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CHAPTER 40
Lumbar Disc Replacement: Current Model, Results, and the Future
Robert D. Fraser
More than 100 different designs for a disc prosthesis hav e
been patented or described in publications (1) since F ernstrom first replaced the nucleus with a metal bearing ball
in the late 1950s (2). Very few of these devices have been
used clinically, however, reflecting the difficulty of translating the success of hip and knee arthroplasty to the
spine. The prostheses that ha v e been implanted in humans
can be classified as (a) nucleus devices or spacers, (b)
mechanical devices with moving parts, and (c) elastomeric implants that aim to reconstruct the normal elastic properties of the disc.
NUCLEUS PROSTHESES
The nucleus replacement device used most extensively
in humans is the Prosthetic Disc Nucleus, or PDN
(Raymedica, Minneapolis, MN). This consists of a h ydrogel pellet contained within a woven polyethylene jacket.
The hydrophilic properties of hydrogel provide this
device with the capacity to absorb fluid and expand.
Hydrogel is a copolymer of polyacrylonitrile (nonhydrophilic) and polyacrylamide (hydrophilic); the ability to absorb and bind water is determined by the ratio of
these polymers. The current design per mits the pellets to
absorb 80% of their weight in water, giving the PDN
device the potential to restore or maintain disc height.
The aim of the woven polyethylene jacket is to limit
swelling and to minimize horizontal spreading (3).
The recommended technique involves the coronal
placement of two parallel devices within the enucleated
disc space, either by the posterior (hemilaminotomy) or
lateral (transpsoas) routes. The developers state that this
device is not intended for use in cases of severe disc
degeneration or where end plate defects are present (3).
Initial trials during the mid-1990s, with two implants
placed side by side in a sagittal plane, demonstrated a
high expulsion rate with 38% of subjects requiring revi-
sion (4). The PDN shape and surgical protocol were subsequently modified to minimize the extrusion rate, and
further clinical trials are in progress. Encouraging results
are claimed with the use of PDN (3–5), although in each
paper details of the methodology are insufficient to permit critical review.
Other nuclear replacements implanted clinically include
the Aquarelle (Stryker Howmedica, Mahwah, NJ), a polyvinyl alcohol material and the Newcleus (Sulzer SpineTech, Edina, MN), a polycarbonate urethane elastomer, but
to date there are no published results of these devices.
Nucleus prostheses may be inserted through a relatively minimally invasive or potentially percutaneous
approach. While this increases the appeal of the procedure, nucleus replacement does not address pathology
related to the annulus or end plate, both of which may be
important components of a painful degenerative process.
Furthermore, nucleus devices are intended to work in
conjunction with the annulus to restore the biomechanical function of the disc. To insert such a device, an annulus lesion must either be made or already exist. Although
special dilators to minimize annulus damage have been
used for the phase IV trials of the PDN, it is unclear
whether the benefits of nucleus replacement will outweigh the effects of damage caused to the annulus.
MECHANICAL DISC PROSTHESES
The prime aim of mechanical disc replacement is to
restore the normal kinematics of the motion segment. The
designs of two mechanical disc prostheses tested in clinical trials are based on the principle of low-friction polyethylene on metal articulations developed for total hip
and knee arthroplasty. A third device, similarly designed
on the basis of the success of prostheses developed for
large synovial joints but with a metal/metal (chrome
cobalt) interface and a posterior rotation axis, is being
393
394 /SECTION V/SPECIFIC CLINICAL ENTITIES
tested in a multicenter clinical trial (6). In general these
prostheses are intended to replace almost the entire disc,
are inserted using an approach similar to an anterior lumbar interbody fusion, and rely on large spikes or fins plus
bony ingrowth for stability against the vertebral end
plates. Mechanical artificial discs lack elasticity and cannot replicate the normal compressive stiffness of the natural disc. Instead they depend on the restoration of a
mobile lordosis to absorb compressive loads across the
lumbar spine (7). Moreover their articulating surfaces
provide little resistance to torsion, a function of the
motion segment that is impaired by the necessary
removal of most of the annulus.
