Effect of supraspinatus tendon repair technique on the infraspinatus tendon

Supraspinatus tendon tears are common and often propagate into larger tears that include the infraspinatus tendon, resulting in loss of function and increased pain. Previously, we showed that the supraspinatus and infraspinatus tendons mechanically interact through a range of rotation angles, potentially shielding the torn supraspinatus tendon from further injury while subjecting the infraspinatus tendon to increased risk of injury. Surgical repair of torn supraspinatus tendons is common, yet the effect of the repair on the infraspinatus tendon is unknown. Since we have established a relationship between strain in the supraspinatus and infraspinatus tendons the success of a supraspinatus tendon repair depends on its effect on the loading environment in the infraspinatus tendon. More specifically, the effect of transosseous supraspinatus tendon repair in comparison to one that utilizes suture anchors, as is commonly done with arthroscopic repairs, on this interaction through these joint positions will be evaluated. We hypothesize that at all joint positions evaluated, both repairs will restore the interaction between the two tendons. For both repairs, (1) increasing supraspinatus tendon load will increase infraspinatus tendon strain and (2) altering the rotation angle from internal to external will increase strain in the infraspinatus tendon. Strains were measured in the infraspinatus tendon insertion through a range of joint rotation angles and supraspinatus tendon loads, for the intact, transosseous, and suture anchor repaired supraspinatus tendons. Images corresponding to specific supraspinatus tendon loads were isolated for the infraspinatus tendon insertion for analysis. The effect of supraspinatus tendon repair on infraspinatus tendon strain differed with joint position. Altering the joint rotation did not change strain in the infraspinatus tendon for any supraspinatus tendon condition. Finally, increasing supraspinatus tendon load resulted in an increase in average maximum and decrease in average minimum principal strain in the infraspinatus tendon. There is a significant difference in infraspinatus tendon strain between the intact and arthroscopically (but not transosseous) repaired supraspinatus tendons that increases with greater loads. Results suggest that at low loads neither supraspinatus tendon repair technique subjects the infraspinatus tendon to potentially detrimental loads; however, at high loads, transosseous repairs may be more advantageous over arthroscopic repairs for the health of the infraspinatus tendon. Results emphasize the importance of limiting loading of the repaired supraspinatus tendon and that at low loads, both repair techniques restore the interaction to the intact supraspinatus tendon case.     

The effect of altered loading following rotator cuff tears in a rat model on the regional mechanical properties of the long head of the biceps tendon

Biceps tendon pathology is a common clinical problem often seen in conjunction with rotator cuff tears. A previous study found detrimental changes to biceps tendons in the presence of rotator cuff tears in a rat model. Therefore, the objective of this study was to utilize this model along with models of altered loading to investigate the effect of altered loading on the initiation of these detrimental changes. We created supraspinatus and infraspinatus rotator cuff tears in the rat and followed these tears with either increased or decreased loading. Mechanical properties were determined along the length of the biceps tendon 4 and 8 weeks following injury. At the insertion site, stiffness increased with decreased loading, while detrimental changes were seen with increased loading 4 weeks following detachments. Increased loading resulted in decreased mechanical properties along the entire tendon length at both time points. Decreased loading resulted in both increased and decreased tendon properties at different regions of the tendon at 4 weeks, but by 8 weeks, there were no differences between decreased loading and detachment alone. We could not conclude where changes begin in the tendon with altered loading, but did demonstrate that regional differences exist. These results support that there is an effect of altered loading, as decreased loading resulted in variable changes at 4 weeks that were no different from detachment alone by 8 weeks, and increased loading resulted in detrimental properties along the entire length at both 4 and 8 weeks.     

