Prof Dr Klaus Schaser
Charité Hospital, Berlin, Germany
Dr Alessandro Luzzati
Istituto Ortopedico Galeazzi, Milano, Italy
Dr Paul Fayada
Hôpital de la Roseraie, Paris, France
Mr Robert Lange*
Composite Implant Developer,
Zurich, Switzerland
* Corresponding author:
Utoquai 43, 8008 Zurich, Switzerland,
Almost one out of two people in the industrialised world will experience lumbar pain each year.(1) Many come for professional consultations. In the United States alone, and at a cost of 33 billion dollars,(2) 400,000 will receive spinal fusions (arthrodesis). Here two or more vertebrae are fixed and made to fuse together with the objective to relieve pain and restore function. As a population, we are growing older and living longer, so these numbers and costs will increase. The spinal surgeon who uses implants to perform this spinal fusion is faced with a dilemma of patient needs that is as old as spine surgery itself.
On one hand, patient needs in the short-term are mechanical: correct and stabilise the spine to start bony fusion. Here, the implant must first be strong. On the other, in the mid-to-long-term, patient-needs are biologic. Living bone must then grow between the vertebrae so that it takes over the implant’s initial stabilisation duties. Ideally, this stabilised spine and bone should receive multiple weight-bearing loads to create the ‘osteogenic strains'(3) that regulate bone’s formation and renewal. The stabilising implants that first had to be strong cannot be too stiff. An implant (or any other reconstruction) that is too rigid can prevent the development of osteogenic strains known to promote bone formation,(4) in this case for intervertebral fusion. For the mid-to-long-term biologic patient needs, the implant must allow enough deformation to strain surrounding bone.
Furthermore, as they age, spine fusion patients will return from time to time for consultation and treatment. Health care practitioners must regularly perform radiologic examinations to control implant positioning, restoration of spinal alignment, correction of deformities, effects of decompression and last but not least the result of the intervertebral fusion. Very often such serial radiological studies are also required because of recurrent symptoms. This can be caused by either progression of disease in the adjacent segments or because of insufficient intervertebral fusion that ultimately increases the risk for implant failure. Independently from the use of CT and/or MRI scans, metal implants (cages, rods and screws) create image artifacts that profoundly impair image quality. This interferes with the ability to detect and specify a diagnosis.
Implant material choices
Industry equips the spinal surgeon most often with two types of implant biomaterials to perform spinal fusions. These are metals (titanium and steel) or plastic, (often a synthetic, high crystalline thermal plastic PEEK or PEKEKK.) Each has performance more appropriate for one side of the spine surgeon’s dilemma between the patient’s short- and mid-to-long-term needs.
Metals (titanium and steel) are intrinsically strong, which is good for correction and stabilisation. But as the vertebrae start to fuse later on, metals become less suited for building and sustaining living bone. Strong metal can be too stiff, carrying too much of the load that a bone should ultimately carry, preventing the living tissue strains that bone needs to initiate and maintain its formation. Over the years, a too-stiff metal implant can erode the strength of the bone it was supposed to stabilise in a phenomenon called stress shielding. When X-rays or radiation therapy are later required, metals can obstruct diagnosis and treatment.
Plastics like PEEK are radiolucent, so soft tissue and bone can be seen through and around it in CT or standard X-ray. Yet under regular biomechanical loads, plastic materials tend to creep and loose their corrective form, as in the case when they are used in implants such as pedicle fixation. This makes PEEK plastic less suited to hold spinal stabilisation and correction.
In many situations, the choice of implants made from either metals or PEEK will force the surgeon to favour one set of patients needs to the detriment of the other. Performance limitations in metal and plastic material have lead engineers to deploy composite implants as a design solution more tailored to living bone in spinal fusions. For over 18 years, long fibre carbon composite continues to be used in an ever-growing number of spinal applications.
Strength and flexibility: the nature of ostaPek long fibre carbon composite
ostaPek long fibre carbon composite is in fact two materials: long fibre carbon in controlled orientation and a matrix of PEKEKK. Technically called a LCFRP (long carbon fibre reinforced polymer,) carbon strands comprise 66.6% and the encapsulating matrix PEKEKK 33.3%. The high carbon-to-plastic matrix ratio is used to achieve maximum strength.
