Duke Orthopedic Journal

: 2019  |  Volume : 9  |  Issue : 1  |  Page : 15--25

Anterior versus posterior lumbar interbody fusion: Does cage geometry matter more than surgical approach?

Sean P Ryan1, Rachel Nash2, Nyle Larson3, Anthony A Catanzano1, Brian L Dial1, Bethany Harpole4, Andrew J Pugely3, Sergio A Mendoza-Lattes1,  
1 Duke University Medical Center, Durham, NC, USA
2 Department of Orthopaedic Surgery, Beaumont Hospital, Detroit, MI, USA
3 University of Iowa Medical Center, Iowa City, IA, USA
4 Department of Orthopaedic Surgery, University of Kansas, Wichita, KS, USA

Correspondence Address:
Dr. Sergio A Mendoza-Lattes
Department of Orthopaedic Surgery, Duke University Medical Center, 40 Duke Medicine Circle, Durham, NC 27710


Background: For patients undergoing lumbar fusion, a variety of interbody arthrodesis techniques and devices exist, but few studies have evaluated the effect of cage geometry on radiographic outcomes. Thus, the purpose of this study is to compare the performance of expandable lordotic posterior lumbar interbody fusion (ePLIF) cages to lordotic anterior lumbar interbody fusion (ALIF) cages and to compare the early radiographic outcomes of different cage designs through review of the available literature. Materials and Methods: This is a retrospective case–control study, including 31 ePLIF and 36 ALIF levels, for the treatment of lumbar radiculopathy. Three-dimensional computed tomography scans were used to measure disc height, interbody angle, and foraminal height, both pre- and postoperatively. Implant geometry and positioning were then correlated with radiographic outcomes. The available ALIF and PLIF literature was then analyzed to determine the radiographic outcomes for each surgical technique based on cage geometry. Results: ePLIF cages increased foraminal height (P < 0.001), which was comparable to lordotic ALIF cages (P < 0.001). ePLIF and ALIF provided similar restoration of disc height; however, ALIF cages provided a significant increase in interbody angle (P < 0.001). Across the available literature, ALIF correlated with greater changes in interbody angle relative to PLIF regardless of cage geometry (lordotic vs. nonlordotic), while PLIF trended toward greater restoration of foraminal height. Conclusion: ePLIF cages are able to restore foraminal and disc height comparable to ALIF cages. However, lordotic ALIF cages should be utilized if sagittal restoration is a priority. Future studies are necessary to further explore the value of different implant design options. Level of Evidence: Level III. Clinical Relevance: Patients with abnormal sagittal balance should undergo a lordotic ALIF procedure. Patients who are sagittally balanced, however, can achieve fusion and decompression with either ALIF or ePLIF.

How to cite this article:
Ryan SP, Nash R, Larson N, Catanzano AA, Dial BL, Harpole B, Pugely AJ, Mendoza-Lattes SA. Anterior versus posterior lumbar interbody fusion: Does cage geometry matter more than surgical approach?.Duke Orthop J 2019;9:15-25

How to cite this URL:
Ryan SP, Nash R, Larson N, Catanzano AA, Dial BL, Harpole B, Pugely AJ, Mendoza-Lattes SA. Anterior versus posterior lumbar interbody fusion: Does cage geometry matter more than surgical approach?. Duke Orthop J [serial online] 2019 [cited 2021 Mar 2 ];9:15-25
Available from: https://www.dukeorthojournal.com/text.asp?2019/9/1/15/279430

Full Text


Circumferential constructs such as anterior lumbar interbody fusion (ALIF) combined with posterolateral fusion (PLF) and posterior lumbar interbody fusion (PLIF) are advantageous as these load-bearing constructs have higher union rates, lower reoperation rates, and improved clinical outcomes compared to isolated PLF.[1],[2],[3],[4] Prior studies have suggested several advantages of ALIF over posterior-only constructs, including larger exposure for endplate preparation, larger implants for superior disc height restoration, and increased segmental and global lordosis.[5],[6],[7] These advantages may be secondary to the anterior approach and ability to release the anterior longitudinal ligament (ALL), thus earning the ALIF the reputation of the “gold standard” of interbody fusion. PLIF, however, is an isolated posterior procedure, which offers 360° of fusion with shorter operating times, less surgical dissection, and is more cost-effective.[8]

