In situ compressive loading and correlative noninvasive imaging of the bone-periodontal ligament-tooth fibrous joint

Characterizing the Mechanical Behavior of Bone and Bone Surrogates in Compression Using pQCT

Many axial and appendicular skeleton bones are subjected to repetitive loading during daily activities. Until recently, the structural analysis of fractures has been limited to 2D sections, and the dynamic assessment of fracture progression has not been possible. The structural failure was analyzed using step-wise micro-compression combined with time-lapsed micro-computed tomographic imaging. The structural failure was investigated in four different sample materials (two different bone surrogates, lumbar vertebral bodies from bovine and red deer). The samples were loaded in different force steps based on uniaxial compression tests. The micro-tomography images were used to create three-dimensional models from which various parameters were calculated that provide information about the structure and density of the samples. By superimposing two 3D images and calculating the different surfaces, it was possible to precisely analyze which trabeculae failed in which area and under which load. According to the current state of the art, bone mineral density is usually used as a value for bone quality, but the question can be raised as to whether other values such as trabecular structure, damage accumulation, and bone mineralization can predict structural competence better than bone mineral density alone.

Functional adaptation of interradicular alveolar bone to reduced chewing loads on dentoalveolar joints in rats

OBJECTIVES

The effects of reduced chewing loads on load bearing integrity of interradicular bone (IB) within dentoalveolar joints (DAJ) in rats were investigated.

METHODS

Four-week-old Sprague Dawley rats (N = 60) were divided into two groups; rats were either fed normal food, which is hard-pellet food (HF) (N = 30), or soft-powdered chow (SF) (N = 30). Biomechanical testing of intact DAJs and mapping of the resulting mechanical strains within IBs from 8- through 24-week-old rats fed HF or SF were performed. Tension- and compression-based mechanical strain profiles were mapped by correlating digital volumes of IBs at no load with the same IBs under load. Heterogeneity within IB was identified by mapping cement lines and TRAP-positive multinucleated cells using histology, and mechanical properties using nanoindentation technique.

RESULTS

Significantly decreased interradicular functional space, IB volume fraction, and elastic modulus of IB in the SF group compared with the HF group were observed, and these trends varied with an increase in age. The elastic modulus values illustrated significant heterogeneity within IB from HF or SF groups. Both compression- and tension-based strains were localized at the coronal portion of the IB and the variation in strain profiles complemented the observed material heterogeneity using histology and nanoindentation.

SIGNIFICANCE

Interradicular space and IB material-related mechanoadaptations in a DAJ are optimized to meet soft food related chewing demands. Results provided insights into age-specific regulation of chewing loads as a plausible "therapeutic dose" to reverse adaptations within the periodontal complex as an attempt to regain functional competence of a dynamic DAJ.

Mechanoadaptive strain and functional osseointegration of dental implants in rats

Spatiotemporal implant-bone biomechanics and mechanoadaptive strains in peri-implant tissue are poorly understood. Physical and chemical characteristics of an implant-bone complex (IBC) were correlated in three-dimensional space (along the length and around a dental implant) to gather insights into time related integration of the implant with the cortical portion of a jaw bone in a rat. Rats (N = 9) were divided into three experimental groups with three rats per time point; 3-, 11-, and 24-day. All rats were fed crumbled hard pellets mixed with water (soft-food diet) for the first 3 days followed by a hard-food diet with intact hard-food pellets (groups of 11- and 24-day only). Biomechanics of the IBCs harvested from rats at each time point was evaluated by performing mechanical testing in situ in tandem with X-ray imaging. The effect of physical association (contact area) of a loaded implant with adapting peri-implant tissue, and resulting strain within was mapped by using digital volume correlation (DVC) technique. The IBC stiffness at respective time points was correlated with mechanical strain in peri-implant tissue. Results illustrated that IBC stiffness at 11-day was lower than that observed at 3-day. However, at 24-day, IBC stiffness recovered to that which was observed at 3-day. Correlative microscopy and spectroscopy illustrated that the lower IBC stiffness was constituted by softer and less mineralized peri-implant tissue that contained varying expressions of osteoconductive elements. Lower IBC stiffness observed at 11-day was constituted by less mineralized peri-implant tissue with osteoconductive elements that included phosphorus (P) which was co-localized with higher expression of zinc (Zn), and lower expression of calcium (Ca). Higher IBC stiffness at 24-day was constituted by mineralized peri-implant tissue with higher expressions of osteoconductive elements including Ca and P, and lower expressions of Zn. These spatiotemporal correlative maps of peri-implant tissue architecture, heterogeneous distribution of mineral density, and elemental colocalization underscore mechanoadaptive physicochemical properties of peri-implant tissue that facilitate functional osseointegration of an implant. These results provided insights into 1) plausible "prescription" of mechanical loads as an osteoinductive "therapeutic dose" to encourage osteoconductive elements in the peri-implant tissue that would facilitate functional osseointegration of the implant; 2) a "critical temporal window" between 3 and 11 days, and perhaps it is this acute phase during which key candidate regenerative molecules can be harnessed to accelerate osseointegration of an implant under load.