Rotator cuff tendon strain correlates with tear propagation

Rotator cuff tears are a common tendon injury often requiring surgical treatment. Understanding the relationships between tear size, tendon loading, and tendon strain adjacent to a rotator cuff tear can provide important insights into predicting the likelihood of propagation to larger tears which would influence clinical treatment. Previous studies assume that an increase in strain correlates with an increase in risk of tear propagation. However, these studies did not explicitly investigate these important relationships. Therefore, the objective of this study was to quantify two-dimensional strain fields adjacent to a rotator cuff tendon tear under loading to failure and to assess the relationship between tendon strain and tear size. Sheep infraspinatus tendons were used to evaluate the effect of tear size on principal strains in the region adjacent to the tear. The relationship between strain, tear propagation, and the direction of tear propagation was quantified. Results showed that principal strains linearly correlated with tear propagation and that tear propagation began at strains as low as 1.7%. In addition, tears propagated in the direction of highest maximum and lowest minimum principal strain. Finally, maximum and minimum principal strains were higher and lower, respectively, adjacent to larger tears compared to smaller tears. Findings from this study validate the use of local strain adjacent to a rotator cuff tear as an indicator of the risk and direction of tear propagation.     

Long durations of immobilization in the rat result in enhanced mechanical properties of the healing supraspinatus tendon insertion site

Rotator cuff tears frequently occur and can lead to pain and decreased shoulder function. Repair of the torn tendon back to bone is often successful in relieving pain, but failure of the repair commonly occurs. Post-operative activity level is an important treatment component that has received minimal attention for the shoulder, but may have the potential to enhance tendon to bone healing. The objective of this study was to investigate the effect of short and long durations of various activity levels on the healing supraspinatus tendon to bone insertion site. Rotator cuff tears were surgically created in Sprague-Dawley rats by detaching the supraspinatus tendon from its insertion on the humerus and these tears were immediately repaired back to the insertion site. The post-operative activity level was controlled through shoulder immobilization (IM), cage activity (CA), or moderate exercise (EX) for durations of 4 or 16 weeks. The healing tissue was evaluated utilizing biomechanical testing and a quantitative polarized light microscopy method. We found that activity level had no effect on the elastic properties (stiffness, modulus) of the insertion site at four weeks post injury and repair, and a decreased activity level had a positive effect on these properties at 16 weeks (IM>CA=EX). Furthermore, a decreased activity level had the greatest positive effect on these properties over time (IM>CA=EX). The angular deviation of the collagen, a measure of disorganization, was decreased with a decrease in activity level at 4 weeks (IM相似文献   

The role of repair tension on tendon to bone healing in an animal model of chronic rotator cuff tears

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.     

Effects of rotator cuff tears on muscle moment arms: a computational study

Rotator cuff tears cause morphologic changes to cuff tendons and muscles, which can alter muscle architecture and moment arm. The effects of these alterations on shoulder mechanical performance in terms of muscle force and joint strength are not well understood. The purpose of this study was to develop a three-dimensional explicit finite element model for investigating morphological changes to rotator cuff tendons following cuff tear. The subsequent objectives were to validate the model by comparing model-predicted moment arms to empirical data, and to use the model to investigate the hypothesis that rotator cuff muscle moment arms are reduced when tendons are divided along the force-bearing direction of the tendon. The model was constructed by extracting tendon, cartilage, and bone geometry from the male Visible Human data set. Infraspinatus and teres minor muscle and tendon paths were identified relative to the humerus and scapula. Kinetic and kinematic boundary conditions in the model replicated experimental protocols, which rotated the humerus from 45 degrees internal to 45 degrees external rotation with constant loads on the tendons. External rotation moment arms were calculated for two conditions of the cuff tendons: intact normal and divided tendon. Predicted moment arms were within the 1-99% confidence intervals of experimental data for nearly all joint angles and tendon sub-regions. In agreement with the experimental findings, when compared to the intact condition, predicted moment arms were reduced for the divided tendon condition. The results of this study provide evidence that one potential mechanism for the reduction in strength observed with cuff tear is reduction of muscle moment arms. The model provides a platform for future studies addressing mechanisms responsible for reduced muscle force and joint strength including changes to muscle length-tension operating range due to altered muscle and tendon excursions, and the effects of cuff tear size and location on moment arms and muscle forces.     

