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CHAPTER 35/DEGENERATIVE DISC DISEASE / 343
ative management including activity modif ication, medication, and active rehabilitation before considering surgical intervention. If the patient fails nonoperative management and has a history and physical examination that
correlates with imaging studies for disc-related pain, then
surgery may be considered. Discography is a provocative
and confir matory test that generally is helpful in determining the source of pain and helps determine which
level(s) should be included in the fusion. The anatomy of
the patient’s disc space needs to be evaluated with respect
to the placement of a pair of fusion cages. In some cases
it is difficult or impossible to properly place a pair of
cages. Sometimes this is the case in patients with a relatively tall disc space, requiring large-diameter cages.
Patients with a significant behavioral component to
their pain are generally poor surgical candidates for cage
fusions. In patients with a presentation that is not clear or
suggests a significant behavioral component, preoperative psychological screening can be very helpful. Block
has reported on the use of such an instrument that we use
in our clinic (13). This instrument was found to predict
success or failure in a high percentage of spine surgery
patients.
BIOMECHANICS OF FUSION CAGES
There have been many reports on the biomechanical
testing of various lumbar fusion cages. However, the
results can be difficult to compare because of differences
in the type of cage used, the specifics of the test parameters used, the manner in which the data are presented and
analyzed, and the fact that most studies suffer from a
small number of specimens tested. The testing can determine the stiffness of a cage, but the ideal stiffness is
unknown; that is, the stiffness that provides optimal stability for fusion to occur, but has the least adverse effect
on the adjacent segment. In other words, the ideal cage
must provide strength, yet still allow load sharing with
the vertebral bodies. As with any laboratory testing, there
is great difficulty in trying to extrapolate the results to the
clinical situation since the role of the musculature has
been eliminated. In addition, testing provides information
only about the immediate effect of the implants. The
device performance once scarring, and bony changes
have occurred cannot be ev aluated. Also, testing provides
comparative data on segmental motion, but cannot determine the optimal amount of permissible motion.
One important parameter is the compressive strength
of the devices. Laboratory testing of fusion cages shows
that they are unlikely to fail from compressive loading.
Jost et al. compared the compressive strength of a carbon
cage, a porous titanium implant, and a metallic threaded
fusion cage with and without posterior fixation (14).
They found that in some specimens, the carbon fiber cage
failed at levels within the range of physiologic load, but
the other two cages did not. P osterior fixation did not sig-
nificantly increase the compressive strength of any of the
devices. Low bone mineral density was associated with
construct failure by device displacement into the vertebral bodies.
Oxland and Lund performed a comprehensive review
of the biomechanics of interbody fusion cages in human
cadaveric specimens (15). They found that, as standalone devices, cages provided stability in flexion but did
not perform as well in extension. Interestingly, there was
no difference in the stability achieved by anterior versus
posterior approach to implanting the devices. This f inding showed that the destruction of the anterior longitudinal ligament by an anterior approach did not result in
reduced stability in extension. Anterior implantation was
associated with more stability in axial rotation than with
posterior surgery . This most likely results from the loss of
the lamina and part of the facet joints required to allow
access to the disc space with posterior implantation of the
devices. The anterior approach also was associated with
greater stability in lateral bending. The authors found that
the addition of posterior fixation increased the stability of
the operated segments.
EARLY FUSION CAGES
One of the earliest cages commonly used was a
femoral ring allograft packed with cancellous bone.
Although this graft was used in the 1980s before the term
“fusion cages” was in vogue, this graft configuration was
similar to that of later cylindrical cages. That is, the dense
bone of the femoral ring provided early support to the
operated segment. There was concern whether the density
of the femoral ring would allow bony ingrowth. The rings
were later packed with a plug of cancellous bone to provide scaffolding for bone growth. In our own experience,
using this graft type in the late 1980s, an 80% fusion rate
was reported for the entire group, including both single
and multiple level fusions (16). In the series of 112
patients, there were no cases of graft collapse. There w ere
seven cases of graft migration, three of which were significant and required reoperation. The migration problems occurred in patients before cutting grooves in the
inferior and superior surfaces of the femoral ring. Subsequently, a cancellous screw was sometimes placed anteriorly as a “doorstop” to prevent anterior graft mig ration.
