Extracellular matrix (ECM) mechanical cues guide healing in tendons. Yet, the molecular mechanisms orchestrating the healing processes remain elusive. Appropriate tissue tension is essential for tendon homeostasis and tissue health. By mapping the attainment of tensional homeostasis, we aim to understand how ECM tension regulates healing. We hypothesize that diseased tendon returns to homeostasis only after the cells reach a mechanically gated exit from wound healing.

We engineered a 3D mechano-culture system to create tendon-like constructs by embedding patient-derived tendon cells into a collagen I hydrogel. Casting the hydrogel between posts anchored in silicone allowed adjusting the post stiffness. Under this static mechanical stimulation, cells remodel the (unorganized) collagen representing wound healing mechanisms. We quantified tissue-level forces using post deflection measurements. Secreted ECM was visualized by metabolic labelling with non-canonical amino acids, click chemistry and confocal microscopy. We blocked cell-mediated actin-myosin contractility using a ROCK inhibitor (Y27632) to explore the involvement of the Rho/ROCK pathway in tension regulation.

Tissue tension forces reached the same homeostatic level at day 21 independent of post compliance (p = 0.9456). While minimal matrix was synthesized in early phases of tissue formation (d3-d5), cell-deposited ECM was present in later stages (d7-d9). More ECM was deposited by tendon constructs cultured on compliant (1Nm) compared to rigid posts (p = 0.0017). Matrix synthesized by constructs cultured on compliant posts was less aligned (greater fiber dispersion, p = 0.0021). ROCK inhibition significantly decreased tissue-level tensional forces (p < 0.0001).

Our results indicate that tendon cells balance matrix remodeling and synthesis during tissue repair to reach an intrinsically defined “mechanostat setpoint” guiding tension-mediated exit from wound healing towards homeostasis. We are identifying specific molecular mechanosensors governing tension-regulated healing in tendon and investigate the Rho/ROCK system as their possible downstream pathway.

Tendon tissue equilibrium very heavily depends on appropriate mechanical loading within a narrow, and still poorly defined, physiological range. We will present an overview of our recent work on the tendon cell-matrix interactions that drive tissue homeostasis, matrix remodelling and eventual tissue degeneration, and discuss a roadmap for unravelling these mechanically regulated signalling pathways for the development of effective treatment strategies. Our data suggest that tissue damage accumulates in the tendon until “intrinsic repair mechanisms” are overwhelmed. At this point, the metabolic cost of extracellular matrix remodeling exceeds the locally available nutrient supply. We hypothesize that upon reach S43.1 ing this “Metabolic Tipping Point”, the vascular system is recruited along with accompanying nerve supply (and pain) and the tissue enters into a chronic disease state characterized by high matrix turnover and increasingly poor tissue quality. In this paradigm, a delicate mechanically regulated balance exists between recruitment and suppression of the extrinsic vascular system by the resident tendon core cells. Upon injury or damage, this regulation in turn steers the tissue towards either functional remodeling or chronic tendon disease.

Tear pattern and tendon involvement are risk factors for the development of a pseudoparalytic shoulder. However, some patients have similar tendon involvement but significantly different active forward flexion. In these cases, it remains unclear why some patients suffer from pseudoparalysis and others with the same tear pattern show good active range of motion. Moment arms (MA) and force vectors of the RC and the deltoid muscle play an important role in the muscular equilibrium to stabilize the glenohumeral joint. Biomechanical and clinical analyses were conducted calculating different MA-ratios of the RC and the deltoid muscle using computer rigid body simulation and a retrospective radiographic investigation of two cohorts with and without pseudoparalysis and massive RC tears. Idealized MAs were represented by two spheres concentric to the joints centre of rotation either spanning to the humeral head or deltoid origin of the acromion. Individual ratios of the RC /deltoid MAs on antero-posterior radiographs using the newly introduced Shoulder Abduction Moment (SAM) Index was compared between the pseudoparalytic and non-pseudoparalytic patients.

Decrease of RC activity and improved glenohumeral stability (+14%) was found in simulations for MA ratios with larger diameters of the humeral head which also were consequently beneficial for the (remaining) RC. Clinical investigation of the MA-ratio showed significant risk of having pseudoparalysis in patients with massive tears and a SAM Index <0.77 (OR=11). The SAM index, representing individual biomechanical characteristics of shoulder morphology has an impact on the presence or absence of pseudoparalysis in shoulders with massive RC tears.

Introduction

ACL reconstruction using hamstring tendons has gained general acceptance. However, it has been recommended to seek a tight fit of the tendon in the bone canal in order to provide circumferential contact and healing of the graft, and to prevent secondary tunnel widening. Recent findings show, that the graft dynamically adapts to pressure in the canal resulting in a potentially loose graft-bone contact. It was the goal of this study to understand the viscoelastic behaviour of hamstring grafts under pressure and to develop a new method for tendon pre-conditioning to reduce the graft volume before implantation, in order to reduce the necessary bone canal diameter to accommodate the same graft.

Introduction: Additional tendon length is occasionally needed for the surgical reattachment of retracted tendons and for lengthening of intact but contracted tendons. To achieve additional length with the known techniques such as the z-plasty, the tendon needs to be cut through entirely and loses its continuity. The purpose of this study was to develop a new method for tendon lengthening, where continuity is preserved and a high amount of additional length is achievable.

Methods: Calf Achilles tendons (n=35) were harvested immediately after slaughter and 5 tendons were assigned to groups I to VII. Angles of 60° (group I and IV), 45° (group II and V) and 30° (group III and VI) were cut. In group IV to VI mattress suture stitches were made along the cutting lines. The mean length increase of the helical cuts was used to define the intended length of group VII, where a z-plasty was performed. Maximal tensile force (Fmax) and additional achieved lengthening at Fmax (LFmax) were determined for each tendon using a materials testing machine. Data were statistically analyzed using ANOVA for inter-group differences and Spearman-correlation for cut angle to additional length relations at a significance level of p< 0.05.

Results: Tendons which were cut helically and sutured (group IV to VI) could achieve higher Fmax than the helically cut tendons without suturing (group I to III). The length and tensile force could be partially controlled by choice of the angle of the helical cut; In the groups for which the cut tendons were not sutured, LFmax was negatively correlated to the cut angle (r=−0.66, p=0.010) and positively correlated to the Fmax (r=0.72, p=0.003). If the helical cut tendons were sutured, there was no correlation of LFmax and cut angle (r=−0.01, p=0.96), but strong positive correlation of Fmax and cut angle (r=0.89, p< 0.0001). Helical cut tendon could achieve higher amount of additional length and tensile strength than tendons lengthened using z-plasty; in group VII, a LFmax of 72%±10% was achieved by a Fmax of 70N±15N. Other than in groups III and IV, where the cut angle was 30°, resulting in 179%±80% and 113%±10%, respectively, significant higher tensile force capacities (from a minimum of 80N±54N in group II to maximally 222N±62N in group IV) was achieved.

Discussion: Helical cutting of tendons allows lengthening tendons to an amount not possible with conventional methods. The lengthened coil-shaped tendon remains in continuity and has the potential to withstand considerable loads also without additional suture reinforcement. The behavior of the helical cut tendon in vivo is not known. However, the preservation of continuity might be favorable not only in regard to high tensile forces but also to healing.