A strand of carbon fibre alone is stronger than steel, but brittle when bent. Plastic PEKEKK, when not reinforced, bends readily but will not hold its form unless made to thicker dimensions that are not always possible in anatomical constraints of the spine.
Carbon and PEKKEK, when brought together in a purposeful orientation of the carbon strands, borrows one material property from the other to create a dual nature material. Its bi-material structure provides implant properties that are new, such as an increase in both strength and flexibility that would be a contradiction in traditional metals or plastic.
ostaPek long fibre carbon composite has also been shown to have an osteo-conductive surface(5) and is completely radiolucent. This means bone and soft tissue can be seen adjacent and through the implants for subsequent radiographic evaluations and treatment.
These material performance qualities make ostaPek carbon composite particularly suited to the opposing needs of spinal fusion and for what scientific literature has called a better balance between the short-term stability (implant strength) and the mid-to-long-term biology, where an implant and bone construct must be less rigid.(6)
How composites function
To understand how ostaPek carbon fibre composite works, it is useful to think of the carbon fibre strands as filaments in a rope. The filaments are oriented to be in tension along the directions where strength is desired. Strands of the rope alone, however, will not keep a supporting form; for this, a matrix must be added. The ostaPek thermal plastic matrix holds the carbon strands in place when load passes through the carbon fibres, while at the same time, allows a tiny bit of matrix stretching between each fibre. This brings more flexibility and spring-like qualities. The engineer can change the orientation of the carbon fibre at different spots in the implant to make it stronger, stiffer or more flexible according to the requirements for treatment. Thus ostaPek carbon fibre orientation in PEKEKK matrix can be used to obtain both strength and flexibility in hopes of better addressing the needs of bony fusion. That is strength for short-term stabilisation and flexibility for mid-to-long term strains in biology.
Using controlled fibre orientation, the engineer can even program different strengths, stiffness and flexibilities at different locations within the same implant without changing the outer geometry. This enables the surgeon to control implant mechanical performance at different levels of the spine in a manner most suited and tailored to the patient’s needs.
Architectural optimisation
Nature teaches us how effective composite strategies can be and at the same time gives clues for new implant design. In living structures that bear load, from the branch of a tree to a section of bone, the dominant internal architecture is purpose-built and oriented according to loads they must support. Because they are living, the strains upon them initiate new structures, guiding their direction and assuring that new supporting cells replace old ones. In bone, an internal structure that provides such support is called a trabeculae. It works like a tiny beam or strut with a spatial orientation exactly as biomechanically required. Trabeculae contribute to bone’s strength and flexibility. In living bone, they actively align themselves throughout life according to the stresses and strains they experience in a manner now called Wolff’s law.(7) The surgeon sets out to form bone trabeculae in a spinal fusion. The engineer attempts to design implants with properties that favor their formation.
In composite implant design, the optimal carbon fibre orientation for an implant is often similar to the dominant trabeculae of bone. These guide the engineer to build composites for a specific location in the spine. To provide optimal strength and flexibility, the implant designer uses ostaPek composite’s bi-material design to follow nature.
Application 1: interbody vertebral cages
With their dominate fibres oriented like trabecular bone in a vertebral body, interbody fusion cages made from ostaPek long fibre carbon composite can be made strong and tall to replace several levels, but with thin walls to make a device that is both flexible and has a large inner volume for uptake of bone graft. Engineers can maximise this cage architecture for more viable bone graft to enhance osteogenesis,(8) yet with planned flexibility for osteogenic strains. Large bone ports in a slightly flexible interbody cage structure are particularly useful for reconstructive corpectomy and tumour surgery.
Application 2: posterior pedicle fixation
Pedicle fixation rods can be made in ostaPek composite to be sufficiently strong for scoliosis correction, yet flexible, in order to store and release energy, somewhat like a micro shock absorber that flexes under load and then returns to form. It is hoped this design will provide osteogenic strains and will better protect the anchoring screws from dislodging and eventually, the adjacent segment from progression of degeneration. ostaPek composite rods are now being used in progressive stiffness, with the stiffer section at the lumbar spine and the less stiff in the thoracic region, to perform spinal fusions for degenerative scoliosis.
