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1999;24:1820–1832.
CHAPTER 26
Spinal Instrumentation Overview in Lumbar Degenerative Disorders: Cages
Kenneth M.C. Cheung and John C.Y. Leong
ROLE OF CAGES IN THE LUMBAR SPINE
Cages are interbody spacers used to bridge or reconstruct
the interval between tw o vertebral bodies (1–4). There are
many different designs, but in general, they all provide a
mechanically strong scaffold inside of which osteoinductive or osteoconductive materials can be placed. Such
material maybe autogenous bone graft, allograft, or more
recently bone morphogenetic proteins (5–10).
Cages are used in either degenerative conditions of the
spine requiring fusion, or as a form of anterior column
reconstruction after destruction by tumor, infection, or
trauma. This overview concentrates on its application in
degenerative spinal conditions, in which cages are either
used as an anterior load-sharing device after posterior
spinal instrumentation and fusion (1,4,7,11,12); or as a
replacement for bone graft after anterior lumbar interbody fusion (ALIF) (13), sometimes referred to as a
“stand-alone” cage.
TYPES OF CAGES
There are no universally accepted classifications for
the cages available today. However, they can be broadly
divided into different types by their design or material
(1).
Three general designs are used, as follows.
Horizontal Cylinders
One example is the Bagby and Kuslich lumbar
(BAK/L) interbody fusion cage (Sulzer Spine-Tech, Minneapolis, MN) (Fig. 26-1). These devices generally are
made from titanium. Usually a pair is inserted together b y
either the anterior or posterior approach, and by use of
either open or minimally invasive techniques (11,13–19).
Vertical Rings
An example is the titanium mesh cage manufactured
by DePuy AcroMed (Johnson & Johnson, Raynham,
MA), sometimes referred to as the Harms cage (Fig. 26-
2). They are designed to allow the length to be cut as
desired; therefore, they can be used to span either a single disc space or multiple segments of the lumbar spine.
Because of the presence of sharp edges, they are usually
and more safely inserted via an open technique (20,21).
Open Boxes
An example is the Brantigan carbon cage (Fig. 26-3)
(22–24). The bo x es are designed either with conical superior and inferior surfaces for an anatomic f it into the disc
space, or as wedges to recreate lumbar lordosis. Some are
designed as a single large cage, which requires an open
technique for insertion; whereas others involve two
smaller rectangular cages, which can be inserted posteriorly using minimally invasive techniques.
Cage Materials
In general, these are usually made from titanium,
carbon, or a carbon composite (polyetheretherketone
[PEEK]). Titanium is a mechanically strong and bio-inert
material that is magnetic resonance imaging (MRI) compatible. Therefore, cages made using this material can
have relatively thin struts that allow more room for bone
graft in its interior. The presence of a metallic shadow on
radiographs makes the assessment of bony fusion within
the cage difficult. Some surgeons attempt to overcome
this by placing bone graft anterior to the cage as well as
within it (5). Carbon and PEEK cages are radiolucent and
allow for an easier assessment of bone fusion. Addition-
286
FIG. 26-1. Bagby and Kuslich lumbar interbody fusion cage,
an example of a threaded horizontal cylinder.
ally, they hav e a Young’s modulus that nearly matches that
of cortical bone. This allows the compressive load on the
cage to be shared by the bone graft inside the cage and
may facilitate a more consistent and rapid bone fusion.
Both carbon and PEEK cages are also MRI compatible.
INDICATIONS FOR CAGES
Before the advent of cages, bone grafts such as tricortical iliac crest grafts and fibula strut grafts were used
very successfully as a form of anterior column support
and to promote spinal fusion (25,26). Thus, there are no
absolute indications for the use of cages, and the value of
cages as an alternative to autogenous bone graft should
be judged with the latter as a gold standard. Accordingly,
the indication for use of a cage is the same as that for
bone graft, namely, anterior column reconstruction and
fusion.
