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ORIGINAL ARTICLE
Year : 2020  |  Volume : 69  |  Issue : 4  |  Page : 237-242

Considering the surface area and sagittal angle in a pair of lumbosacral facets: Determining the structural relevance of asymmetric facets at the lumbosacral junction


1 Department of Anatomy, Faculty of Basic Medical Sciences, College of Medicine, Abia State University, Uturu; Department of Anatomy, Alex Ekwueme Federal University Ndufu Alike Ikwo, Ebonyi State, Enugu, Nigeria
2 Department of Anatomy, Faculty of Basic Medical Sciences, College of Medicine, Abia State University, Uturu; Department of Surgery, College of Medicine, Chukwuemeka Odumegwu Ojukwu University, Uli Campus, Anambra State, Enugu, Nigeria
3 Department of Orthopaedic, National Orthopaedic Hospital, Enugu, Nigeria
4 Department of Anatomy, Faculty of Basic Medical Sciences, College of Medicine, Abia State University, Uturu; Department of Anatomy, Enugu State University of Science and Technology, Enugu, Nigeria

Date of Submission13-May-2019
Date of Acceptance13-Jan-2020
Date of Web Publication29-Dec-2020

Correspondence Address:
Dr. Uchenna Kenneth Ezemagu
Department of Anatomy, Alex Ekwueme Federal University Ndufu Alike Ikwo, Ebonyi State, P.M.B. 1010
Nigeria
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/JASI.JASI_53_19

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  Abstract 


Introduction: The mechanism of spine dysfunction that was linked to asymmetry in facet joint planes remains poorly understood. We determined the surface area and sagittal angle in a pair of L4, L5, and S1 vertebral facets. We aimed to explain the structural relevance of asymmetric facets at the lumbosacral junction. Material and Methods: Vertebral columns of 45 adult male human cadavers were cut at the L3–L4 intervertebral disc. Each section was macerated and tied together in a sequence to obtain the value of sagittal angle of the superior facets of L4, L5, and S1 vertebrae and area of the inferior facets of L4 and L5 vertebrae, using a modified protractor and graph paper method, respectively. Asymmetry was determined using the formula propound by Plochocki (2002). Results: The mean value of surface area of the left and right inferior facets of L4 and the left and right inferior facets of L5 was 161 ± 24 and 168 ± 23 mm2 and 200 ± 28 and 218 ± 33 mm2, respectively. The mean value of sagittal angle of the left and right superior facets of L4, L5, and S1 was 37.71° ± 4.38°, 36.18° ± 4.8°, 46.96° ± 6.49°, 48.51° ± 6.25°, 52.49° ± 5.1°, and 54.67° ± 5.25°, respectively. The degree of asymmetry in the area of the inferior facets of L4 and L5 ranges from 0% to 30% and 0%–32.26%, respectively, and that for sagittal angle of the superior facets of L4, L5, and S1 was 0%–37.93%, 0%–30.95%, and 0%–26.32%, respectively. Discussion and Conclusion: This study would suggest that despite the statistically significant mean differences in the paired variables, the vertebrae were free of any pathological change but with consequent adaptive features. However, the stress effects would suggest that the left lumbosacral facet joints are predisposed to dysfunction of mechanical origin.

Keywords: Adaptation, back pain, biomechanics, bipedal posture, body weight


How to cite this article:
Ezemagu UK, Akpuaka F C, Iyidobi EC, Anibeze CP. Considering the surface area and sagittal angle in a pair of lumbosacral facets: Determining the structural relevance of asymmetric facets at the lumbosacral junction. J Anat Soc India 2020;69:237-42

How to cite this URL:
Ezemagu UK, Akpuaka F C, Iyidobi EC, Anibeze CP. Considering the surface area and sagittal angle in a pair of lumbosacral facets: Determining the structural relevance of asymmetric facets at the lumbosacral junction. J Anat Soc India [serial online] 2020 [cited 2021 Jan 24];69:237-42. Available from: https://www.jasi.org.in/text.asp?2020/69/4/237/305378




  Introduction Top


The extent of asymmetry in a pair of vertebral facets that becomes clinically relevant remains challenging to orthopedic surgeons and chiropractors. We cannot overemphasize the significance of facet asymmetry in medical practice. It may precipitate disc degeneration and hernia,[1] contributes to the development of different types of idiopathic scoliosis,[2] and predicts the occurrence of facet cyst and low back pain.[3],[4] Although the incidence of asymmetry in a pair of the superior facets at the thoracolumbar region of the spine was rare,[5] some authors[6] queried its relevance in facet joint disease at the lumbosacral junction.

Second, in anthropology, lumbosacral weakness[7] and certain spine derangements around the lumbosacral junction of a modern man were considered to be consequences of skeletal modifications to enhance stability, yet the reason remains vague.[7],[8] Although little research has focused on asymmetry in the lower body,[9] this study could suggest a clear reason for the above proposal and explain in part some factors in the puzzle of lumbosacral adaptation to bipedal posture.