A musculoskeletal shoulder simulation of moment arms and joint reaction forces after medialization of the supraspinatus footprint in rotator cuff repair

A non-anatomical reinsertion of the supraspinatus medially to the original footprint to avoid over-tensioning of the tendon in large and retracted tears is one surgical option in rotator cuff (RC) repair. The purpose of the study was to determine the biomechanical effects on the glenohumeral joint with regard to this surgical technique. A modified musculoskeletal computational shoulder model was used to evaluate the change in moment arms and muscle forces of the RC and the co-contracting muscles and the alteration of the joint reaction forces (compressive and shear forces) after reinsertion of the supraspinatus 5?mm, 10?mm, 15?mm and 20?mm medially to the original footprint. A medialization of the supraspinatus reduces its moment arm in glenohumeral abduction. In case of a medialization of the attachment of 15?mm and 20?mm, the supraspinatus restricts glenohumeral abduction at 54° and 68°. In glenohumeral forward flexion and in lower degrees of internal rotation the moment arm of the supraspinatus increases for a medialized tendon attachment and decreases in external rotation in relation to the anatomical condition. A medialization of the supraspinatus insertion point yields in an increase in muscle force for abduction, internal and external rotation. In the present model a medially non-anatomic reinsertion reduces significantly the compressive glenohumeral joint reaction and the glenohumeral stability. Moreover, the results show that a medialization of the supraspinatus leads to a reduction of the supraspinatus moment arm especially in abduction. This leads to an increase of a compensatory supraspinatus load for stabilization the humerus in space, which may potentially cause a postoperative overload of the tendon-bone-complex.     

Supraspinatus tendon organizational and mechanical properties in a chronic rotator cuff tear animal model

Rotator cuff tears of the shoulder are a common cause of pain and disability. The successful repair of rotator cuff tendon tears depends on the time from onset of injury to the time of surgical repair. However, the effect of time from injury to repair remains poorly understood. A rat model was used to investigate the supraspinatus tendon organizational and mechanical property changes that occur with time post-injury to understand the natural injury response in the absence of repair. It was hypothesized that increased time post-injury would result in increased detrimental changes to tendon organizational and mechanical properties. Tendons were detached at the insertion on the humerus without repair and the quantitative organizational and mechanical properties were analyzed at 1, 2, 4, 8, and 16 weeks post-detachment. Tendon detachment resulted in a dramatic decrease in mechanical properties initially followed by a progressive increase with time. The quantitative collagen fiber orientation results provided corroborating support to the mechanical property data. Based on similarities in histology and mechanical properties to rotator cuff tears in humans, the animal model presented here is promising for future investigations of the tendon's natural injury response in the absence of repair.     

Tendon to bone healing: differences in biomechanical,structural, and compositional properties due to a range of activity levels

Little knowledge exists about the healing process of the tendon to bone insertion, and hence little can be done to improve tissue healing. The goal of this study is to describe the healing of the supraspinatus tendon to its bony insertion under a variety of loading conditions. Tendons were surgically detached and repaired in rats. Rat shoulders were then immobilized, allowed cage activity, or exercised. Shoulders that were immobilized demonstrated superior structural (significantly higher collagen orientation), compositional (expression of extracellular matrix genes similar to the uninjured insertion), and quasilinear viscoelastic properties (A = 0.30 +/- 0.10 MPa vs. 0.16 +/- 0.08 MPa, B = 17.4 +/- 2.9 vs. 15.1 +/- 0.9, and tau 2 = 344 +/- 161 s vs. 233 +/- 40 s) compared to those that were exercised, contrary to expectations. With this knowledge of the healing response, treatment modalities for rotator cuff tears can be developed.     