Early interbody cages offered few options and consisted predominantly of rectangular or cylindrical cages inserted using a posterior approach. The implantation of these devices resulted in incomplete sagittal restoration and risked iatrogenic flatback syndrome. This subsequently led to an expansion in the number of available cage options with various geometries, sizes, and materials, in addition to different surgical approaches (transforaminal lumbar interbody fusion, oblique lumbar interbody fusion, and lateral lumbar interbody fusion).[9] Therefore, the type of interbody spacer used in modern spine surgery may have a larger role in defining sagittal alignment and foraminal height than previously recognized.[10],[11],[12],[13] Although many studies have compared various approaches and their outcomes, only a few have looked at the correlation between cage design and radiographic outcomes.[14] Of these studies, most fail to provide the reader with a detailed description of cage sizing and geometry, making direct comparison of both surgical technique and cage design extremely difficult.

The newest generation of cage designs includes vertical expansion capacity, thereby offering the versatility of insertion of a smaller implant with subsequent expansion. There is a paucity of data on these new implants and no comparisons with the ALIF, “gold standard,” results. The purpose of this study is to determine whether expandable PLIF (ePLIF) provides changes in disc space geometry and neuroforaminal (NF) decompression comparable with ALIF. We further sought to perform a thorough review of the literature to compare both surgical technique and cage geometry and their effects on radiographic outcomes.

 Materials and Methods

Study design

This is a retrospective case–control study (Level-III evidence) of prospectively collected data aimed at determining the effect of surgical approach and cage geometry on immediate radiographic outcomes. Study participants were identified from one major orthopedic referral center following the Institutional Review Board approval. Inclusion criteria consisted of symptomatic lumbar foraminal stenosis with clinical radiculopathy, at least 3 months of failed conservative care including physical therapy and at least one selective nerve root injection to confirm the level and side of symptoms. Patients failing to meet any of these criteria were excluded. All procedures were performed consecutively. Patients were either treated with a static ALIF cage (Continental®, Globus Medical Inc., Audubon, PA, USA) – 36 levels or with Expandable PLIF (ePLIF) (Caliber® or Rise®, Globus Medical Inc., Audubon, PA, USA) – 31 levels by a fellowship-trained surgeon at a single institution from July, 2012 to December, 2014.

Radiographic measurements

Pre- and postoperative three-dimensional computed tomography (CT) scans were analyzed using Vitrea 6.0 Image Software (Vital Images, Franklin, WI, USA) [Figure 1]a, [Figure 1]b, [Figure 1]c. Measurements of the symptomatic levels included: (1) anterior disc space height – measured from the anterior edge of the inferior endplate of the superior vertebra to the edge of the superior endplate of the inferior vertebra; (2) posterior disc space height – measured between the posterior endplate edges; (3) the interbody angle – the angle between the two endplates adjacent to the foramen of interest; (4) foraminal height – measured as the vertical distance between the two corresponding pedicles; (5) Foraminal area – the contours of the inferior border of the widest portion of the superior pedicle (point of inflexion of the nerve root), the superior articular process, superior border of the inferior pedicle, and the posterior margin of the intervertebral disc [Figure 1]. The research team was blinded to the side and level of symptoms.{Figure 1}

Surgical technique

Patients in the ALIF group were placed supine and an anterior midline retroperitoneal exposure was performed. The anterior annulus/ALL was excised, followed by removal of the intervertebral disc and posterolateral corner decompression with a curette. Implants were then trialed and final size and degree of lordosis determined. At L5-S1, implants with 15° lordosis were preferred, and at L4–L5 or cephalad, 8° implants were typically utilized. Subsequently, anterior buttress plate was utilized for stability. The patients were then repositioned prone and posterior instrumentation was used in all cases. Finally, a foraminotomy was performed on the symptomatic side and compression was applied across the motion segment.

Patients who underwent ePLIF were placed prone and a standard midline approach was utilized. Full posterior decompression was performed including laminectomy and facetectomy. Pedicle screws (Revere®, Globus Medical Inc., Audubon, PA, USA) were placed at the fusion level and a 10-mm wide annulotomy was performed. Following endplate preparation and trialing, a Globus Medical Caliber® expandable and lordotic PLIF cage was placed and expanded to torque under fluoroscopic guidance. A similar procedure was carried out on the contralateral side. At L5-S1, 12° implants were preferred, and at L4-L5 or higher, 4° implants were commonly chosen. Finally, pedicle screws were compressed across the motion segment. A preoperative and postoperative CT scan for representation is shown in [Figure 1]b and c, respectively.

