Surgeons now use long fiber carbon composites to address the challenges of long degenerative spinal deformities – a change in technology, surgical technique and training
Dr Paul Fayada
Centre Nollet
Paris, France
Dr Steve Hansen
Luxembourg
Mr Robert Lange
Composite Implant Developer,
Zurich, Switzerland
Each week they can be found in the spine surgery clinics of the world: the elderly adult patient with degenerative scoliosis and kyphosis. Their spines are deformed, a source of pain so acute they can hardly walk out of the hall and back. They approach their surgeon with the hope that treatment will stop progression of their disease, return them to active life and perhaps restore their normal posture. Their desire can be to carry groceries, climb the stairs or, as one father put it, ‘It is my daughter’s wedding. I must walk her down the aisle standing straight’.
Their numbers are increasing. As our population lives longer (one of our triumphs in public health), we share the growing duty to treat adult degenerative deformity, which at times calls for spinal surgery.
A destructive cascade
Most degenerative deformation is described as scoliosis and kyphosis. Scoliosis is derived from the Greek word skolios, meaning crooked, and describes an intervertebral rotation and side-to-side curvature of the spine. Degenerative scoliosis occurs as a chain of destructive events that, over time, pass through the many articulations that comprise the human spinal column.(1) For example, as we age, lumbar intervertebral discs degrade and collapse. Encouraged by an asymmetric muscle contractions in the trunk, the lower vertebra can tilt and turn to one side, creating a vertebral segment that is also asymmetric and not parallel to the horizon. In the spinal column, what is tilted below skews everything above. The trunk translates to the side and the patient is out of balance. To compensate, the patient contracts their muscles to bend the upper portion of their spine sideways in the opposite direction, back to centre. This produces the scoliotic curve.
Degenerative kyphosis, also from a Greek root meaning bent, describes a thoracic spine that is abnormally bowed forward, due to a loss of anterior support. For example, several vertebral discs can all collapse. A vertebra can also lose its mechanical integrity from osteoporosis, then fracture or settle. Some of these patients can barely stand and can only walk forward while looking at their toes.
In degenerative scoliosis and kyphosis, the spine is overloaded in its struggle to stay upright. From the outside of the deformity curves, muscles incessantly strain to straighten the spine. At the same time, the spinal articulations at the inside of the deformity curves are compressed and stressed beyond their physiologic limits.
The spine’s asymmetric overload works over the years to destroy its functioning structures. Compensatory muscles continuously contract. Through degeneration, the posterior joints, intervertebral discs and even the vertebral bodies themselves become inflamed and arthritic in an attempt to stabilise the spinal column. This can cause muscular pain and mechanical pain from the grinding of bone where an articulation is lost. Nerves can be blocked (yet another possible source of pain) or the cause the loss of sensation and muscle function.
The spine with degenerative deformation becomes locked out of balance, more painful, its muscles over straining to prevent the progression of the disease.(2)
For some patients, this is a minor condition that is stable, hardly noticeable and unfelt. For others, degenerative spinal deformity is an agony of years, growing in severity and debilitation.
Most patients are conservatively treated, for example, with non-steroidal anti-inflammatory drugs rehabilitation and/or bracing. The more acute cases resisting other methods are treated with long spinal fusion surgery, where several vertebrae are fused together to form one continuous bone with the help of spinal implants. These are most often rod and pedicle screw fixation anchored to the posterior bony elements and intervertebral cages that provide anterior column support.
Long spinal fusion for degenerative deformity
The surgeon seeks to reverse the curve and prevent its progression by performing a long spinal fusion. The goal for the patient is to relieve pain, and to restore spinal balance with less deformity. Nerve roots are decompressed. Portions of the posterior joints are trimmed to unlock and then remobilise the spine so it can be corrected. Cages filled with bone are placed where anterior disc has been removed, in order to align, level and stabilise a vertebral body in relation to the others. Screws and hooks are attached to posterior vertebral bone and connected with rods and bone graft that span the entire construct. All this is designed to hold the instrumented vertebrae in place so they may grow together, forming a stable column of living bone, within a surgically balanced spine that prevents strain of the muscles.
It is a major intervention that challenges the spine surgeon’s skill, technology and art.
Surgical challenges
A long spinal fusion for degenerative deformity is, in fact, several operations performed on several joints, in one surgery. The first question is: from where does the pain come? Is it muscular, mechanical instability of a vertebra or the impingement of a nerve? Is it a combination of all three? What vertebral levels are the sources of pain? In a spine that is degenerative from thorax to sacrum, all eighteen levels must be considered.(3) A vertebral motion segment or level has three joints each: the anterior disc and two posterior facet articulations. Any can generate pain or cause neurological deficit. Is surgical decompression required? Can the arthritic spine be made mobile enough so it may be re-balanced in surgery?
