# Factors Influencing Stem Subsidence in 135-Degree Inlay Reverse Shoulder Arthroplasty

## TL;DR

This study examines why some 135-degree inlay reverse shoulder implants sink into the bone, finding that poor fit at the top of the implant is a key risk factor.

## Contribution

The study identifies proximal filling ratio as a novel independent risk factor for stem subsidence in 135-degree inlay RSA.

## Key findings

- Subsidence occurred in 13.6% of cases with greater stem alignment and lower proximal filling ratio.
- Proximal filling ratio <80% was an independent risk factor for subsidence.
- Proper stem sizing and alignment may improve initial stability and outcomes.

## Abstract

Background/Objectives: Reverse shoulder arthroplasty (RSA) using a 135-degree inlay-type humeral stem has recently gained popularity due to its bone-preserving design. However, stem subsidence (hereafter, subsidence) and its contributing factors are poorly understood. We aimed to investigate the incidence of subsidence and its associated factors in patients undergoing RSA using a 135-degree inlay-type stem. Methods: A total of 44 shoulders treated with uncemented Tornier Perform® Reversed Stems were retrospectively analyzed. Radiographic evaluations included stem alignment and canal filling ratio at three levels. The outcome, subsidence, was defined as >5 mm inferior migration of the stem. Results: Subsidence was observed in 6 shoulders (13.6%), which showed significantly greater stem alignment and lower proximal filling ratio. Logistic regression analysis identified proximal filling ratio <80% as an independent risk factor (odds ratio: 70.0, 95% confidence interval: 3.6–1342.6). Conclusions: Although the findings remain exploratory due to the small sample size and short follow-up period, they suggest that inadequate proximal fit may contribute to subsidence in 135-degree inlay RSA. Ensuring proper stem sizing and alignment during implantation may be essential to improving initial stability and clinical outcomes. Larger, long-term studies are required to generalize these conclusions.