Surface-based modeling of muscles: Functional simulation of the shoulder

Constitutive Models for Active Skeletal Muscle: Review, Comparison, and Application in a Novel Continuum Shoulder Model

The shoulder joint is one of the functionally and anatomically most sophisticated articular systems in the human body. Both complex movement patterns and the stabilization of the highly mobile joint rely on intricate three-dimensional interactions among various components. Continuum-based finite element models can capture such complexity and are thus particularly relevant in shoulder biomechanics. Considering their role as active joint stabilizers and force generators, skeletal muscles require special attention regarding their constitutive description. In this contribution, we propose a constitutive description to model active skeletal muscle within complex musculoskeletal systems, focusing on a novel continuum shoulder model. Based on a thorough review of existing material models, we select an active stress, an active strain, and a generalized active strain approach and combine the most promising and relevant features in a novel material model. We discuss the four models considering physiological, mathematical, and computational aspects, including the applied activation concepts, biophysical principles of force generation, and arising numerical challenges. To establish a basis for numerical comparison, we identify the material parameters based on experimental stress-strain data obtained under multiple active and passive loading conditions. Using the example of a fusiform muscle, we investigate force generation, deformation, and kinematics during active isometric and free contractions. Eventually, we demonstrate the applicability of the proposed material model in a novel continuum mechanical model of the human shoulder, exploring the role of rotator cuff contraction in joint stabilization.

The stabilizing potential of the supraspinatus is inhibited in tear-associated scapula shapes but can be modulated by kinematic adjustments

Introduction

Specific scapula shapes are associated with full-thickness tears of the supraspinatus tendon. A primary role of the supraspinatus is to actively stabilize the glenohumeral joint against muscles that generate destabilizing shear forces. Mechanisms that increase the supraspinatus load required to perform this stabilizing function may increase an individual's susceptibility to tears. Previous work has shown that tear-associated scapula shapes increase the destabilizing action of the deltoid during arm-raising, but no work has investigated whether tear-associated shapes inhibit the stabilizing potential of the supraspinatus itself.

Methods

We combined statistical shape modeling, kinematics-driven simulations of the glenohumeral joint, and a finite element model of the supraspinatus to investigate the interactions among shape, kinematics, and the stabilizing potential of the supraspinatus. First, we identified tear-associated 3D scapula shapes using partial least squares discriminant analysis. Second, we examined how tear-associated shapes alter the stabilizing potential of the supraspinatus given the same kinematic path. Finally, we examined the extent to which kinematic perturbations could modulate differences in stabilizing potential.

Results

Relative to asymptomatic controls, individuals with full-thickness tears possessed a suite of 3D shape differences including narrower supraspinous fossae and anteverted glenoids. For the same abduction path, tear-associated scapula shapes caused supraspinatus fibres to act more anteriorly and less compressively compared to the control shapes, potentially inhibiting the supraspinatus' ability to stabilize the humeral head. When the abduction path of the tear-associated scapula was internally rotated and shifted anteriorly, the supraspinatus line-of-action closely resembled that of the control-associated scapula; however, the tear-associated shape still possessed a narrower breadth in its supraspinatus line-of-action.

Discussion

Our findings indicate that tear-associated scapula geometry may inhibit the stabilizing potential of the supraspinatus, but this shape-driven change could be partially modulated when the abduction path of the tear-associated shape was perturbed. The magnitude of kinematic perturbations required to modulate function exceeded the magnitude of shape differences, indicating that the perturbations are not correcting for a simple offset, but rather complex changes in muscle geometry that occur due to 3D shape differences.

