Long-term radiographic outcomes of expandable versus static cages in transforaminal lumbar interbody fusion

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  • 1 Departments of Neurosurgery and
  • | 2 Orthopaedic Surgery, University of California, San Francisco, California;
  • | 3 Department of Neurosurgery, Neurological Institute, Taipei Veterans General Hospital, Taipei, Taiwan;
  • | 4 University of California, San Francisco, California; and
  • | 5 School of Medicine and
  • | 6 Department of Biomedical Engineering, National Yang-Ming University, Taipei, Taiwan
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OBJECTIVE

Potential advantages of using expandable versus static cages during transforaminal lumbar interbody fusion (TLIF) are not fully established. The authors aimed to compare the long-term radiographic outcomes of expandable versus static TLIF cages.

METHODS

A retrospective review of 1- and 2-level TLIFs over a 10-year period with expandable and static cages was performed at the University of California, San Francisco. Patients with posterior column osteotomy (PCO) were subdivided. Fusion assessment, cage subsidence, anterior and posterior disc height, foraminal dimensions, pelvic incidence (PI), segmental lordosis (SL), lumbar lordosis (LL), pelvic incidence–lumbar lordosis mismatch (PI-LL), pelvic tilt (PT), sacral slope (SS), and sagittal vertical axis (SVA) were assessed.

RESULTS

A consecutive series of 178 patients (with a total of 210 levels) who underwent TLIF using either static (148 levels) or expandable cages (62 levels) was reviewed. The mean patient age was 60.3 ± 11.5 years and 62.8 ± 14.1 years for the static and expandable cage groups, respectively. The mean follow-up was 42.9 ± 29.4 months for the static cage group and 27.6 ± 14.1 months for the expandable cage group. Within the 1-level TLIF group, the SL and PI-LL improved with statistical significance regardless of whether PCO was performed; however, the static group with PCOs also had statistically significant improvement in LL and SVA. The expandable cage with PCO subgroup had significant improvement in SL only. All of the foraminal parameters improved with statistical significance, regardless of the type of cages used; however, the expandable cage group had greater improvement in disc height restoration. The incidence of cage subsidence was higher in the expandable group (19.7% vs 5.4%, p = 0.0017). Within the expandable group, the unilateral facetectomy-only subgroup had a 5.6 times higher subsidence rate than the PCO subgroup (26.8% vs 4.8%, p = 0.04). Four expandable cages collapsed over time.

CONCLUSIONS

Expandable TLIF cages may initially restore disc height better than static cages, but they also have higher rates of subsidence. Unilateral facetectomy alone may result in more subsidence with expandable cages than using bilateral PCO, potentially because of insufficient facet release. Although expandable cages may have more power to induce lordosis and restore disc height than static cages, subsidence and endplate violation may negate any significant gains compared to static cages.

ABBREVIATIONS

ADH = anterior disc height; FA = foraminal area; FH = foraminal height; LL = lumbar lordosis; MIS = minimally invasive surgery; PCO = posterior column osteotomy; PDH = posterior disc height; PI = pelvic incidence; PI-LL = pelvic incidence–lumbar lordosis mismatch; PT = pelvic tilt; SL = segmental lordosis; SS = sacral slope; SVA = sagittal vertical axis; TLIF = transforaminal lumbar interbody fusion.

OBJECTIVE

Potential advantages of using expandable versus static cages during transforaminal lumbar interbody fusion (TLIF) are not fully established. The authors aimed to compare the long-term radiographic outcomes of expandable versus static TLIF cages.

METHODS

A retrospective review of 1- and 2-level TLIFs over a 10-year period with expandable and static cages was performed at the University of California, San Francisco. Patients with posterior column osteotomy (PCO) were subdivided. Fusion assessment, cage subsidence, anterior and posterior disc height, foraminal dimensions, pelvic incidence (PI), segmental lordosis (SL), lumbar lordosis (LL), pelvic incidence–lumbar lordosis mismatch (PI-LL), pelvic tilt (PT), sacral slope (SS), and sagittal vertical axis (SVA) were assessed.

RESULTS

A consecutive series of 178 patients (with a total of 210 levels) who underwent TLIF using either static (148 levels) or expandable cages (62 levels) was reviewed. The mean patient age was 60.3 ± 11.5 years and 62.8 ± 14.1 years for the static and expandable cage groups, respectively. The mean follow-up was 42.9 ± 29.4 months for the static cage group and 27.6 ± 14.1 months for the expandable cage group. Within the 1-level TLIF group, the SL and PI-LL improved with statistical significance regardless of whether PCO was performed; however, the static group with PCOs also had statistically significant improvement in LL and SVA. The expandable cage with PCO subgroup had significant improvement in SL only. All of the foraminal parameters improved with statistical significance, regardless of the type of cages used; however, the expandable cage group had greater improvement in disc height restoration. The incidence of cage subsidence was higher in the expandable group (19.7% vs 5.4%, p = 0.0017). Within the expandable group, the unilateral facetectomy-only subgroup had a 5.6 times higher subsidence rate than the PCO subgroup (26.8% vs 4.8%, p = 0.04). Four expandable cages collapsed over time.

CONCLUSIONS

Expandable TLIF cages may initially restore disc height better than static cages, but they also have higher rates of subsidence. Unilateral facetectomy alone may result in more subsidence with expandable cages than using bilateral PCO, potentially because of insufficient facet release. Although expandable cages may have more power to induce lordosis and restore disc height than static cages, subsidence and endplate violation may negate any significant gains compared to static cages.

In Brief

Transforaminal lumbar interbody fusion (TLIF) is an effective treatment for lumbar degenerative conditions. Although an expandable TLIF cage could theoretically affect foraminal height and lumbar lordosis, the outcomes on spinopelvic parameters have not been well addressed. The objective of this study is to compare the radiographic outcomes of patients who underwent a TLIF with expandable versus static cages. The results demonstrated that subsidence and endplate violation may negate any significant gains compared to static cages and provided long-term results of expandable cages, and identified factors that influence the surgical result when expandable cages were used.

