Hydroxyapatite (HAP) is an inorganic mineral compound and a fundamental building block in biology and materials science. This naturally occurring substance is a form of calcium phosphate, chemically represented by the formula \(Ca_{10}(PO_4)_6(OH)_2\). It is a crystalline solid whose structure grants it exceptional hardness and chemical stability. HAP is largely responsible for the rigid architecture found in many biological systems and can be replicated in a laboratory setting for numerous applications.
The Unique Chemical Structure
Hydroxyapatite’s precise chemical composition combines calcium, phosphate, and hydroxyl ions. The formula, \(Ca_{10}(PO_4)_6(OH)_2\), indicates that ten calcium ions are interlocked with six phosphate groups and two hydroxyl groups. This arrangement forms a highly ordered, repeating crystal structure known as a hexagonal lattice. The hexagonal symmetry of the lattice confers HAP its physical properties, including hardness and low solubility in water.
The lattice structure is capable of substitution, meaning other ions can replace the native calcium, phosphate, or hydroxyl components. For instance, ions such as carbonate, fluoride, or strontium can be incorporated. The ability to accept these substitutions results in slight modifications to the crystal structure and can alter the mineral’s solubility or biological behavior.
The Role of Hydroxyapatite in Bones and Teeth
Hydroxyapatite is the primary inorganic constituent of vertebrate hard tissues, providing rigidity and strength. It makes up approximately 65 to 70% of the dry mass of bone and 70 to 80% of the mass of dental enamel and dentin. Bone crystals are typically small (40 to 60 nanometers) and plate- or needle-shaped. Conversely, HAP crystals in tooth enamel are larger and more tightly packed, contributing to enamel being the hardest substance in the human body.
The creation of these tissues involves mineralization, a regulated process where HAP crystals are deposited within an organic matrix. Osteoblasts, the bone-forming cells, release matrix vesicles containing high concentrations of calcium and phosphate ions. These ions combine to form HAP crystals, often nucleating within specialized “hole zones” found in the collagen fibers that scaffold the bone. An enzyme called alkaline phosphatase facilitates this process by breaking down a natural inhibitor of mineralization, increasing the local availability of phosphate.
The body maintains its skeletal structure through bone remodeling, a continuous process involving two specialized cell types. Osteoclasts break down old or damaged HAP by releasing acid and enzymes into the bone matrix. Following this breakdown, osteoblasts deposit new collagen and fresh HAP mineral. This constant turnover ensures the structural integrity of the skeleton and regulates the body’s balance of calcium and phosphorus.
Synthetic Uses in Medicine and Dentistry
Hydroxyapatite can be manufactured for various clinical applications due to its near-identical composition to natural bone and tooth mineral. Synthetic HAP is valued for its biocompatibility—interacting with host tissue without causing inflammatory reactions or rejection. It is also osteoconductive, providing a suitable surface or scaffold for bone-forming cells to attach and proliferate.
In orthopedic surgery, HAP is frequently used as a coating on metallic implants, such as hip and knee replacement prostheses. Techniques like plasma spraying apply a thin layer of the mineral onto the metal surface, enhancing “biologic fixation.” This coating allows natural bone to bond directly to the implant, a process called osseointegration, which improves the stability and long-term success of cementless devices. HAP is also used as a bone graft material to fill defects, where its porous structure encourages tissue ingrowth and provides a temporary scaffold that is slowly replaced by new bone.
In dentistry, nano-hydroxyapatite (nHA) has emerged for preventative and restorative care, primarily due to its particle size of 20 to 100 nanometers. These nanoparticles penetrate and bind electrostatically to demineralized areas of the enamel surface. Nano-HAP rebuilds damaged enamel by diffusing calcium and phosphate ions into microscopic lesions and cracks, effectively reversing early-stage demineralization. For tooth hypersensitivity, nHA particles physically seal exposed dentinal tubules, blocking the fluid movement that transmits external stimuli and causes pain.