High variability in the degree of liner stability ratio among glenoid components of different anatomical total shoulder arthroplasty systems

Abstract

Background: Anatomic total shoulder arthroplasty (aTSA) is a viable option for select patients with favorable long-term outcomes. However, instability with static decentering and eccentric wear remains a concern. An important factor contributing to the stability of aTSA is the liner-generated stability ratio, constituted by jump-height and radius of curvature. This study aimed to measure jump height and radius of curvature as well as to assess the liner stability ratio (LSR) of various aTSA glenoid components, enabling comparisons of the degree of constraint between implant systems.

Methods: Using manufacturer-independent planning software, glenoid component height, jump height, and radius of curvature in the longitudinal and transverse axes were measured across 28 aTSA systems from 14 companies by two independent raters, with transverse measurements taken at the broadest diameter (t1) and at the corresponding midpoint level of the longitudinal axis (t2); data were validated by comparison with manufacturer-provided specifications. LSR were calculated using a previously validated mathematical formula. The inter-rater reliability was determined using the intraclass correlation coefficient. Visual diagrams illustrated the relationship between glenoid component height, jump height, and LSR across glenoid components of various aTSA designs.

Results: The mean glenoid component height was 32.8 ± 4.9 mm (range, 22.3-43.8 mm), while the mean jump height and radius of curvature were 5.1 ± 1.5 mm (range, 2.4-9.9 mm) and 29.7 ± 3.6 mm (range, 21.7-38.1 mm) in the longitudinal axis, 2.7 ± 0.7 mm (range, 1.5-5.1 mm) and 29.5 ± 3.9 mm (range, 19.9-38.5 mm) in the transversal axis (t1), and 2.6 ± 0.7 mm (range, 1.3-4.7 mm) and 29.7 ± 3.9 mm (range, 20.1-39.7 mm) for t2, respectively. Calculated LSR ranged from 39% to 115% (68% ± 15%) for the longitudinal axis, 31% to 71% (47% ± 8%) for t1, and from 28% to 65% (45% ± 8%) for t2, across available aTSA systems. Manufacturer-provided specifications from two companies showed high concordance with the obtained measurements. Regarding LSR consistency, only 2 systems were consistent (≤5% variation) in both axes. Slight inconsistencies (>5-10%) appeared in 3 systems longitudinally and 5 transversely, while most showed >10% variation across sizes. Inter-rater reliability demonstrated near-perfect agreement between testers.

Conclusion: This study highlights significant variability in the LSR across different glenoid components of aTSA systems, with inconsistencies often even observed within the same system. While the direct clinical impact remains uncertain, the LSR of the glenoid component in aTSA may have an effect on aTSA stability, component wear and loosening attributed to the rocking horse phenomenon. Further research is needed to clarify the biomechanical consequences of LSR variations on aTSA.

Keywords: Anatomic shoulder arthroplasty; Degree of constraint; Glenoid components; Jump height; Liner stability ratio; Radius of curvature.

Figure 1

Example of a polyethylene glenoid component designed for anatomic total shoulder arthroplasty (Univers VaultLock Glenoid System; Arthrex, Naples, FL, USA).

Figure 2

Measurement workflow using the mediCAD 3D Shoulder software. In the sagittal plane view (Left), the longitudinal axis was aligned along the maximal superoinferior diameter of the glenoid component, generating the corresponding coronal plane view for measurements along the longitudinal axis (Right). The corresponding jump height (d) and radius of curvature (r) measurements are displayed in the Right Panel. 3D, three-dimensional.

Figure 3

Measurement workflow using the mediCAD 3D Shoulder software. In the sagittal plane view (Left), the transversal axis was aligned along the maximal anteroposterior diameter of the glenoid component, generating the corresponding axial plane view for measurements along the transversal axis (Right). The associated jump height (d) and radius of curvature (r) measurements to are shown in the Right Panel. 3D, three-dimensional.

Figure 4

Measurement workflow using the mediCAD 3D Shoulder software. In the sagittal plane view (Left), the transversal axis was aligned along the midpoint level of the longitudinal axis of the glenoid component, generating the corresponding axial plane view for measurements along the transversal axis (Right). The associated jump height (d) and radius of curvature (r) measurements to are shown in the Right Panel. 3D, three-dimensional.

Figure 5

Graphs illustrating calculated liner stability ratios (LSRs; y-axis), derived from manually measured jump heights and radii of curvature of various anatomic total shoulder arthroplasty systems, plotted against jump height measurements. (A) Jump height plotted against LSR calculated along the longitudinal axis. (B) Jump height plotted against LSR calculated along the transverse axis (t1). (C) Jump height plotted against LSR calculated along the transverse axis (t2). Symbols of the same color represent LSR values from the same aTSA system but with different radii of curvature. Jump height values are not strictly proportional to glenoid component size. aTSA, anatomic total shoulder arthroplasty.

Figure 6

Graph illustrating the relationship between glenoid component height and LSR across aTSA systems. Considerable variability in LSR is observed both between and within systems, without a consistent linear correlation to glenoid component height. aTSA, anatomic total shoulder arthroplasty; LSR, liner stability ratio.