It is not uncommon to encounter the term hydroxyapatite (or its abbreviations, HA or HAp) when reviewing information about dental or orthopedic implants. This short primer offers a basic overview of why HA coatings have become a much sought after surface treatment by medical device manufacturers seeking to optimize their implant designs.

A Common, But Critical, Calcium Phosphate

Hydroxyapatite is a naturally occurring calcium phosphate and is the primary inorganic component of teeth and bone. In fact, a carbonated form of HA makes up a large portion of the bones in our body as well as the bones of mammals, fish, and the egg shells of birds. It’s also found in shrimp, seashells, certain rocks like limestone, and sea coral.

The HA used to coat dental and orthopedic implants can be derived from these sources or chemically synthesized. Both methods produce a substance that is invaluable for use in bone regrowth, bone repair, and successful integration of medical implants.

Why is Hydroxyapatite Important?

For over a century HA has been used to repair or rebuild missing or damaged portions of bone in bone grafts. It’s very stable and not excessively soluble, meaning it doesn’t negatively react with our bodies, and it doesn’t easily break down and disappear. Equally important, HA readily interacts with our bones in a positive, compatible way, which has been thoroughly demonstrated through multiple scientific studies designed to gauge its bioactive, biocompatible, and osteoconductive properties.

More recently, HA has been used not just for its ability to replace bone, but for its ability to bind with, and even help regenerate, tissue where it is applied. That’s why, in addition to its use as a bone replacement, HA is often employed as a surface coating for medical implants. When applied correctly it can support more successful implant integration by reducing the potential for implant failure or rejection.

How Does Hydroxyapatite Help with Implants?

Hydroxyapatite is used as a bone substitute because of its innate chemical similarities to natural bone. When an implant is coated with HA, it provides an osteophilic surface, which helps the natural bone affix to the implant. This occurs in part because the coating provides a better textured contact area, and partly because the HA itself has osteoinductive properties that have been shown to increase the activity of osteoblasts; the cells that create bones.

Obviously, when working with a new implant, either dental or orthopedic, it’s very desirable to have the surrounding bone not only accept the implant, but to integrate the implant into the body.

However, while HA is excellent at integrating with the body, an implant made entirely from hydroxyapatite isn’t strong enough mechanically for load bearing or functional parts. That is why titanium (or Ti) is a typical material choice for many medical implants. Recognizing that the osteointegration levels of Ti aren’t ideal, a coating of HA on Ti implants presents an ideal solution: the titanium implant affords the mechanical strength while the dense HA coating provides better biocompatibility.

But not all coatings are created equal…

Hydroxyapatite Coatings Done Right

Coating an implant with HA can be performed in numerous ways, from plasma spraying, dip coating, electrochemical deposition and more. Plasma spraying is the most widely applied method and has been successfully employed by Himed since its founding in 1991. Critical considerations include the thickness of the coating, the phase (or state) of the HA that’s applied, and the texture of the coating. All of these will have an impact on the implant performance.

Coating Thickness

While it may seem like more of a good thing can’t be bad, research has shown that there’s an ideal thickness for the application of hydroxyapatite. Too thin and there may not be enough to react with the surrounding tissue, too thick and the coating can suffer mechanical failure known as delamination. Therefore, an ideal thickness in relation to the implant’s function and design must be identified and applied. Such a determination tends to involve both the implant device manufacturer and a company specializing in biomaterials like Himed. Once parameters have been set a coating methodology is programmed and tested. Himed, for example, uses a robotically controlled, atmospheric plasma spray system that adds a consistent coating of HA at exceedingly specific thicknesses.

Uniformity

While maintaining a certain thickness for a coating is important, so is making sure the same thickness is maintained throughout. Matrix HA™ is the system Himed uses to coat implants with a highly-controlled level of uniformity. Through careful testing, validation, and customizable automation it is possible to produce consistent coatings on small or large lots, with the parameters easily replicated when a manufacturer wishes to scale up production.

Surface Texture

The roughness and porosity of the implant surface also matter. With a dental or orthopedic implant, how well the bone grows and blood vessels form at the site of the implant can be determined by the texture of the implant’s surface.

At Himed, we’ve found that an optimal surface morphology on titanium medical implants involves the controlled use of our resorbable apatitic abrasive on the titanium surface prior to the application of hydroxyapatite. That's why our MATRIX HA™ is actually a multi-step process where the application of our proprietary resorbable blast media (RBM) is followed up by the necessary number of passes with HA to achieve the exact specifications required by the device manufacturer.

Composition

Himed’s synthetic hydroxyapatite powder is specially formulated to produce an uncommonly pure and highly crystallized coating on metallic substrates. These high quality coatings ensure optimal bioactivity and performance.

Antibacterial Innovations

One common cause of failure for dental and orthopedic implants is infection. So scientists are now making HA with antibacterial properties. Using nanoparticle composites, incorporating antibiotics with HA, and even using silver nanoparticles have all been effective in inhibiting bacterial growth in implants, assuring they have a better chance of integrating with a patient’s body.

HA Coatings and Medical Implants: A Winning Combo

As medical science continues to progress, hydroxyapatite continues to play a vital role in the field of dental and orthopedic implants, with new research ongoing. By supporting an implant’s biocompatibility and osteointegration, HA-coated implants are helping more people improve their quality of life.