Microcracks and osteoclast resorption activity in vitro

Biological mechanisms underlying complications related to implant site preparation

Implant site preparation is a critical stage of implant surgery that may underpin various complications related to implant surgery. This review discusses the latest available scientific information on risk factors related to implant site preparation. The role of the drilling process in relation to the density of the available alveolar bone, the effects of insertion torque on peri-implant osseous healing, and implant-related variables such as macrodesign and implant-abutment connection are all factors that can influence implant success. Novel information that links osteotomy characteristics (including methods to improve implant initial stability, the impact of drilling speed, and increase of the implant insertion torque modifying the bone-implant interface) with the appropriate instrumentation techniques will be discussed, as well as interactions at the bone-biomaterial interface that may lead to biologic complications mediated by implant dissolution products.

Histological Review of Demineralized Dentin Matrix as a Carrier of rhBMP-2

In 2007, recombinant human bone morphogenetic protein-2 (rhBMP-2) was approved for use in humans at a concentration of 1.5 mg/mL with absorbable collagen sponges as an alternative to autogenous bone grafts for alveolar ridge augmentation, defects associated with extraction sockets, and sinus augmentation. However, the use of supraphysiological doses and the insufficient retention of rhBMP-2, when delivered through collagen sponge, result in dose-dependent side effects related to off-label use. Demineralized dentin matrix (DDM), an osteoinducing bone substrate, has been used as an rhBMP-2 carrier since 1998. In addition, DDM has both microparticle and nanoparticle structures, which do not undergo remodeling, unlike bone. In vitro, DDM is a suitable carrier for BMP-2, with the continued release over 30 days at concentrations sufficient to stimulate osteogenic differentiation. In this review, we discuss the histological outcomes of DDM loaded with rhBMP-2 to highlight the biological functions of exogenous rhBMP-2 associated with the DDM carrier in clinical applications in implant dentistry. Impact Statement Demineralized dentin matrix (DDM) has been used as an recombinant human bone morphogenetic protein (rhBMP-2) carrier and osteo-inducing bone substrate to facilitate continued release and stimulate osteogenic differentiation. In this review, we discuss the histological outcomes of DDM loaded with rhBMP-2 in order to highlight the biological functions of exogenous rhBMP-2 associated with the DDM carrier in clinical applications in implant dentistry.

TCIRG1 Transgenic Rescue of Osteoclast Function Using Induced Pluripotent Stem Cells Derived from Patients with Infantile Malignant Autosomal Recessive Osteopetrosis

BACKGROUND

Osteoclasts are hematopoietic stem cell-derived multinucleated cells necessary for bone remodeling and resorption. TCIRG1 encodes a protein that is an adenosine triphosphate (ATP)-dependent vacuolar proton pump required for this process. Recessive loss-of-function mutations in both copies of this gene lead to impairment of osteoclast function, with increased bone density, increased skeletal mass, and early mortality.

METHODS

We isolated fibroblasts from a patient with the compound heterozygous TCIRG1 mutations c.1549G>A (p.517D>N) and c.2236C>T (p.746Q>X), and reprogrammed them into iPS (induced pluripotent stem) cells. The function of osteoclasts derived from these cells was then rescued by transgenic expression of TCIRG1 cDNA.

RESULTS

In addition to the known effects of TCIRG1 loss of function, iPS cell-derived osteoclasts from this patient had reduced expression of the bone remodeling enzymes cathepsin K (CTSK) and tartrate-resistant acid phosphatase (TRAP), leading to reduced in vitro bone remodeling. Expression of both genes and pit formation were restored in iPS cell-derived osteoclasts following transgenic restoration of TCIRG1 expression.

CONCLUSIONS

Transgenic overexpression of TCIRG1 was sufficient to restore osteoclast function in iPS cell-derived osteoclasts from a patient with infantile malignant autosomal-recessive osteopetrosis.

CLINICAL RELEVANCE

This work provides a proof of concept for an autologous approach to treating osteopetrosis, potentially avoiding the risks associated with hematopoietic stem cell transplantation in a young patient population.

