This retrospective review was approved by the appropriate institutional review board prior to initiation. This study met guidelines of the responsible local governing agency and complied with the principles of the Declaration of Helsinki. The patients or their families were informed that the data from the cases would be submitted for publication, and their written consent was obtained.
Materials
This retrospective cohort study includes 17 consecutive patients (21 fused vertebral levels) with degenerative lumbar kyphoscoliosis or kyphosis, who underwent the LIR procedure as part of their ASD surgery between January 2014 and March 2020. All included individuals had completed at least 1-year follow-up following the procedure.
Surgical procedure
LIR was performed on fused vertebrae resulting from advanced degenerative changes in the lumbar spine (Fig. 1a). All patients undergo pre-operative scoliosis series, computed tomography (CT) with multiplanar reconstruction and MRI imaging. We performed LIR when there was radiographic evidence of fused vertebrae but low signal intensity in the interbody space was observed on T2 weighted magnetic resonance imaging (MRI), indicating a remnant of the disc space (Fig. 1b). We would not perform LIR if vital structures such as the aorta and the vena cava were appreciated to be in the path of the interbody approach on preoperative radiological images. The approach side was determined by whether interbody access was feasible at the L4/5 level. In addition, the LIR approach is easier from the non-fused side of fused vertebrae.
LIR was performed when it was necessary to release the fused vertebrae in order to obtain the target lumbar lordosis (LL) determined using the Scoliosis Research Society-Schwab criteria [2]. Patient positioning follows the standard technique for transpsoas interbody approaches, as does the initial surgical access which employs the standard retractor (Maxcess®, NuVasive, San Diego, CA, USA). Once the operative level has been accessed, a Cobb elevator with 18 mm width was inserted into the disc space from non-fused side. The Cobb elevator was penetrated to the contralateral side of the fused vertebrae under a fluoroscope (OEC 9900 series, GE Health Care Japan co., Hibi, Tokyo) (Fig. 1c). A straight chisel with 7 mm width was sometimes used when there was resistance to advance the Cobb elevator. Mobility between the fused vertebrae may be appreciated by gently rotating the Cobb elevator at this time. This will also confirm that a release has been achieved.
Next, a 6 mm thick Paddle Sizer® (NuVasive, San Diego, CA, USA) was inserted to completely accomplish the release between the fused vertebrae (Fig. 1d). This is followed by the insertion of a Cobb elevator and Paddle Sizer with 18 mm width in sequence to complete release. After inserting trial devices, the cage with an anteroposterior width of 18 mm was inserted according to the usual extreme lateral interbody fusion (XLIF) procedure. At this time, the cage was inserted so as to partially rest on the released osteophyte or the osteosclerotic end plate on the convex side of the curvature. Autologous iliac crest bone and artificial bone made of hybridized hydroxyapatite and type I collagen (ReFit®, HOYA Technosurgical Co., Tokyo, Japan) were mixed (50/50 proportion) and inserted into the cage.
After the anterior procedure including LIR was completed, the patient was placed in the prone position and the posterior corrective fusion surgery was performed as a single staged surgery. The posterior fusion was typically performed from the thoracic spine to the pelvis including L5/S1 posterior lumbar interbody fusion (PLIF). If there was poor flexibility of the spinal motion, grade 1 or 2 osteotomy, as suggested by the Scoliosis Research Society-Schwab criteria [21], was also performed. Spinal kyphosis was corrected by using the cantilever technique with bilateral S1 pedicle screws and bilateral S2 alar iliac screws. Two bent titanium rods with 5.5 mm diameter were connected to the pedicle screws. If the target LL was achieved after the anterior procedure and there were no severe atrophy of the back muscles and no severe degenerative changes of the cranial and caudal discs adjacent to the range of the anticipated fusion construct, short fusion within the lumbar spine was selected. The patient was mobilized as soon as possible following the surgery and wore a hard corset for 3 months postoperatively.
