The concept of spine arthrodesis surgery was given by Albee over hundred years ago for Pott’s spine. Since the inception, this surgery has been the most widely performed spine surgery procedure with an incidence of around 400,000 fusion surgeries conducted in the US alone every year.Over the years, the indications for fusion surgery have been increasing drastically. Early success rates of fusion surgery during the evolution masked the long-term complications of the surgery. Reoperation rates following the fusion surgery were reported to be around 20% at 5 years from the primary surgery.[3,4] Studies suggest that adjacent segment disease (ASD) forms the major indication for reoperation in the long-term accounting for up to 51% of reoperations.[5,6] Initially, the cause for ASD was attributed to increased stress on the adjacent segment following fusion. Recent studies have theorized that damage to the paraspinal muscle and ligaments in open surgical procedures, as compared to percutaneous procedures, is one of the reasons for ASD.[8,9] However, the big question is whether ASD is actually an entity, and one school of thought believes that changes in adjacent segments are due to the natural course of the degenerative process.[10,11] There are few evidences of genetical predispositions for degenerative disease supporting this debate of biological etiology.
As the number of fusion surgeries increases in number so are the number of revision surgeries and also the surgeries to prevent ASD, that is, motion preservation technologies. Even though there are various indications for their usage, their results are inconsistent.[13,14]
The objective of this review is to give the reader a brief overview of adjacent segment pathology and motion preservative technologies, their current status, and long-term outcome through evidence-based literature review.
MATERIALS AND METHODS
A search was conducted in PubMed using the terms (“adjacent segment”) AND (“disease” OR “degeneration” or “pathology”). The articles were then shortlisted based on time of publication (2005 onward), publication in English and inclusion of human subjects. This resulted in 253 articles. Another search for ((“Motion preservation”) AND (“Spine”)) OR (“Adjacent segment disease”) OR (“Adjacent segment pathology”) OR (“Adjacent segment degeneration”) yielded 76 articles. The manuscripts were excluded if they were letters, editorials, commentaries, case series, etc. Once studies had been identified, duplicates were removed, and then the titles and abstracts of the remaining articles were screened by two independent reviewers. They checked the title and abstract of all the studies for relevance. In case of disagreement, consensus was obtained by mutual discussion.
Adjacent segment degeneration (ASDgen) refers to the radiographic changes in the intervertebral disc (IVD) adjacent to the operated spinal level, regardless of the presence of symptoms. Radiographic criteria, which can be considered ASDgen, are given in Table 1. Symptomatic ASDgen is called adjacent segment disease (ASDz). Because of the lack of precision in describing these terminologies. Lawerence et al., in their systematic review, proposed the term adjacent segment pathology (ASP), which covers both clinical adjacent segment pathology (CASP) and radiological adjacent segment pathology (RASP) [Table 1].
The biomechanics of spine is complex. IVDs behave like biological shock absorbers transmitting forces from one level to the next for appropriate spinal column mobility. It is subjected to diverse loading conditions. The complex functional orchestra of supporting structures results in physiological segmental movement, which is compatible in protecting neurological structures.
According to a meta-analysis, which included 55 studies, the incidence of lumbar ASP requiring reoperation was up to 8%, and in cervical spine, it was up to 2.4%–3% per year.
Motion segment fusion increases stress in the adjacent disc due to the nonphysiological center of motion and long lever arm. A 20% increase in shear strain in the adjacent disc was seen following multiple-level fusion surgery. Flexion and extension moment increases intra discal pressure by 73.2% and 45.3% proximal and distal to fusion segment, respectively.
Sagittal alignment of the spine has got an important role to play in the pathogenesis of ASP. Park et al. stated that malalignment at cervical fusion level increased the chances of CASP. Lumbar spine fused in hypo lordosis showed an increase in shear forces by 29% in the adjacent segment.