In vivo effects of different orthodontic loading on root resorption and correlation with mechanobiological stimulus in periodontal ligament

Orthodontic root resorption is a common side effect of orthodontic therapy. It has been shown that high hydrostatic pressure in the periodontal ligament (PDL) generated by orthodontic forces will trigger recruitment of odontoclasts, leaving resorption craters on root surfaces. The patterns of resorption craters are the traces of odontoclast activity. This study aimed to investigate resorptive patterns by: (i) quantifying spatial root resorption under two different levels of in vivo orthodontic loadings using microCT imaging techniques and (ii) correlating the spatial distribution pattern of resorption craters with the induced mechanobiological stimulus field in PDL through nonlinear finite-element analysis (FEA) in silico. Results indicated that the heavy force led to a larger total resorption volume than the light force, mainly by presenting greater individual crater volumes ( p < 0.001) than increasing crater numbers, suggesting that increased mechano-stimulus predominantly boosted cellular resorption activity rather than recruiting more odontoclasts. Furthermore, buccal-cervical and lingual-apical regions in both groups were found to have significantly larger resorption volumes than other regions ( p < 0.005). These clinical observations are complemented by the FEA results, suggesting that root resorption was more likely to occur when the volume average compressive hydrostatic pressure exceeded the capillary blood pressure (4.7 kPa).

Study of Osteoclast Adhesion to Cortical Bone Surfaces: A Correlative Microscopy Approach for Concomitant Imaging of Cellular Dynamics and Surface Modifications

Bone remodeling relies on the coordinated functioning of osteoblasts, bone-forming cells, and osteoclasts, bone-resorbing cells. The effects of specific chemical and physical bone features on the osteoclast adhesive apparatus, the sealing zone ring, and their relation to resorption functionality are still not well-understood. We designed and implemented a correlative imaging method that enables monitoring of the same area of bone surface by time-lapse light microscopy, electron microscopy, and atomic force microscopy before, during, and after exposure to osteoclasts. We show that sealing zone rings preferentially develop around surface protrusions, with lateral dimensions of several micrometers, and ∼1 μm height. Direct overlay of sealing zone rings onto resorption pits on the bone surface shows that the rings adapt to pit morphology. The correlative procedure presented here is noninvasive and performed under ambient conditions, without the need for sample labeling. It can potentially be applied to study various aspects of cell-matrix interactions.

Age-Related Effects of Advanced Glycation End Products (Ages) in Bone Matrix on Osteoclastic Resorption

Advanced glycation end products (AGEs) accumulate in bone extracellular matrix as people age. Previous studies have shown controversial results regarding the role of in situ AGEs accumulation in osteoclastic resorption. To address this issue, this study cultured human osteoclast cells directly on human cadaveric bone slices from different age groups (young and elderly) to warrant its relevance to in vivo conditions. The cell culture was terminated on the 3rd, 7th, and 10th day, respectively, to assess temporal changes in the number of differentiated osteoclasts, the number and size of osteoclastic resorption pits, the amount of bone resorbed, as well as the amount of matrix AGEs released in the medium by resorption. In addition, the in situ concentration of matrix AGEs at each resorption pit was also estimated based on its AGEs autofluorescent intensity. The results indicated that (1) osteoclastic resorption activities were significantly correlated with the donor age, showing larger but shallower resorption pits on the elderly bone substrates than on the younger ones; (2) osteoclast resorption activities were not significantly dependent on the in situ AGEs concentration in bone matrix, and (3) a correlation was observed between osteoclast activities and the concentration of AGEs released by the resorption. These results suggest that osteoclasts tend to migrate away from initial anchoring sites on elderly bone substrate during resorption compared to younger bone substrates. However, such behavior is not directly related to the in situ concentration of AGEs in bone matrix at the resorption sites.

Bone microdamage, remodeling and bone fragility: how much damage is too much damage?

