Patient selection
The case records of 94 patients who underwent OLIF were reviewed, and as a result, a total of 66 patients were included. The average follow-up period was 22.6 ± 6.6 months (range 13–34 months). Among the 66 patients included, 41 patients who received stand-alone OLIF were set as the stand-alone group, and 25 patients who received OLIF combined with percutaneous pedicle screw fixation were set as the combined group. Among the 28 excluded patients, 17 were lost to follow-up or had incomplete follow-up data, 8 had incomplete radiography data, and 3 were withdrawn from the cohort because of secondary pedicle screw fixation after primary stand-alone surgery due to unbearable low back pain. The demographic baseline characteristics of the patients in both groups are described in Table 1. There was no significant difference in age, sex or BMI between the two groups, but the BMD in the combined group was significantly lower than that in the stand-alone group (-1.9 ± 1.3 vs -0.9 ± 1.4, p = 0.005), which also reflected the surgeon's preference for the selection of a fixation scheme for patients with a low BMD (Table 1).
Table 1
Baseline characteristics of included patients
Baseline data
|
Combined
(n = 25)
|
Stand-alone
(n = 41)
|
P value
|
Age(y)
|
62.16 ± 8.65
|
59.46 ± 8.46
|
0.217
|
Gender(M/F)
|
12/13
|
22/19
|
0.800
|
BMI(kg/m2)
|
24.32 ± 1.59
|
24.33 ± 2.40
|
0.984
|
BMD(T value)
|
−1.9 ± 1.3
|
−0.9 ± 1.4
|
0.005*
|
BMI: body mass index, BMD: bone mineral density. * The difference was statistically significant (p < 0.05) |
Clinical outcomes
Before surgery, there was no significant difference in the VAS (6 [5.5, 7] vs 6 [5.5, 7], p = 0.826) or ODI (62.22 [53.33, 68.89] vs 57.78 [53.33, 64.44], p = 0.551) scores between the combined group and stand-alone group. The postoperative VAS and ODI scores decreased significantly in each group and were well maintained 3 months and 6 months after surgery as well as at the last follow-up. Postoperatively, the VAS score of the stand-alone group was slightly better than that of the combined group (3 [3, 4] vs 4 [3, 5], p = 0.012). However, the VAS score in the combined group was better than that in the stand-alone group 3 months after the operation (3 [1.5, 3] vs 3 [2, 4], p = 0.038). The difference disappeared 6 months after the operation and at the last follow-up. There was no significant difference in ODI scores between the groups at any of the time points (Table 2, Fig. 2).
Table 2
Clinical out-comes of the two groups.
Groups
|
VAS score
|
ODI(%)
|
Pre-op
|
Post-op
|
3 m
|
6 m
|
Last f-u
|
Pre-op
|
3 m
|
6 m
|
Last f-u
|
Combined
|
6 [5.5, 7]
|
4 [3, 5]
|
3 [1.5, 3]
|
2 [2, 3]
|
2 [1.5, 2]
|
62.22 [53.33, 68.89]
|
24.44 [17.78, 28.89]
|
17.78 [15.56, 20.00]
|
15.56 [13.33, 17.78]
|
Stand-alone
|
6 [5.5, 7]
|
3 [3, 4]
|
3 [2, 4]
|
2 [2, 3]
|
2 [2, 3]
|
57.78 [53.33, 64.44]
|
26.67 [18.89, 38.89]
|
20.00 [16.67, 22.22]
|
15.56 [13.33, 21.11]
|
Z value
|
−0.229
|
−2.528
|
−2.075
|
−1.657
|
−1.823
|
−0.603
|
−1.348
|
−1.134
|
−0.960
|
P value
|
0.826
|
0.012*
|
0.038*
|
0.100
|
0.067
|
0.551
|
0.180
|
0.260
|
0.341
|
VAS: visual analog scale, ODI: Oswestry disability index, * The difference was statistically significant (p < 0.05) |
Radiography outcomes
In general, the DH increased by 3.11 [2.12, 4.45] mm after the operation, but there was no significant difference in the ΔDH between the two groups (p = 0.848). The FH values preoperatively and 3 postoperatively were comparable between the two groups. The combined group showed good FH maintenance, and the stand-alone group had more obvious FH loss. There was a significant difference of FH between the two groups 6 months after the operation (18.37 ± 1.98 vs 17.26 ± 2.29, p = 0.049), and the difference was more significant at the last follow-up (18.22 ± 2.00 vs 16.87 ± 2.34, p = 0.019). The combined group had significantly lower tCS than the stand-alone group at all time points after the operation (all p ≤ 0.001). There was no significant difference in the fusion rate between the two groups 6 months after the operation or at the last follow-up (92% vs 90.2%, 100% vs 95.1%, respectively) (Table 3, Fig. 2).
Table 3
Radiological results the two groups.
