Lumbar lordosis is signficantly greater on intraoperative prone X-ray than on preoperative standing or supine images: a retrospetive study of lordosis measurement from the United States
Article information
Abstract
Study Design
A retrospective cohort study.
Purpose
We aimed to compare intraoperative prone X-ray (IPXray) lumbar lordosis (LL) measurements with preoperative standing lateral X-rays (SLXray) and computed tomography (CT)/magnetic resonance imaging (MRI) and to assess the value of these preoperative imaging measurements in predicting the required lordosis correction for multilevel posterior spinal fusion.
Overview of Literature
Attaining sagittal alignment is critical in multilevel thoracolumbar fusion. The optimal preoperative imaging technique for accurately planning the required intraoperative sagittal correction and how those preoperative measures change with various degrees of spinal deformity remain unelucidated.
Methods
A retrospective review of multilevel posterior fusion surgeries involving thoracolumbar arthrodesis of ≥4 fusion segments was conducted. After positioning under anesthesia, LL was measured and compared between IPXray and SLXray or CT/MRI using paired t-test. Lordosis difference (LD) was calculated as LL on IPXray minus LL on SLXray (Standing LD) or CT/MRI (Supine LD). The association between pelvic incidence (PI)–LL mismatch and different LL measures was analyzed by Pearson coefficient and multivariate logistic regression.
Results
Average LL was significantly greater on IPXray than on SLXray (p<0.001) or CT/MRI (p<0.001) but not significantly different in LD when comparing SLXray versus CT/MRI (p=0.730). Standing LD and Supine LD were similar (p=0.600). Patients with greater amounts of preoperative PI–LL mismatch had significantly increased LD (X-ray: R2=0.180, p≤0.001; CT: R2=0.019, p<0.04). Additionally, Standing LD became significantly greater than Supine LD with PI–LL mismatch of >20°.
Conclusions
Approximately 10° of additional LD is generated by prone positioning under anesthesia, compared with standing or supine preoperative imaging. Among patients with greater sagittal imbalance, the increased LD suggested they can achieve more correction through positioning. These values should be considered when estimating the amount of lordosis needed to achieve sagittal alignment through surgical maneuvers.
Introduction
Maintaining or improving lumbar spine sagittal alignment is important when performing lumbar fusion, particularly multilevel thoracolumbar fusion [1–3]. Careful preoperative planning and determination of the required amount of intraoperative sagittal plane correction are needed to achieve the desired postoperative alignment. Preoperative sagittal alignment may be assessed by physical exam, X-rays, and advanced imaging modalities, such as computed tomography (CT)/magnetic resonance imaging (MRI) [4], and standing lateral X-ray (SLXray) of the lumbar spine has historically served as the primary modality for such calculations [2,5]. However, the pain associated with the upright posture, as well as variable patient effort, can cause sagittal alignment to appear worse on SLXray than the underlying flexibility that the spine actually allows [6,7].
Supine sagittal images on CT and MRI are alternative means of calculating the amount of lordosis needed during surgery [8]. Given that CT and MRI are taken on supine, they may better reflect the underlying spine flexibility by eliminating the effect of gravity on the spine, relying less on patient effort, and reducing any pain associated with standing upright. However, one limitation is the fact that sagittal alignment on CT/MRI can be different between supine and prone under anesthesia [9]. Consequently, both SLXray and CT/MRI are imperfect predictors of baseline sagittal alignment, which is the basis of the required surgical correction when performing posterior multilevel fusion.
True baseline lordosis is reflected not by alignment on SLXray or CT/MRI but by the alignment achieved intraoperatively when the patient is placed prone under anesthesia, which can likely reflect more lordosis as long as mobile disc segments are present. Under anesthesia, patient guarding and pain responses can be eliminated, potentially enhancing lordosis and sagittal alignment [10–12]. Moreover, the lumbar spine can sag into further lordosis when positioned appropriately on a Jackson-type surgical table. Compared with an upright position, prone position minimizes the effects of gravity and weight bearing on sagittal alignment, thereby, potentially enhancing the sagittal plane.
