SPINE SURGERY-PROBLEMS AND INNOVATIONS. ARTICULATED LUMBAR CAGE.

Author: Munir Elias. M.D, Ph.D. in neurosurgery and human neurophysiology.

Abstract
Background:
Spine surgery is the most common problem facing the neurosurgeon and certain group of orthopedics. It consist over 90% of the activity of the general neurosurgeon in private sector. The most important and most common operations are the lumbar disc surgery and among the most common complications are recurrent disc and discitis. Recurrent lumbar disc, which needs reoperation is around 3-18%. Recurrent cervical disc surgery is almost near zero. All the available today methods to resolve this problem are not acceptable, because transpedicular fixation of the segment have its complications and negative drawbacks by itself. A trail to perform bilateral cleaning of the disc space increase the rate of recurrence. Bilateral cleaning with insertion of 2 TLIF cages from each side also dynamically have its hazards. The below mentioned data and suggestions are the answer to provide the best solution to make the lumbar disc recurrence around zero as in the cervical area without causing further bony destruction to the surrounding anatomical structures. 

Methods:
Since in the industry, there is no such device, the author start to acquire engineering knowledge about the industry of spinal instrumentations and the biomechanics of the lumbar spine and using software such as SolidWorks, Autodesk Inventor and ORS Visual to resolve the problem. Starting from August 2016 up to now, several versions of cage construction were improvised and improved over more than 40 versions. The cage must be MRI compatible without artifacts, slim, not migrating and have several fixating points within the disc space and providing the normal lordotic anatomy of the area.    

Results:
More than 300 cases of the lumbar area were sent for CT-scan L2-S1 with thin continuous cuts and using ORS Visual software and area was studied with accuracy. Each segment of the the lumbar area have its unique characteristic, but L2-3, L3-4 and L4-5 share some similarity. L5-S1 have a little different characteristics. The lordosis in these segments was ranging from 6 to 14 degrees. The inner shape of the disc space had some depth around the elevated center in the inferior wall of the disc space. This trough is around 30 mm in diameter from the outer circumference for most of them with tiny deviation.

Conclusions:
Lumbar articulated cage with double self-locking mechanism is the most suitable solution to minimize the postoperative lumbar disc surgery recurrence rate without the need to violate further anatomical structures.

Keywords: Articulated lumbar cage, recurrent lumbar disc, neurosurgery, spine surgery

INTRODUCTION

Neurosurgery and spine surgery have a lot of challenges. Recurrent prolapsed lumbar disc requiring surgery, is among the most frequent problems annoying the life of the patient and the neurosurgeon, for what many articles were written and among them by myself. ¹ 
A group of 18 patients were operated by me with insertion of Medtronic Satellite sphere ended with complications, for what it was abandoned. In retrospective analysis of this group, the sphere cause avascular necrosis of the upper and lower vertebrae. It then creating tunneling inside the disc space, permitting the sphere to migrate inside the disc space with in these tunnels and the patient coming with different scoliotic posture every postoperative visit. Insertion of TLIF PLIF ALIF must be accompanied with transpedicular fixation of the adjacent involved segments. Some authors suggested insertion of 2 TLIF from each side after bilateral cleaning of the disc space.² The suggested trabicular cage has many disadvantages, among them, no respect to the lordotic nature of the area and no connection between the 2 devices and  and massive MRI artifacts.

Pedicular screw fixation for simple prolapsed lumbar disc is relatively unacceptable to prevent the recurrence and the complications with some authors reach up to 54%. ³  The improvement of the pedicular screws will not be acceptable in simple lumbar disc prolapse, even if the complications reach 0%, because this technique will violate further anatomical structures.

Schmorl's nodules have common place in the vertebral spine and some authors claim it could cause back pain, for what some surgeons perform transpedicular fixation of the involved segment.⁴  With the 40 years experience of the author, this kind of herniation mostly is not the cause of pain by itself, but the disturbance of biodynamics of the segment can be the cause. I never paid attention to this kind of "pathology", because it is a frequent finding and most of the patients coming with other causes of their essential pathology. This fact was mentioned, because most of the Schmorl's nodules are central in location and the cage will avoid their location.

For pain management of the radicular pain, the author have now wide experience with application of bipolar mode radiofrequency to the involved roots during discectomy and it yield a good result.⁵  

In high index of suspicion for discitis or presence of infection in other parts of the body, the patient usually is treated for his infection before surgery. Despite this fact infection will resume even several weeks after surgery in some cases, necessitating long term treatment with antibiotics. The insertion of the cage with the bone graft and the area aided with Vancomycin powder could decrease this possibility. The escalation of osteomyelitis of the adjacent vertebrae could enforce the surgeon to remove the construct, for what the device was constructed to be easily removed.

