Adult spinal deformity (ASD) is a complex condition defined by alteration of normal physiologic spinopelvic parameters. Correction of these parameters results in improvement in quality of life measures.1–5 In adult patients, recent literature has shown that correction of sagittal plane deformity, rather than coronal plane deformity, is more closely correlated with improved quality of life measures.3 Surgical correction of sagittal plane deformity requires proper preoperative planning, and is often achieved via posterior spine fusion and osteotomy.6,7 However, ideal sagittal plane correction can be difficult to achieve. There is the risk of over-correction or undercorrection, leading to complications such as proximal junctional kyphosis. The goals of surgical correction regarding certain spinopelvic parameters have been defined in the literature: sagittal vertical axis (SVA) of less than 50 mm anterior, T1 spinopelvic inclination of less than 0°, pelvic tilt (PT) of less than 20°, and pelvic incidence minus lumbar lordosis (PI-LL) of less than 10° in adults.8,9
Several mathematical and graphical methods of preoperative surgical planning have been proposed. The mathematical methods vary in their goals and level of complexity.1,10–14 Some formulas have been shown to be useful in predicting the angle of osteotomy resection needed to restore sagittal balance, whereas others have provided targets for postoperative spinopelvic parameters. Overall, these mathematical methods have determined that pelvic parameters are important in predicting adequate postoperative alignment, but these formulas may be too cumbersome for routine clinical use.1,7,9–13 Previously described graphical methods have provided a visual approach that may be more suitable for the clinical setting. These approaches have also had several disadvantages, such as neglecting the role of the pelvis in sagittal alignment11 or necessitating the visualization of femoral shafts in the full balance integrated technique.15
Preoperative surgical planning software has recently been developed, providing a visual method for surgical planning.7 This method can help determine if the proposed procedure (eg, osteotomy type, degree of wedge, and level) will result in adequate global correction, and correction of individual parameters. Furthermore, this has led to the development of patient-specific rods matching the contour of the software-generated surgical plan. To the authors' knowledge, no study to date has described the initial clinical outcomes of this approach to ASD correction. The objectives of this study were to (1) describe the process of preoperative planning and creation/utilization of patient-specific spine rods, (2) perform a preliminary analysis of correspondence between planned and postoperative correction, and (3) determine if this approach results in adequate postoperative spinopelvic alignment.
Materials and Methods
After institutional review board approval was obtained, a retrospective case series was performed. Medical records and radiographic images were analyzed from consecutive patients with ASD who underwent instrumented posterolateral spinal fusion and corrective lumbar or thoracic osteotomy following preoperative planning and the creation of patient-specific spine rods. Patients were screened from a database including the surgical logs of 3 surgeons, between 2014 and 2015, at a single institution. The following variables constituted the inclusion criteria: (1) age older than 18 years; (2) diagnosis of ASD including the etiologies of scoliosis or kyphosis, sagittal imbalance, post-traumatic kyphosis, idiopathic flat back syndrome, and ankylosing spondylitis; (3) operation consisting of greater than 5 level instrumented posterolateral fusion with patient-specific spine rods, contoured to specifications obtained through preoperative planning software; (4) lumbar and/or thoracic osteotomy including Smith-Peterson osteotomy (SPO), pedicle subtraction osteotomy (PSO), or vertebral column resection (VCR); and (5) any amount of follow-up as long as immediate (<1 month) postoperative imaging was available. Exclusion criteria consisted of (1) poor sagittal radiograph quality making spinopelvic parameters imprecise or absence of proper long-standing sagittal radiographs, (2) error in or loss of preoperative planning data, or (3) failure to implant the patient-specific rod for any reason. Patients with any amount of follow-up were included because this was an initial study analyzing the immediate correction in relation to the preoperative planning, using the planning tools and subsequent rod contouring for the specific patient. An independent expert performed preliminary analysis and extraction of the demographic, medical, and radiographic data.
