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Section 12, Chapter 5: Motion Preservation for Lumbar Disc Degeneration

Prasanna K. Venkatesh, Catherine Moran, and Ciaran Bolger

INTRODUCTION

Low back pain is one of the most prevalent problems in industrialized countries, affecting nearly 80% of adults at some time in their lives.1 A variety of pathologies cause low back pain, most common among them degenerative disc disease. It has been postulated that annular tears, disc collapse, abnormal motion segment, and biochemical instability results in discogenic pain.

Goals of Motion Preservation Surgery

The goal of motion preservation surgery is to closely replicate normal or near normal biomechanics in an effort to restore patient mobility and minimize the development of clinically significant adjacent-segment disc disease.

Approaches to Motion Preservation Surgery

Dynamic stabilization devices

Total disc replacement

DYNAMIC POSTERIOR STABILIZATION

Introduction to Dynamic Posterior Stabilization

Spinal fusion is believed to accelerate the degeneration of the vertebral segment above or below the fusion site leading to adjacent segment disease (ASD) and pseudoarthrosis.2 The premise of dynamic stabilization is that motion preservation allows for less loading on the discs and facet joints at the adjacent non-fused segments. With dynamic stabilization, implants would stay anchored to the bone despite allowing movement and would provide lifelong stability of the joint. The advantages of these techniques are decreased adjacent level disease, avoidance of pseudarthrosis, no need for bone graft, and decreased surgical morbidity.

Historical Consideration

The Graf ligament, introduced in 1992 by Henry Graf, was first used in spine stabilization.3 It gained wide popularity during this period.4 Graf utilized braided polyester cords looped around pedicle screws to stabilize the spine. He advocated that a supporting posterior extension band was good enough in the treatment of instabilities. However, these implants encountered trouble in the form of loosening of the ligament with time, foraminal narrowing, and flat back.5

Considering the weak points and disadvantages of the Graf ligament, Schmoelz et al. developed the Dynamic transpedicular system (DYNESYS).6 This novel system was introduced to the market in 1994, promoting the concept of stabilization without fusion. In comparison to rigid pedicle screw fixation, the DYNESYS system offered more flexible and rigid internal fixation with preservation of motion in the adjacent levels. Schnake et al. evaluated elastic stabilization of the DYNESYS system following decompression for spinal stenosis with degenerative spondylolisthesis.7 They reported significant improvement in leg pain and increased walking distance with no radiological evidence of progression of spondylolisthesis. Subsequently, Stoll et al. reported the results of a prospective, multicenter study evaluating the safety and efficacy of DYNESYS in the treatment of lumbar instability.8 Stoll et al. reported significant improvements in pain and mobility. They concluded dynamic stabilization is a safe and effective alternative surgical procedure.8 Dynamic stabilization was found be very useful in preventing the segmental degeneration following discectomy. At mean follow up of 34 months, Putzier et al. reported significant increases in Oswestry Disability Scores and the Visual Analog Scale. Putzier et al. concluded that dynamic stabilization is very much beneficial to prevent progression of disc degeneration following discectomy.9

Total Facet Replacement System

Total Facet Replacement Systems such as the TOPS system was the first to be used in clinical trials.5 TOPS (Total Posterior Arthroplasty System), TFAS (Total Facet Arthroplasty System), and ARFS (Anatomic Facet Replacement System) are total facet arthroplasty systems and differ from the remaining dynamic stabilization devices. These implants are designed to replace the entire posterior element while providing flexible restabilization. It was promising in early studies; however, technical difficulty and lack of successful clinical trials prevented its clinical use.10

Posterior Interspinous Distractor System

Posterior interspinous distractor systems came into practice after the work of Sénégas et al., who thought that distracting the spinal process and widening the canal in patients with degenerative spondylolisthesis would result in good clinical outcomes. He developed the Wallis system.11 Many similar systems were later designed as well. Currently, X-Stop is the most widely used interspinous implant. This titanium spacer device can be used in a minimally invasive approach under local anesthesia, so elderly patients with medical comorbidities can be considered especially good candidates for this device. Biomechanical studies of X-Stop have determined that, while correcting the shift in the vertebral column, interspinous processes reduce and facilitate load transfer by forming the rigid bridge. Therefore, it is used in back of the disc with axial back pain, and after discectomies that show the risk of instability. Its main concern is the risk for osteoporosis and that it is effective only for a short duration. It is ideal for elderly patients with comorbid conditions and may be inserted under local anesthesia.10

