Large variability in degree of constraint of reverse total shoulder arthroplasty liners between different implant systems

Abstract

Aims: The liner design is a key determinant of the constraint of a reverse total shoulder arthroplasty (rTSA). The aim of this study was to compare the degree of constraint of rTSA liners between different implant systems.

Methods: An implant company's independent 3D shoulder arthroplasty planning software (mediCAD 3D shoulder v. 7.0, module v. 2.1.84.173.43) was used to determine the jump height of standard and constrained liners of different sizes (radius of curvature) of all available companies. The obtained parameters were used to calculate the stability ratio (degree of constraint) and angle of coverage (degree of glenosphere coverage by liner) of the different systems. Measurements were independently performed by two raters, and intraclass correlation coefficients were calculated to perform a reliability analysis. Additionally, measurements were compared with parameters provided by the companies themselves, when available, to ensure validity of the software-derived measurements.

Results: There were variations in jump height between rTSA systems at a given size, resulting in large differences in stability ratio between systems. Standard liners exhibited a stability ratio range from 126% to 214% (mean 158% (SD 23%)) and constrained liners a range from 151% to 479% (mean 245% (SD 76%)). The angle of coverage showed a range from 103° to 130° (mean 115° (SD 7°)) for standard and a range from 113° to 156° (mean 133° (SD 11°)) for constrained liners. Four arthroplasty systems kept the stability ratio of standard liners constant (within 5%) across different sizes, while one system showed slight inconsistencies (within 10%), and ten arthroplasty systems showed large inconsistencies (range 11% to 28%). The stability ratio of constrained liners was consistent across different sizes in two arthroplasty systems and inconsistent in seven systems (range 18% to 106%).

Conclusion: Large differences in jump height and resulting degree of constraint of rTSA liners were observed between different implant systems, and in many cases even within the same implant systems. While the immediate clinical effect remains unclear, in theory the degree of constraint of the liner plays an important role for the dislocation and notching risk of a rTSA system.

Fig. 1

Example of a humeral liner for reverse total shoulder arthroplasty made out of polyethylene (Univers Reverse; Arthrex, USA).

Fig. 2

Screenshot of a measurement on mediCAD 3D shoulder. Left: axial view; the liner is positioned strictly parallel to the axial plane; the centre of the liners’ concavity is determined by using a best-fit circle. Right: frontal view; first a tangential line is drawn on top of the concavity (blue line). Then, the jump height is determined by drawing an orthogonal line (red) from the middle to the bottom of the concavity.

Fig. 3

Illustration of a reverse total shoulder arthroplasty: radius (r) of the glenosphere and concavity depth (d) or jump height of the liner are required to calculate the liner stability ratio (LSR) using the aforementioned formula. Yellow area: the extent of the glenosphere covered by the liner; yellow striped line: angle of coverage (degree of glenosphere coverage by the liner).

Fig. 4

Diagrams illustrating the liner stability ratio of different reverse total shoulder arthroplasty systems across glenosphere/cup sizes. The reference lines (shades of grey) represent the standard change in stability ratio per increase in jump height for different cup/glenosphere sizes. a) Standard liners. b) Constrained liners.