Tortuosity of the iliac artery is associated with ipsilateral low back pain – a case-control study

Tortuosity of the iliac artery is associated with ipsilateral low back pain – a case-control study | medRxiv /* */ /* */ <!-- <!-- /*! * yepnope1.5.4 * (c) WTFPL, GPLv2 */ (function(a,b,c){function d(a){return"[object Function]"==o.call(a)}function e(a){return"string"==typeof a}function f(){}function g(a){return!a||"loaded"==a||"complete"==a||"uninitialized"==a}function h(){var a=p.shift();q=1,a?a.t?m(function(){("c"==a.t?B.injectCss:B.injectJs)(a.s,0,a.a,a.x,a.e,1)},0):(a(),h()):q=0}function i(a,c,d,e,f,i,j){function k(b){if(!o&&g(l.readyState)&&(u.r=o=1,!q&&h(),l.onload=l.onreadystatechange=null,b)){"img"!=a&&m(function(){t.removeChild(l)},50);for(var d in y[c])y[c].hasOwnProperty(d)&&y[c][d].onload()}}var j=j||B.errorTimeout,l=b.createElement(a),o=0,r=0,u={t:d,s:c,e:f,a:i,x:j};1===y[c]&&(r=1,y[c]=[]),"object"==a?l.data=c:(l.src=c,l.type=a),l.width=l.height="0",l.onerror=l.onload=l.onreadystatechange=function(){k.call(this,r)},p.splice(e,0,u),"img"!=a&&(r||2===y[c]?(t.insertBefore(l,s?null:n),m(k,j)):y[c].push(l))}function j(a,b,c,d,f){return q=0,b=b||"j",e(a)?i("c"==b?v:u,a,b,this.i++,c,d,f):(p.splice(this.i++,0,a),1==p.length&&h()),this}function k(){var a=B;return a.loader={load:j,i:0},a}var l=b.documentElement,m=a.setTimeout,n=b.getElementsByTagName("script")[0],o={}.toString,p=[],q=0,r="MozAppearance"in l.style,s=r&&!!b.createRange().compareNode,t=s?l:n.parentNode,l=a.opera&&"[object Opera]"==o.call(a.opera),l=!!b.attachEvent&&!l,u=r?"object":l?"script":"img",v=l?"script":u,w=Array.isArray||function(a){return"[object Array]"==o.call(a)},x=[],y={},z={timeout:function(a,b){return b.length&&(a.timeout=b[0]),a}},A,B;B=function(a){function b(a){var a=a.split("!"),b=x.length,c=a.pop(),d=a.length,c={url:c,origUrl:c,prefixes:a},e,f,g;for(f=0;f<d;f++)g=a[f].split("="),(e=z[g.shift()])&&(c=e(c,g));for(f=0;f<b;f++)c=x[f](c);return c}function g(a,e,f,g,h){var i=b(a),j=i.autoCallback;i.url.split(".").pop().split("?").shift(),i.bypass||(e&&(e=d(e)?e:e[a]||e[g]||e[a.split("/").pop().split("?")[0]]),i.instead?i.instead(a,e,f,g,h):(y[i.url]?i.noexec=!0:y[i.url]=1,f.load(i.url,i.forceCSS||!i.forceJS&&"css"==i.url.split(".").pop().split("?").shift()?"c":c,i.noexec,i.attrs,i.timeout),(d(e)||d(j))&&f.load(function(){k(),e&&e(i.origUrl,h,g),j&&j(i.origUrl,h,g),y[i.url]=2})))}function h(a,b){function c(a,c){if(a){if(e(a))c||(j=function(){var a=[].slice.call(arguments);k.apply(this,a),l()}),g(a,j,b,0,h);else if(Object(a)===a)for(n in m=function(){var b=0,c;for(c in a)a.hasOwnProperty(c)&&b++;return b}(),a)a.hasOwnProperty(n)&&(!c&&!--m&&(d(j)?j=function(){var a=[].slice.call(arguments);k.apply(this,a),l()}:j[n]=function(a){return function(){var b=[].slice.call(arguments);a&&a.apply(this,b),l()}}(k[n])),g(a[n],j,b,n,h))}else!c&&l()}var h=!!a.test,i=a.load||a.both,j=a.callback||f,k=j,l=a.complete||f,m,n;c(h?a.yep:a.nope,!!i),i&&c(i)}var i,j,l=this.yepnope.loader;if(e(a))g(a,0,l,0);else if(w(a))for(i=0;i (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0];var j=d.createElement(s);var dl=l!='dataLayer'?'&l='+l:'';j.src='//www.googletagmanager.com/gtm.js?id='+i+dl;j.type='text/javascript';j.async=true;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-P4HH5NV'); Skip to main content Home About Submit ALERTS / RSS Search for this keyword Advanced Search Tortuosity of the iliac artery is associated with ipsilateral low back pain – a case-control study View ORCID Profile Jens Maier doi: https://doi.org/10.1101/2025.02.28.25322883 Jens Maier 1 Dept. of Radiology, Sygehus Lillebælt Kolding DK MD Find this author on Google Scholar Find this author on PubMed Search for this author on this site ORCID record for Jens Maier For correspondence: jensmaier{at}dadlnet.dk Abstract Full Text Info/History Metrics Preview PDF Abstract Objectives Thrust owing to the deflection of flow in the bend of an artery has been shown to be associated with clinical symptoms such as neurovascular compression syndromes or vocal cord palsy. This study investigated the association between tortuous iliac arteries and lateral low back pain, hypothesizing that the enhanced pulsatility found in tortuous iliac arteries causes pain by inflicting pulsatile