Reduced functional loads alter the physical characteristics of the bone-periodontal ligament-cementum complex

Abstract

Background and objective: Adaptive properties of the bone-periodontal ligament-tooth complex have been identified by changing the magnitude of functional loads using small-scale animal models, such as rodents. Reported adaptive responses as a result of lower loads due to softer diet include decreased muscle development, change in structure-function relationship of the cranium, narrowed periodontal ligament space, and changes in the mineral level of the cortical bone and alveolar jaw bone and in the glycosaminoglycans of the alveolar bone. However, the adaptive role of the dynamic bone-periodontal ligament-cementum complex to prolonged reduced loads has not been fully explained to date, especially with regard to concurrent adaptations of bone, periodontal ligament and cementum. Therefore, in the present study, using a rat model, the temporal effect of reduced functional loads on physical characteristics, such as morphology and mechanical properties and the mineral profiles of the bone-periodontal ligament-cementum complex was investigated.

Material and methods: Two groups of 6-wk-old male Sprague-Dawley rats were fed nutritionally identical food with a stiffness range of 127-158 N/mm for hard pellet or 0.3-0.5 N/mm for soft powder forms. Spatio-temporal adaptation of the bone-periodontal ligament-cementum complex was identified by mapping changes in the following: (i) periodontal ligament collagen orientation and birefringence using polarized light microscopy, bone and cementum adaptation using histochemistry, and bone and cementum morphology using micro-X-ray computed tomography; (ii) mineral profiles of the periodontal ligament-cementum and periodontal ligament-bone interfaces by X-ray attenuation; and (iii) microhardness of bone and cementum by microindentation of specimens at ages 6, 8, 12 and 15 wk.

Results: Reduced functional loads over prolonged time resulted in the following adaptations: (i) altered periodontal ligament orientation and decreased periodontal ligament collagen birefringence, indicating decreased periodontal ligament turnover rate and decreased apical cementum resorption; (ii) a gradual increase in X-ray attenuation, owing to mineral differences, at the periodontal ligament-bone and periodontal ligament-cementum interfaces, without significant differences in the gradients for either group; (iii) significantly (p < 0.05) lower microhardness of alveolar bone (0.93 ± 0.16 GPa) and secondary cementum (0.803 ± 0.13 GPa) compared with the higher load group insert bone = (1.10 ± 0.17 and cementum = 0.940 ± 0.15 GPa, respectively) at 15 wk, indicating a temporal effect of loads on the local mineralization of bone and cementum.

Conclusion: Based on the results from this study, the effect of reduced functional loads for a prolonged time could differentially affect morphology, mechanical properties and mineral variations of the local load-bearing sites in the bone-periodontal ligament-cementum complex. These observed local changes in turn could help to explain the overall biomechanical function and adaptations of the tooth-bone joint. From a clinical translation perspective, our study provides an insight into modulation of load on the complex for improved tooth function during periodontal disease and/or orthodontic and prosthodontic treatments.

Figure 1

A) Compression testing system. B) Individual unconfined hard pellets loaded in compression to determine stiffness of hard food. C) Powdered soft food confined within a hard plastic container and compressively loaded to determine stiffness of soft food. D) Stiffness of the hard food (HF 1–3) and soft food (SF 1–3) was determined by the slope of the linear portion of the load versus displacement curve. Mean stiffness of hard food: 150 ± 15 N/mm and soft food: 0.4 ± 0.1 N/mm.

Figure 2

A) 3D reconstruction of mandibular molar region. B–D) Reconstructed 2D slices representing coronal, middle and apical regions, buccal-lingual (B), mesial-distal (C), and occusal-apical (D) planes respectively. The coronal region (CR), all primary cementum, in red, the mid-root region (MR), spanning the interface of the primary and secondary cementum, in purple, and the apical region (AR), all secondary cementum, in blue are shown. Scale bar = 1mm

Figure 3

Light microscope images of distal root of molar 2 (M2/D) stained with H&E at 10× to illustrate the variations in PDL-orientation. Representative insets of polarized light micrographs of picrosirius red stained bone-PDL-cementum attachment sites of M2/D at 40×. A, B) M2/D subjected to lower loads illustrates less organized PDL-collagen fibrils, patchy and dim PDL-birefringence. C, D) M2/D subjected to higher loads illustrates organized PDL-collagen fibrils, distinct and bright appearance of the PDL. AB = alveolar bone, Cem = cementum, Den = dentin, PDL = periodontal ligament. Scale bars = 100 μm, inset scale bars = 50 μm