Quantitative comparison of three rat models of Achilles tendon injury: A multidisciplinary approach

The Achilles tendon, while the strongest and largest tendon in the body, is frequently injured. Inconclusive evidence exists regarding treatment strategies for both complete tears and partial tears. Well-characterized animal models of tendon injury are important for understanding physiological processes of tendon repair and testing potential therapeutics. Utilizing three distinct models of rat Achilles tendon injury, the objective of this study was to define and compare the effects and relative impact on tendon properties and ankle function of both tear severity (complete tear versus partial tear, both with post-operative immobilization) and immobilization after partial tear (partial tear with versus without immobilization). We hypothesized that a complete tear would cause inferior post-injury properties compared to a partial tear, and that immediate loading after partial tear would improve post-injury properties compared to immobilization. All models were reproducible and had distinct effects on measured parameters. Injury severity drastically influenced tendon healing, with complete tear causing decreased ankle mobility and tendon mechanics compared to partial tears. One week of plantarflexion immobilization had a strong effect on animals receiving a partial tear. Tendons with partial tears and immobilization failed early during fatigue cycling three weeks post-injury. Partial tear without immobilization had no effect on ankle range of motion through dorsiflexion at any time point compared to the pre-surgery value, while partial tear with immobilization demonstrated diminished function at all post-injury time points. All three models of Achilles injury could be useful for tendon healing investigations, chosen based on the prospective applications of a potential therapeutic.     

Quantitative Ultrasound Facilitates the Exploration of Morphological Association of the Long Head Biceps Tendon with Supraspinatus Tendon Full Thickness Tear

Backgrounds

Pathology of the long head biceps tendon (LHBT) is associated with rotator cuff tears but whether the LHBT texture changes following supraspinatus tendon full thickness tear (SSFT) can be detected at the extra-articular segment remains unknown. This cross-sectional study aimed to explore the morphological differences of the LHBT in shoulders with and without deficient rotator cuffs by using quantitative ultrasound.

Materials and Methods

We selected 145 cases with SSFT and 145 age-and- gender-matched controls. The width, thickness, flattening ratio, cross-sectional area, and echogenicity ratio of the LHBT were measured and a general linear model was used to clarify the relationship between rotator cuff pathology and LHBT morphology. The receiver operating characteristic curves of each parameter were constructed for SSFT discrimination and the maximal Youden indexes were used to define the best cut-off points.

Results

We found increased thickness and cross-sectional area but decreased flattening ratio in shoulders with SSFT, and no between-group differences in the width and echogenicity ratio. The LHBT appearance was modified by biceps peritendinous effusion and medial subluxation, but not by the size of SSFT. The flattening ratio was the best discriminator for SSFT with an area under curve of 0.81 (95% confidence interval, 0.76–0.86). The cut-off values to differentiate between the non-tear and tear groups were 2.00 mm of the thickness, 1.73 of the flattening ratio and 10.53 mm² of the cross-sectional area.

Conclusion

Quantitative ultrasound facilitated the detection of the LHBT morphological changes following SSFT and demonstrated its potential utility in discriminating rotator cuff deficiency.     

Biceps tenotomy in the presence of a supraspinatus tear alters the adjacent intact tendons and glenoid cartilage

A rotator cuff tear is a common injury in athletes and workers who repeatedly perform overhead movements, and it is not uncommon for this demographic to return to activity shortly after treatment. A biceps tenotomy is often performed in the presence of a rotator cuff tear to help reduce pain and improve joint function. However, the effect of this procedure on the surrounding tissues in the glenohumeral joint is unknown. Therefore, the purpose of this study was to investigate the effect of a biceps tenotomy in the presence of a supraspinatus rotator cuff tear followed by overuse activity on ambulatory function and mechanical and histologic properties of the remaining rotator cuff tendons and glenoid cartilage. 46 rats underwent 4 weeks of overuse activity to create a tendinopathic condition, then were randomized into two groups: unilateral detachment of the supraspinatus tendon or detachment of the supraspinatus and long head of the biceps tendons. Ambulatory measurements were performed throughout the 8 weeks prior to euthanasia, followed by analysis of the properties of the remaining intact tendons and glenoid cartilage. Results demonstrate that shoulder function was not effected in the biceps tenotomy group. However, the intact tendons and glenoid cartilage showed altered mechanical and histologic properties. This study provides evidence from an animal model that does not support the use of tenotomy in the presence of a supraspinatus tendon rotator cuff tear, and provides a framework for physicians to better prescribe long-term treatment strategies for patients.     

