Radiological morphology variances of osteotomized vertebra-disc complex following pedicle subtraction osteotomy for ankylosing spondylitis with thoracolumbar kyphosis: the incidence, mechanisms, and prognosis

Accepted Manuscript Title: Radiological morphology variances of osteotomized vertebra-disc complex following pedicle subtraction osteotomy for ankylosing spondylitis with thoracolumbar kyphosis: the incidence, mechanisms and prognosis Author: Sai-hu Mao, Zong-xian Feng, Bang-ping Qian, Yong Qiu PII: DOI: Reference:

S1529-9430(17)31213-5 https://doi.org/10.1016/j.spinee.2017.12.005 SPINEE 57557

To appear in:

The Spine Journal

Received date: Revised date: Accepted date:

1-7-2017 16-11-2017 11-12-2017

Please cite this article as: Sai-hu Mao, Zong-xian Feng, Bang-ping Qian, Yong Qiu, Radiological morphology variances of osteotomized vertebra-disc complex following pedicle subtraction osteotomy for ankylosing spondylitis with thoracolumbar kyphosis: the incidence, mechanisms and prognosis, The Spine Journal (2017), https://doi.org/10.1016/j.spinee.2017.12.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

1

Radiological morphology variances of osteotomized vertebra-disc

2

complex following pedicle subtraction osteotomy for ankylosing

3

spondylitis with thoracolumbar kyphosis: the incidence, mechanisms

4

and prognosis

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Authors Sai-hu Mao1, MD; Zong-xian Feng2, MD; Bang-ping Qian1,2*, MD; Yong Qiu1,2, MD 1

Spine Surgery, the Affiliated Drum Tower Hospital of Nanjing University Medical School, Nanjing, China; 2 Spine Surgery, Drum Tower Clinical Medical College of Nanjing Medical University, Nanjing, China *To whom correspondence should be addressed. E-mail: [email protected]; Address: The Affiliated Drum Tower Hospital of Nanjing University Medical School, Zhongshan Road 321, Nanjing 210008, China; Telephone number: (86)025-68182222-61011 Acknowledgements: This work was financially supported by the National Natural Science Foundation of China (Grant No. 81372009) and the Jiangsu Provincial Key Medical Center.

23

The Manuscript submitted does not contain information about medical

24

device(s)/drug(s).

25

Abstract

26

BACKGROUND CONTEXT: Inaccurate osteotomy cut along with incomplete or even

27

subluxated bone-on-bone closure of osteotomy gap following pedicle subtraction osteotomy (PSO)

28

may be disastrous, hampering the lordosing effect and increasing the likelihood of complications.

29

The inelastic yet osteoporotic spine in ankylosing spondylitis (AS) is specially predisposed to such

30

suboptimal osteotomy, while the relevant data concerning this issue is scarce. 1

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PURPOSE: To analyze the incidence of radiological morphology variances (RMV) of

2

osteotomized vertebra-disc complex (OVDC) following PSO in kyphotic AS patients,

3

conceptualize the mechanisms of the deviated morphology and investigate the prognosis.

4

STUDY DESIGN: Retrospective radiological data analysis.

5

PATIENT SAMPLE: The sample being screened comprises 71 thoracolumbar kyphotic AS

6

patients who underwent single level PSO at our hospital between March 2006 to February 2014.

7

They were stratified by the presence of bridging syndesmophytes (BS) locating within OVDC.

8

OUTCOME MEASURES: Any irregular radiological configuration of OVDC other than the

9

wedge morphology would be considered as RMV and were studied with care to fully describe and

10

classify the spectrum of deviated morphological features. Multiple spino-pelvic sagittal

11

parameters were measured to assess both the regional lordosing effect and the global realignment

12

of sagittal spinal profile.

13

METHODS: For each selected patient with confirmed RMV, the radiological morphology was

14

assessed, defined and categorized. The prognosis involving surgical corrections and maintenance

15

of spino-pelvic sagittal parameters, as well as the remodeling in disordered osteotomized vertebral

16

shape over time, were also investigated.

17

RESULTS: The incidence of RMV was 21.9% in positive BS Group (PG) and 30.8% in negative

18

BS Group (NG). Inappropriate angle and range of osteotomy accounted for the largest share (1 no.

19

for PG and 10 no. for NG, 57.9%) of mechanisms responsible for RMV, followed by vertebral

20

subluxation (VS) (5 no. for PG and 2 no. for NG, 36.8%) and failed osteotomy gap closure (1 no. 2

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for PG, 5.3%). For these patients, the mean bony lordosing effect per PSO segment was 36.0±8.9°

2

postoperatively, and decreased to 34.7±8.7° by a mean follow-up of 3yrs (p=0.076). The

3

magnitude of neighboring disc opening was significantly higher in NG (10.2±6.5 vs. 2.4±3.2°, p=

4

0.009). The global kyphosis and sagittal vertical axis were significantly corrected (77.0±21.2 vs.

