The effects of length and width of the stem on proximal humerus stress shielding in uncemented primary reverse total shoulder arthroplasty

Abstract

Introduction: To preserve humeral bone during RTSA, stems have been made shorter and cement avoided whenever possible. However, with the increased use of uncemented RTSA, a phenomenon comparable to the stress shielding of the hip has been described for the proximal humerus. The aim of this study was to investigate the influence of stem length and width on proximal humeral bone resorption after primary uncemented RTSA.

Materials and methods: The prospective shoulder arthroplasty database of our institution was reviewed for all primary uncemented RTSAs from 2017 to 2020 in osteoarthritis and cuff tear arthropathy cases with > 2-year follow-up. We compared the clinical and the radiographic 2-year outcome of the short and standard length stems of the same prosthesis design. This allowed us to assess the effects of stem length and width with regard to stress shielding. Furthermore, we defined a cut-off value for the filling ratios to prevent stress shielding.

Results: Fifty patients were included in the analysis, nineteen were in the short stem group (SHORT) and thirty-one in the standard stem group (STANDARD). After 2 years, SHORT showed a relative Constant Score of 91.8% and STANDARD of 98.3% (p = 0.256). Stress shielding was found in 4 patients (21%) in SHORT and in 16 patients (52%) in STANDARD (p = 0.03); it occurred more frequently in patients with higher humeral filling ratios (p < 0.05). The calculated cut-off to prevent stress shielding was 0.7 (± 0.03) for the metaphyseal and distal filling ratio.

Conclusion: While short and standard stems for RTSA have good results after 2 years, we found a significant negative effect of higher length and width of the stem with regard to stress shielding. Even though the clinical effects of stress shielding have to be assessed, short stems should be chosen with a filling ratio at the metaphyseal and distal position below 0.7.

Level of evidence (a retrospective case-control study): III.

Keywords: Cuff tear arthropathy; Filling ratio; Omarthritis; Short stem reverse total shoulder arthroplasty (RTSA); Stress shielding.

Fig. 1

Consort diagram with patient selection and exclusion criteria

Fig. 2

Measurements of distal and metaphyseal filling ratios in the short stem (left) and long stem (right) group. A filling ratio of stem width (red) to the distance from inner cortex boundaries (FRinnerCortices, blue) and the distance from outer cortex boundary (FRouterCortices, yellow) was calculated for the distal and metaphyseal measurements. The metaphyseal measurement was taken at the calcar level, and the distance to the stem tip was halved for the position of the distal measurement. The measurements are made perpendicular to the longitudinal axis of the prosthesis stem (green)

Fig. 3

The grading system for stress shielding effect on the proximal humerus. The left side shows lateral stress shielding grade one (cortical thinning) and on the right side, there is lateral stress shielding grade 2 (complete bone loss down to the prosthesis)

Fig. 4

The result of the ROC is shown graphically. On the left side is the result of the distal filling ratios measured at the inner cortices (IC) and on the right side the same measurements of the metaphyseal filling ratios at the inner cortices (IC) (ROC receiver operating characteristic; AUC area under the curve)

Fig. 5

The graphic shows the effect on the occurrence of stress shielding effects when the cut-off values for the filling ratios at the inner cortices are observed. From left to right, a tenfold increase of stress shielding effects can be detected. (FR filling ratio)