Abstract
Aims
The aim of this study was to identify factors associated with poor outcome following coccygectomy on patients with chronic coccydynia and instability of the coccyx.
Methods
From the Danish National Spine Registry, DaneSpine, 134 consecutive patients were identified from a single centre who had coccygectomy from 2011 to 2019. Patient demographic data and patient-reported outcomes, including pain measured on a visual analogue scale (VAS), Oswestry Disability Index (ODI), EuroQol five-dimension five-level questionnaire, and 36-Item Short-Form Health Survey questionnaire (SF-36) were obtained at baseline and at one-year follow-up. Patient satisfaction was obtained at follow-up. Regression analysis, including age, sex, smoking status, BMI, duration of symptoms, work status, welfare payment, preoperative VAS, ODI, and SF-36 was performed to identify factors associated with dissatisfaction with results at one-year follow-up.
Results
A minimum of one year follow-up was available in 112 patients (84%). Mean age was 41.9 years (15 to 78) and 97 of the patients were female (87%). Regression showed no statistically significant association between the investigated prognostic factors and a poor outcome following coccygectomy. The satisfied group showed a statistically significant improvement in patient-reported outcomes at one-year follow-up from baseline, whereas the dissatisfied group did not show a significant improvement.
Conclusion
We did not identify factors associated with poor outcome following coccygectomy. This suggests that neither of the included parameters should be considered contraindications for coccygectomy in patients with chronic coccydynia and instability of the coccyx.
Cite this article: Bone Jt Open 2021;2(7):540–544.
Take home message
Patient characteristics traditionally associated with poor surgical outcome should not discourage surgeons from performing coccygectomy when deciding on the best care for patients with persistent coccydynia and instability of the coccyx.
Introduction
Coccydynia is a painful condition with symptoms confined to the tailbone and the surrounding tissue,1 affecting primarily females.2 The aetiologies of common coccydynia are diverse and risk factors for its development are direct coccygeal trauma, obesity, childbirth, and female sex.3-6 The anatomical configuration and mobility of the coccyx may also contribute to the development of coccydynia, as instability in the form of hypermobility or subluxation in coccygeal joints can force the tailbone into a sharply forward angulated position, especially when patients are sitting.7-10 Pain is primarily present when sitting, or when moving from sitting to standing position, and can cause difficulty when defecating.9,11
Current available treatment options consist of noninvasive therapy ranging from physiotherapy, specialized sitting cushions, and non-steroidal anti-inflammatory drugs (NSAIDs), to steroid injections, radiofrequency ablation, and surgical removal of the coccyx.12
Coccygectomy is usually considered only when a patient’s symptoms are unresponsive to both noninvasive treatments and steroid injections. Previous studies have shown good results from coccygectomy in comprehensive series of patients refractory to other treatment options.11,13-19 Several studies investigating the effectiveness of coccygectomy on patients with chronic coccydynia conclude that careful patient selection is critical in order to achieve successful relief of symptoms.10,20-22
Some prognostic factors relevant to the success of treatment for coccydynia have previously been investigated,22,23 showing that obesity might be associated with a poor outcome, whereas traumatic aetiology may be associated with a good outcome. As research on the topic is very limited, further research on larger prospective series, specific to painful instability, is needed to evaluate the association between prognostic factors and a successful outcome on surgical removal of the coccyx.
The aim of this study is to investigate the association of age, sex, smoking status, BMI, duration of symptoms, work status, and welfare payment, as well as the degree of pain, disability, and quality of life prior to surgery, with the success of coccygectomy in patients with coccydynia due to instability of the coccyx.
Methods
Patients
This is a retrospective study of prospectively collected data on 134 consecutive patients from the Danish National Spine Registry, DaneSpine.24 Patients included in this study were all diagnosed with primary coccydynia and were shown to have instability of the coccyx, either as hypermobility or subluxation, through rectal examination; with distinct sitting pain reproducible through intrarectal coccygeal manipulation. Patients referred were examined in the outpatient clinic with bimanual palpation of the coccyx by an experienced surgeon. If instability was present and manipulation reproduced the patient’s symptoms, the patient was referred to steroid blocks performed with the patient laying in the prone position placed in the CT scanner. All patients underwent three steroid blocks, with an interval of eight to 12 weeks. If the patients only experienced temporary relief, they were offered surgery. All patients were examined and treated by a single surgeon (AS) at the Spine Centre of Southern Denmark from September 2011 to February 2019.
