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00007632-201104200-0001400007632_2011_36_e569_mahato_transitional_9miscellaneous-article< 78_0_7_4 >Spine© 2011 Lippincott Williams & Wilkins, Inc.Volume 36(9)20 April 2011p E569–E573Facet Dimensions, Orientation, and Symmetry at L5–S1 Junction in Lumbosacral Transitional States[ANATOMY]Mahato, Niladri Kumar MBBS, MS, DNBDepartment of Anatomy, Sri Aurobindo Institute of Medical Sciences (SAIMS), Indore, Madhya PradeshAddress correspondence and reprint requests to Niladri K. Mahato, MBBS, MS, DNB, Department of Anatomy, Sri Aurobindo Institute of Medical Sciences (SAIMS), Indore-Ujjain Highway, Bhawrasala, Indore, Madhya Pradesh, India 452010; E-mail: mahatonk@yahoo.co.inAcknowledgment date: May 21, 2010. Revise date: July 9, 2010. Accepted date: August 2, 2010.The manuscript submitted does not contain information about medical device(s)/drug(s).No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.AbstractStudy Design. Study analyses dimensions, angular orientations, and symmetry of articular facets at the L5–S1 junction associated with transitional anomalies comprising (1) L5–S1 accessory articulations; (2) complete sacralization of the L5 vertebrae; and (3) completely lumbarized S1 sacral segments. Data were compared with same parameters evaluated in normal sacrum.Objective. Investigating probable morphologic differences in facet joints at the normal and transition affected lumbosacral junctions.Summary of Background Data. Facet joints at the L5–S1 play an important role in load sharing and imparting stability at this area as well as are involved in conditions such as isthmic spondylolysis, degenerative spondylolisthesis, and osteoarthritis, giving rise to low back pain situations. Several morphologic variations at the lumbosacral junction have been studied and inconsistently alluded to be associated with these painful conditions. Lumbosacral transitional anomalies, on the other hand, have also been linked to low back pain situations. L5–S1 transitions have been shown to be associated with altered L5–S1 articular morphology as well as load-sharing pattern at the region. The analysis of L5–S1 zygapophysial anatomy in context of transitional anomalies at this region has rarely been documented.Methods. Both the superior articulating facets in the normal as well as in transition associated sacra were measured for (1) height, (2) width, (3) surface area, and (4) angulation of the articular surfaces with reference to the midsagittal plane. Data were also analyzed to find structural asymmetry between the two sides.Results. Facets demonstrated smallest linear dimensions, surface areas, and maximum coronal orientation in lumbarized specimens. Facets areas associated with accessory articulations were also smaller and coronally oriented. Sacralization induced insignificant alteration in the facet morphology. Asymmetry (tropism) was observed in facets predominantly associated with accessory L5–S1 articulations.Conclusions. L5–S1 transitions possess altered facet morphology. These alterations are possibly related to low back pain situations.Facet or zygapophysial joints are true diarthrodial articulations.1 These joints and their articular processes acquire their final shape and spatial orientation through childhood depending on the pattern of load distribution acting within them.2 Facet joints in the upper lumbar region are directed relatively in the sagittal plane, gradually changing into a more coronal orientation, as one proceeds to the L5–S1 junction.3,4 The orientation of the facets has been precisely analyzed by identifying their inclinations relative to the frontal, the sagittal, and the horizontal planes.5,6 Structural asymmetries (tropism) in the facets of lumbar vertebrae are frequently encountered.7,8 Tropism has been implicated in altered biomechanical properties at the facet joints possibly leading to degenerative changes in the adjoining intervertebral discs and osteoarthritis of the facet joint.9–11 Altered facet orientation has also been linked to spondylolytic afflictions at the lumbar spine.12,13 Biomechanically, the lumbar facets are highly loaded structures in a normally oriented spine. The load share at these facets may increase many folds in conditions such as in extension of the spine, osteoarthritis of the facet articulations, and relative position of an external load while lifting the same, to name a few.14–16 Despite bearing significant load, the facet joints are endowed with certain range of rotatory, lateral bending, and flexion-extension movements. These movements, nevertheless, are restricted in accordance to the orientation of the facet articulating surface.17–19 The facet joints at the L5–S1 junction are relatively coronal in their disposition and on an average measure approximately 10 mm in height, 18 mm in width, and have a surface area of 158 mm2.20,21Transitional anomalies at the L5–S1 junction are quite frequently encountered in a clinical setting of low back pain situation or detected accidentally in an asymptomatic individual during workup related to an unassociated condition. Lumbosacral transitional vertebra may range from