Comparison of stereolithography (STL) and polygon file format (PLY) for intraoral scans: from chairside to archive

Abstract

Objectives: As dentistry transitions from analog to digital, efficient storage of intraoral scans remains underexplored. This study investigated the file size, geometric fidelity, and suitability of STL and PLY formats for long-term clinical storage.

Methods: In total, 1800 intraoral scans (maxillary and mandibular, with varying partial edentulism) acquired using three intraoral scanners were analyzed. Original STL files were converted into PLY and then reconverted into STL using four open-source tools: Trimesh, PyMeshLab, Blender 3.6, and Blender 4.4. To simulate color data, random vertex colors were added according to vendor-specific encodings. Original and ZIP-compressed file sizes were compared. Vertex coordinates and Hausdorff distances were assessed for geometric fidelity. The Wilcoxon signed-rank test was used for file size comparisons. A code-free batch converter, Plastl, was developed based on Trimesh.

Results: All PLY files were significantly smaller than their STL counterparts (p < 0.001). For non-colored PLY, the median PLY/STL size ratios ranged from 0.38-0.50, corresponding to a 50%-62% reduction. For colored PLY with vendor-specific encodings, files were 39%-59% smaller than STL. After ZIP compression, PLY remained significantly smaller than STL (p < 0.001). Minor vertex deviations (1-5 vertices) were found for only 68 scans (3.8%) with the 99^th percentile Hausdorff distance of zero. These deviations stemmed from dangling vertices removed by PyMeshLab, Blender (3.6/4.4). Trimesh preserved all vertex coordinates.

Conclusions: PLY reduces storage requirements while maintaining geometric accuracy, even for color data. Its adoption in hybrid STL-PLY workflows may increase digital sustainability in dental practice.

Keywords: Intraoral scan; data compression; dental informatics; digital dentistry.