Holte also reported on the use of femoral ring allograft
packed with autogenous graft (17). Supplemental posterior fixation was used in the majority of these cases, and
a fusion rate as high as 96%, depending on the number of
levels fused , w as reported. In another series with the same
graft construct without posterior f ixation, and with the
addition of an anterior screw to prevent graft mig ration,
the fusion rate was 84% to 97% depending on the definition of fusion used (18). In recent years, there has been
renewed interest in the use of these cages made from allograft femoral rings packed with cancellous bone.
344 /SECTION V/SPECIFIC CLINICAL ENTITIES
THREADED CYLINDRICAL CAGES
Perhaps the best-known fusion cages are threaded
cylindrical fusion cages. The introduction of these devices in the 1990s sparked widespread interest in interbody fusion devices. Two cages are typically placed at
each operated disc level. The cages have fenestrations,
with the largest ones being on the inferior and superior
surfaces to allow bone to grow from the vertebral bodies,
into the bone graft packed inside the cage. These cages
can be placed into the disc space using either an anterior
or posterior surgical approach. With the anterior approach, either an open or a laparoscopic technique may
be used. One such cervical threaded metal cage, the
Bagby and Kuslich (BAK) device (Sulzer-SpineTech,
Minneapolis, MN) evolved from cervical fusion cages
used to treat wobbler’s syndrome in horses (19). The cage
designed for that application was a rectangular cage filled
with bone graft. A veterinarian (Bagby) teamed with an
orthopedic surgeon (Kuslich) to design a device for
human implantation. The result was the BAK cage.
Fusion Rates
Determining the presence or absence of fusion is difficult. The only reliable way to determine if a patient has a
solid union is by reoperation and direct exploration of the
fusion mass. Trying to determine fusion from radiographic images is unreliable. One study inv estigated radiographic imaging and computed tomography (CT) scanning of four different interbody devices that had been
implanted into cadaveric specimens (20). Eight of each
cage type were implanted, and radiographs and CT failed
to identify lucency around some of the devices. The
authors also reported that chips packed into the cages
could look identical to bridging bone through the center
of the cage, thereby making the determination of fusion
difficult. The ability of CT to detect fusion in patients
with metal fusion cages has been questioned by other
authors (21).
In addition to the difficulty in detecting fusion, the
actual definition of fusion itself is not uniform across
studies. In some studies of metal cylindrical fusion cages,
up to 5° of motion on flexion-extension radiographs was
considered acceptable for fusion (22). In one study
involving mesh cages combined with anterior buttress
plates and posterior fixation, 3.5° of motion was permitted for solid fusion (23). Our own experience is similar to
that of McAfee, who noted that a good indication of
fusion with open-ended cages is bridging of bone anterior
to the disc space (Fig. 35-1), the so-called “sentinel
fusion” (24).
Another difficulty encountered in many studies is the
lack of data concerning the repeatability of measurements. This factor could greatly influence the reliability
of the reported fusion rates.
FIG. 35-1. Radiograph showing abundant bone growth anterior to the threaded fusion cages, frequently termed a “sentinel fusion.”
Bone biopsies have been performed on a small number
of patients who received carbon fiber or mesh cages and
who appeared to have a solid fusion on radiographic
imaging (25). The authors reported that there was much
variability in the results of the biopsy; however, all
showed histologic evidence of bone graft incorporation.
In most of the biopsies, there was fibrous and necrotic tissue, suggesting that incorporation was not complete.
They also noted particles of carbon f iber or metal within
the biopsies, although these did not cause bone resorption
or inflammatory reactions.
Thalgott et al. reported a 95% fusion rate in patients
whom instrumentation was not removed for continued
symptoms (23). The criteria for a solid fusion in this
study were rather liberal for a combined anterior posterior fusion procedure, allowing up to 3.5° of motion on
flexion extension films. The authors provided no data on
the reproducibility of their measurement method.
The reported fusion rates for threaded metal cylindrical cages have been as high as 95% or greater for singlelevel fusions (22,26). The fusion rates for two-level procedures are low er, in the range of 71% to 80%. High rates
of fusion of 86% to 100% also have been reported for
carbon cages supplemented with pedicle screws (27–29).
