Current Concepts Review
`
`Interbody Fusion Cages in
`Reconstructive Operations on the Spine*
`
`BY PAUL C. MCAFEE, M.D.†, TOWSON, MARYLAND
`
`During the last five years, surgeons around the
`world have inserted more than 80,000 lumbar interbody
`fusion cages; in the United States alone, an estimated
`5000 such devices are implanted each month. The re-
`cent interest in performing lumbar interbody arthro-
`desis with use of cages is attributable to three factors:
`the high rate of failure associated with use of bone
`graft alone3,22,26,45,46,71,82,84,94,96,106,107; the high rate of failure
`associated with use of posterior pedicle-screw instru-
`mentation39,97,102; and the high rate of success associ-
`ated with use of so-called stand-alone anterior fusion
`cages and autogenous bone graft, obviating the need
`to perform a 360-degree (combined anterior and pos-
`terior) lumbar arthrodesis with use of posterior instru-
`mentation77.
`The purpose of the current review is to summarize
`the information in the literature with regard to the back-
`ground, rationale, indications, techniques, results, and
`possible future developments of interbody arthrodesis
`for reconstruction of the spine.
`
`Background
`Early techniques of arthrodesis with use of allo-
`graft or autogenous graft and without instrumentation
`were associated with a high rate of failure. In a classic
`study, Stauffer and Coventry96 reported on eighty-three
`patients who had had an anterior interbody arthro-
`desis between 1959 and 1967. Of seventy-seven pa-
`tients who were followed clinically for an average of
`3.75 years after the procedure, twenty-eight (36 per-
`cent) had good (76 to 100 percent) relief of pain, fif-
`teen (19 percent) had fair (26 to 75 percent) relief, and
`thirty-four (44 percent) had poor (0 to 25 percent) relief.
`Thirty (44 percent) of sixty-eight patients who were
`evaluated radiographically at a minimum of eighteen
`months postoperatively had a pseudarthrosis. Stauffer
`and Coventry defined radiographic fusion as “a pattern
`of continuous trabeculae traversing the grafted region
`and the adjacent vertebral bodies, with no evidence of
`motion when the patient was bending.” These results,
`
`*The author has received or will receive benefits for personal
`or professional use from a commercial party related directly or indi-
`rectly to the subject of this article. No funds were received in sup-
`port of this study.
`†7505 Osler Drive, Suite 104, Towson, Maryland 21204. E-mail
`address: bcspine@aol.com.
`
`Copyright 1999 by The Journal of Bone and Joint Surgery, Incorporated
`
`and the equally unfavorable results reported by other
`investigators20,26,33,45,56,57,82, prompted investigation into and
`development of various augmentation devices to im-
`prove the long-term outcome of spinal arthrodesis.
`
`Technology of Interbody Fusion Cages
`
`History
`Bagby2 was responsible for the early development
`of the lumbar interbody fusion cage. Working with a
`veterinarian, Grant, and a series of thoroughbred horses
`that had wobbler syndrome (a form of spondylitic
`myelopathy that leads to ataxia), he found that the
`Cloward technique20, which requires obtaining bone
`from the iliac crest, resulted in unacceptable morbidity.
`Bagby then developed a novel device, the first interbody
`stainless-steel basket (the Bagby basket), which was a
`thirty-millimeter-long, twenty-five-millimeter-diameter
`cylinder that had two-millimeter fenestrations in its
`walls to allow bone ingrowth. During a standard ante-
`rior cervical decompression and reaming procedure,
`cancellous-bone chips were removed from the posterior
`aspects of the cervical vertebrae. These chips then were
`packed inside the basket to promote anterior interbody
`cervical fusion.