Although true baseline alignment, which is the basis for the required additional surgical correction, can be reflected by intraoperative prone position, it cannot be precisely predicted preoperatively. More accurate preoperative prediction of intraoperative lordosis would be beneficial by reducing unnecessary surgical morbidity associated with additional procedures, such as osteotomy, anterior and posterior interbody fusion, and longer fusion constructs, to obtain more lordosis than actually required to achieve alignment goals. Therefore, the purpose of this study was to compare lordosis on preoperative SLXray and supine sagittal advanced images (CT/MRI) with intraoperative prone X-rays (IPXray) to help determine calculations of the actual lordosis needed in preparation for multilevel posterior spinal fusion.
Materials and Methods
Study design
We conducted this study in compliance with the principles of the Declaration of Helsinki. The study protocol was reviewed and approved by the Institutional Review Board of Emory University (IRB #2025P010901). A wavier of informed consent was obtained prior to study initiation. After obtaining IRB approval, a retrospective review was performed on patients who underwent thoracolumbar arthrodesis of ≥4 vertebral levels through an all-posterior approach over a 5-year period at a single institution. These patients were chosen, because they most likely represented the population on which sagittal alignment correction would be performed. Patients were initially identified from the practices of five fellowship-trained orthopedic spine surgeons using CPT (Current Procedural Terminology) codes for thoracic and lumbar posterior osteotomy (22206, 22207, 22208, 22212, 22214, and 22216); >7-segment posterior instrumentation (22843 and 22844); and pelvic fixation (22848). The operative notes of identified patients were individually evaluated if they met the inclusion criteria.
Patients included in this study were >18 years of age; underwent primary or revision thoracolumbosacral fusion through an all-posterior approach for a variety of preoperative diagnoses, including spinal stenosis, scoliosis, and kyphosis; and had preoperative standing X-ray and CT or MRI of the lumbar spine. The exclusion criteria were age <18 years old; acute fracture or other traumatic injury; underwent combined anterior-posterior deformity correction; and missing or inadequate preoperative standing radiographs, preoperative CT or MRI, or intraoperative prone lateral radiographs. Charts were reviewed for patient demographics and surgical details. All preoperative and intraoperative images were accessed, reviewed, and measured using the GE PACS Healthcare imaging system (GE HealthCare, Chicago, IL, USA).
Sagittal curve measurements
Study inclusion required preoperative images obtained within 6 months before surgery. After anesthetic induction and patient positioning prior to any surgical incision, lateral IPXrays were routinely taken for spine level localization. All patients were placed on prone position on an OSI frame (Orthopaedic Systems Inc., Union City, CA, USA). The chest pad was placed on the sternum to allow the abdomen to hang freely and avoid compression on the axillae. The caudal pads were placed just distal to the anterior superior iliac spine, with the thighs supported in extension. Using the Cobb method, lumbar lordosis (LL) was measured as the angle of intersection between the lines parallel to the L1 superior endplate and S1 superior endplate [13,14]. Pelvic incidence (PI) was measured on 36-inch (=91.44 cm) standing preoperative lateral X-rays, according to the parameters described by Duval-Beaupère et al. [15]. Sagittal vertical axis (SVA) was measured as the distance between a vertical C7 plumb line and a perpendicular line from the posterior-superior corner of S1 on 36-inch (=91.44 cm) SLXrays [13,14].
We introduced lordosis difference (LD) as a new variable and measure of the difference in LL between preoperative images and intraoperative prone positioning images. Standing LD was calculated as lordosis on IPXray minus lordosis on SLXray. Supine LD was calculated as lordosis on IPXray minus lordosis on CT/MRI.
Statistical analysis
Statistical analyses were performed using IBM SPSS Statistics ver. 22.0 (IBM Corp., Armonk, NY, USA). Results were reported as mean±standard deviation when applicable. Paired t-test was used to analyze the differences between the LL measures (X-ray vs. CT/MRI). The association between PI–LL mismatch and different LL measures was analyzed by Pearson coefficient and multivariate logistic regression, which controlled for patient demographics and comorbidities. p<0.05 was considered significant.
Results
This study reviewed and included 163 patients, who had an average age of 60 years (range, 20–78 years) and comprised 75% women. Sixteen (9.8%) had pure sagittal plane deformity (Table 1). The average C7 SVA was 4.45 cm, and the average PI–LL mismatch was 19°. The average number of vertebral levels fused during surgery was 9 (range, 6–12). Fifty-five patients (34%) underwent revision and extension of a prior fusion.
