title: Nonconforming glenoid increases posterior glenohumeral translation after a total shoulder replacement.
abstract: BACKGROUND
  The major complication in nonconforming total shoulder replacement (TSR) is glenoid loosening and is attributed to posteriorly directed humeral head translations. Whether the posterior translations observed clinically are induced by radial mismatch is unclear. The objective of our study was to explain the posterior glenohumeral translations observed clinically after TSR by determining the glenohumeral translation and contact force as a function of radial mismatch. We hypothesized that the posterior direction of glenohumeral translation during scaption would be related to the radial mismatch and that the joint contact force would increase as the radial mismatch increased.
  
  
  METHODS
  A 6-degrees-of-freedom computational model of the glenohumeral joint was developed. We determined the muscle forces, joint contact force, and glenohumeral translation for radial mismatches from 1 mm to 20 mm with the shoulder positioned from 20° to 60° of scaption.
  
  
  RESULTS
  As the radial mismatch increased, the contact location of the humeral head moved posteriorly and inferiorly. The middle deltoid force decreased by 3%, while the supraspinatus and infraspinatus muscle forces increased by 9% and 11%, respectively. The joint contact force remained relatively constant.
  
  
  CONCLUSIONS
  Increased posterior glenohumeral translations were observed with increased radial mismatch. Clinical observations of posterior translation may be attributed to the balancing forces of the middle deltoid, infraspinatus, and supraspinatus muscles. High radial mismatches may lead to eccentric posterior loading on the glenoid component, which could lead to implant loosening and failure.
authors.affiliation: Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY, USA.
authors.forename: Radhika J
authors.initials: RJ
authors.lastname: Patel
authors.affiliation: Department of Biomechanics, Hospital for Special Surgery, New York, NY, USA.
authors.forename: Daniel
authors.initials: D
authors.lastname: Choi
authors.affiliation: Department of Biomechanics, Hospital for Special Surgery, New York, NY, USA.
authors.forename: Timothy
authors.initials: T
authors.lastname: Wright
authors.affiliation: Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY, USA. Electronic address: yg75@cornell.edu.
authors.forename: Yingxin
authors.initials: Y
authors.lastname: Gao
mesh_terms: D019643:Arthroplasty, Replacement / Q000009:adverse effects; D001696:Biomechanical Phenomena; D003198:Computer Simulation; D019544:Equipment Failure Analysis; D006801:Humans; D058430:Humeral Head / Q000503:physiopathology; D018482:Muscle, Skeletal / Q000503:physiopathology; D012540:Scapula / Q000503:physiopathology* / Q000601:surgery; D012785:Shoulder Joint / Q000503:physiopathology / Q000601:surgery*
keywords: 6-degrees-of-freedom computational model; Nonconforming total shoulder replacement; joint contact force; muscle forces; physical shoulder simulator; posterior glenohumeral translation