Does the Change of Acetabular Anteversion Result From Lumbar Pedicle Subtraction Osteotomy in Ankylosing Spondylitis-Related Kyphosis After Primary Total Hip Arthroplasty?

Abstract

Study Design:

Retrospective study.

Objectives:

To analyze the change in acetabular anteversion (AA) after lumbar pedicle subtraction osteotomy (PSO) in ankylosing spondylitis (AS) patients with thoracolumbar kyphosis who have already undergone total hip arthroplasty (THA).

Methods:

AS patients with thoracolumbar kyphosis following lumbar PSO from January 2005 to January 2020 were retrospectively reviewed. Only the patients performed with THA prior to the PSO were included. AA was measured on the full-length standing posterior-anterior radiographs using the ellipse method.

Results:

Twenty patients (34 hips) with a mean age of 36.8 years (range, 22 to 63 years) were included. After lumbar PSO, AA was reduced from 18.59° to 5.85° (P < 0.001). Postoperative change in AA was correlated with the spinal deformity correction. Additionally, the change in AA postoperatively was correlated with pelvic incidence (PI) (R = 0.346, P = 0.045). Although the postoperative change in sagittal vertical axis (SVA) was larger in the patients after L2 or L3 PSO (153.97 mm vs 70.03 mm, P = 0.006), no difference was found in the postoperative change in AA (12.83° vs 10.96°, P = 0.540) compared with the patients following L1 PSO.

Conclusions:

AA was significantly decreased following lumbar PSO and the postoperative change in AA was correlated with the magnitude of spinal deformity correction. Notably, the effect of osteotomy level on the postoperative change in AA was limited in the AS patients underwent 1-level PSO.

Keywords

acetabular anteversion ankylosing spondylitis pedicle subtraction osteotomy spinal deformity correction thoracolumbar kyphosis total hip arthroplasty

Introduction

Ankylosing spondylitis (AS) is an inflammatory rheumatic disease that mainly affects hip and spine. The spine is characterized by the progression of spinal sagittal malalignment and syndesmophytes formation. Furthermore, hip involvement in AS usually leads to gradual hip ankylosis. The incidence of hip involvement could be up to 36%, and nearly 90% of patients were bilateral involvement.¹ In the advanced stage of hip disease, total hip arthroplasty (THA) has been considered as the most common surgical intervention for the hip refractory pain or disability.^2-4

Despite the improved hip function and relieved pain after THA, hip instability has been widely reported.^2-4 The occurrence of hip instability following THA was greatly associated with the functional cup orientation. Since 1978, safe zone has been proposed to minimize the likelihood of dislocation and to provide the best range of motion.⁵ According to the safe zone, cup orientation should be positioned with anatomical acetabular anteversion (AA) between 5° and 25° referred to the anterior pelvic plane. However, in the AS patients with thoracolumbar kyphosis, pelvic backward rotation occurred to maintain the sagittal balance. As a result, despite the cup was anatomically placed inside the safe zone, more anteverted cup would be expected in the standing functional posture. Therefore, the higher rate of hip instability in AS patients following THA occurred.^6,7

In clinical practice, hip surgeons proposed that the intraoperative cup positioning angle should be decreased in advance due to the expected pelvic backward rotation in standing posture.⁸ Based on the constant pelvic incidence (PI), physical pelvic tilt (PT) could be calculated using the equation of PT = 0.24*PI.⁹ For each 10° increase from physical PT to the measured PT, 8.4° decrease in AA during the cup positioning should be considered.⁸ Although the functional AA might be ensured inside the safe zone after THA, the following spinal deformity correction surgery was not taken into account in their surgical planning.

In the AS patients following THA concurrent with severe thoracolumbar kyphosis, pedicle subtraction osteotomy (PSO) would be also performed to restore the sagittal alignment and the horizontal gaze.^3,7 Considering the standing functional AA might be changed following spinal realignment,^10-12 the functional AA might be removed outside the safe zone after PSO.

