Biomechanical evaluation of different designs of glenospheres in the SMR reverse total shoulder prosthesis: range of motion and risk of scapular notching.
BACKGROUND
Reverse total shoulder arthroplasty is a treatment option for cuff tear arthropathy. Scapular notching remains a concern. This biomechanical study compared the range-of-motion in different designs of glenospheres and hence the relative risk of scapular notching.


METHODS
A precision coordinate device was used to investigate four different designs of glenospheres (SMR prosthesis); 36 mm concentric (Standard), 36 mm eccentric, 44 mm concentric, and 44 mm eccentric glenospheres. The centre of rotation in each design was first established. The position of the humeral prosthesis was recorded in the plane of the scapula to compare the degree of adduction and the total range-of-motion.


RESULTS
Eccentric glenospheres were found to improve range-of-motion by allowing a higher degree of adduction. Larger diameter glenospheres were found to improve range-of-motion by increasing adduction and abduction. Compared to the 36 mm concentric (standard) glenosphere, the 36 mm eccentric glenosphere improved adduction by 14.5 degrees, the 44 mm concentric glenosphere improved adduction by 11.6 degrees, the 44 mm eccentric glenosphere improved adduction by 17.7 degrees.


CONCLUSIONS
Eccentric glenospheres with a center-of-rotation placed more inferiorly were shown to improve adduction. This design may reduce the clinical incidence of scapular notching.
Chou
Justin
J
Department of Orthopaedic Surgery, North Shore Hospital, North Shore City, New Zealand.
Malak
Sharif F
SF
Anderson
Iain A
IA
Astley
Tim
T
Poon
Peter C
PC
D000704:Analysis of Variance; D019643:Arthroplasty, Replacement / Q000379:methods*; D001696:Biomechanical Phenomena; D003198:Computer Simulation; D006801:Humans; D007593:Joint Instability / Q000517:prevention & control; D007595:Joint Prosthesis; D011336:Probability; D011474:Prosthesis Design; D016059:Range of Motion, Articular / Q000502:physiology; D012540:Scapula / Q000503:physiopathology*; D012680:Sensitivity and Specificity; D019534:Shoulder Impingement Syndrome / Q000517:prevention & control; D012785:Shoulder Joint / Q000503:physiopathology / Q000601:surgery*; D013314:Stress, Mechanical