`Subsequent studies revealed that horses treated
`with the Bagby technique had improved neurological
`function; some not only survived for many years but also
`won races38. Other investigators began making modi-
`fications of this technique, including threads in the
`basket72,108, adaptation of the cage for use in posterior
`lumbar interbody arthrodesis, and increases in the pull-
`out and compressive strength72; a two-cage technique
`also was developed, in 198825. In another study of horses,
`DeBowes et al.30 compared the results of arthrodesis
`with use of bovine xenograft with those of arthrode-
`sis with use of autogenous graft inside a Bagby basket;
`they found that the rate of fusion was better when the
`Bagby basket had been used and that this device did
`not collapse. After the arthrodesis, the gross appear-
`ance of the bovine xenograft was usually pale, and seven
`of eight sites that were investigated were composed
`of fibrous tissue. The autogenous graft and the Bagby
`basket contained little or no fibrous tissue. Maceration
`studies, with use of maggots to decompose the soft-
`tissue component, indicated that only two of eight bo-
`vine xenografts contained enough ossified tissue in the
`intervertebral space in order to hold the vertebrae to-
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`860
`
`P. C. MCAFEE
`
`FIG. 1-A
`Photograph showing some of the devices studied by Kanayama et al.47, who used silicone elastomer gel inside cages to measure intracage
`pressures under in vitro loading conditions in an investigation of the forces acting on bone graft within different cage geometries. Bottom left,
`Brantigan cage; top left, Harms vertical cage; center, elastomer gel; top right, threaded femoral bone dowel; and bottom right, BAK cage.
`
`gether, whereas seven of eight autogenous grafts in the
`Bagby baskets contained enough tissue.
`
`Current Types of Fusion Cages
`A variety of cages are currently available, each with
`its own indications, advantages, and disadvantages. This
`review will focus on five devices: the Bagby-and-Kuslich
`device51 (BAK; Sulzer Spine-Tech, Minneapolis, Minne-
`sota), the threaded interbody fusion device (TIBFD;
`Medtronic Sofamor-Danek Group, Memphis, Tennes-
`see), the Ray cage (U.S. Surgical, Norwalk, Connecti-
`cut), the Harms titanium-mesh cage (DePuy-AcroMed,
`Cleveland, Ohio), and the Brantigan rectangular and
`rounded cages (DePuy-AcroMed). Most of these de-
`vices have been approved only for limited, investiga-
`tional applications in humans because the long-term
`effects are not yet known. Thus, the BAK device may
`be used only for posterior, anterior, or lateral laparo-
`scopic procedures; the TIBFD device, only in Food and
`Drug Administration-Investigational Device Exemp-
`tion studies; the Ray cage, only as a posterior device; and
`the Brantigan cages, only as posterior devices and only
`in conjunction with posterior pedicle-screw instrumen-
`tation. Only the Harms cage has been approved for
`widespread, unrestricted use to date.
`
`Mechanical, Biological, and
`Physiological Roles of Fusion Cages
`In an effort to establish a baseline for the compari-
`son of investigations of the role of fusion cages, Den-
`nis et al.31 studied thirty-one patients who had had
`an anterior interbody arthrodesis at a total of forty lev-
`els with use of autogenous graft or allograft but not
`metal cages. The height of the disc space was measured
`in each patient preoperatively, early postoperatively,
`
`and at an average of twenty-nine months postopera-
`tively. Although immediate postoperative radiographs
`showed an average increase in the disc-space height of
`9.5 millimeters (89 percent), use of graft alone did not
`provide long-term distraction of the disc space or in-
`creased neuroforaminal height. At the time of the latest
`follow-up examination, the disc-space height had de-
`creased in every patient; at nineteen of the forty verte-
`bral levels, the height at the most recent examination
`was less than the preoperative height. That study dem-
`onstrated that autogenous graft or allograft alone can-
`not maintain neuroforaminal distraction. Maintaining
`this distraction is important because it promotes ante-
`rior load-sharing, increases the amount of space for the
`nerve roots, and prevents flatback syndrome.
`
`Mechanical Role
`Rapoff et al.76 compared the mechanical effects of
`the TIBFD and BAK cages in six fresh-frozen, thawed
`spines from human cadavera and found that the inser-
`tional torque and maximum pushout loads were similar
`for the two cages. Other authors have determined that
`the amount of interspace distraction is as important to
`the overall stability of the construct as the individual
`characteristics of the fusion cage15,36,37,87,99.
`Kanayama et al.47 used bench-top mechanical tests
`to assess different types of fusion cages in sixty func-
`tional calf-spine units, each consisting of one vertebral
`disc space and the adjacent vertebrae (Fig. 1-A). There
`were six specimens in each treatment group. The meth-
`ods of preparation of the cage, anterior discectomy, and
`annular distraction with use of sized distraction plugs
`before insertion of the cage were similar for all ten
`constructs. The devices that were tested included two
`BAK cages, two BAK proximity cages, two Ray cages,
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`
`FIG. 1-B
`Bar graph showing the intracage pressure measurements for the ten cage constructs. The four threaded titanium designs (BAK [Bagby and
`Kuslich], BAK proximity, Ray, and TIBFD [threaded interbody fusion device]) had significantly lower (more favorable) intracage pressures
`than did the other implants (p < 0.05, one-way analysis of variance). * = cage alone was significantly different from Group-A devices, ^ = cage
`alone was significantly different from BAK and TIBFD devices (F = 8.15, p < 0.001), and ^^ = cage alone was significantly different from cage
`with pedicle screws (p < 0.05). One pound per square inch = 6.89 kilopascals.
