Dr. Michael S. Stosich outlines the basic premises and biology of bone related to orthodontics
The various influences of bone physiology underlying the phenomenon of orthodontic tooth movement need to be examined thoroughly for a true understanding of how we, as orthodontists, are capable of exerting orthodontic and orthopedic effects on the dentofacial complex.
Long thought of as a static structure of the body used mainly to support and protect, bone has been shown to be a very dynamic system in constant modeling and remodeling to optimize its strength and mass. For such an optimization process to exist, sensor and feedback mechanisms must be present to sense changes in the state of the environment (loading forces) and send signals to an effector that can begin the process of optimizing bone.
Macroscopic anatomy of bone
The structure of bone is best examined by first looking at the anatomy of a standard long bone such as the femur. The two ends of the bone, epiphyses, form joints with other bones and are covered with a layer of hyaline, or articular, cartilage. Between the two ends of the bone lies the diaphysis, or shaft of the bone. It contains many holes called nutrient foramina through which nerves and blood vessels enter into the bone. Enclosing the bone is a layer of tough, vascular covering of fibrous tissue called the periosteum. The periosteum is firmly attached to the bulk bone, and its fibers are continuous with the various tendons and ligaments connected to the bone. The periosteum is made of two main layers, the fibrous layer and osteogenic layer. The fibrous layer is made of dense connective tissue while the osteogenic layer contains osteoprogenitor cells that play important roles in the formation and repair of bone tissue1.
Mineralized bone is composed of two main types. Cortical (or compact) bone is tissue that is very tightly packed resulting in a substance that is solid, strong, and resistant to bending. Cortical bone composes the walls of the diaphysis and also forms a thin wall around the epiphyses. The epiphyses, however, are mainly composed of cancellous, or spongy, bone. Spongy bone is composed of numerous branching bony plates called trabeculae. The trabeculae are arranged so that irregular interconnecting spaces occur between them, thus reducing the overall weight of the bone while also providing the needed strength. The spongy bone is densest in areas of the epiphyses, subjected to the largest forces of compression. In the larger bones, the diaphysis contains a hollow tube down the center of the cortical bone called the medullary cavity. This area is lined with a thin membrane, called the endosteum, and filled with a specialized type of connective tissue called marrow, which is also continuous into the spongy bone of the epiphyses1.
Microscopic anatomy of bone
Despite being seemingly static and inert, bone is a very dynamic tissue containing many active cells and processes at all times. Bone tissue itself is composed of bone tissue cells (osteocytes, osteoblasts, and osteoclasts) surrounded by insoluble extracellular matrix proteins. The protein matrix consists of both mineralized matrix to provide stiffness and strength, and collagen to provide flexible reinforcement for the mineral components.
In cortical bone, bone matrix is deposited in thin layers called lamellae. The lamellae are arranged in concentric circles around tiny longitudinal tubes called Haversian canals. The Haversian canals contain mostly capillaries and nerves surrounded by loose connective tissue. Evenly spaced and trapped within the lamellae are osteocytes, also arranged in concentric patterns. The osteocytes themselves are located in pockets of fluid called lacunae. The osteocytes and lamellae together form a cylindrical-shaped unit called an osteon, which is the main structural unit of cortical bone (Figure 1). Each osteon is on the order of 200-250 µm in diameter1.
Bone cells receive nutrients via blood vessels running through the Haversian canals. The Haversian canals run longitudinally through bone tissue and are interconnected by transverse communicating canals called Volkmann’s canals. These canals contain larger blood vessels by which vessels in the Haversian canals communicate with the surface of the bone and medullary cavity. The Haversian and Volkmann canals are on the order 10 µm in diameter.1
Spongy bone is also composed of osteocytes in bone matrix. However, the bone cells are not arranged around Haversian canals. Instead, the cells are found within the trabeculae.1
Bone cells include the osteoblasts, osteoclasts, osteocytes, and bone lining cells. Osteoblasts are cuboidal-like cells that originate from mesenchymal stem cells; they are involved in bone formation by forming osteoid that they later mineralize by releasing alkaline phosphatase. Osteoclasts are multinucleated giant cells that originate from hematopoietic stem cells; they are involved in bone resorption. Bone lining cells are located on the periosteal surface and are osteoprogenitor cells that can differentiate into osteoblasts. Lastly, osteocytes are embedded in bone matrix and located in lacunae. Each osteocyte contains numerous cytoplasmic processes that extend outward and pass through fluid-filled tubes of bone called canaliculi, which are on the order of 100 nm in diameter. These cellular processes are attached to the membranes of neighboring cells by gap junctions and connect all osteocytes in the bone matrix as well as the bone lining cells at the periosteal surface, which are very important in cell signaling. Due to these processes, osteocytes are thought to be the main sensor cells of bone to mechanical forces.2
Molecular pathways for bone remodeling in orthodontics
Remodeling of bone can be broken into three steps, each step involving many different cell types coming from different cell lineages. Something must first sense the mechanical force (shear stress) and signal osteoblasts, and osteoclasts to the remodeling area. The osteoclasts and osteoblasts must then migrate to the remodeling area via some sort of chemotactic signal. Finally, when the cells reach the damaged zone, they must begin the remodeling process.3
Osteocytes as sensor cells
Fluid flow could occur at three different levels of the bone porosity. The Haversian and Volkmann canals that contain blood vessels and nerves are the first possibility but, being on the order of 10 µm, are too large to have an effective shear stress. The lacunar and canalicular network that contains the osteocytes is the second possibility. Being on the order of 0.1 µm, these networks correspond with the primary porosity scale associated with the relaxation of the excess pore pressure due to mechanical loading. The third possibility would be the movement of fluid through the collagen-hydroxyapatite matrix of the bone, which is on the order of 20-60 nm. Experiments have shown this to be unlikely because the crystals bind tightly to water and heavily restrict flow.4,5
By modeling the canalicular spaces as cell processes surrounded by a protein/carbohydrate matrix called glycocalyx, levels of shear stress from physiological levels of loading have been estimated between 8-30 dynes/cm.2,4 Even though exact values have not been measured in vivo, new techniques have been developed to show that increased flow through the lacunar and canalicular space does occur as a result of mechanical loading.6
Because of their location within the fluid-filled lacunae of bone, osteocytes have been proposed as the mechanical sensors that sense the fluid flow caused by mechanical loading and produce signals initiating bone remodeling and modeling. The processes can, therefore, sense the shear stress created by fluid flow, which can set off intracellular pathways. The processes of the osteocytes are used to communicate with other osteocytes of the bone as well as bone lining cells located on the bone surface. The osteocytes can, therefore, also send signals to other osteocytes as well as osteogenic bone lining cells about their mechanical state.7
How the fluid flow and shear stresses are sensed by the osteocytes is currently believed to be along two distinct pathways. Cytoskeletal changes with integrins being the mechanotransducer is one possibility. The activation of G-proteins that then trigger intracellular pathways is a second.