Then comes the matter of the levels to instrument and fuse. From where is it safe to begin and end? Too few levels fixed will accentuate imbalance, worsen the deformity and increase rigidity and pain. Too many levels blocked will prevent proper spinal function.(4) The elderly patient’s bone is soft, compromising screw and hook anchorage that is more likely to pull out after surgery. The longer the fusion, the longer the lever-arm at the extremities of the implant construct. With each level fused, the greater the loads upon the implants anchored to bone. With this comes the risk for implant pullout and pseudoarthrosis. Finally, if the surgeon must extend fixation construct to the sacrum, where the bone is porous, pullout and implant breakage become more prevalent.(5)
The long fusion construct is a challenge to implant resilience that is driving the quest for improved material performance.
Long spinal fusions: the limits of metal
Structure
Traditional spine implants are made from titanium and steel. They have set today’s standard of care for thousands of successful fusions. But metals are monolithic and crystalline in structure, and therefore subject to cracking in fatigue. Over the months after surgery and prior to bony fusion, the loads at the end of the long fusion construct are extreme. Rod breakage from fatigue is still too common in the lumbar area or at the extremity of a long fusion construct.(6)
Stiffness
The strength of a metal implant that holds spinal correction can be at odds with the needs of living bone tissue. Spinal implants that are strong enough, yet too stiff, can cause what is called implant induced osteopenia (loss in bone density) and result in a poor quality fusion. This is when a rigid implant prevents the osteogenic strains through bone that are required not only for fusion, but for bone renewal over time.
The spine’s varying needs at different levels
The mechanical requirements for implants can change at different levels of the spine. In a fusion construct that must span from the lumbar spine to thorax, many are convinced that the implant should be less flexible at the lumbar base and more flexible at the upper thoracic region. This is difficult to achieve in metal, without complex geometry or connections that could further complicate the intervention or create a new potential for implant failure.
The limitations of metal for spinal fixation have compelled the surgeon to explore long fiber carbon composite for the treatment of degenerative spinal deformity.
ostaPek long fiber composite adapts to deformity
Over the past 19 years, surgeons and industry have expanded the applications of ostaPek long fiber composite. Intervertebral ostaPek composite cages were first used to replace the disc and later evolved as the treatment for vertebral body replacements after en-bloc resections of spinal tumours.(7) In pedicle fixation of the posterior elements, ostaPek composite plates and titanium screws have been used to perform short fusions since 2005. Recent advances in composite manufacturing, combined with clinical experience, have enabled surgeons to adapt ostaPek composite rods to the complex forms and the biomechanical requirements that are needed to treat degenerative spinal deformity. This has driven change in ostaPek carbon composite design, form, surgical strategy and training.
Changes in material
ostaPek composite combines two intrinsic materials (carbon fibers and a PEKEKK polymer matrix) to give an implant elastic strength and fatigue properties not possible in monolithic materials such as titanium or steel. Pure carbon fibers are woven into strands that are aligned to make their dominant longitudinal axes follow the spinal implant rod. Carbon fibers are also crosshatched in strategic combinations to resist spinal loads in tension and meet the implant designer’s needs for strength and elastic displacement.
Under great pressure and heat, a PEKEKK matrix is polymerised and made to encapsulate each carbon fiber. The resulting ostaPek composite is 66.66% carbon fiber and 33.33% PEKEKK matrix, a high-fiber content whose strands are controlled to make a resultant material that is not only strong, but more elastic and resistant to fatigue than titanium. When the implants are subject to a load, the carbon fibers become taught like the strands of rope, while the PEKEKK matrix allows minute strand movement, still holding their general alignment. This microscopic displacement of the carbon fibers within the PEKEKK polymer matrix creates the material’s elastic capacity to move, absorb energy, return to form and resist fatigue.
Furthermore, through new fabrication techniques, the carbon fiber strand orientation can be changed in different locations within the same implant. This modifies mechanical properties within the same contiguous structure. It is now possible to tailor the mechanical needs of the rod for different regions in the spine, without changing the implant’s outer diameter. For example, using composite fiber control, an ostaPek spinal rod can be made less flexible but elastic at the lumbar region and then more flexible at the thoracic segment, without changing outer geometry. In this way, the surgeon can tailor one contiguous ostaPek rod with a uniform diameter to the different flexibility needs at the sacral, lumbar and thoracic spine.