Schellnack and Buttner-Janz were responsible for the
development of the first version of the Link SB Charité
(Waldemar Link GmbH & Co., Hamburg) artif icial disc
in 1982 (8). Over the next five years there were two revisions of the design. Consisting of a biconvex polyethylene spacer articulating with two concave cobaltchromium alloy end plates, the prosthesis allows rotation
in all three planes. Changing centers of rotation are
allowed by the sliding of the polyeth ylene core, similar to
that which is achieved in mobile-bearing total knee
replacements. First implanted in 1987, the latest model of
the Link SB Charité has been used more extensively than
any other disc prosthesis. Approximately 4,000 prostheses have been implanted (7); the report from the largest
case series of 105 patients describes a satisfactory outcome with the procedure (8).
The ProDisc (Spine Solutions, New York, NY) was
first described by Marnay in 1991 (9). It consists of a
polyethylene cap articulating with two titanium alloy end
plates. The cap over cup design of the metal-polyethylene-metal articulation permits motion in all three planes.
Bertagnoli and Kumar reported on a series of 108
patients with follow-up ranging between 3 months to 2
years (10). In this, the only published paper on the outcome with the ProDisc, an overall success rate of 90.8%
was claimed, increasing to 98% in patients considered to
have “prime” indications.
ELASTOMERIC DISC PROSTHESES
The attraction of elastomeric discs is their potential to
replicate the elasticity of the normal human disc. Not
only would it be feasible to restore the normal compressive stiffness of the natural disc, but also if f irm attachments to the bony end plates could be achieved, it would
provide resistance to torsion and shear.
With these goals in mind, Steffee designed the AcroFlex (DePuy, Acromed Corporation, Cleveland, OH) artificial disc using a polyolef in-based rubber core vulcanized between two titanium end plates. Not only did the
rubber core provide range of motion, but it also enabled
replication of normal disc elasticity . In theory, this design
should allow for better absorption of loads. However, this
advantage of elastomers is offset b y their potentially inferior wear characteristics as demonstrated by the relatively
high rates of failure. In the original series of six patients,
two were reported as failures due to debonding of the
rubber core (11). A second-generation device, using a silicone core instead of rubber, was implanted in eight
patients with one mechanical failure. All failures occurred at levels with increased stress, either due to adjacent fusion levels or scoliosis. The AcroFlex is now in its
third generation of design, reverting back to the rubber
core optimized with improved processing and bonding
techniques, and refined indications for surgery. Although
functional outcomes following implantation have been
generally satisfactory, further trials with the third generation prosthesis were abandoned with the detection of
early failure of the rubber core on thin-section computed
tomography (CT) scans (12).
The thin-section CT scans used in the AcroFlex study
also identified a significant number of patients with
periprosthetic heterotopic ossification. Limiting range of
motion may be another potential source of failure for any
disc replacement. It is therefore recommended that future
studies include the use of thin-section CT and relate this
to range of motion on standing flexion and extension
radiographs.
BASIS FOR CURRENT INDICATIONS
For any spinal operation, including total disc arthroplasty, patient selection should be based on a careful consideration of many factors. A successful outcome is more
likely to be achieved with the precise correlation of the
patient’s histor y, physical examination, and radiographic
investigations, in conjunction with psychosocial and
medical backgrounds. Bearing this in mind, indications
were developed for a trial of the AcroFlex lumbar disc
prosthesis (12). Only patients with one- or two-level
symptomatic disc degeneration at either L4-5 or L5-S1
were included. Patients had to complain of disabling low
back pain, with or without referral type leg symptoms
that had been present for a minimum of 12 months and
had failed to respond to nonoperative treatment. Furthermore, the symptomatic degenerative level had to be convincingly localized by provocative discography. For
inclusion in the study discography had to demonstrate (a)
internal disc disruption at the target level, (b) reproduction of the patient’s typical pain at the target level, and (c)
failure to reproduce typical pain at the control levels adjacent to the target level. Because of uncertainty about the
long-term results only patients between the ages of 30
and 55 were considered (12).