Proponents of cages suggest that they may ha v e a number of advantages over conventional bone graft material
alone. Such advantages mainly stem from the separation
of their mechanical and biological roles and may include:
1. Enhanced mechanical stability
2. Maintenance of intervertebral disc height
3. Avoidance of bone graft donor site morbidity by
using alternative osteoconductive and osteoinductive
materials inside the cage.
FIG. 26-2. Titanium mesh cage (Harms), an example of a
vertical cylinder. End-rings (right side) can be added to the
reduce stress concentration at the ends of the cage, and
therefore reduce the chance of sinking into the end plate.
CHAPTER 26/SPINAL INSTRUMENTATION OVERVIEW / 287
FIG. 26-3. Brantigan carbon cage, an example of an open
box design.
Cages are designed to have better mechanical stability
than bone graft by virtue of their material and design.
Cylindrical cages have a threaded design (Fig. 26-1) and
use a screw-in technique for insertion (14,27), whereas
box cages have a serrated surface (Fig. 26-3) (13,28).
Both help to increase motion segment stiffness and resistance to pull-out. It is claimed that this may improve the
chance of a successful fusion, although this point remains
to be proved scientifically.
Cages may be better at maintaining disc height than bone
graft alone. In our long-term follow-up study (mean duration of 14 years) of 67 patients that underwent anterior lumbar interbody fusion at L4-5 with autologous iliac crest
graft (29), we demonstrated that there was an initial distraction of the disc space by 34% (4.1 mm) followed by an
eventual partial loss of this distraction in 86% of cases
despite a successful fusion. The mean preoperative disc
space height was 12.1 mm, increased immediately after
surgery to 16.2 mm, but settled to 12.6 mm at the latest follow-up. The reduction in distraction occurred within the
first 3 months after surgery and was correlated with age,
but not with recurrence of symptoms, the amount of initial
distraction or sex of the individual. L4-5 segmental angulation followed a similar trend. The early loss of disc space
distraction likely resulted from moulding of the bone graft
into the created space, and bone graft softening and resorption during remodeling. Studies have sho wn that restoration
of disc space height tightens the posterior ligaments and
opens up the intervertebral foramina, thereby indirectly
decompressing the neural foramina exiting nerve roots (30).
Although this has been used as a justification for using
cages, it should be borne in mind that our study showed that
the loss of disc space distraction did not correlate with
return of the patients’ symptoms, and no long-term clinical
study is available to demonstrate that disc space height is
maintained with cages. Indeed, there is some evidence in
both animal and human studies that disc space reduction
and loss of lordosis also occur with threaded cylindrical
cages (31,32). Special considerations should be given to
patients with reduced bone density because there is a risk of
end-plate fracture by the implanted cage, which may result
in loss of distraction (29,33).
288 /SECTION IV/SURGERY
Avoidance of donor site morbidity (34,35) is often
cited as another reason for using a cage. However, until
recently only osteoconductive materials, such as allograft
bone and hydroxyapatite b locks, were availab le as replacements for autogenous bone. We caution that the use of
such materials as the ultimate aim is still to achieve a
fusion and autogenous bone graft is still currently the best
material for this purpose. There have been several studies
examining the use of bone morphogenetic proteins
placed inside cages in human patients (8,10). Both studies concluded that recombinant bone morphogenetic protein-2 (rhBMP-2), can effectiv ely achieve a spinal fusion,
although the number of patients involved were small, and
the follow-up short. Large-scale randomized controlled
trials with long-term follow-up are required to prove this
definitively.
CHOICE OF CAGES
The ideal cage should rigidly immobilize the spine in
all directions, be strong enough to withstand repeated
loading, and have a modulus of elasticity close to that of
cortical bone. It should also be easy to insert by either an
open or minimally in vasive approach, and should be clinically effective by randomized controlled trials and longterm follow-up.
It is difficult to advise readers on the optimal choice of
cage because of the wide variety of cages currently available, with more likely to come on the market in the
future. However, users should be aware of some biomechanical and clinical considerations with the currently
available designs.