Patients who had surgery due to malignant or congenital disorders or reoperations due to infection, fracture after previous amputation of the coccyx, or soft-tissue complications from any surgery in the area were excluded from the study.
Data use approval was acquired from the Danish Data Protection Agency (reference number 16/1586). The study was reviewed and approved by the Research Board of the Centre for Spine Surgery and Research at Lillebaelt Hospital. Waiver of informed consent for this retrospective review of data was granted. Subjects provided consent for use of their data at the time that they completed the questionnaires and were enrolled in DaneSpine.
Surgical procedure
The procedure was performed under general anaesthesia. Patients were placed in a prone position and the skin of the buttocks was distracted using wide surgical tape. A marking in line with the mobile segment was made, and a 4 cm skin incision was made in line with the coccyx, gaining access to the soft-tissue and the coccyx. The small vein on the posterior side of the coccyx was ligated, and a longitudinal incision, corresponding to the mobile segment, was made in the soft-tissue sack. This allowed the mobile segment including the segments below to be removed. If the disc above the mobile segment was intact, then it was preserved at the end of the resection. The soft-tissue sack was closed, followed by a skin closure in three layers, using absorbable sutures.
Measures
Baseline characteristics were obtained from DaneSpine, modified by including questions regarding coccygeal pain. The questionnaire included demographic data and patient-reported outcome measures (PROMs), including the visual analogue scale (VAS) measure of pain,25 the Oswestry Disability Index (ODI) measure of disability,26-28 the EuroQol five-dimension five-level questionnaire (EQ-5D) measure of quality of life,29 and the 36-Item Short-Form Health Survey questionnaire (SF-36)30 measure of physical and mental health comprising the physical component score (PCS) and mental component score (MCS). Based on the questionnaire in which patients were asked about their attitude towards the result of their surgery, patient satisfaction was divided into three categories: those who replied to be satisfied with surgery were categorized as ‘Satisfied’, those who replied to be dissatisfied as ‘Dissatisfied’, and those who replied to be neither satisfied nor dissatisfied as ‘Undecided’. Potential prognostic factors associated with a poor treatment outcome based on clinical relevance were considered and collected from the questionnaires.
Statistical analysis
All analyses were performed using SPSS v. 26.0 (IBM, USA). Patients in the Satisfied, Undecided, and Dissatisfied groups were compared using analysis of variance (ANOVA) for continuous variables, presented as means and standard deviations (SDs), and Fisher’s exact test for categorical variables, presented as amounts and percentages, to describe differences in key characteristics. Logistic regression analysis was performed to determine associations with patient dissatisfaction. Factors included in the model were age, sex, BMI, smoking status, duration of symptoms, work status, receipt of welfare payment, baseline VAS, ODI, and the SF-36 PCS and MCS. The p-value threshold for significance was set at 0.05.
Results
Of the 134 patients who met inclusion criteria, a minimum of one-year follow-up was available in 112 patients (84%) (Table I).
Table I.
Baseline characteristics of respondents.
| Characteristic | Respondents |
|---|---|
| Total, n (%) | 112 (84) |
| Females, n (%) | 97 (87) |
| Mean age, yrs (SD) | 41.9 (11.4) |
| Mean BMI, kg/m2 (SD) | 26.7 (5.0) |
| Smokers, n (%) | 22 (20) |
| Mean VAS pain (SD) | 70.0 (20.1) |
| Mean ODI (SD) | 32.3 (13.0) |
| Mean EQ-5D (SD) | 0.52 (0.29) |
| Mean SF-36 PCS (SD) | 37.9 (9.1) |
| Mean SF-36 MCS (SD) | 46.0 (11.2) |
-
EQ-5D, EuroQol five-dimension questionnaire; ODI , Oswestry Disability Index; SD, standard deviation; SF-36 MCS, Short Form 36 v1 Mental Component Summary Score; SF-36 PCS, Short Form 36 v1 Physical Component Summary Score; VAS, visual analogue scale (0 to 100).
For patients with a minimum one-year follow-up, “Satisfied” patients had statistically significant better baseline ODI, SF-36 MCS, and PCS scores. No other significant differences were detected between the three groups at baseline (Table II).
Table II.