Biochemical factors are also a reason for the development of ASDgen. A static compression promotes catabolic response in the adjacent disc and the other way round in the dynamic compression mode.[25,26] Abnormal loading in the adjacent disc triggers pro-inflammatory cytokines IL-1b and TNF-alpha, which degrades the proteoglycans and activates catabolic enzymes. Disc is an inherently hypoxic structure. A small increase in compression or an abnormal mobility reduces the diffusion of oxygen and nutrients across the endplates, accelerating disc degeneration and in turn ASD.[28,29]
There are also studies that propose ASP is a part of multilevel degenerative spine disease.[10,11] Wang and Ding conducted a meta-analysis that compared the corelation between the presence of degeneration in adjacent disc preoperatively using Pfirrmann’s classification and the incidence of ASD in these cases postoperatively. They concluded that the incidence of ASD increases in postoperative period with a higher grade of Pfirrmann’s classification. This has been supported by the fact that the rate of ASD in spine fused for traumatic and congenital etiology was less compared to degenerative etiology. Hence, preexisting disease also plays a role in its etiopathogenesis. But whether ASD is a consequence of fusion or it is related to ongoing degeneration is still a debate.
Of late, there has been a lot of research on genomic studies, which has shown predisposing genes for disc degeneration.
In future, it might facilitate defining a high-risk population with the help of genetic mapping.
Risk factors for ASP after lumbar and cervical fusion surgery have been mentioned in Tables 2 and 3, respectively.
Depicting ASD following lumbar and cervical fusion with literature review [Tables 4 and 5].
STRATEGIES TO AVOID ADJACENT SEGMENT PATHOLOGY IN LUMBAR AND CERVICAL SPINE: MOTION PRESERVING SURGERIES
It is a long-term controversy whether ASPs, like ASDz and ASDgen, are separate entities or simply reflections of the natural history of cervical and lumbar degenerative disease or iatrogenic induced. Now focus has shifted to prevent future ASPs due to the index surgery, which lead to the concept of motion-preserving surgeries.
The aim of motion-preserving surgeries is to replicate the near-normal biomechanics with a view to minimize the chances of the development of ASPs. It is simply restoring the function of motion segment. In the last decade, there has been a significant increase in the use of motion preservation surgery, especially cervical disc replacement, with up to 17% increase in cervical disc replacement per year.
Motion-preserving devices can be categorized into four main types [Table 6]:
- (1) Total disc replacement (TDR),
- (2) Prosthetic nucleus replacement,
- (3) Posterior dynamic devices, and
- (4) Facet replacement.
TDRs necessitate complete removal of the disc. It contains endplate portions, which is usually metallic component and are fixed to the vertebral endplates. The articulating portion of the device is interposed between the endplates portion and can be composed of various materials.
Lumbar total disc replacement (LTDR)
Biomechanical and kinematic studies have depicted that increased load and movement in adjacent segment leads to early degeneration of adjacent disc. The benefits of motion preservation and protection of adjacent discs from nonphysiologic loading make prosthetic replacement of the IVD a potentially attractive choice.
The LTDR must restore motion, have the ability to transmit and absorb loads and permit translation to accommodate changes in the axis of rotation. It should restore foraminal, disc height, and sagittal alignment.
There are three main LTDR designs for the lumbar spine [Figures 1-3]:
- (1) the three-component ball and socket design with gliding surfaces (Charite, InMotion, Kineflex-L),
- (2) the two-component ball and socket design with gliding surfaces (Maverick, Flexicore), and
- (3) the one-piece design (Freedom, M6-L).
Indications of LTDR
Lumbar TDR is indicated for patient with severe discogenic low back pain who have failed conservative trial. Patient selection remains an important aspect in attaining optimal outcomes.
- Age 18–60 years,
- Male or female with visual analog scale (VAS) > 4 and Oswestry score > 40%, and
- Symptomatic DDD with radiographic evidence: vacuum disc sign, Modic changes, contained herniated disc, high-intensity zone signal, absence of facet degeneration.
Contraindications of LTDR
There are various contraindications for the use of lumbar TDR
- Posterior element disease (facet joint arthritis or previous facet joint resection),
- Spondylosis, and
- Fixed deformity.
