Lumbar fusion, especially interbody fusion, can provide stability to painful and unstable motion segments, provide indirect decompression for foraminal stenosis, and improve spinal balance and deformity. Interbody placement allows for the correction of sagittal parameters including lordosis and restoration of disc height, both of which are frequently lost in the degenerative spine. Several techniques have achieved widespread acceptance for interbody fusion. Anterior lumbar fusion techniques offer significant and reliable benefits to lordotic correction that remain controversial in other approaches, such as transforaminal lumbar interbody fusion (TLIF) and posterior lumbar interbody fusion (PLIF).1 TLIF procedures have been suggested to induce kyphosis or lordosis depending on various patient or surgical technique factors,2–6 while PLIFs are now less commonly performed.
Minimally invasive lateral lumbar interbody fusion (LLIF), another option for lumbar interbody fusions, offers the reliability of lordotic correction observed in anterior lumbar surgery while also supporting a minimally invasive approach that can reduce complications.4,7,8 These minimally invasive approaches have been reported by some investigators to be associated with decreased blood loss, lower postoperative readmissions, reduction in opioid use, and shorter length of hospital stay.4,9–11
There has been significant effort in the literature to better understand the sagittal correction provided by LLIFs, but no optimal technique or implant has been found. Cage geometry, including lordotic angle and implant size, and cage placement have both been suggested to play integral roles in radiographic outcomes.7,12–16 However, recent studies have not demonstrated an association between degree of cage lordosis and correction of sagittal parameters following interbody fusion.13,14,17 The role of cage geometry and placement during LLIF procedures continues to remain unclear. Therefore, the primary objective of our study was to evaluate patient and surgical factors that predict increased overall lumbar lordosis (LL) and segmental lordosis correction following LLIF procedures, specifically evaluating cage geometry and positioning.
Methods
Patient Selection
After approval from the IRB, a retrospective cohort study was conducted on all patients older than 18 years of age who underwent an LLIF, using a Structured Query Language search. Patients were excluded if their surgery included a simultaneous osteotomy or a fusion of more than two levels, or if they underwent a subsequent spine surgery during the 6-month follow-up period. Patients were also excluded if their surgery was indicated for malignancy, infection, or trauma.
Patient Characteristics and Radiographic Parameters
Demographic parameters including age, sex, BMI, smoking status (current, former smoker, never), diabetes history, and history of prior spine surgery were collected via chart review. Surgical details that were recorded included the specific levels fused, interbody cage lordosis, cage dimensions, cage material, cage placement, whether the LLIF was a primary or revision surgery, and if posterior screws were placed via a percutaneous or open approach. Surgical outcomes included 90-day readmission and 90-day complication rates. Included complications were those defined as Clavien-Dindo grade 3 or 4 requiring invasive intervention or organ failure (e.g., reoperation, stroke, myocardial infarction, renal failure, and pulmonary embolism) and surgical complications associated with the procedure or approach (e.g., hardware fracture, dural tear, surgical site infection, arterial/venous injury, hematoma/seroma, and wound dehiscence).
Preoperative, initial postoperative, 6-month postoperative, and Δ (postoperative minus preoperative) measurements were collected for each patient with intact imaging records using our PACS. Standing lateral lumbar radiographs were used for all measurements to ensure standard measurement techniques for each patient. The following pre- and postoperative measurements were recorded: LL, segmental lordosis, anterior disc height, and posterior disc height. LL was measured as the angle between the superior endplate of L1 and the superior endplate of S1. Segmental lordosis was defined as the angle between the superior endplate of the superior vertebra and the inferior endplate of the inferior vertebra. Anterior and posterior disc heights were measured as the distance between the inferior endplate of the superior vertebra and the superior endplate of the inferior vertebra both anteriorly and posteriorly, respectively. Cage placement was defined by measuring the center point ratio (CPR) on the initial postoperative standing lateral radiograph.17,18 CPR is the distance between the midpoint of the cage and the posterior edge of the superior endplate of the inferior vertebral body divided by the entire length of the superior endplate of the inferior vertebral body. In effect, cages with a greater CPR were placed more anteriorly, while those with a lower CPR were located more posteriorly; a CPR of 0.50 represented a cage placed exactly in the middle of the inferior vertebral body.
Statistical Analysis
After we collected radiographic parameters, patients were evaluated to determine if there were any significant predictors of postoperative change in segmental lordosis immediately and at the 6-month follow-up. The patient population was divided equally based on the median degree of immediate segmental lordosis correction, which was determined to be 3°. Patients were subsequently stratified into three subgroups based on the cage positioning CPR (anterior, central, or posterior). Because we expected clustering around more central values with fewer comparisons, we divided groups equally based on the cage positioning within our study population. For all these comparisons, continuous variables were assessed using either an independent t-test or Mann-Whitney U-test for parametric and nonparametric data, respectively. A Shapiro-Wilk score was calculated for each continuous variable to assess normality of distribution. All categorical variables were compared using a Pearson chi-square analysis or Fisher’s exact test. We then conducted a Spearman correlation coefficient analysis to assess associations between cage lordosis and radiographic parameters. Multivariate linear regression was performed to assess independent predictors of Δ LL, Δ segmental lordosis, Δ anterior disc height, and Δ posterior disc height. All statistical tests were conducted using Stata SE (release 17, StataCorp). A p value < 0.05 was considered significant.
