shearing mechanical stress. We used a novel technique to study material deformation, dynamic shear analysis (DSA).

Improved understanding of the biomechanics and biology of rotator cuff tendons (RCT) may help reduce high re-rupture rates following repairs, particularly amongst larger tears. This study aims to use novel methods for quantitatively determining differences in the mechanical and thermal properties of intact healthy RCTs compared to torn ‘diseased’ tendons. A common problem in the mechanical testing of small tendon samples is that stress risers at the clamp-tendon interface can obscure measurements. As the shoulder is subject to shear, tension and compression, we developed a novel solution using Dynamic Shear Analysis (DSA), a form of rheology which studies material deformation. As collagen is the main component of RCT, the structure and mechanical properties may be affected by collagen conformational changes. Both dermis and rat tail tendon with increased collagen cross-linking exhibit stronger mechanical properties. Thermal changes detected by differential scanning calorimetry (DSC) can help to quantify collagen structural differences in torn RCT, and has been previously used to study muscle, cartilage and vertebral discs.

There were 79 tears (mean age 65.2 years), which were classified according to the size of the tear as small, medium, large and massive. Two separate 3mm-sized biopsy samples were taken and subjected to DSA using oscillatory deformation under compression. The storage modulus (G') was calculated and used as an indicator of mechanical integrity. 18 control tendon specimens were obtained from patients aged between 22-89 years (mean age 58.8 years) during shoulder hemiarthroplasties and stabilisations. Additionally 7 normal, 7 small and 7 massive frozen specimens were thermally characterized. 3 samples per patient were heated between 20-80oC in hermetically sealed vessels. Useful thermal parameters were measured such as the melting temperature (TM) which apparently represents breaking of the amide-amide bonds and protein chains mobility, the denaturation temperature (TD) which supposedly corresponds to proteins falling out of solution and the denaturation enthalpy (ΔH) which reflects the relative amount of triple helical structure.

Healthy tendons had a significantly higher modulus than torn tendons, indicating that torn tendons are mechanically weaker than normal tendons (p = 0.032). Normal tendons had significantly higher mean shear modulus than tendons with small and massive tears (p<0.01). Overall there was a negative correlation between moduli and severity of tendon tear (r = −0.698, p=0.189). The moduli did not significantly correlate with age, sex, hand dominance, or length of preservation in formalin. Massive RCT tears had significantly higher TM and TD when compared to normal RCT (p < 0.05), unlike small RCT tears. No significant difference was detected between the denaturation enthalpy of the different RCT groups. This case control study has demonstrated that normal RCTs have a significantly higher modulus than torn tendons, indicating that torn tendons have less mechanical integrity. Our study further demonstrated a trend between increasing tear size and decreasing mechanical integrity. This study has also demonstrated differences in some of the thermal properties of normal and torn RCTs. These are likely due to collagen structural changes. A decrease in the denaturation temperature of torn tendons, suggests that the material is intrinsically less stable. Torn tendons with reduced storage modulus and collagen integrity may be less able to withstand mechanical loads following repair. This pilot study provides some preliminary insight into the mechanisms that may contribute to, or represent adaptations to high rates of failure of RCT repairs.

Background: Up to one third of adults have been estimated to have rotator cuff tendon (RCT) tears. Larger RCT tears are associated with poorer scores and function, and are more likely to re-rupture after surgical repairs, hence there is a need for earlier identification and treatment. The aim of this study was to identify biomarkers of RCT tear pathologies to aid accurate identification and monitoring of disease progression. FTIR provides unique biochemical fingerprints of tissue specimens. All molecules are excited to higher vibrational states at specific wavelengths, which can be used to identify the chemical composition of tissues.

Methods: The chemical composition of 55 formalin-fixed RCTs was measured from patients aged between 20 and 89. RCT tears were classified according to size (Post et al.); 10 each of small, medium, large and massive and 5 partial tears. These torn RCTs were compared to 10 uninjured RCTs. A diamond attenuated total reflectance accessory was used with a FTIR spectrometer to collect spectra for each sample. The spectra were reduced and classified using standard multivariate analysis; principal component analysis (PCA), partial least square (PLS) and discriminant function analysis (DFA). Data pre-processing was applied to ensure accurate quantitative data analysis.

Results: Hierarchical cluster (HCA) demonstrated that normal and torn tendons could be clearly differentiated, and RCT could also be distinguished by their tear size. Partial tears were clearly distinguishable from normal RCT. Using a genetic algorithm we identified the following spectral regions of importance which accounted for most of the features which discriminated between normal and torn tendons:

1030–1200cm-1: carbohydrates, phospholipids,

Partial tears were distinguishable from other stages of tendon pathology based on a spectral region which correlated with collagen III.

Conclusions: FTIR can clearly distinguish normal and different sized RCT tears. This prospective non-randomized study indicates that the onset of RCT tear pathology is mainly due to an alteration of the collagen structural arrangements, with associated changes in lipids and carbohydrates. Partial tears show early onset of chemical changes, particularly in collagen III, which could be used to identify earlier stages of disease. The approach described is rapid and has the potential to be used per-operatively to determine the quality of the tendon and extent of disease, thus guiding surgical repairs or allowing monitoring of disease progression or response to treatments.