Methods for improving the fusion rate of threaded
fusion cages are being investigated. There have been no
CHAPTER 35/DEGENERATIVE DISC DISEASE / 345
clinical studies investigating the possible role of bone
growth stimulators to enhance fusion with cages. However, in a study involving sheep implanted with titanium
threaded fusion cages packed with autograft, bone
growth stimulation was associated with a greater fusion
rate (30). Other possible methods to enhance the fusion
rate achieved with cages is the use of recombinant human
bone morphogenetic protein-2 (rhBMP-2) (31–33), local
gene therapy (34), and recombinant human osteogenic
protein-1 (rhOP-1) (35). These materials have yielded
high fusion rates in studies using animal models (31,35).
In a small patient series, a 100% fusion rate was seen in
a group of patients receiving BMP packed in tapered
cylindrical cages (36). This material appear to be capable
of achieving a high fusion rate and has the additional benefit of eliminating risk of complications and pain associated with harvesting iliac crest autograft. Larger series of
patients are needed to determine if these promising early
results can be maintained.
CLINICAL RESULTS
The clinical outcome achieved with fusion cages are
difficult to evaluate due to inconsistency of the outcome
measures used, use of nonstandardized and nonvalidated
measurement tools, mix of diagnoses in the various studies, and different techniques and devices used within
individual studies.
It should be noted that several of the large studies discussed in the following were performed as part of the
approval process for sale of the devices in the United
States. These studies ha v e the benefits of enrolling a large
number of patients, being closely monitored, and having
a minimum of 2-year follow-up. However, such studies
also tend to have some shortcomings that make it dif ficult
to extrapolate the results to broader future applications.
These include rigorous inclusion-exclusion criteria and
their use by highly specialized and experienced surgeons
recruited to participate in such studies.
Threaded Cylindrical Titanium Cages
In a large study of patients undergoing BAK cage
fusion, Kuslich et al. reported a significant decrease in
postoperative pain scores (22). However, only 32% of the
group from which the preoperative scores were derived
was included in the 24-month follow-up data. Four-year
follow-up data on a subgroup of 185 patients has been
reported (37). Pain and function were significantly improved at the 3-month follow-up compared to the preoperative v alues and did not deteriorate at 4-year follow-up.
Work status improved from 44.1% preoperatively to
71.2% postoperativel y. The results of this study ha ve been
criticized because some of the 17 patients who required
additional surgery after the cage procedure were counted
as having a good clinical outcome, and only 185 of the
original 947 patients enrolled in the original study were
included in the analysis.
Results also have been reported on the 2-year followup of 226 of 236 patients who received the Ray threaded
fusion cages (TFC) (26). Sixty-five percent of patients
had good to excellent results and 65% had good to excellent function. Unfortunately, this study provided no data
comparing the preoperative to postoperative function, so
that conclusions concerning clinical improvement were
not possible. Poor results were reported with the use of
this cage in a series of only 13 patients, all operated at the
L5-S1 level (38). Seven of the 13 patients went on to
reoperation for symptomatic pseudarthrosis. The authors
felt that the use of oversized cages, destruction of the
anterior longitudinal ligament, and removal of part of the
annulus contributed to the poor outcome.
Threaded Bone Dowels
Threaded bone dowels are similar in design to metal
threaded fusion cages. Their potential benefit is that radiographic assessment is easier because of the lack of ar tifact created by cages. However, there are concerns about
the consistency of the strength of these cages, which are
made from allograft bone. Barnes et al reported 1-year
follow-up on a series of 28 patients undergoing interbody
fusion using threaded cortical bone dowels packed with
autogenous iliac crest bone graft (39). Patients with discrelated pain alone underwent anterior interbody fusion,
using the cages as stand-alone devices. In patients with
concomitant spinal stenosis, decompression and posterior
interbody fusion with supplemental posterior pedicle
screws without bone graft was used. There was a relatively low rate of follow-up in the anterior interbody
fusion group (67%). The rate of fusion was less in the
anterior group than in the posterior group (13% versus
95%). Similarly, patient satisfaction was greater in the
posterior group than in the anterior group (38% versus
70%). Based on their results, the authors strongly advocated the use of posterior fixation in addition with the
threaded bone dowels.
There have been a few studies comparing the use of
bone dowels to other fusion procedures. In one such
study, laparoscopic anterior interbody fusion using
threaded cortical bone dowels was compared to posterior
fusion using pedicle screws (40). The laparoscopic group
had shorter hospital stay, less blood loss, and less operative time than the posterior fusion group. However, it
should be noted that the study was not randomized; therefore, there may have been differences in the patients
treated with the two procedures. In a prospective randomized study, Schofferman et al. compared threaded
titanium cylindrical cages packed with autograft to
threaded bone dowel cages packed with demineralized
bone matrix (41). At the 12-month follow-up, both
groups improved significantly based on Oswestry and
346 /SECTION V/SPECIFIC CLINICAL ENTITIES
pain scores, and there were no significant differences in
outcome between the two groups.
