The correlation between lumbosacral transitional anatomy and pars defect

Gabriel S Gonzales-Portillo; Mauricio J Avila; Omar Rizvi; Travis M Dumont

doi:10.4103/isj.isj_59_21

ORIGINAL ARTICLES

Year : 2022 | Volume : 5 | Issue : 2 | Page : 209-214

The correlation between lumbosacral transitional anatomy and pars defect

Gabriel S Gonzales-Portillo¹, Mauricio J Avila², Omar Rizvi¹, Travis M Dumont²
¹ College of Medicine, Tucson, AZ, USA
² Department of Neurosurgery, University of Arizona, Tucson, AZ, USA

Date of Submission	02-Jul-2021
Date of Decision	07-Sep-2021
Date of Acceptance	16-Oct-2021
Date of Web Publication	08-Jun-2022

Correspondence Address:
Travis M Dumont
1501 N Campbell Avenue, Room 4303, Tucson, AZ 85724-5070
USA

Source of Support: None, Conflict of Interest: None

DOI: 10.4103/isj.isj_59_21

Abstract

Background: Transitional anatomy and pars defects are two common incidental findings seen on imaging of the lumbosacral spine. The purpose of this study was to investigate whether there is a correlation between these two lumbar spine phenomena. Materials and Methods: A retrospective review of spinal imaging was conducted of patients presenting with thoracolumbar fractures at our Level I Trauma Center between 2017 and 2018. Computed tomography scans from 260 patients were obtained and assessed for the presence of lumbosacral transitional anatomy and pars defect. Results: From the 260 patients reviewed, 16 patients had transitional anatomy (6%) and 20 patients had lumbar non-traumatic pars defect (8%). Only one patient presented with both transitional anatomy and pars defect. Overall, there was no difference in incidence of pars defect whether transitional lumbosacral anatomy was present (5%) or not (6.25%), P = 1.00, Fisher’s exact test. Conclusion: The findings suggest that patients with transitional anatomy do not have an increased association with lumbar pars defects.

Keywords: Lumbosacral transitional anatomy, lumbosacral transitional vertebrae, pars defect, spine surgery, spine trauma

How to cite this article:
Gonzales-Portillo GS, Avila MJ, Rizvi O, Dumont TM. The correlation between lumbosacral transitional anatomy and pars defect. Indian Spine J 2022;5:209-14

How to cite this URL:
Gonzales-Portillo GS, Avila MJ, Rizvi O, Dumont TM. The correlation between lumbosacral transitional anatomy and pars defect. Indian Spine J [serial online] 2022 [cited 2023 Apr 1];5:209-14. Available from: https://www.isjonline.com/text.asp?2022/5/2/209/346971

Introduction

The pars interarticularis is the portion of the spine between the superior and inferior articular processes of the facet joint. A pars defect or spondylolysis is defined as a unilateral or bilateral fracture of the pars interarticularis of the posterior vertebral arch.^[1],[2] This defect first appears in adolescents as they begin to ambulate, with the incidence increasing with age until adulthood.^[3],[4] The incidence in the general adult population is around 6%^[1],[5] with incidence as high as 50% of athletes with back pain.^[6] The underlying etiology remains controversial; however, majority of theories postulate a hereditary disposition to spondylolysis. There is a variety of empirical evidence for this idea such as an extremely high incidence of spondylolysis among family members as well as strong associations with spina bifida occulta.^[7] These observations have led to the idea that spondylolysis is due to progression from underlying genetic dysplasia.^[8] A genetic weakness within the pars interarticularis is then further susceptible to either direct trauma or overuse injury in populations such as athletes. Overuse injury results in stress reactions within the pars which may already be dysplastic.^[9] The mechanism is likely multifactorial with a combination of both genetically predisposed patients and repetitive stress, which then leads to an acute fracture of the pars and eventually to a chronic stage of non-union of the pars.^[1],[3],[9] Pars defects occur almost exclusively in the lumbar region and most specifically at L5, as high as 95% at this level followed by L4 as the second most common site.^[1],[4]

Similar to a pars defect, lumbosacral transitional vertebra (LSTV) is a common congenital spinal variation.^[10] The two main manifestations of LSTV are either lumbarization of S1 or sacralization of L5 (fusion between L5 and the sacrum).^[10],[11] This congenital variation exists on a wide spectrum with two extremes: (1) an L5 that completely fuses to the sacrum resulting in four lumbar vertebra or (2) an S1 that completely separates from the sacrum resulting in six lumbar vertebra.^[12],[13] The incidence of such anomalies has been reported in a wide range from 4% to 30% in the general population and as high as 15.8% in those with chronic back pain.^[10],[14]

Little is known about their simultaneous occurrence, and the association or increased incidence between the two conditions is not examined in the current literature. Due to the high incidence of pars defect at the level of L5 and the importance of the transition between the lumbar spine and the sacrum for load-bearing forces of the spine, it becomes even more relevant if there are LSTVs, as the combination of these two processes may predispose for worse symptoms. Given that both processes have congenital variants, we hypothesized that there is a relationship between transitional anatomy and pars defect.