Degenerative lumbar pathologies are common among the aging population and can often lead to pain, disability, and poor quality of life. Loss of lumbar lordosis (LL) is a common feature in the degenerative lumbar spine, and there is a growing body of evidence that restoration of sagittal alignment is associated with better clinical outcomes.1–7 Surgical goals should not be limited to only decompression and stabilization of the spine, but should also include optimization of the sagittal alignment whenever possible. Transforaminal lumbar interbody fusion (TLIF) has proven to be a safe and effective treatment for symptomatic lumbar degenerative disease.2–4,8–11 Although the clinical outcomes of TLIF are generally favorable in well-selected patients, there is still controversy regarding its ability to restore sagittal alignment.2–4,8,10,12–16 An important limiting factor for lordosis restoration via TLIF is the limited, relatively small surgical corridor to access the disc space posteriorly. Inserting an excessively large cage via a TLIF approach to correct sagittal alignment may cause injury to nerve roots or result in violation of the endplates. Thus, many spine surgeons consider TLIF as a “kyphogenic” procedure due to the limited ability of static TLIF cages to induce lordosis.

To address this issue, expandable TLIF cages have been developed and launched with the hope of overcoming some of the aforementioned limitations. Expandable cages are designed to be inserted in a low-profile, nonexpanded state into the disc space, and then expanded to a desired height. The theoretical advantage of an expandable cage is the improved ability to restore disc height and segmental lordosis (SL) despite the small corridor through Kambin’s triangle (especially in minimally invasive cases) without injuring the exiting or traversing nerve roots. However, it is not entirely clear how durable or effective the cage expansion results in disc space lordosis and distraction. We aimed to compare the radiographic outcomes of expandable and static TLIF cages.

Methods

Study Design and Patient Inclusion

A cohort of patients with symptomatic lumbar degenerative disc pathology who underwent TLIF surgery at the University of California, San Francisco, from 2007 to 2018 were retrospectively studied. Inclusion criteria were patients who underwent 1- or 2-level TLIF for lumbar degenerative pathologies (prior decompressions were included) and had a minimum follow-up period of 12 months. Patients were excluded if they had prior spinal fusion, infection, or tumor. The study was approved by the IRB.

Patients were then divided into the expandable or static cage groups. The two groups were further stratified based on whether a posterior column osteotomy (PCO) was performed at the TLIF level. The 4 subgroups were: 1) static cage with PCO, 2) static cage without PCO, 3) expandable cage with PCO, and 4) expandable cage without PCO. The choice of the surgical procedure and cage type was based on the clinical assessment by the treating spine surgeon and the patient’s pathology.

Clinical and Radiographic Variables

Patient data were collected through the electronic medical records. Demographic variables included age, sex, diagnosis, and minimally invasive surgery (MIS) versus open surgical treatment. Surgical variables included type of surgery, cage used, osteotomies performed, unilateral versus bilateral facetectomies, and graft material. Pre- and postoperative variables included pelvic incidence (PI), LL, SL, PI and LL (PI-LL) mismatch, sacral slope (SS), pelvic tilt (PT), sagittal vertical axis (SVA), anterior disc height (ADH), posterior disc height (PDH), and foraminal height (FH) and foraminal area (FA) on the non-TLIF side. Spinopelvic parameters were measured on 36-inch long-cassette lateral radiographs (Fig. 1), and disc and foraminal parameters were measured on either lateral radiographs or CT sagittal reconstructions (Fig. 2). Radiographic variables were measured by an attending spinal neurosurgeon (C.-C.C.), and every parameter was measured twice at different time points to increase intrarater reliability.

FIG. 1.
FIG. 1.

Demonstration of sagittal parameter measurements on a 36-inch long-cassette lateral radiograph.

FIG. 2.
FIG. 2.

Demonstration of disc and foraminal parameter measurements on preoperative (A) and postoperative (B) lateral radiographs of a patient with L4–5 spondylolisthesis and 8.2-mm anterior displacement. Demonstration of FA measurement on a sagittal CT image (C). Figure is available in color online only.

Fusion Assessment

Fusion was assessed via an analysis of the latest follow-up lumbar flexion/extension lateral radiographs and CT images. Fusion was defined as the presence of continuous trabecular bone formation through or outside the cages and less than 3° movement on lateral flexion and extension radiographs.12,17–19 Endplate violation and cage subsidence were evaluated by comparing intraoperative fluoroscopic images after cage insertion with lateral radiographs at the latest follow-up. If cage subsidence was seen immediately after cage placement (either intraoperatively or on the immediate postoperative radiographs), it was classified as an endplate violation. If the endplate was intact intraoperatively and at the immediate postoperative lateral radiograph, and subsidence was observed during the follow-up period, it was considered cage subsidence. Subsidence was defined as the cage subsiding into the endplate more than 3 mm.

Statistical Analysis

Descriptive statistics were reported as means with standard deviations and as frequencies and percentages where appropriate. Continuous variables were compared using an unpaired Student t-test and a paired Student t-test. Categorical variables were compared using Pearson’s chi-square test. One-way ANOVA was used for comparison among multiple continuous variables. Two-tailed probability values with an alpha of 0.05 were considered statistically significant.

Results

Demographics

A total of 178 patients (with a total of 210 levels) who met the inclusion criteria were included. There were 148 levels in the static cage group and 62 levels in the expandable cage group (Table 1). The mean age was 60.3 ± 11.5 years and 62.8 ± 14.1 years for the static and expandable groups, respectively (p = 0.18). The mean follow-up was 40.1 ± 27.9 months. There were 51 male patients (34.5%) in the static cage group compared to 31 male patients (50%) in the expandable cage group. In the static cage group, the pathology was isthmic spondylolisthesis in 14 patients (9.5%), recurrent stenosis with spondylosis in 16 patients (10.8%), degenerative spondylolisthesis with stenosis in 29 patients (19.6%), and severe spondylosis with intractable back pain and/or leg pain in 89 patients (60.1%). In the expandable cage group, the pathology was isthmic spondylolisthesis in 6 patients (9.7%), recurrent stenosis with spondylosis in 1 patient (1.6%), degenerative spondylolisthesis with stenosis in 46 patients (74.2%), and severe spondylosis with intractable back pain and/or leg pain in 9 patients (14.5%).

TABLE 1.