Literature review

PubMed was queried for a total of 242 PLIF and 189 ALIF publications. Search terms included: Lumbar Interbody Fusion, PLIF, ALIF, and interbody cages. Abstracts were reviewed and articles with relevant radiographic measures were further analyzed. Data including implant geometry, NF height, lumbar lordosis, and disc height were collected, when available. Articles reporting other measurements, included osteotomies, had alternative surgical approaches, or reported a “change” in interbody angle or NF height rather than pre- and postoperative values, were excluded. Thus, a total of 35 manuscripts included the necessary data for analysis. The average of anterior and posterior disc height was used to compare with reports including only a single mid-disc space height. When the interbody angle was reported separately for different fusion levels, the values were averaged and presented as a single value for all motion segments. This was done to allow the analysis of the change in lordosis based on cage geometry rather than based on level. Cage descriptions were then used to categorize the available literature into lordotic versus nonlordotic implants, in addition to surgical approach. Very few manuscripts presented consistent data on cage lordosis. Articles with incomplete cage information were not included in the data analysis. Of the 431 abstracts reviewed, relevant and complete data were only found in 35 papers. [Figure 2] summarizes the literature search.{Figure 2}

Statistical methods

Demographic information was obtained from the electronic medical record. Statistical analysis was performed with SPSS Statistics v21.0 (Armonk, NY, USA) and Microsoft Excel 2011. Data are presented as an average with 95% confidence intervals (CIs). Two members of the research team performed measurements independently and interobserver reliability was calculated with Cohen's Kappa. Paired t-tests were used to compare continuous variables between the two groups. Chi-square and Pearson's coefficients were also calculated. P < 0.007 using a Bonferroni correction was considered statistically significant.


The ALIF cohort included 36 levels in 30 patients with a mean age of 43.3 years (95% CI 39.9–46.7) and average body mass index (BMI) of 29.4 (95% CI 27.3–31.5). The ePLIF cohort included 31 levels in 19 patients with a mean age of 61.8 years (95% CI 57.8–65.8) and average BMI of 32.0 (95% CI 29.1–34.9). There was a significantly increased age of the ePLIF cohort (P < 0.001) but no difference in BMI (P = 0.157). The average number of levels for the ALIF and ePLIF were 1.2 and 1.74, respectively. ALIF levels included 2 L3/4, 12 L4/5, and 22 L5/S1. ePLIF levels included 1 T12/L1, 2 L1/2, 1 L2/3, 8 L3/4, 10 L4/5, and 9 L5/S1. Interobserver reliability as measured by Cohen's K varied between 0.6092 and 0.8679, indicating substantial agreement of the measurements. The mean overall ePLIF disc height measured preoperatively was 6.55 mm (95% CI: 5.5–7.5 mm) and postoperatively increased to 12.8 mm (95% CI: 11.7–14 mm), P < 0.0001. The mean ALIF disc height measured preoperatively was 6.7 mm (95% CI: 5.0–7.4 mm) and postoperatively increased to 9.85 mm (95% CI: 9.3–10.4 mm), P < 0.001.

The interbody angle (lordosis) for ePLIF levels was −7.5° (95% CI: −5.4°–−9.7°) preoperatively and −5.5° (95% CI: −3.1°–−7.8°) postoperatively, P = 0.1971. This angle changed from −9.1° (95% CI: −7.7°–−10.5°) preoperatively to −12.5° (95% CI: −11.1°–−13.9°) postoperatively (P < 0.0001) in levels treated with ALIF. The ePLIF-treated discs had foraminal height increase from 16.2 mm (95% CI: 14.9–17.6 mm) preoperatively to 20.6 mm (95% CI: 18.6–-22.6 mm) after surgery, P < 0.0001. ALIF-treated foraminal height went from 8.6 mm (95% CI: 7.5–9.8 mm) to 13.2 mm (95% CI: 12.4–14.0 mm), P < 0.0001. A comparison of the change in radiographic parameters for each surgical approach pre- and postoperatively is shown in [Table 1]. The radiographic results from our cohort as well as the available literature review are presented in [Table 2] and [Table 3]. Data from literature review showing PLIF and ALIF cage geometry (lordotic vs. nonlordotic) were reviewed according to the previously described radiologic parameters. The results are shown in [Figure 3].{Table 1}{Table 2}{Table 3}{Figure 3}

