Objectives 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%). Conclusion 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