Baseline characteristics of respondents between groups of satisfaction.
| Characteristic | Satisfied | Undecided | Dissatisfied | p-value |
|---|---|---|---|---|
| Total, n (%) | 78 (70) | 20 (18) | 14 (13) | |
| Females, n (%) | 68 (87) | 18 (90) | 11 (79) | 0.607 |
| Mean age, yrs (SD) | 42.8 (10.2) | 41.2 (15.0) | 37.9 (12.2) | 0.325 |
| Mean BMI, kg/m2 (SD) | 26.5 (4.5) | 28.2 (6.2) | 25.9 (5.7) | 0.333 |
| Smokers, n (%) | 14 (18) | 6 (30) | 2 (14) | 0.416 |
| Mean VAS pain (SD) | 69.0 (19.7) | 73.1 (21.1) | 79.1 (19.4) | 0.199 |
| Mean ODI (SD) | 30.1 (12.0) | 35.8 (15.2) | 39.3 (12.5) | 0.021 |
| Mean EQ-5D (SD) | 0.55 (0.28) | 0.45 (0.32) | 0.44 (0.33) | 0.260 |
| Mean SF-36, PCS (SD) | 39.4 (8.5) | 34.3 (9.0) | 34.6 (10.9) | 0.041 |
| Mean SF-36, MCS (SD) | 48.2 (10.5) | 41.7 (10.1) | 40.1 (13.0) | 0.011 |
-
EQ-5D, EuroQol five-dimension questionnaire; ODI, Oswestry Disability Index; SD, standard deviation; SF-36 MCS, Short Form 36 v1 Mental Component Summary Score; SF-36 PCS, Short Form 36 v1 Physical Component Summary Score; VAS, visual analogue scale (0 to 100).
The one-year outcome measures showed a statistically significantly greater improvement in the satisfied group. In the dissatisfied group the change in outcome measure was not significant from baseline to one-year follow-up (Table III).
Table III.
Difference in patient-reported outcomes baseline to one-year follow-up across the three groups.
| Outcome | Satisfied | Undecided | Dissatisfied | p-value* |
|---|---|---|---|---|
| One-year follow-up, mean (SD) | ||||
| VAS pain | 20.8 (24.3) | 61.8 (19.7) | 78.5 (26.9) | < 0.001 |
| ODI | 12.5 (16.1) | 28.2 (12.5) | 36.9 (16.8) | < 0.001 |
| EQ-5D | 0.84 (0.21) | 0.57 (0.29) | 0.36 (0.36) | < 0.001 |
| SF-36, PCS | 48.7 (9.8) | 37.7 (8.8) | 36.2 (9.4) | < 0.001 |
| SF-36, MCS | 52.0 (9.4) | 45.9 (10.8) | 38.5 (15.0) | < 0.001 |
| Change from baseline, mean (SD) | ||||
| VAS pain | 48.2 (28.8) | 11.3 (28.1) | 0.6 (25.4) | < 0.001 |
| ODI | 17.9 (11.1) | 6.2 (9.7) | 2.4 (11.0) | < 0.001 |
| EQ-5D | 0.29 (0.28) | 0.17 (0.26) | -0.1 (0.18) | < 0.001 |
| SF-36, PCS | 9.3 (8.7) | 0.6 (5.7) | 1.0 (7.3) | < 0.001 |
| SF-36, MCS | 3.7 (11.6) | 0.9 (9.0) | -3.4 (10.0) | 0.104 |
-
*
One-way analysis of variance.
-
EQ-5D, EuroQol five-dimension questionnaire; ODI, Oswestry Disability Index; SD, standard deviation; SF-36 MCS, Short Form 36 v1 Mental Component Summary Score; SF-36 PCS, Short Form 36 v1 Physical Component Summary Score; VAS, visual analogue scale (0 to 100).
Regression, including adjusted odds ratios (ORs) and related 95% confidence intervals, showed that none of the investigated prognostic factors had a statistically significant association on the outcome of coccygectomy (Table IV).
Table IV.
