The school of thought surrounding most orthopedic implant coatings is that the more porous and “rough” the surface is, the better the implant (hip, knee, etc.) will grip to, and connect with, the bone. As a result of this bone ingrowth, the implant will be more stable, requiring fewer repairs and repeat surgeries.
But this mindset didn’t evolve over night. And many still argue that “aggressive, highly porous” coatings are not applicable to many orthopedic implant situations. In addition, a reoccurring challenge over the years has been finding a balance between a coating’s cost and its effectiveness.
Consequently, only a few orthopedic medical device companies have been able to adapt to the many advances in coating technology while consistently offering competitively priced solutions.
By exploring the past, present, and future evolution of orthopedic medical device coatings, we can better appreciate the art and the science behind coatings used today, and what innovations lie ahead.
Twenty-five years ago, the particles (beads) used to form a coating were much larger (0.024 average diameter) than what’s used today. The challenge with larger beaded coatings is that they had minimal porosity, and the resulting surface was quite smooth. This was thought to limit the stability of the implant.
To facilitate higher porosity, and thus better bone ingrowth, innovation led to the development of coatings formed with smaller beads (0.010 average diameter). However, the issue of bone grip still remained.
Titanium plasma spray (TPS) coating was also a viable option at this time. Due to its application process, TPS resulted in a rougher, more “aggressive” coating surface. The challenge with TPS, however, was just the opposite of the small beaded coating. TPS lacked the porosity that beaded coatings offered.
A popular technique that evolved as a solution was to use the porous, beaded coating, but randomly distribute the final layer of beads to create a rough, “stucco” effect. The end result, referred to as “scratch fit,” gave characteristics that were closer to what TPS provided, while still allowing space for bone ingrowth.
Approximately ten years ago, metal foam products hit the market. Metal foam coatings are highly porous and produce a rougher surface. In addition, because of the composition of the metal foam, stand alone products can be made using this technique (e.g., an ankle wedge implant that’s completely foam). This diverse application differentiates a metal foam coating from a bead or TPS coating. However, this method also makes metal foam coatings more expensive, which limits their marketability.
In recent years, trends in health care reform, reimbursement reductions, tighter budgets, and the growing ambition to enter emerging markets like China and India, have forced everyone to tighten their collective belt to maintain competitiveness.
Many of these previously mentioned innovations, like TPS and beaded coatings, are still used today. However, these coating characteristics also evolved into a popular asymmetric particle coating that’s become a popular present-day solution. Asymmetric coatings essentially combine the stabilizing scratch-fit of TPS, the high porosity of metal foam, and the low cost of beaded coating.
For example, the Asymmatrix™ coating offered by Orchid Orthopedic Solutions for both cobalt chromium and titanium implants is highly porous (55% ± 10%), with 300 ± 125μm pore size; and its ultimate tensile strength (UTS), or maximum stress that the material can withstand while being stretched or pulled, equals that of spherical bead coatings. (See Figure 1)
This type of coating is an impressive union of technologies that meet all the requirements of today’s orthopedic implant market. Furthermore, it also enables select companies to offer a bone ingrowth coating solution at a price that’s attractive to low-cost markets like China and India.
In order to remain competitive in today’s market, coating solutions providers require a unique skill set. They need to possess extensive in-house capabilities, emphasize high quality and exceptional customer service, and offer competitive pricing.
A coatings company must also be able to offer a diverse array of coating solutions for every orthopedic implant imaginable. This demands higher level technology, expertise, and an appreciation for more artistic forms of manufacturing.
In addition, a coating partner must be able to provide solutions above and beyond coating, such as design, engineering, and manufacturing. Such diversification and flexibility helps ensure that a design can be efficiently coated and manufactured, and prevents the customer from wasting time and money.
Furthermore, having extensive knowledge and capabilities enables a coatings company to offer non-biased recommendations on what coating best meets their customer’s manufacturing requirements, budget, and time-to-market.
While past and present coating techniques have greatly enhanced the stability and effectiveness of orthopedic implants, new, impressive innovations promise even greater value. Here are a few noteworthy ones on the horizon.
The concept behind antimicrobial coatings is that they help fight infection. These types of coatings work by making the surface of the coating incompatible with harmful microorganisms that cause disease.
Antimicrobial elements are applied throughout the coating at a level dependent upon the chemistry used, for example: silver, organosilane compounds, and iodine, to name a few.
Each chemical compound has a unique mode of antimicrobial activity to combat bacteria, mold, mildew, and other infection causing pathogens that tend to be especially rampant in private and public healthcare environments.
According to a 2012 market research report by Global Industry Analysts, Inc., the forecast is for the market to reach US $1.2 billion by the year 2017.
This growth is mainly driven by increasing demand from hospitals looking for ways to improve health, sanitation, and hygiene, reduce hospital-acquired infections, and prevent the loss of valuable reimbursement dollars.
Direct Laser Metal Sintering (DLMS)
In a traditional medical device machining process, a manufacturer usually gets a metal piece (e.g., cup-shaped) and then puts the coating on it. With DLMS, a special machine employing a high-powered fiber optic laser creates a nearly complete product, for example, an already-coated hip socket cup.
This allows for highly complex geometries to be created in hours directly from 3D computer-aided design data without any tooling. With no special tooling requirements, instead of making one unit at a time, a manufacturer can use DLMS for full production manufacturing, and as a cost-effective method to simplify assemblies and complex geometries.
In addition, since most alloys can be used in the DLMS process, functional prototypes can be made out of the same material as production components. This enables more rigorous testing of prototypes while decreasing the development time for new products.
At present, DLMS is most suitable for companies that have a lot of customer diversity as it grants them the ability to keep the machine running and experience a higher, faster return on investment. Small to mid-sized companies with a more focused customer base may find a DLMS machine cost prohibitive.
Nanotech for Hydroxyapatite coating
The most important parameters for orthopedic implants, especially for weight bearing joints that require a lot of movement, are that they have adequate wear resistance, allow for stable, secure bone attachment, and promote elasticity and strength.
The solution to these pertinent requirements may lie in macro- and micro-textured implants that can be coated with nanoscale bone-like hydroxyapatite (HA), a calcium phosphate complex that is the primary mineral component of bone.
The idea behind using nanotechnology with HA coating is that it’s believed to enhance bioactivity and provide good adhesion between the implant and the bone.
With its relative ease of production and the ability to form a chemically and physically uniform coating over complex geometric shapes, nanocoating has opened up new opportunities to design superior biocompatible coatings for implants — including orthopedic implants made of material other than titanium or cobalt chromium, like plastics or pyrolytic carbon.
A Coat Above
The rise of orthopedic maladies over the past three decades — especially those associated with hips and knees — creates significant opportunity for medical device designers and manufacturers to improve not just implant technologies, but their coatings as well.
As hospitals grow increasingly cost-conscious, they are more inclined to invest in medical devices that support their ability to cost-effectively, reliably, and safely enhance a patient’s quality of life.
Whether it’s HA coatings that use nanotechnology, asymmetric coatings that support stability and bone ingrowth, or antimicrobial coatings that help fight infection, the end goal remains the same: to elevate the performance of the orthopedic implant in the most effective, efficient way possible.
Orthopedic medical device companies that understand why implant coatings have evolved in the way they have are at an advantage. They are better positioned to offer solutions that balance cost and capability to their present and future coatings, and add more value to their customers.
This article was written by Tony Crivella, the senior manager of implant sales at Orchid Orthopedic Solutions, Holt, MI, a contract designer and manufacturer of implants, instruments, and innovative technologies for the orthopedic, dental, and cardiovascular markets. For more information, Click Here