The contraindications to disc replacement surgery are
made up of technical and patient selection considerations.
Patients should be excluded if there is a history of previ-
CHAPTER 40/LUMBAR DISC REPLACEMENT / 395
ous lumbar infection or an active infection elsewhere.
Because disc replacement surgery does not address
posterior element pathology, patients with spondylitic
spondylolisthesis, significant facet arthritis, lateral recess
stenosis, or central stenosis should be considered unsuitable. Technical considerations that impede or prevent disc
arthroplasty include patients with a steep lumbosacral
angle at the target level, osteopenia, previous abdominal
radiation or vascular graft, and morbid abdominal obesity. Lastly, patient factors such as significant medical
comorbidity, ongoing litigation or compensation issues,
substance abuse, presence of three or more Waddell
behavioral signs (13), or psychiatric illness may be the
major factors influencing eventual outcomes. The presence of any of these factors should be regarded a contraindication to disc replacement surgery.
Despite the suggestion that in time disc replacement
will be a solution for multilevel degeneration or degeneration adjacent to a fused segment, this has not been
considered an ideal indication. Additionally, given the
increased stresses placed on the prosthesis in a patient
with structural scoliosis or adjacent-level fusion, such
conditions are considered relative contraindications.
Bertagnoli and Kumar tried to correlate preoperative
clinical findings to outcome with a view to formulating
appropriate indications for disc replacement (10). They
conducted a retrospective review of 108 patients who
underwent total disc arthroplasty with the ProDisc prosthesis. The patients were separated into accordingly into
four groups, those who were considered to have “prime”,
“good”, “borderline”, or “poor” indications for surgery.
Patients with a “prime” indication had a disc height
greater than 4 mm, absence of facet joint arthritis, no
adjacent-level degeneration, and intact posterior elements. Patients with adjacent-level fusions were considered to have “borderline” indications. While there
appears to be a gradient of improved successful outcomes
in patients with better indications, no statistical analyses
were performed. How e v er , it also seems that patients with
“better indications” were those with minimal degeneration and they may well have achieved better outcomes no
matter what form of treatment was employed.
REPORTED CLINICAL RESULTS
In a critical assessment of the evidence related to lumbar disc replacement, Wai et al. (14) carried out a thorough search of both the Pubmed and Ovid Medline
databases up to October 2002, and identified papers concerned with the current clinical use of disc prostheses.
Their assessment, summarized in Table 40-1, recorded
factors important to the outcome of disc replacement,
namely: (a) restoration of disc function, (b) preservation
of adjacent levels, (c) overall clinical function, and (d)
complications. The follow-up for all papers reviewed
averaged just less than 2 years with the conclusions often
based on patients followed up for a much shorter period.
Although a follow-up of 2 years is generally considered acceptable for publishing surgical results, it is quite
inadequate when assessing the outcome of disc replacement surgery. This is particularly the case when assessing
the safety of implants used for total disc replacement.
Their large dimensions and location close to major vessels makes anterior low-lumbar revision surgery hazardous, particularly when this is performed for complications related to mechanical failure.
In their critical review, Wai et al. (14) found a wide discrepancy in the definitions of clinical outcome, many of
which were poorly def ined, with reported success rates
ranging from 63% to 95% after disc replacement. Overall, these results are similar to those from case series
reports for spinal fusion (15–21), and for common forms
of nonoperative care (22–26). Certainly, randomized controlled trials, using validated and independent assessments of outcome and safety are necessary to establish
the efficacy of disc replacement compared with the current standard of care. Clearly, because of the large influence on function of factors other than disc pathology, a
major effect from disc replacement would be needed to
reach statistically significant differences. The Swedish
Lumbar Spine Study Group published a report of a randomized controlled trial on fusion for back pain (15).
