SINUX (Sinitec)

Jan Zoellner

In Vitro Investigation of Lumbar Disk Implants (SINUX)

Degenerative changes to the spinal column and disk-related disorders of human beings were described in antiquity by Hippocrates (460–377B.C.) as hip pain. Even before the age of 30 people begin to suffer degenerative changes of the intervertebral disks, and four out of five people suffer from back pain at some time in their lives. However, 80 to 90% of all cases of acute backache clear up within 6 weeks regardless of whether they are treated or of the type of therapy received. The disk is the largest unvascularized structure in the body. In children, disks have a water content of up to 90%, but as age increases, this figure falls to less than 70%. The solid constituents of the disk, such as proteoglycans, form up to 50% of its dry weight. The water is not free, rather being bound to structural components of the macromolecules. In vivo, the pressure in the nucleus pulposus is generated via the osmotic gradients of the proteoglycans and the water-binding molecule hyaluronic acid. With the exception of organ pain, pain syndrome in the lumbar spine region is generally due to premature disk deterioration, particularly at L4–L5 and L5–S1. Atrophy and compression of the disk lead to loosening of the vertebral motor segments. Degeneration involves the disturbance of fluid transfer due to sclerosis of the end plates and basal plates. As a result, the disk loses more and more water and the intradiskal pressure falls, leading to reduction in the intervertebral space. Kolditz et al¹ were able to show that the disk loses water under increasing strain, and in consequence the osmotic gradient increases until equilibrium between osmotic and mechanical pressure is reached. When the strain on the disk is relieved there is a corresponding influx of fluid. The normal intradiskal pressure (L5–S1), without any additional external load, has been put at up to 5 atm (500 kPa).² In their paper, Wilke et al³ showed that, under unfavorable load when lifting heavy weights, the intradiskal pressure in segment L4–L5 can rise as high as 2300 kPa.

In an intact segment the outer fibers are aligned with the longitudinal ligaments, whereas the fibers in internal layers are arranged at a 60 degree angle.⁴ This orientation is the principal factor producing torsional strength.⁵ An important element in the degenerative process is the microsystem for transporting matter via the end plates. As the disk becomes drier, the end plates transform into a sclerotic barrier. The segment reduces in height, with attendant parallelization of the type I collagen fibers in the region of the external annulus fibrosus. This parallelization causes the segment to lose torsion resistance.

This process may be regarded as a vicious circle. The degeneration of the nucleus pulposus leads to reduction in segmental height and accompanying loss of torsion resistance. This, in turn, entails an increase in the segment’s physiological mobility and accelerated degeneration. Degeneration involving radial fissures in the annulus fibrosus can lead to a prolapsed disk and a subsequent need for nucleotomy. In this vicious circle the segmental degeneration and the nucleotomy are intimately linked because the nucleotomy has a similar effect to the degeneration just described. However, unlike that process, the changes after nucleotomy can proceed at a significantly faster rate, so that mobility compensation mechanisms such as osteophytic bone spurs, which produce a secondary reduction in mobility, do not cut in until later. The removal of disk tissue reduces the intradiskal pressure and the intervertebral space, thus increasing segmental mobility. This process causes considerable wear of the vertebral arch joints, which can lead in turn to significant symptoms. The pain caused by these degenerative symptoms, such as narrowing of the spinal canal, is often difficult to distinguish from pain resulting from disk problems. Furthermore, if age leads to a natural stiffening of the segment the symptoms will rapidly diminish. However, all too often this process takes many years and is not complete until advanced old age, or indeed it may not occur at all. Meanwhile, as long as mobility is retained the accompanying pain can be expected to persist.

In the presence of the relevant indications, lumbar nucleotomy may be performed. However, the procedure is associated with a significant rate of postoperative problems. A meta-analysis of the literature conducted by Schulitz et al,⁶ involving a total of 20,148 patients, found rates of poor postoperative outcomes ranging from 7 to 27%. These postoperative problems in connection with disk surgery are often described either as postdiskectomy syndrome (PDS) or as failed back surgery syndrome (FBSS). Two major causes of these problems are postoperative instability and postoperative recidivism, although they may also be due to narrowness of the recessus lateralis, epidural fibrosis, arachnoiditis, facet syndrome, operating on the wrong segment, or poor surgical technique. The changed postoperative mobility of the vertebral motor segment is a significant causative factor for the occurrence of postoperative pain.7 Increasingly, diskectomy was found to lead to decreased height of the intervertebral space and reduced intradiskal pressure. White and Panjabi⁸ describe a variable pivot in the dorsal region of the segment. Increasing instability can lead to a displacement and widening of this pivot, which means in turn that this increase causes complex changes in the overall biomechanical structure. Because there is no precise definition of the term instability in relation to the vertebral motor segment, this state of affairs is better described as “abnormal mobility” or “increasing mobility,” and most closely resembles the situation after a nucleotomy. Others, though, describe the postnucleotomy situation as an increased range of motion (ROM) between adjacent vertebral bodies.

The field of spinal surgery, and in particular the surgical treatment of degenerative disease of the lumbar spine, is currently undergoing a paradigm shift. Both disk operations in their technical variants and fusion methods are being viewed in an increasingly critical light. In a leading article for Scientific American entitled “New Thinking about Back Pain,” the author notes the minimal correlation of pathological findings and imaging techniques, of the simultaneous frequency of back pain syndrome and the consequent problems in determining whether surgical or conservative therapy is indicated.⁹ A new “old” concept involves the development of a flexible disk implant, which aims to stabilize the affected segment while also reconstructing the physiological mobility of the vertebral motor segment. The principal requirements of a disk implant are well-functioning biomechanical reconstruction of the intervertebral space, good biocompatibility, and long-term load resistance coupled with ease of handling conducive to implantation via minimally invasive methods of surgical access.