Microanatomical changes and biomolecular expression at the PDL-entheses during experimental tooth movement

The novel aspect of this study was to contextualize the co-localization of biomolecular expression in widened and narrowed periodontal ligament (PDL)-space within a mechanically activated periodontal complex. The PDL is unique as it is the only ligament with both innervation and vascularization. Maxillary molars in 6-week-old male C57BL/6 mice (N = 5) were experimentally translated for 2 weeks using an elastic spacer. Contralateral teeth were used as controls. Mechanical testing of the periodontal complex of a mouse in situ and imaging using X-ray micro-computed tomography (micro-XCT) illustrated deformations within blood vessels (BV) of the PDL. PDL-bone and PDL-cementum entheses at the widened and narrowed PDL-spaces following experimental tooth movement (ETM) illustrated osterix (OSX), bone sialoprotein (BSP), cluster of differentiation 146 (CD146), and protein gene product 9.5 (PGP9.5), indicating active remodeling at these sites. PGP9.5 positive nerve bundles (NBs) were co-localized with multinucleated cells (MCs), Howship's resorption lacunae, and CD146 positive BVs. Association between nerves and MC was complemented by visualizing the proximity of osmium tetroxide stained NBs with the ultrastructure of MCs by performing scanning transmission electron microscopy. Spatial association of NB with BV, and NB with MC, provided insights into the plausible co-activation of NBs to initiate osteoclastic activity. Resorption of mineral occurred as an attempt to restore PDL-space of the load-bearing complex, specifically at the PDL-entheses. Mapping of anatomy-specific structural elements and their association with regenerative molecules by correlating light and electron micrographs provided insights into the use of these extracellular matrix molecules as plausible targets for pharmacological interventions related to tooth movement. Within the realm of tissue regeneration, modulation of load can reverse naturally occurring mineral formation to experimentally induced resorption, and naturally occurring mineral resorption to experimentally induced formation at the enthesial sites to permit tooth translation.

A Force on the Crown and Tug of War in the Periodontal Complex

The load-bearing dentoalveolar fibrous joint is composed of biomechanically active periodontal ligament (PDL), bone, cementum, and the synergistic entheses of PDL-bone and PDL-cementum. Physiologic and pathologic loads on the dentoalveolar fibrous joint prompt natural shifts in strain gradients within mineralized and fibrous tissues and trigger a cascade of biochemical events within the widened and narrowed sites of the periodontal complex. This review highlights data from in situ biomechanical simulations that provide tooth movements relative to the alveolar socket. The methods and subsequent results provide a reasonable approximation of strain-regulated biochemical events resulting in mesial mineral formation and distal resorption events within microanatomical regions at the ligament-tethered/enthesial ends. These biochemical events, including expressions of biglycan, decorin, chondroitin sulfated neuroglial 2, osteopontin, and bone sialoprotein and localization of various hypertrophic progenitors, are observed at the alkaline phosphatase-positive widened site, resulting in mineral formation and osteoid/cementoid layers. On the narrowed side, tartrate-resistant acid phosphatase regions can lead to a sequence of clastic activities resulting in resorption pits in bone and cementum. These strain-regulated biochemical and subsequently biomineralization events in the load-bearing periodontal complex are critical for maintenance of the periodontal space and overall macroscale joint biomechanics.