5

24.4±18.8°; 160.6±72.4 vs. 48.2±38.6 mm, all p<0.001) and remained stable by the ultimate

6

follow-up (p>0.05). No devastating neurological deficits were noticed. Patients with VS and failed

7

osteotomy gap closure exclusively showed solid bone healing and adaptive remodeling without

8

rod breakage at final follow-up.

9

CONCLUSIONS: Radiological morphology variances of OVDC were a high occurrence

10

following PSO in AS, being mainly attributed to inaccurate osteotomy cut and vertebral

11

subluxation. Neighboring disc opening and rotational or translational subluxation were major

12

available remedial mechanisms strengthening the lordosing effect when that of vertebral wedging

13

was impaired and insufficient. The bone fusion and remodeling concerning the subluxated or

14

dislocated osteotomized vertebra was utterly favorable, maintaining the kyphosis correction and

15

preventing instrumentation failure.

16

Key words: ankylosing spondylitis, pedicle subtraction osteotomy, radiological morphology

17

variances, vertebral subluxation, remodeling

18

19

Introduction

20

Ankylosing spondylitis (AS) is a common cause of inelastic kyphotic spine with significant

3

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1

deterioration in physical function [1, 2]. A chain of compensation mechanisms can intervene at

2

early stages to help maintaining the sagittal balance, yet may be surpassed by when a corrective

3

surgery is indicated. A meticulous and customized lordosing osteotomy can effectively restore the

4

disturbed sagittal spinal-pelvic alignment and horizontal gaze, as well as to rectify the associated

5

disabling symptoms fully [2-6]. Currently, the use of pedicle subtraction osteotomy (PSO) has

6

become the mainstay [4, 5] .

7 8

There is an acknowledged association between range and shape of osteotomy, morphology of

9

osteotomized vertebra-disc complex (OVDC) and the resulting lordosing capability[7]. Accurate

10

osteotomy cut as scheduled is the premise for creating the best lordosing effect with minimal risk.

11

Generally, there are two main transforms of PSO being termed as closing wedge osteotomy (CWO)

12

and closing-opening wedge osteotomy (COWO)[4], and normal morphological transition in

13

between occurred frequently according to the amount of kyphosis correction warranted. However,

14

given the complexity and technical demands associated with performing PSO, the unexpected

15

disordered morphological transitions of OVDC beyond CWO and COWO are one burgeoning

16

clinical problem following extensive utility of PSO on osteoporotic and rigid spine in AS. The

17

decreased bone strength and density may make controlled angle and range of osteotomy, hinge

18

formation and closure of bone-on-bone osteotomy gap a big challenge[8]. This is particularly true

19

for spine surgeons initially contemplating a PSO procedure in AS. These unexpected and even

20

unwanted morphological transitions of OVDC may result in undesirable and, sometimes,

21

irreversible consequences involving instability of spinal column[9], impaired lordosing capability,

22

unsatisfying restored sagittal spinal alignment, increasing rates of perioperative complications and 4

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more medical cost. Currently, this issue hasn’t been analyzed thoroughly. We thus argue that the

2

non-existence of pre-established categories of unexpected disordered morphological transitions of

3

OVDC

4

complication-prevention strategies and prognosis of miscontoured OVDC for PSO in AS.

may

have

profound

consequences

for

insufficient

recognition

of

both

5 6

To the best of our knowledge, there is currently only one available study documenting surgeons’

7

experience with the mechanisms and prognosis of radiological morphology variances (RMV) of

8

OVDC following PSO for AS with thoracolumbar kyphosis[9]. This study was thus designed to

9

analyze the incidence of unexpected morphological transition between standard wedged and

10

disordered osteotomy in PSO procedures of kyphotic AS, conceptualize the deviated morphology

11

of OVDC and investigate the prognosis.

12

Methods

13

Subjects

14

In this retrospective radiographic study, consecutive thoracolumbar kyphotic AS patients

15

receiving corrective osteotomy at our institution from March 2006 to February 2014 were

16

screened according to the enrollment criteria as follows: (1) with thoracolumbar/lumbar kyphosis;

17

(2) PSO performed at lumbar vertebrae; and (3) a minimum 2-year follow-up. The indications for

18

osteotomy were poor cosmetic appearance due to a persistent stooping spine, difficulty of

19

horizontal gaze, early fatigue caused by muscle strain and restricted personal hygiene[10, 11]. The

20

exclusion criterions were applied to those with (1) additional polysegmental Smith-Petersen

21

osteotomy (SPO), continuous or interrupted two-level transpedicular wedge osteotomy; (2) PSO 5

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through pseudarthrosis; and (3) disco-vertebral destructive lesions, namely the so-called

2

Andersson lesions[12], locating within OVDC. After ethics approval was obtained from the

3

institutional review board, the medical records, radiographs and operative records were reviewed.

4

Patient demographic information were recorded including age at surgery, sex, apex of kyphosis,

5

level of osteotomy, ossification status of anterior longitudinal ligaments (ALL) within OVDC,

6

length of instrumentation, postoperative neurological status and surgical complications.