Therefore, if the THA was planned to be performed prior to the PSO in the AS patients concurrent with thoracolumbar kyphosis and hip involvement, not only the degree of pelvic backward rotation but also the future change in AA following PSO should be considered in the adjustment of AA during the primary THA. However, there has been no study focused on investigating the spinal deformity correction on the change in cup orientation in AS-realted thoracolumbar kyphosis. Thus, the aim of the current study was to analyze the change in AA after lumbar PSO in AS patients with thoracolumbar kyphosis who have already been performed with THA.

Materials and Methods

Patients

This was a single-center retrospective designed study performed among AS patients with thoracolumbar kyphosis following lumbar PSO from January 2005 to January 2020. Only the patients already have been performed with THA prior to the PSO were included. Inclusion criteria were: (1) diagnosis of AS according to the Modified New York Criteria¹³; (2) age older than 18 years; (3) available full-length standing posterior-anterior and lateral radiographs at baseline, immediately after surgery and at the last follow-up; and (4) adequate visualization of the acetabular component on the full-length standing posterior-anterior and lateral radiographs. Exclusion criteria were: (1) patients with a Brooker class IV of the ectopic ossification following THA¹⁴; and (2) patients with the native hip classified as the bath ankylosing spondylitis radiology hip index (BASRI-Hip) grade 4.¹⁵ Surgical plan was made according to the full-length standing lateral radiographs aimed to restore the horizontal gaze and balanced standing posture.^16,17 This study has been approved by the institutional review board of Affiliated Drum Tower Hospital of Nanjing University Medical School (IRB approved no., 2 011052). Informed consent was obtained from all patients.

Data Collection

Radiographs were analyzed using Surgimap software (Nemaris, NewYork, NY, USA). AA was measured on the full-length standing posterior-anterior radiograph using the Lewinnek method (Figure 1).⁵ Spinal morphological parameters included thoracic kyphosis (TK), lumbar lordosis (LL), C7 pelvic angle (C7PA), spinosacral angle (SSA) and global kyphosis (GK).^18,19 Spinal positional parameters included sagittal vertical axis (SVA) and C7 tilt. Pelvic parameters included PI, sacral slope (SS) and PT (Figure 2). A positive angle indicates kyphosis. Health-related quality of life (HRQoL) questionnaires including the Oswestry Disability Index (ODI) and visual analogue scale (VAS) for back pain were also collected for the patients with completed questionnaires at baseline and the last follow-up. To investigate the effect of osteotomy level on the postoperative change in AA, for the patients following one-level PSO, radiographic parameters were compared between the patients underwent L1 PSO and the patients underwent PSO at L2 or below. Time interval from primary THA to PSO was also collected. For the patients underwent bilateral THA, mean value of the time intervals was calculated for the further statistical analysis.

Figure 1.

A full-length standing posterior-anterior radiograph showing the measurement of AA using the Lewinnek method. D1 and D2 are the minor and major axes of the elliptical appearance of the cup, respectively. AA = arcsin (D1/D2).

Figure 2.

The measurements of spinopelvic parameters. (A) C7 tilt was the angle between the vertical and the line connecting C7 and the femoral axis (point a). GK was the Cobb angle between the most tilted vertebra cranially (vertebra A) and the most tilted vertebra caudally (vertebra B). C7PA was defined as the angle between the line from point a to C7 and the line from point a to the center of sacral endplate (point b). (B) SVA was the horizontal distance between the C7 plumb line and the posterior superior corner of S1 (point c). SSA was measured as the angle between the line connecting point b and C7 and the sacral endplate. The measurements of TK and LL were also presented. (C) Pelvic parameters including PI, SS and PT were also measured.

Statistical Analysis

Data analyses were performed using SPSS version 20.0 software (SPSS, Chicago, IL, USA). Differences in continuous variables were assessed using Student t tests while the differences in categorical variables were analyzed using Chi-square tests. Correlations between the postoperative changes in AA and spinopelvic parameters were calculated via linear regression analysis. Additionally, correlations between the preoperative spinopelvic parameters and postoperative changes in AA were analyzed using bivariate correlation analysis. P values less than 0.05 were considered statistically significant.