Periodontal ligament entheses and their adaptive role in the context of dentoalveolar joint function

OBJECTIVE

The dynamic bone-periodontal ligament (PDL)-tooth fibrous joint consists of two adaptive functionally graded interfaces (FGI), the PDL-bone and PDL-cementum that respond to mechanical strain transmitted during mastication. In general, from a materials and mechanics perspective, FGI prevent catastrophic failure during prolonged cyclic loading. This review is a discourse of results gathered from literature to illustrate the dynamic adaptive nature of the fibrous joint in response to physiologic and pathologic simulated functions, and experimental tooth movement.

METHODS

Historically, studies have investigated soft to hard tissue transitions through analytical techniques that provided insights into structural, biochemical, and mechanical characterization methods. Experimental approaches included two dimensional to three dimensional advanced in situ imaging and analytical techniques. These techniques allowed mapping and correlation of deformations to physicochemical and mechanobiological changes within volumes of the complex subjected to concentric and eccentric loading regimes respectively.

RESULTS

Tooth movement is facilitated by mechanobiological activity at the interfaces of the fibrous joint and generates elastic discontinuities at these interfaces in response to eccentric loading. Both concentric and eccentric loads mediated cellular responses to strains, and prompted self-regulating mineral forming and resorbing zones that in turn altered the functional space of the joint.

SIGNIFICANCE

A multiscale biomechanics and mechanobiology approach is important for correlating joint function to tissue-level strain-adaptive properties with overall effects on joint form as related to physiologic and pathologic functions. Elucidating the shift in localization of biomolecules specifically at interfaces during development, function, and therapeutic loading of the joint is critical for developing "functional regeneration and adaptation" strategies with an emphasis on restoring physiologic joint function.

Strain mapping and correlative microscopy of the alveolar bone in a bone-periodontal ligament-tooth fibrous joint

This study details a method to calculate strains within interradicular alveolar bone using digital volume correlation on X-ray tomograms of intact bone-periodontal ligament-tooth fibrous joints. The effects of loading schemes (concentric and eccentric) and optical magnification on the resulting strain in alveolar bone will be investigated with an intent to correlate deformation gradients with data sets from other complementary techniques. Strain maps will be correlated with structural and site-specific mechanical properties obtained on the same specimen using atomic force microscopy and atomic force microscopy-based nanoindentation technique. Specimens include polydimethylsiloxane as a standard material and intact hemi-mandibles harvested from rats. X-ray tomograms were taken at no-load and loaded conditions using an in situ load cell coupled to a micro X-ray computed tomography unit. Digital volume correlation was used to calculate deformations within alveolar bone. Comparison of strain maps was made as a result of different loading schemes (concentric vs eccentric) and at different magnifications (4× vs 10×). Virtual sections and strain maps from digital volume correlation solutions were aligned with structure and reduced elastic modulus to correlate datasets of the same region within a specimen. Strain distribution between concentrically and eccentrically loaded complexes was different but illustrated a similar range. Strain maps of homogeneous materials (polydimethylsiloxane) resulting from digital volume correlation at different magnifications were similar. However, strain maps of heterogeneous materials at lower and higher magnification differed. The digital volume correlation technique illustrated a dependence on optical magnification specifically for heterogeneous materials such as bone. The results at a higher optical magnification highlight the potential for extracting deformation at higher resolutions. Correlation of data spaces from different complementary techniques is plausible and could provide insights into biological and physicochemical processes that lead to functional adaptation of tissues and joints.

Strain-guided mineralization in the bone-PDL-cementum complex of a rat periodontium

Objective

The objective of this study was to investigate the effect of mechanical strain by mapping physicochemical properties at periodontal ligament (PDL)–bone and PDL–cementum attachment sites and within the tissues per se.

