Clinical success was achieved in 23 patients (88%). This included 3 excellent (12%), 9 good (35%), and 11 fair (42%) results. Clinical success was not achieved in 3 with poor results (12%). Table 2 correlates the clinical and fusion success of these patients.
At the final radiographic evaluation, fusion success was achieved in all 30 unilateral-cage levels. There were no pseudarthroses, instrumentation failures, or significant subsidence at any of the unilateral cage levels. Disc space height and foraminal height were restored by the surgery and maintained at last follow-up. However, there were 3 patients with single level, 2 cage radiographic pseudarthrosis. Fusion success was then 23/26 (88%) for all patients. The first pseudarthrosis was the L2-3 retropulsed cage removed at 8 months and subsequently fused on radiograph and computed tomography (CT) scan. The second was an L2-3 level persistent pseudarthrosis from an initial L2-S1 pseudarthrosis reconstruction that was explored and treated with pedicle screw implant removal and bone morphogenic protein augmentation of posterolateral fusion and subsequently was healed at last follow-up with x-rays and CT scan. The third patient had a pseudarthrosis level reported at L3-4 on CT in a L1-5 fusion for degenerative scoliosis, and did not agree with fusion exploration, accepting a fair clinical result. Table 2 correlated the clinical status with fusion success.
The goal of lumbar interbody fusion is to relieve pain caused by neurologic compression and achievement of a stable surgical construct. Compared with posterolateral instrumented fusion, adding an interbody fusion produces a significantly stiffer construct that protects the posterior instrumentation from failure, and provides a circumferential fusion mass with an increased rate of successful fusion.7,23,33 Both the PLIF and the TLIF approach offer circumferential fusion from a posterior approach.
With the commonality of unilateral interbody fusion with bilateral pedicle screws and posterolateral graft, the comparison to reported TLIF studies is appropriate. Lowe et al38 reported 90% fusion rate and 80% clinical success in TLIF with 2 titanium-mesh cages. Hee et al21 reported 96% fusion rate with the same TLIF procedure augmented with autologous growth factors. McAfee et al19 in 120 spondylolisthesis patients treated with TLIF with a unilateral CFRP cage reported a 97.5% fusion rate. Potter et al29 reported 93% fusion rate and 79% clinical success with interbody structural allograft or bioabsorbable spacers. Hackenberg et al20 reported 89% fusion rate and significant improvement in Oswestry disability index in TLIF with a unilateral CFRP. Molinari reported 19 active-duty military personnel were treated with 2 CFRP cages and 16 with a unilateral cage. When only a single cage was used, bone graft was inserted from a bilateral approach.27 Molinari et al27 reported that the results were generally good, and that patients having a unilateral cage had equal fusion and clinical success as those having 2 cages. The fusion and clinical results of TLIF with unilateral interbody support and bone grafting were not significantly different by χ2 analysis from this unilateral cage series.
During the design of the CFRP cages, mechanical testing assumed use of bilateral cage placement and bilateral pedicle screw fixation.14,39 Biomechanical fatigue strength of 2 cages had a 2-fold safety factor over the maximal loads of daily living.40 The single cage fatigue strength had a small or nonexistent safety factor over the maximal loads of daily living.40 Closkey et al41 in an in vitro analysis found >30% of the vertebral endplate surface was required for load transmission across structural interbody grafts. Recent published mechanical data related to single interbody cage in TLIF found no statistical difference in initial stability and stiffness with a single cage compared with 2 cages.15–17 Ames et al15 in human cadavers, found no significant difference in motion between PLIF with 2 allograft bone spacers and TLIF with a single spacer as long as there were bilateral pedicle screws. Chen et al16 in a porcine model found unilateral cage and unilateral pedicle screws had similar stability to the unilateral cage with bilateral pedicle screws. Kettler et al17 in a human cadaver found a unilateral cage is as stable as bilateral cages. Position of the graft did not change the stiffness or stability.15,42 Ames et al15 varied the position of the unilateral graft between the anterior and middle column without diminution of the stability or stiffness of the construct. Harris et al42 in a human cadaver study placed an oblique single CFRP cage, and found the addition of bilateral pedicle screws matched the flexibility of an intact motion segment. Heth et al43 compared anterior and transverse placement of threaded cylindrical cages and found no difference in stability with transverse positioned cages. Wang et al44 in human cadavers compared sagittal versus oblique placement of cylindrical cages and found any differences between cage position was normalized with bilateral pedicle screw fixation. Several key points are made by these studies. A unilateral cage is as stable and stiff as bilateral cages. Cage position does not decrease the stability of the construct. All constructs are improved by bilateral pedicle screw fixation.