microtrauma on the psoas muscle or its embedded nerve roots. Methods In this retrospective case-control study, participants in the main study (aged 40 - 80 years) were recruited on the basis of existing outpatient computed tomography (CT) scans of the abdomen showing no significant abnormality (exclusion criteria: malignant disease; degenerative lumbar disease; lumbar surgery, fracture, or structural deformity; axial arthritis; aneurysms of the abdominal aorta or iliac arteries; femoral hernia), selecting subjects with normal iliac arteries (control group, arteries bending >135°, n= 52), and markedly tortuous iliac arteries (study group, arterial bends ≤ 90°, n=47). Marked iliac tortuosity was subdivided into bends targeting the psoas muscle and bends passing in front of or behind the muscle. Due to significant differences in age and sex, calculations of the participants’ self-reported frequency and localization of low back pain were performed on matched pairs (n = 31; 17 male and 14 female pairs, mean age of 62.5 years in the control group and 63.2 years in the study group). Concomitantly, an estimate of the frequency of marked iliac tortuosity was obtained by screening 500 consecutive CT scans. Results Marked iliac tortuosity on one or both sides was found in 39.2% of subjects aged 40 - 80 years, more often in males (p<0.00001), and more often on the left side (p<0.0001). Lateral low back pain was reported more often by participants in the study group (n=14) than by those in the control group (n=6) (p=0.031). In the matched study group, lateral low back pain was reported on 11 of the 20 sides of a markedly tortuous iliac artery targeting the psoas muscle. This was more often than on the side of a markedly tortuous iliac artery passing in front of or behind the muscle (four of 27 sides, p=0.005) and more often than in the matched control group (8 of 62 sides, p=0.0003). Conclusion Iliac artery tortuosity targeting the psoas muscle is associated with ipsilateral low back pain. This may explain low back pain in some cases currently regarded as nonspecific. Arterial tortuosity can cause symptoms when targeting vulnerable structures. In cases of otherwise unexplained focal neurological symptoms, screening imaging for presence and orientation of arterial tortuosity in the affected area is recommended. Introduction Despite being a frequent finding on imaging studies, arterial tortuosity is often not mentioned in reports, the assumption being that it is not associated with any significant pathology. This is probably correct in most cases. However, there are exceptions. Arterial tortuosity has been associated with age, elastin deficiency, genetic defects, and cardiovascular risk factors [ 1 – 3 ]. The main factor in the development of arterial tortuosity is vascular elongation between two fixed points, which forces the vessel into a tortuous shape [ 4 ]. While clinically silent in most cases [ 5 ], arterial tortuosity has been associated with intra-arterial and downstream pathologies, such as dissection, leukoaraiosis, or stroke [ 6 ; 7 ]. However, tortuous arteries can also affect their surrounding structures by thrust owing to the deviation of flow in the bend of the vessel. Thrust is an angle- and mass-flow-dependent vector that can be calculated using trigonometric functions. Hydrodynamically, an arterial curvature can be regarded as a pulsatory lens that causes a significant increase in pulsatility and focuses the pulsatility in the direction of the peak of the bend ( Fig.1 ). Download figure Open in new tab Fig. 1. Simplified vector diagram illustrating the concept of thrust Arrow 1 - impulse carried by a defined volume heading towards the bend; arrow 2 - recoil impulse generated by the same volume leaving the bend in the new direction; horizontal arrow - resulting thrust. Reducing the angle of a bend from 135° to 90° (as depicted) while maintaining the same flow increases the resulting thrust by 85%. Reducing the angle from 170° to 90° causes an >8-fold increase in thrust. This example neglects static pressure (in vessels that are not in the horizontal plane) and blood pressure. In most cases, this repetitive unidirectional stimulus is innocuous and is absorbed by the surrounding tissue or fluid, yet