Demographic and operative characteristics
Lumbar lordosis
As shown in Table 2, LL did not significantly differ between preoperative SLXray and CT/MRI in all patients (39.8° vs. 39.6°, respectively; p=0.730) and in the subsets of patients who underwent primary surgery and had greater lordosis (42.7° vs. 43.2°, respectively; p=0.592) and those who underwent revision and had less lordosis (34.3° vs. 32.5°, respectively; p=0.153). Lordosis was significantly greater on IPXray under anesthesia (47.8°) than on preoperative SLXray (39.8°, p<0.001) and CT/MRI (39.6°, p<0.001). Similar trends were observed in the primary and revision subgroups.
Lumbar lordosis measurements
Lordosis difference
A Standing LD of 10.2° implied that the IPXray had an average of 10.2° additional lordosis, compared with that of SLXray. A Supine LD of 9.9° meant that the IPXray had an average of 9.9° additional lordosis, compared with that of CT/MRI (Table 3). The Standing and Supine LD values were not significantly different (p=0.600). The results indicated that approximately 10° of additional lordosis was obtained intraoperatively after prone positioning, compared with those measured on SLXray or CT/MRI. The primary and revision subsets had similar LD with the overall population. The Standing and Supine LD values did not significantly differ in the subset of patients who underwent primary surgery (9.8° vs. 8.9°, respectively; p=0.306) and in the revision cohort (11.0° vs. 11.8°, respectively; p=0.549).
Lumbar lordosis difference
Lordosis difference vs. PI–LL mismatch
As shown in Fig. 1, PI–LL mismatch had a weakly positive correlation with Standing LD (R2=0.18, p<0.001) and a small but statistically significant correlation (R2=0.019, p=0.042) with Supine LD. Table 4 shows LD as a function of PI–LL mismatch. Two findings are noted. First, consistent with Fig. 1, greater PI–LL mismatch was associated with greater Standing and Supine LD. Therefore, greater overall sagittal malalignment was associated with greater LD. When the PI–LL mismatch was >20°, Standing LD was 13.5° and Supine LD was 10.7°. When the PI–LL mismatch was >30°, Standing LD increased to 15.3° and Supine LD increased slightly to 11.2°. Second, with greater amounts of PI–LL mismatch, the Supine LD was significantly lower than the Standing LD. This result meant that compared with SLXray, CT/MRI more closely approximated the lordosis on IPXray and gave a closer estimation of the baseline lordosis that needed intraoperative correction. Patients with a PI–LL mismatch >20° showed a small but significant 2.8° difference between the Standing and Supine LD values (13.5° vs. 10.7°, respectively; p=0.022). At a PI–LL mismatch of >30°, the difference between the Standing and Supine LD values increased to 4.1° (15.3° vs. 11.2°, respectively; p<0.001).
Scatterplot of the relationship between pelvic incidence (PI)–lumbar lordosis (LL) mismatch and the lordosis difference (LD) measured on (A) standing X-ray (Standing LD) or (B) supine computed tomography/magnetic resonance imaging (Supine LD).
Lordosis difference vs. PI–LL mismatch
Discussion
In this study on patients who underwent multilevel posterior-only fusion, we measured LL on standing and supine preoperative images and compared them with IPXrays under anesthesia. On average, L1–S1 lordosis was approximately 10° greater on IPXrays than on preoperative SLXray and CT/MRI images; this implied that LD was approximately 10°, regardless of whether the imaging was taken standing or supine. Therefore, intraoperative maneuvers would need approximately 10° less lordosis than preoperatively predicted, because simply positioning the patient on prone under anesthesia provided additional 10° of lordosis. Although the exact LD will vary according to individual patient characteristics, such as number of segments with solid fusion or stiffness of unfused segments, this 10° rule of thumb is important to consider during preoperative planning of the amount of lordosis that is actually needed using intraoperative surgical maneuvers.
