In this lesson we will cover the anatomical structure of bone, alongside its physiology and clinical relevance to us in the dental world. Thank you to @mydentalnotebook for the wonderful images.
An adult skeleton consists of 206 bones, which can be split into the axial and appendicular skeleton. Our skeleton, alongside our joints (cartilage & ligaments) allows us to go about our daily activities and lives. Bone is a remarkable tissue, and highly active. In this lesson we will cover the anatomical structure of bone, alongside its physiology and clinical relevance to us in the dental world.
Protect: Bones are both strong and importantly, they are light. Their structure allows them to resist both compressive and tensile forces. Our vital organs are often encased in a bony structure e.g. the brain within our skull.
Support and Movement: Without bone, locomotion would be very difficult. Bones allow muscular tendons and ligaments to attach, providing a structural frame to be acted upon.
Storage and Mineral Homeostasis: The most abundant resources our bones store are: adipose tissue (fat) and minerals; in particular, calcium and phosphate. Bone plays a crucial role in calcium homeostasis. When plasma calcium levels fall, cells within our bone are activated via various mechanisms (e.g. hormones), allowing mineral resorption and release to occur. Calcium is an important component for our nervous and muscular system to function and also plays an important role in clotting.
Cell Formation: Haematopoiesis occurs in our bone marrow. Red blood cells, alongside, our immune cells (B and T cells) are formed in our bone and may subsequently travel to various organs to mature (e.g. Thymus).
Important note:
Plasticity: For bone to be able to carry out the functions listed above it must have the ability to adapt. The coupling of resorption and remodelling occurs under tight control, allowing both homeostasis to take place and allows the skeleton to modify appropriately for its function (e.g. exercise) or lack of use (e.g. alveolar bone resorbs when a tooth is lost).
Cortical (Compact/ Lamellar) Bone:
Compact bone is the bone "shell", arranged in rings, aiding it to resist bending under force.
Trabecular (Cancellous/ Spongy) Bone:
Trabecular bone is arranged in rods/ plates which aids the resistance against compressive forces. Trabecular bone plays a more dominant role in mineral homeostasis. Therefore, metabolic diseases of bone have a greater impact on trabecular bone compared to cortical, as seen in osteoporosis.
Woven (young) Bone:
Woven bone is seen more commonly in the developing bone and has an unorganised appearance. It will also be seen at fracture sites during the healing process (see OMFS fracture lesson). Woven bone can also be seen in disease, where there is a regulatory breakdown of bone remodelling, as seen in the secretory/ blastic phase of Paget's disease.
Bone can also be classified by its shape/ size:
Bone is a specialised connective tissue. It is one of the hardest materials within the human body that can withstand tensile and compressive forces. In addition to it's strength it manages to remain relatively light.
Vasculature:
Bone is very well nourished due it having a rich blood supply that penetrates the periosteum via Volkmann's canals. There is usually one major vessel called the nutrient vessel which penetrates compact bone at the diaphysis, splitting into a proximal and distal vessel within the bone. Vessels are also found at the metaphysis and epiphysis which nourish the joint.
Extracellular Matrix (ECM) Constituents:
Bone has a varied structure dependent on its type and location in the body. However, in general the ECM consists of collagen (30%), mineral salt crystals (55%), and many other traces of organic constituents along with water.
The adult skeleton consists of 80% compact bone and 20% spongy bone. The majority (90%) of the collagen found in bone's ECM is Type 1, forming a network which has become mineralised with inorganic hydroxyapatite crystals Ca(10)(PO4)6(OH)2. The ECM also consists of water and many other organic proteins, such has: growth factors, fibronectin, osteocalcin and glycoproteins.
Linings
Bone is lined externally with the periosteum; a highly vascular and innervated tissue that also contains bone lining cells. It penetrates the bone with sharpey's fibres becoming securely adhered. The sensory innervation of the periostium is what makes breaking a bone so painful; it is particularly sensitive to tearing and tensional forces. Bone on its internal surface has a lining known as the endosteum, which is a strong connective tissue.
Long Bone Example:
Long bones have a cortical shell which surrounds the inner marrow (Yellow: adipose or Red: haematopoietic) in its shaft. Trabecular bone can be found nearer the articulating/ proximal ends.
Compact bone:
Compact bone consists of lamellae. These are circumferential layers (sheets/ plates) containing paralleled concentric rings known as the Osteon. The Osteon forms around a central neurovascular system (and lymph) contained in a canal; Haversian canal. Volkmann's canals branch off these systems (perpendicular) to form a network of vessels that penetrates though the bone.