Third, the lumbosacral junction transmits the weight of the upper body to the pelvis, and it is the most frequent site of back pain and facet cyst.[3],[4] Mechanical forces, static and dynamic stress,[10],[11] higher body mass index,[12],[13] and a smaller relative cross-sectional area of the paraspinal muscle[14] were linked to disc degeneration and back pain. Similarly, authors[8],[15],[16] explained the pattern of weight transmission at the lumbosacral junction and noted that lumbosacral facets were subjected to mechanical stress.

Given these findings, necessitated studies designed to determine the extent of asymmetry in surface area and sagittal angle in a pair of vertebral facets at the lumbosacral junction and explain how each facet responds to the stress of upper body weight. It would be of benefit in the management of spine derangement of mechanical origin.


  Material and Methods Top


After institutional review board approval for this study in line with the conditions of the Medical Research Ethics Committee, a total of 45 adult male human cadavers (age range: 25–35 years, mean: 28 years) were involved since there were no female cadavers available during the study. The cadavers were being dissected by medical students during gross anatomy dissection curriculum.

The vertebral columns were cut at the L3–L4 intervertebral disc, using a hand saw [Figure 1]. Vertebrae that belong to each section were obtained through the process of maceration. All the vertebrae were reported by a pathologist to be fully ossified and free of any pathological change or inborn abnormality. The value of the left and right inferior facet areas of the fourth and fifth lumbar vertebrae was obtained in a sequence using graph paper method.[8],[15] The number of squares covered on the graph paper was independently counted and determined by the three researchers to minimize interobserver error. The value of the sagittal angle of L4, L5, and S1 vertebrae was obtained in a sequence using a modified protractor.[5],[17]
Figure 1: Anterior (A) and posterior (B) views of lower portion of the spine (L4 – coccyx) and pelvic girdle of a dissected cadaver, showing the Lumbosacral junction(L) and right L5/S1 facet joint(F)

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The extent of asymmetry in facet area and sagittal angle was calculated as follows: ([left side − right side] ÷ right side) × 100 as adopted by Plochocki.[9] The descriptive statistics of the data showed that the continuous random variables were normally distributed as shown in [Figure 2]a, [Figure 2]b, [Figure 2]c. Therefore, we adopted a paired sample t-test to compare the two means of paired inferior facet area and sagittal angle to establish the level of significance of the mean differences. Furthermore, regression analyses were conducted on paired variables.
Figure 2: (a-c) Frequency distribution of degree of facet joint tropism of L4 (a), L5 (b), and S1 (c), showing a normal distribution curve

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  Results Top


The statistics in [Table 1] showed that the mean values of surface area of the left facet of the fourth lumbar vertebra (L4LF), the right facet of the fourth lumbar vertebra (L4RF), the left facet of the fifth lumbar vertebra (L5LF), and the right facet of the fifth lumbar vertebra (L5RF) were 161 ± 24, 168 ± 23, 200 ± 28, and 218 ± 33 mm2, respectively. The mean values of the sagittal angle of the left (L4LSA) and right (L4RSA) superior facets of L4, the left (L5LSA) and right (L5RSA) superior facets of L5, and the left (S1LSA) and right (S1RSA) superior facets of S1 were 37.71° ± 4.38°, 36.18° ± 4.8°, 46.96° ± 6.49°, 48.51° ± 6.25°, 52.49° ± 5.1°, and 54.67° ± 5.25°, respectively. The result was characterized by increase in sagittal angle in a sequence from the fourth lumbar to the first sacral vertebra.
Table 1: Descriptive statistics of L4, L5, and S1 variables

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The range of asymmetry in the area of corresponding inferior facets of L4 and L5 was –23.16%–30.00% and −32.26%–20.99%, respectively, and that for sagittal angle of L4, L5, and S1 was −14.29%, 37.93%; -30.95%, 25.58% and -26.32%, 20.00%, respectively. [Figure 2]a, [Figure 2]b, [Figure 2]c. Having adopted the formula propound by Plochocki[9] to determine the proportion of asymmetry in a pair of corresponding structures, the negative value implied that the right parameter was greater than the left counterpart in some cases.

The correlation matrices and mean differences between the paired variables in [Table 2] showed that the mean value of surface area of L4RF and L5RF was significantly (P < 0.05) higher than that of L4LF and L5LF, respectively. Likewise, the mean value of L4LSA, L5RSA, and S1RSA was significantly (P < 0.05) higher than that of L4RSA, L5LSA, and S1LSA, respectively.
Table 2: The test of mean differences between L4, L5, and S1 corresponding variables

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The regression analyses in [Table 3] showed that the regression coefficients of the predictors were highly significant (P < 0.01), indicating that predictions made on the resultant regression equations are reliable. Likewise, the multiple regression analyses revealed a linear relationship of the sagittal angle in a sequence from L4 to S1, which could be predicted with the following equations:
Table 3: Regression analysis summary for L4, L5, and S1 corresponding variables

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L5LSA (°) = −9.689 + 0.946 L4LSA + 0.399 S1LSA

L5RSA (°) = 4.873 + 0.706 L4RSA + 0.331 S1RSA.