Demographic and surgical information in 178 patients (210 levels)

Cage Type
VariableStatic (n = 148)Expandable (n = 62)p Value
Mean age ± SD, yrs60.3 ± 11.562.8 ± 14.10.18
Males51 (34.5)31 (50)0.03
Mean follow-up ± SD, mos42.9 ± 29.427.6 ± 14.10.0001
Pathology<0.0001
 Isthmic spondylolisthesis14 (9.5)6 (9.7)
 Recurrent stenosis16 (10.8)1 (1.6)
 Degenerative spondylolisthesis w/ stenosis29 (19.6)46 (74.2)
 Severe spondylosis w/ intractable low-back pain or leg pain89 (60.1)9 (14.5)
Level0.96
 L2–32 (1.4)1 (1.6)
 L3–420 (13.5)8 (12.9)
 L4–593 (62.8)41 (66.1)
 L5–S133 (22.3)12 (19.4)
Previous decompressive surgery23 (15.5)5 (8.1)0.15
Surgical type<0.0001
 MIS/mini open65 (43.9)4 (6.5)
 Open83 (56.1)58 (93.5)
PCO0.13
 w/ PCO35 (23.6)21 (33.9)
 w/o PCO113 (76.4)41 (66.1)
Complications
 Screw malposition20
 CSF leak01
Endplate violation9 (6.1)11 (17.7)0.009
Cage subsidence8 (5.4)12 (19.4)0.0017
Expandable cage height loss4
Fusion rate90.5%88.7%0.68
Revision
 CSF leak repair01
 Screw revision20
 Pseudarthrosis122
 Adjacent-segment disease102
 Expandable cage height loss w/ pseudarthrosis01

Data given as number (%) unless otherwise indicated. Boldface type indicates statistical significance.

The distribution of surgical levels was as follows. The static cage group had 2 patients (1.4%) with surgery at L2–3, 20 (13.5%) at L3–4, 93 (62.8%) at L4–5, and 33 (22.3%) at L5–S1; the expandable cage group had 1 (1.6%) at L2–3, 8 (12.9%) at L3–4, 41 (66.1%) at L4–5, and 12 (19.4%) at L5–S1. There was no statistically significant difference in the distribution of surgical levels between groups (p = 0.96). Sixty-five patients (43.9%) in the static cage group and 4 patients (6.5%) in the expandable cage group had MIS or mini-open surgery (p < 0.0001). There were 35 patients (23.6%) who had PCOs in the static cage group compared to 21 patients (33.9%) in the expandable cage group (p = 0.13). The endplate violation rate was 6.1% in the static cage group versus 17.7% in the expandable cage group (p = 0.009). The cage subsidence rate was significantly higher in the expandable cage group (19.7% vs 5.4%, p = 0.0017). The fusion rate was similar in both groups (90.5% vs 88.7%, p = 0.68). There were 4 patients who experienced cage height loss secondary to cage collapse, but only 1 of them was symptomatic and required revision surgery (Table 1).

Cage Specifications

All static cages had a 5° lordotic angle with an average cage height of 9.68 mm. The expandable cages were more lordotic, and most had a larger cage height compared to the static cages. All 62 expandable cages had lordotic angles greater than 10°. In addition, there were 30 cages with lordotic angles larger than 12°. The average cage height in the expandable cage group was 16 mm.

Spinopelvic Parameters After TLIF

Tables 2 and 3 demonstrate the change in spinopelvic parameters after 1- and 2-level TLIFs. In the 1-level TLIF group, using a static cage with a PCO yielded a statistically significant improvement in LL (56° ± 14.4° to 57.4° ± 12.3°, p = 0.016), SL (4.1° ± 4.4° to 6.2° ± 4.7°, p = 0.03), PI-LL (7.8° ± 13.7° to 5.2° ± 11.4°, p = 0.03), and SVA (5.8 ± 4.1 cm to 2.5 ± 2.9 cm, p = 0.01). In addition, static cages without a PCO had statistically significant improvement in SL (4.9° ± 3.2° to 6.2° ± 4.4°, p = 0.008) and PI-LL (5.6° ± 13.6° to 4.3° ± 12.1°, p = 0.05). The SL had statistically significantly improvement in the expandable cage with PCO group (4.6° ± 3.5° to 6.6° ± 4.2°, p = 0.03). There was no statistically significant difference in any parameter after 1-level TLIF using an expandable cage without a PCO. Most parameters showed nonsignificant improvement in the 2-level TLIF group except for the SVA in the static cage with PCO group.

TABLE 2.

Comparison of pre- and postoperative sagittal parameters: 1-level TLIF

ParameterStatic w/ PCO (n = 22)Static w/o PCO (n = 86)p ValueExpandable w/ PCO (n = 16)Expandable w/o PCO (n = 22)p Value
PI (°)
 Preop62.1 ± 12.554.8 ± 12.50.0257.1 ± 13.156.5 ± 9.30.87
LL (°)
 Preop56 ± 14.449.7 ± 13.10.0750.6 ± 9.747.5 ± 18.70.54
 Postop57.4 ± 12.351.3 ± 12.10.0453.5 ± 9.550.7 ± 18.60.58
 p value0.0160.060.060.11
SL (°)
 Preop4.1 ± 4.44.9 ± 4.20.424.6 ± 3.54.1 ± 6.10.74
 Postop6.2 ± 4.76.2 ± 4.40.996.6 ± 4.26.3 ± 3.40.82
 p value0.030.0080.030.07
PI-LL (°)
 Preop7.8 ± 13.75.6 ± 13.60.566.4 ± 10.39.0 ± 19.70.64
 Postop5.2 ± 11.44.3 ± 12.10.793.6 ± 9.75.8 ± 19.20.67
 p value0.030.050.060.11
SVA (cm)
 Preop5.8 ± 4.13.1 ± 2.90.034.5 ± 3.44.1 ± 2.80.78
 Postop2.5 ± 2.94.2 ± 3.30.074.3 ± 3.63.9 ± 3.50.89
 p value0.010.170.640.81
SS (°)
 Preop41.9 ± 9.435.8 ± 8.30.0238.4 ± 10.436.2 ± 8.90.49
 Postop39.6 ± 8.536.3 ± 8.20.1237.4 ± 8.937.3 ± 9.10.98
 p value0.910.260.430.42
PT (°)
 Preop23.5 ± 9.718.2 ± 13.30.1818.7 ± 9.620.3 ± 8.70.59
 Postop23.1 ± 8.819.1 ± 10.20.1319.7 ± 8.119.1 ± 9.20.85
 p value0.910.30.430.42

Postop = the latest follow-up.

Boldface type indicates statistical significance.

TABLE 3.