Results
Baseline Characteristics
A total of 106 levels in 78 unique patients met inclusion criteria (Table 1). Most of the cages were placed in male patients (62.3%), with a mean overall patient age of 65.2 years. Most procedures involved an LLIF at one level (n = 50, 64.1%), while 28 (35.9%) were two-level LLIF procedures. The most common levels fused were L3–4 (46.2%), followed by L2–3 (26.4%) and L4–5 (22.6%). The L1–2 vertebral level was the least commonly fused (4.7%). No stand-alone LLIF procedures were identified, and all patients had posterior instrumentation. Overall, 49.1% of all levels were associated with percutaneous screw placement via a minimally invasive or robotic approach, while 51.9% of screws at fused levels were placed via an open approach (Table 1). One complication and readmission was noted in our cohort, in which the same obese patient experienced wound dehiscence and required incision and drainage of their posterior incision site. The posterior fusion in this patient was performed via an open approach, and the surgery was a revision procedure.
Demographics on a per-level basis (n = 106 levels)
| Variable | Value |
|---|---|
| Mean age (SD), yrs | 65.2 (9.3) |
| Sex, n (%) | |
| Male | 66 (62.3) |
| Female | 40 (37.7) |
| Mean BMI (SD) | 30.4 (6.0) |
| Diabetes history, n (%) | 21 (19.8) |
| Smoking history, n (%) | |
| Never smoker | 61 (57.6) |
| Former smoker | 29 (27.4) |
| Current smoker | 16 (15.1) |
| Revision surgery, n (%) | 31 (29.3) |
| No. of levels fused, n (%) | |
| 1 level | 50 (47.2) |
| 2 levels | 56 (52.8) |
| Levels fused, n (%) | |
| L1–2 | 5 (4.7) |
| L2–3 | 28 (26.4) |
| L3–4 | 49 (46.2) |
| L4–5 | 24 (22.6) |
| Percutaneous screws, n (%) | 52 (49.1) |
| Mean CLA (SD), ° | 9.45 (2.3) |
| Mean cage height (SD), mm | 10.4 (2.1) |
| Cage material, n (%) | |
| PEEK | 40 (40.4) |
| Titanium | 59 (59.6) |
CLA = cage lordotic angle; PEEK = polyetheretherketone.
Immediate Postoperative Segmental Lordosis
We first divided our cohort by the median improvement in postoperative segmental lordosis, which was 3.0°. Fifty-two cages improved segmental lordosis by < 3°, while 54 provided > 3° of segmental lordosis. Only 8 patients experienced any loss of segmental lordosis postoperatively. Patients who experienced greater improvement in segmental lordosis after fusion were found to have worse preoperative segmental lordosis (mean 9.21° vs 13.18°, p = 0.0072) but similar overall LL (mean 34.9° vs 38.6°, p = 0.3617). There were no other differences in patient or operative characteristics based on initial postoperative improvement. Patients with greater segmental lordosis correction had a more anterior cage placement (mean CPR 0.586 vs 0.516, p = 0.0097), but no significant differences in overall cage dimensions, lordosis angle, or material (Table 2).
Association of patient factors with overall segmental correction in the immediate postoperative period
| Factor | Δ Segmental Lordosis | p Value | |
|---|---|---|---|
| <3.0°, n = 52 | >3.0°, n = 54 | ||
| Mean age (SD), yrs | 66.6 (9.2) | 63.8 (9.2) | 0.1023 |
| Sex, n | — | — | 0.515 |
| Male | 34 | 32 | |
| Female | 18 | 22 | |
| Mean BMI (SD) | 30.3 (5.6) | 30.5 (6.4) | 0.9885 |
| Diabetes history, n | 11 | 10 | 0.734 |
| Smoking history, n | — | — | 0.877 |
| Never smoker | 30 | 31 | |
| Former smoker | 15 | 14 | |
| Current smoker | 7 | 9 | |
| Primary vs revision | 15 | 16 | 0.929 |
| No. of levels fused, n | — | — | 0.552 |
| 1 level | 23 | 27 | |
| 2 levels | 29 | 27 | |
| Percutaneous screws, n | 24 | 28 | 0.557 |
| Mean CLA (SD), ° | 9.85 (2.1) | 9.06 (2.5) | 0.0997 |
| Mean cage height (SD), mm | 10.4 (2.2) | 10.4 (2.0) | 0.7496 |
| Cage material, n | — | — | 0.367 |
| PEEK | 22 | 18 | |
| Titanium | 27 | 32 | |
| Mean CPR (SD) | 0.516 (0.12) | 0.586 (0.14) | 0.0097 |
| Mean preop LL (SD), ° | 38.60 (10.1) | 34.91 (15.1) | 0.3617 |
| Mean preop segmental lordosis (SD), ° | 13.18 (6.9) | 9.21 (8.0) | 0.0072 |
Boldface type indicates statistical significance.
Six-Month Postoperative Segmental Lordosis
On 6-month follow-up standing radiographs, 61 fused levels (57.5%) demonstrated > 3° postoperative improvement in segmental lordosis. Patients experiencing maintained improvement in segmental lordosis were more likely to be female (49.2% vs 22.2%, p = 0.005) and were more likely to have percutaneously placed posterior pedicle screws compared with open placement (57.4% vs 37.8%, p = 0.046). Patients with greater improvement in segmental lordosis were still more likely to have worse preoperative segmental lordosis (mean 9.11° vs 13.93°, p = 0.0012) and more likely to have anteriorly placed interbody cages (mean CPR 0.578 vs 0.519, p = 0.039; Table 3).