From the published data, Wai et al. (14) estimated that
more than 500 subjects would be needed to determine a
10% improvement in outcome for a 2:1 study design with
a power of 0.8.
Even though there may be no significant difference in
clinical outcomes between fusion and disc replacement in
the short term, the latter has the theoretical advantage of
restoring the function of the motion segment, hence protecting the adjacent levels. Almost all of the published
studies have described restoration of disc height and
return of motion for both nuclear and total disc replacement surgeries (14). Although protection of adjacent disc
levels is considered of prime importance to the rationale
for disc replacement, only three studies hav e assessed this
potential benefit. Cinotti et al. perfor med magnetic resonance imaging (MRI) on 10 patients from their original
cohort of Link SB Charité III prostheses and found no
adjacent level degeneration (27). As more than 75% of
their study population has not been assessed, it is not
valid to draw any conclusions from their report. In the
report of their experience with the ProDisc, Bertagnoli
and Kumar mention that 4.6% of patients dev eloped radiographic evidence of adjacent-level degeneration within
3 to 24 months of implantation (10). With such a short
follow-up and without a control group, it is unclear if this
represents the natural history of disc degeneration or is
significantly less than would have occurred had the patients undergone a lumbar fusion. Many investigations of
failure
in back pain dislocation; 1% core
eventual fusion
on MRI for excellent, 17% ROM affected if
10 patients required malpositioned
>50% (1 subsidence,
restoration of 2 ossification)
disc height
and lordosis
fair results
technical adjacent level fusion
radiographic OR
result
level
degeneration
medication
lordosis by 7° in pain
in Oswestry
b
avg. of 1.8 mm scores reported in
reported in 7 patients
7 patients
VQ Increase in disc N/A 43.7% improvement 10% overall explant rate
a
size follow-up
Sample Months of
TABLE 40-1. Summar y of published peer-reviewed clinical papers on disc replacement arthroplasty
(no. of (range Study Restoration of Preservation of Clinical
Author/year Disc discs) if stated) quality normal mechanics adjacent levels success Complications
Enker, 1993 Acroflex 6 40 — 8° avg. ROM N/A 66% success 33% implant failure
Griffith, 1994 Char ité SB III 93 (139) 12 (1–37) — N/A N/A 65% improvement 4.3% migration/
III trials disc height for migration
Cinotti, 1996 Char ité SB III 46 (56) 38 (48–60) IR, NC 12° avg. ROM No degeneration 63% good or 26% placed too anterior;
Lemaire, 1997 Charité SB III 105 51 NC 7° avg. ROM, N/A 79% excellent 2.9% related to implant
Zeegers, 1999 Charité SB III 46 (75) 48 NC 74% good 24% required 70% improved 1 required conversion to
Sott, 2000 Charité SB III 15 48 (18–68) NC N/A N/A 80% good or None related to implant
Bertagnoli, 2002 ProDisc 108 (134) (3–48) VQ, NC 10° avg. ROM 4.6% adjacent 90.8% excellent None related to implant
Hopf, 2002 Charité SB III 35 15 NC Correction of avg. N/A 80% reduction None related to implant
Bertagnoli, 2002 PDN—phase 168 6 (3–24) VQ Preservation of N/A 88% success 26% required revision
Hochshuler, 2002 Charité SB III 22 12 VQ N/A N/A 40% improvement None related to implant
phases height by an of Oswestry
Klara, 2002 PDN—all 423 Up to 48
IR, reported that outcomes assessed independently; MRI, magnetic resonance imaging; NC, no conflict
Buttner-Janz, 2002 Charité SB III 20 46 (6–156) VQ 7° avg. ROM N/A 95% reduction in pain None related to implant
of interest stated; OR, operation; ROM, range of motion; VQ, validated questionnaire (e.q., SF36,
Clinical results of patients with shorter follow-up not reported.bNot clearly stated for radiographic details but implied since 7 patients reported for Oswestry.