Figure 4

A) Representative resorption sites on mesial-distal 2D slice of a mandibular second molar. Areas of resorption are colored in both mesial-distal 2D reconstructed slices and corresponding tomographs for all groups. Extensive resorption can be observed in the CR and MR regions for all groups. However, in the lower functional load groups (B) there is little or no resorption in the AR region compared to the higher functional load groups (C) (white arrows).

Figure 5

A) Representative tomograph and 2D sagittal section illustrating the examined regions of the bone-PDL-cementum complex subjected to lower (B) and higher (C) loads. B, C) Light micrographs of M2/D at 20× illustrating variations in reversal lines (black lines) in AB. B) Reversal lines increase with age in the CR region opposing areas of primary cementum resorption (white arrows). C) Reversal lines increase with age in the AR region opposing areas of secondary cementum resorption (white arrows). AB = alveolar bone, Cem = cementum, PDL = periodontal ligament. Scale bars = 100 μm

Figure 6

A,D) Transverse sections illustrating trailing and leading envelopes of the tooth as darker and lighter attenuating regions (white arrows). A-represents bone-tooth complex subjected to lower loads and D-represents bone-tooth complex subjected to higher loads. X-ray intensity profiles illustrate gradients (normalized) due to mineral variation in alveolar bone near the PDL-bone interface, and PDL-cementum interface of secondary cementum for lower (B and C) and higher (E and F) functional loads at all time points in bone and secondary cementum. Note: The 6 week higher load group is also shown in the lower load for ease of comparison. For all time points and locations, the intensity gradually increases in the first 25μm (for bone) or 15 μm (for cementum) from the PDL-space into mineralized tissue with no significant differences between the higher and lower load groups. An apparent decrease in X-ray attenuation in bone and cementum subjected to lower loads (B and C) compared to higher (E and F) loads was observed. M2=molar 2, PDL-Min. Tissue Enth. = Enthesis of PDL-respective mineralized tissue

Figure 7

A) Mean hardness values of alveolar bone for higher and lower load groups at each time point. B) Mean hardness values of secondary cementum determined by microindentation for higher and lower load groups at each time point. Range of hardness values are represented by gray lines above (maximum) and below (minimum). At all experimental time points (8, 12, and 15 weeks) respective bone and cementum hardness values in the higher load group was greater than in the lower load group. Asterisks indicate statistical significance (p<0.05) between groups at the 12 and 15 week time points. C) Light microscope images illustrate microindents in alveolar bone (AB) and secondary cementum (CEM). PDL = Periodontal Ligament, DEN = Dentin. Scale bar = 100μm

Figure 8

A) Virtual slice of M2 taken from a tomograph illustrating the bone-PDL-tooth complex. Schematic of a biomechanical model illustrating dominant forces on the second mandibular rat molar and resulting load-mediated adaptation of tissues in the complex subjected to higher and lower functional loads. B) The hard pellet diet provides a high functional load and apically directed forces in addition to the distally directed forces due to innate tooth drift. Together, these forces cause the tooth to rotate, with the inter-radicular bone as the fulcrum point (green dot). C, D) The rotation redirects the apical and distal forces, causing PDL compression between the alveolar bone and the tooth and resulting resorption in the coronal, midroot, and apical regions with time. E) The soft, powdered diet decreases the functional load, so that the distally directed forces of tooth drift dictate tooth movement. The anatomy of the inter-radicular bone also creates a fulcrum point (*) around which some rotational movement could exist. F) The distal tooth drift together with some rotation causes regional PDL compression between the alveolar bone and the coronal portion of the tooth, resulting in resorption. G) In the absence of substantive functional loading, the rotational force on the tooth is minimized, and little compressive force acts on the apical portion of the root. Meanwhile, subsequent tooth drift acts to translate the tooth distally, causing regional PDL compression between the alveolar bone and the tooth in the coronal and midroot regions. En = Enamel, Den = Dentin, Cem = Secondary Cementum, PDL = Periodontal Ligament, AB = Alveolar Bone.