Association of patient factors with overall segmental lordosis correction at 6 months postoperatively
| Variable | Δ Segmental Lordosis | p Value | |
|---|---|---|---|
| <3.0°, n = 45 | >3.0°, n = 61 | ||
| Mean age (SD), yrs | 67.1 (8.3) | 63.8 (9.8) | 0.1375 |
| Sex, n | — | — | 0.005 |
| Male | 35 | 31 | |
| Female | 10 | 30 | |
| Mean BMI (SD) | 30.6 (5.7) | 30.2 (6.2) | 0.7484 |
| Diabetes history, n | 8 | 13 | 0.652 |
| Smoking history, n | — | — | 0.491 |
| Never smoker | 24 | 37 | |
| Former smoker | 15 | 14 | |
| Current smoker | 6 | 10 | |
| Primary vs revision, n | 14 | 17 | 0.717 |
| No. of levels fused, n | — | — | 0.204 |
| 1 level | 18 | 32 | |
| 2 levels | 27 | 29 | |
| Percutaneous screws, n | 17 | 35 | 0.046 |
| Mean CLA (SD), ° | 9.88 (2.3) | 9.13 (2.4) | 0.1218 |
| Mean cage height (SD), mm | 10.42 (1.8) | 10.39 (2.2) | 0.6972 |
| Cage material, n | — | — | 0.990 |
| PEEK | 17 | 23 | |
| Titanium | 25 | 34 | |
| Mean CPR (SD) | 0.519 (0.15) | 0.578 (0.13) | 0.039 |
| Mean preop LL (SD), ° | 37.84 (11.0) | 35.90 (14.3) | 0.8003 |
| Mean preop segmental lordosis (SD), ° | 13.93 (6.2) | 9.11 (8.1) | 0.0012 |
Boldface type indicates statistical significance.
Cage Placement and Segmental Lordosis
To evaluate the effect of cage placement on segmental lordosis, we then used the CPR to divide cage placement into anterior, central, and posterior. Patients were considered to have anteriorly placed cages at a CPR > 0.61 and posteriorly placed cages with a CPR < 0.485. All groups experienced improvement (as noted by positive Δ values) in overall LL, segmental lordosis, and anterior and posterior disc height both immediately postoperatively and at 6 months. Despite showing no differences in baseline segmental lordosis, patients with anteriorly or centrally placed cages experienced the greatest segmental lordosis correction immediately (mean anterior 4.81° and central 4.46° vs posterior 2.47°, p = 0.0315) and at 6 months postoperatively. Patients with anteriorly placed cages also had greater overall lordosis correction postoperatively (mean 6.30°) compared with those with cages placed centrally (mean 3.71°) or posteriorly (mean 3.36°, p = 0.0338), but these differences disappeared at 6 months (mean 6.11° vs 5.17° vs 4.11°, p = 0.3408; Table 4).
Association of cage placement with radiographic parameters
| Parameter | Anterior, n = 35 | Central, n = 36 | Posterior, n = 35 | p Value |
|---|---|---|---|---|
| Mean overall LL (SD), ° | ||||
| Preop | 34.77 (13.8) | 38.53 (11.8) | 36.81 (13.4) | 0.6234 |
| Postop | 41.07 (11.9) | 42.24 (13.2) | 40.18 (12.2) | 0.8220 |
| Δ postop | 6.30 (6.5) | 3.71 (5.0) | 3.36 (5.5) | 0.0338 |
| 6 mos | 40.89 (12.2) | 43.70 (13.1) | 40.92 (12.5) | 0.6116 |
| Δ 6 mos | 6.11 (7.3) | 5.17 (5.9) | 4.11 (6.6) | 0.3408 |
| Mean segmental lordosis (SD), ° | ||||
| Preop | 11.21 (6.9) | 11.11 (8.0) | 11.15 (8.2) | 0.9986 |
| Postop | 16.02 (6.8) | 15.58 (7.9) | 13.62 (8.1) | 0.3788 |
| Δ postop | 4.81 (4.3) | 4.46 (4.6) | 2.47 (2.9) | 0.0315 |
| 6 mos | 16.83 (7.1) | 15.68 (7.7) | 13.47 (7.9) | 0.1722 |
| Δ 6 mos | 5.62 (4.2) | 4.56 (4.8) | 2.32 (2.8) | 0.0024 |
| Mean anterior disc height (SD), mm | ||||
| Preop | 8.71 (2.6) | 7.20 (3.3) | 8.15 (7.2) | 0.1439 |
| Postop | 15.33 (3.6) | 12.38 (2.8) | 12.67 (2.8) | 0.0004 |
| Δ postop | 6.63 (4.3) | 5.18 (4.0) | 4.5 (2.8) | 0.0602 |
| 6 mos | 14.95 (2.7) | 12.73 (3.1) | 11.85 (3.2) | 0.0001 |
| Δ 6 mos | 6.24 (3.3) | 5.53 (4.1) | 3.69 (3.4) | 0.0122 |
| Mean posterior disc height (SD), mm | ||||
| Preop | 4.71 (2.0) | 4.37 (1.7) | 4.61 (1.7) | 0.7154 |
| Postop | 6.51 (2.4) | 8.08 (2.8) | 9.52 (3.0) | 0.0001 |
| Δ postop | 1.80 (2.2) | 3.71 (2.8) | 4.91 (3.0) | 0.0001 |
| 6 mos | 6.78 (2.1) | 7.87 (2.5) | 8.79 (3.3) | 0.0236 |
| Δ 6 mos | 2.06 (2.1) | 3.50 (2.4) | 4.18 (3.4) | 0.0255 |
Boldface type indicates statistical significance.
