Structure, Composition and Mechanical Properties of Overlap-Cementum at the CEJ

Introduction: In this study the commonly observed cervical cementum adjacent to enamel is defined as overlap-cementum (OC). It is unclear to what extent OC is afibrillar acellular cementum (AAC); lacking fibrous collagen or acellular extrinsic fiber cementum (AEFC). In bovine and equine teeth, cementum is integrated with enamel via microscallop-like structure. Hypothesis: The OC containing collagen is integrated with enamel via microscallops and forms a poorly adherent interface with enamel. Methods: Twenty primary teeth were cut in half longitudinally. The specimens were trimmed to ~5 mm³, mounted on atomic force microscope (AFM) stubs and polished through 1µm. The exposed surface of OC in some specimens was etched using EDTA for 10 minutes to aid in collagen fiber identification. The etched and unetched OC were characterized for structure, chemical composition and mechanical properties using AFM, Raman microspectroscopy and a Triboindenter. The integration of OC with enamel was studied using a scanning electron microscope (SEM). Additionally, 150-300 μm thick ground sections were used for detailed structural analysis of the OC integration with enamel using micro-X-ray computed tomography (μ-XCT). Results: AFM images of OC showed collagen fibers running oblique or parallel to cementum-enamel interface and Raman microspectroscopy of areas of OC contained organic peaks (C-H stretch at 2940 cm^-1). A significant difference (t-test, P < 0.05) in modulus of dry (15.3 ± 8.4 GPa) and wet (10.9 ± 5.8 GPa) OC was observed. In most cases SEM illustrated a space between coronal OC and enamel. However, in some cases, μ-XCT illustrated OC near the cervix was attached to enamel by well-spaced microscallops. Conclusions: The most commonly observed OC appeared to be AEFC adjacent to AAC. Although OC forms a poor interface in some regions, along the periphery of the tooth it could be attached to enamel via microscallops. Support: NIH/NIDCR Grant P01DE09859