Patient demographic, clinical, and surgical data
Patient charts were abstracted to obtain baseline demographic and surgical characteristics of include patients. These consisted of biologic sex, age, body mass index (BMI), bone density (T-score), injection of teriparatide 2 months or more before surgery, follow-up periods, and treated intervertebral levels, surgical access side (convex or concave) and range of fusion levels. Preoperative fusion ratio (FR) of auto-fused vertebrae was assessed according to the ratio of the length of the autofused portion to the total height of the intervertebral space on coronal plane imaging using multiplanar reconstruction (MPR) of the CT scan (Fig. 2).
Evaluation of postoperative LIR segments
The time from installation to removal of the Maxcess® retractor was measured at 17 fused vertebrae treated with LIR and 28 intervertebral segments operated with usual XLIF in the non-fused interbody. An amount of blood loss in the anterior surgery was investigated from an operation record and an amount of blood loss per single segment was calculated.
The segment operated with LIR was evaluated radiologically. Segmental lordotic angle (SLA, the angle measured between the superior endplate of the fused vertebra cranially and inferior endplate of the fused vertebra caudally in the sagittal plane) and segmental coronal angle (SCA, the angle measured between the superior endplate of the fused vertebra cranially and inferior endplate of the fused vertebra caudally in the coronal plane) was measured preoperatively and at the final observation. SLA was measured by creating an exact sagittal image of the center of the vertebral body using MPR of CT. SCA was similarly measured by creating an exact coronal image of the center of the vertebral body. Bone union was investigated using CT 1-year after surgery. Patterns of bone union were assessed in the fused segments treated by LIR according to the Proietti classification [17].
To control for intraobserver variability, the measurements of SLA and SCA in all 21 fused vertebrae were evaluated by the same observer, > 4 weeks after the first reading. The measurements were also evaluated by two spine surgeon supervisors certified by the Japanese Society for Spine Surgery and Related Research to determine interobserver variability.
Evaluation of global spine alignment
For the spinopelvic parameter, pelvic incidence (PI), lumbar lordosis (LL), pelvic tilt (PT), sacral slope (SS), PI-LL mismatch, and sagittal vertical axis (SVA), and as coronal alignment parameters Cobb angle (the angle measured between the superior endplate of the most tilted vertebra cranially and the inferior endplate of the most tilted vertebra caudally) and C7-CSVL (deviation of the C7 plumb line from the central sacral vertical line) were evaluated in 14 patients (82.4%) with minimum 2-years follow-up before surgery and at the final observation. All spinopelvic and coronal parameters were measured using standard standing position X-rays which were performed before surgery and at the final observation.
Evaluation of clinical variables
Oswestry Disability Index (ODI), low back and leg pain based on visual analog scale (VAS) score, and the short form 36 health survey questionnaire (SF-36) were evaluated postoperatively and at the final observation.
Complications
Neurological deterioration including the muscle weakness and numbness of the leg, and the other complication during surgery were assessed from inpatient records, outpatient visit records, operative records, and the radiographic images including MPR of CT. The intraoperative endplate injury of 2 mm over and the anterior longitudinal ligament injury were evaluated from the radiographic images including MPR of CT within 1 week after surgery. Proximal junctional kyphosis (PJK) and hardware failures such as cage subsidence according to the Marchi classification [22], screw breakage, and rod fracture were evaluated from the radiographic images including MPR of CT at the final observation. Finally, the causes of reoperation and its proportion were investigated.
Statistical analyses
All t-tests were performed after confirming the normality of investigated data by the Shapiro-Wilk test. If the variables included in this study were not normally distributed, the Wilcoxon signed rank test was employed accordingly. Comparison of the required time to perform LIR at the fused vertebrae and XLIF at the other non-fused intervertebral spaces was conducted using the t-test. A paired t-test was used to evaluate the SLA and SCA at intervertebral segments treated with LIR pre-operatively and at 1-year after surgery. SLA 1-year after surgery was compared with the lordotic angle of inserted cage using a paired t-test. Spinopelvic parameters and global spinal alignment were compared between using pre-operative and final post-operative measurements using a paired t-test. ODI, VAS, and SF-36 were compared using a paired t-test or the Wilcoxon signed rank test. All statistical analyses were performed using JMP data analysis software, version 14 (SAS Institute Inc., Cary, NC, USA). P-values < 0.05 were considered statistically significant.