Changes in interbody angle and NF area in the ePLIF cohort were further analyzed in our cohort. The relative position in which the ePLIF implants were placed within the disc space was additionally considered. Anteriorly placed ePLIF cages seemed to favor lordosis restoration (P = 0.0711) [Figure 4], while posteriorly placed implants trended toward improved foraminal height restoration (P = 0.1900) [Figure 5].{Figure 4}{Figure 5}


Interbody arthrodesis has evolved as a common and effective technique to treat select spinal pathology, and it has been shown that a solid fusion in the lumbar spine is most reliably achieved with a load-bearing construct.[1],[2],[3],[4] However, variations in approach and implants persist. The choice of surgical technique must be tailored to various patient factors, including regional/global alignment, neurologic compression, bone quality, and patient symptoms. ALIF has previously been advocated as the most efficacious method for restoration of lordosis, but also provides increased disc height, and potential for increased foraminal decompression.[5],[7] A further factor to consider, which is often overlooked, is implant geometry. As shown in our literature review, cage geometry is infrequently reported, leaving surgeons to speculate on which procedure and implant offer the best results.

According to our findings, both lordotic ALIF and lordotic ePLIF cages resulted in a significant increase in the disc height and NF height. However, ePLIF overall generated a small degree of local kyphosis while ALIF consistently resulted in improved lordosis. This difference was partially driven by location of ePLIF cage placement, with anteriorly placed cages generating local lordosis. Even with anteriorly placed ePLIF cages, however, ALIF had greater restoration of lordosis, which is consistent in the available literature. While both ALIF and ePLIF provided significant improvements in FH and disc height, ALIF generated significantly more lordosis (interbody angle) as shown in [Figure 3].

Both restoration of motion segment lordosis and improvement in NF dimensions are vital to clinical success. Failure to restore sagittal alignment contributes to breakdown at the segments adjacent to that of the fusion, as well as adjacent segment disease.[14],[24] Inadequate NF decompression may also occur, which leads to persistent symptoms. Previous authors have cautioned that failure of indirect foraminal decompression can occur in approximately 6.5% of the cases[19] and is more prevalent in obese patients or in the presence of severe facet joint arthropathy. Schlegel et al. reported that 10-mm symmetric distraction of the intervertebral disc can result in 40% improvement in the foraminal area for indirect decompression.[42],[43] However, unrecognized NF stenosis may persist,[30],[44] potentially stemming from disc height collapse, foraminal disc extrusion, facet hypertrophy, ectopic ossification or calcification of ligamentum flavum, and/or disc-osteophyte complex. Given the significance of adequate NF decompression, the question of “how much NF decompression is adequate?” becomes relevant when deciding which surgical approach (and cage) will be utilized.

Several authors have shown the effect that positional changes have on the NF dimensions and that symptoms are usually triggered in extension. Therefore, supine imaging may only partially reflect the reality of the anatomical geometry creating symptoms. Recent studies have found that there is an average 30% difference in the NF area between flexion and extension in the lumbar spine.[45] While Fujiwara et al. stated that flexion of the spine opened the NF by 11.3%,[46] Singh et al. found that where angular motion exceeded 15°, the average NF area varied by 75 mm2 between flexion and extension.[45]

Hasegawa et al.[47],[48] reviewed the critical height of the NF and found that nerve-root compression occurred more frequently when the NF height was ≤15 mm. If we consider 15 mm as the critical NF height,[47] and then account for the reported 30% decrease that may occur between flexion and extension,[45] the NF height could reduce to 10.5 mm. This should, therefore, be the minimum threshold for acceptable NF height intraoperatively to accommodate the dorsal root ganglia (which have an average height 4.3 ± 0.9–8.3 ± 1.2 mm).[48] In the current study, as well as in the literature review, the foraminal height postoperatively for both ALIF and PLIF reached the acceptable threshold by this calculation. The symptomatic patients in our study also had a NF height of 8 mm on average prior to surgery, further supporting this theory.

