Cage Obliquity in Oblique Lumbar Interbody Fusion—How Common Is It and What Are the Effects on Fusion Rates, Subsidence, and Sagittal Alignment? A Computed Tomography-Based Analysis

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Figure 1
Obliquity of the cage was measured as an angle (A) between a horizontal line (yellow) down the length of the cage and a line along the posterior border of the vertebral body (orange) using the computerized software. The blue line represents the outline of the interbody cage.
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Figure 2
Computerized tomographic image of the lumbar spine showing complete fusion on both the lateral (A) and anteroposterior (B) views.
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Figure 3
Radiographs of the lumbar spine in the lateral view. At the L3/4 level, there is subsidence of the entire cage (red arrow), while at the L4/5 level, there is no subsidence (white arrow).
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Figure 4
Computed tomography images showing different degrees of fusion. (A) Complete fusion (Bridwell 1). (B) Intact graft without complete remodeling (Bridwell 2). (C) Lucencies are seen in both superior and inferior aspects (Bridwell 3) at the L2/3 level as indicated by the white arrow.
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Figure 5
Intraoperative photo showing the author’s maneuver for orthogonal rotation of the cage. Note that while the retractor is placed obliquely, the maneuver is performed outside the disc space.

Table 1

Summary of patient and surgical details.

Variables	Value
Patient Details
No. of patients	53
No. of operated levels	97
Age, y, mean ± SD	67.5 ± 7.93
Gender
Men	16 (30.2%)
Women	37 (69.8%)
Body mass index, mean ± SD	25.8 ± 4.56
American Society of Anesthesiologists classification
1	2 (3.77%)
2	36 (67.9%)
3	15 (28.3%)
Surgical details
No. of levels operated
1	19 (35.8%)
2	24 (45.3%)
3	10 (18.9%)
Levels operated
L1–L2	3 (3.09%)
L2–L3	18 (18.6%)
L3–L4	40 (41.2%)
L4–L5	36 (37.1%)
Cage length, mm
40	3 (3.09%)
45	27 (27.8%)
50	41 (42.3%)
55	23 (23.7%)
60	3 (3.09%)
Cage height, mm
8	24 (24.7%)
10	60 (61.9%)
12	11 (11.3%)
14	1 (1.03%)

Table 2

Positioning of interbody cages.

Cage Positioning	Value
Cage orientation (obliquity)
0°–5°, n (%)	69 (71.1%)
5°–10°, n (%)	28 (28.9%)
Mean ± SD degree of obliquity (°)	4.17 ± 2.84
Cage positioning in the sagittal plane
Anterior one-third of the vertebral body, n (%)	8 (8.25%)
Middle one-third of the vertebral body, n (%)	89 (91.8%)
Mean ± SD distance anterior to midline, mm	1.86 ± 2.51

Table 3

Postoperative fusion and subsidence rates.

Postoperative Outcome Measure	Value
Fusion
Bridwell 1 and 2 (fusion achieved), n (%)	96 (99.0%)
Bridwell 3 and 4 (nonunion), n (%)	1 (1.03%)
Bridwell score, mean ± SD	1.14 ± 0.352
Correlation with cage obliquity	No (r_s = 0.16; P = 0.293)
Correlation with sagittal placement	No (r_s = 0.03; P = 0.0638)
Subsidence
Marchi 0, n (%)	81 (83.5%)
Marchi 1, n (%)	14 (14.4%)
Marchi 2, n (%)	2 (2.06%)
Marchi 3, n (%)	0 (0%)
Marchi score, mean ± SD	0.186 ± 0.44
Correlation with cage obliquity	No (r_s = −0.11; P = 0.81)
Correlation with sagittal placement	No (r_s = −0.05; P = 0.474)

Table 4

Measure of sagittal alignment correction.

Sagittal Alignment Parameters	Value	P
Disc height, mm
Preoperative, mean ± SD	9.63 ± 8.76
Postoperative, mean ± SD	13.3 ± 3.14
Change, mean ± SD	4.46 ± 3.77	<0.0001
Correlation with cage obliquity	No (r_s = 0.21)	0.111
Correlation with sagittal placement	No (r_s = 0.13)	0.461
Segmental lordosis angle (°)
Preoperative, mean ± SD	5.95 ± 5.13
Postoperative, mean ± SD	10.2 ± 4.30
Change, mean ± SD	4.23 ± 5.67	<0.0001
Correlation with cage obliquity	No (r_s = 0.04)	0.931
Correlation with sagittal placement	No (r_s = −0.004)	0.561