Reproduced from Wai E, Selmon G, Fraser RD. Disc replacement arthroplasties: can the success of hip
a
Oswestry) used in clinical assessment.
and knee replacements be repeated in the spine? Semin Spine Surg 2003;(15)4:473–482.
CHAPTER 40/LUMBAR DISC REPLACEMENT / 397
adjacent-level degeneration after fusion have used MRI
or have longer follow-up times (28–31).
The work of K umar et al. is the most similar in methodology to the study by Bertagnoli et al. but with a much
longer follow-up period (32). They found an 8% incidence
of radiographically determined adjacent-level disease in
patients 5 years following lumbar fusion with a normal
sagittal alignment. Furthermore, in an MRI study we carried out 10 years after anterior lumbar interbody fusion
(28), the adjacent disc was found to be free of degeneration
in 68% of cases. We examined the patter n of degeneration
in the remainder of the lumbar spine, including the presence of skip lesions, comparing this with the reports of
MRI findings in normal asymptomatic populations. Based
on this comparison, we concluded that adjacent-level degeneration was determined more by constitutional factors
than by the presence of a solid fusion.
Further challenging the ability of disc replacement to
preserve the adjacent levels, a study by Zeegers found
that 24% of patients required adjacent-level surgery
within 2 years of having an SB Charité III prosthesis
inserted (33). While this high incidence may represent a
failure to adequately rule out adjacent-level pathology
before performing the index procedure, it is much higher
than any reported for lumbar fusion. In any ev ent, there is
clearly no evidence to date that disc replacement protects
against adjacent level degeneration.
FOOD AND DRUG ADMINISTRATION STATUS
At the present time, the U.S. Food and Drug Administration (FDA) has not approved disc replacement devices
for routine marketing, and currently the use of these
devices is for investigational purposes only. To obtain
approval for general use the manufacturer is required to
demonstrate to the FDA’s satisfaction that their prosthesis
is as substantially equivalent (as safe and effective) to an
already approved device. Specif ic guidelines concerning
the indications, in vivo biomechanical testing, and clinical results of devices have been published for spinal
implants (34).
Two separate multicenter, randomized controlled clinical trials are in progress in the United States under the
FDA’s Investigational Device Exemption. The SB Charité
III is being compared with anterior interbody fusion
using the Bagby and Kuslich (BAK) device, while the
ProDisc is being tested against combined anterior and
posterior fusion using pedicle screws and an interbody
fusion construct (35). Both trials intend to recruit
between 300 and 500 patients and the manufacturers
hope to achieve FDA approval within the next few years.
The initiation of a FDA-approved trial in the United
States is pending. The PDN has been given approval by
the Canadian Therapeutic Directorate to begin a clinical
trial and three Canadian centers are recruiting patients for
a prospective evaluation of the PDN (4).
It is expected that these trials will address short-term
safety issues related to the prostheses and whether the
clinical efficacy is similar to fusion at 2 years. As mentioned previously, these studies are unlikely to have sufficient pow er to detect a dif ference with fusion unless there
is a large clinical effect. It is therefore essential that longterm follow-up of the studies be carried out to determine
efficacy, the overall safety of the implants, and their ability to protect adjacent levels.
THE FUTURE OF DISC REPLACEMENT
It is the spectacular success of arthroplasty of the hip
and knee that has continued to drive the development of
artificial disc technology. Further enthusiasm has been
generated by the demonstration that a number of prostheses have restored range of motion to the disc and by the
clinical use of the Link SB Charité device for over 10
years. However, this measure of technical success and
implant longevity does not necessarily mean that the
future of disc replacement is certain. It is only if it can be
shown in the long term to perform at least as well as
fusion, without compromise from implant failure, heterotopic bone formation or excessive facet degeneration
leading to stenosis, that the place of disc replacement will
be finally assured.