Lacunae, found within the lamellae, harbour osteocytes. Caniculi branch from the lacunae, providing a nutrient network for the cell and pathways of communication with other cells within bone. This transport system alongside the Haversian systems is what provides bone with great healing capacity and adapting capabilities.
There is a vast collagen framework found in bone which switches orientation between each lamellae. This cross hatching effect, provides bone with its ability to withstand tensile forces.
Trabecular:
Trabecular bone is arranged accordingly to the stress distribution experiences by the bone. Therefore it is NOT randomly placed, although it may look/ seem that way.
Trabecular bone surrounds the internal marrow spaces. As mentioned above, marrow can be yellow or red. Yellow marrow predominantly consists of adipose tissue, whereas red marrow contains stem cells in a vascular network with the haematopoietic capacity to form many cell types, important for developing our immune system, healing and oxygen carrying abilities of the blood. Red marrow has a greater distribution seen in children and adolescences.
Compact Bone:
Note:
Structural lines: can be seen on bone microscopic/ histology slides, which refer mainly to the osteoblast and osteoclast activity.
Bone has many functions as mentioned above; therefore it needs a large number of cells to keep it operational. The main roles bone cells carry out are: bone synthesis, bone maintenance and bone resorption.
Main role:
Life:
Specialised connective tissue cell:
Osteoblasts aid appositional growth and regulate mineral homeostasis. They respond to multiple signalling pathways, such as parathyroid hormone (PTH), Sex hormones, Glucocorticoids, Vitamin D, Inflammatory cytokines and growth factors. They regulate bone resorption by the expression of RANKL which activates osteoclast activity or they secrete Osteoprotegrin (OPG) which inhibits RANKL expression decreasing osteoclast activity.
Main Role:
Life:
Specialised connective tissue cell:
Osteocytes are osteoblasts that have become incorporated into the ECM during bone formation. They are thought to be the major controller of bone biology by inducing osteoclast activity. They have receptors like osteoblasts e.g. PTH
which aid their ability to maintain the body's homeostasis of calcium and phosphate metabolism. Osteocytes also secrete sclerostin at the completion of the osteon which inhibits further bone formation; antagonising bone morphogenesis proteins (BMP) growth factors. In addition they regulate localised remodelling of the bone by responding to tension and strain.
Post proliferation, osteoblasts can flatten and become incorporated with the periosteum. They form a protective barrier which prevents osteoclasts coming into contact with the bone surface (exposed bone is vulnerable as it is thought osteoclasts can become active via direct contact). Like osteoblasts, bone lining cells aid mineral homeostasis, and like osteocytes they can initiate bone remodelling.
Stem cells are also found in bone. As osteoprogenitor cells divide they produce one daughter cell and one cell differentiates, maintain the supply of stem cells whilst contributing to the needs of the bone.
Main Role:
Life Span:
Specialised connective tissue cell:
When osteoblasts express RANKL, the RANK receptor on osteoclasts is activated. Osteoclasts can also become activated by macrophage colony stimulating factor (M-CSF), and, if the local environment becomes hypoxic and/or acidic; they are also thought to activate with direct contact with bone mineral. When active, they differentiate, becoming multinucleate, express integrins on their cell surface to provide bone adherence, and develop villi, creating their ruffled border.
Ossification/ Osteogenesis has 4 main phases throughout development and life:
Embryo bone development can be seen to begin around the 6th week of intrauterine life is the form of mesenchyme hyaline cartilage.
Most of the hyaline cartilage framework becomes ossified, but some does remain and is found in the nose, ribs and articulate surfaces of joints. (Note: In flat bones, a difference to note is that is is a fibrous framework).
When ossification occurs, a bone collar is laid down surrounding the cartilage, and then the enclosed model is digested forming the medullary cavity. An OUT to IN processes.
Birth and Childhood - cartilage remains at the epiphyseal plate
Direct bone formation is layed down as mesenchyme membranous sheets. Examples of where this occurs are: the flat bones of the skull, mandible and many of the facial bones e.g. Maxilla.
Note: at birth soft spots known as fontanelles are present between the bones. this allows a degree of flexibility of the head to aid the frothing process.
Bone is a highly dynamic and active organ. Its regulation is controlled by: Hormones (PTH), Mineral balance (plasma calcium) and growth factors.