Postoperatively, patients with anteriorly placed cages had a greater mean anterior disc height (15.33 mm) compared with those placed centrally (12.38 mm) or posteriorly (12.67 mm, p = 0.0004), despite similar mean baseline anterior disc heights (p = 0.1439). At the 6-month follow-up, the overall mean anterior disc height was highest in cages placed anteriorly (p = 0.0122), and these patients experienced the greatest increase in anterior disc height (mean anterior 6.24 mm vs posterior 3.69 mm, p = 0.0122). In contrast, cages placed more posteriorly increased the Δ posterior disc height both postoperatively (mean posterior 4.91 mm vs anterior 1.80 mm, p = 0.0001) and at 6 months (mean posterior 4.18 mm vs anterior 2.06 mm, p = 0.0255; Table 4). Spearman correlation analysis found weak positive relationships between increasing (more anterior) CPR and Δ LL (correlation 0.1943, p = 0.046), Δ segmental lordosis (correlation 0.3053, p = 0.0015), and Δ anterior disc height (correlation 0.3494, p = 0.0002). More posterior cage placement was moderately correlated with a greater Δ posterior disc height (correlation 0.4603, p < 0.0001; Table 5).
Spearman correlation of CPR and radiographic parameters
| Independent Variable (x) | Dependent Variable (y) | Spearman Correlation | p Value | Relationship |
|---|---|---|---|---|
| LL | ||||
| CPR | Preop LL, ° | −0.0823 | 0.4016 | No relationship |
| CPR | Initial postop LL | −0.0278 | 0.7770 | No relationship |
| CPR | Δ initial LL | 0.1943 | 0.0460 | Weak positive |
| CPR | 6-mo LL | −0.0511 | 0.6209 | No relationship |
| CPR | Δ 6-mo LL | 0.1045 | 0.2865 | No relationship |
| Segmental lordosis | ||||
| CPR | Preop segmental lordosis, ° | −0.0112 | 0.9089 | No relationship |
| CPR | Initial postop segmental lordosis | 0.1149 | 0.2409 | No relationship |
| CPR | Δ initial segmental lordosis | 0.2221 | 0.0221 | Weak positive |
| CPR | 6-mo segmental lordosis | 0.1453 | 0.1373 | No relationship |
| CPR | Δ 6-mo segmental lordosis | 0.3053 | 0.0015 | Weak positive |
| Anterior disc height | ||||
| CPR | Preop anterior disc height, mm | 0.1120 | 0.2530 | No relationship |
| CPR | Initial postop anterior disc height | 0.3494 | 0.0002 | Weak positive |
| CPR | Δ initial anterior height | 0.2178 | 0.0249 | Weak positive |
| CPR | 6-mo anterior disc height | 0.4132 | <0.0001 | Moderate positive |
| CPR | Δ 6-mo anterior height | 0.2407 | 0.0130 | Weak positive |
| Posterior disc height | ||||
| CPR | Preop posterior disc height, mm | 0.0735 | 0.4539 | No relationship |
| CPR | Initial postop posterior disc height | −0.4399 | <0.0001 | Moderate negative |
| CPR | Δ initial posterior height | −0.4603 | <0.0001 | Moderate negative |
| CPR | 6-mo posterior disc height | −0.3069 | 0.0014 | Weak negative |
| CPR | Δ 6-mo posterior height | −0.3210 | 0.0008 | Weak negative |
Boldface type indicates statistical significance.
Evaluation of cage lordotic angle found that an increasing degree of cage lordosis demonstrated no correlation with LL, segmental lordosis, or anterior or posterior disc height at any time point (Table 6).
Correlation of cage lordosis and radiographic parameters
| Independent Variable (x) | Dependent Variable (y) | Spearman Correlation | p Value | Relationship |
|---|---|---|---|---|
| LL | ||||
| CLA | Preop LL, ° | −0.0601 | 0.5609 | None |
| CLA | Initial postop LL | −0.1494 | 0.1463 | None |
| CLA | Δ initial LL | −0.0668 | 0.5177 | None |
| CLA | 6-mo LL | −0.1807 | 0.0780 | None |
| CLA | Δ 6-mo LL | −0.1092 | 0.2894 | None |
| Segmental lordosis | ||||
| CLA | Preop segmental lordosis, ° | 0.0239 | 0.8174 | None |
| CLA | Initial postop segmental lordosis | −0.0535 | 0.6050 | None |
| CLA | Δ initial segmental lordosis | −0.1801 | 0.0792 | None |
| CLA | 6-mo segmental lordosis | −0.0171 | 0.8687 | None |
| CLA | Δ 6-mo segmental lordosis | −0.1346 | 0.1910 | None |
| Anterior disc height | ||||
| CLA | Preop anterior disc height, mm | −0.0836 | 0.4180 | None |
| CLA | Initial postop anterior disc height | 0.0711 | 0.4910 | None |
| CLA | Δ initial anterior height | 0.1269 | 0.2179 | None |
| CLA | 6-mo anterior disc height | 0.1037 | 0.3144 | None |
| CLA | Δ 6-mo anterior height | 0.1379 | 0.1803 | None |
| Posterior disc height | ||||
| CLA | Preop posterior disc height, mm | −0.0949 | 0.3575 | None |
| CLA | Initial postop posterior disc height | −0.0234 | 0.8210 | None |
| CLA | Δ initial posterior height | −0.0215 | 0.8350 | None |
| CLA | 6-mo posterior disc height | 0.0132 | 0.8986 | None |
| CLA | Δ 6-mo posterior height | 0.0414 | 0.6889 | None |
Multivariate Linear Regression
Multivariate linear regression showed that older age predicted less overall improvement in lordosis at 6 months (β = −0.20, p = 0.007), but CPR, cage lordotic angle, or number of levels fused did not affect lordosis. CPR independently predicted a greater 6-month improvement in segmental lordosis (β = 7.27, p = 0.015), while greater preoperative segmental lordosis predicted less improvement in segmental lordosis (β = −0.12, p = 0.030). Increasing CPR was associated with a decrease in posterior disc height (β = −8.43, p < 0.001) and an increase in anterior disc height (β = 4.68, p = 0.082), although its relationship to anterior disc height did not reach statistical significance. On multivariate regression, percutaneous screw placement, cage lordotic angle, and cage height did not predict any changes in radiographic outcomes (Table 7).
