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Hydroxyapatite Crystals / Prosthetic Coating

- 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;
    - cautions:
           - 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.

Hydroxyapatite coatings.

Comparison of Proximal Porous-Coated and Grit-Blasted Surfaces of Hydroxyapatite-Coated Stems.