From the reported results of case series described
herein, it seems likely that current FDA trials will demonstrate short-term eff icacy and safety, leading to approval
of these devices for routine marketing. Because of anticipated demand, the expected FDA approval is likely to be
followed by extensive and widespread clinical usage of
these devices. Based on the experience associated with
the release of other spinal implants as well as the knowledge gained from hip and knee arthroplasty, it will be
only a matter of time before surgeons are confronted with
problems created by implant failure. There is, however,
one major advantage for the spinal surgeon compared
with the hip or knee surgeon undertaking revision procedures for a failed prosthesis. Unlike surgery for the hip
and knee, there is unlikely to be a great functional disadvantage in converting spinal ar throplasty to arthrodesis;
moreover, it may be possible to circumvent the surgical
site by retrieving the situation with a posterolateral
fusion. However, when it is necessary to remove a large
lumbar interbody device anteriorly, the proximity of the
major vessels and other vital organs, the scarring from
the original surgery, and the pathology associated with
implant failure all combine to make the revision procedure extremely difficult and potentially disastrous (12).
Clearly the introduction of these devices into the community should be with caution. The current “gold standard” investigations for diagnosis of discogenic back
pain, MRI, and discography have signif icant high falsepositive rates.A large potential for misdiagnosis resulting
in surgery for pain not arising from the disc exists; hence
398 /SECTION V/SPECIFIC CLINICAL ENTITIES
the unsatisfactory outcomes often reported for treatment
of discogenic back pain.
Combined anterior and posterior fusion has been promoted by many spinal surgeons for the management of
back pain since it deals with many different potential pain
sources, including not only the disc but also the ligaments
and facet joints. In contrast, disc arthroplasty targets only
the disc. It is, therefore, more vulnerable to the effects of
misdiagnosis leading to the replacement of a nonpainful
disc rather than dealing with the actual pain source. This is
countered by the restoration of the functional spinal unit’s
range of motion. However, it is unclear whether the extra
few degrees of range of motion offered by a disc replacement over fusion is functionally significant, especially
given the motion a v ailab le at adjacent le v els and hip joints.
Future models of spinal arthroplasty will no doubt aim
to replicate the elastic properties of the disc without
incurring the problem of wear particles, and also will
attempt to address facet joint pathology. It will take a
great deal of time and carefully controlled long-term trials to determine whether or not these devices offer the
best alternative for patients undergoing surgery for discogenic low back pain.
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2002:11:S106–S110.
CHAPTER 41
Disc Herniation: Definition and Types
Tom Bendix
Disc herniation used to be considered as a local bulge on
a disc surface, causing pressure on a nerve root. This was
based largely on the fact that m y elo gram was the onl y w a y
to establish the diagnosis. As myelogram is a highly invasive procedure, which also exposes the patient to signif icant radiation levels, it is considered ethically unacceptable in the examination of asymptomatic individuals.
Today, computed tomography (CT) and magnetic resonance imaging (MRI) studies in asymptomatic individuals
have shown that the presence of a local bulge on the disc
surface certainly does not correlate convincingly with the
classic symptoms of disc herniation (Fig. 41-1) (1–6).
Such studies have sparked a totally new era for this
diagnosis. It is likely that disc herniation symptoms are
predominantly initiated by nuclear tissue coming into
physical contact with the nerve root, whereas a (local)
bulge does not normally cause nerve damage (Fig. 41-1).
Moreover, after a passed clinical cause the bulge consists
of scar tissue, forming the morphologic “herniation” (7).
Several terms have been used to describe this condition: disc herniation, herniated nucleus pulposus (HNP),
prolapse, and slipped disc are those most often used.
Time has come, however, to redef ine the condition as a
syndrome characterized by nerve damage, primarily
caused by irritation from nuclear tissue, giving rise to a
production of a variety of cytokines and other inflammatory or autoimmune components (8–21), leading to secondary pressure hypersensitivity (22,23).