Multivariate linear regression of radiographic outcomes at 6 months
| Variable | Beta Coefficient | 95% CI | p Value |
|---|---|---|---|
| Δ LL | |||
| Levels fused | 2.42 | −0.53 to 5.37 | 0.107 |
| CPR | 0.52 | −8.73 to 9.78 | 0.911 |
| CLA | −0.4 | −1.0 to 0.19 | 0.179 |
| Percutaneous screws | 1.94 | −1.00 to 4.89 | 0.194 |
| Sex (male) | 1.92 | −0.82 to 4.66 | 0.167 |
| Age | −0.20 | −0.35 to −0.06 | 0.007 |
| Δ segmental lordosis | |||
| CPR | 7.27 | 1.43 to 13.12 | 0.015 |
| Cage height | −0.25 | −0.69 to 0.18 | 0.253 |
| CLA | −9.17 | −0.54 to 0.20 | 0.371 |
| Percutaneous screws | 0.79 | −1.00 to 2.59 | 0.383 |
| Preop segmental lordosis | −0.12 | −0.23 to −0.01 | 0.030 |
| Age | −0.09 | −0.17 to 0.003 | 0.059 |
| Δ anterior disc height | |||
| CPR | 4.68 | −0.60 to 9.97 | 0.082 |
| CLA | 0.26 | −0.07 to 0.59 | 0.125 |
| Percutaneous screws | 0.88 | −0.67 to 2.43 | 0.263 |
| Δ posterior disc height | |||
| CPR | −8.43 | −12.28 to −4.58 | <0.001 |
| CLA | 0.09 | −0.15 to 0.33 | 0.477 |
| Cage height | −0.05 | −0.74 to 0.22 | 0.696 |
| Percutaneous screws | 0.44 | −0.74 to 1.61 | 0.461 |
| Sex (male) | −1.08 | −2.19 to 0.02 | 0.054 |
Boldface type indicates statistical significance.
Discussion
Several contemporary surgical techniques, particularly anterior lumbar interbody fusion and minimally invasive LLIF, allow spine surgeons to offer significant lumbar lordotic improvement and indirect neural decompression. There are multiple considerations that affect surgical decisions in conjunction with many options for cage selection and placement technique, but no clear guidelines exist. For example, if a patient presents primarily with a kyphotic deformity, surgeons may fear that placing fewer lordotic cages may not help the deformity, or that posterior placement of the cage may worsen the deformity. But because persistent leg pain is associated with lower posterior disc height following interbody fusion,13 surgeons may instead feel a need to place the cage more posteriorly to provide indirect decompression.19 Our study aimed to help answer some of these uncertainties on postoperative radiographic outcomes.
In our evaluation of 106 levels fused via LLIF, we identified that all patients, on average, experienced greater improvement in sagittal parameters and disc space restoration, regardless of patient, surgical, or cage characteristics. Additionally, we identified that LLIFs were associated with a low complication and readmission rate. In TLIFs, another commonly utilized surgery for degenerative spinal conditions, both overall and segmental lordosis have been shown to inconsistently improve, with as many as 42.3% of patients exhibiting kyphotic alignment changes following the surgery.2 After a TLIF, fewer lordotic segments appear to be more likely to become more lordotic postoperatively, and highly lordotic segments may lose lordosis.2 However, in our study, only 7.5% of fused levels became kyphotic, consistent with the literature on the reliability of LLIFs.1,4,16,20,21 Moreover, cages placed more anteriorly provided greater improvements in overall lordosis, segmental lordosis, and anterior disc height, while those placed more posteriorly provided greater posterior disc space restoration, which may indicate better indirect decompression. These findings demonstrate that patient-specific factors will likely determine the ideal cage placement.
