Calculation of correction angle required for spinal osteotomy in ankylosing spondylitis patients with thoracolumbar kyphosis: comparison of FBI method and hilus pulmonis (HP) method.

doi:10.21203/rs.3.rs-3086812/v1

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Calculation of correction angle required for spinal osteotomy in ankylosing spondylitis patients with thoracolumbar kyphosis: comparison of FBI method and hilus pulmonis (HP) method.

https://doi.org/10.21203/rs.3.rs-3086812/v1

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Study Design：Retrospective case–control radiographic study

Purpose

The purpose of this study is to compare hilus pulmonis (HP) method and full balance integrated (FBI) method in calculating the amount of correction angle needed to achieve a satisfactory sagittal balance for ankylosing spondylitis (AS) patients with thoracolumbar kyphosis, then, to explore if HP method can be safely used instead when C7 vertebra is invisible.

Methods

38 AS patients (34 men, 4 women; mean age, 38.4 yr) who underwent one-level PSO for thoracolumbar kyphosis deformity were retrospectively collected. The planned osteotomy angles were calculated on preoperative full-length spinal radiographs using both methods (HP and FBI). Pre- and post-operative sagittal parameters were also assessed, including Cobb T1- S1, global tilt, sagittal vertical axis (SVA), pelvic tilt, lumbar lordosis, femoral obliquity angle (FOA).

Results

All radiographical parameters had significantly improved from preoperative to postoperative. The T1–S1 sagittal Cobb angles significantly improved from 43.5 ° (range, -12.1° ~84.5°) preoperatively to 9.2° (range, −15.7°~33.7°) postoperatively (P < 0.001). All radiographical

parameters had significantly improved from preoperative to postoperative, including global tilt (52.3°~21.8°, P<0.001), SVA (139.4 ~ 58.1mm, P < 0.001), PT (33.6° ~16.8°, P<0.001),

lumbar lordosis (1.9°~-36.3°, P<0.001) and femoral tilt (22.0° ~10.1°, P<0.001). The mean osteotomy angle was 28.2° (range 20.4°~41.9°). The average predicted correction angles with FBI method and HP method were 43.70° and 51.80°, respectively. There was significant difference between these two methods in terms of predicted correction angle (P=0.001).

Conclusions

In AS patients with only thoracolumbar kyphosis, HP method had greater estimated correction angle than FBI method. Since our experience confirmed that all patients achieved the best equilibrium when C7 plumb line closed to the sacral plateau according to FBI method, HP method could not be safely used instead to calculate the estimated correction angle.

Health sciences/Diseases/Rheumatic diseases/Musculoskeletal abnormalities

Health sciences/Diseases/Rheumatic diseases/Spondyloarthritis

Ankylosing spondylitis

Thoracolumbar kyphosis

Hilus pulmonis method

FBI method

Ankylosing spondylitis (AS) is a chronic systematic inflammatory disease that primarily affects the axial skeleton, including ankylosis of the sacroiliac joints and spine [1]. About 30% of AS patients can have rigid kyphotic deformity if left untreated and cause severe daily functional impairment [2].

It has been shown that corrective surgeries in AS can achieve sagittal realignment, improve daily disability and health related quality of life [3, 4]. Correction of spinal deformity with spine osteotomy is one of the surgical options to restore sagittal balance. To get good restoration of sagittal balance, it is necessary to set a correct osteotomy angle. There are several ways to calculate the angle required for spinal osteotomy, such as “exact” angle method proposed by Yang and Ondra et al [5], full balance integrated (FBI) method presented by Le Huec et al [6], spinofemoral angle (SFA) method proposed by Lamartina et al [7] and hilus pulmonis (HP) method provided by Song et al [8].

