by Elizabeth Hofheinz, M.P.H., M.Ed., September 6, 2019
The Barrow Neurological Institute in Phoenix, Arizona is a world-renown facility serving patients with numerous spine-related conditions. Brian Kelly, Ph.D. is Assistant Professor of Spinal Biomechanics in the Division of Neurological Surgery at Barrows and directs a Barrows lab that focuses on the mechanical behaviors of the human spine—and more.
Discussing their latest work, Dr. Kelly commented to OSN, “At the moment, a lot of companies are trying to determine the right parameters for 3D printed spinal interbody cages. Some of the most recent advances have been in the area of optimizing porosity/surface texture in order to create the most inviting environment for bony ingrowth. With traditional interbody cages bony growth has to bridge the implant before fusion between two vertebrae can occur. With porous 3D printed technologies bone growth integrates directly with the implant.”
“The primary goal is to optimize the quality and speed of bony ingrowth, while the other aim is mechanical in nature. With 3D printing you can mechanically tailor the stiffness or elastic modulus of a device. So, in theory you can create something with a stiffness that more closely matches that of human bone. However, there is a lower limit. If the stiffness and strength are dialed down too much there is the risk of failure. Under load, if a device deforms substantially, there may be too much relative motion between the implant and mating bone surfaces. That is not a good match to promote bone ingrowth.”
Advanced strain measurement…
“We have recently been performing a lot of studies that investigate stress and strain in spinal instrumentation. These are assessments of what happens when a load-bearing object deforms under load; and it may only be measurable in micrometers. If you have pedicle screws and long spinal rods, for example, then surgeons have questions regarding how to optimize those in order to avoid failures such as rod fracture. At Barrow, we are putting strain gauge sensors on rods in order to measure the extent of deformation with specific construct designs. (stress/strain). Strain gauges are…small film-type elements of various sizes that glue to the rod’s surface. When the rod stretches and the surface deforms, the gauge deforms with it changing the resistance and voltage of a built-in electrical circuit. This allows for a calibrated measurement of the surface deformation.”
While strain gauges are the gold standard for measuring strain on anything, says Dr. Kelly, there is a newer option. “Another possibility is to use digital imaging correlation. For example, if you have a spinal rod, you can put a speckle coating on it, then load the rod and have two optical cameras track the speckled pattern. Custom software analyzes how the dots move in relation to each other and converts the speckled patterned movements into engineering strain measurements. What’s intriguing about this is that it can capture much bigger surface areas and can examine not only hard metal objects but soft tissue as well, assessing the strain distribution under different conditions. We hope to be able to coat a 3D printed cage, put it under load, and then track the speckle pattern on its structure so that we can register the full distribution of strain across the device. This could serve to better understand how well the cage is holding up to the load, quantify local deformations, and also identify potential areas of weakness.”
Surgeons seeking science…
“Fundamentally,” says Dr. Kelly, “Surgeons want to know what really matters in terms of device performance. They are wary of situations where a particular product or product feature becomes ‘hyped’. So, we try to work with them to obtain more scientific-based information. For example, a surgeon may want to know how modulus matching affects device subsidence because any interbody device they implant needs to be able to maintain height and alignment. Currently some people are enthused about the idea of more closely matching the modulus of an interbody device to that of human bone. However, there are a number of mechanical variables at play including device placement, degree of endplate removal, and footprint size of the device for example. More research is needed to isolate and identify which variables matter the most clinically.”
“To this end, we are hoping to partner with a company currently developing 3D printed interbody device technologies. It would be great if we could distill things down to isolate what is fundamentally most important with these new technologies and assist surgeons by putting the science behind it. Studies using animal models for example, could assess how well a 3D printed device integrates with bone. Does it integrate sooner? If so, does that earlier integration result in earlier strength? Is it possible to create a bony bridge that provides mechanical stability and leads to fusion sooner than with existing technology?”
R2D2 loading spines?
“While standardized testing has been valuable, one problem with it is that the loading scenario is simplistic. Many of the loads that an implant might see in-vivo are not represented. To address this we have built a custom-designed robot and can apply any load or combination of loads to spinal test specimens. So, in addition to traditional flexion and extension bending moments for example, we can simulate much more complex activities of daily living that involve combined movements and loads. Standardized testing has been traditionally confined within anatomical planes, but humans do not move in anatomical planes!”
But can they strain map a 3D printed cage? “I think we can,” says Dr. Kelly. “And I think we can strain map these under increasingly complex loads. We need to understand what really matters in terms of the different features available with 3D cages under in-vivo like conditions. Our efforts have always been aimed at shortening the distance between bench top research and the patient.”