Preoperative Planning/Patient-Specific Spine Rod Approach
The perioperative approach in this study consisted of the following: (1) preoperative planning using the software Surgimap (Nemaris Inc, New York, New York); (2) ordering patient-specific spine rods, UNiD (MEDICREA Group, New York, New York), contoured to the preoperative plan software-generated specifications; and (3) performing a postero-lateral instrumented spine fusion with corrective osteotomies and implantation of UNiD patient-specific rods. Minor in situ rod contouring and rod shortening was performed as needed. The rod company, MEDICREA Group, and the surgical planning software company, Nemaris Inc, are integrated so that patient-specific posterior rods can be ordered through the software to match the material, diameter, length, and exact contour (including LL and thoracic kyphosis [TK]) specified by the surgical plan. The authors used preoperative long-standing sagittal radiographs for preoperative planning in all cases. The software enabled them to plan the location, number, type, and angle of the osteotomies. The software also allowed them to simulate the correction, which enabled them to design the contour of the desired rod for the anticipated correction of the spine.
The surgical plan was determined by the individual surgeon and confirmed at a multidisciplinary indications conference. All of the patients' preoperative spinopelvic parameters were initially mapped. In patients with severe sagittal imbalance, generally greater than 10 cm anterior, the authors planned a PSO and/or multiple SPOs often with anterior intervertebral cages to gain additional correction. The Surgimap Wedge Osteotomy Tool feature was used to cut and splice the preoperative imaging, displaying a visual representation of global sagittal alignment and corresponding predicted spinopelvic parameters (Figure 1). The authors used the Surgimap Wedge Osteotomy Tool feature to simulate the appropriate osteotomy size (generally around 30° for a PSO and approximately 12° for a SPO or Ponte osteotomy). In the thoracic regions, they used smaller angular values (on average, 20° to 25° wedges for PSOs and 7° wedges for SPOs).
Preoperative planning analysis. Preoperative imaging with corresponding baseline spinopelvic parameters (A). Surgical planning imaging with a 31° pedicle subtraction osteotomy simulated at L4 and corresponding predicted spinopelvic parameters. Surgical planning Wedge Osteotomy Tool (Nemaris Inc, New York, New York) feature was used to splice imaging to simulate predicted postoperative imaging (B). Postoperative imaging with corresponding spinopelvic parameters. Completed operation deviated from surgical plan; planned L4 pedicle subtraction osteotomy vs completed L3 pedicle subtraction osteotomy (C). Abbreviations: LL, lumbar lordosis; PI, pelvic incidence; PI-LL, pelvic incidence minus lumbar lordosis; PT, pelvic tilt; SVA, sagittal vertical axis; TK, thoracic kyphosis.
Full-length anteroposterior and lateral radiographs were obtained in a standard fashion preoperatively and at a routine follow-up in the postoperative period. Immediate postoperative (within 1 month) standing radiographs were obtained for all patients. The following spinopelvic measurements were obtained for evaluation and planning of the spinal deformity correction: SVA, PI, sacral slope, PT, LL, TK, and PI-LL.8,16
Following osteotomy planning via the Surgimap Wedge Osteotomy Tool feature, surgeons approximated postoperative PT and SVA. The pelvis was positioned such that the desired PT was obtained, and the correction planned allowed for an anticipated SVA of less than 50 mm anterior. The SVA and PT as well as other parameters were recorded. This method is based on the notion that the pelvis normally compensates for inadequate spinal alignment.6,7,17 Therefore, if the planned osteotomy allows for a corrected SVA, then the patient's pelvis will relax, leading to a decreased PT. However, in a patient with a severely retroverted pelvis (PT >35°) and at high risk for loss of pelvic mobility (ie, of an older age or having hip flexion contractures),6 a lack of adequate pelvis relaxation was considered in surgical planning. In such cases, the authors planned a more conservative osteotomy to prevent overcorrection.
Because PT and SVA are intimately related, increases in PT directly result in decreases in SVA.6–8,17 Thus, it is difficult to simultaneously predict both the exact postoperative PT and the exact corresponding SVA values. Therefore, the authors calculated their correspondence between planned and postoperative SVA/PT by analyzing the postoperative radiographs using the Surgimap Wedge Osteotomy Tool feature again. Their primary goal was to achieve adequate spinopelvic balance, regardless of the exact PT and SVA values. Once the preoperative plan was finalized, rods were contoured to the corrected patient image. The authors ordered 6.0-mm titanium rods. Intraoperatively, slight in situ contouring was conducted as needed. The rods were routinely ordered long to allow for patient variance and to accommodate the learning curve of the surgeons in designing the rods.