Microdamage resulting from fatigue or 'wear and tear' loading contributes to bone fragility; however, the full extent of its influence is not completely understood. Linear microcracks (∼50-100 μm) and diffuse damage (clusters of sublamellar-sized cracks) are the two major bone microdamage types, each with different mechanical and biological consequences. Healthy bone, due to its numerous microstructural interfaces and its ability to affect matrix level repair, deals effectively with microdamage. From a material standpoint, healthy bone behaves much like engineering composites like carbon-fiber reinforced plastics. Both materials allow matrix damage to form during fatigue loading and use microstructural interfaces to dissipate energy and limit microcrack propagation to slow fracture. The terms fracture toughness and 'toughening mechanism', respectively, describe mechanical behavior and microstructural features that prevent crack growth and make it harder to fracture a material. Critically, toughness is independent of strength. In bone, primary toughening features include mineral and collagen interfaces, lamellae and tissue heterogeneity among osteons. The damage tolerance of bone and other composites can be overcome with sustained loading and/or matrix changes such that the microstructure no longer limits microcrack propagation. With reduced remodeling due to aging, disease or remodeling suppression, microdamage accumulation can occur along with loss of tissue heterogeneity. Both contribute additively to reduced fracture toughness. Thus, the answer to the key question for bone fragility of how much microdamage is too much is extremely complex. It ultimately depends on the interplay between matrix damage content, internal repair and effectiveness of matrix-toughening mechanisms.

Surface microcracks signal osteoblasts to regulate alignment and bone formation

Microcracks are present in bone and can result from fatigue damage due to repeated, cyclically applied stresses. From a mechanical point, microcracks can dissipate strain energy at the advancing tip of a crack to improve overall bone toughness. Physiologically, microcracks are thought to trigger bone remodeling. Here, we examine the effect of microcracks specifically on osteoblasts, which are bone-forming cells, by comparing cell responses on microcracked versus non-microcracked hydroxyapatite (HA) specimens. Osteoblast attachment was found to be greater on microcracked HA specimens (p<0.05). More importantly, we identified the preferential alignment of osteoblasts in the direction of the microcracks on HA. Cells also displayed a preferential attachment that was 75 to 90 μm away from the microcrack indent. After 21 days of culture, osteoblast maturation was notably enhanced on the HA with microcracks, as indicated by increased alkaline phosphatase activity and gene expression. Furthermore, examination of bone deposition by confocal laser scanning microscopy indicated preferential mineralization at microcrack indentation sites. Dissolution studies indicate that the microcracks increase calcium release, which could contribute to osteoblast responses. Our findings suggest that microcracks signal osteoblast attachment and bone formation/healing.

Osteoclasts on bone and dentin in vitro: mechanism of trail formation and comparison of resorption behavior

The main function of osteoclasts in vivo is the resorption of bone matrix, leaving behind typical resorption traces consisting of pits and trails. The mechanism of pit formation is well described, but less is known about trail formation. Pit-forming osteoclasts possess round actin rings. In this study we show that trail-forming osteoclasts have crescent-shaped actin rings and provide a model that describes the detailed mechanism. To generate a trail, the actin ring of the resorption organelle attaches with one side outside the existing trail margin. The other side of the ring attaches to the wall inside the trail, thus sealing that narrow part to be resorbed next (3–21 lm). This 3D configuration allows vertical resorption layer-by-layer from the surface to a depth in combination with horizontal cell movement. Thus, trails are not just traces of a horizontal translation of osteoclasts during resorption. Additionally, we compared osteoclastic resorption on bone and dentin since the latter is the most frequently used in vitro model and data are extrapolated to bone. Histomorphometric analyses revealed a material-dependent effect reflected by an 11-fold higher resorption area and a sevenfold higher number of pits per square centimeter on dentin compared to bone. An important material-independent aspect was reflected by comparable mean pit area (μm²) and podosome patterns. Hence, dentin promotes the generation of resorbing osteoclasts, but once resorption has started, it proceeds independently of material properties. Thus, dentin is a suitable model substrate for data acquisition as long as osteoclast generation is not part of the analyses.