MATERIALS AND METHODS
 

Anatomical background:

The disc is a fibrocartilage that lies between bony vertebral bodies, conferring flexibility, load transfer, and energy dissipation to the spine. It is comprised of the central gelatinous nucleus pulposus (NP) surrounded circumferentially by the annulus fibrosus (AF). The hyaline cartilage endplate forms an interface between the disc and adjacent vertebral bodies. The NP is structurally and mechanically isotropic and contains a network of type II collagen interspersed with proteoglycans, resulting in a high water content within the tissue. The osmotic swelling that results is a defining feature of NP mechanics. Each lamella of the multi-lamellar AF consists of highly aligned collagen fibers whose orientation alternates above and below the transverse axis of the spine by approximately 30° in adjacent lamellae. While the AF can be approximated as an angle-ply laminate ring, its true architecture is more complex: lamellae are circumferentially discontinuous and traversed by fibrous elements that run radially outward.⁶  The authors in their article mentioned the importance of the annulus fibrosis and nucleus fibrosis and described in detail the biomechanics and bioengineering and they mentioned the following: Fusion is the surgical standard for the treatment of axial low back pain. This treatment is highly invasive and is intended to stop pain by eliminating motion across the joint space. Despite the frequency of its practice, fusion often fails to alleviate pain and may accelerate degenerative changes in adjacent discs. Total disc arthroplasty is a recently approved surgical option that aims to maintain segmental motion; however, its long term efficacy has not been established and mechanical wear may challenge its the long-term success.

Fig-1: Normal anatomy of the lumbar spine: midsagittal section.

The mean thickness of the end plate of the lumbar disci was 1.03±0.24 mm for cranial (to disk) endplates and 0.78±0.16 mm for caudal endplates. For lumbar intervertebral disks, the cranial endplate was significantly thicker and denser than the caudal endplate (p<0.001-0.05). Thickness and BMD of endplates were independent of age. Based on discography, a trend of more severe disk degeneration associated with greater thickness in both the cranial and caudal endplates was observed, and was most marked in severely degenerated disks (p<0.05). However, no evidence was detected for a link between more severe disk degeneration and elevated endplate BMD (p>0.05).⁷ 

Biodynamic background:

The lordotic curve of the lumbar spine is around 50±10 degrees according to different factors. It is supposed to be equal to the pelvic incidence which is fixed and equal to the sum of sacral slope and pelvic tilt. These data are of concern when dealing with multilevel fusion and presence of gross deformity of the spine with discrepancies of the pelvic tilt exceeding more than 25 degrees.⁸  Considering these data the acquisition of the intradiscal configuration is more logic than taking these data to decide the degree of lordotic degree of the construct. Never the less, the lumbar lordotic curve is more heavily dependant at the L4 to S1 levels, constituting 75% of the global value of the lumbar lordosis.

Patients

Imaging

Fig-2: Measurements taken for L4-5 disc space in one patient.

Fig-3: According to these data obtained by ORS Visual software, the patient needs a cage total width 30 mm, 10mm height with 6 degrees lordosis.

Fig-4: This case needs articulated cage 32 mm total diameter, 11 mm height and 12 degrees lordosis according to the ORS Visual data.

RESULTS


DISCUSSION
.
Advantages of this technology:
1. The recurrence rate will go down almost to zero, because the device will aid support to the height of the disc space, preventing the outer most part of the annulus fibrosis to slip to the vertebral canal.
2. Restoring of the lordotic alignments and the height of the collapsed disc space will augment the degree of postoperative recovery. The LBP and radicular pain will be less than the standard methods.
3. Working in fresh area is more easier than working in postoperative recurrent disc with massive scars.
4. In certain cases the disc space height is different on both sides. This technology will correct this problem.
5. There is no need to perform transpedicular screw fixation, because the construct will yield this mission from within the disc space with the use of the 2 lockers embedded in the articulated cage. They have about 1.5 mm depth blade, sharp from all edges to minimize surgical trauma to the endplates,
6. There will be no migration, nor loosening of the construct, since it is placed at the anatomical groove in the disc space, avoiding by this the escalation of avascular necrosis.
7. Posterior migration of the construct will be prevented by the lordotic configuration of the construct. Anterior migration will be prevented by the lockers and presence of some curve at the most anterior part of the construct.
8. The MRI artifacts will be minimal, since the titanium lockers are small and far from the dura.
9. In retrolisthesis, this technology can be used and can resolve the retrolisthesis, by reducing the height of the disc space and distracting the adjacent facets.
10. The harvested bone from the spinous process can be melted and inserted at the site of the cavities, were the lockers situated, avoiding during that the use of artificial bone graft and minimizing the postoperative infection, inflammatory reaction or rejection.
11. The slim configuration of the construct with maximum width 7 mm prevent dural force traction during insertion and keeping the isthmus and facets intact with most stenotic anatomy of the area of interest. The usual TLIF has a width of 10 mm.
12. The annulus fibrosis will be preserved, and can provide its function at least partially.

Limitations and disadvantages of this technology:
1. This technology is not suitable for spondylolisthesis with gross overmobility. In these cases transpedicular screw fixation of the mobile segments is mandatory.

Footnotes
Disclaimer: The author of this paper have received no outside funding, and have nothing to disclose.


REFERENCES
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