To our knowledge, there are only four prior studies that have explored the geometric analysis and radiographic outcomes of ePLIF. Park et al. mentioned that in their study of 34 patients that disc height was improved from 8.11 to 10.02 mm; however, angle of lordosis and foraminal height were not mentioned.[22] Kim et al. reported on 57 patients and noted that the disc height increased from 9.9 to 12.2 mm, and the angle of lordosis went from 3.5° to 6.4°, but there was no mention of foraminal height.[23] Foraminal height, however, is felt to be a more reliable measurement of indirect decompression than disc height due to the concern for the effect that the position of the cage leads to variable results, as shown in our study. Coe et al. analyzed the ePLIF in 32 patients and noted improvements in disc height by 4.5 mm, local disc angle by 6.1 degrees, and foraminal height by 5.6 mm.[49] However, they did not have a control group for comparison or provide preoperative or postoperative radiographic values for analysis. In addition, no mention was made as to whether these changes were significant. Finally, Neely et al.[50] in a review of 470 cases with the ePLIF found high fusion rates, low reoperation rates, and good clinical outcomes. However, they did not specifically look at disc height, foraminal height, or local lordosis. Other reports on expandable cages are available for TLIF with similar results to PLIF.[51]

Unlike prior studies on ePLIF implants, our data provide a control group through the “gold standard” ALIF cage and focuses on radiographic outcomes based on different implant designs. Our data support that both devices improve NF height effectively, which should provide adequate nerve root decompression and decrease the rate of surgical failure and symptom recurrence. Importantly, our results for the lordotic ALIF and ePLIF cage data are similar to prior studies, which strengthen our findings. None of the positions with the ePLIF cage provided a better sagittal profile (interbody angle) than ALIF cages. This may be explained by the release of the ALL that occurs during the ALIF procedure, as described by previous authors[30],[35] However, our findings show that the anterior disc space had significantly greater height increase with ePLIF compared to ALIF, which argues against release of the ALL as the sole driving factor. We propose that releasing the facet joints during ePLIF may allow an increased expansion of the disc space (anteriorly and posteriorly). When left intact during ALIF, they may act as a tether posteriorly, decreasing the relative disc space height achievable, but improving segmental lordosis when performed in conjunction with release of the ALL.

This study has several limitations, as one would expect with any retrospective study. It was noted that in the ALIF cohort, there was a bias toward younger patients. Therefore, the older ePLIF cohort may have had more rigidity and resistance to interbody expansion. Since this cohort showed greater anterior and posterior disc height expansion compared to the ALIF cohort, we feel this was not a factor that significantly influenced our outcomes. In addition, the sample size was small; however, our results are comparable to previously published data for both ALIF and PLIF implants. In the literature review, a relatively small number of papers reported the necessary data points to be included, which limited our analysis. For example, based on [Figure 3], it would appear that nonlordotic ALIF cages provided more lordosis than lordotic ALIF cages. [Table 3], however, reveals that only one paper by Hsieh et al.[11] contained enough information to be included in the evaluation of nonlordotic cages. A final weakness to this paper is that we did not examine individual lumbar levels in relation to cage geometry. This also has a potential to contribute to patient selection bias, as higher vertebral levels (i.e. T12–L1) were more unlikely to undergo an ALIF. It may be that at caudal lumbar levels with greater lordosis, radiographic outcomes differ based on approach and implant design more than at cephalad lumbar levels. Patient selection bias, however, certainly has the potential to confound any retrospective studies comparing two different procedures.


Both devices were able to surpass the critical NF height necessary for the exiting nerve root, with superior lordosis correction noted with the lordotic ALIF group. The question still remains, however, as to what the correct amount of NF height restoration is to alleviate stenosis without overloading the adjacent endplates leading to subsidence of the implant, or surpassing the physiologic loads at adjacent levels causing adjacent segment pathology. Based on this data, our recommendation is that patients with abnormal sagittal balance undergo a lordotic ALIF procedure. Patients that are sagittally balanced, however, can achieve fusion and decompression of the NF with either ALIF or ePLIF. Given the cost-effectiveness of a single posterior approach and decreased operative time, a posterior approach and expandable implant should be considered for this patient population. Further prospective studies will be needed to assess the objective goals relating to direct and indirect decompression of the NF as well as long-term clinical outcomes of patients undergoing lumbar interbody fusion.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


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