Binary logistic regression identification of prognostic factors.
| Variable | Adjusted OR (95% CI) | p-value |
|---|---|---|
| Age | 0.964 (0.906 to 1.025) | 0.240 |
| BMI | 0.899 (0.770 to 1.049) | 0.177 |
| Sex | 2.855 (0.414 to 19.708) | 0.287 |
| Smoking | 0.391 (0.055 to 2.760) | 0.346 |
| Duration of pain | 1.290 (0.418 to 3.982) | 0.657 |
| Work status | 0.736 (0.121 to 4.481) | 0.739 |
| Receipt of welfare payments | 0.000 (0.000 to 0.000) | 0.998 |
| ODI | 1.055 (0.958 to 1.161) | 0.280 |
| SF-36 PCS | 1.036 (0.912 to 1.178) | 0.585 |
| SF-36 MCS | 0.951 (0.880 to 1.027) | 0.201 |
| VAS pain | 1.028 (0.978 to 1.080) | 0.283 |
-
CI, confidence interval; ODI, Oswestry Disability Index; OR, odds ratio; SD, standard deviation; SF-36 MCS, Short Form 36 v1 Mental Component Summary Score; SF-36 PCS, Short Form 36 v1 Physical Component Summary Score; VAS, visual analogue scale (0 to 100).
Discussion
Although the objective of this study was to identify prognostic factors associated with poor outcomes after coccygectomy in patients with painful instability of the coccyx, the results of the current study suggest that none of the investigated parameters may present as a contraindication to perform coccygectomy in our cohort of patients. We were unable to identify any statistically significant associations between the baseline parameters and a poor outcome of coccygectomy in patients with painful instability of the coccyx.
It is the experience of the surgeon that patients do not tend to improve noticeably beyond the one-year mark following surgery, why the authors are inclined to reject the notion that a longer period of follow-up would result in a greater difference between outcomes and thus in a better analysis of prognostic factors. In a large study on coccygectomy, Hanley et al13 reported no significant difference between patient-reported outcomes at one and two years postoperatively.
The strict indications for coccygectomy in the current cohort, primary coccydynia with hypermobility, pain reproduction on rectal examination and transient pain relief from three local steroid injections prior to surgery, may have contributed to the low incidence of dissatisfied patients. This is in line with the rationale brought forth by Maigne et al10 that detection of either hypermobility or a certain degree of subluxation provides objective evidence of an organic lesion in chronic coccydynia patients which may be amenable to coccygectomy. Surgical removal eliminates the mechanical stress caused by the mobile segment, resulting in pain relief. Considering that traumatic aetiology has been suggested to be a prognostic factor related to a successful outcome,22,23 coccydynia with a confirmed diagnosis of coccygeal instability may be a prognostic factor for a successful outcome in its own right.
A previous article concerning prognostic factors on coccygectomy in patients with coccydynia found that high BMI might be correlated with a poor outcome after surgery.23 However, the current study did not find an association of BMI with dissatisfaction. This is despite the fact that the mean BMI in the entire cohort was in the overweight category (26.7 kg/m2) and 25% of the patients in our cohort had a BMI greater than 30 kg/m2.
The major limitations of the present study are the use of retrospective analysis using data from a registry with patients enrolled prospectively, a lack of full one-year follow-up on all patients and the limit on length of follow-up set by the database. Although this may introduce selection bias, a previous drop-out analysis on the same database by Højmark et al31 in 2016 found that non-respondents often were younger smokers who were back at work and with better self-reported outcomes. Thus, lack of complete follow-up is not likely to negatively affect the results. Although both the EQ-5D and SF-36 were collected, only the SF-36 summary scores were included in the analysis as both are generic measures of health-related quality of life. The binary logistic regression model for identifying prognostic factors was limited to include 11 variables due to the sample size available for analysis. Another included measure, ODI, is not a validated measure of disability regarding the coccyx, but is designed specifically for lower back disease. Nevertheless, we find it to be the best approximate to a measure of disability in patients with coccygeal disorders available. This is the largest study to investigate prognostic factors related to the outcome of coccygectomy and the first study to evaluate prognostic factors in a cohort solely containing patients with painful instability of the coccyx. Future studies should aim to investigate the correlation between different aetiologies and the outcome of coccygectomy in order to improve patient selection.
In a large consecutive cohort of patients with coccydynia and instability of the coccyx, no baseline parameters were identified as a prognostic for a poor outcome following coccygectomy. The majority had satisfactory results of their surgery. It is clinically relevant to consider that these factors, which might be suspected to entail a poor surgical outcome, should not influence the decision to perform coccygectomy on patients with coccydynia and instability of the coccyx.