Several studies have evaluated the importance of cage characteristics across different interbody fusion techniques, primarily in TLIF procedures. A recent analysis of 126 single-level TLIFs demonstrated no association between cage lordosis or cage placement in sagittal parameters or disc height.17 Another study of 45 single-level TLIFs found that anterior cage placement contributed to greater posterior disc height but did not affect LL.13 This finding is counterintuitive and contrary to the findings of our study, as we found that cage positioning increases the height in the area wherein it is placed. Part of the reason for these contrasting findings may come from differences inherent to the approach. The lateral approach allows a surgeon to make a larger discectomy and insertion of large interbody grafts. An examination of 309 operative levels with degenerative spondylolisthesis found that patients undergoing anterior or lateral lumbar spine surgery experienced significantly greater improvement in lordosis than patients undergoing a TLIF.22 LLIF cages have a larger profile than TLIF cages, which allows them to increase disc space throughout the intravertebral space regardless of positioning, while simultaneously augmenting disc height at the site of placement. The larger interbody LLIF cages allow the devices to rest on the lateral margins of the ring apophysis, which may help provide a larger footprint to counteract the resistance of the anterior ligament. Conversely, the smaller TLIF cages may be unable to distract the disc space anteriorly due to tethering of the intact anterior longitudinal ligament.23
Cage positioning significantly impacted disc space restoration in our study. If cage height or lordosis were the primary factor for vertebral body distraction, then one could imagine that placing a hyperlordotic cage posteriorly would increase segmental lordosis by acting as a fulcrum for the superior vertebral body. However, based on radiographic review, anterior cage placement appeared to allow the anterior portion of the superior vertebra to sit atop the cage, while posterior cage placement led to a greater increase in disc height posteriorly than anteriorly. The reason for this finding may be partially because the tension of the anterior longitudinal ligament may resist this increased motion in posteriorly placed cages, limiting the effects of cage lordotic angle. Increasing CPR showed significant moderate negative correlations with Δ posterior disc height, suggesting that greater posterior disc height is associated with more posteriorly placed cages. This association remained significant with a large effect size independent of other patient factors, including age, BMI, cage height/lordosis, and percutaneous screw placement. Moreover, anterior cage placement resulted in significantly less improvement in posterior disc height. Increasing posterior disc height is particularly important because it consequently expands the foraminal space, which can relieve compression of the existing nerve root. However, LLIF cages still significantly increased disc height throughout the disc space regardless of where the cage was placed, suggesting that even anterior placement provides patients with some degree of neural decompression.
LLIF reliably increases lordosis for patients regardless of cage parameters. While patients with worse preoperative segmental lordosis experienced greater improvement in lordosis correction, patients with better baseline segmental lordosis still benefited from the additional lordosis provided via the LLIF approach. Open or mini-open anterior approaches may further expand lordosis, likely due to resection of the anterior longitudinal ligament that acts as a tether on the vertebral column.5 In the posterior and minimally invasive lateral approaches to the spine, the anterior longitudinal ligament remains intact and other surgical strategies are needed to further increase lordosis.24,25 Smith-Petersen and pedicle subtraction osteotomies allow for significant correction in overall lordosis following interbody fusion and may be indicated for severe spinal deformity correction. Patients undergoing PLIF or TLIF may also undergo bilateral facetectomies at the fused level, which may effectively act as an osteotomy in lordosis correction.18 These procedures may have greater effects on alignment than cage placement alone and frequently explain some of the findings in studies that suggest that TLIF and PLIF significantly increase overall lordosis.2,3,18,26 However, osteotomies may lead to increased blood loss, increased complications, prolonged hospital stay, and increased pseudarthrosis rates,27–32 so the reliability of LLIF in lordosis correction may set the stage for an increase in utilization among those patients in need of smaller corrections in lumbar deformity.
We found strong associations for cages placed more anteriorly, with improved segmental and overall lordosis through 6 months of follow-up. While percutaneous screws appeared to contribute to sustained lordosis improvement at 6 months, percutaneous screws did not independently predict improvement in radiographic outcomes. Additionally, cage lordosis and cage height demonstrated no independent associations with lordosis improvement. Cage height has been speculated to contribute to increasing disc space height, which is intuitive, as a larger cage may increase disc space to match the height of the cage. Similarly, hyperlordotic cages are believed to incrementally increase the segmental angle after placement. In our study, cages had an average lordosis of 9.45° and height of 10.4 mm. Neither of these cage parameters impacted outcomes in patients immediately postoperatively. One prior study of 61 LLIF levels found that lordotic cages increased segmental lordosis more than parallel cages but did not affect foraminal disc height or overall LL.33 However, this study did not account for the different placements of each cage. Multiple other studies that also evaluated lordosis in LLIF and TLIF found that cage lordosis did not independently predict radiographic improvement.14,17 Moreover, surgeons should feel comfortable with cage choice without attempting to place progressively larger or more lordotic cages and instead focus on cage placement. Cages should be appropriately sized to a patient’s anatomy rather than to impact radiographic outcomes, which may significantly reduce vertebral endplate disruption and increase the chance of subsequent cage subsidence.12
Limitations of the Study
The results of this study must be interpreted in the context of its limitations, including those inherent to any retrospective study, which include missing data and bias in data availability. Second, the length of follow-up in our study precludes us from commenting on the effects of subsidence and long-term fusion rates. While these are clinically important, they may impact conclusions drawn as to the sagittal alignment and disc space changes from different cage parameters and surgical technique alone. Further long-term studies are needed to evaluate whether cage placement can affect cage subsidence. Despite these limitations, our study findings add significantly to the literature by showing that LLIF cages significantly improve radiographic parameters regardless of placement, but that placement can further positively augment certain findings with calculated placement techniques.
Conclusions
Surgeons should feel comfortable that a minimally invasive LLIF will contribute to improved lordosis and increased disc space height, regardless of where the cage is placed or the features of the cage that are utilized. During LLIF, anterior cage placement improves the lordosis angle greater than posterior placement, while posterior cage placement provides greater restoration in posterior disc space height. Cage placement should be guided by the primary purpose of surgery and whether the patient will benefit more from lordotic correction or indirect decompression.