Integrins are transmembrane, het-erodimer receptor proteins located on almost every cell in the body. They are of extreme importance in cell adhesion and bind to extracellular proteins containing the RGD sequence. Integrins have the unique property of being able to bind extracellular proteins and being attached directly to the intracellular actin cytoskeleton via binding proteins. Because of this unique property, integrins are thought to be able to function as mechanotransducers by transmitting mechanical signals from the extracellular matrix to the cytoskeleton.8
Cyclic adenosine monophosphate (cAMP) is another second messenger that has been shown to increase in response to mechanical forces.9 Unlike the PKA pathway, cAMP production can be inhibited by use of indomethacin, suggesting a prostaglandin dependent pathway.9,10 The production of cAMP has been shown to inhibit ERK activation, and therefore, cAMP may play a negative regulatory function.10
Another important pathway stimulated by both G-proteins and integrins is production of nitric oxide (NO). Increases in intracellular Ca2+ activate nitric oxide synthase (NOS), particularly the isoform eNOS.11 The NO produced may also stimulate the production of more prostaglandins or may be a paracrine factor for osteoblasts.8
Signaling osteoblasts
Since the sensor-cell osteocytes are unable to directly make bone themselves, a paracrine factor must be released to stimulate bone formation, either by direct activation of osteoblasts or differentiation, and activation of bone lining cells. The intracellular pathways described above produce two possible paracrine factors, NO and prostaglandins (PGE2). Insulin-like growth factor (IGF) in the form IGF-1 has also shown to be a possible paracrine factor for the bone forming cells.12 It is also possible that PGE2 is an upstream regulator of IGF-1; inhibition of prostaglandin release using indomethacin has significantly inhibited IGF-1 (Chow, 2000).
Recent work has shown that PGE2 may also act on the osteocytes as well. Jiang and Cheng13 demonstrated that PGE2 has a stimulatory effect on the formation of gap junctions among osteocytes in a parallel plate flow chamber model. They proposed that PGE2 stimulates the synthesis of connexin 43 (Cx43); the connexin involved in gap junctions between osteocytes, and formation of more functional gap junctions. By this method, greater numbers of signaling molecules can pass between osteocytes to increase the efficiency of cell signaling and ultimately bone remodeling.
After the signal is received from the osteocytes, the osteoblasts begin the production of new bone. This process generally occurs about 72 hours after the mechanical stimulation and is evidenced by increases in osteocalcin (OCN) and collagen at the bone-forming surface,14 along with a measurable increase in mineral apposition rate8. The process has been described on both the periosteal and endosteal sides of cortical bone as well as in trabecular bone.8
Endocrine factors such as parathyroid hormone (PTH) also appear to function as a signaling factor for osteoblasts. Instead of directly stimulating the osteoblasts to form bone, PTH seems to play a permissive role in the anabolic response of bone to mechanical loading and may play a key role in sensitizing the threshold of bone formation. Turner, et al.,15 hypothesize that PTH gives bone cells “memory” that allow them habituation to certain magnitudes of mechanical forces. Supporting this hypothesis is the fact that each bone in the body seems to have its own threshold for remodeling, with one example being long bones exhibiting different sensitivities to mechanical loading than craniofacial bones.
Concluding remarks
In this article, I have attempted to outline the basic premises and biology of bone as it relates to orthodontics in order to further delve into fundamental questions in orthodontics and dentofacial orthopedics.
In the next issue, clinical questions will be posed and answered on the biology of tooth movement.
Michael S. Stosich, DMD, MS, MS, has performed orthodontic and craniofacial reconstruction work throughout the world, but his first priority is his patients at iDentity Orthodontics in the Chicagoland area. With educational credentials and training twice that required of an orthodontist, Dr. Stosich has published and lectured throughout the U.S. and abroad. His sincere interest and dedication toward the study of stem cell tissue engineering, combined with a rare creativity toward scientific discovery, paved the way for Dr. Stosich to serve as lead scientist in a variety of studies. This yielded numerous publications that lead to important advancements in craniofacial cases. His achievements were also awarded by the National Institutes of Health, which endowed grants toward future study. Dr. Stosich is also faculty at the University of Chicago Medicine. Dr. Stosich believes in giving back to the communities he serves and focuses on charitable giving where it can do the most good by treating underserved and unprivileged children through his involvement in the Smiles Change Lives foundation, Smiles for Service, and his work on the Chicago craniofacial team. Dr. Stosich is also involved in local community programs linking orthodontics to philanthropy.
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