One area of concern with the use of cages compared to
bone graft is that the stiff struts of the cage stress shield
the bone graft placed inside the cage. Because bone heals
best under compression, this could result in reduced
fusion rates. In a study by Kanayama et al. (37), a calf
spine model was used to quantify the stress-shielding
effects of 11 lumbar interbody fusion cages by measuring
pressure within the cages. This was achieved by injecting
the cages with a silicon elastomer before insertion, and
intracage pressures were measured using pressure needle
transducers. The authors concluded that threaded fusion
cages demonstrated significantly lower intracage pressures compared with nonthreaded cages and structural
allografts. Whether this is clinically relevant is not
known, because there are no comparati ve studies e xamining fusion rates among the various types of cages.
The possibility of stress shielding and the presence of
motion within the spine segment despite the use of cages
raise the question of whether they can be used as standalone devices, without supplemental fixation. Because of
a lack of clinical studies with long-term follow-up, this
question cannot be currently answered. There are certainly advocates who recommend supplemental posterior
fixation to improve the chance of fusion (38,42–46). It
should be borne in mind that iliac crest bone grafting
without instrumentation has been used successfully to
perform anterior lumbar interbody fusion with fusion
rates for a single level fusion of up to 96% (26,29,47,48).
This should be the benchmark against which fusions with
cages are compared.
Clinical Considerations
Biomechanical Considerations
In general, studies comparing the stiffness of the spine
segment after cage insertion have shown no signif icant
differences among the various designs (13,36–38). All
are effective at stabilizing the spine, compared with its
intact condition, in flexion, axial rotation, and lateral
bending. This effectiveness is dependent on their ability
to distract the surrounding soft tissues and their contact
with the host bone. Hence, the cage needs to be appropriately sized for height as well as fit (13,39). Differences
within a particular category of cage design may have an
effect on mechanical stability. For instance, cages with
sharp teeth have higher “pull-out” forces (13), whereas
Harms cages without the addition of the end-rings impart
only marginally better stability in rotation when compared to bone graft (40). All stand-alone cages, however,
are less effectiv e in stabilizing the spine in e xtension, and
motion between the cage–bone junction is present unless
supplemental fixation is used (41). Addition of f ixation,
whether anterior or posterior, and by translaminar or
pedicle screws, will significantly improve the fusion segment stiffness (3,36,38,42).
Interbody fusion in degenerative disc disease serves a
number of purposes. First, by removing the disc it
removes a potential source of pain, and by removing herniated fragments it decompresses the nerve roots. Second, fusion stabilizes the segment and augments the anterior column. Third, restoration of disc space height
tightens up the posterior ligaments and opens up the
intervertebral foramina, thus indirectly decompressing
the nerve roots (30,31). The first two aims can be adequately achiev ed by anterior interbody fusion with tricortical iliac crest bone graft, whereas a cage may be
required for the third aim. However, before a rational
choice of fusion devices can be made, surgeons should be
aware of the clinical track record of cages.
There are very few reports of long-term results of
cages. One of the longest and earliest experiences in the
use of interbody spacer was by the senior author in 1994
(49). A titanium mesh interbody spacer was inserted as a
stand-alone device without bone grafting in 23 patients
with an average follow-up period of 8 years (range, 5 to
12 years). This study demonstrated that bony ingrowth
into the titanium mesh occurred in 18 of 23 patients, as
shown by lack of a radiolucent line at the bone–implant
CHAPTER 26/SPINAL INSTRUMENTATION OVERVIEW / 289
junction, and was subsequently confirmed in retrieval
studies. However, six of these 18 implants failed by a
mid-substance disruption of the mesh, with three implants developing a crack, and three becoming
deformed. In all six patients, movement could be demonstrated between the adjacent vertebral bodies, despite the
lack of a radiolucent line. The authors postulated that
with the solid metal–bone interface, the mesh became
subjected to more stress during flexion and extension,
such that if solid bone fusion did not occur, the implant
failed in its mid-substance. One of the best ways to
demonstrate bone fusion is the presence of anterior
bridging bone (Fig. 26-4). This occurred in four of the 23
cases, and when this was seen, no further movement
between the adjacent vertebral bodies could be demonstrated on flexion and extension, and no mesh failures
occurred.
