| Rotation of the humeral head relative to the scapula is required for activities of daily living and manual labor. Tearing one or more tendons of the rotator cuff can result in reduced strength for some subjects, but for other subjects there is little or no strength reduction. The long standing paradigm of deterministic modeling in musculoskeletal biomechanics has not been able to explain variation in strength, or variability in strength deficit. One hypothesis is that due to variation in several factors some patients retain strength following cuff tear, while others do not retain strength. This dissertation aimed to measure variability of muscle force and torque generating potential for muscles causing external rotation of the humerus relative to the scapula: supraspinatus, infraspinatus and teres minor. Then, using a probabilistic model we wanted to determine if measured parameter variability explains distributions of measured external rotation strength in healthy normals, and if differences in parameter variability could explain strength distributions in rotator cuff tear subjects.; Parameters describing muscle force and joint torque generating capacity, muscle physiologic cross sectional area (PCSA), and length-tension relationship, and muscle moment arm, were measured in 10 human cadaver specimens. Significant variability was demonstrated across specimens, and between muscles.; A Monte Carlo model was constructed with multivariate Gamma distributions fit to summary statistics and the covariance matrix of measured parameters. The model predicted the variability of healthy normal external rotation strength with small error. Mean PCSA for infraspinatus and teres minor was decreased 51%, and standard deviation of PCSA was increased 16% for the supraspinatus tendon tear case. For the infraspinatus-supraspinatus tendon tear case, mean PCSA of the remaining muscle, teres minor, was increased 93%, and standard deviation of teres minor PCSA was increased by a factor of 3. Median muscle forces and distributions were significantly different for all muscles between healthy normal and rotator cuff tear cases.; The results agree with clinical findings of infraspinatus degradation via fatty infiltration in supraspinatus tendon tear cases. Additionally, clinical observations of teres minor hypertrophy in cases of supraspinatus-infraspinatus tendon tear are supported. Results and insights found here are previously un-elucidated with deterministic models and demonstrate strong potential for probabilistic modeling. |