may cause further vessel elongation over time. However, thrust can cause symptoms if the bend of the artery is oriented towards a vulnerable structure. In a 2019 paper [ 8 ], my group described an association between tortuosity of the internal carotid artery and vocal cord palsy if the bend of the carotid tortuosity is directed towards the expected position of the vagus nerve, suggesting pulsatile thrust as the underlying pathophysiological mechanism. In my clinical practice, I have observed several cases in which a complaint of lateral low back pain appeared to be related to iliac artery tortuosity. Imaging in such cases suggests that pain is caused by the iliac tortuosity inflicting pulsatile microtrauma on the psoas muscle and/or its embedded lumbar nerve roots. Therefore, I hypothesized that iliac tortuosity targeting the psoas muscle is associated with ipsilateral low back pain. A typical case is shown in Fig. 2 . Download figure Open in new tab Fig. 2. Case report Elderly patient referred to magnetic resonance imaging (MRI) of the lumbar spine and pelvis with a four-week history of left sided low lumbar pain irradiating to the front of the thigh, and reduced force on knee extension. MRI of the lumbar spine (not depicted) only showed mild spondylosis. MRI of the pelvis showed marked tortuosity of both common iliac arteries. Left: axial T2 image at level L5/S1 showing the right arterial tortuosity (R) passing in front of the psoas muscle (P), while the left (L) artery’s bend is targeting the psoas muscle, causing an impression in the muscle. Right: coronal short tau inversion recovery (STIR) image showing focal muscular edema (seen as high signal intensity) in the left iliac (I) and psoas (P) muscle at the level of the impression. The angle of this left iliac tortuosity measured on adjacent coronal images (not depicted) was 110°. Low back pain is highly prevalent and the main cause of years lived with disability [ 9 ]. Consequently, it is a major contributor to health care costs [ 10 ]. European guidelines for the management of low back pain in primary care define low back pain as ‘‘pain and discomfort’’ localized below the costal margin and above the inferior gluteal folds, with or without leg pain [ 11 ]. In 85 - 90% of cases, low back pain cannot be attributed to a recognizable, known specific pathology and is hence classified as nonspecific [ 12 ]. If an association exists between iliac artery tortuosity and low back pain, patients with this condition are currently categorized as having nonspecific low back pain. The design of a prospective study investigating this association would be complicated by several factors. Since routine imaging is not recommended in patients with nonspecific low back pain [ 13 ], the inclusion of participants with confounding lumbar pathologies would be inevitable. Even with previous imaging of the lumbar spine, the iliac artery status of the participants would be indeterminate at the time of inclusion. Finally, sample size calculations would be hampered by the fact that iliac artery tortuosity is not a well-defined entity, and its frequency unknown. Therefore, I chose a retrospective approach. Materials and methods This retrospective case-control study was performed at the Department of Radiology of a Danish Regional Hospital between August 2021 and July 2022 by a consultant radiologist with >20 years of experience. The Danish National Review Board waived the need for approval (NVK 2107163). Imaging All imaging data were obtained from the department’s abdominal computed tomography (CT) scans performed on outpatients. CT imaging (acquisition thickness, 0.625 mm; pitch, 0.984; 120 kV) was performed using GE Revolution scanners (GE Healthcare, Chicago, IL, USA) in the arterial or venous phase after intravenous injection of 100 ml Visipaque 320 (GE Healthcare, Chicago, IL, USA). Reconstructions in the three standard planes had a thickness of 2.5 mm and spacing of 2 mm. Tortuosity of the iliac artery Arterial tortuosity is mainly observed in the elderly population. Therefore, this study focused on individuals aged 40 - 80 years old. The threshold for tortuosity had to be marked without an existing definition of iliac artery tortuosity. Iliac artery tortuosity typically