While lifetime prevalence of sciatica of any etiology is
about 40% (24), no authors have been able to give a serious frequency of lifetime prevalence of lumbar disc herniation (LDH). This may be in agreement with the vague
definitions described later in the chapter that the most
frequent rough estimate is about 3% to 4%. The 1-year
incidence has been estimated at 0.1% to 0.5% (25). Of
patients with acute low back pain, only 1% have nerveroot symptoms (26). Age distribution has its peak close to
40, and male: female ratio is probably close to 1:1 for all
LDH, whereas for those operated upon it is 1.5:1 to 2:1
(5,27,28).
In this chapter only lumbar annular (not end plate) disc
herniation is addressed.
The onset of annular rupture is usually the end point of
gradual disc degeneration, which in turn seems more
related to genetic issues than to physical loads (29). Of
physical factors, flexion plays the greatest role, but even
this factor is not impressive (30).
DEFINITIONS
Considering the aforementioned poor correlation
between clinical and imaging findings, different aspects
of definitions have to be considered.
Patho-Anatomic Types
In accordance with today’s knowledge (as discussed
later in the chapter), the aspect of the nucleus being contained or not seems most relevant as the primary nerve
damage is most likely associated with the inflammatory
influence of free nucleus pulposus tissue on the nerve
root. If nerve damage resulted primarily from pressure, it
is not likely that so many silent disc herniations are seen
in MRI studies. Pressure is, how e ver , ob viousl y important
secondarily, as pressure on any inflamed structure causes
pain. But as the primary lesion, pressure alone only rarely
seems to be relevant (Fig. 41-2).
Contained
The disc bulges locally, but the outermost layer is still
intact. In most cases it is questionable whether this condition causes root damage, rather than referred discogenic pain. If the bulge compresses the dorsal ganglion of
the intervertebral nerve root, however, it is likely that the
presence of nuclear material directly touching the root
399
400 /SECTION V/SPECIFIC CLINICAL ENTITIES
FIG. 41-1. The three disc herniations shown here are
identical on computed tomography or magnetic resonance imaging. Clinically only the one with chemical irr itation of the nerve root (bottom, left) produces symptoms. Bottom right: A healed, but now clinically silent
herniation.
may not be necessary (31,32). This is substantiated by
research demonstrating that the threshold for compression-induced neuronal firing is about half that for other
parts of the nerve root (33). Likewise, if a chemically
inactive bulge causes compression over such a wide area
of the nerve root that the root suffers ischemia (33), this
may also sensitize the nerve root without any chemically
induced lesion (34–36) (Fig. 41-3).
It seems as if contained disc herniations or more dif-
fuse disc herniation displacements (5,37,38) represent a
poorer prognosis as opposed to a more well-defined herniation, probably because the first condition is only a
small part of a more substantial degeneration (Fig. 41-6).
Some authors also divide “contained disc herniations”
into “soft” and “hard” categories. Thus, pain from a primary contained disc herniation is most likely rather a
“simple” discogenic pain, which causes back pain that
dominates over optional leg pain. Such leg pain can be
“radicular”/dermatomal in its perceived location (39), but
will most often be diffuse in its distribution.
FIG. 41-2. The different types of disc herniations.
CHAPTER 41/DISC HERNIATION: DEFINITION AND TYPES / 401
FIG. 41-3. The main basic pathology in disc herniation is
inflammation. Pressure on the ganglion and ischemia (widespread compression) may also cause a primary lesion, but
plays a greater role as secondary irritation when existing
inflammation has caused nerve damage.
Complete = Protruded = Free
The nuclear material has broken through the outermost
layer of annulus fibrosus, and can be seen in the canal,
and is therefore no longer contained. It is extruded from
the fissure of the disc, whether or not it is in continuum
with the central nuclear tissue. It is not quite clear from
literature if a herniation that has passed the outermost
annulus layer, but still remains subligamentous (40), is
considered “free”. Those protruding through the ligament
are called “transligamentous” (40).
