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Spinal fusion implant stiffness is a topic of great debate and if technology continues to advance, how can this be reconciled
with the treatment of patients today?
coLigne Design and Development Team,
It is the question that continues to be asked, but no one quite wants to be last to formulate its answer: how stiff should the spinal fusion implant be?
It is not that medicine and industry have been short on proposals. Quite the contrary. Implant stiffness has been discussed, argued and at times, ignored with vigour for over 100 years. If technology continues to deliver more solutions, as well as the means for their study, the clear response keeps spinning out of reach. How can this be reconciled with the treatment of today’s spinal patient?
This question is enormous because in spinal fusion surgery, implant stiffness brings an initial stability that enables bone to form between vertebrae and buttress an unhealthy spine. But over time, that same stiffness can prevent bone cells from renewing. Excessive rigidity can even remove the very bone that a spinal fusion surgery is intended to build. This is because implants disrupt a bone’s mechanical self-regulation system that lays bone down, renews it or takes it away over time.
So here lies the implant designer’s dilemma: how to achieve implant rigidity that is stable enough to start the spinal fusion that over time, will not be so stiff such that later it erodes the quality of bone. Surgeons and industry have struggled with the question for as long as there have been implants and spinal fusions. How much stiffness in a spinal construct is enough, not enough or too much? Can this differ in each region of the spine? Should it be the same for each patient? How do the needs for implant stiffness change as the patient heals?
Long fibre carbon composite, in which fibres are oriented to control implants’ mechanical properties, may bring some new solutions to the questions of rigidity for spinal implants and their affect upon vertebral bone over the functional life of the implant.
Spinal fusion as the surgeon’s last resort: a growing treatment with questions to be solved
They are performed by the hundreds of thousands world wide each year and could be considered the surgeon’s final solution to treat pain generating abnormal motion between two or more vertebral bones. Certain types of spinal pathology can become so unstable and painful that the spinal surgeon resolves to weld vertebral bones together to make one solid block of living bone. Spinal fusions are state of the art for certain forms of degenerative disk diseases, spondylolisthesis, spinal stenosis, scoliosis, fractures and tumours.
As a part of the fusion procedure, pedicle bone screws are implanted into the vertebra and connected with spinal rods, next to a bone graft medium. Together these serve as an internal brace and biologic scaffold with the purpose of creating an optimal site for fusion to occur. The pedicle screw and rods are usually intended to stay in the patient next to the bone bridge for the patient’s lifetime because implant removal is yet another surgery, with accompanying potential risks.
At first, a stiff and stable implant construct is considered good for fusion because it holds a spinal alignment and allows the vascular bone graft to ‘take’, usually within the first six months. But as the bone bridge continues to mature, it must then become capable of bearing its own load replacing the load bearing function of the implant which could otherwise fail due to fatigue. But for the bone bridge to become stronger, it must be strained, that is slightly deformed under load to trigger cell level mechanisms that tell any bone that it must reinforced. Therefore, at a microscopic level, over time the implant and bony bridge must deform slightly to assure long term viable bone formation and renewal.
How does the implant designer create an implant that at first is mechanically stiff and stable enough to allow spinal fusion to begin, but still allow some deformation of the surrounding bone to allow it to renew itself over time? This is the conflict of implant mechanics and bone biology over time.
CSSO and the trade-off between mechanics over time
Implant design that contributes to initial stability mechanics can over time work against fusion biology. When considering technical solutions, it is useful to describe trade-offs with an acronym: CSSO
The implant must:
Correct and Stabilise can be considered the mechanical goals for initial fusion, while the Stimulate and Observe bone the longer term biological counterparts that must continue the fusion process.
Bone renewal mechanical formation regulator. Frost’s mechanostat
Science proposes some explanation for this. It is theorised that over time resorption, renewal, and new formation is regulated by micro strain upon bone cells. It must be remembered that to keep bone tissue viable, it gets naturally removed and replaced. How much is removed and put back down is in part regulated by bone strain.
