gms | German Medical Science

Objectives: Tendon injuries are accountable for around 30% of all musculoskeletal complaints, no satisfactory therapeutic targets have been developed. Tendon healing process is characterized for being long lasting and oftentimes inefficient. Aim: To uncover Tenomodulin (Tnmd) role during late tendon repair by implementing an Achilles tendon injury model in Tnmd-knockout (KO) and wild type (WT) animals. Based on previous data (inferior scar organization and hypervascular KO tendons) we hypothesized a further deterioration of the phenotype over time, resulting in heterotopic ossification (HO) due to persistent hypervascularity.

Methods: Skeletally mature 6-month old male and female Tnmd-KO and WT mice were used in this study (n=32/genotype). After skin incision, Achilles tendons were fully resected and reconstructed using a Kirchmayr-Kessler suture, followed by cerclage insertion through the tibiofibular fork and calcaneus ensuring load transmission and protecting suture failure. One hundred days post-repair, Micro-CT imaging was used to quantify HO content (n=7-8). Prior to biomechanical testing (Hochstrat E. et al., PLoS One, 2019) the cross-sectional areas of injured and uninjured control tendons were measured and then the parameters stiffness, static and dynamic E-modules were assessed and analyzed (n=8-14). Finally, in order to test tissue functionality, animals were allowed to voluntarily run (overnight) at day 100 and the total distance was recorded. Statistical testing was performed with unpaired t-tests and/or one-way ANOVA.

Results and Conclusion: HO was identified in both genotypes, with the total (*p=0.04) and enthesial (*p=0.02) HO surface being significantly larger in Tnmd-deficient mice. HO distribution was primarily bipolar, at the myotendinous junction and enthesis. Biomechanical testing revealed that injured tendons were less stiff, and both static and dynamic E-modulus were significantly decreased compared to uninjured controls. Interestingly, higher stiffness (p=0.0543) and significantly increased static (**p<0.01) and dynamic E-modulus were measured in uninjured Tnmd-KO tendons at 4% (***p<0.001), 6% (**p<0.01) and 8% strain (***p<0.001). Injured Tnmd-deficient animals ran at day 100 significantly less than their WT littermates. Tendon biomechanical properties were highly compromised upon injury, which we propose is due to the formation of HO. Significantly higher HO was quantified in Tnmd-KO tendons; this novel result can be explained with the augmented vasculature detected at day 8 post-injury. Moreover, uninjured Tnmd-KO tendons were stiffer and possessed significantly higher static and dynamic E-modulus, which is in line with our previous nanomechanical data. Last, voluntary running tests demonstrated that Tnmd-KO animals ran significantly less than WTs 100 days after injury, an indicative of diminished tissue remodeling. Based on these new findings, we suggest that Tnmd plays an indispensable role in tendon healing by protecting against post-injury HO formation.