Understanding the hierarchical structure of collagen fibers of the human periodontal ligament: Implications for biomechanical characteristics

The periodontal ligament (PDL) is a unique fibrous connective tissue that regulates periodontal homeostasis mechanisms. Its biomechanical properties primarily reside in the hierarchical and non-uniform collagenous network. This study aimed to investigate the region-specific structure and composition of collagen fibers in the PDL at various scales and to explore their relationship with mechanical properties in a split-mouth design. Fresh human cadaver transverse PDL specimens of maxillary anterior teeth were categorized into cervical, middle, and apical groups. These specimens were analyzed via Masson's trichrome staining, scanning electron microscopy, picrosirius red (PSR) staining, three-dimensional (3D) reconstruction, Raman spectroscopy, and uniaxial tensile test. Statistical analyses were performed to compare the structural, compositional, and tensile properties among the groups. Notably, the middle PDL samples exhibited superior tensile strength and higher fiber area fraction than the other two transverse sections. Despite a higher mineral-to-matrix ratio and a different collagen secondary structure, the apical PDL demonstrated a relatively weaker tensile strength, possibly associated with its discovered sparser collagen fiber areal fraction. The cervical region, characterized by a mediocre fiber areal fraction, displayed diminished tensile strength. The 3D reconstructed collagenous network model and PSR staining exposed the fiber interaction and the micropores. Microscale porosity and variations in collagen secondary structure, particularly in the apical region, suggest adaptive mechanisms for accommodating compressive forces and maintaining functional integrity. Variance in the tensile properties of samples in different force directions indicated the significant influence of fiber orientation and root level on tissue mechanics. STATEMENT OF SIGNIFICANCE: : This study provides critical insights into the biomechanical and structural properties of the human periodontal ligament (PDL), particularly focusing on the underexplored anterior teeth. Through advanced techniques like SEM, histological staining, 3D reconstruction, Raman spectroscopy, and tensile testing, we reveal significant regional variations in PDL collagen organization, composition, and biomechanical properties. Our findings address a crucial knowledge gap concerning the material mechanics of the PDL, offering a foundational understanding for future periodontal tissue engineering and biomimetic material development. This multi-scale analysis underscores the importance of both mesoscale structural characteristics and nanoscale molecular structures in maintaining PDL mechanical integrity.