- See: bone mineralization
- Basic Science:
- compound that is similar to bone mineral, sharing the formula Ca10 (PO4 )6 OH2;
- crystal lattice of hydroxyapatite is hexagonal or pseudohexagonal;
- as shown by radiographic diffraction analysis, calcium and phosphate mineralize in bone in the form of an apatite, which is similar to hydroxyapatite [Ca10(PO4)6(OH)2;
- other configurations may include brushite (CaHPO4 2H2O) or octacalcium phosphate [Ca8H2(PO4)65H2O;
- crystalline structure can be changed by manipulation of the calcium-to-phosphate ratio, as well as changing the content of carbonate and flourine;
- higher concentrations of flourine and a higher calcium-to-phosphate ratio, increases the biologic stability of the molecule;
- exchangeable pool of mineralized calcium:
- only about 0.65% of human bone calcium is part of readily exchangeable pool;
- in living bone, the exchangeable areas of mineralized bone may occur at the lining of haversian canals or resorption cavities;
- at sites of new bone deposition (subperiosteal or newly mineralizing osteons) there is not significant exchange reactions;
- mineralization of soft tissues:
- Strategies for Implant Fixation: Hydroxyapatite Coatings:
- clinical investigations of the performance of hydroxyapatite-coated femoral stems continue to yield encouraging results;
- these coatings are susceptible to resorption by osteoclast-like cells;
- as much as 20% of coating might be removed within two years;
- hydroxyapatite is brittle and may fracture if the coating is too thick;
- alternative strategies:
- use of porous-coated prostheses as the substrate for hydroxyapatite coating;
- hence, if the hydroxyapatite coating is resorbed, the bone ingrowth into porous coating can serve as attachment site for the prosthesis;
- Results of Press Fit Hydroxyapatite Coated Stems:
- they seem to work, but are they necessary?
- WN Capello et al, JBJS 1997.
- 152 THR in 133 patients w/ an average age of 39 years;
- femoral stem: double wedged grit blasted collarless, straight, titanium alloy, a 50 micrometer thick circumferential coating
of hydroxyapatite (applied to the proximal third of the component);
- after a minimum 5 year follow up (avg 6.4 years), only 4 stems required revision, but none of these were for loosening
Multicenter trial of Collagraft as bone graft substitute.
Hydroxyapatite-coated total hip prostheses. Two-year clinical and roentgenographic results of 100 cases.
Porous hydroxyapatite as a bone-graft substitute in metaphyseal defects. A histometric study.
The Influence of a Hydroxyapatite and Tricalcium-Phosphate Coating on Bone Gowth into Titanium Fiber-Metal Implants.
The effect of operative fit and hydroxyapatite coating on the mechanical and biological response to porous implants.
Hydroxylapatite-coated hip implants. Multicenter three-year clinical and roentgenographic results.
Early clinical experience with hydroxyapatite-coated femoral implants.
Comparison of Proximal Porous-Coated and Grit-Blasted Surfaces of Hydroxyapatite-Coated Stems.