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    Evaluating the Torque-Compression Relationship in RSA Baseplate Design

    Reverse shoulder arthroplasty (RSA) has become increasingly popular in the past 2 decades, with its indications expanding substantially. As surgeons have become more familiar with the procedure, rates of reoperation and revision associated with RSA have decreased. However, implant-related complications – such as glenoid baseplate loosening – are still significantly more common with RSA designs than with anatomic shoulder arthroplasty designs.

    Glenoid baseplate loosening can have a number of causes, poor bone quality, inadequate initial biomechanical stability, and lack of bone ingrowth onto the prosthesis. The baseplate design can affect initial fixation and long-term implant survivorship, and several biomechanical studies have explored options to improve fixation and minimize micromotion with various types and configurations of peripheral screws, various baseplate positions and orientations, and multiple baseplate designs.

    The initial fixation strength of a glenoid baseplate construct, as well as tactile feedback to the surgeon, can be affected by the applied torque during insertion of a central-screw baseplate and the resultant axial compressive forces on the glenoid bone. The relationship between torque and compression can theoretically be affected by a number of baseplate design features, including modularity, locking and non-locking features of a central screw, shape of the baseplate, length and diameter of the central screw, and component surface roughness.

    The relationship between insertion torque and baseplate compression in a central screw baseplate design has been recently evaluated. However, data quantifying the relationship between the clinically relevant variables of torque and compression with different baseplate constructs is currently lacking in the literature.

    In a recently published study, the translational research team at the Foundation for Orthopaedic Research and Education collaborated with surgeons from multiple centers in the US to investigate the role of the compressive forces generated by varying levels of torque using 3 glenoid baseplate designs (Figure 1):

    • An established monoblock central screw design
    • A 2-piece baseplate with a locking central screw
    • A 2-piece baseplate with a nonlocking central screw

    Figure 1. Illustration of 3D direct metal laser sintering baseplate designs: monoblock (A), 2-piece non-locking (B), and 2-piece locking (C).

    Below is the abstract for this study. You can read the full text here.

    Background

    A linear relationship between baseplate insertion torque and compression force in reverse shoulder arthroplasty (RSA) baseplates with central screw design has been recently established. In this study, we evaluated 3 different baseplate designs and their influence on the torque-compression relationship.

    Methods

    Three different RSA baseplate designs were evaluated through biomechanical testing using a glenoid vault, bone surrogate model. A digital torque gauge was used to measure insertion torque applied to the baseplate, whereas compression data were collected continuously from a load cell (Figure 1). Additionally, 2 predictive models were developed to predict the compression forces of each baseplate design at varying levels of torque.

    Figure 2. Detailed illustration of biomechanical setup (A) and actual experimental setup with a close-up of the top view (B). Each tested foam block was fixed to the load cell, which continuously collected data. Baseplates fit flush on the epoxy sheet, which was fixed to a rigid platform at 5 mm. Torque was applied with a digital torque gauge (maximum 6.8 Nm [60 in.lbf]).

    Results

    A linear relationship was found between baseplate compression and insertion torque for all 3 baseplate designs. Both the monoblock and 2-piece locking designs achieved the goal torque of 6.8 Nm, whereas the 2-piece nonlocking design did not due to material strip-out. No significant difference in maximum compression was found between the monoblock and 2-piece locking designs. However, the 2-piece nonlocking design achieved significantly higher compression. Both predictive models were shown to adequately predict compressive forces at different torque inputs for the monoblock and 2-piece locking designs but not the 2-piece nonlocking design.

    Conclusion

    The torque-compression relationship of a central screw baseplate is significantly affected by baseplate design. A 2-piece nonlocking baseplate reaches higher compression levels and risks material strip-out at lower insertional torques compared with a monoblock and 2-piece locking design. This has implications both on component design and on surgeon tactile feedback during surgery.

    Source

    Diaz MA, Hsu JE, Ricchetti ET, Garrigues GE, Gutierrez S, Frankle MA. Influence of reverse total shoulder arthroplasty baseplate design on torque and compression relationship. JSES Int. 2020 Apr 28;4(2):388-396. doi: 10.1016/j.jseint.2020.02.004. eCollection 2020 Jun.

    Note: This is the continuation of previous study, and it is part of a long-term project to evaluate the relationship between torque and compression in reverse shoulder arthroplasty. The reference for the previous study is Diaz MA, Garrigues GE, Ricchetti ET, Gutierrez S, Frankle MA. Relationship between insertion torque and compression strength in the reverse total shoulder arthroplasty baseplate. J Orthop Res. 2020;38(4):871-879. doi:10.1002/jor.24506.