In situ fibril stretch and sliding is location-dependent in mouse supraspinatus tendons

Tendons are able to transmit high loads efficiently due to their finely optimized hierarchical collagen structure. Two mechanisms by which tendons respond to load are collagen fibril sliding and deformation (stretch). Although many studies have demonstrated that regional variations in tendon structure, composition, and organization contribute to the full tendon׳s mechanical response, the location-dependent response to loading at the fibril level has not been investigated. In addition, the instantaneous response of fibrils to loading, which is clinically relevant for repetitive stretch or fatigue injuries, has also not been studied. Therefore, the purpose of this study was to quantify the instantaneous response of collagen fibrils throughout a mechanical loading protocol, both in the insertion site and in the midsubstance of the mouse supraspinatus tendon. Utilizing a novel atomic force microscopy-based imaging technique, tendons at various strain levels were directly visualized and analyzed for changes in fibril d-period with increasing tendon strain. At the insertion site, d-period significantly increased from 0% to 1% tendon strain, increased again from 3% to 5% strain, and decreased after 5% strain. At the midsubstance, d-period increased from 0% to 1% strain and then decreased after 7% strain. In addition, fibril d-period heterogeneity (fibril sliding) was present, primarily at 3% strain with a large majority occurring in the tendon midsubstance. This study builds upon previous work by adding information on the instantaneous and regional-dependent fibrillar response to mechanical loading and presents data proposing that collagen fibril sliding and stretch are directly related to tissue organization and function.     

Collagen fiber orientation at the tendon to bone insertion and its influence on stress concentrations

The tendon to bone insertion serves the mechanical role of transferring loads from a relatively compliant tendon to a relatively rigid bone. The details of the mechanism of load transfer are of great importance, since current surgical procedures for tendon reattachment have high failure rates. We hypothesized that the microscopic structure of the insertion is optimized to minimize stress concentrations associated with this load transfer. To explore this, collagen fiber orientation distributions were measured in the supraspinatus tendons of rats. The angular deviation of fibers was fairly uniform across the insertion, and the mean angles of the local distributions deviated mildly from the tendon axis. To explore how these observed property distributions could influence load transfer, these distributions were used to derive material properties for an idealized two-dimensional mechanical model of an insertion. Comparison between stress concentrations in this idealized model and those in three comparison models suggests that the microstructure serves to (1) simultaneously reduce stress concentrations and material mass, and (2) shield the insertion's outward splay from the highest stresses.     

Determination of the compressive material properties of the supraspinatus tendon

A methodology was developed for determining the compressive properties of the supraspinatus tendon, based on finite element principles. Simplified three-dimensional models ure re reated based on anatomical thickness measurements of unloaded supraspinatus tendons over 15 points. The tendon material was characterized as a composite structure of' longitudinally arranged collagen fibers within an extrafibrillar matrix. The matrix was formulated as a hyperelastic material described by the Ogden form of the strain energy potential. The hyperelastic material parameters were parametrically manipulated until the analytical load-displacement results were similar to the results obtaizned from indentation testinrg. In the geometrically averaged tendon, the average ratio of experimental to theoretical maximum indentation displacement was 1.00 (SD: 0.01). The average normalization of residuals was 2.1 g (SD: 0.9 g). Therefjore, the compressive material properties of the supraspinatus tendo'n extrafibrillar matrix were adequately derived with a first-order hyperelastic formulation. The initial comnpressive elastic modulus ranged from 0.024 to 0.090 MPa over the tendon surface and increased nonlinearly with additional compression. Using these material properties, the stresses induced during acromional impingement can be analyzed.     