affects the common iliac/external iliac main axis in conjunction. Marked iliac artery tortuosity was arbitrarily defined as the occurrence of any ≤90° bend in the main axis of the artery. Normal iliac arteries were defined as angles in all iliac artery main axis curvatures >135° in the coronal and sagittal planes. Iliac arteries bending between 90° and 135° were classified as borderline tortuosity and were excluded. The iliac main axes were evaluated by examining standard coronal and sagittal CT reconstructions, without identifying the origin of the internal iliac arteries. No supplementary CT reconstructions were performed ( Fig. 3 ). Download figure Open in new tab Fig. 3. Normal and tortuous iliac arteries CT maximum intensity projections, thickness 25 mm. A (coronal): normal iliac artery main axes. B (coronal): 135° tortuosity of left iliac artery is classified as borderline normal. C (coronal): marked right sided iliac tortuosity oriented laterally, passing in front of the psoas muscle. D (sagittal): marked right sided iliac tortuosity near midline, oriented posteriorly. Hypothesizing that the psoas muscle or its embedded nerve roots are the vulnerable structures, the CT scans of participants with marked tortuosity were reviewed in the axial plane at the level of the peak of the arterial convexity to evaluate whether the tortuosity targets the psoas muscle, or passes in front of or behind the muscle ( Fig. 4 ). Download figure Open in new tab Fig. 4. Marked iliac tortuosity and its relation to the psoas muscle CT maximum intensity projections, thickness 25 mm. A and B are images from the same subject with marked tortuosity in the left iliac artery main axis. The thin white line on A (coronal) indicates the level of axial reconstruction (B), showing the left iliac tortuosity targeting the center of the left psoas muscle. The thin white line on B indicates the coronal reconstruction level (A). C and D are axial images obtained from different subjects. C: Right iliac tortuosity passing in front of the psoas muscle. D: Left iliac tortuosity passing behind the psoas muscle. Participants Participants in the main study were identified by continuously screening my department’s outpatient production for abdominal CT scans showing no significant abnormality and selecting individuals with marked tortuosity of the iliac arteries on one or both sides (study group) or normal iliac arteries on both sides (control group). The following exclusion criteria (notion in referral or signs at CT) were applied: malignant disease; degenerative lumbar disease (defined as reduced height of intervertebral discs, herniated disc, neuroforaminal or spinal stenosis); lumbar surgery, fracture, or structural deformity; axial arthritis; aneurysms of the abdominal aorta or iliac arteries; femoral hernia. Concomitantly with the screening of outpatient CT scans for the main study, an estimate of the frequency of marked iliac tortuosity in the background population was obtained by reviewing 500 consecutive outpatient CT scans in the age group of 40 - 80 years for the presence of marked iliac tortuosity without applying any exclusion criteria. Within two weeks of CT, subjects qualifying for inclusion in the main study received an unsolicited invitation to participate. After providing written consent, the participants completed a REDCap-based questionnaire (REDCap, Vanderbilt University, TN, USA) for low back pain designed for this study ( Table 1 ). The questionnaire was kept as simple as possible to facilitate participation. View this table: View inline View popup Download powerpoint Table 1. Questionnaire Statistics The iliac arteries and psoas muscles are paired organs. An affection of the psoas muscle on one side was hypothesized to cause ipsilateral symptoms. The occurrence of this phenomenon on both sides was considered possible. Thus, the participants’ right and left sides were regarded as independent entities, and additional statistics were performed to compare sides, not patients. The age distribution of the study and control groups was compared using Student’s t-test. All remaining comparisons were performed on 2 x 2 contingency tables analyzed with the chi-square test, or alternatively Fisher’s exact test for discrete variables