A systematic review was conducted by Cui et al. in which 13 studies were considered for the determination of the mid and long-term outcomes of total lumbar disc replacement. It showed that patients had a satisfactory clinical outcome ranging from good to excellent in 75.5%–93.3% (in the last follow-up). 65.9% of the patients had part-time or full-time job following the LTDR. Amongst the 13 studies, 10 had reported complication rates, which ranged from 0% to 34.4%, and reoperation rates of 0%–39.3%. It is to be noted that 8 out of 13 studies showed reoperation rates of less than 10%.
However, two studies showed reoperation rates of more than 30%. These studies were on the use of AcroFlex prosthesis with high reoperations rates between 33% and 39.3%. Even though the mid and long-term results of this study were favorable, they failed to establish superiority over fusion procedures.
The majority of experts concur that one of the most crucial elements influencing TDR outcomes is patient selection. Lumbar TDR may be a good alternative to lumbar fusion, particularly for young patients with DDD who don’t have any major facet joint degeneration, deformity, instability, or osteopenia/osteoporosis. However, worries about long-term results, implant durability, and potential extremely late problems may still prevent lumbar TDR from being widely used.
CERVICAL TOTAL DISC REPLACEMENT (CTDR)
Both CTDR and LTDR rely on the same principle of neural decompression in addition to maintaining near-normal motion, maintaining segmental lordosis, and maintaining anatomical disc height. Although the outcomes of ACDF are safe and effective over decades, CTDR counters the negative aspect of the ACDF.
Indications for CTDR are:
- Requirement of surgical treatment at one to two levels from C3 to T1 after a failed conservative treatment lasting at least 6 weeks for any one or more of the following: (1) PIVD with radiculopathy, (2) spondylotic radiculopathy, (3) PIVD with myelopathy, or (4) spondylotic myelopathy.
Contraindications for CTDR are:
- Ankylosing spondylitis, rheumatoid arthritis, OPLL, or diffuse idiopathic skeletal hyperostosis,
- Insulin-requiring diabetes mellitus,
- Prior cervical spinal infection,
- Chronic steroid use or a medical condition requiring,
- Chronic steroid administration, and
The artificial cervical discs, which are approved, include Bryan disc, Prestige ST, Mobi-C, ProDisc C PCM, Secure C, and Prestige LP [Figure 4].
A meta-analysis by Peng et al. included 30 RCT’s comparing outcomes of CTDR and ACDF showed that prolonged surgical time, better overall neurological success, and neck disability index (NDI) success rates were found in CTDR in short-, mid-, and long-term follow-up periods, with lower dysphagia/dysphonia during short-term follow-up. CTDR group showed a lower incidence of ASD, and reoperation rates in long-term follow-up periods. Comparable length of hospital stay and blood loss were found in both groups.
Wang et al. in their meta-analysis of 11 RCT’s concluded in comparison to ACDF, CTDR achieved significantly higher overall success rates, improvement of NDI, neurological success, patients’ satisfaction. Functional outcome measures such as VAS for neck pain and upper limb pain, the Short Form-36 (SF-36 PCS), and the Short Form-36 (SF-36 MCS) revealed superiority in the CTDR group. CTDR group had a lower rate of symptomatic adjacent segment, resurgery at the index level, and surgery at the adjacent level.
Meta-analysis by Gao et al., Luo et al., Rao et al., and Findlay et al. showed similar findings with a superiority of ADR over ACDF.
CTDR is a great surgical choice since it preserves segmental mobility, has the potential to lessen ASP, and doesn’t require plating or bone graft harvesting. The body of research supporting the efficacy and durability of arthroplasty is expanding, and it is a crucial technique in a spine surgeon’s toolbox.
Prosthetic nucleus replacement
On the contrary to TDR, prosthetic nucleus replacements necessitate evacuation of only the nucleus pulposus through an annulotomy. Otherwise, the annulus remains largely intact. The implant is usually a deformable material that is inserted into the disc space with or without a fixation method.