7 8

Operative techniques

9

The patient was operated in prone position on a special bow-type frame, which accommodated the

10

kyphotic spine. Pedicle screws were inserted at the planed vertebral levels. A laminectomy

11

covering the extent of the whole lamina of osteotomized vertebrae along with the caudal and

12

cephalad half of the adjacent lamina was performed. Subsequently, a wedge-shaped resection from

13

bilateral pedicles extending into the anterior vertebral column was conducted. During osteotomy

14

procedures, a provisional short rod was placed across the osteotomized vertebra to prevent early

15

collapse of the osteotomy gap and vertebral subluxation. Following completion of wedge

16

decancellation and excision of the posterior and partial lateral cortical vertebral walls, the special

17

bow-type frame was gradually straightened to achieve direct bone-on-bone contact and closure of

18

the osteotomy gap. At this very moment, if the patient’s shoulder and pelvis were in the same

19

horizontal line, the realignment of kyphotic sagittal profile was considered to be acceptable;

20

otherwise, by pushing the osteotomy site manually and compressing the adjacent pedicle screws,

21

anterior opening through disc wedging or fracture of anterior cortex would be obtained to further

22

strengthen the lordosing effect[4]. Finally, rods were contoured and fixed to maintain the kyphosis 6

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correction. Continuous monitoring of both somatosensory-evoked and motor-evoked potentials

2

(SEP and MEP) were performed during surgery.

3 4

Radiographic assessments of the morphology of the osteotomized vertebra-disc

5

complex

6

Standing lateral radiographs of the entire spine taken before surgery, before discharge and at the

7

final follow-up were obtained in all the patients. The patients were stratified into either positive

8

BS Group (PG) or negative BS Group (NG) based on the pre-operative presence of bridging

9

syndesmophytes at the disc level adjacent to the osteotomy vertebra, namely bamboo spine. The

10

morphology of OVDC was radiologically assessed, defined and categorized in PG and NG,

11

respectively. A wedging radiographic shape of the OV with or without opening of anterior

12

vertebral cortex was the anticipated optimal contour most frequently seen following a standard

13

PSO, and was termed as CWO and COWO, respectively. Any irregular radiological configuration

14

would be considered as RMV and were studied with care to fully describe the spectrum of

15

deviated morphological features. The conceptualization was then constructed and classified based

16

on a review of both the variability in deviated morphological patterns and the types of potential

17

causalities. The prognosis of numerous disordered morphological changes would be elucidated by

18

examining surgical corrections and maintenance of spino-pelvic sagittal parameters, as well as the

19

remodeling in disordered osteotomized vertebral shape over time.

20 21

For those with RMV within OVDC, multiple radiological spino-pelvic sagittal parameters were

22

assessed preoperatively, postoperatively and at the final follow-up including global kyphosis (GK), 7

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lumbar lordosis (LL), sagittal vertical axis (SVA), pelvic incidence (PI), pelvic tilt (PT) and sacral

2

slope (SS). In order to quantitatively analyze the bony- and disc-originated lordosing effect, two

3

additional sagittal parameters regarding the wedging of OVDC were evaluated (Figure 1A,B): (1)

4

Osteotomized vertebra angle (OVA): the angle between the superior and inferior endplates of the

5

osteotomied vertebra[4]; (2) The pedicle subtraction angle (PSA): the angle formed between the

6

caudal endplate of 1 suprajacent vertebra above the osteotomy vertebra and the cranial endplate of

7

1 infrajacent vertebrae below the osteotomy vertebra[13]. The total kyphosis correction obtained

8

per PSO segment was termed as osteotomy angle and defined as the difference between pre- and

9

post-operative PSA. The vertebral contribution was calculated as the post-operative OVA being

10

subtracted by pre-operative OVA, and the remaining correction angle was considered to be

11

attributed to disc wedging. We also evaluated the wedging index of osteotomized vertebrae (OV),

12

a parameter being defined as the ratio of posterior to anterior height of vertebral body. All the

13

angles were recorded as negative if being lordosis, otherwise it was positive. One orthopedic

14

resident performed the measurements on the PACS (Picture Archiving and Communications

15

Systems, PACS) workstation. Duplicate measurements were obtained for each parameter, and the

16

average values were used for the final analyses.

17 18

Statistics Analysis

19

The statistical analysis was performed using SPSS 17.0 statistical software (SPSS, Inc, USA).

20

Descriptive statistics was performed to analyze patients’ demographics. The intergroup

21

pre-operative values of multiple spino-pelvic sagittal parameters were compared using

22

independent sample T test. Paired-sample t test was applied to compare the operative and 8

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follow-up changes of multiple spino-pelvic sagittal parameters. Comparisons of the incidence of

2

either anticipated or disordered morphology of OVDC between PG and NG were made using

3

crosstab analysis. A p-value of less than 0.05 was considered to be statistically significant.