Results

Finally, 336 AS patients following lumbar PSO were reviewed, 20 patients (6.0%) were performed with prior THA. All of the 20 patients were included in the current study. Of the included 20 patients (34 hips), 14 were performed with bilateral THA, whereas the remaining 6 were treated with unilateral THA. There were 3 patients performed with 2-level PSO (L1 and L4) and all of the 3 patients were treated with bilateral THA. Demographic and surgical data is summarized in Table 1. There were 9 hips included in the 6 patients underwent L1 PSO, 14 hips in the 8 patients underwent L2 PSO and 5 hips in the 3 patients underwent L3 PSO. There was no significant difference in sex distribution, age, follow-up period and fusion levels between the patients underwent L1 PSO and the patients underwent PSO at L2 or below.

Table 1.

Comparison of the Demographic and Surgical data.*

	Total	Range	L1 PSO	Range	PSO at L2 or below	Range	P value
Sex (male,%)	85.3	-	100	-	73.7	-	0.144
Age (years)	36.8 ± 10.6	22-63	31.1 ± 4.7	22-37	39.9 ± 11.2	27-63	0.061
Mean follow-up (months)	25.4 ± 25.7	3-81	23.5 ± 23.2	3-66	24.5 ± 25.3	3-62	0.920
Time interval between THA and PSO (months)	64.9 ± 49.8	9-180	48.0 ± 14.7	24-60	56.8 ± 43.3	9-144	0.433
Fusion levels	9.3 ± 1.7	6-13	9.7 ± 2.2	8-13	8.7 ± 0.8	6-10	0.086

PSO, pedicle subtraction osteotomy; THA, total hip arthroplasty.

* Mean values are presented ± SD.

AA and spinopelvic parameters of the whole hips at baseline, immediately after PSO and at the last follow-up are presented in Table 2. Linear regression correlations between the postoperative changes in AA and spinopelvic parameters are shown in Table 3. Postoperative change in AA was correlated with the postoperative changes in LL (R = 0.438, P = 0.010), C7PA (R = 0.521, P = 0.002), SSA (R = 0.476, P = 0.004), SS (R = 0.628, P < 0.001) and PT (R = 0.825, P < 0.001). Furthermore, 1.0° decrease of AA occurred with 4.0° increase of LL (R² = 0.192, P = 0.010), 2.8° decrease of C7PA (R² = 0.272, P = 0.002), 3.6° increase of SSA (R² = 0.227, P = 0.004), 1.5° increase of SS (R² = 0.395, P < 0.001) and 1.0° decrease of PT (R² = 0.681, P < 0.001). Regarding the correlations between preoperative spinopelvic parameters and postoperative change in AA, only the preoperative PI (R = 0.346, P = 0.045) was correlated with the postoperative change in AA.

Table 2.

Comparison of the Radiographic Parameters at Baseline, Immediately After PSO and at the Latest Follow-Up.*

	Baseline	After PSO	P value^†	Latest follow-up	P value&
AA(°)	18.6 ± 11.8	5.9 ± 11.4	0.000	8.2 ± 10.3	0.010
TK(°)	52.5 ± 18.4	51.1 ± 18.9	0.186	51.1 ± 16.4	0.886
LL(°)	-1.4 ± 15.1	-47.5 ± 13.4	0.000	-43.0 ± 12.7	0.051
C7PA(°)	53.4 ± 15.3	24.6 ± 11.4	0.000	28.0 ± 9.5	0.028
SSA(°)	73.5 ± 13.6	108.9 ± 9.0	0.000	105.4 ± 8.6	0.066
GK(°)	88.0 ± 17.1	67.0 ± 15.8	0.000	67.6 ± 14.6	0.112
SVA(mm)	184.5 ± 76.7	55.4 ± 37.7	0.000	52.3 ± 34.4	0.861
C7 tilt(°)	14.0 ± 10.0	-0.4 ± 4.8	0.000	-1.1 ± 3.9	0.895
PI(°)	48.3 ± 8.1	48.7 ± 8.5	0.571	49.1 ± 6.8	0.460
SS(°)	9.0 ± 7.9	23.7 ± 9.6	0.000	20.2 ± 7.4	0.055
PT(°)	39.3 ± 11.2	25.0 ± 12.4	0.000	29.1 ± 9.0	0.033