Disclosures
Dr. Kurd reported personal fees from Stryker Spine, Durastat LLC, Spinal Elements, and Camber Spine outside the submitted work. Dr. Rihn reported personal fees from Globus Medical and stock ownership in Xtant Medical outside the submitted work. Dr. Hilibrand reported receiving royalties for intellectual property from Zimmer Biomet and CTL Amedica outside the submitted work. Dr. Vaccaro reported personal fees from Aesculap, Globus, Stryker Spine, and Medtronic; being an independent contractor for AO Spine; being a board member for the National Spine Health Foundation; and having stock options with Advanced Spinal Intellectual Properties, Atlas Spine, Avaz Surgical, Bonovo Orthopaedics, Computational Biodynamics, Cytonics, Deep Health, Dimension Orthotics LLC, Electrocore, Flagship Surgical, FlowPharma, Jushi, Innovative Surgical Design, NuVasive, Orthobullets, Paradigm Spine, Parvizi Surgical Innovation, Progressive Spinal Technologies, Replication Medica, Sentryx, Spine Medica, Spineology, Spine Wave, Stout Medical, Vertiflex, ViewFi Health, AVKN Patient Driven Care, Accellus, and Harvard Medtech outside the submitted work. Dr. Schroeder reported personal fees from consults for Zimmer, Stryker, Medtronic, Teledoc, Astura, ISD, Bioventus, NuVasive, Camber, and Surgalign; and being Editor in Chief of Clinical Spine Surgery outside the submitted work.
Author Contributions
Conception and design: Lee, Issa, Lambrechts, Tran, Kurd, Woods, Canseco, Vaccaro, Kepler, Schroeder. Acquisition of data: Issa, Tran, Trenchfield, Baker, Fras, Yalla, Kurd. Analysis and interpretation of data: Issa, Tran, Fras, Yalla, Woods, Canseco, Hilibrand, Kepler. Drafting the article: Issa, Lambrechts, Tran, Fras, Yalla, Canseco, Hilibrand. Critically revising the article: Lee, Issa, Tran, Kurd, Woods, Rihn, Canseco, Hilibrand, Vaccaro, Kepler, Schroeder. Reviewed submitted version of manuscript: Lee, Issa, Lambrechts, Tran, Trenchfield, Fras, Yalla, Kurd, Canseco, Hilibrand, Vaccaro, Kepler, Schroeder. Approved the final version of the manuscript on behalf of all authors: Lee. Statistical analysis: Issa, Tran. Administrative/technical/material support: Tran, Vaccaro, Schroeder. Study supervision: Lee, Lambrechts, Tran, Kurd, Canseco, Vaccaro, Kepler.
References
- 1↑
Rothrock RJ, McNeill IT, Yaeger K, Oermann EK, Cho SK, Caridi JM. Lumbar lordosis correction with interbody fusion: systematic literature review and analysis. World Neurosurg. 2018;118:21–31.
- 2↑
Liu J, Duan P, Mummaneni PV, et al. Does transforaminal lumbar interbody fusion induce lordosis or kyphosis? Radiographic evaluation with a minimum 2-year follow-up. J Neurosurg Spine. 2021;35(4):419–426.
- 3↑
Jagannathan J, Sansur CA, Oskouian RJ Jr, Fu KM, Shaffrey CI. Radiographic restoration of lumbar alignment after transforaminal lumbar interbody fusion. Neurosurgery. 2009;64(5):955–964.
- 4↑
Kono Y, Gen H, Sakuma Y, Koshika Y. Comparison of clinical and radiologic results of mini-open transforaminal lumbar interbody fusion and extreme lateral interbody fusion indirect decompression for degenerative lumbar spondylolisthesis. Asian Spine J. 2018;12(2):356–364.
- 5↑
Hsieh PC, Koski TR, O’Shaughnessy BA, et al. Anterior lumbar interbody fusion in comparison with transforaminal lumbar interbody fusion: implications for the restoration of foraminal height, local disc angle, lumbar lordosis, and sagittal balance. J Neurosurg Spine. 2007;7(4):379–386.
- 6↑
Gelfand Y, Benton J, De la Garza-Ramos R, Yanamadala V, Yassari R, Kinon MD. Effect of cage type on short-term radiographic outcomes in transforaminal lumbar interbody fusion. World Neurosurg. 2020;141:e953–e958.
- 7↑
Ebata S, Ohba T, Haro H. Adequate cage placement for a satisfactory outcome after lumbar lateral interbody fusion with MRI and CT analysis. Spine Surg Relat Res. 2018;2(1):53–59.
- 8↑
Billinghurst J, Akbarnia BA. Extreme lateral interbody fusion—XLIF. Curr Orthop Pract. 2009;20(3):238–251.
- 9↑
Sembrano JN, Tohmeh A, Isaacs R. Two-year comparative outcomes of MIS lateral and MIS transforaminal interbody fusion in the treatment of degenerative spondylolisthesis: part I: clinical findings. Spine (Phila Pa 1976). 2016;41(suppl 8):S123-S132.
- 10
Park Y, Ha JW. Comparison of one-level posterior lumbar interbody fusion performed with a minimally invasive approach or a traditional open approach. Spine (Phila Pa 1976). 2007;32(5):537–543.
- 11↑
Acosta FL, Liu J, Slimack N, Moller D, Fessler R, Koski T. Changes in coronal and sagittal plane alignment following minimally invasive direct lateral interbody fusion for the treatment of degenerative lumbar disease in adults: a radiographic study. J Neurosurg Spine. 2011;15(1):92–96.
- 12↑
Park SJ, Lee CS, Chung SS, Kang SS, Park HJ, Kim SH. The ideal cage position for achieving both indirect neural decompression and segmental angle restoration in lateral lumbar interbody fusion (LLIF). Clin Spine Surg. 2017;30(6):E784–E790.