Carbon Fiber Cages
Carbon fiber cages are designed to be used in pairs and
with posterior fixation. The carbon fiber allows for easier
assessment of fusion status because this material is not
visible on radiographic images and does not create artifact on CT scans. One disadvantage of carbon fiber cages
is that they may fracture or collapse, which may result in
release of some of the carbon fibers. Brantigan repor ted
2-year follow-up data for 221 patients receiving carbon
fiber cages and pedicle screw fixation as part of a multicenter study (28). In the subgroup of 92 patients with a
diagnosis of DDD, the fusion rate was 100% and 86%
had a good clinical outcome; however, there were numerous complications in the series.
There have been other reports on the use of carbon
fiber cages. One series included 71 patients with carbon
fiber cages and posterior fixation (27). It reported a 90%
fusion rate, but at the median follow-up of 28 months,
only 66% of patients w ere satisfied with the results of the
surgery and would have the procedure again for the same
result. Two studies on the use of carbon fiber cages have
reported fusion rates of 82% and 86%, but provided no
clinical outcome data (29,42).
Mesh Cages
Mesh cages, also sometimes referred to as a type of
vertical cage, can be cut to the height desired to f it into
the disc space. The cages are packed with bone graft and
inserted into the disc space. Reinforcement rings may be
placed around the superior and inferior ends of the cage
to provide axial support. There have been only a few
reports on the clinical results of mesh cages, none of
which deal specifically with disc-related pain. The
patient population in these studies was typically a mixed
group of patients with pseudoarthrosis, deformity, postlaminectomy syndrome, or disc-related pain. One study
described the results of 50 patients undergoing combined
antero-posterior lumbar fusion using mesh cages packed
with coralline hydroxyapatite mixed with demineralized
bone matrix (23). A buttress plate was placed anteriorly
over the operated segment to prevent potential displacement of the cages. Pedicle screws or facet screws were
used with the posterior fusion. The mean follow-up was
50 months, ranging from 36 to 64 months. The authors
reported good results with the procedure, although there
were several cases requiring posterior fixation removal.
One study evaluated the outcome following nonthreaded cages (either Brantigan or Harms) for the treatment of single-level, disc-related pain w as performed in a
group of 15 active-duty servicemen (43). The cages were
inserted posteriorly and packed with autogenous iliac
FIG. 35-2. Modular rectangular fusion cage.
crest graft and were supplemented with pedicle screw
fusion. The authors reported that 12 of the 15 servicemen,
80%, returned to full duty. This compared favorably to
the 36% who returned to active duty in a selected group
who elected not to have surgery for their single-level
symptomatic disc degeneration.
Modular Rectangular Cages
There is a rectangular, modular cage (InFix; Spinal
Concepts, Austin, TX) (Fig. 35-2) designed to be used as
a stand-alone device and to be implanted using an anterior approach to the spine. It consists of two plates with
struts that are placed on the periphery of the plates to
control the height and angulation between the plates,
thereby allowing anatomic restoration of the disc height
and lordosis. The plates have holes to allow bony
ingrowth from the vertebral bodies to unite with the bone
packed inside the cage. This device is being evaluated in
a large, multicenter study, but currently no results from
this study have been reported.
LAPAROSCOPIC LUMBAR INTERBODY FUSION
The introduction of fusion cages came at a time when
laparoscopic spinal fusion was being developed. The
cages complemented this evolving surgical technique.
They were small enough to be passed through the cannulas used in the laparoscopic procedures. The role of
laparoscopic spinal fusion has gained some acceptance
but some still question if it reduces morbidity, reduces
hospital stay, and reduces recovery time. The endoscopic
technique has been criticized as having an increased complication rate, being too expensive, being too diff icult to
perform at the L4-5 level, and offering no benefit over
open surgery.