The slight microscopic deformation of bone cells under load signals bone formation to turn itself on and to replace bone that is naturally removed. In the opposite sence, the absence of strain sends signals to take bone away. Increased strain can create extra bone up to a point, but too much load and strain overloads bone formation and bone goes into micro stress fracture. Harold Frost called this mechanism a mechanostat.1
Like a thermostat that keeps room temperature at a certain level by regulating hot and cold air flow, the mechanostat uses measurable levels of micro strain to initiate bone formation and to take it away when micro strain gets low. This is bone’s machinery that keeps bone viable. Another more recent observation of bone and strain has shown that both more and less strain upon the bone accelerates the rate of formation and removal.2 Strain upon bone enables formation, renewal and removal. Any implant that is permanent, such as spine pedicle screws and rods, affect that bone strain and over time the quality of bone.
Metals and plastic: opposite extremes to address the same problem
Stainless steel and more recently titanium alloy have been used to manufacture spinal fusion implants for decades, with titanium the more recent preference due to its compatibility with magnetic resonance imaging (the Observe bone aspect of the CSSO acronym). Metal alloys are also malleable, that is, they can be bent to form, giving them adaptability to spinal curves. In 5.5 or 6mm diameter they can be made initially strong enough to hold position (Correction) and Stabilise for initial fusion. But bending Titanium to spinal curves creates microscopic fractures that can propagate.
Furthermore, if they are stiff enough to initiate fusion, they may be preventing bone to form long term. In clinical series that studied spinal fusions to correct degenerative deformity long term, evidence has emerged that suggests that metal rods are too stiff, preventing bone to take over its load bearing role on the fused vertebrae and causing late pseudoarthrosis, (that is, a non successful fusion). If the percentages were relatively low, under 5%, some fusion failures occurred as late as five years when bone should be bearing load, suggesting the titanium rod implant may be preventing viable bone renewal. Authors recommend ‘long term follow up of at least 5–15 years to assess the true outcome for deformity surgery’.3 These observations have led spinal surgeons to wonder if metal rods are too stiff over time, even if at the onset of fusion, they provide sufficient stability to start the formation of bone.
Some surgeons also postulate that stiff fixation can propagate forces upon the adjacent non fused vertebral levels and accelerate degeneration of comparative healthy areas of the spine.
The need to stimulate bone over time, the quest to protect vertebral levels beyond a fusion segment, has led industry and surgeons to develop pedicle fixation that is less rigid or with varied degrees of rigidity at different levels.
Dynamic systems and PEEK rods
In the hopes of providing micro strain for bone to improve bone formation over time and perhaps protect the adjacent non fused levels of the spine, more dynamic fixation have been introduced. Hinge mechanisms have been proposed to go above a solid fusion. In addition, plastic rods, particularly PEEK (poly ether ether ketone) have also been proposed. One of the main theoretical advantages has been load sharing where the implant allows the surrounding bone to receive load, particularly of the anterior spine where additional bone inside cages may be placed.
However, while much more flexible, there is concern that at the early stage of bone formation they may not be stiff enough to be an adjunct for fusion leading to loss of correction and rod failure. There is usually a very big difference in strength for correction and stability of such systems as compared to metal rods. Perhaps the difference is too abrupt. Due to the complexity of their designs, or the softness of materials, hinge and PEEK rod systems are most often limited to only a few levels of stabilisation.
Limitations of metals and plastic as well as the abrupt mechanical difference of hinge mechanisms has led industry and surgeons to explore complex materials, in particular long carbon fibre reinforced composites that are, in the long process of medical development, a recent design proposal for spinal implants.
Long fibre carbon composite: programing 100,000 filaments
A carbon filament is many times stronger than the same-dimensioned structure in steel. Using recently developed manufacturing methods, their orientation can be controlled and locked within a PEKEKK (poly ether ketone ether ketone ketone) matrix that produces desired strength and stiffnesses in different directions, something not possible in simple metals or plastics. In a 6mm diameter spinal rod, there are over 100,000 carbon filaments that can be potentially oriented for desired strength and stiffness control. This creates some particularly attractive implant performance capabilities that from a design standpoint becomes the moment where the development team says: “okay, all this capability is great. Now what do we do?
It is possible to get strength for Correction that is close to metals and considered desirable as an adjunct to fusion. But composite rods can be programmed to bring a larger elastic zone, meaning it can deform a bit more and return to its original shape for Correction, more like a spring. This can be good for two things in a fusion construct, straining the surrounding bone, without sacrificing Correction and absorbing some of the shock upon the implant from abrupt movements of the spine. This characteristic should also be helpful for the Stabilisation aspect of the design, holding form, but still straining bone.