Growth Zones:
As adolescence finishes the epiphyseal plate has become fused and becomes the epiphyseal line. Growth plate fuses around 18-21yrs (females sooner)
Appositional growth (increases the bone diameter)
Periosteal cells (ostoeblasts) lay down collagen matrix forming new bone known as the osteon (when mineralised = osteoid). They become surrounded by ECM now becoming osteocytes. The new matrix forms around the outer vessels, forming a tunnel, creating the Haversian system and lamellae are produced.
Endosteal cells (osteoclasts) resorb the inner surface of the bone, increasing the medullary diameter.
Having the ability to remodel allows bone turnover for repair, functional adaptation (e.g. exercise) and homeostasis regulation. Orthodontists utilise the bones ability to remodel allowing the desired tooth movements to occur.
The rate of remodelling is dependent on the both the site and type of the bone. In general compact bone turns over 4% per year, whilst spongy bone turns over 20% per year.
It is important to note that osteoclast activity is faster than osteoblast activity; hence the natural ageing process.
Rest - formation - Reversal (cement line) - resorption front
Bone has many markings, projections, depressions and openings due to neurovascular pathways, articulating surfaces, as well as muscle tendon and ligament attachments.
The mandible is a great bone to use as an example, because it covers many types of bone markings. Below are numbered diagrams showing the complexity and range one bone can contain.
More detail will be shown in the bones of the skull anatomy lesson.
As bone ages, we decrease in stature, bone can become deformed, causing pain, and from an oral perspective: there is an increased risk of tooth loss.
Mesial Drift - As teeth wear, they eventually narrow. Therefore space should be created, yet inter-proximal contacts still remain. This is because the bone can remodel, allowing the ability of the teeth to drift (commonly in a mesial direction). One theory regarding our wisdom teeth; many years ago, when our diet was tougher and rougher, more wear would have occurred; therefore, more space created alongside more mesial drift, allowing the 8s to erupt into position.
Localised bone loss - If alveolar bone doesn't have a purpose it will be lost. Sometimes, due to the spread of load created by a denture, the process of bone resorption can occur at a faster rate than expected.
Lack of PDL proprioception
Orthodontics - note: osteoclasts don't differentiate between bone, root and cementum; hence, the associated root resorption (shortening) that occurs.
Implants - Osseointegration (rigid) Note: XLA and bone grafting
Distraction osteogenesis - Bone can be manipulated e.g. orthognathic surgery. Basic description: If the bone is slightly left apart and secured, the healing process (like in fracture repair) can occur, forming new bone at the junction; therefore, increasing the bones length.
Osteoporosis and many other metabolic bone disease. Bone can become weaker, and care must be taken to avoid procedural fractures (e.g. of the mandible). The opposite can occur, where bone becomes extremely dense, making extractions very difficult to carry out (without surgical intervention) e.g. osteopetrosis.
Gene Mutations: Collagen type 1 seen in osteogenesis Imperfecta - also affects the enamel and dentine.
MRONJ When bone doesn't heal, this can be a serious complication for the patient, significantly impacting their quality of life.
Understanding the normal function of our systems is important, especially when treating patients with disease; because, if we know where the functional break down has occurred, we can hopefully treat of manage their situation appropriately. We may not necessarily be asked what a Haversian system in the clinical setting is, but, we must be aware that bone is highly vascularised, so if bleeding were to occur from a socket, we need to find out where from, and bone should not be left out from the assessment. I hope you found this lesson useful.
General References used to create this lesson:
Berkovitz BK, Holland GR, Moxham BJ. Oral Anatomy, Histology and Embryology E-Book. Elsevier Health Sciences; 2017 Jul 11.
Nanci A. Ten Cate's Oral Histology-e-book: development, structure, and function. Elsevier Health Sciences; 2017 Aug 15.
Tortora GJ, Derrickson BH. Principles of anatomy and physiology. John Wiley & Sons; 2018 May 15.
Marieb EN, Keller SM. Essentials of Human Anatomy and Physiology: Books a la Carte Edition. Benjamin-Cummings; 2011.
Norton NS. Netter's head and neck anatomy for dentistry e-book. Elsevier Health Sciences; 2016 Sep 13.
Logan BM, Reynolds P, Rice S, Hutchings RT. McMinn's Color Atlas of Head and Neck Anatomy E-Book. Elsevier Health Sciences; 2016 Oct 21.
Atkinson ME. Anatomy for dental students. Oxford University Press; 2013 Mar 14.
Moore AE, Renton T, Taylor T, Popat S, Jasani MK. Relevance of bisphosphonate therapy in osteoporosis and cancer–no cause for alarm in dentistry. Dental update. 2016 Apr 2;43(3):235-42.