presenting as (1) unilateral or bilateral accessory articulations between an extended transverse process of the L5 vertebra, (2) complete unilateral or bilateral fusion of the L5 and S1 segments at the transverse process and the vertebral bodies (sacralization), and (3) to featuring complete separation of the S1 from the rest of the sacral mass (lumbarization).22 L5–S1 transitional anomalies are associated with altered morphologies of all articular surfaces (including facet surface area) of the sacrum.23–25 These alterations change the load-bearing stress patterns at the facets probably exposing them to higher risk of degeneration causing low back pain.26–28Morphologic alterations associated with L5–S1 transitions have been reported, with probable biomechanical explanations pertaining to their frequent involvement in low back pain situations. It is however, difficult to find studies that focus on the facet morphology, its orientation, and bilateral symmetry in context of all types of L5–S1 transitional anomalies. This study investigates probable structural differences in the facets in L5–S1 transitions, as presented in the configuration of the superior articular facets in the associated sacra.MATERIALS AND METHODSNormal sacra (n = 150), sacra with accessory articulation between an extended L5 transverse process and the alae of sacrum (n = 25) [(unilateral articulation, n = 15), (bilateral articulations n = 10)], sacra with complete and bilaterally fused L5 (sacralized) (n = 20), and sacra with complete separation of the S1 segment (n = 5) were selected for the study. These dried human sacra belonged to the age group 40 to 65 years, represented samples from both the sexes and included samples from different population groups. Superior articular facets in all specimens on both the sides were measured for: (1) the maximum available vertical distance across the articular surface, (2) maximum transverse distance between the ventral and dorsal margins of the surface, (3) the articular surface area, and (4) the angle subtended by the plane of the articular facet with the sagittal plane in the midline (Figures 1 and 2). Linear measurements were recorded using a digital vernier caliper [sensitivity 0.01mm]. Surface areas were traced on thin cellophane paper and the measured with the help of a digital planimeter. Angles were determined with a device made up of two thin straight wires. The wires were joined with each other at a flexible angle that changed its value in accordance to the separation between the two wires. The two wires could be slid over each other in either direction, at the flexible junction. To obtain the facet angulation, one arm of the wire was positioned in the midsagittal plane (on the midline over the sacral body and over the root of the spinous process, coinciding with the direction of the spine of the sacrum). The other arm was approximated along the facet articular surface, touching the ventral and the dorsal ends of the surface simultaneously. Descriptive statistics and differences in the mean values of all the parameters between the groups were calculated.Figure 1. A normal superior sacral articular facet. The asterisk denotes the angular orientation of the facet. FW, facet width. The inset shows the top of an L5 sacralized specimen. Note the similarity of the facets in the two sacra.Figure 2. Facets associated with unilateral accessory articulation (left), on the sacral ala. Note the coronal orientation, small area, and flatness of the facets. Similar features in the facets can be observed in facets in a bilateral accessory L5–S1 articulated situation (inset). FH, facet height; Acc Art, accessory articulation.RESULTSThe absolute values for all parameters were found to be greatest for the normal sacra except for the angular orientation with respect to the sagittal plane (Table 1). The heights of the facets were smallest for facets with bilateral L5–S1 accessory articulations. Bilateral fusion of the L5 (complete fusion of the L5 to the sacral mass) appeared not to have altered the heights of the facet to any significant extent. Complete lumbarization (separation of the S1 sacral segment from the rest of the sacrum), however, was associated with shorter facet heights. The distribution of width values across the sacral samples demonstrated a similar pattern as seen for the facet height. The facet surface areas showed maximum values in the normal sacra. Transitions with bilateral accessory articulations demonstrated facets with the smallest surface areas. Complete sacralization reduced the surface areas minimally. Lumbarization marginally decreased the facet articular surface. The angular orientation of the facets exhibits perceptible differences. The plane of the facet articular surfaces when measured against the midsagittal plane yielded minimum values for the normal facets. Slightly greater values were seen in the sacralized specimen. The angles presented higher values in facets associated with unilateral and bilateral accessory articulations. The maximum values for the angle were seen in the samples where the top of the sacra was represented by the superior articular facet of the S2 sacral segment (in