Comparison of pre- and postoperative sagittal parameters: 2-level TLIF

ParameterStatic w/ PCO (n = 13)Static w/o PCO (n = 27)p ValueExpandable w/ PCO (n = 5)Expandable w/o PCO (n = 19)p Value
PI (°)
 Preop56.9 ± 14.659.2 ± 9.80.6957 ± 7.861.2 ± 15.40.66
LL (°)
 Preop38.9 ± 10.749.8 ± 11.80.0651.7 ± 19.454.6 ± 12.70.77
 Postop44.4 ± 14.449.5 ± 10.60.3947 ± 12.255.5 ± 12.30.33
 p value0.080.350.40.75
SL (°)
 Preop4.3 ± 4.64.8 ± 2.80.77.2 ± 1.84.6 ± 6.10.35
 Postop6.5 ± 2.95.5 ± 3.90.437.4 ± 2.45.1 ± 3.10.14
 p value0.140.430.920.81
PI-LL (°)
 Preop18.0 ± 9.19.4 ± 12.40.125.3 ± 11.78.5 ± 12.20.71
 Postop12.5 ± 14.59.7 ± 13.10.6710 ± 4.45.8 ± 11.70.57
 p value0.090.810.40.75
SVA (cm)
 Preop5.8 ± 3.73.3 ± 3.10.282.4 ± 2.73.6 ± 3.40.63
 Postop3.5 ± 3.53.7 ± 3.80.943.6 ± 3.24.1 ± 2.90.81
 p value0.020.560.580.63
SS (°)
 Preop35.6 ± 9.235.9 ± 10.10.9640.3 ± 9.742.8 ± 11.30.75
 Postop36.9 ± 11.938.00 ± 9.90.8337.3 ± 8.141.3 ± 11.30.59
 p value0.590.220.091.0
PT (°)
 Preop21.3 ± 11.123.9 ± 9.10.6316.7 ± 3.820.4 ± 8.80.51
 Postop20.0 ± 12.221.2 ± 10.80.8319.9 ± 7.819.7 ± 3.50.96
 p value0.590.220.091.0

Boldface type indicates statistical significance.

Foraminal Measurements

Table 4 demonstrates the foraminal changes after TLIF. The use of a static cage with PCO improved ADH (7.8 ± 3.1 to 9.7 ± 2.3 mm, p = 0.002), PDH (4.7 ± 1.9 to 6.1 ± 2.1 mm, p = 0.006), and FH (15.3 ± 3.8 to 16.6 ± 3.5 mm, p = 0.0009). In addition, the static cage without PCO also improved ADH (8.2 ± 3.2 to 9.6 ± 2.5 mm, p < 0.0001), PDH (4.6 ± 1.7 to 5.9 ± 1.9 mm, p < 0.0001), FH (15.8 ± 3.4 to 17.2 ± 3.2 mm, p < 0.0001), and FA (98.4 ± 25.5 to 120.7 ± 27.2 mm2, p < 0.0001). The use of expandable cages with PCO also increased ADH (9.1 ± 3.1 to 12.1 ± 2.3 mm, p = 0.0006), PDH (5.2 ± 1.7 to 7.4 ± 2.2 mm, p = 0.0003), and FH (16.5 ± 3.5 to 18.4 ± 2.9 mm, p = 0.005). The use of an expandable cage without PCO also improved ADH (7.9 ± 3.4 to 10.4 ± 2.6 mm, p = 0.0001), PDH (4.8 ± 2.3 to 6.8 ± 2.2 mm, p < 0.0001), FH (15.5 ± 3.5 to 16.6 ± 3.4 mm, p = 0.009), and FA (100.8 ± 32.9 to 127.7 ± 38.9 mm2, p < 0.0001).

TABLE 4.

Comparison of pre- and postoperative foraminal parameters

ParameterStatic w/ PCOStatic w/o PCOp ValueExpandable w/ PCOExpandable w/o PCOp Value
ADH (mm)
 Preop7.8 ± 3.18.2 ± 3.20.639.1 ± 3.17.9 ± 3.40.34
 Postop9.7 ± 2.39.6 ± 2.50.8212.1 ± 2.310.4 ± 2.60.05
 p value0.002<0.00010.00060.0001
PDH (mm)
 Preop4.7 ± 1.94.6 ± 1.70.655.2 ± 1.74.8 ± 2.30.51
 Postop6.1 ± 2.15.9 ± 1.90.487.4 ± 2.26.8 ± 2.20.49
 p value0.006<0.00010.0003<0.0001
FH (mm)
 Preop15.3 ± 3.815.8 ± 3.40.4616.5 ± 3.515.5 ± 3.50.63
 Postop16.6 ± 3.517.2 ± 3.20.6218.4 ± 2.916.6 ± 3.40.06
 p value0.0009<0.00010.0050.009
FA (mm2)
 PreopNA98.4 ± 25.5NA100.8 ± 32.9
 PostopNA120.7 ± 27.2NA127.7 ± 38.9
 p valueNA<0.0001NA<0.0001

NA = the foraminal area cannot be measured after PCO.

Boldface type indicates statistical significance.

Surgical Characteristics and Outcomes

The overall fusion rate was similar in both groups (90.5% vs 88.7%, p = 0.68; Table 5). Comparing fusion rates in patients with and without cage subsidence, the fusion rates in the static cage group were 75.0% and 91.4%, respectively, and 75.0% and 92.0% in the expandable cage group, respectively. The expandable cage group had a significantly higher subsidence rate than the static group (19.7% vs 5.4%, p = 0.001). In the static cage group, the subsidence rate was significantly higher in patients who had unilateral facetectomy than in those who had PCO (5.3% vs 2.9%, p = 0.049). In the expandable cage group, the subsidence rate was even higher with unilateral facetectomy than with PCO (26.8% vs 4.8%, p = 0.04).

TABLE 5.

Comparison of fusion and cage subsidence status

VariableStatic Cagep ValueExpandable Cagep Value
Fusion assessment
 Fusion rate90.5%88.7%0.68
 Fusion rate in case
  w/ cage subsidence75%0.123*75%0.09*
  w/o cage subsidence91.4%92%
Subsidence assessment
 Subsidence rate5.4%19.7%0.001
 Subsidence rate in case
  w/ unilateral facetectomy5.3%0.04926.8%0.04
  w/ PCO2.9%4.8%

Boldface type indicates statistical significance.

Although the fusion rate is lower in patients who have cage subsidence, the sample size is small, which may be underpowered to make a conclusion.