Based on this experience, the authors feel that one
should make the distinction between bone ingrowth into
the periphery of the cage, and bone fusion, which extends
from one end plate to the other. If the latter does not
occur, then the cage may fail in the long term. Thus, in
reading the literature on the results of cages, the reader
should make this distinction and note the duration of follow-up. Any study with less than 5 years of follow-up
is very unlikely to see cage failure because of bone
ingrowth only and the lack of a solid bone fusion.
One study examined needle biopsies from tissue within
radiographically successful intervertebral body fusion
cages filled with autograft (6). Five cages were implanted
anteriorly, one with additional fixation, and four cages
were implanted as part of a posterior lumbar interbody
fusion. Five were carbon cages (Brantigan cage), whereas
four were titanium mesh (Harms Cage), with a mean
postimplantation biopsy duration of 28 months (range, 8
to 72 months). Biopsies were obtained from within the
center of the cages and showed small fragments of
FIG. 26-4. Implanted titanium mesh block showing anterior
bone bridging and solid fusion.
necrotic bone associated with viable bone and restoration
of hematopoietic bone marrow. Numerous cement lines
demarcated the edges of previous cycles of remodeling,
and the ratio of necrotic to viable bone varied greatly
among cases. Small particles of debris were found in four
of the five carbon-fiber cages and in one of the four specimens from titanium cages, but there was no visible bone
resorption or inflammation. It could be interpreted that,
despite stress-shielding by the cage, solid bone fusion
eventually may occur, although bone grafts contained
within these cages are still undergoing remodeling
beyond the 3 to 6 months that autograft is normally
expected to fuse and remodel.
In 1997, Ray (50) reported his initial results of using a
stand-alone threaded cage packed with autogenous bone
graft and inserted via a posterior approach. Fusion, as
assessed by plain lateral radiographs and lack of movement on flexion-extension radiographs, was said to occur
in 91%. Lack of movement may result only from bony
ingrowth into the superior and inferior faces of the cage
for a limited distance. The word “fusion” should be used
only if there is evidence of continuous bony ingrowth
through the entire extent of the cage. To date, this has not
been definitively demonstrated in any study.
Kuslich et al. reported on the 4-year follow-up results
of the BAK cage in 25.6% of the original study population (18). Their overall “fusion rate” was 91.7% and
95.1% at 2 and 4 years, respectively. This was from a
combination of anterior and posterior approaches.
Whether or not additional instrumentation was used is
unknown. The late-occurring complication rate was
13.8%. Complications necessitating a second operation
occurred in 8.7%, and reoperations directly related to the
device occurred in 3.1%. This study has been criticized
for the small number of patients available for follow-up
assessment (51,52).
Studies examining the use of other cages have tended
to be combined with posterior fixation, and they all
report “fusion rates” in the range of 90% to 100%
(7,12,20,23,24,53). However, it should be noted that no
movement would be detectable on flexion-extension radiographs with solid posterior f ixation, thereby hampering
fusion assessment. It requires long-term follow-up to
demonstrate nonunion from loosening or implant failure
(54).
One additional criterion in the choice of cages is the
surgeon’s familiarity with the techniques of insertion.
Although the majority of the box cages are used in a
similar manner to bone graft, the threaded cylindrical
cages require a specialized technique of insertion. Attention to detail is important, because complications arising
from inadequate distraction of the anulus fibrosus,
under sizing of the cages, and dural tears from a posterior approach all have been described (55). Additionally,
some cages are designed for anterior minimally invasive
insertion (15–17,56–58), whereas others are designed for
290 /SECTION IV/SURGERY
use as a PLIF only. These are all important issues to consider, and an y sur geon intending to use such cages should
be thoroughly familiar with their indications and design
considerations.