Laparoscopic spine surgery requires developing new
skills not typically learned by spine surgeons. They must
CHAPTER 35/DEGENERATIVE DISC DISEASE / 347
manipulate the instruments based on what is viewed on a
video monitor. Several authors have reported on the
learning curve associated with laparoscopic spine surgery
(44–46). In several reports, laparoscopic fusion is reported to have longer operating time but less blood loss
than open anterior fusion (47,48). In both of these studies, laparoscopic surgery was also associated with
reduced hospitalization time. There is debate concerning
the effectiveness and safety of using endoscopic techniques to fuse the L4-5 level. Vraney reviewed radiographic vascular examinations performed for vascular
conditions, and recorded the feasibility of accessing the
L4-5 disc using laparoscopic techniques (49). He suggested that in only one-third of patients could the disc
space be accessed safely because of the vascular anatom y.
Zdeblick also expressed concern about the safety of
accessing the L4-5 disc due to a greater incidence of
complications (50). He advocated that laparoscopic
fusion was feasible at L4-5 if the bifurcation of the vessels was above the L4-5 space and that an open approach
should be used if the bifurcation was at or belo w the disc.
Kathkouda et al. reported the feasibility of laparoscopic
surgery at the L5-1 level (51), but discouraged its use for
multilevel procedures because of difficulty accessing the
L4-5 disc. However, it should be noted that they performed only 24 cases in the 3 years of their study. This
small a number of cases may suggest that they had not
overcome the learning curve associated with the technique. The laparoscopic approach to the L4-5 space was
analyzed by Regan et al. (52), who found that by varying
the approach to the L4-5 disc space, based on the location
of the bifurcation of the great vessels, the disc space
could be accessed safely laparoscopically. They noted
that no patients were denied laparoscopic fusion because
of vascular anatomy at the L4-5 level. The ability to
safely assess the L4-5 disc laparoscopically has been
reported by other authors as well (47,53).
Results from an animal study using mesh cages
showed that laparoscopic fusion resulted in a less stiff
spinal segment than that achieved by an open procedure
(54). They attributed this difference primarily to the fact
that less of a discectomy and less decortication of the end
plates were performed laparoscopically than with an open
procedure. The authors felt that these factors contributed
to less bone growth into the cages, thereby reducing the
stiffness of the fused segment. These laboratory findings
were supported in a clinical study by McAfee et al. comparing the fusion rate achieved with partial versus complete discectomy (55). The y found that the fusion rate was
significantly greater in the group in whom a complete
discectomy was performed than in patients having a partial discectomy. These results suggest that it is desirable
to perform a complete discectomy when using fusion
cages. This has been confirmed in another study (47).
There has been one published report of a large number
of laparoscopic spine fusion cases performed using cages
(48). The authors of this study reported on a multicenter
series of 240 consecutive patients. The series was compared to a historical cohort of 591 consecutive patients
undergoing open anterior lumbar interbody fusion using
the same design of fusion cages. They found that the
laparoscopic technique was associated with reduced hospital stay, less blood loss, but had greater operative time.
Complications in the two groups were comparable.
There has been one prospective, randomized study
directly comparing the results of laparoscopic to open
anterior lumbar interbody fusion using the same design
of interbody fusion cages (47). The authors reported that
the laparoscopic procedure was associated with a longer
operating time but reduced hospital stay. The hospital
costs for the two techniques were similar. The laparoscopic group had a greater percentage of patients who
returned to work and they did so more quickly than
patients undergoing open fusion.
Surgeons considering performing laparoscopic fusions
must have appropriate training in the technique and
should do enough of the procedures to overcome the
learning curve in order to attain and maintain a high skill
level with this procedure.
ANTERIOR VERSUS POSTERIOR APPRO A CH
FOR DEVICE IMPLANTATION
Several interbody fusion cages can be implanted from
either an anterior or posterior approach to the disc space.
There appears to be no recommended preference of approach based on clinical results from the procedure.
Therefore, the decision regarding the approach generally
is based on several other factors. One important factor is
the training and experience of the surgeon. If there is
neural compression, a posterior approach is preferable in
order to address these problems. A posterior approach
reduces the risk of injury to the major anterior vascular
structures and also avoids damage to neural structures
that can result in retrograde ejaculation. If the patient has
significant calcification of the vessels or prior abdominal
surgery in the vicinity of the disc level to be operated,
anterior surgery may be risky. However, there are several
advantages to the anterior approach. It provides a wider
access to the disc space allowing more room to work.