4 5

Results

6

A comprehensive review of our spinal surgery database identified 71 AS patients fulfilling the

7

above-mentioned inclusion and exclusion criteria. Among these patients, 32 were stratified into

8

the group with bridging syndesmophytes locating within OVDC. After a meticulous review of the

9

recruited patients, 7 patients in PG and 12 patients in NG were identified to be associated with

10

RMV. There were 15 males (78.9%) and 4 females (21.1%). For these patients, the average age at

11

surgery was 34.4±9.1 years (range from 21 to 49 yrs). The follow-up period averaged 36.2±12.0

12

months (range, 24-60 months). RMV patients in PG demonstrated significantly larger PI

13

(57.9±13.2° vs. 40.1±8.9°, p=0.003) and older age at surgery (40.3±6.6 vs. 31.0±8.7 yrs, p=0.027)

14

when compared with patients from NG (Table 1). The wedging index of OV was larger in NG, yet

15

the difference was not statistically significant (p=0.097). The length of instrumented segments was

16

similar between two groups (p>0.05, Table 2).

17 18

Radiological morphology variances of OVDC

19

The standard and anticipated morphological appearances being classified as CWO or COWO were

20

the mainstream geometric shapes of OVDC following PSO. And the incidences of CWO and

21

COWO were 28.1% and 50.0% in PG and 66.7% and 2.6% in NG, respectively, which differed

22

significantly from each other (p<0.05, Table 1). The remaining isolates being classified as RMV 9

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accounted for 30.8% in NG and 21.9% in PG, and no significant intergroup difference was

2

detected (p=0.400, Table 1). Thereinto, most RMV (11 no., 57.9%) fell into the categories of

3

inappropriate angle and range of osteotomy. These irregular radiological shapes were

4

subsequently conceptualized as follows (Table 2):

5

1. True parallel osteotomy with resultant vertebral subluxation (VS) and (or) disc opening

6

(Figure 2C): The directions of transpedicular osteotomy were nearly mutually parallel,

7

creating a miscontoured quadrate shape osteotomy and resulting in significantly impaired

8

lordosing capability. Sagittal translational VS and (or) disc opening occurred during

9

osteotomy closure to produce some degree of compensatory kyphosis correction. This type of

10

RMV was a combined consequence following parallel collapse and shortening in vertebral

11

height with limited sagittal translation and (or) disc opening.

12

2. False parallel osteotomy with disc opening (Figure 2D): A transpedicular wedge osteotomy

13

was performed in a significantly wedged vertebra, creating a quadrate shape OV. This RMV

14

was termed as false parallel osteotomy since there truly existed acceptable lordosing effect.

15

Meanwhile, the corresponding AS patients were usually associated with small PI and

16

non-ossified ALL locating within OVDC. Thus the adjacent disc opening within OVDC could

17

occur concomitantly to further strengthen the lordosing capability.

18

3. Corner wedge osteotomy with disc opening (Figure 2E): The transpedicular wedge osteotomy

19

was restricted within the posterior superior corner of OV. The hinge shifted

20

posterior-superiorly accordingly. In this situation, typically, the anterior cortex of OV was not

21

bended or fractured, preserving an intact straight profile. Meanwhile, the superior adjacent

22

disc opening was inevitable, and rupture of ossified ALL could be noticed in cases with 10

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1

bridging syndesmophytes.

2

4. CWO accompanied by fracture of lower end plate (Figure 2F): The PSO cut excessively into

3

the lower part of OV, causing fracture of lower end plate during osteotomy closure. This was

4

in contrary to common sense that the upper end plate of OV got fractured more frequently

5

during PSO.

6

5. Rat-tail sign of OV secondary to excessive wedge osteotomy (Figure 2G): Excessive

7

decancellation in osteoporotic OV resembled eggshell osteotomy, and consequently the cortex

8

on cortex collapse of posterior vertebral body following osteotomy closure resulted in string

9

morphology of OV resembling rat-tail.

10 11

Vertebral subluxation at osteotomy level was the second largest subgroup of RMV (7 no.,

12

36.8%) noticed in this case series. Anterior VS, being defined as ventral translation of the cranial

13

spinal segments, was solely observed for 3 cases in PG, while posterior VS, being defined as

14

ventral translation of the caudal spinal segments, was equally detected in both groups (Table 2,

15

Figure 2A, B). Failed closure of osteotomy gap resembling Chance fracture was the third-ranking

16

type of RMV (1 no., 5.3%) in this study (Figure 2H). For this patient, the lordosing effect was

17

achieved through rotational wedging, yet the attempt to actualize bone-on-bone closure of

18

osteotomy gap was failed, leaving a wide bony gap resembling Chance fracture.

19 20

Prognosis concerning the surgical corrections and maintenance of spino-pelvic

21

sagittal parameters, as well as vertebral remodeling

22

The radiological assessments of spino-pelvic sagittal parameters before surgery, after surgery and 11

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1

at the ultimate follow-up were summarized in Table 3. The RMV groups with and without

2

bridging syndesmophytes were equivalent with regard to preoperative GK, LL, SVA and SS (all

3

p>0.05) but not PT (p=0.029, Table 3). Significant pre to post-operative improvements were

4

observed for both groups in terms of correction of GK, LL, PT, SS and SVA (all p<0.001, Table

5

3). At the final follow-up, only PT got deteriorated significantly in the NG but not PG when

6

compared with those data immediately after surgery (p=0.002, Table 3).