The patients' medical records were evaluated by a single individual. Patient age, gender, body mass index, and primary diagnosis were recorded. Surgical variables including the number of fusion levels, location of fusion levels, pelvic fixation, and osteotomy type and location were also collected. Spinopelvic measurements performed by the surgeons pre- and postoperatively were gathered and analyzed (Figure 1).
Outcome analysis included assessment of preoperative, planned, and postoperative variables; evaluation of a treatment effect (difference between postoperative and preoperative values of all studied characteristics)18; and assessment of the correspondence between planned and achieved results (difference between postoperative and planned values). The values between the planned and the achieved treatment effects were defined. Categorical data were used to assess the predictive capability of the planning approach, in particular the ability to predict normal/abnormal postoperative sagittal balance.
Mean and SD were used to define preoperative, planned, and postoperative values; mean difference and corresponding SD were used to evaluate treatment effects as well as the difference between postoperative and planned values. Statistical significance was defined via paired t test.19 The correspondence between planned and achieved treatment effects was defined by linear regression analysis using the coefficient of determination (R2), root mean square error, and P value by F test.19 Sensitivity, specificity, and positive and negative predictive values were defined to assess predictive capability of the planning method to obtain the “ideal” sagittal balance. The sagittal balance was considered normal if the SVA was less than 50 mm; statistical significance was defined using the chi-square test.20 The correlation between the treatment effects of the parameters was performed to determine which parameters provide the most significant impact on the sagittal balance correction.
The initial patient population meeting the inclusion criteria totaled 26 patients. Eight patients were excluded from the study: 5 patients with poor imaging and 3 patients for whom preoperative planning was used but, for various reasons, patient-specific spine rods were not implanted.
Eighteen patients were included in the final analysis: 8 men and 10 women with ages ranging from 43 to 65 years (Table 1). The number of fused levels ranged from 6 to 15; 12 cases had fixation to the ilium, 5 to S1, and 1 to L4. Preoperative body mass index ranged from 20 to 52 kg/m2. The following osteotomies were performed: PSO (n=12), SPO (n=1), PSO and SPO (n=4), and VCR (n=1). Preoperative parameters are provided in Table 2.
Demographics of the Total Population (N=18)
Spinopelvic Parameters Before and After Surgical Correction
Planning achieved normal parameters in 16 of 18 cases. Continued anterior imbalance with staged correction was expected in 2 cases (preoperative SVAs of 189 mm and 156 mm). The planned spinopelvic characteristics are provided in Table 2.
Normal sagittal balance was observed in 13 patients. Continued anterior imbalance was seen in 5 patients, being anticipated in 2. The spinopelvic characteristics that were reached are presented in Table 2. All spinopelvic parameters showed a significant difference postoperatively compared with preoperatively (P≤.003), except for PI (Table 2). Only TK and LL showed a significant (P≤.03) difference between planned and postoperative values, exceeding the level of the planned correction (Table 2).
The authors found a statistically significant (P≤.011) correspondence between the planned and the postoperative treatment effects for SVA, LL, and PI-LL. The coefficient of determination (R2) ranged from 0.36 to 0.45 (Table 3).
Correspondence Between Planned and Real Postoperative Changes of Spinopelvic Parameters by Linear Regression Analysis
A statistically significant correlation was found between the treatment effects of SVA, LL, and PI-LL (|0.75|>R<|0.99|; P<.001). The treatment effects of LL and PI-LL had a significant correlation with changes of PT (|0.51|>R<|0.55|; P<.03).
The predictive capability of the planning for postoperative sagittal balance had a sensitivity of 60%, a specificity of 100%, a positive predictive value of 100%, and a negative predictive value of 87% (P=.023) (Table 4).
Predictive Capability of the Evaluated Method Concerning Postoperative Sagittal Imbalance
Etiologies of the Difference Between Postoperative and Planned Outcomes
An analysis of the differences between postoperative and planned outcomes revealed several findings. Three patients had unplanned, continued sagittal plane imbalance (Figure 1). Four patients appeared to have a larger angle osteotomy performed than what was originally planned, with additional contouring of the patient-specific rods (Figure 2). Six patients had greater than the mean absolute PT variance (mean, 5.8°). Two of 6 patients with PT variance had deviations of the plan in which the planned osteotomy differed from the completed osteotomy and 3 of 6 patients appeared to have a larger PSO performed than what was originally planned. One of these cases was attributed to an inflexible pelvis, with the pelvis not relaxing as expected. Nine patients had a greater than mean absolute TK variance (mean, 9.8°). The majority of these seemed to be at least partially attributed to failure to account for immediate development of mild proximal junctional kyphosis, and/or reciprocal change in the nonfused thoracic region (Figure 3). Eight patients had greater than the mean absolute SVA variance (mean, 34.9 mm). In 4 of the 8 patients, the outcomes deviated slightly from the plan, including failed PT prediction, failed prediction of changes in TK, and osteotomy plan mismatch.