In this study, we retrospectively reviewed computed tomography (CT) images of patients who presented after traumatic injuries to the spine to our trauma hospital and evaluated the presence of LSTV and concomitant pars defect. As LSTV is congenital in nature and pars defect can also be congenital, we hypothesized that they could be related processes.

Materials and Methods

After approval of Institutional Review Board (IRB), we conducted a retrospective review of radiological images, and charts of patients presenting after a trauma with thoracolumbar fractures at our Level 1 Trauma Emergency Department between 2017 and 2018. Our hospital is the only Level 1 Trauma center in the city and has a coverage of about 1.2 million people including the suburbs. Patients were identified using ICD-9 codes, and then individual patients’ charts were filtered to remove miscoded patients. In our trauma hospital, all patients presenting to the Emergency Room with suspicion of polytrauma undergo a CT scan of their head, neck, chest, abdomen, and pelvis with reconstruction of the images for the spine. We purposefully selected trauma patients as they underwent CT scans of their entire body as part of the Emergency Room/Trauma workup making their CT images accessible for review and comparison in the hospital electronic medical record. During the study time, the CT protocol was the same and all images were taken in the same CT machine available in the emergency department of our hospital. The spine reconstructions are obtained from the original CT of the chest and abdomen, and the imaging software creates the spine sequences. The CT slices are 2.0 mm cuts which allow for adequate bone inspection.

Sagittal, axial, and coronal views of the CT scans in a “bone window” were then analyzed individually by two authors. Lumbosacral transitional anatomy was determined by the definition of Castellvi et al.^[15] Each author reviewed the images and disagreement was resolved with discussion between reviewers. Pars defect was identified in the sagittal plane and could either be unilateral or bilateral. Given that the patient presented after a trauma, we elected to only include patients in the “pars defect” category if the borders of the defect were hyperdense and corticated signifying a chronic defect rather than an acute one following their injury; if the borders were not hyperdense, we assumed this was an acute fracture and the patient was not included as one of the “pars defect” patients for this study.

Since patients presented to the emergency room in our hospital as a trauma emergency, we were unable to determine their symptomatology before their presentation.

Data were then collected and tabulated in Excel and later processed in STRATA statistical software. Following data collection, observational statistics were obtained and tests of significance via Fisher’s exact test were calculated. A P-value of 0.05 was deemed statistically significant.

Results

In total, 260 patients met the inclusion criteria, and their respective CT images and charts were assessed. From the 260 patients reviewed, 16 patients had transitional anatomy (6%) and 20 patients had pars defect (8%). Eighteen patients (90%) had pars defects located at the L5 vertebral body and 2 patients at L4.

[Table 1] presents the demographic information of the two main patient groups (pars defect and LSTV). [Table 2] shows the demographics of the whole cohort (260 patients), and [Table 3] shows the demographics of patients without any spine defects.

Table 1: Characteristics of the patients with transitional anatomy and pars defect

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Table 2: Characteristics of the whole cohort of analyzed patients

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Table 3: Patients without any LSTV or pars defect

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Of the 260 patients evaluated, only one patient presented with both transitional anatomy and pars defect (0.4%). Overall, there was no difference in incidence of pars defect whether transitional lumbosacral anatomy was present (5%) or not (6.25%), P = 1.00, Fisher’s exact test.

With regard to sex, for LSTV, 56% of the patients were male, whereas for pars defect the majority (80%) were male patients.

For the LSTV, with regard to the Castellvi classification, there were six type IIIa patients, three type IV, two type Ia, two type IIIb, two type IIa, and one type Ib.

Given that this is a trauma cohort with thoracolumbar fractures, all patients’ fractures were classified according to the morphology of the fracture. Of the 16 patients with transitional anatomy, 7 presented with a compression fracture, 7 presented with a burst fracture, and 2 presented with a distraction fracture.

With regard to patients with pars defect, of the 20 patients, 9 had a compression fracture, 6 had a burst fracture, and 5 had a distraction fracture.