`
`two TIBFD cages, one Harms titanium-mesh cage, two
`Harms vertical titanium-mesh cages, two Brantigan rec-
`tangular carbon-fiber cages, a larger rounded Branti-
`gan anterior lumbar interbody fusion cage shaped to fit
`within the interbody disc space, one femoral ring allo-
`graft, and two bone-dowel allografts. The modes of test-
`ing included axial compression (500 newtons), torsion
`(three newton-meters), flexion (five newton-meters),
`and lateral bending (five newton-meters). Intracage
`pressures were measured with pressure-needle trans-
`ducers throughout the various loading conditions after
`a silicone elastomer gel had been injected into the cages
`and allowed to polymerize. The purpose of the gel was
`to provide a homogeneous material, simulating bone
`graft, inside each cage for measurement of strain. Pilot
`studies had shown that it was not useful to measure the
`strain on actual bone graft as such strain proved to be
`extremely variable and depended on the amount of
`force used to pack the bone graft inside the cage. With
`the numbers available for study, no significant differ-
`ences were detected among the ten cage constructs with
`regard to functional stability (p > 0.05, one-way analysis
`of variance). Intracage pressure was not found to be
`significantly different among the Harms titanium-mesh
`vertical cages, the Brantigan cages, the femoral ring al-
`lograft, or the bone-dowel allografts; however, the four
`threaded cages (the BAK, BAK proximity, Ray, and
`TIBFD devices) had significantly lower intracage pres-
`sures than did the other implants (p < 0.05, one-way
`analysis of variance) (Fig. 1-B). These findings were sup-
`ported by those of Oxland et al.73, who found no differ-
`
`ence in bench-top mechanical loading between two po-
`rous bilateral BAK implants and a central contoured
`SynCage implant with end-plate fit.
`
`Biological Role
`To date, to the best of my knowledge, the only study
`of long-term results with use of fusion cages was re-
`ported by Cunningham et al.27. After an average of four-
`teen years (range, eight to fifteen years), histological
`analysis of six vertebral specimens from horses that had
`had an anterior interbody arthrodesis with insertion of
`a stainless-steel Bagby basket revealed successful fusion
`with mature trabecular bone spanning the sites of the
`arthrodesis. There was a significant decrease in bone-
`mineral density (p < 0.05) at the fusion site compared
`with that of the adjacent vertebral bodies, but this stress-
`shielding had no adverse clinical consequences. Sagittal
`microradiographs showed complete remodeling of the
`entire disc space, including the end plate and residual
`posterior remnants of the interbody disc posterior to the
`basket (Figs. 2-A and 2-B). Whether this equine model
`can be equated with the human situation remains to be
`determined.
`
`Physiological Role
`In a study of nine fresh-frozen lumbar spines from
`the cadavera of individuals who had had neuroforami-
`nal stenosis, Chen et al.18 found that placement of sili-
`cone molds in the neuroforamina after the application
`of a fusion cage significantly increased the neuroforami-
`nal volume (by 23 percent at the fourth and fifth lumbar
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`862
`
`P. C. MCAFEE
`
`FIG. 2-B
`FIG. 2-A
`Microradiographs showing the extent of trabecular remodeling fourteen years after treatment with a Bagby basket (Fig. 2-A) and a
`bone-dowel allograft (Fig. 2-B, arrows) from the study by Cunningham et al.27, who examined six equine specimens at an average of
`fourteen years after a successful anterior interbody arthrodesis and insertion of a Bagby stainless-steel basket.
`
`level and by 22 percent at the fifth lumbar and first
`sacral level) and the posterior disc height (by 37 percent
`at the fourth and fifth lumbar level and by 45 percent at
`the fifth lumbar and first sacral level) (p < 0.001 for both).