in those comparisons where one or more of the variables were <5. All analyses were two-sided. The level of statistical significance was set at p<0.05. Chi-square and Fisher’s exact tests were performed using medcalc.org. The remaining statistics were performed using spreadsheet software (Microsoft Excel). Data management was provided, and REDCap was hosted by OPEN (Open Patient data Explorative Network, Odense University Hospital, Region of Southern Denmark). Results Frequency of marked iliac tortuosity The frequencies of marked iliac artery tortuosity on one or both sides in the analysis of 500 consecutive CT scans are shown in Table 2 . Tortuosity frequency increases with age. In individuals aged >70 years, marked tortuosity on one or both sides was found in >50% of cases. Overall, marked iliac tortuosity was more frequent in males (p<0.00001). View this table: View inline View popup Download powerpoint Table 2. Frequency of marked iliac artery tortuosity in 500 consecutive outpatient CT scans Analysis based on individual A total of 304 individuals who qualified for inclusion in the main study were invited to participate. A total of 99 participants accepted and returned the completed questionnaire. Most participants in the main study were referred for CT by their general practitioner for suspicion of malignant disease or nonspecific abdominal symptoms. A small proportion of the participants were scanned for follow-up of an aneurysm of the thoracic aorta. All questionnaires were returned within four weeks from CT. 52 of the participants had normal iliac arteries (control group), 47 had marked iliac tortuosity on one or both sides (study group). Of the 47 participants in the study group, 27 had bilateral marked iliac tortuosity and 18 (2) had unilateral marked tortuosity on the left (right) side; marked iliac tortuosity was more frequent on the left side (p<0.0001). All iliac tortuosities targeting the psoas muscle did this on the same side. Significant differences in age (p<0.0001) and sex (p<0.0001) were observed. To control for confounding due to these differences, all comparisons between the control and study groups were based on subgroups consisting of matched pairs (matching for sex and age; one control per case; age difference in one pair ≤2 years). 17 male and 14 female pairs could be matched. There was no significant difference in age between the matched groups. Table 3 shows the demographic data and questionnaire results of the original and matched groups. Lateral low back pain on one or both sides was reported more often by participants in the study group (n=14) than by those in the control group (n=6) (p=0.031). View this table: View inline View popup Download powerpoint Table 3. Demographic data and low back pain questionnaire results in the original groups and the matched groups Analysis based on sides Considering the participants’ right and left sides as independent entities, statistics were also performed to compare the sides. Thus, both the control and study groups comprised 31 x 2 = 62 sides. First, the frequency of lateral low back pain in the study group was compared with that in the control group. However, on 15 of the 62 sides of the study group, the iliac arteries were not tortuous. Therefore, a second comparison was performed, including only the 47 sides with marked tortuosity, and comparing them with the 62 sides of the control group. Finally, a third comparison, focusing only on the 20 marked tortuosities targeting the psoas muscle, was performed. The results of these comparisons are presented in Table 4 . View this table: View inline View popup Download powerpoint Table 4. Sides-based comparisons of the frequency of lateral low back pain. R: right; L: left; OR: odds radio; LL: lower limit; UL: upper limit). The odds ratio for a complaint of lateral low back pain is considerably higher in tortuous arteries that target the psoas muscle. This was also reflected by a direct comparison of the sides within the study group; in the 47 sides with marked iliac tortuosity, lateral low back pain was found on 11 of 20 sides of tortuosity targeting the psoas muscle, but only on four of 27 sides of tortuosity not targeting the psoas muscle; marked tortuosity targeting the psoas muscle