Bone grafting of the available surface area of the disc space is important for fusion success. Prolo36 found successful fusion filled 77% of the available disc space with bone. A clinical study using CT scan to demonstrate disc removal showed more than 56% of the cross-sectional area of the endplate could be cleared from a unilateral TLIF approach.45 The surface area of exposed bone graft in a single CFRP cage is 138 mm2 and the surface area of a typical L5 lumbar endplate is 1259 mm2. A single CFRP cage will fill only 10% of the endplate.40 Additional bone grafting to fill all available surface area is recommended. Placement of additional bone graft around the single cage may account for the undiminished high rate of fusion success in this series.
A shortcoming of this study is that a small population will result in poor specificity or underestimation of the actual pseudarthrosis rate. To improve the statistical power of the study would require a large number of patients that may be impractical. This study demonstrates successful fusion and clinical results when the patient selection criteria are expanded to include complex revision surgical problems. The results of this study support the use of a unilateral interbody cage combined with bilateral posteriolateral fusion and pedicle screws. A prospective randomized trial comparing unilateral with bilateral CFRP cages is warranted. The authors emphasize that when a fusion is done with an unilateral approach to the disc space, it is particularly important for the surgeon to pack additional bone in the disc space around the cage.
Fusion and clinical success rates are not diminished by the use of a unilateral CFRP cage rather than the recommended 2 cages. Clinical success with the unilateral single cages was not statistically different from the clinical success rates of the IDE study. Fusion success was achieved in 100% of 30 single cage levels and 23 of 26 patients (89%) at all fusion levels. Mechanical failure did not occur with the single cage. A single cage with bilateral pedicle screws provides adequate alignment, balance, and mechanical stability, and allows the maximal amount of autologous graft to fill the disc space.
1. Cloward RB. The treatment of ruptured lumbar intervertebral discs by vertebral body fusion. I. Indications, operative technique, after care. J Neurosurg. 1953;10:154–168.
2. Cloward RB. Lesions of the intervertebral disks and their treatment by interbody fusion methods. The painful disk. Clin Orthop. 1963;27:51–77.
3. Cloward RB. Spondylolisthesis: treatment by laminectomy and posterior interbody fusion. Clin Orthop. 1981;74–82.
4. Cloward RB. Posterior lumbar interbody fusion updated. Clin Orthop. 1985;16–19.
5. Brantigan JW, Cunningham BW, Warden K, et al. Compression strength of donor bone for posterior lumbar interbody fusion. Spine. 1993;18:1213–1221.
6. Brantigan JW. Pseudarthrosis rate after allograft posterior lumbar interbody fusion with pedicle screw
and plate fixation. Spine. 1994;19:1271–1279; Discussion 1280.
7. Brantigan JW, Steffee AD. A carbon fiber implant to aid interbody lumbar fusion. Two-year clinical results in the first 26 patients. Spine. 1993;18:2106–2107.
8. Tullberg T, Brandt B, Rydberg J, et al. Fusion rate after posterior lumbar interbody fusion with carbon fiber implant: 1-year follow-up of 51 patients. Eur Spine J. 1996;5:178–182.
9. Hashimoto T, Shigenobu K, Kanayama M, et al. Clinical results of single-level posterior lumbar interbody fusion using the Brantigan I/F carbon cage
filled with a mixture of local morselized bone and bioactive ceramic granules. Spine. 2002;27:258–262.