Sequestered
A free nucleus fragment, no longer in contact with the
annular canal it originated from, is clinically meaningful
in the interpretation of fluctuating symptoms, and during
surgery.
“No-Bulge Herniation”
FIG. 41-4. The location in the segment can be intraspinal
(a), foraminal (b), or extraforaminal (c).
The intraspinal herniation is most often paramedian
as shown on Fig. 41-4, but can be median as well.
“Extraforaminal” herniation is also known as “extreme
lateral”, and seems to cause a higher degree of pain than
the other types (41).
Clinical T ypes
The classic disc herniation starts after a period with
only back pain, or back pain that dominates over leg
pain/sciatica. When leg pain takes over and dominates,
the course of a disc herniation begins (Fig. 41-5).
It may be argued that the clinical disc herniation begins
with the initial back pain. However, when a period of
(dominating) back pain of discogenic origin starts, it is
only seldom that it is follo w ed by a disc herniation. Moreover, dominating leg pain may follow shortly after the
onset of back pain, or after months or years with back
pain. Probably the short course corresponds to a single,
and “clean” annular rupture, whereas a long-lasting “prodrome” may correspond to a herniation in a disc that is
highly degenerated (Fig. 41-6).
It seems likely that some nucleus-tissue–induced nerve
damage is caused by leakage of nuclear chemicals that do
not physically form a bulge on the disc (Olmarker, personal communication). As with other nerve-root lesions,
a mechanical component is needed to cause the leakage.
Radiologic T ypes
Radiologically, LDH is defined as a localized bulge on a
disc. Several researchers have tried to make a strict distinction between prolapse and disc protrusion by means of
deciding how much of the entire intervertebral-disc circumference is taken up by the bulging “dome”. The reason
why no consensus has ever been made is most likely that it
has become clearer that the correlation between a certain
“dome” and the clinical symptoms is small (1).
Another radiologic categorization refers to the location
of the LDH on the circumference (Fig. 41-4).
FIG. 41-5. A typical course of a disc herniation. See text.
402 /SECTION V/SPECIFIC CLINICAL ENTITIES
FIG. 41-6. The clinical course of a disc herniation in relation to the different stages of what happens in
the disc. See text.
The distribution of leg pain usually follows the root
leaving the spinal canal one level caudal to the herniation
(Fig. 41-7). If the herniation is more centrally located,
the root leaving two levels below will occasionally be
affected. A central herniation may even damage the roots
that exit several le v els belo w, as seen in the “cauda equina
syndrome” (see below). Or it may simply cause—or take
part in—spinal stenosis, most often when the canal is
already narrow at that level (see Chapters 1 and 48).
If the herniation is foraminal or extraforaminal, the
nerve root taking off at the same level may be affected.
About 95% of lumbar herniations are located in the
two lo w er discs, 5% at L3-4, and onl y v ery few abo v e that
level.
The symptoms of “cauda equina syndrome”, where
several sacral roots are involved, include flatus incontinence, urinary incontinence, and groin hypoanesthesia.
The influence on the vesica urticaria sphincter is most
often retention. Urinary incontinence may either be seen
because of retention in terms of overflow, or as a primary
neurologic disturbance. Groin hypoanesthesia can be unilateral or bilateral.
NATURAL HISTORY
The natural course of LDH has certainly not been
accurately stated (42). It varies with:
• different intensity of clinical symptoms including the
degree of paresis
• coexisting pathology, particularly whether the actual
disc is highly degenerated or not, but also recess or
spinal stenosis, spondylolisthesis, and so forth (43)
• psychosocial factors (e.g., employment and higher
social group correlates to a faster recovery) (44).
FIG. 41-7. The typical root irritations with paramedian lumbar disc herniation at various levels.
The course does not seem influenced by sex or age
(44), the latter at least not with complete herniation (5).
It is problematic that the clinical courses described in
various studies represent a variety of definitions of LDH,
most likely including many without actual nuclear herniation. The literature makes no conclusions on an average
course of the different phases.