title: Biomechanical analysis of the effect of congruence, depth and radius on the stability ratio of a simplistic 'ball-and-socket' joint model.
abstract: OBJECTIVE
  The bony shoulder stability ratio (BSSR) allows for quantification of the bony stabilisers in vivo. We aimed to biomechanically validate the BSSR, determine whether joint incongruence affects the stability ratio (SR) of a shoulder model, and determine the correct parameters (glenoid concavity versus humeral head radius) for calculation of the BSSR in vivo.
  
  
  METHODS
  Four polyethylene balls (radii: 19.1 mm to 38.1 mm) were used to mould four fitting sockets in four different depths (3.2 mm to 19.1mm). The SR was measured in biomechanical congruent and incongruent experimental series. The experimental SR of a congruent system was compared with the calculated SR based on the BSSR approach. Differences in SR between congruent and incongruent experimental conditions were quantified. Finally, the experimental SR was compared with either calculated SR based on the socket concavity or plastic ball radius.
  
  
  RESULTS
  The experimental SR is comparable with the calculated SR (mean difference 10%, sd 8%; relative values). The experimental incongruence study observed almost no differences (2%, sd 2%). The calculated SR on the basis of the socket concavity radius is superior in predicting the experimental SR (mean difference 10%, sd 9%) compared with the calculated SR based on the plastic ball radius (mean difference 42%, sd 55%).
  
  
  CONCLUSIONS
  The present biomechanical investigation confirmed the validity of the BSSR. Incongruence has no significant effect on the SR of a shoulder model. In the event of an incongruent system, the calculation of the BSSR on the basis of the glenoid concavity radius is recommended.Cite this article: L. Ernstbrunner, J-D. Werthel, T. Hatta, A. R. Thoreson, H. Resch, K-N. An, P. Moroder. Biomechanical analysis of the effect of congruence, depth and radius on the stability ratio of a simplistic 'ball-and-socket' joint model. Bone Joint Res 2016;5:453-460. DOI: 10.1302/2046-3758.510.BJR-2016-0078.R1.
authors.affiliation: Department of Orthopaedics and Traumatology, Paracelsus Medical University, Salzburg, Austria and Department of Orthopaedics, Balgrist University Hospital, University of Zurich, Switzerland, Zurich, Switzerland lukas.ernstbrunner@stud.pmu.ac.at.
authors.forename: L
authors.initials: L
authors.lastname: Ernstbrunner
authors.affiliation: Mayo Clinic, 200 First Street SW, Rochester, MN 55905, USA.
authors.forename: J-D
authors.initials: JD
authors.lastname: Werthel
authors.affiliation: Mayo Clinic, 200 First Street SW, Rochester, MN 55905, USA.
authors.forename: T
authors.initials: T
authors.lastname: Hatta
authors.affiliation: Mayo Clinic, 200 First Street SW, Rochester, MN 55905, USA.
authors.forename: A R
authors.initials: AR
authors.lastname: Thoreson
authors.affiliation: Department of Orthopaedics and Traumatology, Paracelsus Medical University, Salzburg, Austria.
authors.forename: H
authors.initials: H
authors.lastname: Resch
authors.affiliation: Mayo Clinic, 200 First Street SW, Rochester, MN 55905, USA.
authors.forename: K-N
authors.initials: KN
authors.lastname: An
authors.affiliation: Department of Orthopaedics and Traumatology, Paracelsus Medical University, Salzburg, Austria and Center for Musculoskeletal Surgery, Charite Universitaetsmedizin, Berlin, Germany.
authors.forename: P
authors.initials: P
authors.lastname: Moroder
keywords: Bony shoulder stability ratio; Shoulder instability; Stability ratio