Nanocrystalline spherical hydroxyapatite granules for bone repair: in vitro evaluation with osteoblast-like cells and osteoclasts

Conventionally sintered hydroxyapatite-based materials for bone repair show poor resorbability due to the loss of nanocrystallinity. The present study describes a method to establish nanocrystalline hydroxyapatite granules. The material was prepared by ionotropic gelation of an alginate sol containing hydroxyapatite (HA) powder. Subsequent thermal elimination of alginate at 650 °C yielded non-sintered, but unexpectedly stable hydroxyapatite granules. By adding stearic acid as an organic filler to the alginate/HA suspension, the granules exhibited macropores after thermal treatment. A third type of material was achieved by additional coating of the granules with silica particles. Microstructure and specific surface area of the different materials were characterized in comparison to the already established granular calcium phosphate material Cerasorb M(®). Cytocompatibility and potential for bone regeneration of the materials was evaluated by in vitro examinations with osteosarcoma cells and osteoclasts. Osteoblast-like SaOS-2 cells proliferated on all examined materials and showed the typical increase of alkaline phosphatase (ALP) activity during cultivation. Expression of bone-related genes coding for ALP, osteonectin, osteopontin, osteocalcin and bone sialoprotein II on the materials was proven by RT-PCR. Human monocytes were seeded onto the different granules and osteoclastogenesis was examined by activity measurement of tartrate-specific acid phosphatase (TRAP). Gene expression analysis after 23 days of cultivation revealed an increased expression of osteoclast-related genes TRAP, vitronectin receptor and cathepsin K, which was on the same level for all examined materials. These results indicate, that the nanocrystalline granular materials are of clinical interest, especially for bone regeneration.

Osteoporosis - a current view of pharmacological prevention and treatment

Postmenopausal osteoporosis is the most common bone disease, associated with low bone mineral density (BMD) and pathological fractures which lead to significant morbidity. It is defined clinically by a BMD of 2.5 standard deviations or more below the young female adult mean (T-score =−2.5). Osteoporosis was a huge global problem both socially and economically – in the UK alone, in 2011 £6 million per day was spent on treatment and social care of the 230,000 osteoporotic fracture patients – and therefore viable preventative and therapeutic approaches are key to managing this problem within the aging population of today. One of the main issues surrounding the potential of osteoporosis management is diagnosing patients at risk before they develop a fracture. We discuss the current and future possibilities for identifying susceptible patients, from fracture risk assessment to shape modeling and in relation to the high heritability of osteoporosis now that a plethora of genes have been associated with low BMD and osteoporotic fracture. This review highlights the current therapeutics in clinical use (including bisphosphonates, anti-RANKL [receptor activator of NF-κB ligand], intermittent low dose parathyroid hormone, and strontium ranelate) and some of those in development (anti-sclerostin antibodies and cathepsin K inhibitors). By highlighting the intimate relationship between the activities of bone forming (osteoblasts) and bone-resorbing (osteoclasts) cells, we include an overview and comparison of the molecular mechanisms exploited in each therapy.

The effect of rotative stress on CAII, FAS, FASL, OSCAR, and TRAP gene expression in osteoclasts

This study was designed to explore the effects of rotative stress on carbonic anhydrase II (CAII), TNF receptor superfamily member 6 (FAS), FAS ligand (FASL), osteoclast-associated receptor (OSCAR), and tartrate-resistant acid phosphatase (TRAP) gene expression in osteoclasts. Osteoclasts were induced from RAW264.7 cells cultured in medium containing recombinant murine soluble receptor activator of NF-Kβ ligand (sRANKL). The mRNA and protein expression of CAII, FAS, FASL, OSCAR, and TRAP genes in osteoclasts was detected by RT-PCR and Western blot, respectively, after osteoclasts were loaded at various rotative stress strengths and times. No significant differences in mRNA and protein expression were observed between any of the control groups (P > 0.05). Importantly, rotative stress had a significant effect on the mRNA and protein expression of these genes (P < 0.05). We found a negative relationship between rotative stress strength and prolonged loading time and the expression of FAS/FASL genes in osteoclasts. In addition, there was a positive relationship between rotative stress strength and prolonged loading time and the expression of CAII, OSCAR, or TRAP genes in osteoclasts. Based on these results, rotative stress has a significant effect on CAII, FAS, FASL, OSCAR, and TRAP gene expression in osteoclasts.