Questions arise after a string of failures of 3D printed cages
written by an anonymous spine expert
Additive manufacturing (AM) is growing quickly in the orthopedic industry as suppliers seek to reduce manufacturing costs, inventory costs, and lead times. Some of the largest spine companies have made significant investments in AM, including Stryker’s announcement of their intention to invest more than $400 million in AM production machines and facilities.
Recently, makers of spinal interbody fusion devices have turned to AM to exploit the market’s shift toward surface-enhanced titanium implants that are purported to accelerate the rate of osteogenesis and subsequent fusion. 3D-printed interbody devices are primarily aimed at creating highly-porous structures that are intended to allow for boney “in-growth” at the bone-implant interface. There are now several spinal implant makers that employ additive-manufacturing techniques.
However, with all of the hype surrounding 3D-printed porous titanium interbody devices, one very important question remains.
Are they safe?
The question is surprisingly difficult to answer. The reason for the uncertainty is that there currently are no ASTM standards that look at how interbody implants are structurally affected by forces created during simulated impaction into the disc space. The current ASTM standard shear test for interbody implants is at a rate that is approximately 60,000 times slower than the rate an interbody device is subjected to during impaction.1 Thus, all interbody devices, including those made by AM, are gaining clearance despite the uncertainty of how well they can truly withstand impaction forces. Perhaps of notable concern are porous 3D-printed implants, since, after all, increased porosity means the replacement of metal substrate with air that inherently weakens the construct. Some of the current technologies are stating porosity as high as 80%, which means that the device contains only 20% metal to endure impaction and subsequent loading with no clear means of testing its ability to do so.
A cursory look at the FDA’s Manufacturer and User Facility Device Experience (MAUDE) database does, indeed, show that there is a significant number of 3D-printed porous cages that have been reported to the FDA for breakage during insertion or in situ postoperative collapse. One manufacturer alone has reported 39 such events on their porous interbody implants, 11 of which occurred postoperatively. Of particular concern are the postoperative breakages/collapses that often require secondary surgery and may occur due to implant weakening during insertion. In fact, many of these types of devices now carry contraindication warnings in their Indications for Use not to be implanted in obese patients.
Summary of MAUDE reported breakages to date can be found here – List of Stryker and K2M failures 2
Of further concern is the possibility of generating particulate debris during impaction into the disc space, since additive printing techniques create residual microscopic particles on the surface of the implant. Unless these particles are removed during secondary processing, they may be prone to coming loose during impaction into the disc space and possibly beginning an insidious osteolytic reaction. A recent biomechanical study that simulated impaction showed that titanium particulate was generated from a titanium-plasma sprayed device made from AM processes, while none was produced by machined implants made from subtractive methods.1
Further investigation is required to determine if 3D-printed porous implants are indeed safe following implantation, both in terms of strength and particulate generation. And until the FDA alters its ASTM testing standards to better represent the forces an interbody implant actually endures during impaction – or unless manufacturers conduct and publish the results of their own legitimate impaction testing – the answer will continue to remain unclear.
1 Kienle, A., Graf, N., Wilke, H.J., Does Impaction of Titanium-Coated Interbody Fusion Cages into the Disc Space Cause Wear Debris and/or Delamination? The Spine Journal. 2016 Feb; 16 (2): 235-42.
