Advantages of Hydroxyapatite The beneficial biocompatible properties of hydroxyapatite are well documented. It is rapidly integrated into the human body, while at the same time the body is none the wiser as to the invasion by a foreign body, albeit a friendly invasion. Perhaps its most interesting property is that hydroxyapatite will bond to bone forming indistinguishable unions. Disadvantages of Hydroxyapatite However, poor mechanical properties (in particular fatigue properties) mean that hydroxyapatite cannot be used in bulk form for load bearing applications such as orthopaedics. Hydroxyapatite Coatings Coatings of hydroxyapatite have good potential as they can exploit the biocompatible and bone bonding properties of the ceramic, while utilising the mechanical properties of substrates such as Ti6Al4V and other biocompatible alloys. While the metallic materials have the required mechanical properties, they benefit from the hydroxyapatite which provides an osteophilic surface for bone to bond to, anchoring the implant and transferring load to the skeleton, helping to combat bone atrophy. Coating Techniques Some techniques that have been tried are summarised in the following table. | | | Dip Coating | 0.05-0.5mm | Inexpensive Coatings applied quickly Can coat complex substrates | Requires high sintering temperatures Thermal expansion mismatch | | Sputter Coating | 0.02-1μm | Uniform coating thickness on flat substrates | Line of sight technique Expensive Time consuming Cannot coat complex substrates Produces amorphous coatings | | Pulsed Laser Deposition | 0.05- 5μm | As for sputter coating | As for sputter coating | | Hot Pressing and Hot Isostatic Pressing | 0.2-2.0mm | Produces dense coatings | HP cannot coat complex substrates High temperature required Thermal expansion mismatch Elastic property differences Expensive Removal/Interaction of encapsulation material | | Electrophoretic Deposition | 0.1-2.0mm | Uniform coating thickness Rapid deposition rates Can coat complex substrates | Difficult to produce crack-free coatings Requires high sintering temperatures | | Thermal Spraying | 30-200μm | High deposition rates | Line of sight technique High temperatures induce decomposition Rapid cooling produces amorphous coatings | | Sol-Gel | <1μm | Can coat complex shapes Low processing temperatures Relatively cheap as coatings are very thin | Some processes require controlled atmosphere processing Expensive raw materials | Information in the table above has been sourced from various scientific publications. Some of the coating techniques have been purely experimental. Plasma Sprayed Hydroxyapatite Coatings Of the techniques outlined above, only thermal spraying, in particular, plasma spraying is the only commercially accepted method for producing hydroxyapatite coatings. While plasma spraying is a well understood process, control of variables is quite complicated. Small changes to processing variables can vastly affect the properties of the final coating. In the case of hydroxyapatite coatings, this type of problem can be exacerbated due to its thermal instability. Variations in such things as powder feedstock may produce soluble coatings. The reason for this is the high processing temperatures encountered by the hydroxyapatite induce some decomposition to soluble calcium phosphate compounds. Clinical Behaviour Plasma sprayed hydroxyapatite coated implants have become widely used over the last twenty years, with several companies manufacturing devices for orthopaedic and dental applications. In general, the implants have performed well. In fact, when removed from patients (sometimes post humously) and separated from the surrounding bone, it has been the the bone/coating interface that has failed. This indicates that the bond between the bone and the coating has formed a stronger bond than that which exists between the metal substarte and hydroxyapatite coating. |