10. Sears W. Posterior lumbar interbody fusion for lytic spondylolisthesis: restoration of sagittal balance using insert-and-rotate interbody spacers. Spine J. 2005;5:161–169.
11. Molinari RW, Bridwell KH, Lenke LG, et al. Anterior column support in surgery for high-grade, isthmic spondylolisthesis. Clin Orthop Relat Res. 2002;109–120.
12. Kwon BK, Berta S, Daffner SD, et al. Radiographic analysis of transforaminal lumbar interbody fusion for the treatment of adult isthmic spondylolisthesis. J Spinal Disord Tech. 2003;16:469–476.
13. Brantigan JW, Neidre A. Achievement of normal sagittal plane alignment using a wedged carbon fiber reinforced polymer fusion cage
in treatment of spondylolisthesis. Spine J. 2003;3:86–196.
14. Brantigan JW, Steffee AD, Geiger JM. A carbon fiber implant to aid interbody lumbar fusion. Mechanical testing. Spine. 1991;16(suppl 6):S277–S282.
15. Ames CP, Acosta FL Jr, Chi J, et al. Biomechanical comparison of posterior lumbar interbody fusion and transforaminal lumbar interbody fusion performed at 1 and 2 levels. Spine. 2005;30:E562–E566.
16. Chen HH, Cheung HH, Wang WK, et al. Biomechanical analysis of unilateral fixation with interbody cages. Spine. 2005;30:E92–E96.
17. Kettler A, Schmoelz W, Kast E, et al. In vitro stabilizing effect of a transforaminal compared with two posterior lumbar interbody fusion cages. Spine. 2005;30:E665–E670.
18. Lowe TG, Tahernia AD. Unilateral transforaminal posterior lumbar interbody fusion. Clin Orthop Relat Res. 2002;64–72.
19. McAfee PC, Devine JG, Chaput CD, et al. The indications for interbody fusion cages in the treatment of spondylolisthesis: analysis of 120 cases. Spine. 2005;30:S60–S65.
20. Hackenberg L, Halm H, Bullmann V, et al. Transforaminal lumbar interbody fusion: a safe technique with satisfactory three to five year results. Eur Spine J. 2005;14:551–558.
21. Hee HT, Majd ME, Holt RT, et al. Do autologous growth factors enhance transforaminal lumbar interbody fusion? Eur Spine J. 2003;12:400–407.
22. Zhao J, Wang X, Hou T, et al. One versus two BAK fusion cages in posterior lumbar interbody fusion to L4-L5 degenerative spondylolisthesis: a randomized, controlled prospective study in 25 patients with minimum two-year follow-up. Spine. 2002;27:2753–2757.
23. Wang JC, Mummaneni PV, Haid RW. Current treatment strategies for the painful lumbar motion segment: posterolateral fusion versus interbody fusion. Spine. 2005;30(suppl 16):S33–S43.
24. Coe JD, Vaccaro AR. Instrumented transforaminal lumbar interbody fusion with bioresorbable polymer implants and iliac crest autograft. Spine. 2005;30(suppl 17):S76–S83.
25. Harms J, Jeszenszky D, Stolze D. True spondylolisthesis reduction and more segmental fusion in spondylolisthesis. In: Bridwell KH, DeWald RL, eds. Textbook of Spinal Surgery. Philadelphia: Lippincott-Raven; 1997:1337–1347.
26. Humphreys SC, Hodges SD, Patwardhan AG, et al. Comparison of posterior and transforaminal approaches to lumbar interbody fusion. Spine. 2001;26:567–571.
27. Molinari RW, Sloboda J, Johnstone FL. Are 2 cages needed with instrumented PLIF
? A comparison of 1 versus 2 interbody cages in a military population. Am J Orthop. 2003;32:337–343; Discussion 343.
