Glenoid concavity has a higher impact on shoulder stability than the size of a bony defect

Abstract

Purpose Surgical treatment of shoulder instability caused by anterior glenoid bone loss is based on a critical threshold of the defect size. Recent studies indicate that the glenoid concavity is essential for glenohumeral stability. However, biomechanical proof of this principle is lacking. The aim of this study was to evaluate whether glenoid concavity allows a more precise assessment of glenohumeral stability than the defect size alone. Methods The stability ratio (SR) is a biomechanical estimate of glenohumeral stability. It is defined as the maximum dislocating force the joint can resist related to a medial compression force. This ratio was determined for 17 human cadaveric glenoids in a robotic test setup depending on osteochondral concavity and anterior defect size. Bony defects were created gradually, and a 3D measuring arm was used for morphometric measurements. The influence of defect size and concavity on the SR was examined using linear models. In addition, the morphometrical-based bony shoulder stability ratio (BSSR) was evaluated to prove its suitability for estimation of glenohumeral stability independent of defect size. Results Glenoid concavity is a significant predictor for the SR, while the defect size provides minor informative value. The linear model featured a high goodness of fit with a determination coefficient of R2 = 0.98, indicating that 98% of the SR is predictable by concavity and defect size. The low mean squared error (MSE) of 4.2% proved a precise estimation of the SR. Defect size as an exclusive predictor in the linear model reduced R2 to 0.9 and increased the MSE to 25.7%. Furthermore, the loss of SR with increasing defect size was shown to be significantly dependent on the initial concavity. The BSSR as a single predictor for glenohumeral stability led to highest precision with MSE = 3.4%. Conclusion Glenoid concavity is a crucial factor for the SR. Independent of the defect size, the computable BSSR is a precise biomechanical estimate of the measured SR. The inclusion of glenoid concavity has the potential to influence clinical decision-making for an improved and personalised treatment of glenohumeral instability with anterior glenoid bone loss.

The Anatomy of Glenoid Concavity—Bony and Osteochondral Assessment of a Stability-Related Parameter

“…The anatomy of glenoid concavity is a crucial factor contributing to glenohumeral stability. Loss of concavity, which was recently mathematically represented by the BSSR, is directly and linearly related to the loss of SR, a widely used biomechanical estimate for glenohumeral stability [ 4 , 5 ]. A deeper knowledge of the bony and osteochondral concavity distribution is essential to understand the important stability effect of the anatomy.…”

“…A few studies have proposed formulas to estimate native glenoid width based on the length [ 24 , 25 ]. However, it has been shown that the width of bony defects has inaccurate information about stability [ 4 ]. The approximation of initial anteroposterior concavity by measuring only the superoinferior concavity in terms of BSSR or OSSR may be a more precise approach to manage the loss of bony glenoid.…”

“…When these force distributions are broken down to the glenohumeral morphology, the BSSR is obtained [ 11 ]. As recently shown, the BSSR has a high linear correlation with the measured SR [ 4 ]. Thus, the BSSR is a stability-related parameter that can clinically be assessed in radiologic data from a measurement of the sphere radius and the glenoid depth [ 18 ].…”

“…However, the concavity may differ between patients. Recent finite element methods and biomechanical studies demonstrated that the extent of curvature has a high impact on glenohumeral stability [ 4 , 5 ]. Moroder et al concluded that the biomechanical effect of a bony defect depends on intraindividual differences in concavity [ 5 ].…”

The unstable shoulder: what soft tissue, bony anatomy and biomechanics can teach us

“…Recently, a rethinking process involving the impact of glenoid concavity has been initiated, challenging the current concept of determining a general threshold value for critical glenoid bone loss as a criterion for the use of a bone augmentation procedure in shoulder instability [ 16 , 22 ]. Ironically, previous work already demonstrated that the concave shape of the articular surface of the glenoid should be considered the crucial component for determining glenohumeral stability, which is based on the synergism with the rotator cuff via the “concavity compression” principle [ 14 , 26 ].…”