Scientific Basis

Cadaveric studies, finite element analyses and animal studies have demonstrated fusion increases intradiscal pressure, endplate stress and annular stress at the adjacent segment. Adjacent level degeneration is thought to result from increased motion and stress at the adjacent level due to restricted motion at the fused segment. In addition, fused segments cannot accommodate the regional alignment changes during different postures. This suboptimal sagittal alignment has been linked to the development of adjacent level degeneration.2 Posterior stabilization devices unload the disc and facets by load sharing. These devices restrict some motion and alter load transfer through functional spinal unit. These biomechanical properties of devices are well reviewed in the 2005 article by Sengupta and Mulholland.12

Graf ligamentoplasties are the initial soft stabilization devices used in clinical practice. These implants consist of titanium pedicle screws connected by prosthetic ligaments that are tensioned to hold the FSU (functional spinal unit) in lordosis. These implants are intended to maintain lordotic opposition of the facet joints and eliminate the pathological motion. Because the FSU is fixed in lordosis, load transfer to the posterior annulus and disc are significant. The other potential problem from the forced extension from lordosis is lateral recess stenosis.13

DYNESYS systems are similar to the Graf System with the addition of compression resistant polycarbonate urethane sleeves around the ligaments. The motion segment can be stabilized in a more neutral alignment instead of lordosis. These prosthetic ligaments provide resistance to flexion and the sleeves prevent excessive lordosis and bear compressive loads. With kyphotic movement, prosthetic ligaments prevent excessive motion with lordotic movement and plastic sleeves assume load-bearing function. These in turn reduce the overload on the posterior annulus.

Problems encountered in DYNESYS systems are:

  • Compression sleeves limit lordosis if placed in excessive distraction.
  • High possibility of implants becoming kyphotic.
  • Breakage and loosening of the screws due to compressive loads on the sleeve causing excessive movements on the pedicle.
  • Compression sleeves significantly increase rigidity of the construct.

Schmoelz et al.6 performed a study in vitro of intradiscal pressure to determine the influence of dynamic stabilization on the load bearing of the bridged disc. He studied four different conditions such as intact, destabilized, DYNESYS-stabilized, and rigidly fixed with an internal fixator. In the neutral position, there were no significant differences in the disc pressure for the four conditions. During the course of loading, both the DYNESYS and the internal fixator significantly reduced the pressure change from neutral to extension in comparison to the intact spine. However, there were no significant pressure changes noted from neutral to flexion. DYNESYS showed no significant difference in axial rotation disc pressure when compared to the intact spine.

Recently, Yeager et al.14 reported an in vitro study of two flexible constructs: DYNESYS Dynamic Stabilization System with PEEK (Polyether Ether Ketone) rod versus a rigid titanium rod. These were tested in flexion extension, lateral bending, and axial rotation to evaluate the biomechanical strength of intact, DYNESYS-PEEK rod, titanium rod, and destabilized conditions. Statistically significant decreases in range of movements were seen in instrumented spines during flexion extension and lateral bending. The results of this study support previous findings that DYNESYS and PEEK constructs behave similarly to a titanium rod in vitro.14

TABLE 5-1. Devices
Pedicle Screw-Based System Facet Replacement Devices Posterior Interspinous Stabilization
  • Graf System
  • DYNESYS
  • Talin Rod
  • Scient’X Isobar
  • Accuflex Transition Stabilization System
  • CD Horizon
  • TOPS (Total Posterior Arthroplasty System)
  • TFAS (Total Facet Arthroplasty System)
  • ARFS (Anatomic Facet Replacement System) STABILIMAX NZ
  • DIAM
  • Wallis System
  • X-Stop (Interspinous process distraction device)