Abbreviations: PSO, pedicle subtraction osteotomy; AA, acetabular anteversion; TK, thoracic kyphosis; LL, lumbar lordosis; C7PA, C7 pelvic angle; SSA, spinosacral angle; GK, global kyphosis; SVA, sagittal vertical axis; PI, pelvic incidence; SS, sacral slope; PT, pelvic tilt.

* Mean values are presented ± SD. Bold type represents statistical significance.

^† The P value represents the comparison between the baseline value and immediate postoperative value, &The P value represents the comparison between the immediate postoperative value and the latest follow-up value.

Table 3.

Linear Regression Correlations Between the Postoperative Changes in AA and Spinopelvic Parameters.*

	Unstandardized coefficient	Standardized coefficient	R²	P value
TK(°)	-0.051	-0.045	0.002	0.802
LL(°)	0.248	0.438	0.192	0.010
C7PA(°)	0.364	0.521	0.272	0.002
SSA(°)	0.282	0.476	0.227	0.004
GK(°)	0.194	0.242	0.058	0.169
SVA(mm)	0.005	0.051	0.003	0.774
C7 tilt(°)	0.030	0.037	0.001	0.836
PI(°)	0.309	0.180	0.033	0.307
SS(°)	0.668	0.628	0.395	0.000
PT(°)	0.972	0.825	0.681	0.000

AA, acetabular anteversion; TK, thoracic kyphosis; LL, lumbar lordosis; C7PA, C7 pelvic angle; SSA, spinosacral angle; GK, global kyphosis; SVA, sagittal vertical axis; PI, pelvic incidence; SS, sacral slope; PT, pelvic tilt.

* Bold type represents statistical significance.

Comparison of the baseline radiographic parameters between the patients underwent L1 PSO and the patients underwent PSO at L2 or below are shown in Table 4. Moreover, comparison of the postoperative changes in radiographic parameters between the patients underwent L1 PSO and the patients underwent PSO at L2 or below are summarized in Table 5. Postoperative changes in LL(P = 0.019), C7PA (P = 0.008), SSA(P = 0.021), SVA(P = 0.006) and C7 tilt (P = 0.044) were significantly larger in the patients underwent PSO at L2 or below.

Table 4.

Comparison of the Baseline Radiographic Parameters Between the Patients Underwent L1 PSO and the Patients Underwent PSO at L2 or Below.*

	L1 PSO	PSO at L2 or below	P value
AA(°)	13.6 ± 9.0	19.5 ± 13.5	0.249
TK(°)	57.0 ± 24.2	44.3 ± 11.8	0.069
LL(°)	-14.0 ± 12.8	4.1 ± 12.6	0.001
C7PA(°)	39.7 ± 15.0	55.3 ± 10.2	0.003
SSA(°)	84.6 ± 15.6	72.0 ± 9.3	0.012
GK(°)	87.6 ± 28.5	84.3 ± 10.2	0.649
SVA(mm)	120.5 ± 81.4	194.0 ± 62.2	0.014
C7 tilt(°)	6.2 ± 9.0	13.9 ± 8.3	0.035
PI(°)	43.7 ± 8.6	49.2 ± 7.5	0.099
SS(°)	10.4 ± 5.6	7.7 ± 9.3	0.440
PT(°)	33.4 ± 10.5	41.5 ± 10.6	0.072

PSO, pedicle subtraction osteotomy; AA, acetabular anteversion; TK, thoracic kyphosis; LL, lumbar lordosis; C7PA, C7 pelvic angle; SSA, spinosacral angle; GK, global kyphosis; SVA, sagittal vertical axis; PI, pelvic incidence; SS, sacral slope; PT, pelvic tilt.