correlated with ipsilateral pain, p=0.005. Performing this analysis in the original study group (75 sides with marked tortuosity), comparing sides with marked tortuosity targeting the psoas muscle (pain reported on 13 of 27 sides) and sides with marked tortuosity not targeting the psoas muscle (five of 48 sides) yielded p=0.0003. Other results Analyses of the participants’ intensity and frequency of low back pain and symptoms in the lower extremities did not reveal any significant findings (data not shown). Nine of the 31 participants in the matched study group had marked iliac artery tortuosity targeting the psoas muscle on one or both sides and reported lateral low back pain. Marked iliac tortuosity being found in 39,2 % of subjects aged 40 - 80 years, the estimated prevalence of lateral low back pain associated with iliac tortuosity in this age group is 11.4 %. Discussion Any change in the direction of a fluid moving through a pipe elbow or tortuous vessel creates thrust. This is an everyday issue in engineering, where the restraint of a pipe elbow (a so-called thrust block) must be properly dimensioned to hold the pipe stationary [ 14 ]. Despite being a ubiquitous phenomenon in the human body, thrust due to flow deflection in tortuous arteries tends to be neglected in medical science. Thus, tortuosity found in imaging studies is often not reported. In a prospective study of carotid tortuosity in 345 patients, Togay et al. [ 15 ] found no association between tortuosity and vascular risk factors, atherosclerotic lesions, ischemic stroke, TIA, or carotid stenosis. The authors concluded that carotid tortuosity appears to be more of curiosity than a clinically significant finding. However, in a recent review, Ciurica et al. [ 16 ] suggested that arterial tortuosity may have considerable clinical and research utility as a biomarker for various vascular conditions, and made a plea for the establishment of a multidisciplinary initiative on arterial tortuosity. This study found an association between iliac artery tortuosity targeting the psoas muscle and ipsilateral low back pain, suggesting that the augmented pulsatility of the arterial bend causes pain by inflicting pulsatile microtrauma on the psoas muscle. This provides further evidence of the pathophysiological significance of thrust if the bend of a tortuous artery targets a vulnerable structure. The association between pulsatile thrust and clinical symptoms is reminiscent of intracranial neurovascular compression syndromes. Key features for the diagnosis of neurovascular compression syndromes are the localization of the contact between the vessel and cranial nerve at the transition zone between the central and peripheral myelin, and the displacement and atrophy of the affected nerve [ 17 ]. However, thrust has also been associated with the pathogenesis of these syndromes, as reflected by the suggestion that vessel contact should be perpendicular to the axis of the affected nerve [ 18 ]. Thus, in a series of seven trigeminal neuralgias [ 19 ], all causative vessels performed loops oriented towards the nerve. In neurovascular compression [ 20 ], as well as in the present study, a significant number of vessel juxtapositions are found in asymptomatic patients. This suggests that hemodynamic phenomena alone may not be sufficient to account for the observed symptoms [ 21 ]. Arterial tortuosity has been classified using various methods, including the number of consecutive curvatures [ 22 ], visual appearance [ 23 ], ratio of vessel curve length over the line distance between the two ends (tortuosity index) [ 24 ], or sum of angles [ 25 ]. Considering the highly complex morphology of arterial tortuosity, it is reasonable to use such simplifications. In studies of the significance of augmented pulsatility in tortuous arteries, the key feature is the orientation of the arterial bend towards the hypothesized vulnerable structure. The degree of angulation appears to be less important as long as there is sufficient elongation to form an arterial bend. Thus, the calculation of an 11.4 % prevalence of lateral low back pain associated with iliac tortuosity targeting the psoas muscle in subjects aged 40 - 80 years may be a conservative estimate because the association