Berjano [9] et al compared between “exact” angle method, FBI method and SFA method to verify the accuracy of planned osteotomy angle. The FBI and SFA methods obtain equivalent calculations of the amount of needed correction. The estimated correction angle with both methods is higher than that calculated with the exact trigonometric method. However, among all of these three methods, C7 centroid is needed. The visibility of C7 vertebra would highly influence the estimated osteotomy angle. It is reported that high as 24.6% of C7 inferior endplates were invisible [10]. The calculated angle would probably be inaccurate if C7 is invisible, which may lead to unsatisfactory surgical and clinical outcomes.

On the other hand, with HP method, HP rather than C7 centroid is used. It claimed that by using the planned angle calculated with HP method, health related quality of life scores had

significantly improved after surgical correction of thoracolumbar kyphosis in AS patients [8]. However, HP method was based on the theory that HP is the body’s gravity center, which still needs to find scientific evidences to support. To the best of our knowledge, there is still no report to compare HP method with other methods in terms of accuracy of estimated osteotomy angle. The difference between HP method and others is still unknown.

The aim of this study is to compare HP method with other methods in calculating the amount of correction needed to achieve a satisfactory sagittal balance; Then, to explore if HP method can be safely used instead when C7 vertebra is invisible. Since SFA method obtained equivalent value of planned correction angle when compared to FBI technique [9], we would do the comparison between HP method and FBI method.

Ethics approval

by Clinical Research Ethics Committee Beijing Tongren Hospital, Capital Medical University was obtained before the beginning of this study, and all of the methods were performed in accordance with the guidelines and regulations of the ethics review board. The request for waivers of informed consent was approved by Clinical Research Ethics Committee Beijing Tongren Hospital, Capital Medical University. The current study retrospectively reviewed patients with thoracolumbar kyphosis secondary to AS, who underwent one-level pedicle subtraction osteotomy in one institution between July 2017 and September 2019. Patients with kyphotic Cobb angle of T1-S1 greater than 90°, concurrent ankylosed cervical spine were excluded [8]. Also, patients with unclear either C7 vertebra or HP were ruled out, only patients with clear both C7 vertebra and HP were included.

Standing full-length spine radiographs of patients in neutral, unsupported position were taken both pre-operatively and post-operatively, including the whole spine and pelvis. On these radiographs the following parameters were measured by two experienced spine surgeons. Cobb T1–S1, as the angle between upper endplate of T1 and S1[8]; sagittal vertical axis (SVA), as the distance measured between C7 plumb line (C7PL) and posterosuperior corner

of S1[11], considered positive if the C7PL was anterior to the posterosuperior corner of S1,

and negative if the C7PL was posterior to the posterosuperior corner of S1; global tilt, as the

angle formed by a line connecting C7 centroid to the midpoint of the S1 endplate and the other line connecting hip axis to the midpoint of the S1 endplate, a parameter least affected by patient positioning[12], was also measured; Pelvic incidence (PI), as the angle between the line perpendicular to the S1 endplate at its midpoint and the line connecting this point to hip axis[13]. Pelvic tilt (PT), as the angle formed by a line connecting hip axis and the midpoint of S1 endplate and the vertical line [13]; lumbar lordosis, as the angle between the upper endplate of T12 and S1 [14]. Femoral obliquity angle (FOA), as the angle between the femoral axis and the vertical line [6]. Osteotomy angle was defined the change in Cobb angle of targeted vertebra pre- and post-operatively (Fig. 1).

On the pre-operative X-rays the predicted surgical correction angle in each patient was calculated by two experienced spine surgeons using FBI technique and HP method. Then the values of predicted correction angle were compared.

FBI method [6] is to calculate the sum of three angles: angle of C7 translation (C7TA), femoral obliquity angle (FOA), and angle of pelvic tilt compensation (PTCA). C7TA is the angle of posteriorly translate C7 centroid over the posterior superior corner of S1 endplate through osteotomy at targeted vertebra. FOA reflects the amount of hip/knee flexion. PTCA is set based on their experience, 10⁰ are added when pelvic tilt is more than 25⁰, and 5⁰ are added when pelvic tilt is 15⁰−25⁰ (Fig. 2).