Radiographic Follow-Up

Follow-up of foraminal measurements in the expandable cage group with subsidence demonstrated gradual disc and foraminal height loss over time. At the 24-month follow-up, the ADH, PDH, and FH were 64%, 56%, and 76% of their original values. Among the 12 patients who had cage subsidence in the expandable cage group, there were 3 patients who developed symptomatic pseudarthrosis, which required revision surgery. There were 4 patients who had their expandable cages lose height secondary to loss of the expansion. One case had cage height loss at 2 months after surgery, but the posterior fixation remained intact and the patient was asymptomatic. Another case had pseudarthrosis, implant failure, and underwent revision with an anterior approach (Fig. 3). The 2 other cases had radiographic cage height loss, but the patients remained asymptomatic.

FIG. 3.
FIG. 3.

Lateral radiographs 1 month after TLIF with an expandable cage (A), 12 months after TLIF with an expandable cage demonstrating cage collapse/loss of height of the cage (B), 40 months after TLIF demonstrating pseudarthrosis and screw fracture (arrow; C), and after revision surgery with anterior lumbar interbody fusion as well as posterior screw-rod replacement, demonstrating solid fusion (D).

Discussion

Restoration of sagittal alignment has been shown to be associated with improved clinical outcomes.5,7 However, restoring LL using static cages via TLIF can be challenging and has not been consistently demonstrated in the literature. The expandable TLIF cage has been developed in an attempt to induce more lordosis, improve disc and foraminal height, and afford a larger interbody implant through the relatively small corridor of Kambin’s triangle. However, long-term clinical and radiographic data directly comparing static and expandable TLIF cages are sparse.

Although many biomechanical studies have demonstrated the ability of static TLIF cages to create lordosis, the clinical efficacy of such an effect is equivocal in the published literature.20–22 Although some studies have shown improvement in radiographic parameters with TLIF,2,3,9,14 others showed no improvement.4,8,10,16 In addition, there are only a few clinical studies specifically examining the outcomes of the expandable TLIF cages. Yee et al.9 retrospectively studied 89 patients (48 with nonexpendable cages and 41 with expandable cages) with degenerative lumbar pathology who underwent TLIF, and found that the use of expandable cages did not result in improved correction of SL and LL compared to the use of static cages. Another study by Hawasli el at.10 reviewed 48 patients who underwent MIS TLIF with either an expandable or static cage. They found that expandable cages increased SL but had no effect on overall LL. They also reported that using expandable cages produced greater restoration of disc height and FH compared to static cages. Massie et al.15 reported a series of 44 patients who underwent MIS TLIF using a crescent-shaped expandable cage and found that expandable cages provided a significant restoration of segmental height and lordosis.

In our study, TLIFs with static cages and a PCO demonstrated greater improvement of LL than TLIFs without a PCO. Interestingly, expandable-cage TLIFs with PCOs did not show better radiographic correction compared to static cage TLIFs with PCOs. In both groups, there was an increase of approximately 2° of SL. This was unexpected, because the expandable cages had heights of up to 17 mm and 15° of lordosis compared to their static counterparts with 10 mm of height and 5° of lordosis. There may be many reasons for these findings. First, the expandable cages had higher rates of subsidence. Thus, although the cages may have been taller and more lordotic, the subsidence may have negated any efficacy these cages would have had on radiographic parameters. Second, because there was no difference in TLIFs with PCOs between groups, the majority of lordosis induction in this subset may have been created via compression over the PCO, not the shape or size of the TLIF cage. Third, with PCOs, larger cages could be inserted, and such larger cages could have resulted in less mobility of the spine, especially when expandable cages were expanded to their maximal dimension. The bigger cage may have resulted in less mobility of the disc space because of maximal distraction of the space. This lack of mobility is especially true when longer (not just taller) cages are placed, leading to decreased angular correction and less ability to compress the posterior disc space. Thus, for these reasons, we did not observe as much angular correction with PCOs in expandable cage TLIFs as might have been predicted given the inherent lordosis of the cages.

The expandable cage group demonstrated superior disc height restoration compared to the static cage group, although height and foraminal parameters improved in both groups. Even though compression is applied over the PCO, we saw an increase in FH after PCO, not a decrease. This may be because of the fairly tall cages used (average of 9.6 mm in the static group and 12 mm in the expandable group), and there may have been more release of the spinal segment with PCOs, allowing for increased distraction and concomitant foraminal disc height. This is consistent with our finding that unilateral facetectomy TLIFs had more subsidence than PCO TLIFs; the PCOs most likely released the segment more, allowing for more distraction and more FH restoration.

Cage subsidence is a potential complication with TLIF that can lead to gradual loss of disc height and foraminal restenosis. Osteoporosis, endplate morphology, implant surface area, surgical technique, and cage position have been shown to be possible risk factors in cage subsidence.23–25 The reported subsidence rates range from 4% to 33% in reported TLIF series with static cages.2–4,8,9 In our study, the subsidence rate was 19.7% in the expandable cage group and 5.4% in the static cage group (p = 0.001). One reason for this may be that the expansion of the TLIF cage results in a stress riser, increasing the risk of imploding into the endplate. Our results suggest that the high subsidence rate may also be associated with the inability to distract the contralateral facet with unilateral facetectomy. In the static cage group, the subsidence rate is almost twice as high in patients with a unilateral facetectomy than in patients with PCO (5.3% vs 2.9%, p = 0.049). In the expandable cage group, the subsidence rate in patients with unilateral facetectomy is 5.6 times higher than that in patients with PCO (26.8% vs 4.8%, p = 0.04). This observation may reflect the decreased ability to distract the disc space with the contralateral facet still intact, which is often arthritic and potentially even partially ankylosed. In patients with unilateral facetectomy only, the contralateral facet holds the disc space together, resisting distraction. Using an expandable cage against a stiff facet not only risks direct endplate implosion during surgery, but it also results in a stress riser because the expansion mechanism of the cages may be very powerful, leading to eventual subsidence over time. Recently, a biomechanical study reported by Snyder et al. found similar results, and they concluded that complete bilateral facetectomy in TLIF had significant improvement in sagittal alignment compared to unilateral facetectomy.26 They also reported that cage subsidence also had an adverse effect on fusion. In our subgroup analysis of cage subsidence and fusion, our observed fusion rates were lower in patients who had cage subsidence in both the static group (75% vs 91.4%, p = 0.123) and the expandable group (75% vs 92%, p = 0.09; Table 5). This decreased arthrodesis rate may be the result of micromotion from the disc space collapse and possible loosening of pedicle screw fixation from the change in angulation after subsidence compared to the angulation at initial pedicle screw placement. In our study, we also observed 4 cases with expandable cage height loss from collapse. The cages were confirmed as expanded in the intraoperative radiographs, and the height loss occurred postoperatively. One such case was symptomatic and required revision surgery, and the collapsed cages were reported to the manufacturer (Fig. 3).

