The role of pectoralis major and latissimus dorsi muscles in a biomechanical model of massive rotator cuff tear.

BACKGROUND: Superior migration of the humeral head after massive rotator cuff tear (mRCT) is thought to lead to cuff tear arthropathy. Previous biomechanical studies have demonstrated the ability of the pectoralis major and latissimus dorsi (PM/LD) muscles to resist this migration. This study examined the role of PM/LD muscles on glenohumeral joint forces and acromiohumeral contact pressures in a mRCT model. METHODS: Six cadaveric shoulders were tested using a custom shoulder-testing system. Muscle insertions of the rotator cuff, deltoid, and PM/LD were preserved and used for muscle loading. Specimens were tested in 3 different humeral rotation positions at 0° abduction and 2 rotation positions at 60° abduction. Testing was performed for intact specimens, after supraspinatus removal, and after supraspinatus/infraspinatus/teres minor removal. PM/LD were loaded or unloaded to determine their effect. Humeral head kinematics, glenohumeral joint forces, and acromiohumeral contact area and pressure were measured. RESULTS: For the mRCT condition at 0° abduction, unloading the PM/LD resulted in superior shift of the humeral head. Acromiohumeral contact pressures were undetectable when the PM/LD were loaded but increased significantly after PM/LD unloading. After mRCT, superior joint forces were increased and compressive forces were decreased compared with intact; loading the PM/LD resolved these abnormal forces in some testing conditions. CONCLUSION: In mRCT, the PM and LD muscles are effective in improving glenohumeral kinematics and reducing acromiohumeral pressures. Strengthening or neuromuscular training of this musculature, or both, may delay the progression to cuff tear arthropathy.

Irradiation does not modify mechanical properties of cancellous bone under compression.

BACKGROUND: Gamma radiation sterilization can make cortical bone allograft more brittle, but whether it influences mechanical properties and propensity to form microscopic cracks in structurally intact cancellous bone allograft is unknown. QUESTIONS/PURPOSES: We therefore determined the effects of gamma radiation sterilization on structurally intact cancellous bone mechanical properties and damage formation in both low- and high-density femoral cancellous bone (volume fraction 9%-44%). METHODS: We studied 26 cancellous bone cores from the proximal and distal femurs of 10 human female cadavers (49-82 years of age) submitted to a single compressive load beyond yield. Mechanical properties and the formation of microscopic cracks and other tissue damage (identified through fluorochrome staining) were compared between irradiated and control specimens. RESULTS: We observed no alterations in mechanical properties with gamma radiation sterilization after taking into account variation in specimen porosity. No differences in microscopic tissue damage were observed between the groups. CONCLUSIONS: Although gamma radiation sterilization influences the mechanical properties and failure processes in cortical bone, it does not appear to influence the performance of cancellous bone under uniaxial loading.