Along with the material choice discussed in Part 1 of our Spinal Implants series, surface treatment is the most important factor affecting the functionality, performance, and longevity of spinal implants and instruments.
During the manufacturing process they may have to undergo multiple finishing operations including burr removal from previous manufacturing operations, rounding of sharp edges, general surface cleaning, surface smoothing, and, frequently, high gloss polishing.
Treatment steps can also include surface texturing/profiling and shot peening, areas Rosler Metal Finishing has extensive experience in.
This blog post will focus on the finishing requirements and challenges of spinal implants as well as the solutions provided by mass finishing and shot blasting processes and the associated costs.
For the comfort of patients and effectiveness of the components, spinal implants must be finished to precise specifications, including:
- The removal of sharp edges and burs to prevent rupturing blood vessels during implantation.
- A smooth, sometimes polished surfaces to prevent osseointegration in some types of implants.
- Textured (somewhat rougher) surfaces to promote osseointegration in some implants or as preparation for coatings.
- Absolutely clean surfaces to prevent infections.
- Improved resistance against tensile and bending loads for longevity. This is achieved with shot peening mainly spinal rods, screws and cervical plates.
Additive manufacturing is rapidly evolving from a purely prototyping method into a full-fledged manufacturing system, bringing challenges and opportunities with it.
Titanium interbody fusion cages are already made using additive manufacturing with great success.
While we expect the trend towards 3D printed orthopedic implants to accelerate over the coming years, additive manufactured components pose enormous finishing challenges.
In addition to successfully removing support structures and sintered metal particles, these implants have significantly higher surface roughness to overcome. The initial surface roughness of a 3D printed component can be as high as Ra = 800 micro inches, while the initial surface roughness of a cast or forged part amounts to Ra = 120-320 micro inches.
Additional processing and time are required to achieve similar finishes between additive manufacturing and forged work pieces.
Many surface treatment technologies are utilized for finishing spinal implants, yet the numerous cost benefits and technical flexibility shot blasting and mass finishing are probably the most widely used treatment systems for surface preparation and finishing of medical components.
Mass finishing works as a grinding system to create pressure and rubbing between work pieces and media. This smoothing, polishing effect can achieve surface finishes as low as Ra = 0.8 micro inches.
Uses include deburring/edge radiusing, surface cleaning (descaling, de-oiling) after casting, forging, blanking, machining, heat treatment, thread rolling for screws, and additive manufacturing.
Shot blasting utilizes the impact of small metal or mineral pellets to produces cleaning, peening, or texturing effects on a work piece. With throwing speeds of 200-800 feet per second, this method can achieve surface finishes of 16-32 micro inches.
Shot blasting is an exceptionally versatile surface treatment technology that is used for deburring/de-flashing, surface cleaning (de-scaling, de-rusting, stripping of coatings), surface preparation for coating, for cosmetic purposes, and surface improvement. Most medical applications utilize air and wet blast systems.
Individually and when combined as a multi-stage process, mass finishing and shot blasting offer a number of benefits.
- They create homogeneous, all-around “isotropic” (multi directional) finishes as opposed to “anisotropic” (mono directional) surface structures produced by machining, belt and wheel grinding, drawing, and extrusion.
- Both can handle all materials, from the toughest metals like titanium and platinum to all kinds of polymers and, even ceramics.
- Whatever the task, mass finishing and shot blasting produce consistent, absolutely repeatable finishing results. They completely eliminate quality fluctuations inherent in manual or other mechanical finishing methods.
- Users can choose from a broad equipment spectrum, from simple, low-cost stand-alone machines to fully automated finishing systems.
- In combination with other methods, both technologies create perfect surface finishes on additive manufactured parts.
In addition to being very cost-effective, both mass finishing and shot blasting are highly adaptable to customer needs. The equipment spectrum ranges from small manual or semi-automatic machines for low production volumes to fully automatic systems for high volume production.
The customer decides, how much they want to spend and what degree of automation is desired. The costs for work piece fixtures – if required – are manageable, and due to the high degree of mechanization and automation the personnel costs are only a small percentage of the total costs.
For relatively simple processes the costs per piece can be as low as a few cents. With more complex, multi-step finishing operations for high value components like surgical instruments the entire finishing costs will amount to no more than just a few dollars.
The big savings are, however, achieved by the stability of the finishing processes, ensuring absolutely repeatable, high-quality finishing results with zero scrap rates!
The Rosler Way
Check back for additional blog posts about spinal implants in the future.