Ray designed prosthetic disc nucleus (PDN) and also its subsequent design changes (PDN; Raymedica, Minneapolis, MN). Indications for PDN are discogenic low-back pain due to degenerative disc disease (DDD) or postdiscectomy. PDN implantation is done through standard laminectomy and annulotomy approach. Hydrophilic gel pillow that absorbs water and expands once implanted form the vital component of PDN. Prevention of device expulsion postimplantation forms the core strategy for implant design [Figure 5].
Currently, in the United States of America, no nucleus replacements or augmentation devices are approved. Complications of nucleus replacement devices typically include (1) implant expulsion, especially with devices without any fixation points, because annulotomy is done to introduce the device; (2) subsidence due to stress shielding in the center of the endplate; (3) endplate remodeling with disc height collapse; and (4) poor disc kinematics resulting in canal stenosis and facet arthritis or fracture.[70,71]
Presently, there is a lack of any level 1 evidence to ascertain the clinical efficacy of nuclear disc replacement. But a retrospective review to determine the long-term results of the DASCOR system (Disc Dynamics, Eden Prairie, MN) by Golan et al. showed that out of 22 patients, mean ODI was 33.4 ± 18.4 and mean VAS was 4 ± 2.8, and both were on improvement side. Among these, 46% patients developed ASDgen in follow-up, revision surgery rates were high with six patients undergoing surgery at index level and three patients needing surgery at adjacent level.
When suggesting nuclear replacement as a treatment for individuals with DDD, the surgeon must evaluate endplate integrity, disc height, endplate form, annular integrity, and BMI.
Posterior Dynamic Devices
Posterior dynamic devices are of many forms. Dynamic, or so-called soft stabilization and semirigid are both typically pedicle screw based. The screws are connected by a flexible longitudinal rod. Currently available devices vary from articulating ball joints to longitudinal Silastic rods.
Interspinous devices can also be considered posterior dynamic stabilization devices. These devices consist of an implant that is interposed between the adjacent spinous processes. Some are held in place by their design alone, whereas others require supplemental tethering fixation.
The dynamic stabilization systems may be classified into four categories as follows:
- Interspinous distraction devices (Minns silicone distraction device Wallis system X-stop),
- Inter-spinous ligament devices (elastic ligament (Bronsard’s ligament across the spinous processes) loop system),
- Ligaments across pedicle screws (Graf ligament, dynesis device), and
- Semirigid metallic devices across the pedicle screws (FASS (fulcrum assisted soft stabilization) system DSS (dynamic soft stabilization) system).
Dynamic fixation of the lumbar spine would favorably alter the movement and load the transmission of a spinal motion segment without the intention of fusion. A dynamic stabilization, also known as soft stabilization or flexible stabilization, leaves the spinal segment mobile, and its intention is to alter the load-bearing pattern of the motion segment, as well as to control any abnormal motion at the segment.
The flexibility of these systems is achieved by either of the two different configurations of these implants: flexible rods or special connections (dampers or hinges) between pedicle screws and rigid rods (Isobar TTL Semi-rigid Spinal System, Cosmic Posterior Dynamic System). As far as flexible rods are concerned, their flexibility is derived from the fact that these rods are made either of elastic materials like polyethylene (Dysnesys, Zimmer) or PEEK (CD-Horizon Legacy PEEK Rod), or of rigid materials manufactured in such a way that some degree of flexibility is possible (i.e., in the case of AccuFlex Rod System the double helical cuts in the rods make the rods to bend) [Figures 6, 7].[74,75]
Indications for this type of implant include: (1) moderate DDD, (2) mild facet arthropathy, and (3) low-grade spondylolisthesis or instability and spinal stenosis (always with an additional decompressive procedure).
Spinal instability and listhesis greater than grade 1, advanced DDD or disc collapse more than 50%, osteoporosis, and scoliotic deformity.
In individuals with degenerative lumbar spine, dynamic stabilization system does provide good and stable clinical mid-term outcomes, and it appears to be a viable option to fusion surgery with similar clinical outcomes but fewer reoperations.
As the name suggests, these are positioned in the interspinous region of the vertebrae and distracts the posterior aspect of the motion segment. This distraction of the posterior elements leads to the decompression of neural elements through three different mechanisms.
