Three-dimensional printing for spinal implants

Richard Guyer

One of the most exciting areas of technology for spine is probably 3D printing which allows for the creation of virtually anything that can be designed or imaged on a computer. Initially developed in 1984, and recently applied to spine, its applications are boundless, writes Richard Guyer.

Creating a device by 3D printing involves the repeated lamination of layers of metal powder according to an image-based pattern. A laser is used to melt the layers together before the next is placed, one after another, until the device is completed.

A computer-aided design file for a custom implant, or a reconstructed image from a computed tomography or magnetic resonance imaging scan, may be imported to the printer to produce a patient-specific device or implant. 

The potential benefits of 3D printing include the production of intricate spinal implant designs, allowing for control of porosity It also allows customisation of size and shape for individual patient needs. The design of the implant surface technology can be altered at a micro- or nano-level to enhance osseointegration. The benefits of this 3D printing include rapid prototyping of new designs and lowering of the cost of custom implants.

Undoubtedly, equipment prices will continue to come down as even home 3D printers can be bought for just a few hundred US dollars.

Currently applied to spinal fusion implants and specialised coatings, 3D printing may be applied in the future to scaffolds for disc degeneration and tissue regeneration, the latter of which is already occurring in the laboratory. The first 3D-printed fusion cage was created by 4WEB in the USA. This cage has 75% porosity for boney ingrowth, and disperses the load throughout the device. The surface was designed to encourage osseointegration. The clinical use of these implants has become routine, with over 3,000 cages implanted thus far. Mechanical testing shows the excellent strength properties and benefits of such a design. Currently, there are several other 3D-printed cages produced by K2M in the USA, Oxford Performance Materials in the UK, and joimax in Germany. 

There have been individual reports of a 3D-printed vertebra for special situations. A thoracic vertebral body replacement has been 3D-printed for a 21-year-old with an osseous fibroma. Another report utilised a 3D-printed cervical vertebra for a 12-year-old patient that had a sarcoma at the C2 level. Finally, 3D printing is being utilised to produce resorbable materials to replace bone voids and osseous defects.

3D printing has been used to enhance the surface of titanium implants by utilising nobium; a soft, grey, ductile transition metal. It has also been utilised for minimally invasive implants as well. 

While the 3D-printing of custom implants is exciting, I am most excited about the non-fusion applications for spine. While not in clinical use, basic design work has already been completed for several concepts for the design of scaffolds for intervertebral disc regeneration. Other researchers are working on nucleus pulposus regeneration, in which case a scaffold would be designed to allow the proliferation of chondrocytes on the scaffolds. This has tremendous implications for potential disc repair. Various materials have also been investigated utilising 3D-printing, such as polycaprolactone scaffolds, polyurethane and chitosan-gelatin. These scaffolds are similar to the native disc, in that there are concentric lamellae with similar thickness and spacing, as in the mechanical properties of the human disc. Cellular attachment and proliferation in alignment with the concentric lamellae has been shown.

Despite this excitement, there are factors affecting the accuracy of 3D-printed implants. The quality of clinical scans used will affect the final product. If a scan reconstruction is not accurate, it will affect the dimensions of the final model. 

We must be careful to use attention given in the lay press for the advancement of implant design, rather than just marketing hype. Of course, proof always lies within the scientific data. Will these implants allow osseointegration? Will the loading characteristics stimulate cell proliferation?

Unfortunately, there is little data available for these new implants.

Computer-related technology continues to impact spine care; 3D printing, I believe, can be used to manufacture more intricately designed fusion cages, as well as custom implants. Early phase research has been initiated for implants facilitating disc regeneration. I believe that someday the process will be simplified to the point that one will send a pre-operative scan and receive a customised implant no more cost than a regular implant. I am optimistic that 3D printing will continue to evolve and allow surgeons to provide the building blocks for disc regeneration.

Richard Guyer is a co-founder of the Texas Back Institute, Plano, USA