Biomechanics of muscle after rotator cuff tear: Multi-scale assessment of spatial and temporal effects

PROJECT SUMMARY/ABSTRACT A rotator cuff tendon tear affects up to 64% of adults, resulting in muscular changes that drive functional disability and poor clinical outcomes. Rotator cuff tears are associated with muscle atrophy and fatty infiltration (accrual of fat in the muscle bulk), non-homogenous distribution of fatty infiltration within the muscle, and reduced muscle strength and contractile function. Changes to muscle morphology likely have a negative effect on the muscle’s architecture (number and organization of muscle fibers) and the mechanical properties dictating the muscle’s overall force-generating capacity. However, little is known about the mechanisms linking muscle structure and function or how these associations change over time, exposing a notable knowledge gap. Establishing the structure-function associations of muscle after rotator cuff tear will expose the targets and key time points for design of treatments to improve outcomes for patients with a rotator cuff tear. Thus, the objective of this project is to determine the spatial and temporal changes to muscle morphology, architecture, and multi-scale mechanics after rotator cuff tear and surgical repair, and develop a predictive model to identify the mechanisms driving biomechanical function. To achieve this goal, longitudinal experimental assessments will be performed in an established rabbit model; a novel computational model will also be developed based on experimental structural measurements of muscle to further probe the mechanisms of muscle function. Specifically, this project aims to: 1) Establish the structure-function relationships of muscle after rotator cuff tear and surgical repair; and 2) Develop and test a predictive model to examine mechanisms driving reduced biomechanical function after rotator cuff tear. A rotator cuff tear will be surgically introduced in rabbits by blunt dissection of the supraspinatus tendon, with assessments at 2, 4, 6, and 8 weeks after injury; surgical repair will be performed 8 weeks after injury, with a final assessment 8 weeks after repair. At each time point, muscle morphology and architecture, and intra- versus extra-cellular location of lipid accumulation will be quantified using magnetic resonance imaging (MRI). Multi-scale mechanics will be assessed at whole muscle and single fiber levels. Structure-function associations and how these associations change over time will be established. Structural measures of muscle morphology, architecture, and mechanics will be used to develop and test a predictive model to probe the mechanistic role of each structural parameter on biomechanical function. Models will be used to determine the mechanism and timing that should be targeted by treatment to improve functional outcomes. This work is significant because it will establish the structure-function relationship of muscle after rotator cuff tear and surgical repair and identify the mechanisms underpinning biomechanical function. Little work has examined both spatial and temporal changes to muscle structure and examined their influence on function, making this work innovative. Study outcomes will expand our understanding of the impact of rotator cuff tear and drive development of novel treatments for the rotator cuff tear patient population.