`
`Selection of Patients for Arthrodesis
`with Use of an Interbody Fusion Cage
`Ray78, in a Food and Drug Administration-approved
`Investigational Device Exemption study, selected pa-
`tients for insertion of a lumbar interbody fusion cage
`with use of six criteria: severe, disabling, intractable back
`pain; degenerated disc spaces with resultant pain; an
`absence of disc-space or systemic infection; no previous
`interbody arthrodesis at the target levels; an absence
`of degeneration at adjacent, neighboring disc spaces,
`whether or not they were painful; and no or Meyerding69
`grade-I spondylolisthesis. In addition, the disabling back
`pain had to have been present for at least one year and
`refractory to extensive nonoperative care and there had
`to be substantial loss of both disc height and mobil-
`ity. Patients who had a disc-space height of more than
`twelve millimeters were excluded.
`I believe that most of these criteria are not selective
`enough; cages have been used for patients who have
`general disc pain or disc spaces that appear dark on
`
`magnetic resonance imaging studies (so-called black-
`disc disease — that is, the earliest changes, on magnetic
`resonance images, caused by degenerative disc disease
`that is due to loss of hydration signal within the nucleus
`pulposus). I prefer a more conservative selection pro-
`cess, with use of cages limited to patients who have
`postlaminectomy syndrome or disc-space collapse with
`neuroforaminal narrowing. I do not use cages for pa-
`tients who have black-disc disease or simply a positive
`discogram. Most patients whom I manage with a cage
`have disease involving only one disc level, and I do not
`use the device for those with involvement of more than
`two levels. If a patient has instability at more than two
`levels, it should be treated with a posterior approach and
`pedicle-screw instrumentation.
`
`Definition of Fusion
`As stated in one review article33, the rate of fusion
`“depends to a great extent on the investigator’s in-
`terpretation.” Because there is no single definition of
`what constitutes fusion, it is difficult if not impossible to
`compare the results of different studies. Moreover, it is
`difficult to determine radiographically if fusion has oc-
`curred. In addition, findings of biomechanical tests of
`stability do not always directly correspond to radio-
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`
`FIG. 3-A
`FIG. 3-B
`Radiographs demonstrating the paradox regarding a solid fusion compared with a so-called functional arthrodesis. Most investigators
`would agree that Fig. 3-A shows a fusion (as indicated by solid, continuous trabecular bone-bridging between the vertebrae) and that Fig. 3-B
`shows a pseudarthrosis according to the criteria of Stauffer and Coventry96 (a two-to-three-millimeter fibrous interface between the vertebral
`bodies). In laboratory testing, however, the flexural, torsional, and axial compressive stiffnesses were greater for the specimen shown in Fig.
`3-B than for that shown in Fig. 3-A59; this was because the cross-sectional area of the hypertrophic pseudarthrosis callus in the specimen shown
`in Fig. 3-B was much greater than the cross-sectional area of the specimen shown in Fig. 3-A. (Reprinted, with permission, from: McAfee, P.
`C.; Regan, J. J.; Farey, I. D.; Gurr, K. R.; and Warden, K. E.: The biomechanical and histomorphometric properties of anterior lumbar fusions:
`a canine model. J. Spinal Disord., 1: 105, 1988.)
`
`graphic evidence of fusion. For example, a radiographi-
`cally solid fusion with continuously bridging trabecular
`bone in a canine specimen (Fig. 3-A) had less mechan-
`ical stiffness than did a specimen that contained a two-
`to-three-millimeter-wide fibrous interface between the
`vertebral bodies (Fig. 3-B).
`The rates of fusion are approximately 20 percent
`higher when the sole criterion is loss of motion (de-
`termined by comparing lateral flexion and extension
`radiographs) rather than continuous trabeculae across
`the graft-vertebrae interfaces4,16,20,23,54-56,59,67,83,85,90,101,104. One
`study of 100 patients included eleven who had “a fi-
`brous fusion . . . with absorption of the grafts”; this
`inclusion resulted in a rate of fusion of 94 percent21.
`My criterion for fusion is the presence of bridging
`trabecular bone between the vertebral bodies. The most
`reliable radiographic indication of fusion postopera-
`tively is the sentinel sign, or the presence of bridging
`bone anterior to the fusion cage (Figs. 4-A, 4-B, and
`4-C). Similar to the late-maturation phases of callus for-
`
`mation in a fracture of the femur, the cross-sectional
`area of an exuberant fracture callus can restore normal
`stability before mature haversian bone is seen in radio-
`graphic continuity. One drawback of a fusion cage in-
`serted after a so-called reamed-channel discectomy is
`that the reparative process is confined to a smaller
`cross-sectional area (the fenestrations in the cage) in
`contrast to uninhibited hypertrophy.