Results

In the mid- and long-term, Graf ligamentoplasty reduces the risk of adjacent segment degeneration. Kanayama et al.15 compared Graf ligamentoplasty to posterolateral fusion with 5-year follow-up in 45 patients. Radiographic evidence of adjacent-segment degeneration at final follow-up was more frequent in the posterolateral-lumbar-fusion group than the Graf group (6% at L1-L2, 6% at L2-L3, 18% at L3-L4, and 18% at L5-S1, respectively in posterolateral-lumbar fusion). Only 6% in the Graf ligamentoplasty group and 18% in the posterolateral fusion group required additional surgeries for the adjacent segment degeneration. This study concluded that in well selected patients, Graf ligamentoplasty lowers the rate of adjacent-segment degeneration.15

Rigby et al.16 reported on mid- and long-term follow up of Graf ligament stabilization. A retrospective review of 51 patients with a mean follow-up time of 4 years was reported. The Oswestry Disability Score only improved an average of six points with longer follow-up. There were 12 complications and four required additional surgery. Seven patients (14%) required additional fusion surgeries. This study concluded that longer-term results of this technique are not as encouraging as earlier studies suggested.16

Hadlow et al.17 reported on a retrospective case-controlled study comparing Graf ligamentoplasty and instrumented posterolateral fusion in a consecutive series of 83 patients operated on by a single surgeon. Patients underwent either soft-tissue stabilization using the Graf ligament or posterolateral fusion with pedicle-screw instrumentation. There was a significantly better outcome, when measured by the Low Back Outcome Score, in the group of patients managed by posterolateral fusion at 1 year (P = 0.02), although at 2 years the difference was less significant (P < 0.34). This study demonstrated that the outcome after soft-tissue stabilization was associated with a worse outcome at 1 year and a significantly higher revision rate at 2 years.2

Putzier et al.9 evaluated the outcome of dynamic stabilization on lumbar discectomy, specifically, on the effect of dynamic stabilization on segmental degeneration after discectomy. Eighty-four patients with initial-stage disc degeneration (Modic 1) underwent discectomy, and 35 of those patients had the addition of DYNESYS stabilization. At mean 34-month follow-up, no progression of disc degeneration was noted in the DYNESYS group at follow-up, whereas radiographic signs of accelerated degeneration were noted in the discectomy-only group. The authors concluded that dynamic stabilization is useful to prevent progression of initial disc degeneration in segments after lumbar discectomy.9

Recently Fay et al.18 reported facet-related arthrodesis in DYNESYS dynamic stabilization. This retrospective study included 80 consecutive patients with 1- or 2-level lumbar spinal stenosis who underwent laminectomy and DYNESYS dynamic fusion with mean follow-up of more than 2 years.18 It was noted that 54.4% suffered unintended facet arthrodesis and subsequent immobile spinal segment.18

Similarly, Lee et al.19 reported facet-joint changes after application of lumbar non-fusion dynamic stabilization. They retrospectively compared monosegmental surgery at L4-5 with non-fusion dynamic stabilization using the DYNESYS system (DYNESYS group) and transforaminal lumbar interbody fusion with pedicle screw fixation (fusion group). Facet joint degeneration was evaluated at each segment using the CT grading system. In the fusion group, significant facet joint degeneration developed on both sides. Oswestry Disability Index scores improved less in the fusion group than in the DYNESYS group. The authors of the study concluded that non-fusion dynamic stabilization using the DYNESYS system had a greater preventative effect on facet joint degeneration than that obtained using fusion surgery.

Pham et al.20,21 recently published the complications associated with the DYNESYS dynamic stabilization system. He concluded that complications associated with this non-fusion system found similar infection rates and reoperation rates when compared with lumbar fusion. There was a higher incidence of pedicle screw loosening although there was a lower incidence of screw fractures. The overall incidence of adjacent segment disease appeared to be lower compared to the fusion group.20,21

Kondrashov et al.22 reported on the X-Stop (Interspinous process distraction device) in 18 patients with spinal stenosis and after a 4-year follow-up. The overall success rate was 78% and significantly improved functional outcomes.

Subsequently, Anderson et al.23 reported the results of X-Stop in neurogenic claudication in patients with degenerative spondylolisthesis. This study was conducted with non-operative control versus surgery using X-Stop. After 2-year follow up, clinical success rate was noted in 63% of patients. However, spondylolisthesis and segmental kyphosis were unaltered. The authors concluded that X-Stop is more effective than non-operative management of neurogenic claudication.