HP method [8] is based on location of the HP as a center of trunk gravity in AS patients. the ideal PT is calculated according to formula of Vialle [15] (PT = (0.37 × PI) − 7°) based on the patient’s PI. Then the HP is shifted to pelvic neutral position line and the predicted osteotomy angle is obtained (Fig. 3).

Statistical Analysis

All data were presented as mean ± standard deviation. The paired sample t test was used to compare preoperative and post-operative data, and to compare between FBI method and HP

method in terms of values of predicted osteotomy angle. All analyses were performed using SPSS computer software (version 26; SPSS, Chicago, IL, USA). A p-value of < 0.05 was considered as statistically significant.

A total of 38 AS patients (34 men, 4 women; mean age, 38.5 year, age range, 24–63 year) who underwent one-level PSO for thoracolumbar kyphosis deformity were collected. Of these patients, the numbers of patients who received one-level PSO performed at T11, T12, L1, L2, L3 were 3, 2, 2, 22, 9 respectively (Table 1).

Table 1

Demographic data of patients
Male	Female	Age (yr)	Osteotomy site and case number
			T11	T12	L1	L2	L3
34	4	38.5 ± 10.5	3	2	2	22	9

The T1–S1 Cobb angles preoperatively and postoperatively were 43.5 ° (range, -12.1° ~84.5°) and 9.2° (range, − 15.7°~33.7°), respectively (P < 0.001). All radiographical parameters had significantly improved from preoperative to postoperative, including global tilt (52.3°~21.8°, P < 0.001), SVA (139.4 ~ 58.1mm, P < 0.001), PT (33.6° ~16.8°, P < 0.001), lumbar lordosis

(1.9°~-36.3°, P < 0.001) and femoral tilt (22.0° ~10.1°, P < 0.001) (Table 2).

Table 2

Comparison of Preoperative and Postoperative Spinopelvic Parameters
	Preop value	Postop value	P-value
CobbT1-S1 (º)	43.5 ± 22.5	9.2 ± 15.1	0.00*
Global tilt(º)	52.3 ± 8.1	21.8 ± 9.9	0.00*
Lumbar lordosis (º)	1.9 ± 13.7	-36.2 ± 13.8	0.00*
Pelvic tilt (º)	33.6 ± 8.0	16.8 ± 9.9	0.00*
FOA (º)	22.0 ± 8.7	10.1 ± 3.0	0.00*
SVA (mm)	139.4 ± 55.4	58.1 ± 37.7	0.00*
*: p < 0.05 (2-tailed); FOA: femoral obliquity angle

The mean osteotomy angle was 28.2° (range 20.4°~41.9°). The average predicted correction angles with FBI method and HP method were 43.70 and 51.80, respectively. There was significant difference between these two methods in terms of predicted correction angle (P = 0.001) (Table 3).

Table 3

Comparison of mean values of planned correction angles with two different methods
planned correction (º)		(Paired t-test P value)
FBI	43.7 ± 14.3
HP	51.8 ± 21.3	0.001

In AS patients with thoracolumbar kyphosis, sagittal imbalance may cause stooped and downward-looking posture, which highly limited patient’s activities of daily living [13, 14]. Sagittal balance is a key point to have a better long-term prognosis.In order to be able to stand

and walk in an upright position, restoration of sagittal spinal balance is necessary. Usually, corrective spinal osteotomy would be considered to restore the sagittal balance of the ankylosed spine, the capacity to look straight forward, and improve the patient’s quality of daily life. PSO is one of the efficient surgical procedures to treat fixed spinal deformities that require major correction. This type of osteotomy enables a gain of average correction 29–38⁰[16]. To achieve sagittal spinal balance, the value of correction angle required at the osteotomy site should be calculated individually for every patient [17]. In the process of preoperative planning, it is essential to determine how much correction is needed.

Predesigned osteotomy angel is an indispensable parameter to restore spine imbalance to avoid insufficient correction.