7 8

For the total RMV cohort, the mean bony lordosing effect per PSO segment, being defined as

9

△OVA, was 36.0±8.9° postoperatively (range: 18-48°), and decreased to 34.7±8.7° (range: 18-46°)

10

by a mean follow-up of 3yrs (p=0.076). Patients in PG and NG accomplished comparable amount

11

of kyphosis correction per PSO segment (△PSA) [42.4±4.9°(33-49°) vs. 43.8±9.3° (29-58°),

12

p>0.05, Table 2]. However, the contribution of adjacent disc wedging to total correction per PSO

13

segment (△PSA-△OVA) was significantly larger in NG [10.2±6.5° (23.3%) vs. 2.4±3.2° (5.7%),

14

p=0.009, Table 2]. At the ultimate follow-up, the contributions of adjacent disc wedging were well

15

maintained in both groups (NG: 10.2±6.5° vs. 8.3±5.3°, p=0.149; PG: 2.4±3.2° vs. 1.4±2.2°,

16

p=0.216). Finally, for patients with VS and failed osteotomy gap closure, the bone fusion and

17

adaptive vertebral remodeling concerning the subluxated or dislocated osteotomized vertebra was

18

utterly favorable, being exclusively confirmed radiologically at the ultimate follow-up (Figure 2A,

19

B, H). No instrumentation failure was detected for all the patients with RMV.

20 21

Complications

22

For this case series, no intra-operative severe vascular or neurological complications occurred. Dural 12

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tear was a potential adverse event, but was not observed for patients with VS and failed osteotomy

2

gap closure. Postoperatively, one patient experienced traumatic cervical fracture with resultant

3

neurological impairments and underwent emergency decompressive surgery.

4 5

Discussion

6

The morphology of osteotomized vertebra-disc complex following PSO was decisive of the

7

lordosing capability. A standard morphological shape following accurate angle and range of

8

wedge osteotomy was the fundamental component for achieving best lordosing capability[14],

9

while any unexpected radiological morphology variances of OVDC deviating from an ideal wedge

10

shape could potentially make the lordosing capability differing substantially. The intravertebral

11

osteoporosis taking place in close vicinity with pathologic new bone formation in AS could further

12

decrease the bone strength and density along the osteotomy line[8], making controlled precise

13

extent of bone resection and reliable hinge formation a big challenge. To our knowledge, no

14

previous studies had thoroughly analyzed the incidence, morphological features and prognosis of

15

RMV following PSO in AS, while such data was essentially needed not only to mitigate patient

16

and

17

complication-prevention strategies and prognosis of miscontoured OVDC during and after

18

surgery.

surgeon

expectations,

but

might

also

help

to

improve

the

recognition

of

19 20

The present study represented the first study investigating this issue. The results revealed that

21

the incidence of RMV were comparable between groups with and without bridging

22

syndesmophytes. As for standard morphology, the occurrence of CWO was significantly larger for 13

Page 13 of 31

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NG when compared to PG. In other words, COWO was more frequently observed in PG. The

2

additional compensatory lordosing capability of the neighboring mobile disc should account for

3

such discrepancy. After a meticulous analysis of the deviated morphological features in this series,

4

the conceptualization of RMV was then constructed and classified. Inappropriate angle and range

5

of osteotomy accounted for the largest share in total. This could be attributed to the difficulty of

6

precise osteotomy in significant osteoporotic vertebrae in AS. It is of paramount importance to

7

identify between the true and false parallel osteotomy. The latter one being endowed with true

8

lordosing capability was typically characterized as a pre-operative wedged OV being cut into a

9

quadrate shape OV with no significant rod bending at osteotomy level. The lordosing effect could

10

further be spontaneously strengthened by anterior opening of mobile adjacent disc to achieve

11

satisfactory sagittal realignment. In the contrary, true parallel osteotomy showed no obvious

12

lordosing effect, and could only be compensated by sagittal translation, anterior disc opening, or

13

both. Corner wedge osteotomy was characterized by an intact and straight profile of anterior

14

cortex of OV, and the upper adjacent disc was opened inevitably to ensure lordosing effect

15

regardless of whether the anterior longitudinal ligaments was ossified or not. String morphology

16

of OV being termed as rat-tail sign was the extreme form of the biconcave morphology resembling

17

fish tail, which was usually a combined consequence of excessive wedge osteotomy in extremely

18

osteoporotic vertebrae. A relatively intact anterior half vertebral body with collapsed posterior half

19

vertebral body was the premise of acceptable lordosing effect in this situation. We also observed

20

one case with a rare occurrence of fracture of lower end plate, which was in contrary to our basic

21

knowledge that the fracture of upper end plate in PSO was a common occurrence. The

22

down-shifting of osteotomy cutting excessively into the lower part of OV was responsible for this 14

Page 14 of 31

1

phenomenon.