`To add to the confusion, the criteria for a success-
`ful fusion in patients who are managed with a cage
`are often different from those used in previous re-
`ports. Kumar et al.50, in a retrospective review of the
`results for thirty-two patients who had had an anterior
`lumbar interbody arthrodesis, found that twenty-one pa-
`tients (66 percent) had radiographic union and stability
`on flexion and extension, whereas four (13 percent) had
`nonunion and instability. The radiographic results for
`the remaining seven patients (22 percent) were am-
`biguous. Those authors coined the phrase “functional
`arthrodesis” to describe such patients, with the term
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`864
`
`P. C. MCAFEE
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`indicating stability with less than 2 degrees of motion as
`seen on flexion and extension radiographs and bridging
`bone anterior or posterior to the femoral allograft al-
`though the fusion was less than complete. The seven
`patients had a translucent line separating the vertebral
`end plate from the bone graft on one or both sides.
`Kumar et al. interpreted the anterior interbody arthro-
`deses in these seven patients as being successful, which
`led them to calculate an overall rate of clinical success
`of 88 percent. They explained that the “functional
`arthrodesis” was stable because of the formation of ex-
`ternal callus surrounding the femoral allograft, which
`increased the cross-sectional area of the fusion at the
`level of the interspace. Similar to many authors who
`have used femoral allografts, they reported a high prev-
`alence of subsidence (average, four millimeters) “due to
`cavitation of the femoral graft into the surrounding bod-
`ies as the femoral graft appeared to maintain its pre-
`operative dimensions.” This shows that the quality of
`bone in the vertebral end plates is important for main-
`taining compressive strength.
`The interpretation of fusion on the basis of radio-
`graphs is also subject to controversy. Stauffer and Cov-
`entry96 defined fusion as bridging and no motion.
`However, in recent studies of BAK cages, fusion was
`considered to have occurred even in the presence of
`as much as 5 degrees of difference (motion) between
`flexion and extension radiographs1,51,108. Some authors
`have thought that more than 5 degrees of motion as seen
`on lateral flexion and extension radiographs indicates a
`failure of fusion11,13,14,78. Others have defined fusion with
`use of stricter criteria (only 2 or 3 degrees of motion as
`seen on flexion and extension radiographs)34,53,57,58. Still
`others have stated that radiolucent areas that are wider
`than two millimeters and extend along at least 50 per-
`cent of the bone adjacent to the implant are indicative
`of failure40,108.
`Deciding whether a patient has a fusion or a failure
`of the arthrodesis on the basis of the amount of motion
`seen on flexion and extension radiographs is difficult for
`several reasons. First, the difference in the range of mo-
`tion among asymptomatic individuals can range from 7
`to 14 degrees13, so it may not be accurate to use a par-
`ticular degree of motion as the baseline with which to
`determine the presence of fusion. Second, radiographs
`may not accurately depict the range of motion that is
`possible because a patient who has pain on bending may
`bend less than he or she can. Third, the measurement of
`angular motion may be unreliable in the presence of
`pedicle-screw instrumentation, which may decrease mo-
`tion even in patients who have a pseudarthrosis and thus
`result in a false-positive finding of fusion. The effect, on
`the findings on stress radiographs, of anterior cage in-
`strumentation alone in the absence of a fusion has not
`been studied, to my knowledge52.
`My colleagues and I66 reported on twenty patients
`who had had additional spinal reconstruction proce-
`
`dures after failure of an arthrodesis with use of a cage.
`Five of these patients had been thought, by their refer-
`ring surgeons, to have had a solid fusion on the basis of
`flexion and extension radiographs made less than two
`years postoperatively; however, on follow-up more than
`two years after the operation, all five were found to have
`gross motion, subsidence, mechanical pain, and migra-
`tion of the cage. Late pseudarthrosis and instability also
`had developed. Thus, that study demonstrated the un-
`reliability of assessment of fusion on the basis of flexion
`and extension radiographs alone. Because of the un-
`reliability of radiographic interpretation of motion, I
`prefer to define fusion as simply the presence of bridg-
`ing trabecular bone between vertebral bodies.