Application 3: ostaPek carbon composite and tumour surgery
The needs of oncological treatment in spinal tumours can require a surgeon to remove one or several entire vertebrae en bloc within healthy tissue margins.(9) This is an ultimate surgical destabilisation that requires subsequent implant fixation that will safely hold the spinal column in place. Today, as the ability to select patients and treat cancer improves, so does the rate and time for survival. These cases mandate both initial implant stability and the subsequent bone formation that will maintain the spine through life.(10,11) Anterior column support is provided by an ostaPek Vertebral Body Replacement, filled with the patient’s bone. Posterior stability is provided by titanium screws that are visible, and ostaPek composite rods that are not visible under normal X-ray. The goal is to stabilise, increase the area of bone for fusion, allow loading, and to see the fused bone.
Nuances and limitations in surgery
Some ostaPek carbon composite qualities preclude certain surgeries or require some special training. Composite structures cannot be reformed or bent in surgery like metals. The surgeon must adapt to preformed composite implants. On the rare occasion when this is not possible, such as a complex scoliosis curve with pedicle fixation and rods, a titanium rod component is used. Spinal surgeons new to composite will feel a generous elastic zone, somewhat like a spring. But unlike metal, when deformed beyond the elastic limit, the implant can split. This is readily detectible in surgery by a sound and the implant is replaced. This occurrence becomes rare with experience and training. When a surgeon experienced with composite uses a slower alignment procedure, employing the spine’s viscoelastic properties as an advantage, ostaPek composite rods can be used to correct and stabilise extensive scoliosis deformities. However, now that ostaPek’s biomechanical and radiolucency properties have been shown, the ongoing clinical studies must answer what this brings in the very long term. This is a question that comes with all new technology.
Conclusions
ostaPek composite enlarges the range of solutions for the spinal surgeon performing a spinal arthrodesis: maximised volume for viable cells(8); osteo-conductive surface(5) in flexible fixation(6) that is designed for osteogenic strains(3) to form bone in a biologic fusion. ostaPek composite brings radiolucency. This envelope of performance allows correction and stabilisation with small changes in technique.
If an 18-year clinical history should be considered as relatively new in medicine, ostaPek’s growing applications and use continue to support that carbon fibre composite will find a wide acceptance in spinal surgery.
References
- Waxman R et al. A prospective follow-up study of low back pain in the community. Spine 2000;25(16):2085–90.
- Rajaee SS et al. Spinal fusion in the United States. Analysis of trends from 1998 to 2008. Spine 2012;37(16):67–76.
- Frost HM. From Wolff’s law to the mechanostat: a new ‘face’ of physiology. J Orthop Sci 1998;3:282–86.
- Claes L. The mechanical and morphological properties of bone beneath internal fixation plates of differing rigidity. J Orthop Res 1989;7(2):170–77.
- Sigot-Luizard MF. Biological evaluation of the Osta-Pek (Carbon-Pekekk) composite used in spinal surgery. J Vertebral Pathol 2000;12:1–5.
- Perren SM. Evolution of the internal fixation of long bone fractures. The scientific basis of biological internal fixation: choosing a new balance between stability and biology. J Bone Joint Surg 2002;84-B:1093–110.
- Wolff J. Das Gesetz der Transformation der Knochen. Berlin: Verlag von August Hirschwald, 1892.
- Togawa D et al. Bone graft incorporation in radiographically successful human intervertebral body fusion cages. Spine 2001;26(24):2744–50.
- Melcher I et al. Primary malignant bone tumors and solitary metastases of the thoracolumbar spine: results by management with total en bloc spondylectomy. Eur Spine J 2007;16(8):1193–202.
- Druschel C et al. Surgical management of recurrent thoracolumbar spinal sarcoma with 4-level total en bloc spondylectomy: description of technique and report of two cases. Eur Spine J 2012;21(1):1–9.
- Disch AC et al. Oncosurgical results of multilevel thoracolumbar en-bloc spondylectomy and reconstruction with a carbon composite vertebral body replacement system. Spine 2011; 1; 36(10):E647-55.