Posterior interbody fusion has a potential risk of injury to
the nerve roots owing to overretraction and requires
remov al of some of the facet joints, which may contribute
to instability if too much bone is removed. Finally, the
posterior approach damages the posterior musculature,
which may be a source of pain and disability.
DISTRACTION AND SUBSIDENCE
Interbody fusion should restore normal disc space
height and prevent future collapse. Most fusion cages are
strong enough to prevent failure and collapse. Disc space
348 /SECTION V/SPECIFIC CLINICAL ENTITIES
height may be lost if the cages subside into the adjacent
vertebral bodies. In a study using a sheep model, it was
found that fusion cages significantly distracted the disc
space (56). Although there was subsidence for 2 months
after surgery, by 4 months the operated levels were
solidly fused and the disc space height was greater than it
was preoperatively. Distraction and maintenance of the
disc space height have been investigated in two recent
clinical studies (57,58). Both studies reported reduction
of initial disc space height following surgery, but the
height remained greater than the preoperative height.
Distraction of the disc space height also results in indirect
decompression of the neural foramen. Results of a laboratory study showed that the implantation of interbody
cages resulted in opening of the foramen (59). This finding was supported in a clinical study investigating
changes in foraminal height when cages were inserted
into collapsed disc spaces (60). These findings paralleled
those of the studies on distraction of the disc space. That
is, although the foraminal height decreased over the 2year study period, it remained greater than the preoperative value.
LUMBAR LORDOSIS
It is desirable to create or maintain normal lumbar lordosis when fusing the spine. However, the impact of
minor variation in alignment on clinical results has not
been established. Klemme et al. compared the amount of
lordosis produced by threaded devices alone to vertical
cages with posterior fixation (61). They found that cylindrical fusion cages placed parallel to the end plates did
not maintain lumbar lordosis as well as vertical mesh
cages combined with pedicle screws. Another study
investigated sagittal alignment with threaded fusion
cages (62). The authors found that there was a significant
decreased in lumbar lordosis at 2-year follow-up in
patients with a posterior interbody fusion, although the
values were within a normal range. They also found that
lumbar lordosis was not related to ultimate clinical outcome. Another study compared the degree of lordosis
achieved with wedged-shaped cages compared to rectangular cages (63). All cages were made of polyetheretherketone. The authors found that both cage designs improved the sagittal alignment and that there was no
particular benef it with the wedge-shaped cages.
CAGES AS STAND-ALONE DEVICES
There is great debate concerning the use of fusion
cages without supplemental posterior fixation. Biomechanical studies show that greater stability is achieved
when posterior fixation is included. However, the question of how much immediate stability is needed to
achieve good long-term outcome remains unanswered.
The potential disadvantages of supplemental posterior
fixation include increased operating time, increased
blood loss, increased cost, damage to the posterior musculature, and risk of mechanical failure and reoperations
associated with the posterior instrumentation. In addition,
the additional stiffness provided by supplemental posterior fixation may be associated with long-term accelerated breakdown of the adjacent segment. Se veral methods
have been proposed to address some of these potential
problems. There include use of pedicle screw fixation
without bone graft, facet screw fusion, and use of translaminar screws without bone graft.
Although there is concern about the use of cages as
stand-alone devices, there is little clinical evidence that
there is a significant problem when using them in this
manner. The clinical outcome reported from the large
series of patients enrolled in the Food and Drug Administration Investigational Device Exemption (IDE) studies
for the BAK and Ray TFC cages indicate that the devices
perform well without supplemental posterior fixation.
Hacker et al. compared the results of stand-alone posterior interbody fusion with titanium threaded fusion cages
with combined antero-posterior fusion using allograft
dowels (64). There was no significant difference in the
percentages of patients reporting excellent or good
results in the two groups. The total costs in the standalone cage group were less than in the combined group,
although the follow-up time for the former was less than
that of the combined antero-posterior fusion group.
O’Dowd et al. reported a revision rate of 31% at a
mean of 17.8 months (range, 2 to 25 months) when using
cages alone in anterior interbody procedures (65). In a
review with 3- to 7-year follow-up of patients in whom
cages were used as stand-alone devices, Tran et al.
reported that 8.1% of patients underwent reoperation at
the same level (66). However, 2.1% of this group underwent reoperation soon after the initial surgery to address
problems with cage placement. Therefore, only 5.2% of
the group underwent reoperation at the same level as the
index surgery for unresolved or new onset pain. Their
results support the concept that cages can be used effectively as stand-alone devices. In the large-scale studies
reported by Kuslich et al. and Ray, in which cages were
used as stand-alone devices, there was not a high reoperation rate to add supplemental posterior fixation (26,37).