There are several limitations to this study. First, this is a single-institution retrospective series over a decade. Surgical techniques, implant design, and surgeon experience may have changed over time. Second, the case types are heterogeneous and open and minimally invasive approaches were not analyzed separately. Third, we did not compare the clinical outcomes because the primary focus of this study was radiographic outcome. Also, there may have been a selection bias in this study because the selection process of expandable or static cages was based on surgeon preference and assessment of patient pathology. The main factors with regard to choosing expandable versus static cages were surgeon preference and patient anatomy. For instance, a very lordotic disc space with a narrow posterior disc height and large anterior disc height may have been more amenable to an expandable cage, to put a small device in posteriorly with subsequent anterior expansion. However, this was not uniformly part of the decision algorithm, and surgeon preference played a large role in the decision-making. Without randomization, a selection bias could have been introduced. Moreover, there was no direct randomization with regard to which patients had PCOs, and this could also have led to selection bias. In addition, the shape (crescent vs straight) and position of the TLIF cage (anterior vs oblique placement) are additional variables that can influence the amount of sagittal correction obtained, which we did not control for. Lastly, we did not compare the relationship between bone density and fusion because such data were not available. Osteoporosis has been associated with implant subsidence, but this factor was not controlled for in this study. Another factor that was not accounted for in this study is the differences in disc space variables such as vacuum changes, bridging osteophytes, and extent of degeneration of the contralateral facet. Such factors could affect the radiographic correction and even subsidence when trying to expand the disc space, and these variables should be considered when interpreting the data. Another issue to consider is the difference in spinopelvic parameter changes with upper lumbar (L2–3) compared to lower lumbar (L4–S1) TLIF. The more distal the lordosis is induced, the more SVA change should be manifested. We have tried to control for this by measuring SL also, but the location of TLIF and overall sagittal balance should be considered.

Within the degenerative pathology, it can be argued that spinopelvic parameters are not as critical. Although the majority of degenerative conditions of the spine can be treated with little attention paid to spinopelvic parameters, there are instances in which fusion in the patient’s in situ position can result in flat-back syndrome. Thus, having knowledge of when to induce more lordosis if possible and when to simply decompress can be helpful even in degenerative pathologies. In addition, previous decompression makes the surgery more difficult because of scar tissue, higher risk of CSF leak, and obscured anatomy, and it could be argued that the results could have been affected by previous surgery. However, in our study, only a minority had previous decompressions (16% static, 8% expandable), and there was no statistical difference between the two groups (p = 0.l5).

Conclusions

Expandable TLIF cages have greater restoration of disc and foraminal height, but also have higher risk of cage subsidence compared to static cages. Static TLIF cages can improve spinopelvic alignment and may not lead to kyphosis postoperatively. Cage subsidence increases the risk of pseudarthrosis regardless of cage type. Bilateral facet releases may allow for increased disc height distraction and potentially decrease the rate of subsidence, especially when using expandable TLIF cages.

Acknowledgments

Chih-Chang Chang received a grant from Yen Tjing Ling Medical Foundation, a nongovernmental, nonprofit organization.

Disclosures

Dr. Chou reports being a consultant to Globus and Medtronic, and receiving royalties from Globus. Dr. Tan reports being a consultant to Medtronic and Stryker, and having ownership in Integrity Implants. Dr. Berven reports having ownership in Green Sun Medical and Providence Medical; being a consultant to Medtronic, Globus, DePuy, Innovasis, and Integrity Spine; and receiving royalties from Medtronic and Stryker. Dr. Mummaneni reports being a consultant to DePuy Synthes, Globus, and Stryker; having direct stock ownership in Spinicity/ISD; receiving support of non–study-related clinical or research effort from the NREF; receiving royalties from DePuy Synthes, Thieme, and Springer; receiving honoraria from Spineart; and receiving a grant from AO Spine.

Author Contributions

Conception and design: Chang, Chou, Tan, Berven, Mummaneni. Acquisition of data: Chang. Analysis and interpretation of data: Chang. Drafting the article: Chou, Pennicooke, Rivera, Tan, Mummaneni. Critically revising the article: Chou, Rivera, Tan, Mummaneni. Reviewed submitted version of manuscript: all authors. Approved the final version of the manuscript on behalf of all authors: Chang. Statistical analysis: Chang. Administrative/technical/material support: Chou, Tan, Berven, Mummaneni. Study supervision: Chou, Mummaneni.

References

  • 1

    Park P, Fu KM, Mummaneni PV, et al. The impact of age on surgical goals for spinopelvic alignment in minimally invasive surgery for adult spinal deformity. J Neurosurg Spine. 2018;29(5):560564.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 2

    Choi WS, Kim JS, Ryu KS, et al. Minimally invasive transforaminal lumbar interbody fusion at L5-S1 through a unilateral approach: technical feasibility and outcomes. BioMed Res Int. 2016;2016:2518394.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 3

    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):1520.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 4

    Lee DY, Jung TG, Lee SH. Single-level instrumented mini-open transforaminal lumbar interbody fusion in elderly patients. J Neurosurg Spine. 2008;9(2):137144.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 5

    Radovanovic I, Urquhart JC, Ganapathy V, et al. Influence of postoperative sagittal balance and spinopelvic parameters on the outcome of patients surgically treated for degenerative lumbar spondylolisthesis. J Neurosurg Spine. 2017;26(4):448453.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 6

    Hikata T, Watanabe K, Fujita N, et al. Impact of sagittal spinopelvic alignment on clinical outcomes after decompression surgery for lumbar spinal canal stenosis without coronal imbalance. J Neurosurg Spine. 2015;23(4):451458.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 7

    Than KD, Park P, Fu KM, et al. Clinical and radiographic parameters associated with best versus worst clinical outcomes in minimally invasive spinal deformity surgery. J Neurosurg Spine. 2016;25(1):2125.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 8

