Becker's Spine Review

55 PRACTICE MANAGEMENT SPINE DEVICE & INNOVATION HEALTHCARE REFORM Challenging the Modulus of Elasticity Paradigm for Interbody Fusion Devices By Av Edidin, PhD C ontemporary spine fusion cages are mostly engi- neered from titanium (usually Ti6Al4V alloy) or polyetheretherketone (PEEK). e latter material, PEEK (a thermoset polymer), was introduced as a material for spinal interbody fusion devices, or cages, in the early 2000's and was suggested to have advantageous imaging, structural, and biocompatibility properties over titanium. From an imaging perspective, PEEK cages are more radiolucent than titanium cages, however this attribute may not be as marked as initially suggested because titanium cages require less material to resist a given load and therefore oen have wider apertures, or win- dows, as well as thinner walls. One of the main benefits that has been attributed to PEEK is a modulus of elasticity ("modulus") that is closer to that of bone, thereby implying a favorable load transfer pattern that loads the gra contained within the device. Alternatively stated, the PEEK cage is suggested to "stress shield" less than a met- al cage with its associated higher modulus. e challenge is that while PEEK indeed has a lower modulus than titanium, modulus is not the metric that drives the response of the bone implant system. e modulus of a material is an intrinsic property, like den- sity or hardness. More deformable materials such as polymers and rubber have low moduli whereas rigid materials such as metals have higher moduli. While PEEK does possess a lower modulus than titanium, the net result is that more material must be used when building a PEEK cage to overcome its lower modulus property to ensure the PEEK device can withstand the overall loads in the spine. As such, the critical property to con- sider when determining the response to an applied load when comparing two cages is not the modulus of elasticity, but rather the structural stiffness of the cage. Whereas modulus is one of the properties that factors into the overall structural response equation, the design of the struc- ture plays a much greater role. In fact, it is far easier to increase the flexibility of a structure such as a cage by slightly reducing wall thickness than by reducing its modulus via selection of an alternative material. For example, a titanium rod of diameter 6 mm has a structural stiffness in bending that is almost 30 per- cent lower than a rod of 7 mm diameter — the extra millimeter has an exponential effect. Another way to illustrate the greater role of dimension on mechanical response is to consider a low modulus mate- rial such as paper. When corrugated and formed into a box, hundreds of pounds may be safely transported even though the base material has a very low modulus. Similarly, aluminum is a material with a modulus close to titanium, yet a thin sheet may be torn as easily as paper and a soda can crumpled with bare hands. us it can be observed that the material's modulus does not drive the response of the system; its form does. To further explore the relationship between modulus and stiffness, consider an idealized spine cage consisting essential- ly of a hollow box with material removed from each side to create windows. For simplicity, it will be assumed that the load transmitted to the top of the cage is concentrated in the center. In addition, the behavior of a single column or strut will be considered. e idealized cage is then just a rectangular prism with windows. Each horizontal strut acts like a beam in bending, essential- ly like a bridge across a river, causing it to deflect downwards when load from the vertebral body is applied. e amount the beam bends or deflects can be easily calculated from the equation: is equation is the key to understanding why changing the modulus of a cage has a limited effect by itself and why a cage made of a lower modulus material will require more of that material to resist the same load. e response is dominated by the moment of inertia ("I"), which tells us how hard it is to bend something based on its geometry. Assuming cages of equal dimension are made from PEEK and from titanium, the deflec- tion of the PEEK cage would be approximately 32 times greater than titanium since the modulus of PEEK is 32 times lower than that of titanium. Such large deflections have two disadvan- tages: they can cause the cage to break under large displacement repeated load and the deflections will both irritate and abrade the underlying bone through constant repeated engagement. erefore, the PEEK cage cannot be made to the same design specifications as the titanium cage and instead must be fabricated with much thicker struts until the overall deforma- tion is reduced to the level exhibited by the titanium cage. In other words, in order to properly support the physical loads placed upon it, the PEEK cage must be made bulkier and thicker than a titanium cage to achieve the same stiffness, thus substantially reducing the advantage of starting with a lower modulus material. In summary, while PEEK cages have a native modulus closer to that of bone, this benefit is substantially mitigated by the need to include significantly more material to sufficiently stiffen the device to resist the applied loads in the spine. Overall, implant design as opposed to implant modulus, determines the structural response of the bone-implant system. n

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