Multiplanar CT-based characterization of offset and positional change in reverse shoulder arthroplasty and validation of radiographic metrics.

BACKGROUND: Accurate quantification of lateralization and distalization in reverse total shoulder arthroplasty (rTSA) is essential for optimizing planning and outcomes. Angular radiographic parameters such as the lateralization shoulder angle (LSA) and distalization shoulder angle (DSA) are widely used and distance-based ratios, including the lateralization index (LI) and distalization index (DI) have recently been introduced. However, the validity of radiographic metrics reflecting implant position and pre- to postoperative positional changes remains unclear. Therefore, this study aimed (1) to characterize the position and positional change of the center of rotation (COR) and humerus for a specific rTSA construct using CT multiplanar reconstruction (MPR) in a standardized plane and (2) to evaluate how well radiographic metrics represent true implant position and positional change.

METHODS: Thirty consecutive shoulders undergoing primary rTSA between July 2022 and March 2024 using Stryker implants (Reverse II with BIO-RSA or Perform augmented baseplate) were prospectively included. Preoperative and one-year postoperative CT scans and radiographs were obtained. In the standardized scapular coronal plane, CT-based lateral and vertical offsets were quantified as landmark-based distances from a fixed acromial reference (ACR) to the COR and the greater tuberosity (GT), defining COR and humeral offset, respectively. Pre- to postoperative changes in these offsets were analyzed and defined as lateralization and distalization. Additionally, the glenoid-implant interface (ITF) was registered, allowing calculation of the total lateral offset (ITF-GT distance). Radiographic LSA, DSA, LI, and DI were measured and correlated with CT-derived measurements using Pearson coefficients.

RESULTS: Postoperatively, the COR was significantly medialized (-12.9 ± 6.5 mm; p<0.0001) and distalized (5.7 ± 4.6 mm; p<0.0001). Humeral lateral offset was maintained (1.7 ± 6.9 mm; p=0.199), whereas humeral vertical offset increased (24.4 ± 5.5 mm; p<0.0001). LSA showed no correlation with COR, humeral or total lateral offset. LI correlated with COR and humeral lateral offset (r = 0.84 and r = 0.82, respectively; both p<0.0001). Neither LSA nor LI were associated with pre- to postoperative positional changes. DSA correlated with humeral vertical offset (r = 0.50; p=0.005), whereas DI correlated with COR vertical offset (r = -0.60; p<0.001) and its pre- to postoperative change (r = -0.44; p=0.015). No other significant correlations were identified.

CONCLUSION: Following rTSA with the studied implants, the COR is consistently medialized and distalized, whereas humeral lateral offset remained unchanged and humeral vertical offset increased. Current radiographic metrics insufficiently capture both true implant position and pre- to postoperative individual positional changes.

LEVEL OF EVIDENCE: Level I; Diagnostic Study.