More length, new S form
Deformity fusions tend to be long and their forms complex. In healthy patients, the thoracolumbar spine is curved to form an S. This allows humans to stand upright and see forward without excessive strain to the muscles. Restoring a degenerative deformation to balance requires composite spinal rods in long anatomic forms. This is a challenge in manufacturing technology. A series of long degenerative fusions instrumentation was studied retrospectively and used to design archetypical forms for the spinal rod in composite. New moulding techniques are now being used to build rods of sufficient length and form to accommodate the complex spinal curves.
Composite extends to sacrum
The degenerative deformity surgeon must often extend the instrumented fusion construct from the thoracic and lumbar spine to reach the sacrum, a place with its own complexity. Sacral bone is more cancellous and, in long spinal fusions, sacral screw pullout at the end of the construct can require another operation.(8) To prevent this, surgeons connect the longitudinal rods laterally, spanning the sacroiliac joint at a location called the iliac crest. This significantly improves fixation of the fusion instrumentation.
This subtle joint moves slightly with every motion of the spine. Thanks to the ligaments and muscles that band the pelvis to the sacrum, the sacroiliac joint absorbs forces between the ground and body. Locking this with metal, although improving the screw anchorage, disrupts the shock absorber role of the sacroiliac joint and to the patient can feel like a jack hammer.
Composite’s long carbon fibers can now be configured to allow an elastic rotation. These qualities can be used to span the sacroiliac joint with an ostaPek rod that connects to titanium screws in the iliac crest. This configuration is designed to improve the fusion construct’s anchorage to the sacral region, while better preserving the subtle movement and shock absorber role of the sacroiliac joint.
The coAlliance: teaching the composite touch
Like the pilot flying an aircraft, spinal surgeons must not only manage instruments and systems, but also develop a situational awareness for their new equipment through feel. There is a different touch in spinal surgery when using carbon composite to correct deformity. ostaPek composite’s elastic qualities feel more spring-like in the surgical wound. It is at first startling, even concerning to the surgeon of experience. But to the surgeon who becomes skilled in composite management during surgery, it becomes an asset for treatment. Composite’s spring can be anticipated and, as the surgeon secures the rod to titanium screws that are anchored in bone, felt during alignment manoeuvres. The composite rod continues to return to form after connections are tight due to the visco-elastic properties that are present in both the spine and ostaPek material. This can be planned as a more gentle correction. This elastic quality is also a consideration after the wound is closed.
The coAlliance now teaches composite skills for surgery. Based on aviation training in simulation, surgeons receive a clinical problem to be solved. In the pre-surgery brief, strategies, potential pitfalls and surgical sequences are discussed with experienced colleagues. The surgeon then simulates the intervention in the anatomy lab and under the guidance of the instructor. The team approach and safety of this setting enables the surgeon to explore and develop the feel for ostaPek composite that will later be deployed in long and complex interventions upon their patients.
Spinal surgery in practice, after all the new technology and theory, remains a hands-on art.
Conclusion
The ability to use our spines to walk and stand upright in part defines us as humans. The loss of this ability through degenerative spinal deformity can become the source of great distress, infirmity and pain. The challenges to treat the most acute deformities with spinal fusions have led surgeons to explore new materials for spinal implants. ostaPek long fiber carbon composite puts new implant performance into the surgeon’s hands.
References
- Hawes M et al. The transformation of spinal curvature into spinal deformity: pathological processes and implications for treatment. Scoliosis 2006;1:3 doi:10.1186/1748-7161-1-3.
- Ploumis A et al. A correlation of radiographic and functional measurements in adult degenerative scoliosis. Spine 2009;34(15):1581-4.
- DeWald C et al. Instrumentation-related complications of multilevel fusions for adult spinal deformity patients over age 65. Spine 2006;31(19):S144–51.
- Shuffelberger H et l. Debate: determining the upper instrumented vertebra in the management of adult degenerative scoliosis: stopping at T10 versus L1.Spine 2006;31:185-94.
- Kim Y et al. Pseudarthrosis in long adult spinal deformity instrumentation and fusion to the sacrum: prevalence and risk factor analysis of 144 cases. Spine 2006;31(20): 2329-36.
- Weiss H et al. Rate of complications in scoliosis surgery – a systematic review of the Pub Med literature. Scoliosis 2008; 3:9 doi: 10.1186/1748-7161-3-9.
- Disch AC et al. Oncosurgical results of multilevel thoracolumbar en-bloc spondylectomy and reconstruction with carbon composite vertebral body replacement system. Spine 2011; 36(10) E647-55.
- Kebaish K. Sacropelvic fixation techniques and complications. Spine 2010; 35(25):2245–51.