- 13↑
Kepler CK, Rihn JA, Radcliff KE, et al. Restoration of lordosis and disk height after single-level transforaminal lumbar interbody fusion. Orthop Surg. 2012;4(1):15–20.
- 14↑
Alimi M, Lang G, Navarro-Ramirez R, et al. The impact of cage dimensions, positioning, and side of approach in extreme lateral interbody fusion. Clin Spine Surg. 2018;31(1):E42–E49.
- 15
Gambhir S, Wang T, Pelletier MH, Walsh WR, Ball JR. How does cage lordosis influence postoperative segmental lordosis in lumbar interbody fusion. World Neurosurg. 2019;126:e606–e611.
- 16↑
Bakare AA, Fessler DR, Wewel JT, Fontes RBV, Fessler RG, O’Toole JE. Changes in segmental and lumbar lordosis after lateral lumbar interbody fusion with different lordotic cage angulations. Int J Spine Surg. 2021;15(3):440–448.
- 17↑
DiMaria S, Karamian BA, Siegel N, et al. Does interbody cage lordosis and position affect radiographic outcomes after single-level transforaminal lumbar interbody fusion? Clin Spine Surg. 2022;35(9):E674–E679.
- 18↑
Landham PR, Don AS, Robertson PA. Do position and size matter? An analysis of cage and placement variables for optimum lordosis in PLIF reconstruction. Eur Spine J. 2017;26(11):2843–2850.
- 19↑
Transfeldt EE, Topp R, Mehbod AA, Winter RB. Surgical outcomes of decompression, decompression with limited fusion, and decompression with full curve fusion for degenerative scoliosis with radiculopathy. Spine (Phila Pa 1976). 2010;35(20):1872–1875.
- 20↑
Uribe JS, Myhre SL, Youssef JA. Preservation or restoration of segmental and regional spinal lordosis using minimally invasive interbody fusion techniques in degenerative lumbar conditions: a literature review. Spine (Phila Pa 1976). 2016;41(suppl 8):S50-S58.
- 21↑
Isaacs RE, Sembrano JN, Tohmeh AG. Two-year comparative outcomes of MIS lateral and MIS transforaminal interbody fusion in the treatment of degenerative spondylolisthesis: part II: radiographic findings. Spine (Phila Pa 1976).2016;41(suppl 8):S133-S144.
- 22↑
Watkins RGI IV, Hanna R, Chang D, Watkins RGI III. Sagittal alignment after lumbar interbody fusion: comparing anterior, lateral, and transforaminal approaches. J Spinal Disord Tech. 2014;27(5):253–256.
- 23↑
Ding Q, Tang X, Zhang R, Wu H, Liu C. Do radiographic results of transforaminal lumbar interbody fusion vary with cage position in patients with degenerative lumbar diseases? Orthop Surg. 2022;14(4):730–741.
- 24↑
Malham GM, Parker RM, Goss B, Blecher CM. Clinical results and limitations of indirect decompression in spinal stenosis with laterally implanted interbody cages: results from a prospective cohort study. Eur Spine J. 2015;24(3 suppl 3):339–345.
- 25↑
Malham GM, Parker RM, Goss B, Blecher CM, Ballok ZE. Indirect foraminal decompression is independent of metabolically active facet arthropathy in extreme lateral interbody fusion. Spine (Phila Pa 1976). 2014;39(22):E1303–E1310.
- 26↑
Tye EY, Alentado VJ, Mroz TE, Orr RD, Steinmetz MP. Comparison of clinical and radiographic outcomes in patients receiving single-level transforaminal lumbar interbody fusion with removal of unilateral or bilateral facet joints. Spine (Phila Pa 1976). 2016;41(17):E1039–E1045.
- 27↑
Dangelmajer S, Zadnik PL, Rodriguez ST, Gokaslan ZL, Sciubba DM. Minimally invasive spine surgery for adult degenerative lumbar scoliosis. Neurosurg Focus. 2014;36(5):E7.
- 28
Qiao J, Xiao L, Sun X, et al. Vertebral subluxation during three-column osteotomy in surgical correction of adult spine deformity: incidence, risk factors, and complications. Eur Spine J. 2018;27(3):630–635.
- 29
Eskilsson K, Sharma D, Johansson C, Hedlund R. Pedicle subtraction osteotomy: a comprehensive analysis in 104 patients. Does the cause of deformity influence the outcome? J Neurosurg Spine. 2017;27(1):56–62.
- 30
Ohba T, Ebata S, Ikegami S, Oba H, Haro H. Indications and limitations of minimally invasive lateral lumbar interbody fusion without osteotomy for adult spinal deformity. Eur Spine J. 2020;29(6):1362–1370.
- 31
Kim YJ, Bridwell KH, Lenke LG, Cheh G, Baldus C. Results of lumbar pedicle subtraction osteotomies for fixed sagittal imbalance: a minimum 5-year follow-up study. Spine (Phila Pa 1976). 2007;32(20):2189–2197.
- 32↑
Yang BP, Ondra SL, Chen LA, Jung HS, Koski TR, Salehi SA. Clinical and radiographic outcomes of thoracic and lumbar pedicle subtraction osteotomy for fixed sagittal imbalance. J Neurosurg Spine. 2006;5(1):9–17.
- 33↑
Sembrano JN, Horazdovsky RD, Sharma AK, Yson SC, Santos ERG, Polly DWJ Jr. Do lordotic cages provide better segmental lordosis versus nonlordotic cages in lateral lumbar interbody fusion (LLIF)? Clin Spine Surg. 2017;30(4):E338–E343.