Planned vs completed osteotomy angle. Preoperative imaging with a simulated 30° pedicle subtraction osteotomy, with a corresponding Cobb angle of 33° drawn across the neighboring vertebrae endplates (A). Postoperative imaging in the same patient, with a corresponding Cobb angle of 45° across neighboring vertebrae endplates (B).
Reciprocal kyphosis. Preoperative Cobb angle encompassing the nonfused thoracic region, superior T10 endplate to superior T4 endplate, with a corresponding angle of 17° (A). Postoperative Cobb angle encompassing the nonfused thoracic region, superior T10 endplate to superior T4 endplate, with a corresponding angle of 36° (B).
This study of 18 consecutive patients with ASD had the primary goal of describing the initial outcomes and challenges of a novel approach to ASD correction: detailed preoperative planning and the subsequent design and implantation of patient-specific rods. On average, the patient population had a significant amount of deformity defined by baseline spinopelvic parameters. Overall, the use of preoperative planning and patient-specific rods resulted in greatly improved postoperative sagittal balance in all 18 operated on cases, with normal postoperative SVA (<50 mm) in 13 of 16 planned cases. The ability to determine whether the amount of correction achieved during surgery, with the patient prone, will match the desired parameters when the patient is erect is a challenge for deformity surgeons. This result was reached by significant correction of LL, PT, and PI-LL. Further, the authors' approach resulted in all 18 patients having PI-LL values of less than 10°. In 4 patients with abnormal postoperative sagittal balance, preoperative SVA exceeded 100 mm. The correction of such severe imbalance may require more than one surgery, leading to insufficient results in the pilot analysis. These 4 patients should not be viewed as having an inferior outcome, but instead are one step closer to an adequate sagittal balance.
The applied approach allowed for good prediction of the postoperative sagittal correction with relatively high sensitivity and near absolute specificity and positive predictive value. The correspondence between planned and postoperative spinopelvic variables was moderate and statistically significant for SVA, LL, and PI-LL. The correlation analysis suggested that corrections of the LL and the PI-LL are likely to provide the most sufficient input in the sagittal plane correction. On average, the authors' approach underestimated the amount of lordosis and PT relaxation that would be obtained through the proposed osteotomy procedure. They did not account for compensatory TK postoperatively, including mild forms of proximal junctional kyphosis and reciprocal changes in the thoracic region (Figure 3).6 However, the impact of the TK on sagittal balance improvement was not significant in this study.
The authors attribute the variance in this case series to several main etiologies. The first and most obvious reason for variance resulted from deviation from the original surgical plan. In 7 of 18 patients, a significant intraoperative change in surgical plan was made due to patient anatomy, patient status, and/or subjective surgeon discretion. These changes included changing the level of the osteotomy, changing the type of osteotomy, or adding/subtracting SPOs. This caused deviation between the planned and the postoperative spinopelvic characteristics, in particular LL and TK (Table 2).
A second etiology for variance included some discrepancy between the angle of the planned osteotomy and the angle of the actual completed osteotomy. This was approximated by calculating the Cobb angle across the most adjacent endplates of neighboring vertebrae to the osteotomy vertebrae on preoperative and postoperative radiographs (Figure 2). Future improvements to this approach may involve methods to determine the exact osteotomy wedge obtained during surgery, or intraoperatively resecting the exact osteotomy wedge as originally planned.
A third etiology for variance pertained to failing to account for changes in kyphosis postoperatively, including proximal junctional kyphosis and reciprocal kyphosis. Recent literature has described the importance of reciprocal kyphosis or changes in kyphosis of the nonfused thoracic spine in postoperative spinopelvic alignment.8,21,22 In one series, this led to poor postoperative spinal alignment in 18 of 34 studied patients.21 If the fusion span was below T4, increases in kyphosis up to T1 could increase both the measured TK (T12-T4) and the SVA, due to the direct relationship between kyphosis and sagittal alignment. If the fusion span included T4, increases in kyphosis affected SVA but not TK because this was calculated from T4 to T12. Future improvements in variance may result from trying to predict increases in kyphosis. One method that the current authors have implemented is using the Surgimap Wedge Osteotomy Tool feature to create small reverse cuts across intervertebral disks above the fusion construct to create a more harmonious thoracic contour (Figure 4). Studies are needed to determine whether this method better approximates postoperative kyphosis and SVA.