    Kim JS, Kang BU, Lee SH, et al. Mini-transforaminal lumbar interbody fusion versus anterior lumbar interbody fusion augmented by percutaneous pedicle screw fixation: a comparison of surgical outcomes in adult low-grade isthmic spondylolisthesis. J Spinal Disord Tech. 2009;22(2):114121.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 9

    Yee TJ, Joseph JR, Terman SW, Park P. Expandable vs static cages in transforaminal lumbar interbody fusion: radiographic comparison of segmental and lumbar sagittal angles. Neurosurgery. 2017;81(1):6974.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 10

    Hawasli AH, Khalifeh JM, Chatrath A, et al. Minimally invasive transforaminal lumbar interbody fusion with expandable versus static interbody devices: radiographic assessment of sagittal segmental and pelvic parameters. Neurosurg Focus. 2017;43(2):E10.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 11

    Kuo CH, Huang WC, Wu JC, et al. Radiological adjacent-segment degeneration in L4–5 spondylolisthesis: comparison between dynamic stabilization and minimally invasive transforaminal lumbar interbody fusion. J Neurosurg Spine. 2018;29(3):250258.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 12

    Alvi MA, Kurian SJ, Wahood W, et al. Assessing the difference in clinical and radiologic outcomes between expandable cage and nonexpandable cage among patients undergoing minimally invasive transforaminal interbody fusion: a systematic review and meta-analysis. World Neurosurg. 2019;127:596606.e1.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 13

    Carlson BB, Saville P, Dowdell J, et al. Restoration of lumbar lordosis after minimally invasive transforaminal lumbar interbody fusion: a systematic review. Spine J. 2019;19(5):951958.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 14

    Galla F, Wähnert D, Liljenqvist U. Georg Schmorl Prize of the German Spine Society (DWG) 2017: Correction of spino-pelvic alignment with relordosing mono- and bisegmental TLIF spondylodesis. Eur Spine J. 2018;27(4):789796.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 15

    Massie LW, Zakaria HM, Schultz LR, et al. Assessment of radiographic and clinical outcomes of an articulating expandable interbody cage in minimally invasive transforaminal lumbar interbody fusion for spondylolisthesis. Neurosurg Focus. 2018;44(1):E8.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 16

    Pereira C, Santos Silva P, Cunha M, et al. How does minimally invasive transforaminal lumbar interbody fusion influence lumbar radiologic parameters? World Neurosurg. 2018;116:e895e902.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 17

    Shah RR, Mohammed S, Saifuddin A, Taylor BA. Comparison of plain radiographs with CT scan to evaluate interbody fusion following the use of titanium interbody cages and transpedicular instrumentation. Eur Spine J. 2003;12(4):378385.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 18

    Aoki Y, Yamagata M, Ikeda Y, et al. A prospective randomized controlled study comparing transforaminal lumbar interbody fusion techniques for degenerative spondylolisthesis: unilateral pedicle screw and 1 cage versus bilateral pedicle screws and 2 cages. J Neurosurg Spine. 2012;17(2):153159.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 19

    Dong J, Rong L, Feng F, et al. Unilateral pedicle screw fixation through a tubular retractor via the Wiltse approach compared with conventional bilateral pedicle screw fixation for single-segment degenerative lumbar instability: a prospective randomized study. J Neurosurg Spine. 2014;20(1):5359.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 20

    Robertson PA, Armstrong WA, Woods DL, Rawlinson JJ. Lordosis recreation in transforaminal and posterior lumbar interbody fusion: a cadaveric study of the influence of surgical bone resection and cage angle. Spine (Phila Pa 1976). 2018;43(22):E1350E1357.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 21

    Qandah NA, Klocke NF, Synkowski JJ, et al. Additional sagittal correction can be obtained when using an expandable titanium interbody device in lumbar Smith-Peterson osteotomies: a biomechanical study. Spine J. 2015;15(3):506513.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 22

    Godzik J, Lehrman JN, Newcomb AGUS, et al. Tailoring selection of transforaminal interbody spacers based on biomechanical characteristics and surgical goals: evaluation of an expandable spacer. J Neurosurg Spine. 2019;32(3):383389.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 23

    Mi J, Li K, Zhao X, et al. Vertebral body Hounsfield units are associated with cage subsidence after transforaminal lumbar interbody fusion with unilateral pedicle screw fixation. Clin Spine Surg. 2017;30(8):E1130E1136.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 24

    Palepu V, Helgeson M, Molyneaux-Francis M, Nagaraja S. The effects of bone microstructure on subsidence risk for ALIF, LLIF, PLIF, and TLIF spine cages. J Biomech Eng. Published online December 5, 2018. doi:10.1115/1.4042181

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 25

    Zhou QS, Chen X, Xu L, et al. Does vertebral end plate morphology affect cage subsidence after transforaminal lumbar interbody fusion? World Neurosurg. 2019;130:e694e701.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 26

    Snyder LA, Lehrman JN, Menon RK, et al. Biomechanical implications of unilateral facetectomy, unilateral facetectomy plus partial contralateral facetectomy, and complete bilateral facetectomy in minimally invasive transforaminal interbody fusion. J Neurosurg Spine. 2019;31(3):447452.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation

Images showing severe global coronal malalignment preoperatively and after correction using posterior instrumentation and a kickstand rod on the side of coronal malalignment. See the article by Buell et al. (pp 399–412).

  • View in gallery

    Demonstration of sagittal parameter measurements on a 36-inch long-cassette lateral radiograph.

  • View in gallery

    Demonstration of disc and foraminal parameter measurements on preoperative (A) and postoperative (B) lateral radiographs of a patient with L4–5 spondylolisthesis and 8.2-mm anterior displacement. Demonstration of FA measurement on a sagittal CT image (C). Figure is available in color online only.

  • View in gallery

    Lateral radiographs 1 month after TLIF with an expandable cage (A), 12 months after TLIF with an expandable cage demonstrating cage collapse/loss of height of the cage (B), 40 months after TLIF demonstrating pseudarthrosis and screw fracture (arrow; C), and after revision surgery with anterior lumbar interbody fusion as well as posterior screw-rod replacement, demonstrating solid fusion (D).

