Postural differences in shoulder dynamics during pushing and pulling

The Influence of Functional Shoulder Biomechanics as a Mediator of Patient-Reported Outcomes following Mastectomy and Breast Reconstruction

BACKGROUND

Postmastectomy breast reconstruction techniques differentially influence patient-reported physical and psychosocial well-being. Objective measures of shoulder biomechanics, which are uniquely influenced by reconstruction technique, may provide insight into the influence of reconstruction technique on patient-reported outcomes.

METHODS

Robot-assisted measures of shoulder strength and stiffness, and five validated patient-reported outcomes surveys were obtained from 46 women who had undergone mastectomy and a combined latissimus dorsi flap plus subpectoral implant, subpectoral implant, or DIEP flap breast reconstruction. Mediation analyses examined the role of functional shoulder biomechanics as a mediator between reconstruction technique and patient-reported outcomes.

RESULTS

Reconstruction technique affected shoulder biomechanics, with latissimus dorsi flap plus subpectoral implant patients exhibiting reduced shoulder strength and stiffness compared with subpectoral implant and DIEP flap patients. Increasing external rotation strength was predictive of improved upper extremity function (p = 0.04). Increasing shoulder stiffness while at rest was predictive of worsened upper extremity function (p = 0.03). Increasing shoulder stiffness at rest and during contraction was indicative of worsened psychosocial well-being (all p ≤ 0.02). Reconstruction technique did not predict survey scores of function directly, or when mediated by functional shoulder biomechanics.

CONCLUSIONS

In the current cohort, latissimus dorsi plus subpectoral implant breast reconstructions significantly reduced shoulder strength and stiffness when compared with the other techniques. In addition, objective measures of shoulder biomechanics were predictive of patient-reported physical and psychosocial well-being. The results emphasize the need for improved perioperative screening for shoulder functional deficits in patients undergoing breast reconstruction.

CLINICAL QUESITON/LEVEL OF EVIDENCE

Therapeutic, II.

Quantifying the Multidimensional Impedance of the Shoulder During Volitional Contractions

The neuromuscular control of the shoulder requires regulation of 3D joint mechanics, but it is unknown how these mechanics vary during tasks that load the shoulder in different directions. The purpose of this study was to quantify how the 3D mechanics of the shoulder change with voluntary torque production. Eleven participants produced voluntary isometric torques in one of six directions along three measurement axes. Impedance was estimated by applying small, pseudorandom angular perturbations about the shoulder as participants maintained steady state torques. The nonparametric impedance frequency response functions estimated from the data were parameterized by a collection of second-order linear systems to model the 3D inertia, viscosity, and stiffness of the shoulder. Each component of the 3D stiffness matrix scaled linearly with volitional torque production. Viscosity also increased monotonically with torque but nonlinearly. The directions of maximal stiffness and viscosity were consistently aligned towards the direction of torque production. Further, the shoulder was least stiff and least viscous in the direction of internal/external rotation, suggesting it may be more prone to injury along this axis. These experimental findings and the corresponding mathematical model summarizing our results provide novel insights into how the neuromuscular system regulates 3D shoulder mechanics in response to volitional muscle activations.

Posture-dependent neuromuscular contributions to three-dimensional isometric shoulder torque generation

This study is the first to identify the muscle synergies underlying three-dimensional isometric shoulder torque generation. Although the overall structure of these synergies was unaffected by arm posture, the weighted contributions of several muscles composing two synergy patterns changed as a function of the elevation or plane of elevation of the shoulder. Our findings provide valuable insight for the development of targeted interventions for the restoration of shoulder function after neuromuscular or orthopedic pathologies.