Design

Accentuated mechanical strain was induced by applying a unidirectional force of 0.06 N for 14 days on molars in a rat model. The associated changes in functional space between the tooth and bone, mineral forming and resorbing events at the PDL–bone and PDL–cementum attachment sites were identified by using micro-X-ray computed tomography (micro-XCT), atomic force microscopy (AFM), dynamic histomorphometry, Raman microspectroscopy, and AFM-based nanoindentation technique. Results from these analytical techniques were correlated with histochemical strains specific to low and high molecular weight GAGs, including biglycan, and osteoclast distribution through tartrate resistant acid phosphatase (TRAP) staining.

Results

Unique chemical and mechanical qualities including heterogeneous bony fingers with hygroscopic Sharpey's fibers contributing to a higher organic (amide III — 1240 cm^− 1) to inorganic (phosphate — 960 cm^− 1) ratio, with lower average elastic modulus of 8 GPa versus 12 GPa in unadapted regions were identified. Furthermore, an increased presence of elemental Zn in cement lines and mineralizing fronts of PDL–bone was observed. Adapted regions containing bony fingers exhibited woven bone-like architecture and these regions rich in biglycan (BGN) and bone sialoprotein (BSP) also contained high-molecular weight polysaccharides predominantly at the site of polarized bone growth.

Conclusions

From a fundamental science perspective the shift in local properties due to strain amplification at the soft–hard tissue attachment sites is governed by semiautonomous cellular events at the PDL–bone and PDL–cementum sites. Over time, these strain-mediated events can alter the physicochemical properties of tissues per se, and consequently the overall biomechanics of the bone–PDL–tooth complex. From a clinical perspective, the shifts in magnitude and duration of forces on the periodontal ligament can prompt a shift in physiologic mineral apposition in cementum and alveolar bone albeit of an adapted quality owing to the rapid mechanical translation of the tooth.

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Load-mediated shifts in mechanical strains will prompt self-governing zones at PDL-cementum and PDL-bone entheses.

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The intensity of strain amplification is predominantly felt at the entheses as it is a region where disparate materials attach.

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Physicochemical observations at the PDL-bone enthesial zone are not directly correlated to the events at PDL-cementum zone.

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Rapid shifts in PDL strain can prompt a shift in mineral apposition at respective entheses albeit of an adapted quality.

Multiscale biomechanical responses of adapted bone-periodontal ligament-tooth fibrous joints

Reduced functional loads cause adaptations in organs. In this study, temporal adaptations of bone-ligament-tooth fibrous joints to reduced functional loads were mapped using a holistic approach. Systematic studies were performed to evaluate organ-level and tissue-level adaptations in specimens harvested periodically from rats (N=60) given powder food for 6 months over 8,12,16,20, and 24 weeks. Bone-periodontal ligament (PDL)-tooth fibrous joint adaptation was evaluated by comparing changes in joint stiffness with changes in functional space between the tooth and alveolar bony socket. Adaptations in tissues included mapping changes in the PDL and bone architecture as observed from collagen birefringence, bone hardness and volume fraction in rats fed soft foods (soft diet, SD) compared to those fed hard pellets as a routine diet (hard diet, HD). In situ biomechanical testing on harvested fibrous joints revealed increased stiffness in SD groups (SD:239-605 N/mm) (p<0.05) at 8 and 12 weeks. Increased joint stiffness in early development phase was due to decreased functional space (at 8 weeks change in functional space was -33 μm, at 12 weeks change in functional space was -30 μm) and shifts in tissue quality as highlighted by birefringence, architecture and hardness. These physical changes were not observed in joints that were well into function, that is, in rodents older than 12 weeks of age. Significant adaptations in older groups were highlighted by shifts in bone growth (bone volume fraction 24 weeks: Δ-0.06) and bone hardness (8 weeks: Δ-0.04 GPa, 16 weeks: Δ-0.07 GPa, 24 weeks: Δ-0.06 GPa). The response rate (N/s) of joints to mechanical loads decreased in SD groups. Results from the study showed that joint adaptation depended on age. The initial form-related adaptation (observed change in functional space) can challenge strain-adaptive nature of tissues to meet functional demands with increasing age into adulthood. The coupled effect between functional space in the bone-PDL-tooth complex and strain-adaptive nature of tissues is necessary to accommodate functional demands, and is temporally sensitive despite joint malfunction. From an applied science perspective, we propose that adaptations are registered as functional history in tissues and joints.