can also be found in borderline iliac tortuosity (bends >90°), which was not included in this study (the patient in Fig. 2 had an angulation of 110°). This study used an untraditional approach, in which participants were a sample of the background population defined by findings on existing imaging, exclusion criteria, and willingness to participate. As low back pain is a very frequent symptom, a considerable proportion of subjects with this complaint can be expected in any sample of the background population. In a review of the worldwide prevalence of low back pain including 165 studies from 54 countries, the 1-month prevalence was 30.8% [ 26 ]. An even higher prevalence of low back pain was found in the present study, with 61 to 64 % of participants reporting to have suffered from low back pain during the preceding 14 days. This is likely a result of response bias, with persons suffering from back pain being more willing to participate in a study investigating this symptom. Participants were blinded to their iliac tortuosity status; thus, response bias cannot explain the association between iliac artery tortuosity and ipsilateral low back pain found in this study. A central aspect of this study is the bilaterality of the involved structures (psoas muscle and iliac artery), allowing an analysis of the body halves. This is reminiscent of within-person trials used in ophthalmology and dermatology, where the paired body sites of a person receive two competing interventions and where inter-person variability is removed by making the comparison within the same person [ 27 ]. However, this study differs from within-person trials in several ways: instead of an intervention, an intrinsic phenomenon is investigated; the phenomenon can only be identified by imaging; and the phenomenon can be uni- or bilateral. Confounding factors might alter the strength of the association between iliac artery tortuosity targeting the psoas muscle and induced pain, and can therefore affect the comparisons between the control group and the study group. Confounding from age and sex differences was addressed by performing these calculations on matched pairs. However, confounding factors cannot alter the fact that the association is bound to be on the same side, allowing for an analysis based on sides, not individuals. Such an analysis based on sides could also be performed within the study group, omitting the reference to the control group. Assuming that there are no confounding factors that simultaneously can cause lateral low back pain and iliac tortuosity targeting the psoas muscle on the same side, this analysis is considered free from confounding. It could therefore be extended to comprise all 47 participants in the original (before pairing) study group. In retrospect, this study could have produced a highly significant result without a control group. Conclusion Tortuosity of the iliac artery targeting the psoas muscle is associated with ipsilateral low back pain, typically in an elderly male on the left side. This association may explain lateral low back pain in a considerable number of cases currently regarded as nonspecific. Demarcating lateral low back pain associated with iliac tortuosity against nonspecific low back pain may lead to a more specific treatment. Thrust is present in all arterial bends. It deserves more attention; in cases of otherwise unexplained focal neurological symptoms, screening imaging for presence and orientation of arterial tortuosity in the affected area is recommended. Associations comparable to the one described in this paper may be found in other areas of the body. Statements and declarations Ethical considerations The Danish national review board waived the need for approval (NVK 2107163). Consent to participate All participants provided written informed consent. Consent for publication Consent for publication of Fig. 2 has been obtained. Declaration of conflicting interest The author declares no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Funding statement This study did not receive any funding. Data availability Imaging data is archived in the PACS system of the Region of Southern Denmark and is not accessible to the public. 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