Planning for reciprocal kyphosis. Preoperative radiograph (A). Surgical planning imaging with 3 wedges, simulating 3° of reciprocal kyphosis per disk level above the fusion construct (B).
A final etiology for variance is related to predictions of pelvic relaxation. Prior studies have aimed to predict postoperative PT. One of the more recent studies has determined a mathematical formula for predicting adequate correction, but this may be too complex for the clinical setting.1 Although the approach of the current study was not able to predict PT perfectly, it resulted in targets of adequate spinopelvic balance in the majority of the patients.
This study reports some of the initial outcomes and challenges of a novel approach to ASD correction. The ability to determine whether the amount of correction achieved during surgery, with the patient prone, will match the desired parameters when the patient is erect is a challenge for deformity surgeons. The data from this pilot study, introducing a way to meticulously plan corrections and personalize the rods for each patient, are promising. Along with the limitations of a small patient population, deviation of surgical plans in some cases, limited patient follow-up, retrospective design with the inherent risk of selection bias, and absence of a comparative group, the biggest challenge for the surgeons was in the contour of the spine to the rods and not the rods to the spine. This paradigm shift was responsible for the variations seen in this pilot study. The group has now performed more than 35 implantations of UNiD patient-specific rods and the intermediate results continue to show improvement in the ability to accurately plan and execute deformity corrections.
Now, the mean parameters must be determined for patients of different age groups, as it is clear that one size does not fit all. With a more physiologic correction, it may be possible to diminish complications such as proximal junctional kyphosis. Although sagittal balance is being improved, the issue is whether the correct parameters are being examined to judge the outcome of patients with ASD. Furthermore, the time saved no longer contouring rods during surgery, along with the impact on the biomechanical strength if metal notching is avoided, should be quantified due to the obvious potential benefits.23–25 Studies are needed to further validate and determine the long-term outcomes of this approach to personalized medicine in spine surgery.
- Lafage V, Schwab F, Vira S, Patel A, Ungar B, Farcy JP. Spinopelvic parameters after surgery can be predicted: a preliminary formula and validation of standing alignment. Spine (Phila Pa 1976). 2011; 36(13):1037–1045. doi:10.1097/BRS.0b013e3181eb9469 [CrossRef]
- Glassman SD, Berven S, Bridwell K, Horton W, Dimar JR. Correlation of radiographic parameters and clinical symptoms in adult scoliosis. Spine (Phila Pa 1976). 2005; 30(6):682–688. doi:10.1097/01.brs.0000155425.04536.f7 [CrossRef]
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- Schwab F, el-Fegoun AB, Gamez L, Goodman H, Farcy JP. A lumbar classification of scoliosis in the adult patient: preliminary approach. Spine (Phila Pa 1976). 2005; 30(14):1670–1673. doi:10.1097/01.brs.0000170293.81234.f0 [CrossRef]
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- Ames CP, Smith JS, Scheer JK, et al. Impact of spinopelvic alignment on decision making in deformity surgery in adults: a review. J Neurosurg Spine. 2012; 16(6):547–564. doi:10.3171/2012.2.SPINE11320 [CrossRef]
- Akbar M, Terran J, Ames CP, Lafage V, Schwab F. Use of Surgimap spine in sagittal plane analysis, osteotomy planning, and correction calculation. Neurosurg Clin North Am. 2013; 24(2):163–172. doi:10.1016/j.nec.2012.12.007 [CrossRef]
- Lafage V, Schwab F, Patel A, Hawkinson N, Farcy JP. Pelvic tilt and truncal inclination: two key radiographic parameters in the setting of adults with spinal deformity. Spine (Phila Pa 1976). 2009; 34(17):E599–E606. doi:10.1097/BRS.0b013e3181aad219 [CrossRef]
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- Ondra SL, Marzouk S, Koski T, Silva F, Salehi S. Mathematical calculation of pedicle subtraction osteotomy size to allow precision correction of fixed sagittal deformity. Spine (Phila Pa 1976). 2006; 31(25):E973–E979. doi:10.1097/01.brs.0000247950.02886.e5 [CrossRef]
- Rose PS, Bridwell KH, Lenke LG, et al. Role of pelvic incidence, thoracic kyphosis, and patient factors on sagittal plane correction following pedicle subtraction osteotomy. Spine (Phila Pa 1976). 2009; 34(8):785–791. doi:10.1097/BRS.0b013e31819d0c86 [CrossRef]
- Schwab F, Lafage V, Patel A, Farcy JP. Sagittal plane considerations and the pelvis in the adult patient. Spine (Phila Pa 1976). 2009; 34(17):1828–1833. doi:10.1097/BRS.0b013e3181a13c08 [CrossRef]
- Yang BP, Ondra SL. A method for calculating the exact angle required during pedicle subtraction osteotomy for fixed sagittal deformity: comparison with the trigonometric method. Neurosurgery. 2006; 59(4)(suppl 2):ONS458–ONS463.