2 3

VS was the second-ranking type of RMV in this case series. In a previous study[9], it was

4

identified that the underlying mechanisms responsible for VS were deviation from a perfect

5

wedged osteotomy gap, inadvertent fracture of anterior cortex, as well as improper

6

cantilever technique. In this patient cohort, when being analyzed separately, the incidence of VS

7

was relatively larger for patients with bridging syndesmophytes locating within OVDC (15.6% vs.

8

5.1%). Moreover, anterior VS was uniquely observed in PG. A relatively larger required amount of

9

correction in PG surpassing the maximum amount of lordosing effect per PSO segment along with

10

limited compensatory mechanism of neighboring anterior disc opening could cause higher risk of

11

sagittal translational subluxation. This might partially account for the discrepancy regarding the

12

variability in incidence of VS between these two groups. We hypothesized that anterior VS

13

contributed to improvement of SVA mainly through hinge antedisplacement and retroversion of

14

the proximal spinal segments while that for posterior VS was mainly achieved through dorsal

15

sagittal translation of the cranial spine. For these cases with irreversible VS yet normal

16

intraoperative neuromonitoring signals, emergency extensive laminectomy was still warranted and

17

adjunctive bone grafting favoring enhancement of bone fusion was pretty important for prevention

18

of instrumentation failure[15]. For postoperative care, external plaster immobilization was

19

essential and beneficial for further enhancing the fusion rate.

20 21

As observed, the bone union and adaptive remodeling of the subluxated or dislocated

22

osteotomized vertebra was utterly favorable. Alterations in mechanical strain enhancing the 15

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1

adaptive bone remodeling process involving bone growth, absorption and reinforcement, as well

2

as the superior fusion capability of AS, should account for the good functional adaptation,

3

resulting in reshaping in its contour and mass[15, 16]. Interestingly, we also observed that failure

4

to actualize bone-on-bone osteotomy closure leaving a wide bony gap resembling Chance fracture

5

could also result in solid bone healing with the premise of sufficient bone grafting, stable

6

instrumentation and external brace immobilization.

7 8

Additionally, this study quantitatively confirmed that, for both groups, the GK and SVA were

9

significantly corrected, and a harmony between pelvic orientation and lumbar lordosis were well

10

restored. The lordosing effect per PSO segment was comparable to the magnitude reported in the

11

literature, which ranged from 26.3°-42.2°[4, 17, 18]. The contribution of adjacent disc wedging to

12

total correction per PSO segment, however, differed substantially between two groups, being more

13

prominent in NG. We noted that the reduced bony lordosing effect secondary to disordered

14

morphology of OV could be well compensated by neighboring disc opening, if being non-ossified.

15

Otherwise, when the lordosing effect was impaired and insufficient along with no disc-originated

16

lordosing effect, intraoperative vertebral subtraction or dislocation might happen to strengthen the

17

improvement of sagittal balance. A passive extension of the rigid osteotomized spine following

18

straightening of the special bow-type frame was the premise and stimulus for creating these

19

remedial mechanisms. Lastly, we noticed a significantly older age with larger PI for patients in PG.

20

This could be explained by the knowledge that the capability to compensate for spinal kyphosis by

21

retroversion of the pelvis was limited by the value of PI[19, 20]. Thus for AS patients with a high

22

PI, this compensative mechanism could sustain a relatively longer time period and be surpassed at 16

Page 16 of 31

1

an older age, resulting in a later onset of sagittal imbalance by when a corrective surgery was

2

indicated.

3 4

Conclusion

5

Judging from the overall aspects, we concluded that disordered and deviated morphology

6

variances of OVDC could be encountered frequently following PSO in AS, and the

7

conceptualization of RMV was well constructed and classified, being mainly attributed to

8

inaccurate and uncontrollable angle and range of wedge osteotomy and vertebral subluxation. A

9

chain of remedial mechanisms mainly involving disc opening and sagittal translation could occur

10

spontaneously following straightening of the special bow-type frame to obtain better realignment

11

of sagittal profile. Sufficient bone grafting, stable instrumentation and external brace

12

immobilization were essential for the types of RMV related to subluxated or dislocated

13

osteotomized vertebrae. The bone fusion and adaptive remodeling during follow-up was

14

surprisingly favorable, maintaining the kyphosis correction and preventing instrumentation failure.

15

For those with unsatisfied sagittal realignment, which could be judged intraoperatively by whether

16

the patient’s shoulder and pelvis were in the same horizontal line, additional osteotomy at distal

17

neighboring levels should be considered. These knowledge were essentially instructional and

18

would assist in counseling inexperienced spine surgeons in regards to their concerns with

19

prognosis of miscontoured OVDC after PSO. Limitation of this study lied in that this was a

20

single-center experience with relatively small patient sample, and might warrant further validation

21

to be used as a proxy.

22 23

References 17

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

1.

Ward MM. Health-related quality of life in ankylosing spondylitis: a survey of 175 patients.

