How a Tendon Tears from Within express
The shoulder tendon is like a cable that holds your arm and lets it rotate. When that cable gets injured, its internal structure changes, and it starts to stretch differently. In this study, they took a rat's supraspinatus tendon and pulled it along its fibers while recording how every point inside it shifted. The result was a complete 3D picture of deformation — for both healthy and injured tendons.
Next came the question: how to describe this behavior with equations? The scientists tested three models of varying complexity. The first, simplest one treated the tissue as a uniform rubber. The second, more sophisticated, accounted for fibers that resist stretching differently along and across. The third was even simpler than the second but included a term for shear. It turned out the simplest model fails: it misses how the injured tendon starts to shear internally. The middle and simplified models performed equally well, even though one uses more parameters.
Why does this matter? If we can accurately predict how a tendon will behave after injury or surgery, doctors can tailor treatments individually. For now, no model captures every nuance, but even the simplified version picks up the main trends. It's like a rough map of the terrain: you won't navigate every rock with it, but it points you in the right direction.
An injured tendon is like a rope where some strands have frayed: when pulled, the remaining fibers slide and shift more than they would in an intact rope. That shear turned out to be key — the simplest model misses it, but more realistic ones catch it.
Original abstract
as written by the authors arXiv:2601.09660
In this work, we characterized the material properties of an animal model of the rotator cuff tendon using full volume datasets of both its intact and injured states by capturing internal strain behavior throughout the tendon. Our experimental setup, involving tension along the fiber direction, activated volumetric, tensile, and shear mechanisms due to the tendon's complex geometry. We implemented an approach to model inference that we refer to as variational system identification (VSI) to solve the weak form of the stress equilibrium equation using these full volume displacements. Three constitutive models were used for parameter inference: a neo-Hookean model, a modified Holzapfel-Gasser-Ogden (HGO) model with higher-order terms in the first and second invariants, and a reduced polynomial model consisting of terms based on the first, second, and fiber-related invariants. Inferred parameters were further refined using an adjoint-based partial differential equation (PDE)-constrained optimization framework. Our results show that the modified HGO model captures the tendon's deformation mechanisms with reasonable accuracy, while the neo-Hookean model fails to reproduce key internal features, particularly the shear behavior in the injured tendon. Surprisingly, the simplified polynomial model performed comparably to the modified HGO formulation using only three terms. These findings suggest that while current constitutive models do not fully replicate the complex internal mechanics of the tendon, they are capable of capturing key trends in both intact and damaged tissue, using a homogeneous modeling approach. Continued model development is needed to bridge this gap and enable clinical-grade, predictive simulations of tendon injury and repair.
The supraspinatus tendon is one of the most commonly injured tendons in humans: it accounts for up to half of all shoulder pain.