* Mean values are presented ± SD. Bold type represents statistical significance.

Table 5.

Comparison of the Postoperative Changes in Radiographic Parameters Between the Patients Underwent L1 PSO and the Patients Underwent PSO at L2 or Below.*

	L1 PSO	PSO at L2 or below	P value
AA(°)	11.0 ± 7.2	12.8 ± 7.6	0.540
TK(°)	5.6 ± 3.4	0.4 ± 6.8	0.039
LL(°)	36.0 ± 14.2	47.1 ± 9.1	0.019
C7PA(°)	19.9 ± 15.2	30.7 ± 4.6	0.008
SSA(°)	26.0 ± 16.2	36.6 ± 6.9	0.021
GK(°)	23.7 ± 12.5	19.9 ± 8.1	0.333
SVA(mm)	70.0 ± 85.3	154.0 ± 61.9	0.006
C7 tilt(°)	6.2 ± 9.1	16.4 ± 7.3	0.004
PI(°)	-0.3 ± 4.6	0.5 ± 3.6	0.643
SS(°)	14.0 ± 7.6	13.8 ± 6.5	0.946
PT(°)	13.7 ± 8.5	14.2 ± 5.7	0.857

* Mean values are presented ± SD. Bold type represents statistical significance.

Compared with the patients underwent bilateral THA, postoperative AA were lower in those after unilateral THA (-4.0° vs 8.0°, P = 0.018). However, the postoperative changes in AA (17.0° vs 11.8°, P = 0.106) and LL (45.8° vs 46.2°, P = 0.935) were not significantly different. Interestingly, the time interval between primary THA and PSO was greater in the patients underwent bilateral THA (33.6 vs 72.0 months, P = 0.011).

Before PSO, 12 hips were out of the safe zone. Of the 12 hips, 8 were excessively anteverted (AA >25°) while the other 4 were excessively retroverted (AA < 5°). In the 22 hips with AA inside the safe zone before PSO,13 hips were decreased outside the safe zone after PSO. 6 of the 8 excessively anteverted hips were decreased inside the safe zone. Due to the great AA before PSO, 2 hips were still excessively anteverted after PSO.

Completed preoperative and the last follow-up HRQoL questionnaires were available for 17 patients. At the last follow-up, ODI (15.8 vs 29.3, P = 0.015) and VAS (2.1 vs 5.4, P < 0.001) were significantly decreased compared with those at baseline. During the follow-up, there was 1 patient underwent revision of THA due to the limited hip range of motion. In this patient, AA was decreased from -6.0° to -18.0° immediately after PSO and the hip range of motion was greatly restricted. One year after PSO, AA was improved to 17° through the revision of THA, and the hip range of motion was correspondingly improved. There were 2 native hips performed with THA due to the progression of hip ankylosis. Three cases of complications occurred including dura tear without postoperative cerebrospinal fluid leakage (N = 1), vertebral subluxation without neurological deficit (N = 1) and transient numbness of left lower limb due to the radiculopathy (N = 1). No revision of PSO occurred.

Discussion

In the end-stage of AS, severe functional disability might be caused by the combination of hip radiographic destruction and thoracolumbar kyphosis. For these patients, both THA and PSO were required to improve the health-related quality of life. The sequence of the 2 operations was mainly dependent on the presence / absence of hip flexion contracture.⁷ In the patients with hip flexion contracture, THA should be performed prior to the PSO. However, THA performed prior to the PSO would lead to the variation in AA after spinal deformity correction. Therefore, the potential change in AA following PSO should be emphasized during the primary THA.

AA in the AS Patients With Thoracolumbar Kyphosis

In the current study, of the 12 hips outside the safe zone, 8 were excessively anteverted and 4 were excessively retroverted before PSO. On one hand, the excessively anteverted hips could be attributed to the AS-related thoracolumbar kyphosis. In these patients, cup positioning angle might not be adjusted enough according to the degree of pelvic backward rotation during THA. Due to the compensated pelvic backward rotation, the standing functional AA were increased outside the safe zone despite the anatomical AA might be inside the safe zone. On the other hand, over-adjustment of AA according to the degree of pelvic backward rotation during THA would result in the excessively retroverted hips.

