Understanding The Function of Hydroxyapatite: Its Role in Bone Health and Regeneration

Hydroxyapatite is an inorganic material that is naturally present in human bones and teeth. It has a role to play in the bone’s structural strength and bone regeneration. Doctors use synthetic hydroxyapatite for bone repair treatments.

Hydroxyapatite forms 65% of the bones and provides rigidity to the bones and teeth. ⁽¹⁾ It forms 70-80% of the weight of human dentin and enamel. ⁽³⁾ It is also known to reduce occasional muscle discomfort from everyday exertion. It is more readily absorbed than calcium. It can get affected negatively by high sugar and acidic drinks. Overall, hydroxyapatite is an important mineral for bone health as it provides structural strength and reduces muscle discomfort. It is a main component of bone regeneration and can be modified to improve the bone’s osteoconductive properties.

Bones are complex tissues that consist of several components including cells, extracellular matrix, and minerals. The extracellular matrix is formed of the following:

• Collagen Fibers: The main organic component of bone that is needed for flexibility and tensile strength. These are synthesized by osteoblasts (the bone-forming cells) and are organized into a 3-D network essential for the mechanical strength of bone.
• Hydroxyapatite Crystals: The inorganic component of bone needed for hardness and compressive strength. It is composed of 40% of calcium and 18% phosphate in form of the insoluble salt forming small crystals deposited in the collagen fibers, giving bones the characteristic hardness. ⁽³⁾

Apart from collagen fibers and hydroxyapatite crystals, bone also contains several types of cells including osteoblast (Bone forming cells), osteoclast (Bone-resorbing cells), and osteocytes (mature bone cells).

Bone has a complex composition including both organic and inorganic components. There are several types of cells playing the role of maintaining bone health and function.

Hydroxyapatite is present in bones and teeth. Its crystals are present in bone in the shape of plates or needles and may vary in number, size, and shape.

The functions of hydroxyapatite include:

• Provides Strength and Stability to the Bone: Hydroxyapatite is an inorganic material that is known to play a role in providing structure, strength, and stability. It provides support to the underlying bones. Hydroxyapatite plays a role in bone regeneration as it helps in forming a scaffold for new bone cells to grow on. Also, it can be used to create biphasic calcium phosphate ceramics, a strong and lightweight material used to replace damaged bone tissue.
• Resisting Compression and Tension Forces: The crystalline structure of hydroxyapatite allows it to resist compressive force. When pressure or compressive force is applied on to the bone, the hydroxyapatite crystals resist the force and prevent the bones from fracturing or breaking. Its arrangement also allows resisting tension forces, if the bone is pulled or stretched. It also distributes the tension throughout the bone, preventing stress on any one area of the bone. ⁽¹⁾
• Storing and Releasing Minerals: Hydroxyapatite plays a role in bone repair and regeneration. It is also used as a composite with polymers or other ceramics or as a coating on orthopedic and dental implants. It acts as a reservoir of trace elements including carbon, oxygen, and strontium, and is used to reconstruct ancient diets, migration, and paleoclimate. Also, it is used in the biomineralization of the skeleton to regenerate and manipulate calcium and phosphate ions on large scale. ⁽²⁾
• Facilitating Bone Growth and Repair: Hydroxyapatite has a crucial role to play in bone growth and repair. In a fractured and damaged bone, the natural healing process involves the deposition of new bone tissue on the affected areas and the process is known as bone remodeling. During bone remodelling, osteoblasts secrete the collagen matrix that gets mineralized with hydroxyapatite crystals. The hydroxyapatite crystals provide the bone with characteristic hardness and strength. Osteoclast then breaks down the old bone tissue and the osteoblast deposit the new bone tissue. The process continues until the bone has fully healed. ⁽⁴⁾.

  Hydroxyapatite is also used in bone grafting procedures in which a small piece of bone is removed from one part of the body and transplanted to another area when the bone is needed. The role of hydroxyapatite is to stimulate the growth of new bone.

The formation of hydroxyapatite is a complex process and involves different cellular and molecular interactions.

Process of Bone Formation

It begins with the deposition of collagen fibers that make up the organic component of the bone. These fibers serve as a framework in which the hydroxyapatite crystals grow. Then the calcium and phosphate ions start accumulating on the collagen fibers forming small crystals of hydroxyapatite.

Role of Osteoclast, Osteoblast, and Osteocytes

Several types of cells including osteoclast, osteoblast, and osteocytes are involved in the formation and maintenance of bone tissue. Osteoblasts synthesize and deposit collagen fibers, and osteocytes regulate the bone remodeling and mineralization and osteoclast breakdown and reabsorb bone tissue. This process is essential for the maintenance of healthy bones. Any disruption in the process can lead to skeletal disorders.

Importance of Vitamin D and Calcium in Hydroxyapatite Formation

Calcium provides structural support to the bones and teeth. Insufficient calcium levels in the blood weaken the bones and increase the risk of fracture. It also helps in maintaining healthy muscles and nerves and regulates many bodily functions.

Vitamin D is also important for the absorption of calcium from the diet and its utilization in the body. Together, calcium and vitamin promote the formation of hydroxyapatite. The deficiency of both can affect the formation of hydroxyapatite and lead to weakened bones, tooth decay, and other health problems.

There are several factors that may affect the formation and maintenance of hydroxyapatite in the bone.

• Ageing: As the body ages, it becomes less efficient at absorbing and utilizing nutrients, including those necessary for the formation of hydroxyapatite. This declines bone density and increases the risk of fracture.
• Hormonal Imbalance: Hormones have a significant role to play in bone health. For example, estrogen help maintain bone density in women and testosterone helps maintain bone health in men. The thyroid hormone also affects bone metabolism.
• Diseases and Conditions Affecting Bone Health: There are several diseases that affect bone health and increase the risk of fracture. These include osteoporosis, osteoarthritis, rheumatoid arthritis, celiac disease, Paget’s disease, celiac disease, and hyperparathyroidism. Maintaining a healthy lifestyle including a nutritious diet, regular exercise, and avoiding smoking and excessive alcohol consumption can be helpful in promoting bone health and reducing the risk of fractures.

Hydroxyapatite is a crucial component of bone tissue, providing bone with strength and rigidity and the ability to resist mechanical stress. It is a complex mineral composed of calcium, phosphorus, and other minerals and forms a dense mineral matrix in bone tissue. It is important for resisting compressive forces.

Several factors influence the formation of hydroxyapatite including nutrition, hormone, physical activity, age, genetics, and medical conditions. Adequate intake of calcium, phosphorus, and other essential nutrients is necessary for the formation of hydroxyapatite.

Taking steps to promote bone health can help ensure the bones remain strong and healthy throughout life.