`In a study of forty-nine patients who had had ex-
`ploration of the fusion mass during removal of the
`hardware, Blumenthal and Gill4 found only 69 percent
`agreement between the radiographic and operative
`
`FIG. 4-A
`Figs. 4-A, 4-B, and 4-C: It is often difficult to confirm a lumbar
`fusion with use of flexion and extension radiographs alone. The most
`reliable criterion of a successful fusion in association with a fusion
`cage is the presence of trabecular bone-bridging in continuity be-
`tween vertebrae.
`Fig. 4-A: Lateral radiograph made six months after an anterior
`laparoscopic arthrodesis of the fifth lumbar to the first sacral verte-
`bra for the treatment of postlaminectomy instability. Additional
`iliac-crest bone graft was packed anterior to the BAK fusion cage
`(arrows).
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`findings; 90 percent of the patients had a successful fu-
`sion. Those authors suggested that, in one of five pa-
`tients, plain radiographs had led to an underestimation
`of the degree of fusion and the premineralized osteoid
`might have been functionally fused while appearing ra-
`diolucent on radiographs.
`Brantigan et al.13 used the most stringent criteria for
`fusion, especially considering that the Brantigan cage
`for posterior lumbar interbody arthrodesis is radiolu-
`cent, allowing better radiographic visualization of the
`dynamics of the bone graft than do cages composed of
`titanium, which creates artifacts. According to those au-
`thors, no motion was acceptable, but it must be remem-
`bered that the devices always were used in conjunction
`with pedicle screws.
`In a prospective, multicenter clinical trial, Yuan et
`al.108 used the following definition of fusion to study the
`BAK cage in a Food and Drug Administration-approved
`Investigational Device Exemption study reported by
`Alpert1 (PMA [Premarket Approval] P950002). The fu-
`sion was considered to be solid if there were no dramat-
`ically obvious radiolucencies and there was less than
`5 degrees of vertebral motion in the sagittal plane as
`assessed with digitization methods. All radiographs
`that demonstrated between 3 and 7 degrees of sagittal
`motion were evaluated by an independent radiologist.
`Patients who had had a two-level procedure were con-
`
`sidered to have a successful fusion only if both levels
`were fused (see Results of Clinical Series).
`In an Investigational Device Exemption study of
`titanium fusion cages, Ray78 defined fusion according to
`six criteria: (1) lack of any visible motion, or less than 3
`degrees of intersegmental change, as seen on flexion and
`extension radiographs; (2) lack of a dark halo around
`the implant; (3) minimum loss of disc-space height, in-
`dicating a resistance to collapse of the cancellous verte-
`bral bone; (4) lack of visible fracture of the device, graft,
`or vertebrae; (5) lack of substantial sclerotic changes
`in the recipient bone bed or the graft; and (6) visible
`bone within the hollow Ray titanium fusion cage as seen
`on anterior, posterior, or Ferguson radiographs32. If the
`radiologist determined that the vertebral bodies were
`fused but the surgeon thought that they were not, fusion
`was considered not to have occurred (see Results of
`Clinical Series).
`
`Hybrid Interbody Grafts: Biological Cages
`Although he was not the first author of the pub-
`lished study, O’Brien is credited with the concept of the
`so-called hybrid interbody graft, a biological fusion cage
`consisting of a femoral cortical allograft ring packed
`with autogenous cancellous bone graft44. Of forty pa-
`tients who were managed with this technique, thirty-
`two had posterior instrumentation and eight did not.
`
`FIG. 4-B
`FIG. 4-C
`Illustration (Fig. 4-B) showing the principle of ream long-cage short66, which ensures that additional cancellous bone (Fig. 4-C, arrows) can
`be visualized anterior to the cage. This is an example of the so-called sentinel sign, which indicates a successful arthrodesis.
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`866
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`P. C. MCAFEE
`
`FIG. 5-B
`FIG. 5-A
`Figs. 5-A through 5-G: Magnetic resonance images and radiographs of a thirty-five-year-old man who was managed with a biological (hybrid)
`threaded femoral bone-dowel allograft packed with autogenous iliac-crest bone graft.
`Fig. 5-A: Magnetic resonance image made just before a posterior laminectomy and discectomy at the fifth lumbar and first sacral levels for
`the treatment of a central disc herniation (arrowhead).