Groups
|
ΔDH
(mm)
|
FH (mm)
|
tCS (mm)
|
Fusion rate
|
Pre-op
|
Post-op
|
3 m
|
6 m
|
Last f-u
|
3 m
|
6 m
|
Last f-u
|
6 m
|
Last f-u
|
Combined
|
3.24 [2.03, 4.55]
|
17.18 ± 2.39
|
19.40 ± 2.00
|
18.71 ± 2.10
|
18.37 ± 1.98
|
18.22 ± 2.00
|
2.50 ± 1.24
|
3.20 ± 1.48
|
3.50 ± 1.50
|
23/25(92%)
|
25/25(100%)
|
Stand-alone
|
3.08 [2.15, 4.33]
|
17.31 ± 2.14
|
19.87 ± 1.93
|
18.36 ± 2.41
|
17.26 ± 2.29
|
16.87 ± 2.34
|
4.15 ± 1.91
|
4.85 ± 2.03
|
5.26 ± 2.11
|
37/41(90.2%)
|
39/41(95.1%)
|
Statistical value
|
Z=−0.192
|
t=−0.226
|
t=−0.955
|
t = 0.599
|
t = 2.004
|
t = 2.397
|
t=−3.837
|
t=−3.522
|
t=−3.641
|
\
|
\
|
P value
|
0.848
|
0.822
|
0.343
|
0.551
|
0.049*
|
0.019*
|
< 0.001*
|
0.001*
|
0.001*
|
1.000
|
0.522
|
DH: discal height, FH: foraminal hight, tCS: total cage subsidence, * The difference was statistically significant (p < 0.05) |
Correlation analysis
Because the correlation between CS and clinical outcomes depends mainly on the severity of individual CS rather than the sum. Thus, we use linear regression to analyze the relationship between mCS and BMD, BMI, DH, VAS, ODI. In the combined group, BMD was mildly negatively correlated (r= -0.602, p = 0.001) with mCS, and BMI was a moderate-intensity positive correlation for mCS (r = 0.400, p = 0.047), while ΔDH (r=-0.059, p = 0.781) were not significantly correlated with mCS. In contrast, in the stand-alone group, the negative correlation between BMD and mCS was significantly enhanced (r= -0.797, p < 0.001), and a correlation between mCS and BMI (r = 0.207, p = 0.195) or ΔDH (r = 0.271, p = 0.086) was not found.
Multiple linear regression showed that only BMD was a risk factor for mCS in the regression model in the combined group (β =-0.535, p = 0.005) and stand-alone group (β =-0.756, p < 0.001) (Table 4). Therefore, BMI and ΔDH were excluded from the multiple linear regression model, and a simple linear regression model of BMD and mCS was used(Table 5). According to the linear regression model, when severe CS was defined as mCS > 4 mm, the BMD threshold in the stand-alone group was − 1.38, and the use of additional implantation expanded the BMD threshold to -4.77 in the combined group.
Table 4
Multiple linear regression results
Groups
|
Variables
|
SE
|
β
|
95%CI
|
P value
|
Combined
|
Constant
|
3.003
|
|
(−9.539, 2.950)
|
0.285
|
BMD
|
0.156
|
−0.535
|
(−0.812,−0.164)
|
0.005*
|
BMI
|
0.125
|
0.268
|
(−0.063, 0.457)
|
0.130
|
ΔDH
|
0.085
|
−0.002
|
(−0.177, 0.176)
|
0.992
|
Stand-alone
|
Constant
|
1.528
|
|
(−3.688,2.505)
|
0.701
|
BMD
|
0.102
|
−0.756
|
(−1.031,−0.618)
|
< 0.001*
|
BMI
|
0.060
|
0.185
|
(−0.002, 0.240)
|
0.054
|
ΔDH
|
0.084
|
0.177
|
(−0.013, 0.329)
|
0.069
|
Dependent variable: mCS, SE: standard error, 95%CI: 95% confidence interval, * The difference was statistically significant (p < 0.05) |
Table 5
Simple linear regression results.
Groups
|
Variables
|
SE
|
β
|
95%CI
|
P value
|
Combined
|
Constant
|
0.342
|
|
(0.675, 2.090)
|
0.001*
|
BMD
|
0.152
|
−0.602
|
(−0.862,−0.234)
|
0.001*
|
Stand-alone
|
Constant
|
0.174
|
|
(2.451, 3.153)
|
< 0.001*
|
BMD
|
0.106
|
−0.797
|
(−1.083,−0.656)
|
< 0.001*
|
Dependent variable: mCS, SE: standard error, 95%CI: 95% confidence interval, * The difference was statistically significant (p < 0.05) |
On the other hand, there was only a low to moderate positive correlation between mCS and VAS (r = 0.427, p = 0.033) or ODI (r = 0.594, p = 0.002) scores in the combined group. In contrast, in the stand-alone group, this positive correlation was much stronger with VAS (r = 0.685, p < 0.001) and DOI (r = 0.616, p < 0.001) .
Complications
No cases of major artery injury or spinal nerve injury occurred. One case of segmental artery injury occurred in the stand-alone group, and lumbar myasthenia was relatively more common, with 4 cases occurring in each group. There were 3 cases of sympathetic nerve injury in the combined group and 2 cases in the stand-alone group. There was 1 case of ureteral injury in the stand-alone group. No cage retropulsion or other complications were observed in either group. Statistical analysis was abandoned due to the small number of complications.