Conclusion

In case of advanced spinal degeneration and gross instability, fusion still remains the operative method of choice. Dynamic stabilization with DYNESYS is usefully in spinal degenerative disease with or without Grade I spondylolisthesis and in patients who require a faster recovery.

LUMBAR TOTAL DISC REPLACEMENT (LTDR)

Introduction

Spinal arthrodeses were once considered the gold standard for surgical management of lumbar degenerative disc disease. Lumbar total disc replace­ment (LTDR) replaced arthrodesis in the management of chronic discogenic back pain because it was expected to reduce various intra- and post-operative complications particularly relat­ed to fusion and adjacent segment disease (ASD). LTDR is in­dicated in chronic back pain arising from the disc itself, so-called discogenic back pain in patients with degenerative disc disease.24

Biomechanical Rationale

Though spinal fusion has stood the test of time, its disadvantages of delayed post-operative recovery, pseudo-arthrosis, bone-graft site morbidity and instrument-related problems are worrying. Incidence of adjacent level degeneration following spinal fusion remains unresolved and biomechanical and kinematic investigations demonstrate increased load and movement adjacent to fused segments. Disc replacement is an option for patients with chronic back pain who meet the selection criteria; the benefits of motion preservation and protection of adjacent levels from non-physiologic loading make prosthetic replacement of the disc a potentially attractive choice.25

With disc degeneration, the nucleus pulposus reduces in volume as a consequence of decreased proteoglycan and water concentration, resulting in a loss of intradiscal pressure and a change in the elastic modulus gradient. Degeneration alters the fatigue-recovery of the disc. There is a reduction in the ability of the disc to attenuate shock and provide an even stress distribution. The load borne by the annulus subsequently increases and it undergoes wear and tear. This phenomenon of disc incompetence transfers the load to the facet joints, which may lead to facet joint degeneration. The solution to re-establish the spinal biomechanics is to replace the degenerated disc with a mechanical device, with the aim of restoration of intervertebral disc height, lumbar facet joint structure and function as well as range of motion of the motion segments.25

Historical Consideration

The first disc prosthesis implant was tried by Nachemson, where he tried to implant a silicon testicular prosthesis into the disc space, however, it was abandoned due to implant disintegration.25 The first artificial lumbar disc, in the form of a steel ball, was implanted using an anterior approach by Fernström in 1960.26 The results of this implant appeared initially encouraging, but disappointed in the long-term follow-up when the ball end­ed up subsiding into the subchondral bone.26 About 20 years later in 1984, Schellnack and Buttner-Janz in Germany implanted the SB CHARITÉ™ prosthesis using an anterior approach. The implant was a semi-constrained type of lumbar artificial disc and comprised two metallic upper and lower plates and a sliding polyethylene core.27 Thereafter, three successive models of this implant have been launched and regularly used by David and Lemaire.24 In 1990, Marnay implanted ProDisc-I, which was a semi-constrained type and comprised two metallic plates and a non-mobile polyethylene core. An upgrad­ed successive model, ProDisc-II was launched in 1999 and has been widely used in the market. Since then, many different designs and composition of lumbar artificial disc have been launched, and mul­titudes of implants are available today.24

Scientific Basis of Lumbar Disc Replacement

The basis for total disc replacement is to eliminate the painful degenerated disc and maintain segmental motion, which in turn reduces the incidence of adjacent level degeneration and allows the implanted segment to adjust its sagittal alignment to accommodate functional demands. The important biomechanical properties of the implant should be as follows:

  • Implant should perform under physiologic cycling loading conditions.
  • Implant kinematics should not lead to progressive facet arthrosis.
  • Implant should retain enough motion to reduce the development of adjacent level degeneration.
  • Fixation and footprints of the implants should be optimized to reduce the incidence of subsidence and loosening.

Total disc replacement devices can be categorized according to their composite biomaterials (metal-on-metal or metal-on-polymer), biomechanics (unconstrained, semi-constrained, constrained), components (one, two, or three-piece designs) or fixation (spike or keel). Constrained devices have a mechanical stop within the range of physiologic motion, while semi-constrained devices have a mechanical stop outside the range of physiologic motion.