`Fig. 5-B: Magnetic resonance image made within one month after the operation, showing pyogenic vertebral osteomyelitis.
`
`The overall rate of fusion was seventy (96 percent) of
`seventy-three levels. The average interbody disc height
`increased postoperatively but returned to preoperative
`values at an average of 1.4 years (range, 1.0 to 2.4 years).
`Because it is composed entirely of bone, the hybrid
`cage is capable of complete remodeling, unlike titanium
`cages. Additionally, it can be used in patients who have
`an infection (Figs. 5-A through 5-G). The femoral allo-
`graft portion of the cage determines the acute or imme-
`diate stability of the construct, whereas the autogenous
`iliac-crest graft determines the long-term stability.
`In a series consisting of forty-five patients, two 6.5-
`millimeter-diameter cancellous-bone screws with wash-
`
`ers were used to prevent the femoral allograft from
`dislodging anteriorly49. The indications for the procedure
`were disc-disruption syndrome in twenty-one patients,
`postlaminectomy syndrome in twelve patients, and a
`so-called mobile nonunion after a failed posterolateral
`arthrodesis in eleven patients. (The forty-fifth patient
`was not accounted for in the report.) Two years postoper-
`atively, thirty-eight patients (84 percent) had complete
`stability as seen on flexion and extension radiographs. Six
`(13 percent) had a radiolucent line on one or both sides
`of the femoral allograft, but fusion was suggested by the
`presence of bridging bone either anterior or posterior to
`the femoral allograft. Those authors concluded that the
`
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`INTERBODY FUSION CAGES IN RECONSTRUCTIVE OPERATIONS ON THE SPINE
`
`867
`
`pain); 729 patients (77 percent) reported marked or
`disabling pain. The average pain score twelve months
`postoperatively was 3.1 points (mild pain), and that at
`twenty-four months was 2.9 points. Twenty-four months
`after the operation, 805 patients (85 percent) reported
`a decrease in pain.
`The functional outcome was evaluated, with use of
`a numerical scale, according to seven parameters: the
`ability to stand, sit, walk, squat, and put on socks and
`shoes; the level of recreational activity; and the level
`of work. The best (lowest) possible score was 7 points,
`and the worst (highest) score was 32 points. (An asymp-
`tomatic, or so-called normal, individual would score be-
`tween 9 and 12 points on this scale.) The average
`functional score was 20.9 points preoperatively, 15.2
`points at twelve months, and 14.4 points at twenty-four
`months.
`Yuan et al.108 also followed the patients with regard
`to their ability to return to work postoperatively. At the
`time of the operation, only 341 (36 percent) of the 947
`patients were working outside the home; 502 (53 per-
`cent) were not working because of disability, and 104 (11
`percent) were homemakers, students, or retirees. Eight
`hundred and forty-three patients were considered to be
`eligible for work after the operation; these included pa-
`tients who either had worked or had been receiving
`disability compensation before the operation. Five hun-
`dred and seventy-three (68 percent) of these patients
`returned to work at twelve months, and 658 (78 percent)
`returned at twenty-four months. Of 283 patients who
`were seen for a two-year follow-up evaluation, 91 per-
`cent had fusion (defined as the absence of substantial
`radiolucencies and less than 5 degrees of vertebral mo-
`tion in the sagittal plane as assessed with digitization
`methods)1.
`To gain perspective on the rates of fusion associated
`with lumbar interbody fusion cages, it is helpful to re-
`view the prospective study reported by Zdeblick110 in
`1993. One hundred and twenty-four patients were ran-
`domly assigned to one of three treatment groups: pos-
`terolateral arthrodesis with use of autogenous bone
`grafts (group I), posterolateral arthrodesis supplemented
`with semirigid pedicle-screw instrumentation (group II),
`or posterolateral arthrodesis with autogenous grafts
`and rigid pedicle-screw-and-rod fixation (group III). The
`overall rate of fusion was 65 percent for group I, 77
`percent for group II, and 95 percent for group III.
`
`Ray Titanium Fusion Cage
`The Investigational Device Exemption study of the
`Ray cage78 comprised 211 patients who were followed
`for a minimum of twenty-four months postoperatively.
`The indications for the procedure were severe, disabling
`back pain (203 patients; 96 percent); major annular de-
`generation (156 patients; 74 percent); disc herniation