- (a) Increase in the dimension of neural foramens, especially in foraminal stenosis,
- (b) Unbuckling and stretching of the buckled ligamentum flavum,
- (c) Distraction of the posterior aspect of the disc and also vertebral body resulting in decompression of the anterior–central part of the spinal canal.
Interspinous devices are broadly classified into restricted and unrestricted devices. Restricted devices (Wallis, Diam, intraspine) control both excessive flexion and extension, acting as dynamic stabilization devices, while unrestricted devices (X-STOP. Aperious, Bacjac, Ellipse, etc.) limit only the excessive extension of the spine. Limitation of spinal flexion in the case of restricted implants is achieved through synthetic laces attached to the cranial and caudal spinous processes [Figure 8].
Tram et al., in their systematic review comparing traditional decompressive surgery versus interspinous devices, showed that there was no collective advantage of one procedure over the other, and both were unique surgical interventions with different therapeutic efficacies and complications.
Wu et al., in their systematic review and meta-analysis, concluded that they obtained some benefits in the interspinous device group, but it was not significant as compared to only decompression group. However, interspinous spacers had a higher incidence of complications, reoperation, and higher cost as compared to the decompression group. Li et al. conducted a systematic review of RCT’s and the pooled analysis showed a higher reoperation rate with interspinous devices. These devices were not superior in giving symptomatic relief in neurological claudication as compared to laminectomy.
Due to a lack of significant clinical and surgical evidence of safety and efficacy in published peer‐reviewed medical research, justification of interspinous devices usage is still questionable. Additional clinical studies are required to demonstrate the effectiveness with reference to postoperative results.
As the disc height reduces in the degenerative cascade of the IVD, facet load increases leading to facet arthritis and osteophyte formation, causing low-back pain. Facet replacement is an invasive procedure as compared to interspinous devices, wherein a laminectomy followed by bilateral facetectomy is performed, and a prosthetic facet device implanted. Facet replacement decompresses the canal and foraminal stenosis and also provides some motion at the affected level.
Total facet arthroplasty system (TFAS) is a pedicle-based sliding ball-in-bowl-type joint system. Its indication includes lumbar canal stenosis. TFAS, unlike fusion procedure, retains segmental motion and provides stability, and restores lumbar lordosis.
TOPS is another facet joint replacement system that is composed of titanium construct, which has interlocking, flexible articulating core, surrounded by polyurethane elastomer cover, which is capable of transmitting tensile and compressive loads in addition to shear forces. Two other systems for facet replacement are ACADIA and Stabilmax NZ [Figure 9].
Four multicentre, prospective, RCT’s analyzing the long-term results of four distinct total facet arthroplasty (TFA) devices were conducted in the US since early 2000. These studies were given permission by the US FDA as investigational device exemption studies. Dismal results of these trials lead to the unavailability of the TFA products commercially.[52,82]
Any recommendations for treatment using artificial facet joints are presently not attainable because clinical trials have not yet been completed, and there is an inadequate amount of clinical data. Position statements and guidelines on motion preserving surgeries of various well known societies has been mentioned in Table 7.
ASDgen is very commonly seen after cervical and lumbar fusion. However, ASDz is still an enigma, and regular follow-up is required. Evidence supports alteration in biomechanical properties in adjacent segments, both proximal and distal to fusion levels. With a shift in focus to restore the near-normal biomechanics of adjacent segments, various types of motion preservation surgeries have been developed, and there is ongoing research with varying levels of evidence for each type. There is Level I evidence in favour of cervical disc replacement, lumbar disc replacement as well as posterior dynamic stabilization and Level II evidence in favour of Interspinous devices. However, Level IV evidence in favour of lumbar facet replacement as well as nucleus pulposus replacement. Thus justification for the use of motion preservation surgeries is still investigational and is subjected to outcomes of well-conducted long-term studies, especially for motion preservation surgeries other than cervical and lumbar disc replacement.
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Conflicts of interest
There are no conflicts of interest.
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