The control of 100,000 filaments within a 6mm rod, makes it possible to change strength and stiffness at different locations in the structure in order to adapt to different needs at different levels of the spine. For example, in fusing a spine to treat adult deformity, there is at least an intuitive logic that the loads at the lower level of the spine are greater than the upper thoracic level. The vertebral bone size difference between the lumbar and thoracic spine support this.
So does the robust surrounding ligaments that connect the sacrum and lowest lumbar vertebra to the pelvis. Rod implant breakage provides another design cue. It is more regularly found at the lower lumbar level than at the upper-thoracic in long fusions. From a design perspective, it is conceivable that more rigidity is needed at the sacro lumbar level than at the thoracic. This is certainly what many surgeons are asking. It has driven the development of alternative stiffnesses in hinge and PEEK rod systems.
But here comes the trouble for the spinal designers, surgeons and industry: if carbon composite fibre control allows strength and stiffness to be programmed independently from implant geometry and can changed in direction in ways not before possible, how stiff should the construct be?
“Well we are not really sure, but what we have had before is probably not right…” This summarises the general consciousness. It must be different; it is not quite right. It has to be safe and reliably used.
These are not the measurements that come from the department of mechanical engineering. In implant design circles, they have made eyes roll to the ceiling. Regulators, whose business is to keep things safe and predictable, are also uncomfortable. There is no quantifiable consensus of how stiff a spinal implant structure should be. Stiff enough for fusion. Not so stiff that it stress shields viable bone.
Using composite’s carbon fibre control to find a safe performance envelope
Carbon fibre control can be used to create new implant performance and establish a safe zone for exploration. Mechanical studies with different carbon fiber orientations showed that with the same ostaPek composite 66.6% carbon fiber to 33.3% PEKEKK matrix,4 it was possible to make composites with stiffness approaching the extremes of either titanium or PEEK. The performance of these materials can be used as known delineators of safety and predictably. Stay within titanium and PEEK stiffness boundaries; program for stiffness that approached titanium for Correction but a bit more elastic for Stimulation.
These composite fibre orientations were tested using the traditional in vitro test models. Constructs performed with strength and fatigue at levels considered acceptable in the literature, including the progressive configurations with more stiffness at the lower lumbar curves than those at thoracic levels. These values were used to build a performance envelope that stayed within the accepted safety levels. Molding methods were created to form the composite rods in pre-defined curves that would fit surgeons’ needs for corrections.
Development in time
It has taken 15 years to bring composite rods to their present level of performance in a surgically usable form. Surgeons began with short fusions and progressed to treat thoraco–lumbar deformity. Early data on these series show fusion levels similar to metals, but so far an improved resistance to fatigue. More follow up will determine if the hoped-for improvement of bone quality will be observed at the long term.
Some problems remain. Working with composite brings a subtle change in the surgeon’s technique. Composite rod instruments used to place the implants must adapt to find a compromise between needs for the composite and surgeons habits. Rods must be pre-curved and if the requirement to change a rod curvature in surgery is rare and different rod forms can be substituted at all times, it takes the surgeon a little more planning to become comfortable with something they cannot modify once implanted. Finally, as Professor Max Aebi mentioned at his summary remarks on a conference about new, less rigid pedicle fixation in Bern, “our clinical science must catch up to this new technology….”5
The future with programmable composites
Exactly how stiff should a spinal fusion be over time? If the question will require many years of work to answer, long fibre carbon composite brings a tool to address it. A new range of performance criteria is emerging for fusions: emphasis on the long term Biology questions in the CSSO design trade off: Correct, Stabilise, Stimulate and Observe bone. Implants designers are being asked to consider the stiffness of their designs to provide the strains that not only initiate healthy bone, but help it endure. Carbon fibre composite fibre allows new performance to be programmed within a safety envelope between PEEK and titanium. This allows surgeons to explore fusion surgery safely with the hopes of producing reliable way to create a bridge of bone that lasts. Continued clinical research will evaluate this, as implants change.