References
1. Thiele GH . Coccygodynia. The mechanism of its production and its relationship to anorectal disease . Am J Surg . 1950 ; 79 ( 1 ): 110 – 116 . Google Scholar
2. Karadimas EJ , Trypsiannis G , Giannoudis PV . Surgical treatment of coccygodynia: An analytic review of the literature . Eur Spine J . 2011 ; 20 ( 5 ): 698 – 705 . Crossref PubMed Google Scholar
3. ÁM M-C , García-García E , Aragúndez-Marcos MP . Coccyx pain in women after childbirth . Enfermeria Clinica . 2019 ; 29 ( 4 ): 245 – 247 . Crossref PubMed Google Scholar
4. Maigne JY , Doursounian L , Chatellier G . Causes and mechanisms of common coccydynia: Role of body mass index and coccygeal trauma . Spine . 2000 ; 25 ( 23 ): 3072 – 3079 . Crossref PubMed Google Scholar
5. Maigne JY , Rusakiewicz F , Diouf M . Postpartum coccydynia: A case series study of 57 women . Eur J Phys Rehabil Med . 2012 ; 48 ( 3 ): 387 – 392 . PubMed Google Scholar
6. Nathan ST , Fisher BE , Roberts CS . Coccydynia: A review of pathoanatomy, aetiology, treatment and outcome . J Bone Joint Surg Br . 2010 ; 92-B ( 12 ): 1622 – 1627 . Crossref PubMed Google Scholar
7. Postacchini F , Massobrio M . Idiopathic coccygodynia. Analysis of fifty-one operative cases and a radiographic study of the normal coccyx . J Bone Joint Surg Am . 1983 ; 65-A ( 8 ): 1116 – 1124 . PubMed Google Scholar
8. Maigne JY , Tamalet B . Standardized radiologic protocol for the study of common coccygodyma and characteristics of the lesions observed in the sitting position: Clinical elements differentiating luxation, hypermobility, and normal mobility . Spine . 1996 ; 21 ( 22 ): 2588 – 2593 . Google Scholar
9. Salar O , Mushtaq F , Ahmed M . Defecation pain and coccydynia due to an anteverted coccyx: A case report . J Med Case Rep . 2012 ; 6 : 175 . Crossref PubMed Google Scholar
10. Maigne JY , Lagauche D , Doursounian L . Instability of the coccyx in coccydynia . J Bone Joint Surg Br . 2000 ; 82-B ( 7 ): 1038 – 1041 . Crossref PubMed Google Scholar
11. Wray CC , Easom S , Hoskinson J . Coccydynia. Aetiology and treatment . J Bone Joint Surg Br . 1991 ; 73-B ( 2 ): 335 – 338 . Crossref PubMed Google Scholar
12. Lirette LS , Chaiban G , Tolba R , Eissa H . Coccydynia: An overview of the anatomy, etiology, and treatment of coccyx pain . Ochsner J . 2014 ; 14 ( 1 ): 84 – 87 . PubMed Google Scholar
13. Hanley EN , Ode G , Seymour R , Jackson BJ . Coccygectomy for patients with chronic Coccydynia: A prospective, observational study of 98 patients . Bone Jt J . 2016 ; 98-B ( 4 ): 526 – 533 . Crossref PubMed Google Scholar
14. Doursounian L , Maigne JY , Faure F , Chatellier G . Coccygectomy for instability of the coccyx . Int Orthop . 2004 ; 28 ( 3 ): 176 – 179 . Crossref PubMed Google Scholar
15. El Mohsen Arafa A , El Rady Mahmoud A , Zayan M , Kotb A . Coccygectomy for posttraumatic coccygodynia: A long-term prospective study . Curr Orthop Pract . 2017 ; 28 ( 5 ): 484 – 488 . Google Scholar
16. Sehirlioglu A , Ozturk C , Oguz E , Emre T , Bek D , Altinmakas M . Coccygectomy in the surgical treatment of traumatic coccygodynia . Injury . 2007 ; 38 ( 2 ): 182 – 187 . Crossref PubMed Google Scholar
17. Gáspár L , Jónás Z , Kiss L , Vereb G , Csernátony Z . Coccygectomy has a favorable effect on the intensity, manifestation, and characteristics of pain caused by coccygodynia: A retrospective evaluation of 34 patients followed for 3-18 years . Eur J Orthop Surg Traumatol . 2009 ; 19 ( 6 ): 403 – 407 . Google Scholar
18. Hellberg S , Strange-Vognsen HH . Coccygodynia treated by resection of the coccyx . Acta Orthop . 1990 ; 61 ( 5 ): 463 – 465 . Crossref PubMed Google Scholar