Unconstrained devices lack a mechanical stop. More strong and perfect anchorage and stability are required in more constrained designs. While in non-constrained designs the plates are highly mobile to protect them against the risks of mechanical stress, this design im­poses greater stress on the posterior joints. LADs (Lumbar Artificial Discs) are made with metal and alloys such as stainless steel, titanium and cobalt alloys, and high molecular weight polyeth­ylene such as ultra-high molecular weight polyethylene (UHM­WPE) for nucleus core and ceramics.24

Preclinical evidence

Since its introduction to the medical market in 1999, PEEK has quickly gained the confidence and acceptance of physicians as a highly reliable and biocompatible material for permanent medical implants. Both in vitro and in vivo biodurability tests demonstrate that PEEK has excellent biodurability for the disc arthroplasty application. Under the flexion load, three types of implanted model showed an intersegmental rotation angle similar to the one measured with the intact model. Under the extension load, all of the artificial disc-implanted models demonstrated an increased extension rotational angle at the operated level, resulting in an increased facet contact load when compared with the adjacent segments. The increased facet load may lead to facet degeneration.25

Devices

Several types of artificial discs are on the market. Most of them have metallic endplates, which are fixed to the adjacent vertebral bodies. Usually the endplates are linked by a ball-and-socket-joint allowing rotation within the artificial disc. Often a polyethylene inlay takes over the ball part. The inlay is fixed to the caudal endplate (i.e., Prodisc), movable in anterior/posterior direction (i.e., Activ-L™), or movable in a plane parallel to the endplate (i.e., Mobidisc™). In some disc types, a metallic ball is fixed to the caudal endplate (Maverick™). The CHARITÉ artificial disc has a lentoid sliding core of polyethylene, encased between the biconcave metallic endplates. Clinical success rates amount to about 80% and apply similarly to all disc types mentioned. The CHARITÉ artificial disc got FDA approval in 2004. Subsequently, Prodisc-L (2006) and the next-generation CHARITÉ device, In Motion (2007), received FDA approval. Several more devices have followed (Maverick, Kineflex, FlexiCore, and Activ-L) with a number of others in various stages of development.28

 
METAL-ON-POLYMER METAL-ON-METAL
CHARITÉ™ Maverick
In Motion Flexicore
Prodisc-L Kineflex
Activ-L Lateral disc
Mobi-L True disc PL
Physio-L
CA-disc
E disc

Clinical evidence

Lemaire et al. reported the 51-month follow-up clinical re­sult of CHARITÉ disc insertion in 105 patients.29 Among them, 79% of patients responded as very satisfied and 87% were able to perform normal labor works.29 Tropiano et al. assessed the outcome of 55 patients treated with ProDisc-L and followed up for average 8.7 years. In this study, 40 of the patients presented notable symptomatic improvement, a 74% success rate was achieved and no implant-related complications were reported.30 Gornet et al. proved the effectiveness of Maveric disc implants in patients with increased in disc space height and larger lumbar lordosis.31 Bertagnoli found no differences in outcomes for patients with prior posterior discectomy or laminectomy vs. those with no previous surgery for both single- and multi-level total disc replacement.32 Leahy et al. found no statistically significant differences in outcomes for patients with no previous lumbar surgery vs. those with a previous discectomy.33 Geisler et al. studied patients from the CHARITÉ IDE trial with and without prior back surgery.34 There were no significant differences in Oswestry Disability Index (ODI) and Visual Analog Scale. At 2-year follow-up, both groups had similar levels of satisfaction and return-to-work. Tropiano et al.30 found satisfactory results in 90% of patients with previous surgery. Tropiano et al. reported patients with failed back surgery experienced notable radicular pain after ProDisc implantation, possibly due to epidural fibrosis resulting in nerve root traction after intervertebral distraction.

Conclusion

In review of the literatures regarding the clinical significance of LTDR in the management of patients with DDD in the lum­bar spine, it can be realized that the clinical value of LTDR still remains unsettled and the selection criteria for LTDR must be restrictive. Longer follow-up should still be necessary to confirm the maintenance of improved surgical outcome and to observe any very late complications including wear-debris os­teolysis and ASD. LTDR still may get a chance to estab­lish itself as a substitute of fusion in the surgical treatment of DDD.

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