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. 2024 Dec 23;10(12):334.
doi: 10.3390/jimaging10120334.

Volumetric Humeral Canal Fill Ratio Effects Primary Stability and Cortical Bone Loading in Short and Standard Stem Reverse Shoulder Arthroplasty: A Biomechanical and Computational Study

Daniel Ritter  1   2 , Patric Raiss  3 , Patrick J Denard  4 , Brian C Werner  5 , Peter E Müller  2 , Matthias Woiczinski  2   6 , Coen A Wijdicks  1 , Samuel Bachmaier  1
Affiliations

Volumetric Humeral Canal Fill Ratio Effects Primary Stability and Cortical Bone Loading in Short and Standard Stem Reverse Shoulder Arthroplasty: A Biomechanical and Computational Study

Daniel Ritter et al. J Imaging. .

Abstract

Objective: This study evaluated the effect of three-dimensional (3D) volumetric humeral canal fill ratios (VFR) of reverse shoulder arthroplasty (RSA) short and standard stems on biomechanical stability and bone deformations in the proximal humerus.

Methods: Forty cadaveric shoulder specimens were analyzed in a clinical computed tomography (CT) scanner allowing for segmentation of the humeral canal to calculate volumetric measures which were verified postoperatively with plain radiographs. Virtual implant positioning allowed for group assignment (VFR < 0.72): Standard stem with low (n = 10) and high (n = 10) filling ratios, a short stem with low (n = 10) and high filling ratios (n = 10). Biomechanical testing included cyclic loading of the native bone and the implanted humeral component. Optical recording allowed for spatial implant tracking and the quantification of cortical bone deformations in the proximal humerus.

Results: Planned filling ratios based on 3D volumetric measures had a good-to-excellent correlation (ICC = 0.835; p < 0.001) with implanted filling ratios. Lower canal fill ratios resulted in significantly higher variability between short and standard stems regarding implant tilt (820 N: p = 0.030) and subsidence (220 N: p = 0.046, 520 N: p = 0.007 and 820 N: p = 0.005). Higher filling ratios resulted in significantly lower bone deformations in the medial calcar area compared to the native bone, while the bone deformations in lower filling ratios did not differ significantly (p > 0.177).

Conclusions: Lower canal filling ratios maintain dynamic bone loading in the medial calcar of the humerus similar to the native situation in this biomechanical loading setup. Short stems implanted with a low filling ratio have an increased risk for implant tilt and subsidence compared to high filling ratios or standard stems.

Keywords: CT imaging; biomechanics; bone deformation; canal fill; micromotion; reverse shoulder arthroplasty; short stem; standard stem; stress shielding.

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Conflict of interest statement

D.R. is an employee of Arthrex. P.R. receives consulting fees and honoraria from Arthrex, receives support for travel to meetings for the study or other purposes from Arthrex. P.D. receives consulting fees and honoraria from Arthrex, receives support for travel to meetings for the study or other purposes from Arthrex, and receives royalties from Arthrex. B.W. receives consulting fees and honoraria from Arthrex, Pacira and Lifenet, receives research support from Arthrex, Zimmer Biomet, Exactech, Pacira and Lifenet and receives support for travel to meetings for the study or other purposes from Arthrex. M.W. declares no conflicts of interests. C.W. is an employee of Arthrex. P.E.M. receives consulting fees and honoraria from BBraun Aesculap and Medacta and the affiliated research institute receives research support from Arthrex. There were no financial payments or other benefits related to the subject of this article. S.B. is an employee of Arthrex.

Figures

Figure 1
Figure 1
Methodical framework, from virtually planning and developing a volumetric measure of the humeral canal which was used in this study for group assignment and planning of low and high filling ratios. Canal fill ratios were controlled using postoperative X-rays after the implantation and before testing the implanted humeral component biomechanically.
Figure 2
Figure 2
Measurement and calculation of the filling ratios by dividing the red marked measure through the respective blue one. The three-dimensional rendered and segmented CT data on the left side allowed for volumetric calculation of the canal fill ratio (3D VFR). Calculation of the canal fill ratios based on two-dimensional plane radiographs (2D Metaphysis FR and 2D Diaphysis FR) is shown on the right side based on current clinical practice [14,16].
Figure 3
Figure 3
The 2D to 3D registration allowedto validate the accuracy of preoperative canal fill measurements with the actual postoperative implant seating: (A). preoperative planning of the humeral implant (purple) and segmentation of the humeral canal (orange), (B). registration of postOP X-rays, (C). correction of the implant position according to postOP position (blue) and (D). calculation of the true postOP canal fill ratio for comparison with the preOP ratio.
Figure 4
Figure 4
(A) Testing protocol shows the loading cycles including the points of data analysis (a–g). (B) Experimental cyclic loading setups and the optical tracking points (green) for data analysis. (C) Evaluated tracking points during cyclic loading force (F) to analyze implant subsidence and tilt measurements between analysis points a and b, d or f, respectively, (simlant and αimlant, Δab, Δad, and Δaf) at the end of each loading block. Bone micromotion (sBoneHW, Δbc, Δde, and Δfg) was evaluated as bone displacement within each final load cycle (hysteresis width (HW)). Total compressive transmission caused deformation of the bone was measured at the end of each loading block (sBoneTot, Δab, Δad, and Δaf).
Figure 5
Figure 5
Boxplot of implant subsidence (A) and tilt (B) at the end of each cyclic loading block (220 N, 520 N, and 820 N) comparing short and standard stem implants, respectively, implanted with high and low filling ratios.
Figure 6
Figure 6
Boxplots of total bone deformation (A) and bone micromotion (B) for each cyclic loading block (220 N, 520 N, and 820 N) comparing low- and high filling ratios to the biomechanical behavior of the native bone.
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