The Case for the Compliant Spinal Device

Category: Blog,Expert's Corner
Mar 21, 2020

With the advent of additive manufacturing and its widespread adoption over the past decade, the orthopedic device sector has been at the forefront of the technology’s utilization. Indeed, the space has always been focused on various kinds of advancements – championed by small and large companies alike – to ensure patient outcomes remain positive and reduce adverse events. Additive manufacturing has provided unparalleled design freedom, which – when combined with advanced computational tools – are allowing for the creation of “biologically relevant” devices. And one key criterion still remains at the forefront – device stiffness.

Many are familiar with Wolff’s Law – the idea that bone in a healthy person or animal will adapt to the loads under which it is placed. Not many know that Julius Wolff actually misinterpreted the mechanical data and rejected the idea of bone resorption (loss of bone during disuse); he was subsequently given credit for the continued work on the subject by the French scientist Wilhelm Roux, who was the first to accurately describe the adaptation of bone to altered load. Work on this subject was further refined by Harold Frost and Webster Jee in their publication Utah Paradigm of Skeletal Physiology – which describes what Harold Frost coined “mechanostat.” [1] It’s easiest to show in a figure, but first let’s go over some quick terms:

  • Strain: the deformation or extension of a body that is subjected to a set of forces, defined as the change in length divided by the initial length.
  • Resorption: The process by which the body breaks down the tissue in bone
  • Apposition: The process by which bone forms and rebuilds, also called remodeling
  • Homeostasis: A dynamic state of equilibrium, i.e. “steady state”
  • Minimum Effective Strain: The smallest amount of strain required to maintain either homeostasis or bone remodeling
The four bone usage windows or zones, according to the Mechanostat hypothesis as popularized by Harold Frost

As you can see above, we have bone mass plotted on a graph with respect to microstrain. Overall, this constitutes a relatively small amount of motion – but you’ll see why it’s important. When the bone is not strained, it enters a period of disuse and resorption. This causes the body to break down the bone and reclaim the available calcium and is a large part of why osteoporosis occurs. As microstrain increases, you get to the “Physiologic Loading Zone” where homeostasis is occurring. In this zone, no bone mass is added or subtracted. As microstrain keeps increasing, you get to the phases of overload and pathologic overload. Pathologic overload creates too much strain on the bone, making the remodeling architecture of bone brittle and less adaptive to future stresses (called “woven bone”). However, the “overload” portion of the graph is where the magic occurs! And here is where an emerging field of science called mechanobiology has continued to study – how do these physical forces and changes in the mechanical properties of cells and tissues contribute to development, physiology, and cell differentiation.

Perhaps there are some who remain skeptical of how such tiny amounts of motion might cause large changes in bone growth. However, there exist many different studies spanning decades which confirm repeatedly. A more recent review of clinical literature was performed in 2014, titled “Vibration Therapy: Clinical Applications in Bone,” which was published in Current Opinion in Endocrinology, Diabetes, and Obesity [2].

In the review, the authors state “for instance, adult sheep exposed to LIV (low-intensity vibrations) [30 Hz, 0.3g, 20 min) showed a 34% increase in femoral trabecular bone by micro-CT and histology at 1 year. In mice, only 3 weeks of LIV [30 Hz, 0.3g, 20 min] increased trabecular bone … These types of studies show that microstrain initiates the bodies anabolic response to encourage bone apposition … which are at least partially conveyed through mechanical regulation of mesenchymal stem cells, which provide progenitors for bone and muscle growth.

It is with these studies in mind that we arrive at the importance of a compliant spinal implant. Up until recently, spinal cages have largely been manufactured out of a solid block of titanium. While there is a long history of use of this metal and it shows excellent biocompatible properties, its stiffness is more than 9 times that of bone. As the idea of stress shielding came into the forefront of implant design, the move to PEEK as a substrate rose to the forefront – since PEEK advertised a stiffness much closer to the bone. Even then, the material must continue to be removed so as to lessen the stiffness of the PEEK – if we are to bring the device within the appropriate strain range to encourage max bone apposition. But here we often run up against a hard stop – for as Class II devices these spinal implants must prove to be structurally equivalent to an already-existing implant in the field. Because these devices must meet these structural requirements, stiffness often takes a back seat to strength. And though patient outcomes have improved, adjacent segment issues due to spinal implant cage stiffness continue to plague the industry. It’s not uncommon for lumbar fusion patients to undergo second and third surgeries to fuse additional spinal segments, further exacerbating the issues.

To illustrate the idea of stiffness reduction to induce microstrain we can use the example of bone scaffolds. A low stiffness scaffold manufactured by additive manufacturing can be intentionally designed computationally to ensure the appropriate stiffness range is achieved, which will maximize bone apposition

Illustration showing the area of a bone defect
Illustration showing the area of an example custom scaffold to fill the space

This type of structure can be custom to form fit the local geometry. And when combined with the body’s natural loading, it creates an environment where bone remodeling happens quickly and effectively.

Illustration of the progression of bone growing onto a bone void scaffold

Although motion preservation devices will continue to gain market share within the spinal surgery segment, the fusion procedure will continue to be the main staple. In order to advance the standard of care, a new class of devices must rise up – combining computational design and patient-specific loading – and challenging the long-standing tradition of “strength trumps stiffness.” In a follow-up article, the mechanism behind the intelligent analysis of these types of implants will be revealed, in order to show how new tools can help augment better device design.


[1] Frost, H. “The Utah paradigm of skeletal physiology: an overview of its insights for bone, cartilage and collagenous tissue organs.” J Bone Miner Metab 18, 305–316 (2000).

[2] Thompson, William R et al. “Vibration therapy: clinical applications in bone.” Current opinion in endocrinology, diabetes, and obesity vol. 21,6 (2014): 447-53. doi:10.1097/MED.0000000000000111

About the Author:

Matt Shomper is Director of Engineering at Tangible Solutions and holds a BSME from Cedarville University with minors in math and biomedical engineering. With tens of thousands of implants across 12 released orthopedic device lines residing in patients worldwide, Matt has been an integral part in releasing cutting-edge products into the field – including one of the first Sacroiliac screws, one of the first interspinous clamps and the first fully sterile-packaged cervical plating set on the market. Now leading the charge on medical device computational design and medical additive serial production process flow, he enjoys working at Tangible almost more than anything!

Reach out at to chat about the current state of the market or for collaborative opportunities.

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