In one small study of interbody fusion using cages
only, cages supplemented with in situ posterolateral bone
graft were compared to cages supplemented with posterolateral pedicle screw fixation (67). Cage subsidence
was more common in the group without pedicle screw
fixation, and device loosening was more common at the
L3-4 and L4-5 levels. They suggested that pedicle screw
fixation should be used when operating at levels above
L5-1. How e ver , this study did not compare clinical results
to determine if there were any differences in outcome.
Carbon fiber cages were designed to be used with
pedicle screw fixation (28). In Brantigan’s study, the
CHAPTER 35/DEGENERATIVE DISC DISEASE / 349
fusion rate was reported to be 100% in a subgroup of
their patients, although their incidence of reoperation and
complications was much greater than in the other studies
using cylindrical titanium cages as stand-alone devices
(26,37,66). There is a trend now among many clinicians
to do combined antero-posterior fusions. Fusion cages in
the proper clinical setting have proven useful as standalone devices. The best candidates are those who have
relativel y narro w disc height compared to those with normal heights.
COMPLETE VERSUS PARTIAL DISCECTOMY
There has been some discussion concerning the optimal preparation of a disc space prior to the implantation
of threaded fusion cages. Some of these devices have
been implanted after the removal of cylindrical plugs of
disc tissue to make a space for the cages to be inserted.
The primary theoretical benefit of this method is less
damage to the anterior longitudinal ligament. The practicality of the situation is that once the surgeon finishes
putting both cages of a dual cage construct, there is negligible anterior longitudinal ligament left. The potential
benefits of complete removal of the disc are elimination
of the pain generator and removal of disc tissue that
might promote growth of fibrous tissue into the cage. The
influence of complete versus partial discectomy in
achieving solid fusion when using threaded titanium
cages has been investigated in a randomized study (55).
At the 2-year follow-up, all patients in whom a total discectomy was performed had a solid fusion compared to
86% fusion rate in group in whom a reamed channel discectomy was done. The importance of total discectomy
was also discussed by Sachs et al., who reported a high
fusion rate using the cages as stand alone devices (47). In
general it makes intuitive sense to carry out a complete
discectomy if a fusion is a desired end point.
COMPLICATIONS
Complications encountered in the treatment of symptomatic disc degeneration are discussed in detail in
another chapter of this book (Blumenthal and Ohnmeiss).
In general, the complications associated with the use of
fusion cages are similar to those reported for other interbody fusion methods. We could not f ind any reports of
frank device failure related to the use of threaded metal
cylindrical cages. As expected, the anterior approach to
the disc space is associated with problems related to
injury of vascular and sympathetic structures as are
encountered with fusion not involving cages. Posteriorly,
complications have been related to retraction of the nerve
roots to allow access to the disc space. There have been
reports of a few cases of device migration or malpositioned cages requiring reoperation for revision. In a
review of 20 patients with cage-related complications,
McAfee concluded that the problems were owing to technical error in all cases (68).
DISCUSSION
In recent years, a variety of fusion cages have been
used as devices for interbody fusion procedures. They can
provide initial stability to the operated segment and can
increase the fusion rate. As with other spine surgery procedures, many questions remain concerning the use of the
cages. There has been much discussion concerning optimal cage design. Items that have been discussed include
the shape and modulus of elasticity of the cages, the optimal bone–implant interface area, the degree of penetration of the cage into the end plates of the vertebral bodies, and the use of posterior fixation. There have been
attempts to address some of the issues in laboratory testing, but they have not been adequately addressed in clinical studies. The use of cages as stand-alone devices
remains controversial. There are reports of good results
with this method. However, although there has been
much discussion concerning the potential problems with
this procedure, there have been little or no data published
on a large series of patients in whom the use of cages as
stand-alone devices was related to poor results. Although
routine use of posterior fixation may possibly minimize
problems related to using cages as stand-alone devices,
this increases costs, operative morbidity, and the potential
for complications related to the additional instrumentation. As with other spine surgery procedures, there is a
need for well-defined clinical outcome studies on cages
investigating the impact of multiple factors such as
approach and technique, posterior fixation, discectomy
technique, graft material, device shape, size, and placement.
