Mechanical response of tendons to common therapies and the effects of tendon injury progression
Restricted (Penn State Only)
- Author:
- Khandare, Sujata
- Graduate Program:
- Bioengineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 23, 2022
- Committee Members:
- Stephen Piazza, Outside Unit & Field Member
Spencer Szczesny, Major Field Member
Meghan Vidt, Chair & Dissertation Advisor
Julianna Simon, Major Field Member
Daniel Hayes, Program Head/Chair - Keywords:
- Dry needling
Focused ultrasound
Tendon
Mechanical properties
Mechanical testing
Rotator cuff
Computational model
Glenohumeral joint
Muscle activation
Force couple
Tear severity
Rotator cuff tear - Abstract:
- Musculoskeletal disorders are reported by over 100 million Americans each year, with an associated economic cost of ~$800 billion. Degenerative disorders of the tendon, or tendinopathies account for ~30 % of musculoskeletal disorders with Achilles and supraspinatus tendons most prone to pathology. Tendinopathies result in mechanically weaker tendons that could lead to tendon rupture if treatments that are designed to promote healing are not assessed mechanically. Given that tendons are load-bearing structures that transfer high amounts of load from the muscle to bone, treatments for tendon injuries should be assessed based on their mechanical effects to ensure tendon mechanical properties are not compromised. However, certain tendon injuries, like the rotator cuff tears, are asymptomatic and progress over time in the absence of treatment. Rotator cuff tears lead to altered movement kinematics and potential joint instability. Therefore, it is necessary to understand how rotator cuff tear progression affects muscle activation patterns that could play a compensatory role in stabilizing the joint. First, we studied the mechanical effects of two treatment modalities on tendon mechanical properties: dry needling (DN), and focused ultrasound (fUS). DN is widely used to alleviate pain and restore function after tendon injury. DN involves repeated “peppering” of the injured tendon which is hypothesized to create micro-damage and facilitate healing. However, repeated needle insertions may disrupt the mechanical integrity of the tendon leading to reduced mechanical properties and predisposal to rupture. Moreover, DN is invasive and high inter-practitioner variability has led to mixed success rates. fUS is a non-invasive medical technology that directs ultrasound energy into a well-defined focal volume. fUS can induce thermal and/or mechanical bioeffects, with bioeffect type controlled through ultrasound parameters. Mechanical bioeffects of fUS can potentially introduce micro-damage, similar to DN, to elicit the healing response; however, tendons have been resistant to mechanical bioeffects. If fUS can induce micro-damage similar to DN while maintaining tendon mechanical properties and facilitating tendon healing, this could represent a promising non-invasive alternative for treating tendinopathies. We mechanically tested ex vivo rat Achilles and supraspinatus tendons treated with either sham, DN, or fUS. Results showed that the elastic modulus of supraspinatus tendons treated with DN was lower than sham. Stiffness and percent relaxation of both tendons treated with DN were lower than sham. Modulus of both tendons treated with fUS were similar to sham. We also performed a pilot study to evaluate and compare the mechanical and healing effects of DN and fUS in a rat tendinopathy model. Elastic modulus, stiffness, and ultimate tensile stress of tendons exposed to DN were lower than the contralateral controls; maximum load was not different; and percent relaxation was significantly higher. Elastic modulus and ultimate tensile stress of tendons exposed to fUS were lower than the contralateral controls; stiffness, percent relaxation, and maximum load were not different. Healing effects of DN and fUS were not different. These results suggest that fUS preserves mechanical properties better than DN. To understand how these treatments affect the distribution of stress and strain in tendons under varying physiological loading conditions, we developed a finite element (FE) model of the rat supraspinatus muscle-tendon unit. FE models were customized with our previous experimentally-derived tendon mechanical properties to represent tendons treated with sham, DN, and fUS. Physiologically increasing loads were applied to the tendon to compute the magnitude and distribution of principal stress and strain. No differences in stress and strain were observed in the different treatment models and a linear increase in maximum stress and strain with increased applied load was observed in all models. Magnitude and location of maximum stress and strain from these analyses could inform predictions of location-specific initiation and progression of tendon injuries. Second, we studied how increased rotator cuff tear (RCT) severity affects glenohumeral joint loading and muscle activation patterns using a computational model. RCT cause decreased muscle forces and disrupt the force balance at the glenohumeral joint, compromising joint stability. Increased RCT severity may affect predictors of joint stability, like joint contact force (JCF), but they are not feasible to measure in vivo. Computational musculoskeletal models provide a tool to predict JCF or track RCT progression in the absence of treatment. Since RCT are highly prevalent in older adults, muscle volume measurements were used to scale a nominal upper limb model’s peak isometric muscle forces to represent force generating characteristics of an average older adult male. Increased RCT severity was represented by systematically decreasing peak isometric muscle forces of supraspinatus, infraspinatus, and subscapularis. Computational simulations were performed for static postures to examine the influence of muscle force imbalance across the glenohumeral joint on predicted muscle activation and predicted magnitude and orientation of glenohumeral JCF. Results revealed that the peak glenohumeral JCF magnitude remained relatively consistent across increased RCT severity and a relative balance of the transverse force couple is maintained even in massive RCT models. Predicted muscle activations of intact muscles, like teres minor, increased with greater RCT severity. The existing upper limb model, however, has certain limitations. The glenohumeral joint is represented as a ball-and-socket joint and the model includes stiffness constraints at end ranges of motion to represent force contributions from ligaments, rather than actual ligaments. This limits the understanding of how humeral head translation could contribute to reduced subacromial space eventually leading to shoulder impingement syndrome or rotator cuff tears or dislocation. Therefore, we developed the model to include more biofidelic representations, specifically through the inclusion of coracohumeral ligaments, glenohumeral ligaments, posterior capsule, and three translational degrees of freedom (DOF) to complement the existing three rotational DOF in the model. Gravity driven simulations were performed to evaluate the function of coracohumeral (CHL) and superior glenohumeral ligament (SGHL) and results were compared to previous studies to validate the model. Effect of shoulder elevation on humeral head translation during static tasks were also evaluated and compared to previous studies. Glenohumeral ligaments and translations were implemented in the model and the model was successfully validated by comparison with literature. Predicted superior-inferior translation of the humeral head reveal that for shoulder elevations of 0°, 15°, and 30°, the humeral head translates inferiorly; however for 45° shoulder elevation, the humeral head translates superiorly, which is consistent with previous experimental evaluations. Inclusion of ligaments and humeral head translation in the model facilitate prediction of conditions like subacromial impingement that could be followed by RCT. Results from our first study suggest that fUS does not negatively affect tendon mechanical properties and we also identified the specific fUS parameters that can create the desired bioeffects in tendons. fUS should be studied further to fully understand its effects to help evaluate fUS as an alternative, non-invasive treatment for tendon injuries. Results from our second study suggest that glenohumeral joint stability is prioritized even with severe RCT as unaffected muscles play a compensatory role. These results combined with evaluation of humeral head translation provide a better understanding of glenohumeral joint stability. Computational modeling analyses is valuable for future development of targeted treatment plans based on patient’s RCT severity. Overall, this research suggests that the effect of treatments on tendon mechanical properties should be considered and understanding effects of tendon injury progression is beneficial in developing targeted treatment plans.