Arthritis care and research : the official journal of the Arthritis Health Professions Association. 1999;12(4):247-55. 2.

Ji ML, Qian BP, Qiu Y, et al. Change in Abdominal Morphology After Surgical Correction of

Thoracolumbar Kyphosis Secondary to Ankylosing Spondylitis: A Computed Tomographic Study. Spine. 2015;40(23):E1244-9. 3.

Fu J, Song K, Zhang YG, et al. Changes in cardiac function after pedicle subtraction osteotomy in

patients with

a

kyphosis

due to

ankylosing spondylitis.

The bone &

joint

journal.

2015;97-B(10):1405-10. 4.

Qian BP, Wang XH, Qiu Y, et al. The influence of closing-opening wedge osteotomy on sagittal

balance in thoracolumbar kyphosis secondary to ankylosing spondylitis: a comparison with closing wedge osteotomy. Spine. 2012;37(16):1415-23. 5.

Qian BP, Qiu Y, Wang B, et al. Pedicle subtraction osteotomy through pseudarthrosis to correct

thoracolumbar kyphotic deformity in advanced ankylosing spondylitis. European spine journal : official publication of the European Spine Society, the European Spinal Deformity Society, and the European Section of the Cervical Spine Research Society. 2012;21(4):711-8. 6.

Fu J, Zhang G, Zhang Y, et al. Pulmonary function improvement in patients with ankylosing

spondylitis kyphosis after pedicle subtraction osteotomy. Spine. 2014;39(18):E1116-22. 7.

Schwab F, Blondel B, Chay E, et al. The comprehensive anatomical spinal osteotomy classification.

Neurosurgery. 2015;76 Suppl 1:S33-41; discussion S. 8.

Klingberg E, Lorentzon M, Mellstrom D, et al. Osteoporosis in ankylosing spondylitis - prevalence,

risk factors and methods of assessment. Arthritis research & therapy. 2012;14(3):R108. 9.

Qian BP, Mao SH, Jiang J, Wang B, Qiu Y. Mechanisms, Predisposing Factors, and Prognosis of

Intraoperative Vertebral Subluxation During Pedicle Subtraction Osteotomy in Surgical Correction of Thoracolumbar Kyphosis Secondary to Ankylosing Spondylitis. Spine. 2017;42(16):E983-E90. 10. Kiaer T, Gehrchen M. Transpedicular closed wedge osteotomy in ankylosing spondylitis: results of surgical treatment and prospective outcome analysis. European spine journal : official publication of the European Spine Society, the European Spinal Deformity Society, and the European Section of the Cervical Spine Research Society. 2010;19(1):57-64. 11. Pigge RR, Scheerder FJ, Smit TH, Mullender MG, van Royen BJ. Effectiveness of preoperative planning in the restoration of balance and view in ankylosing spondylitis. Neurosurgical focus. 2008;24(1):E7. 12. Dihlmann W, Lindenfelser R. [Polysegmental andersson lesion in ankylosing spondylitis (roentgenological-histological synopsis) (author's transl)]. RoFo : Fortschritte auf dem Gebiete der Rontgenstrahlen und der Nuklearmedizin. 1979;130(4):454-60. 13. Kim YJ, Bridwell KH, Lenke LG, Cheh G, Baldus C. Results of lumbar pedicle subtraction osteotomies

for

fixed

sagittal

imbalance:

a

minimum

5-year

follow-up

study.

Spine.

2007;32(20):2189-97. 14. Schwab F, Blondel B, Chay E, et al. The Comprehensive Anatomical Spinal Osteotomy Classification. Neurosurgery. 2013. 15. Qian BP, Mao SH, Jiang J, Wang B, Qiu Y. Mechanisms, Predisposing Factors and Prognosis of Intra-Operative Vertebral Subluxation During Pedicle Subtraction Osteotomy in Surgical Correction of Thoracolumbar Kyphosis Secondary to Ankylosing Spondylitis. Spine. 2016. 16. Rubin CT, Lanyon LE. Kappa Delta Award paper. Osteoregulatory nature of mechanical stimuli: 18

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function as a determinant for adaptive remodeling in bone. Journal of orthopaedic research : official

14

Figure legends

15

Fig 1 Illustration of the measurements of osteotomized vertebra angle (OVA) and pedicle

16

subtraction angle (PSA).

