Bone Physiology

In this section we will discuss aspects of bone physiology that are most relevant to the study of mechanically mediated bone adaptation and its relevance in implant dentistry. These aspects include bone cells and what they do, the different aspects of bone adaptation including osteogenesis, modeling and remodeling, and description of surfaces on which remodeling can occur. We will basically describe what potential exists for alterations of bone structure based on whether the bone is undergoing osteogenesis, modeling and remodeling. This physiologic potential will determine how mechanics may affect bone structure.

Bone physiology is controlled by an interaction of mechanical and metabolic factors. Under physiologic circumstances, bone formation is primarily regulated by functional loading (peak strains). The biochemical mediators of calcium metabolism ( Parathyroid hormone, estrogen, vitamin D) predominate in control of bone resorption. The implant dentist should have firm grasp of the modern concepts of bone physiology,metabolism and biomechanics. This is often the only objective means for designing and implementing an innovative treatment plan.

I. Bone Cells

Bone cells may be divided into two broad classifications depending on whether they make bone or resorb it. Osteoblasts make bone, while Osteoclasts resorb or take away bone. However, there are actually three different sub-categories of bone cells related to osteoblasts: the osteoblasts themselves, bone lining cells and osteocytes.Bone lining cells are basically inactive osteoblasts (in terms of making bone) that line bone surfaces. Osteocytes are osteoblasts that have becomes encased in bone matrix during bone tissue production.

II. Bone Surfaces: There are four major bone surfaces:

1) Periosteal (outer surface of all bones)

2) Endosteal (inner surface of cortical bone)

3) Haversian (inner surface of haversian canals within osteons)

4) Trabecular (outer surface of all individual trabeculae)

All activity that changes bone by direct operation on bone tissue occurs on one of these four surfaces. These four surfaces can be seen in the figure below:

Schematic representation of bone in both cross and longitudinal sections

The periosteum in the picture is the same as the periosteal surface. Likewise, endosteum in the picture is the same as the endosteal surface. The trabeculae have their own surface. Inside the cylindrical haversian systems are the haversian surfaces.

III Structural classification of bone

Macroscopically, bone tissue can be classified into two types, compact and trabecular bone. Compact bone is dense, corticated and makes up the outer surfaces of most of the bones of the skeleton. Trabecular bone is spongy, cancellous and constitutes the inner surfaces of bones, also containing the medullary cavity or marrow.Microscopically they are identified as two types, the immature (woven) and mature (lamellar) bone. Immature bone is formed earlier and more rapidly than mature bone, and is the first type to form in a repair situation or around an implant. In most cases mature bone will then replace the immature bone.

IV. Osteogenesis: Production of Bone on Soft Tissue

Now that we know the players in bone adaptation, we will look at the ways in which bone may be created and modified. There are three major ways bone tissue may be altered: 1) osteogenesis, 2) modeling and 3) remodeling. The alterations differ basically in the tissue on which bone is placed and the way in which osteoblasts and osteoclasts work together. Osteogenesis is the production of bone on soft tissues, either soft fibrous tissue or cartilage. It is the way in which bones are formed during embryonic development and how bone is initially formed at the site of injury, for example in fracture healing.

Embryologically, osteogenesis may be classified as either intramembranous or endochondral. When the ossification occurs directly, it is defined as intramembranous.It is the process by which flat bones like the skull, parts of mandible and clavicle are formed. Embryonic mesenchymal cells with abundant vascular supply develop foci of intracellular collagen deposition. Soon, osteoblasts are identified in these regions,secreting osteoid into which calcium salts are deposited.The second type of osteogenesis is endochondral ossification. This process is different from intramembranous ossification in that it occurs with a cartilage base. Endochondral ossification is responsible for a good deal of formation of the long bones , vertebrae and portion of the mandible. It occurs in the following steps: Embryonic mesenchymal stem cells differentiate into a primitive hyaline cartilage. Blood vessels and bone- forming units, which resorb the cartilage and replace it with osteoid, invade this matrix.In osteogenesis, large amounts of woven bone can be formed very rapidly. This bone is believed to be much more compliant than organized lamellar bone. In osteogenesis, osteoblasts and osteoclasts generally act independently, that is, they are not coupled in their actions.

V. Bone Modeling

It is a surface-specific activity (apposition or resorption) that produces a net change in the size and/or shape of a bone after initial bone formation in the embryonic or initial healing stage . It is an uncoupled process , that is bone structure alterations occur by independent action of osteoblasts and osteoclasts. Modeling represents a process that allows a change in initial bone architecture. It has been suggested that external demands such as load may initiate modeling. Therefore, it means that bone resorption and formation occur on different surfaces.

AFigure depicts the actions of Osteoblasts and Osteoclasts during Bone Modeling

VI Bone Remodeling

Bone remodeling differs from the other means of bone structure alteration in that osteoblasts and Osteoclasts do not act independently but are coupled and bone resorption and formation occur at the same spot on a bone surface. It represents a change that occurs within mineralised bone without concomitant alteration of architecture of the tissue.As with modeling, bone remodeling occurs on existing bone surfaces. However,unlike modeling, remodeling cannot cause large changes in bone structure at a given site.

Remodeling is accomplished by

•  teams of 10 osteoclasts and hundreds of osteoblasts
•  teams are called basic multicellular units (BMUs)

The cellular interactions associated with a remodeling cycle are divided into four main events which follow the resting state(Quiescence); activation, resorption, reversal and formation

Quiescence refers to the resting state of the bone surface. This includes all of the bone surfaces. Activation is the recruitment of osteclasts to a bone surface and signal coupling of osteoblasts. Resorption is the removal of bone by osteoclasts.Reversal is the process by which osteoclasts stop removing bone and osteoblasts fill the defect. Formation is the laying down of bone by osteoblasts. The entire process is illustrated in Fig.




Fig : Figure depicting remodelling stages LC refers to lining cells, POC refers to osteoclast precursors, OC refers to osteoclasts, HL refers to Howship lacunae (the name of a resorption pit), OB refers to osteoblast, CL refers to closed lacunae, and BSU refers to bone structural unit, the newly created piece of bone 

This scenario is most relevant to trabecular bone surface remodeling. In cortical bone the same steps occur in remodeling, but the remodeling occurs in a different shape, known as a cutting cone.

 

Fig: Diagrammatic representation of Cutting Cone. Longitudinal section:1) multinucleated osteoclasts in howship’s lacunae advancing longitudinally  from right to left & radially to enlarge a resorption cavity. 2) perivascular spindle-shaped precursor cells. 3) capillary loop delivering osteoclast precursors & pericytes. 4)mononuclear cellslining reversal zone. 5) osteoblast oppposing bone centripetally in radial closure & its perivascular precursor cells. 6) flattened cells lining haversian canal of completed haversian system or osteon. Transverse Section: A) resorption cavities lined with osteoclasts. B) completed resorption cavities lined by mononuclear cells, the reversal zone. C) forming Haversian system or osteons lined with osteoblasts. D) completed Haversian system or osteon with flattened bone cells lining canal. 

 Following the discussion of normal bone physiology and adaptation, lets move on to understand the events occurring at the bone implant interface.