PPM1D and intervertebral disk degenerative disorder: A total of 33 aging-related differentially expressed genes (DEGs) were screened in IDD, and machine learning combined with ROC curve analysis identified PPM1D, PIK3C2A, and BTG3 as aging-related biomarkers for IDD; Gene Set Enrichment Analysis (GSEA) revealed that these three biomarkers were enriched in gene functions including cellular senescence, multicellular organismal aging, negative regulation of cellular senescence, and Ribosome.