Change in AA Due to Spinal Deformity Correction

The compensated pelvic backward rotation would be released following the spinal realignment after PSO. Consequently, the postoperative standing functional AA would be decreased as well.⁷ In the current study, AA was decreased in all hips due to the spinal realignment. For the hips with AA < 5° before PSO, AA would be much further less than the lower limit of the safe zone following PSO. For the excessively anteverted hips, all hips were corrected toward the safe zone in the current study.

Rather than the hips with AA outside the safe zone before PSO, more considerations should be focused on the hips with preoperative AA inside the safe zone. For these patients, standing functional AA might be decreased outside the safe zone after PSO. Consequently, the satisfying hip range of motion would be deteriorated, for some patients as long as the requirement of revision of THA,²⁰ due to the spinal deformity correction. In the current study, over half of the hips were decreased outside the safe zone after PSO (Figure 3). The relationship between the decrease in AA and spinal alignment was needed to predict the cup orientation after PSO.

Figure 3.

A 28-year-old male AS patient with thoracolumbar kyphosis following primary unilateral THA underwent L1 PSO. (A1) Before PSO, PI and C7PA were 28° and 25°, respectively. (B1) After L1 PSO, LL was increased from -24° to -58°. SVA was restored from 58 mm to 21 mm with a decrease of 37 mm and PT was reduced from 22° to 1° with a decrease of 21°. (A2, B2) After PSO, AA was decreased by 17°. (C1, C2) Five months after PSO, PT was 1°, AA was increased to -4°.

Postoperative change in AA was positively correlated with the magnitude of spinal deformity correction in the present study. Except for PT and SS, the postoperative change in C7PA was the most strongly correlated with the postoperative change in AA. This result implicated that the postoperative change in C7PA could predict the postoperative AA most accurately. Additionally, the post-operative change in AA was also positively correlated with the PI value. PI was an individual constant radiographic parameter describing the pelvic morphology independent on the pelvic posture.^9,21 Through inducing larger pelvic backward rotation, higher PI could provide much more compensation ability to the thoracolumbar kyphosis. Conversely, to restore the ideal spinal sagittal alignment, the amount of deformity correction would be larger in the patients with higher PI.²² Therefore, the postoperative change in AA would be larger because of the greater deformity correction needed in the patients with higher PI.

Notably, postoperative AA was lower in the patients after unilateral THA. Theoretically, physical functional would be impaired more in the patients with bilateral hip ankyloses. Therefore, patients with bilateral hip ankyloses would be performed with THA earlier than those with unilateral hip ankylosis. Due to the longer time interval between THA and PSO, increase of AA due to the progressing thoracolumbar kyphosis in the patients underwent bilateral THA would be greater than those underwent unilateral THA. Considering the comparable spinal deformity correction, postoperative AA was lower in the patients after unilateral THA.

Effect of PSO Level on the Postoperative Change in AA

As previously reported, greater decrease in PT, rather than the SVA, occurred in the adult spinal deformity following more caudal PSO.²³ However, neither the change in AA nor the change in PT was affected by the PSO level in the current study. Inversely, the more caudal PSO level was associated with larger decrease in SVA. The opposite findings could be attributed to the severity of the sagittal deformity. In the current study, the mean preoperative SVA of the patients performed with 1-level PSO was 170 mm which was greatly larger than that of 122 mm in the previous study.²³ Since the restricted magnitude of deformity correction achieved by 1-level PSO, ideal spinopelvic sagittal alignment might not be achieved in the most AS patients with thoracolumbar kyphosis.²⁴ Except for the significant decrease in PT in the AS patients with less baseline SVA or C7 tilt, compensated pelvic backward rotation remained to maintain the sagittal balance (C7 tilt = 0) after PSO.^24,25 Thus, more caudal PSO level would mainly result in the restoration of SVA. The greater decrease in PT or AA might not occur in AS patients following PSO with more caudal osteotomy level (Figures 3, 4). The absent correlation between the postoperative changes in AA and spinal positional parameters (SVA and C7 tilt) in the current study could be also attributed to the compensated change in PT to restore the ideal SVA and C7 tilt.