  • 1

    Park P, Fu KM, Mummaneni PV, et al. The impact of age on surgical goals for spinopelvic alignment in minimally invasive surgery for adult spinal deformity. J Neurosurg Spine. 2018;29(5):560564.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 2

    Choi WS, Kim JS, Ryu KS, et al. Minimally invasive transforaminal lumbar interbody fusion at L5-S1 through a unilateral approach: technical feasibility and outcomes. BioMed Res Int. 2016;2016:2518394.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 3

    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):1520.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 4

    Lee DY, Jung TG, Lee SH. Single-level instrumented mini-open transforaminal lumbar interbody fusion in elderly patients. J Neurosurg Spine. 2008;9(2):137144.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 5

    Radovanovic I, Urquhart JC, Ganapathy V, et al. Influence of postoperative sagittal balance and spinopelvic parameters on the outcome of patients surgically treated for degenerative lumbar spondylolisthesis. J Neurosurg Spine. 2017;26(4):448453.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 6

    Hikata T, Watanabe K, Fujita N, et al. Impact of sagittal spinopelvic alignment on clinical outcomes after decompression surgery for lumbar spinal canal stenosis without coronal imbalance. J Neurosurg Spine. 2015;23(4):451458.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 7

    Than KD, Park P, Fu KM, et al. Clinical and radiographic parameters associated with best versus worst clinical outcomes in minimally invasive spinal deformity surgery. J Neurosurg Spine. 2016;25(1):2125.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 8

    Kim JS, Kang BU, Lee SH, et al. Mini-transforaminal lumbar interbody fusion versus anterior lumbar interbody fusion augmented by percutaneous pedicle screw fixation: a comparison of surgical outcomes in adult low-grade isthmic spondylolisthesis. J Spinal Disord Tech. 2009;22(2):114121.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 9

    Yee TJ, Joseph JR, Terman SW, Park P. Expandable vs static cages in transforaminal lumbar interbody fusion: radiographic comparison of segmental and lumbar sagittal angles. Neurosurgery. 2017;81(1):6974.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 10

    Hawasli AH, Khalifeh JM, Chatrath A, et al. Minimally invasive transforaminal lumbar interbody fusion with expandable versus static interbody devices: radiographic assessment of sagittal segmental and pelvic parameters. Neurosurg Focus. 2017;43(2):E10.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 11

    Kuo CH, Huang WC, Wu JC, et al. Radiological adjacent-segment degeneration in L4–5 spondylolisthesis: comparison between dynamic stabilization and minimally invasive transforaminal lumbar interbody fusion. J Neurosurg Spine. 2018;29(3):250258.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 12

    Alvi MA, Kurian SJ, Wahood W, et al. Assessing the difference in clinical and radiologic outcomes between expandable cage and nonexpandable cage among patients undergoing minimally invasive transforaminal interbody fusion: a systematic review and meta-analysis. World Neurosurg. 2019;127:596606.e1.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 13

    Carlson BB, Saville P, Dowdell J, et al. Restoration of lumbar lordosis after minimally invasive transforaminal lumbar interbody fusion: a systematic review. Spine J. 2019;19(5):951958.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 14

    Galla F, Wähnert D, Liljenqvist U. Georg Schmorl Prize of the German Spine Society (DWG) 2017: Correction of spino-pelvic alignment with relordosing mono- and bisegmental TLIF spondylodesis. Eur Spine J. 2018;27(4):789796.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 15

    Massie LW, Zakaria HM, Schultz LR, et al. Assessment of radiographic and clinical outcomes of an articulating expandable interbody cage in minimally invasive transforaminal lumbar interbody fusion for spondylolisthesis. Neurosurg Focus. 2018;44(1):E8.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 16

    Pereira C, Santos Silva P, Cunha M, et al. How does minimally invasive transforaminal lumbar interbody fusion influence lumbar radiologic parameters? World Neurosurg. 2018;116:e895e902.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 17

    Shah RR, Mohammed S, Saifuddin A, Taylor BA. Comparison of plain radiographs with CT scan to evaluate interbody fusion following the use of titanium interbody cages and transpedicular instrumentation. Eur Spine J. 2003;12(4):378385.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 18

    Aoki Y, Yamagata M, Ikeda Y, et al. A prospective randomized controlled study comparing transforaminal lumbar interbody fusion techniques for degenerative spondylolisthesis: unilateral pedicle screw and 1 cage versus bilateral pedicle screws and 2 cages. J Neurosurg Spine. 2012;17(2):153159.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 19

    Dong J, Rong L, Feng F, et al. Unilateral pedicle screw fixation through a tubular retractor via the Wiltse approach compared with conventional bilateral pedicle screw fixation for single-segment degenerative lumbar instability: a prospective randomized study. J Neurosurg Spine. 2014;20(1):5359.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 20

    Robertson PA, Armstrong WA, Woods DL, Rawlinson JJ. Lordosis recreation in transforaminal and posterior lumbar interbody fusion: a cadaveric study of the influence of surgical bone resection and cage angle. Spine (Phila Pa 1976). 2018;43(22):E1350E1357.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 21

    Qandah NA, Klocke NF, Synkowski JJ, et al. Additional sagittal correction can be obtained when using an expandable titanium interbody device in lumbar Smith-Peterson osteotomies: a biomechanical study. Spine J. 2015;15(3):506513.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 22

    Godzik J, Lehrman JN, Newcomb AGUS, et al. Tailoring selection of transforaminal interbody spacers based on biomechanical characteristics and surgical goals: evaluation of an expandable spacer. J Neurosurg Spine. 2019;32(3):383389.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 23

    Mi J, Li K, Zhao X, et al. Vertebral body Hounsfield units are associated with cage subsidence after transforaminal lumbar interbody fusion with unilateral pedicle screw fixation. Clin Spine Surg. 2017;30(8):E1130E1136.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 24

    Palepu V, Helgeson M, Molyneaux-Francis M, Nagaraja S. The effects of bone microstructure on subsidence risk for ALIF, LLIF, PLIF, and TLIF spine cages. J Biomech Eng. Published online December 5, 2018. doi:10.1115/1.4042181

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 25

    Zhou QS, Chen X, Xu L, et al. Does vertebral end plate morphology affect cage subsidence after transforaminal lumbar interbody fusion? World Neurosurg. 2019;130:e694e701.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 26

    Snyder LA, Lehrman JN, Menon RK, et al. Biomechanical implications of unilateral facetectomy, unilateral facetectomy plus partial contralateral facetectomy, and complete bilateral facetectomy in minimally invasive transforaminal interbody fusion. J Neurosurg Spine. 2019;31(3):447452.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation

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