28. Mummaneni PV, Rodts GE Jr. The mini-open transforaminal lumbar interbody fusion. Neurosurgery. 2005;57(suppl 4):256-261; Discussion 256–261.
29. Potter BK, Freedman BA, Verwiebe EG, et al. Transforaminal lumbar interbody fusion: clinical and radiographic results and complications in 100 consecutive patients. J Spinal Disord Tech. 2005;18:337–346.
30. Rivet DJ, Jeck D, Brennan J, et al. Clinical outcomes and complications associated with pedicle screw
fixation-augmented lumbar interbody fusion. J Neurosurg Spine. 2004;1:261–266.
31. Salehi SA, Tawk R, Ganju A, et al. Transforaminal lumbar interbody fusion: surgical technique and results in 24 patients. Neurosurgery. 2004;54:368–374; Discussion 374.
32. Schwender JD, Holly LT, Rouben DP, et al. Minimally invasive transforaminal lumbar interbody fusion (TLIF
): technical feasibility and initial results. J Spinal Disord Tech. 2005;18(suppl):S1–S6.
33. Brantigan JW, Steffee AD, Lewis ML, et al. Lumbar interbody fusion using the Brantigan I/F cage
for posterior lumbar interbody fusion and the variable pedicle screw
placement system: two-year results from a Food and Drug Administration investigational device exemption clinical trial. Spine. 2000;25:1437–1446.
34. Brantigan JW, Neidre A, Toohey JS. The lumbar I/F cage
for posterior lumbar interbody fusion with the Variable Screw Placement System: 10-year results of a food and drug administration clinical trial. Spine J. 2004;4:681–688.
35. Stromberg L, Toohey JS, Neidre A, et al. Complications and surgical considerations in posterior lumbar interbody fusion with carbon fiber interbody cages and Steffee pedicle screws and plates. Orthopedics. 2003;26:1039–1043.
36. Prolo DJ, Oklund SA, Butcher M. Toward uniformity in evaluating results of lumbar spine operations. A paradigm applied to posterior lumbar interbody fusions. Spine. 1986;11:601–606.
37. Blount KJ, Krompinger WJ, Maljanian R, et al. Moving toward a standard for spinal fusion outcomes assessment. J Spinal Disord Tech. 2002;15:16–23.
38. Lowe TG, Tahernia AD, O'Brien MF, et al. Unilateral transforaminal posterior lumbar interbody fusion (TLIF
): indications, technique, and 2-year results. J Spinal Disord Tech. 2002;15:31–38.
39. Brantigan JW, McAfee PC, Cunningham BW, et al. Interbody lumbar fusion using a carbon fiber cage
implant versus allograft bone. An investigational study in the Spanish goat. Spine. 1994;19:1436–1444.
40. Serhan H. Presentation of mechanical testing of the Brantigan cage
. In: The Orthopaedics and Rehabilitation Devices Advisory Panel Meeting of the Department of Health and Human Services, Public Health Service, Food and Drug Administration. Maryland: Bethesda; 1997.
41. Closkey RF, Parsons JR, Lee CK, et al. Mechanics of interbody spinal fusion. Analysis of critical bone graft area. Spine. 1993;18:1011–1015.
42. Harris BM, Hilibrand AS, Savas PE, et al. Transforaminal lumbar interbody fusion: the effect of various instrumentation techniques on the flexibility of the lumbar spine. Spine. 2004;29:E65–E70.
43. Heth JA, Hitchon PW, Goel VK, et al. A biomechanical comparison between anterior and transverse interbody fusion cages. Spine. 2001;26:E261–E267.
44. Wang ST, Goel VK, Fu CY, et al. Posterior instrumentation reduces differences in spine stability as a result of different cage
orientations: an in vitro study. Spine. 2005;30:62–67.
45. Javernick MA, Kuklo TR, Polly DW Jr. Transforaminal lumbar interbody fusion: unilateral versus bilateral disk removal—an in vivo study. Am J Orthop. 2003;32:344–348; Discussion 348.