Figure 4.

A 35-year-old female AS patient with thoracolumbar kyphosis following primary bilateral THA underwent L2 PSO. (A1) Before PSO, PI and C7PA were 44° and 53°, respectively. PT and SVA were 36° and 220 mm, respectively. (B1) After L2 PSO, LL was increased from 5° to -38°. SVA and PT were decreased by 187 mm and 6°, respectively. (A2, B2) Both left and right AA were decreased by 5° after PSO. (C1, C2) Three months after PSO, PT was 31°, the left AA was increased by 1° while the right AA remained unchanged.

In clinical practice, for the patients requiring THA performed prior to PSO, the potential change in AA following PSO should be further subtracted in the adjustment of AA based on the compensated change in PT during the primary THA. According to the results of our study, 1° of AA should be subtracted for each 2.8° decrease of C7PA, 4.0° increase of LL and 3.6° increase of SSA, respectively. The expected spinal deformity correction could be predicted by spine surgeons based on the surgical planning for thoracolumbar kyphosis. Additionally, the subtracted value of AA should be correspondingly increased in the patients with less baseline SVA or C7 tilt. Finally, the influence of osteotomy level on the postoperative change in AA might not be apparent in the AS patients with thoracolumbar kyphosis underwent 1-level PSO.

For the AS patients with thoracolumbar kyphosis already underwent THA, the specific hip function after PSO could be reflected by AA to some extent. Through our results, AA after PSO could be predicted preoperatively according to the surgical planning. Hip range of motion and stability might be greatly improved when postoperative AA was decreased inside the safe zone.^11,12 In these patients, hip function would benefit from the spinal realignment. Whereas, if the postoperative AA decreased out of the safe zone, the functional range of flexion of the hip might be decreased, risks of anterior impingement, posterior dislocation and edge-loading might be increased. Furthermore, even revision of THA might be required after spinal realignment.²⁰ When preparing PSO for these patients, the potential surgical complications should be informed. Decreased ODI and VAS scores at the last follow-up indicated the improved physical function and relieved back pain due to the spinal realignment. However, the specific questionnaires for hip function was not available.

Limitations

Several limitations in the current study should be addressed. Firstly, the current study was a single-center retrospective designed study. Secondly, due to the low incidence of THA in AS patients with thoracolumbar kyphosis, the sample of patients was too small. Therefore, the correlations between the postoperative changes in AA and spinopelvic parameters were not strong. Finally, as well-known, different degree of AA was required for various specific position, especially in the AS patients without spinal mobility to compensate with positional change. For Asian patients, usually adopting a posture of sitting on the floor with legs crossed during daily living, higher value of AA might be better for hip stability and mobility. However, patients′ postoperative lifestyles were not analyzed in the current study due to the small sample of patients. Future study with larger sample size and more comprehensive analyses are warranted.

Conclusions

Besides the cup position inside the safe zone, both excessively anteverted and retroverted cups were found in the AS patients with thoracolumbar kyphosis following primary THA. AA would be reduced following PSO, and the postoperative change in AA was correlated with the magnitude of spinal deformity correction. Acetabular cup stability following spinal deformity correction could be improved by the adjustment of cup positioning angle during primary THA. Due to the complex clinical situation of the change in AA after PSO, the cup positioning angle should be tailored with consultation between hip and spine surgeons.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: One of the authors (Bang-ping Qian) has received funding from Jiangsu Provincial Medical Talent Program (ZDRCA2016068). Another author (Yong Qiu) has received funding from the Jiangsu Provincial Key Medical Center (YXZXA2016009).

ORCID iD

Bang-ping Qian, MD

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