SUMMARY
Lumbar interbody fusion cages may have some advantages over conventional autogenous bone grafting techniques. They have the theoretical potential to maintain
vertebral distraction, and separate the structural and biological functions of an interbody spacer. With the development of osteoinductive compounds delivered as a
recombinant protein or via gene therapy (59), bone graft
harvesting and donor site morbidity theoretically could
be avoided altogether. However, techniques of insertion
may be demanding and surgeons should be familiar with
the design before using a cage of their choice. Finally,
one should balance the use of such expensive implants
with the low cost and proven effectiveness of autogenous
bone graft, which is the gold standard in anterior interbody reconstruction and fusion to date.
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CHAPTER 27
Translaminar Screw Fixation
Dieter Grob
One of the surgical concepts to reduce pain originating
from the lumbar spine is to immobilize the involved vertebrae. The most natural way to achieve this is to take
advantage of the process of bone healing. By preparing
the parts to be fixed with decortication and additional
placement of bone graft, fusion may be enhanced. The
mechanical situation of the spine, with considerable lever
arms and multiple segmental centers of motion, implies a
relatively low success rate of solid bony bridging.
To improve the fusion rate and reduce the need for rigid
and cumbersome external fixation postoperativ ely, internal
fixation has been introduced to temporarily immobilize the
spine and thus enhance bony calcification. For this purpose
Hadra (1) introduced first metallic wires in 1891. Before
the era of pedicular screw fixation was introduced in
Europe in the early 1960s (2), attempts to stabilize vertebrae posteriorly with screws only were attempted. The use
of facet screws was first reported by King (3), whose technique was to immobilize the lumbosacral joints with short
screws traversing the facets. With this technique he
achieved a fusion rate of 91% without prolonged postoperative external fixation. Boucher (4) adopted this idea and
improved the technique. He achie v ed a 100% fusion rate in
single level fusions by penetrating the ipsilateral pedicle
with the tip of the screw, thus improving the bony purchase. This method of screw insertion implied that the tip
of the screw had to be placed near the foramen and the
nerve root and carried the risk of nerve injury. In addition,
there was a risk of decreased stability of this construct if
the screw broke through the cortex of the posterior aspect
of the facet. These complications of screw f ixation in the
lumbar spine made them unpopular despite the simplicity
of the technique. It was Magerl 1984 (5) who modified and
improved the technique of screw f ixation and popularized
revival of this type of immobilization. By inserting the
screws from the contralateral side through the lamina,
through the facet and ending in the base of the transverse
process, most of the disadvantages of the former techniques were eliminated without losing their advantages.
Bony purchase was increased by the passage of the screw
through the lamina, and the procedure was less risky
because: (a) the insertion of the screw was clearly posterior
to the neural elements; (b) the technique could be performed under direct visualization; and (c) the direction of
screw insertion was parallel to the exiting nerve root,
thereby minimizing the risk of injury to the nerve.
BIOMECHANICAL CONSIDERATIONS
A simplified but practical biomechanical concept of
the lumbar spine with a three-column model was conceived by Louis (6). The anterior column is represented
by the disc, the vertebral body, and the two posterior
columns by the facets. In the course of evolution,
anatomy adapted to the physiologic requirements of the
spine; therefore, it seems reasonable to assume that the
facets developed in response to mechanical necessity.
The importance of the facets has been demonstrated by
several in vitro experiments in which partial or total resec-
tion of the facets led to dysfunction of the functional spinal
motion unit (7,8). Together with the intervertebral disc, the
facets share and support the axial load of the spine.
Although the disc appears to be the primary load-bearing
structure (8), the facets function as an indispensable part of
the three-column concept to transmit part of the axial load,
which varies according the position of the individual
(9,10). Structural and morphologic changes that occur with
destruction of cartilage and osteophyte formation underscore the important mechanical properties of the facets.
Because the load passes partially through the facets, the
lever arm acting on an internal fixation device through the
facets remains small. A low-prof ile fixation device is sufficient to block segmental motion efficiently enough to
enhance solid bony fusion (11,12).
Despite the fact that translaminar screw fixation represents the lowest prof ile implant for the lumbar spine, the
stability in flexion achieved with this technique in vitro is
similar to that provided b y pedicle scre w fixation (13,14).
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