Several methods were proposed to help plan the estimated osteotomy angle for PSO. Yang and Ondra et al. [5] provided an “exact method”, which was based on C7 plumb line passing through posterior superior corner of S1 endplate by osteotomy. However, this “exact” method did not consider parameters of pelvic compensation. High PT suggestive of pelvic retroversion were highly associated with walking disability and poor quality of life. Restoration of PT < 20⁰ is one of the major objectives of sagittal realignment in surgical treatment of adult spinal deformity [13, 18]. Insufficient correction of sagittal deformity might lead to failure of deformity correction in the future.

FBI method, along with SFA method overcome the defects of “exact” method. The FBI index is the sum of angle of C7 translation(C7TA), angle of femur of obliquity (FOA) and angle of tilt compensation (PTCA) [6]. It is based on a global analysis of the full body balance, and took pelvic and lower limb compensation into consideration. The value of the FBI index was highly associated with clinical outcomes [19]. Insufficient correction following PSO is a risk factor for mechanical complications [20]. SFA method is to calculate the angle formed by the femoral axis and the line drawn from the center of C7 to the point where the vertical line from the posterior end of S1 plate intersects the level of planned osteotomy. SFA equals the sum of femoral flexion angle plus the angle of C7 translation [7]. Berjano et al [9] found the FBI and SFA methods obtained equivalent the amount of needed correction after comparison. In our results, the correction angle required with FBI method was 43.70. FBI method had been verified by clinical practice. When C7 plumb line closed to the sacral plateu, the best equilibrium was achieved.

Our results showed that there was significant difference between HP method and FBI method in terms of estimated osteotomy angle in this group of AS patients. This information is important in pre-op planning for AS patients.

HP method was based on the theory that the HP was the so called “gravity center of trunk”, then shifted HP to the pelvic neutral position line by osteotomy to calculate the estimated correction angle [8]. As reported, about 25% of C7 inferior endplates were invisible [10]. If HP method were proved to be accurate, it would be an ideal alternative method to calculate the correction angle required when C7 centroid was invisible.

But in our results, the estimated correction angle with HP method was 51.8°, which was significantly different from that with FBI method (43.7°). That is, HP method had greater estimated correction angle than FBI method. Our findings suggested HP method be problematic. Similarly, Wang et al. [23] also noticed the problems with HP method. They used the HP method for the planning of osteotomy angel. They found that pelvic tilt (PT) was hardly restored to calculated value postoperatively as expected according to HP method in their research. The actual postoperative PT and the theoretical postoperative PT (tPT) have significant difference. This may have important clinical implications. To investigate the reason, we searched several literatures on anatomy and found that how the individual difference of the physique affects on the center of gravity. There is no evidence that the hilum is the center of gravity of the trunk. It is very difficult to determine the center of gravity of the

living body, because the body is non-geometrical and the density of the trunk is not

homogeneous. Katsumasa Fujikawa had researched the center of gravity of the trunk by cadaver. The results were illustrated in Fig. 4. The intersection of the two solid lines was the center of gravity of the trunk. None of the cases did the center of gravity lie at the hilus pulmonis[21]. Even in some cases who are being out of balance, the center of gravity does not necessarily have to be on the body (Fig. 5) .

This current study included AS patients only with thoracolumbar kyphosis, but without concurrent ankylosed cervical kyphosis. For AS patients with cervical kyphosis, other parameters such as CBVA should also be considered when estimated osteotomy angle is planned [16]. Also, for AS patients with thoracolumbar kyphosis and concurrent ankylosed hips, arthroplasty surgeons may be consulted to determine whether total hip replacement or spinal corrective osteotomy would be performed first [22] .

Data availability” statements

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

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No competing interests reported.

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Calculation of correction angle required for spinal osteotomy in ankylosing spondylitis patients with thoracolumbar kyphosis: comparison of FBI method and hilus pulmonis (HP) method.

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Abstract

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Introduction

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Statistical Analysis

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