Although fusion cages have been reported to yield
good results in many patients, surgeons must pay careful
attention to patient selection and surgical technique.
There needs to be a thorough diagnostic evaluation with
correlative imaging studies. Patients should have failed
an adequate course of nonoperative management prior to
being considered a candidate for surgery. Patients with a
poor psychological profile will likely do poorly with
fusion using cages or any other operative intervention.
There is little doubt that the number of future interbody
fusions will decrease as the use of disc prostheses
increases. The indications for some of these devices will
be similar to the indications for fusion cages in the treatment of disc-related pain. However, spinal fusion will
continue to be performed for specif ic conditions and in
patients who are not good disc replacement candidates.
As discussed, the details of spinal fusion cage procedures
needs further investigation, although based on the data
currently available, these devices appear to have a significant role in the treatment of patients with back pain and
remain an important part of the spine surgeon’s arma-
350 /SECTION V/SPECIFIC CLINICAL ENTITIES
mentarium. We believe that in the properly selected patient, cages offer the surgeon a satisfactory option as a
stand-alone device.
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CHAPTER 36
Minimally Invasive Procedures for Anterior Column Fusion and Reconstruction
H. Michael Mayer
The term “minimally inv asive” has been used in the surgical scientific literature since the introduction of microsurgical and endoscopic surgical approaches. It has been
applied in various fields, mainly abdominal, gynecologic,
and thoracic surgery (1–3). Although arthroscopic techniques in the peripheral joints or microsurgical techniques
for discectomy or decompression have been used for
many years in orthopedic surgery, the term “minimally
invasive” was very rarely used or associated with these
procedures. In fact, it has only come to our perception in
recent years, when it was increasingly used to describe or
characterize procedures or surgical approaches for the
treatment of degenerative lumbar disc disorders.
It is important to distinguish between “true” minimally
invasive procedures for diagnostic and therapeutic purposes and minimally invasive approaches for curative
surgical procedures. Typical examples for minimally
invasive diagnostic and therapeutic procedures include
different kinds of infiltrations including epidural catheters, root blocks, facet joint block, discography, intradiscal electrothermal therapy, and others. These procedures,
how e v er , are either “diagnostic instruments” that are used
to supplement information from noninvasive imaging
techniques such as magnetic resonance imaging (MRI),
or represent noncurative modalities with temporary therapeutic effects. They should thus be classif ied as semiinvasive conservative measures.
For the definitive surgical treatment of degenerative
disorders of the lumbar spine, a variety of minimally
invasive techniques have been developed over the last 15
years. All these techniques represent surgical approaches
that are less invasive than the usual standard approaches
(Table 36-1).
This leads to a very fundamental but important concept
which should be appreciated to avoid misunderstandings
and misinterpretations: minimally invasive surgery for
the definitive curative treatment of segmental lumbar disc
degeneration is a minimally in v asive approach to perform
“target surgery” such as disc excision, fusion, or disc
replacement—procedures that are (maximally) invasive.
Wrong indications for surgery, undesired side effects,
complications, and poor results are strongly influenced
by the surgical approach to the target area (4,5). Less
invasive techniques, in general, decrease the degree of
“iatrogenic” surgical trauma. They ameliorate early postoperative morbidity and enable earl y and aggressiv e rehabilitation of the patient without an increase in complications. This chapter describes the rationale for surgery for
degenerative lumbar spine disorders, the goals of sur gical
procedures, and the implementation of minimally invasive techniques into the surgical standard strategies.
RATIONALES FOR SURGERY
There is a long-standing controversy about the surgical
treatment of degenerative lumbar disc “disease.”
Although there are no evidence-based data to support
spinal fusion or reconstruction of the “functional spinal
unit,” surgery is performed worldwide with varying frequency depending on national or continental philosophies. The “gold standard” procedure has always been
segmental spinal fusion. This can be performed by different techniques and has become one of the classic “experience-based” procedures with poorly predictable success
rates due to the lack of an international consensus for
patient selection, surgical approach, fusion technique,
and postoperative management (6–9).
In the last 2 years, there has been a tremendous acceleration in the development and application of a new philosophy that is termed “spine arthroplasty.” This term
encompasses all surgical techniques that aim for a
dynamic reconstruction and preservation of motion without performing a fusion (Table 36-2). Principles of some
of these procedures are described in Chapters 38–40.
352