Scapular muscle dynamic stiffness of asymptomatic subjects and subjects with chronic shoulder pain, at rest and isometric contraction conditions

Muscle stiffness had a crucial role in joint stability, particularly, at the shoulder complex. Although changes in upper trapezius muscle stiffness have been described for shoulder pain, contradictory findings have been obtained. Also, existing data regarding scapular muscles are, majorly, about trapezius. Myotonometry is a method used to assess stiffness; however, the reliability values of scapular muscle stiffness through this method have not been assessed in shoulder pain conditions. The present study aims to compare scapular muscles' stiffness (trapezius, serratus anterior, and levator scapulae) between subjects with and without chronic shoulder pain and to evaluate the related test-retest reliability. Twenty-two symptomatic and twenty-two asymptomatic subjects participated in a cross-sectional study. The dynamic muscular stiffness of scapular muscles, at rest and during an isometric contraction, was measured bilaterally with myotonometry, in two moments. The differences in bilateral averaged values between symptomatic and asymptomatic subjects and the effect of the group (group presenting pain in the dominant or non-dominant side, and asymptomatic group) and of the limb (unilateral painful or asymptomatic limb, and bilateral asymptomatic limbs) were investigated. Test-retest intra-rater reliability was determined. An effect of the group was observed at rest, for middle trapezius stiffness, and during contraction, for middle and lower trapezius stiffness. For middle trapezius, increased values were observed in the group presenting pain in non-dominant side comparing to both groups or to group presenting pain in dominant side. The intraclass correlation coefficient, majorly, ranged between 0.775 and 0.989. Participants with pain in the non-dominant side presented an increased middle trapezius' stiffness. Globally, high reliability was observed for scapular muscles dynamic stiffness.

Computational analysis of subscapularis tears and pectoralis major transfers on muscular activity

BACKGROUND

Pectoralis major is the most common muscle transfer procedure to restore joint function after subscapularis tears. Limited information is available on how the neuromuscular system adjusts to the new configuration, which could explain the mixed outcomes of the procedure. The purpose of this study is to assess how muscles activation patterns change after pectoralis major transfers and report their biomechanical implications.

METHODS

We compare how muscle activation change with subscapularis tears and after its treatment by pectoralis major transfers of the clavicular, sternal, or both these segments, during three activities of daily living and a computational musculoskeletal model of the shoulder.

FINDINGS

Our results indicate that subscapularis tears require a compensatory activation of the supraspinatus and is accompanied by a reduced co-contraction of the infraspinatus, both of which can be partially recovered after transfer. Furthermore, although the pectoralis major acts asynchronously to the subscapularis before the transfer, its activation pattern changes significantly after the transfer.

INTERPRETATION

The capability of a transferred muscle segment to activate similarly to the intact subscapularis is found to be dependent on the given motion. Differences in the activation patterns between intact subscapularis and the segments of pectoralis major may explain the difficulty in adapting psycho-motor patterns during the rehabilitation period. Thereby, rehabilitation programs could benefit from targeted training on specific motion and biofeedback programs. Finally, the condition of the anterior deltoid should be considered to improve joint function.

Influence of rotator cuff integrity on loading and kinematics before and after reverse shoulder arthroplasty

Reverse Shoulder Arthroplasty has become a very common procedure for shoulder joint replacement, even for scenarios where an anatomical reconstruction would traditionally be used. Our hypothesis is that implanting a reverse prosthesis with a functional rotator cuff may lead to higher joint reaction force (JRF) and have a negative impact on the prosthesis. Available motion capture data during anterior flexion was input to a finite-element musculoskeletal shoulder model, and muscle activations were computed using inverse dynamics. Simulations were carried out for the intact joint as well as for various types of rotator cuff tears: superior (supraspinatus), superior-anterior (supraspinatus and subscapularis), and superior-posterior (supraspinatus, infraspinatus and teres minor). Each rotator cuff tear condition was repeated after shifting the humerus and the glenohumeral joint center of rotation to represent the effect of a reverse prosthesis. Changes in compressive, shear, and total JRF were analyzed. The model compared favorably to in vivo JRF measurements, and existing clinical and biomechanical knowledge. Implanting a reverse prosthesis with a functional rotator cuff or with an isolated supraspinatus tear led to more than 2 times higher compressive JRF than with massive rotator cuff tears (superior-anterior or superior-posterior), while the shear force remained comparable. The total JRF increased more than 1.5 times. While a lower shear to compressive ratio may reduce the risk of glenosphere loosening, higher JRF might increase the risk for other failure modes such as fracture or polyethylene wear of the reverse prosthesis.