Regarding treatment options, a χ² analysis showed no significant difference between patients with a pars defect or LSTV when compared with individuals without these defects and the treatment modality chosen (brace, surgery, or neither) (P-value = 0.2786).

The only patient with both LSTV and pars defect is shown in [Figure 1]. This patient had a simple L1 compression fracture that was treated without any intervention. The LSTV consisted of a partially lumbarized S1 that met criteria for Castellvi IIIa.

Figure 1: A patient with unilateral L4 pars defect, L5 pars defect, and LSTV. A. The L4 pars defect. B. The partially lumbarized S1. C. L5 pars defect

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[Figure 2] shows a patient with transitional anatomy who underwent surgery for traumatic L1 distraction spinal fracture.

Figure 2: A patient with transitional anatomy who underwent surgery for traumatic spine injury

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In order to classify the different LSTVs, we used the Castellvi classification. [Figure 3] displays how the Castellvi classification was assigned via the coronal plane. This patient with S1 lumbarization met the criteria for Castellvi IIb.

Figure 3: Castellvi type IIb patient who suffered a traumatic injury (not shown)

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One example of a patient with an L4 pars defect is shown in [Figure 4].

Figure 4: A patient with an L4 pars defect

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Discussion

Our study shows that patients with LSTV anatomy do not have an increased association with having a concomitant pars defect. Overall, we found that transitional anatomy is present in 6% of the patients and a pars defect is found in 8% of the patients. Prior studies have shown that the prevalence of pars defect is 3–6% in the general population and 13.9% in studies on athletes.^[10] Thus, our results for LSTV and pars defect in this trauma population are similar to those published for general adult non-athlete population.

As both pars defect and LSTV are normal congenital variants, we initially hypothesized that they may be associated and could be commonly presented simultaneously. Our results showed that this is not the case as there was no association and only one patient of the total cohort had both findings (0.4%). Although this study found no association between the two, each spine abnormality has potential implications for patients and spine surgeons.

In general, however, the progression of pars defects is not well examined, and it is possible that it may result in progressive listhesis requiring surgical interventions. This can be more relevant if a patient suffers from a traumatic injury and has a baseline pars defect; the two events could potentiate causing additional instability requiring treatment.

Interestingly, our study showed a predominance of pars defects in males, specifically 80% or a 4:1 ratio. Previous studies from Fredrickson et al.^[4] and Roche and Rowe report a male-to-female ratio of nearly 2:1. The higher incidence of pars defects in males from our study may be due to sample size; however, the distribution of male/female patients reviewed was almost equal lending credence to the idea that this may be a true finding. One possible explanation for our particular findings is that the literature reports that males are more commonly involved in motor vehicle crash and other high injury activities.^{[16],[17],[18]}

For particular lumbar spine levels, our study reports the incidence of pars defect occurring at L5 as 90% and followed by L4 with 10% of the cases.

Similar to pars defects, LSTVs are congenital variations that may have significant clinical implications. Transitional anatomy has been shown to be related to chronic lower back pain and radicular pain.^[19] Our study found that the most common LSTV is a Castellvi type IIIa unilateral complete fusion of the transverse process.^[15] Additional implications of LSTV include muscle volume loss and increase in muscle degeneration,^[20] which also have implications for recovery after trauma.

This brings forth another serious clinical implication of transitional anatomy in particular to spine trauma: incorrect level operation. For proper surgical intervention, surgeons must be aware of the correct spinal level that they will operate, which is mainly done through intraoperative counting or perioperative imaging. Recognition of a normal variant in a transitional anatomy is key to avoid wrong level surgery.^[21]

In this study, for the two cohorts of spine variants, pars defects and LSTV, there was no statistically significant difference between different treatments for a variety of thoracolumbar fractures after trauma, with brace being the most common treatment overall.

Limitations include the retrospective nature of the study, which can limit the clinical data; nonetheless, the CT scans were easily accessible in our electronic medical record. This is a population that was selected for convenience as per our hospital protocol, and trauma patients get evaluated with a CT scan; hence, the wide availability of these studies to fully evaluate LSTV and pars defect and their clinical presentation is not back pain per se rather it is the traumatic event. Additional studies may be needed to compare a non-trauma population in a wider population-based study.

Conclusion

We found that in a trauma population, LSTVs are present in 6%, whereas lumbar non-traumatic pars defect is found in 8%. Only one patient had both anomalies simultaneously. The findings suggest that patients with transitional anatomy do not have an increased association with lumbar pars defects.

Patient Declaration Statement

The authors certify that since this is a retrospective review of de-identified images of patients obtained, patient consent was not applicable. IRB approval was taken for the study.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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