Genetic Response of Rat Supraspinatus Tendon and Muscle to Exercise

Inflammation is a complex, biologic event that aims to protect and repair tissue. Previous studies suggest that inflammation is critical to induce a healing response following acute injury; however, whether similar inflammatory responses occur as a result of beneficial, non-injurious loading is unknown. The objective of this study was to screen for alterations in a subset of inflammatory and extracellular matrix genes to identify the responses of rat supraspinatus tendon and muscle to a known, non-injurious loading condition. We sought to define how a subset of genes representative of specific inflammation and matrix turnover pathways is altered in supraspinatus tendon and muscle 1) acutely following a single loading bout and 2) chronically following repeated loading bouts. In this study, Sprague-Dawley rats in the acute group ran a single bout of non-injurious exercise on a flat treadmill (10 m/min, 1 hour) and were sacrificed 12 or 24 hours after. Rats in the chronic group ran 5 days/wk for 1 or 8 weeks. A control group maintained normal cage activity. Supraspinatus muscle and tendon were harvested for RNA extractions, and a custom Panomics QuantiGene 2.0 multiplex assay was used to detect 48 target and 3 housekeeping genes. Muscle/tendon and acute/chronic groups had distinct gene expression. Components of the arachidonic acid cascade and matrix metalloproteinases and their inhibitors were altered with acute and chronic exercise. Collagen expression increased. Using a previously validated model of non-injurious exercise, we have shown that supraspinatus tendon and muscle respond to acute and chronic exercise by regulating inflammatory- and matrix turnover-related genes, suggesting that these pathways are involved in the beneficial adaptations to exercise.     

Value of exact focusing of extracorporeal shock waves (ESWT) in therapy of tendinitis calcarea. A prospective randomized study]

A controlled randomized study was designed to analyse the effect of extracorporeal shockwave therapy (ESWT) focussed on either the calcified region or the insertion of the supraspinatus tendon. The study included 50 patients who were treated with a Storz Minilith SI-1 prototype shockwave generator. In the treated group, 4000 impulses (ED+ 0.78 ml/mm2) were applied, under local anaesthesia to the insertion of the supraspinatus in 2 treatment sessions. Control patients received ESWT focussed on the calcified region. Follow-up examinations were carried out 12 weeks after treatment by an independent observer. We found functional improvement and pain reduction in both groups. Statistical analyses showed significant superiority of ESWT focussed on the calcified region for the parameters constant-score (primary endpoint, p < 0.001) and pain intensity (p = 0.001). For the treatment of calcific tendinitis affecting the supraspinatus, we recommend accurate fluoroscopy-controlled focussing of ESWT on the calcification. Focussing on the calcification rather than on the insertion of the supraspinatus tendon is significantly more effective. On the basis of our results, ESWT requires the use of suitable shockwave generators that permit accurate focussing.     

Quantifying extensibility of rotator cuff muscle with tendon rupture using shear wave elastography: A cadaveric study

Surgical repair for large rotator cuff tear remains challenging due to tear size, altered muscle mechanical properties, and poor musculotendinous extensibility. Insufficient extensibility might lead to an incomplete reconstruction; moreover, excessive stresses after repair may result in repair failure without healing. Therefore, estimates of extensibility of cuff muscles can help in pre-surgical planning to prevent unexpected scenarios during surgery. The purpose of this study was to determine if quantified mechanical properties of the supraspinatus muscle using shear wave elastography (SWE) could be used to predict the extensibility of the musculotendinous unit on cadaveric specimens. Forty-five fresh-frozen cadaveric shoulders (25 intact and 20 with rotator cuff tear) were used for the study. Passive stiffness of 4 anatomical regions in the supraspinatus muscle was first measured using SWE. After detaching the distal edge of supraspinatus muscle from other cuff muscles, the detached muscle was axially pulled with the scapula fixed. The correlation between the SWE modulus and the extensibility of the muscle under 30 and 60 N loads was assessed. There was a significant negative correlation between SWE measurements and the experimental extensibility. SWE modulus for the anterior-deep region in the supraspinatus muscle showed the strongest correlation with extensibility under 30 N (r = 0.70, P < 0.001) and 60 N (r = 0.68, P < 0.001). Quantitative SWE assessment for the supraspinatus muscle was highly correlated with extensibility of musculotendinous unit on cadaveric shoulders. This technique may be used to predict the extensibility for rotator cuff tears for pre-surgical planning.     

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.