17

Fig 2 True parallel osteotomy with VS and disc opening (A, preoperative; B, postoperative; C

18

final follow-up)

19

Fig 3 False parallel osteotomy with disc opening (A, preoperative; B, postoperative; C final

20

follow-up)

21

Fig 4 Corner wedge osteotomy with upper disc opening (A, preoperative; B, postoperative; C final

22

follow-up)

23

Fig 5 CWO accompanied by fracture of lower end plate (A, preoperative; B, postoperative; C final

24

follow-up)

25

Fig 6 Rat-tail sign of OV secondary to excessive wedge osteotomy (A, preoperative; B,

26

postoperative; C final follow-up)

27

Fig 7 Anterior vertebral subluxation (A, preoperative; B, postoperative; C final follow-up)

28

Fig 8 Posterior vertebral subluxation (A, preoperative; B, postoperative; C final follow-up)

29

Fig 9 Open osteotomy gap resembling Chance fracture (A, preoperative; B, postoperative; C final

publication of the Orthopaedic Research Society. 1987;5(2):300-10. 17. Chang KW, Cheng CW, Chen HC, Chang KI, Chen TC. Closing-opening wedge osteotomy for the treatment of sagittal imbalance. Spine. 2008;33(13):1470-7. 18. Kim KT, Lee SH, Suk KS, Lee JH, Im YJ. Spinal pseudarthrosis in advanced ankylosing spondylitis with sagittal plane deformity: clinical characteristics and outcome analysis. Spine. 2007;32(15):1641-7. 19. Johnson RD, Valore A, Villaminar A, Comisso M, Balsano M. Sagittal balance and pelvic parameters--a paradigm shift in spinal surgery. Journal of clinical neuroscience : official journal of the Neurosurgical Society of Australasia. 2013;20(2):191-6. 20. Qian BP, Jiang J, Qiu Y, Wang B, Yu Y, Zhu ZZ. The presence of a negative sacral slope in patients with ankylosing spondylitis with severe thoracolumbar kyphosis. The Journal of bone and joint surgery American volume. 2014;96(22):e188.

19

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1

follow-up)

2 3 4

Table 1 Comparisons of patient demographic information, lordosing capability per PSO segment, and morphological features of OVDC between groups with and without bridging syndesmophytes Parameters NG PG t p Age 31.0±8.7 40.3±6.6 2.426 0.027* PI(°) 40.1±8.9 57.9±13.2 3.526 0.003* Wedging index of OV 1.25±0.23 1.09±0.07 -1.756 0.097 Instrumented segments 9.5±1.4 8. 6±0.8 -1.619 0.124 △PSA(°) 43.8±9.3 42.4±4.9 -0.430 0.672 △OVA(°) 33.7±10.1 40.0±4.6 1.553 0.139 △PSA-△OVA (°) 10.2±6.5 2.4±3.2 -2.932 0.009* Incidence of CWO 26/39(66.7%) 9/32(28.1%) / 0.001* Incidence of COWO 1/39(2.5%) 16/32(50.0%) / 0.000* Incidence of RMV 12/39(30.8%) 7/32(21.9%) / 0.400 Incidence of VS 2/39(5.1%) 5/32(15.6%) / 0.282

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

* Indicates a statistically significant difference between groups with and without bridging syndesmophytes (P < 0.05). OVA indicates osteotomized vertebra angle; PSA, pedicle subtraction angle; OV, osteotomized vertebra; PI, pelvic index; RMV, radiological morphology variances; VS, vertebral subluxation; CWO, closing wedge osteotomy; COWO, closing-opening wedge osteotomy.

20

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Table 2 Conceptualization and comparison of RMV between groups with and without bridging syndesmophytes Morphology of OV

NG (no.)

PG (no.)

CWO COWO Anterior VS Posterior VS True parallel osteotomy with VS and disc opening False parallel osteotomy with disc opening Corner wedge osteotomy with upper disc opening Rat-tail sign of OV secondary to excessive wedge osteotomy CWO accompanied by fracture of lower end plate Open osteotomy gap resembling Chance fracture Summary

26 1 0 2 1 3 2 3 1 0 39

9 16 3 2 0 0 1 0 0 1 32

Abbreviations: refer to Table 2

21

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Table 3. Radiological comparisons of spino-pelvic sagittal parameters before surgery, before discharge and at final follow-up between RMV groups with and without bridging syndesmophytes Parameters

Groups

Preoperative

Postoperative

Final Follow-up

SVA(mm)

NG

146.8±84.3

32.5±37.9*

45.0±34.7

PG NG PG NG PG NG PG NG

184.1±40.9 75.8±20.8 79.1±23.6 8.3±11.7 -0.4±13.3 7.4±9.1 11.4±4.1 32.6±9.7

75.1±22.4* 20.3±11.5* 31.3±26.9* -33.0±16.6* -38.0±14.0* 20.8±8.0* 23.7±6.1* 18.9±8.7*

66.0±37.7 24.1±10.0 20.1±10.7 -33.2±11.0 -37.0±10.5 17.2±9.8 18.6±4.2 23.4±10.6#

PG

46.4±15.8†

30.3±16.3*

36.7±10.2

GK(°) LL(°) SS(°) PT(°) 6 7 8 9 10 11 12 13 14

† Indicates a statistically significant difference intergroup comparing preoperative values ( p < 0.05). * Indicates a statistically significant difference intragroup comparing preoperative and postoperative values (p < 0.05). # Indicates a statistically significant difference intragroup comparing postoperative and final follow-up values (p < 0.05). SVA indicates sagittal vertical axis; GK, global kyphosis ; LL, lumbar lordosis; SS, sacral slope; PT, pelvic tilt.

15 16

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