19. Kleimeyer JP , Wood KB , Lonne G , Herzog T , Ju K , Beyer L . Surgery for refractory Coccygodynia: Operative versus nonoperative treatment . Spine . 2017 ; 42 ( 16 ): 1214 – 1219 . Crossref PubMed Google Scholar
20. Mouhsine E , Garofalo R , Chevalley F , et al. Posttraumatic coccygeal instability . Spine J . 2006 ; 6 ( 5 ): 544 – 549 . Crossref PubMed Google Scholar
21. Cebesoy O , Guclu B , Kose KC , Basarir K , Guner D , Us AK . Coccygectomy for coccygodynia: Do we really have to wait . Injury . 2007 ; 38 ( 10 ): 1183 – 1188 . Crossref PubMed Google Scholar
22. Bayne O , Bateman JE , Cameron HU . The influence of etiology on the results of coccygectomy . Clin Orthop Relat Res . 1984 ; 190 ( 190 ): 266 – 272 . PubMed Google Scholar
23. Kodumuri P , Raghuvanshi S , Bommireddy R , Klezl Z . Coccydynia - could age, trauma and body mass index be independent prognostic factors for outcomes of intervention . Ann R Coll Surg Engl . 2018 ; 100 ( 1 ): 12 – 15 . Crossref PubMed Google Scholar
24. DaneSpine . Årsrapporter . 2020 . http://drks.ortopaedi.dk/aarsrapporter/ Google Scholar
25. McCaffery M , Beebe A . Pain: Clinical Manual for Nursing Practice . Baltimore, USA : Mosby Company . 1993 . Google Scholar
26. Fairbank JC , Couper J , Davies JB , O’Brien JP . The Oswestry low back pain questionnaire . Physiotherapy . 1980 ; 66 : 271 – 273 . Google Scholar
27. Fairbank JC , Pynsent PB . The Oswestry disability index . Spine (Phila Pa 1976) . 2000 ; 25 ( 22 ): 2940 – 2952 . Crossref PubMed Google Scholar
28. Lauridsen HH , Hartvigsen J , Manniche C , Korsholm L , Grunnet-Nilsson N . Danish version of the Oswestry disability index for patients with low back pain. Part 2: Sensitivity, specificity and clinically significant improvement in two low back pain populations . Eur Spine J . 2007 ; 16 ( 1 ): 135 – 136 . Crossref PubMed Google Scholar
29. EuroQol Group . EuroQol - a new facility for the measurement of health-related quality of life . Health Policy . 1990 ; 16 ( 3 ): 199 – 208 . Google Scholar
30. Ware JE , Kosinksi M , Keller SK . SF-36 Physical and Mental Health Summaries Scales: A User’s Manual . Boston, MA : The Health Institute . 1994 . Google Scholar
31. Hojmark K , Stottrup C , Carreon L , Andersen MO . Patient-reported outcome measures unbiased by loss of follow-up. Single-center study based on DaneSpine, the Danish spine surgery registry . Eur Spine J . 2016 ; 25 ( 1 ): 282 – 286 . Crossref PubMed Google Scholar
Author contributions
M. M. Jensen: Writing - original draft, Resources, Formal analysis.
S. Milosevic: Writing - original draft, Resources, Formal analysis.
G. Andersen: Writing - original draft, Study resources, Formal analysis.
L. Carreon: Supervision, Formal analysis, Writing - review and editing.
A. Simony: Data curation, Investigation, Writing - review and editing.
M. M. Rasmussen: Writing - review and editing.
M. Andersen: Supervision, Writing - review and editing.
Funding statement
No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. Open access funding was self-funded through the authors' institution, the Spine Center of Southern Denmark.
Acknowledgements
We would like to thank our research coordinator, Karen Højmark, for facilitating the extraction of data from the DaneSpine database.
© 2021 Author(s) et al. This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives (CC BY-NC-ND 4.0) licence, which permits the copying and redistribution of the work only, and provided the original author and source are credited. See https://creativecommons.org/licenses/by-nc-nd/4.0/