  Discussion Top


The analysis of the work was a setback for lack of a statistical package determining asymmetry in a pair of structures among individuals in a given population. Although manual measurement of vertebral column angulations was valid in assessment of spine derangement,[18] a simple diagnostic tool and methodology determining the sequential relationship of the vertebral facets of an individual is essential. The regression equations in [Table 3] could serve as functional matrices to adopt during identification of an individual sacrum and lower lumbar vertebrae in a forensic anthropology laboratory. Second, the equations might be useful to orthopedic surgeons during reconstructive spine intervention procedures that aim to retain the original alignment of the facets. Such techniques include vertebroplasty and kyphoplasty,[19] in conjunction with pedicle screw-based instrumentation used for treating certain spinal geometric distortion.[20],[21],[22]

In support of the results in [Table 1], some authors[15] observed that average inferior facet area of L4 and L5 was 1.49 ± 0.25 and 1.69 ± 0.31 cm2, respectively. Likewise, the mean value of the left sagittal angle of the superior facets of L4, L5, and S1 was 35° ± 11°, 44° ± 13°, and 55° ± 15°, respectively, and that of the right counterpart was 40° ± 14°, 41° ± 13°, and 36° ± 17°, respectively.[17] The values from this study were within the range of mean value of the figures above except for the value of the right sagittal angle of S1 vertebra. The difference was attributed to race[17] or biomechanical environment.[23],[24] Moreover, inherent error due to approximation and difficulty in locating anatomic landmarks might have contributed to the difference. However, independent assessments by the three researchers were adopted in this study to minimize these sources of error.

Although some authors[7],[25] observed tropism on the lumbosacral joint planes, others[2],[26] had no evidence of asymmetry in the superior or inferior facets of the lumbar vertebrae in children. Most likely, asymmetry in the facet area or facet joint planes at the lumbosacral junction in adults could be an outcome of uneven distribution of upper body weight and stress on the pair of facets during vertebral bone growth and remodel. Similarly, Rubbery[27] reported that the increase in sagittal angle of the lumbar vertebral facets in a sequence inclines the facet joint at the lumbosacral junction toward the coronal plane. Thus, the superior facet of S1 acts as an anterior wedge to the matching inferior facet of L5. The alignment was likely to enable a load on the body of vertebrae to gradually shift to their facets and also resist shear of the spine due to vertically directed axial load on the lumbosacral angle [Figure 1].

Notably, the vertebral column works as a driving shaft, and facet joints guide the extent and direction of its movements. If an axial torque rotation is applied at one end of it, every segment should be subjected to a twisting moment, in a direction perpendicular to the radius of the segment. Therefore, facet asymmetry presents an absence of common center of curvature for the left and right facets during hyperextension, flexion, and rotation of the spine. Precisely, it creates a rotational stress to the facet joint structures, leading to a plastic deformation,[28] especially on the counterpart with smaller radius and surface area. Stress is a measure of the force, an object experience per unit cross-sectional area, indicating an inverse relationship with surface area.

Therefore, mechanical strength of a vertebral facet, subject to the pressure from the pull of gravity on body weight and load, depends on its cross-sectional area. This would suggest that the left facet joint structures of the lower lumbar vertebrae were predisposed to stress effects and facet dysfunction of mechanical origin because of their smaller surface area when compared with the right corresponding facets.

However, the fifth lumbar vertebra gives origin to iliolumbar ligaments and relatively thickest mammillary processes and muscles that reinforce its superior facets.[10] The ligament and muscles attach to the pelvis. Ardently, we observed that the mammillary tubercle of the superior facet of S1 that aligned with the relatively larger inferior facet of L5 was likely to protrude more than its counterpart. The above-observed adaptive features might have reinforced the facets enabling them to accommodate the asymmetrical stress and also transmit it to the pelvis through the ligaments and muscles.

This study would suggest that despite the statistically significant mean differences in the paired variables in [Table 2], the vertebrae were free of any pathological change. It could be an indication that stresses encountered by the pair of facets were within the range of their allowable and working stresses. However, it might have explained in part why earlier researchers in anthropology attributed weakness and low back pain to the lumbosacral junction. Moreover, it revealed a structural variation in the skeletal framework of a modern man, and how it could affect his biomechanical environment, as well as offered insight to this paleontological puzzle.


  Conclusion Top


The increase in sagittal angle of the lumbar vertebral facets in a sequence led to a wedge alignment of the inferior facets of L5 and superior facets of S1. This arrangement could resist shear due to vertically directed axial load on the lumbosacral angle. Second, the mammillary tubercle of the superior facet of S1 that aligns with the relatively larger inferior facet of L5 was likely to protrude more than that of its corresponding facet. Furthermore, the mean value of the right facet area and sagittal angle greater than the left counterpart predisposed the left facets to stress effects and facet dysfunction of mechanical origin.

Acknowledgment

The authors would like to thank Dr. C. O. Ani (MD, FWACS) for proofreading the manuscript. We thank Dr. G. Ndukwe and Dr. S. Danborno for granting us access to the facilities at the Gross Anatomy laboratories of Abia State University and Ahmadu Bello University, Zaria, respectively, and Dr. Felix Aguboshim for the statistical analysis of this work.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

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    Tables

  [Table 1], [Table 2], [Table 3]



 

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