The increased LD in patients with greater sagittal imbalance, as noted by greater PI–LL mismatch, meant that greater correction can be expected after prone positioning under anesthesia. In patients with mismatch of >30°, additional 15° lordosis was generated after positioning under anesthesia, compared with that on SLXray. As the PI–LL mismatch increased, the significantly greater Standing LD than Supine LD meant that the supine preoperative image (CT/MRI) more closely approximated the lordosis on IPXray. These numbers should be taken into consideration when calculating the actual amount of lordosis needed to achieve the desired sagittal alignment during surgical maneuvers. Notably, although inclusion of patients who underwent revision introduced some heterogeneity, similar findings were observed in both primary surgery and revision subgroups. Inclusion of both cohorts was deliberate, because it more accurately represented the spectrum of patients undergoing multilevel posterior fusion in clinical practice.
In this study, preoperative LD was similar between SLXray and CT/MRI. Therefore, the baseline lordosis achieved after intraoperative prone positioning could be estimated by adding 10° to either preoperative SLXray or CT/MRI. In a study by Sharma et al. [16], who evaluated the ability of preoperative MRI to determine flexibility of sagittal imbalance, LD from intraoperative X-ray was 2.93° for MRI and 5.5° for standing X-ray; these LD values were less than the 10.2° in our study. Nevertheless, the previous study also found no difference in LD between standing and supine imaging in the overall group (p=0.1205); however, in their subgroup of patients with flexible curves, defined as >10° LD between standing X-ray and supine MRI, LD was significantly less for MRI than for standing X-ray (3.08° vs. 11.46°, p=0.0009). Harimaya et al. [10] found that preoperative supine X-rays more accurately predicted intraoperative lordosis. In our study, LD was significantly smaller on CT/MRI than on than on SLXray in patients with greater amount of PI–LL mismatch. This result indicated that lordosis on CT/MRI was closer to that on IPXray in patients with greater sagittal plane deformity. Direct comparison between these previous studies and ours is not possible primarily because of differences in patient population, as well as differences in number of patients (n=44 in the study by Harimaya et al. [10]) and surgical approaches (only 35% posterior approach in the study by Sharma et al. [16]).
One limitation of this study was that the study population underwent multilevel posterior fusions for a variety of diagnoses, including spinal stenosis, scoliosis, and kyphosis. The overall population had a C7 SVA of 4.45 cm and a PI–LL mismatch of 19°. Therefore, the 10° LD found in this population may not necessarily apply to those with more severe sagittal plane deformity. In addition, spine flexibility (i.e., amount of correction after prone positioning) varies among patients and depends on a number of factors, including number of previously fused segments and stiffness of any non-fused disc spaces. Moreover, this study did not assess the presence or severity of hip pathology, which was beyond its intended scope. Future studies may consider examining the contribution of hip joint morphology to sagittal alignment and its interaction with positional changes in LL. In addition, the Lumbar Distribution Index (LDI) was not measured in this study, because our focus was on global rather than segmental changes in lordosis. Although inclusion of LDI might have provided additional granularity, it was unlikely to alter the main findings. This remains a limitation and could be incorporated in future prospective analyses.
Conclusions
LL measured on lateral IPXrays under anesthesia was approximately 10° greater than that measured on preoperative standing radiographs or supine CT/MRI. Patients with greater sagittal deformity obtained greater passive correction after prone positioning under anesthesia, reaching up to 15° in those with PI–LL mismatch >30°. In patients with >20° PI–LL mismatch, CT/MRI more closely predicted lordosis on intraoperative prone positioning. These factors can assist surgeons to more accurately estimate the actual amount of lordosis required intraoperatively when performing multilevel posterior lumbar fusion.
Key Points
Lumbar lordosis measured on intraoperative prone lateral X-rays under anesthesia was approximately 10° greater than that measured on preoperative standing radiographs or supine computed tomography (CT)/magnetic resonance imaging (MRI).
Patients with greater sagittal deformity obtained greater passive correction after prone positioning under anesthesia.
In patients with more significant pelvic incidence–lumbar lordosis mismatch, preoperative CT/MRI rather than standing radiograph more closely predicted lordosis on intraoperative prone positioning.
Notes
Conflict of Interest
No potential conflict of interest relevant to this article was reported.
Author Contributions
Conceptualization: IW, RW, SN, JR. Data curation: WM, AG, IW, APS, RW, SN, EK. Formal analysis: WM, AG, IW, APS, RW, SN, EK. Investigation: WM, AG, AS, EK. Project administration: JR. Writing original draft: WM, AG, IW, RW, SN, EK. Writing–review & editing: JR. Supervision: JR. Final approval of the manuscript: all authors.