- Le Huec JC, Leijssen P, Duarte M, Aunoble S. Thoracolumbar imbalance analysis for osteotomy planification using a new method: FBI technique. Eur Spine J. 2011; 20(suppl 5):669–680. doi:10.1007/s00586-011-1935-y [CrossRef]
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Demographics of the Total Population (N=18)
|Age, mean (SD), y||66 (11)|
|Gender, No. (%)|
| Female||10 (55.6)|
| Male||8 (44.4)|
|Body mass index, mean (SD), kg/m2||30.1 (7.3)|
|American Society of Anesthesiologists score, mean (SD)||2.7 (0.5)|
|Previous posterior spine fusion, No. (%)|
| Primary||7 (38.9)|
| Reoperation||11 (61.1)|
|Fusion levels, mean (SD), No.||11.4 (3)|
|Osteotomy type, No. (%)|
| Pedicle subtraction osteotomy||12 (66.6)|
| Smith-Peterson osteotomy||1 (5.6)|
| Pedicle subtraction osteotomy and Smith-Peterson osteotomy||4 (22.2)|
| Corpectomy||1 (5.6)|
Spinopelvic Parameters Before and After Surgical Correction
|Preoperative||Planned||Postoperative||Mean (SD)||P||Mean (SD)||P|
|Sagittal vertical axis, mm||96.8 (56.8)||14.3 (22.4)||21.8 (37.1)||−75.1 (54.4)||<.001||7.5 (43.3)||.478|
|Thoracic kyphosis||37.7° (14.4°)||36.8° (15.2°)||44.9° (13.4°)||7.2° (8.8°)||.003||8.1° (8.4°)||<.001|
|Lumbar lordosis||29.3° (18.9°)||56.1° (16.7°)||62.6° (12.5°)||33.3° (14.7°)||<.001||6.6° (11.8°)||.03|
|Pelvic incidence||58.6° (12.2°)||58.6° (12.2°)||58.4° (13.2°)||−0.1° (1.8°)||.834||−0.1° (1.8°)||.834|
|Pelvic incidence minus lumbar lordosis||29.2° (16.7°)||0.9° (14.7°)||−4.1° (7.5°)||−33.3° (14.7°)||<.001||−5.0° (13.8°)||.147|
|Sacral slope||26.4° (10.8°)||38.9° (11.7°)||40.7° (14.0°)||14.3° (8.0°)||<.001||1.9° (8.4°)||.376|
|Pelvic tilt||32.0° (10.9°)||20.5° (9.6°)||17.7° (8.0°)||−14.3° (7.8°)||<.001||−2.8° (7.7°)||.144|
Correspondence Between Planned and Real Postoperative Changes of Spinopelvic Parameters by Linear Regression Analysis
|Characteristic||Coefficient of Determination (R2)||Root Mean Square Error||P|
|Sagittal vertical axis, mm||0.45||44.1||.002|
|Pelvic incidence minus lumbar lordosis||0.36°||11.4°||.011|
Predictive Capability of the Evaluated Method Concerning Postoperative Sagittal Imbalance
|Predictiona||Postoperative Sagittal Balance||P|
|Abnormal (Anterior/Posterior SVA ≥50 mm)||Normal (Anterior/Posterior SVA <50 mm)|