complete lumbarization of S1). The increased values of the angles indicated a more coronal orientation of the plane of the facet articular surface (a value of 90° indicating a plane absolutely congruent to the coronal plane).TABLE 1. Mean Values of Parameters Measured in the Superior Sacral Articular Facets. Linear Dimensions Are Given in millimeters and Surface Areas in Square Millimeters. Standard Deviations Are Given in Parenthesis (±2SD). “P” Values Indicate the Test of Significance of Difference of Means Between the Group and the Normal Facets. Facet Angles Are in Degrees with Relation to the Midsagittal PlaneAnalysis of the facet area and the angles on both the sides was expected to identify the presence symmetry in the parameters between the two sides, or the lack of it (Table 2). All the facets demonstrated differences in the means of all parameters, across the midline, in all groups of sacra. None of these differences were statistically significant. Samples with unilateral accessory articulations demonstrated marginally greater surface areas on the side of the accessory facet. The value of the facet angle, on the side of the accessory articulation, also showed a slightly increased coronal orientation. The absolute difference between the means between the two sides in the lumbarized group was observed to be large (mean = 5.25 mm). This possibly occurred because of the less number of such anomalies studied here. The difference was not statistically significant.TABLE 2. Statistical Comparison of Facet Joint Areas and Angle on Both Sides. The Data for Facets Related to Unilateral Accessory Articulation Are also Presented as Values Observed on the Side of the Accessory Articulation as well as Those Measured on the Side Without It. Differences in the Means of Linear Dimensions Are Given in millimeters and That of Surface Areas in square millimeters. Standard Deviations of the Differences Are Given in Parenthesis (±2SD)Although statistically not significant, morphologic differences between facets at the same plane were observed to be common. Asymmetry (tropism) often presented as a small, flat, or dysmorphic contralateral facet (Figure 2). Asymmetries in articular surface areas were observed to be the maximum between facets associated with unilateral accessory articulations (mean = 0.23 mm).DISCUSSIONThe facets are loaded structures and the load subjected to the facet joints is proportional to the total articular surface area of the facets.29 Transitional anomalies alter the articular structures and load-bearing patterns at the L5–S1 junctions. These alterations are probably reflected in the changes at the zygapophysial facets that are observed in this study. The facets are the smallest at the L5–S1 junction that demonstrate bilateral accessory L5–S1 articulations. Probably these accessory facets share the total quanta of forces that pass through the L5–S1 interface and results in reduction of the area of the facet joints. Though these facets are smaller, their sizes probably do not compromise their roles in prevention anterior translation of the L5–S1 due to their increased coronal orientation. This type of orientation of facets (in presence of joint capsules of accessory articulations) might restrict or jeopardize normal lateral bending, axial rotation, and flexion extension at the L5–S1 junction predisposing it to an LBP situation.Facets at the L6–S1 level (in lumbarization) demonstrate the most coronal of articular planes. The area at these facet surfaces is also smaller. The lumbosacral facets in these cases actually represent the articulation between the S1 and the S2 sacral segments; the S1 representing a presacral segment here. Thus, the facets at the top of the sacral mass correspond to the superior articulating facets of the S2 segment. The facet area is, therefore, smaller and more coronal. The “coronality” of the facets increases as one moves caudally from the L5–S1 junction towards facet joints within the body of the sacrum. Complete lumbarization increases the number of movable lumbar segments and reduces the size of the sacral mass and its articular surfaces. These transitions may be associated with greater wear and tear, and lesser stability at the lower spine. It can be questioned whether spines with additional lumbar vertebrae are longer at the lumbar region and possibly with an accentuated lordotic curve. If it were true, the L6–S1 junction would be subjected to an increased gradient of a horizontal (translatory) force at the junction and a reduced degree of force from the axial direction. This possibly explains the utility of coronal orientation of facets seen with lumbarization (trying to prevent a spondylolysthesis).Sacralization of L5 is also associated with structural alterations. The size of the sacrum increases with the inclusion of the L5 vertebra at the top. In context of the facets, no significant changes are observed. Spines with L5–S1 fusion present the L4–L5 junction as the new lumbosacral junction. The ligaments associated with the L4 vertebrae are not as well anchored to the ilium (iliolumbar ligament) or to the sacrum (lumbosacral ligament), as with the L5 vertebra at a normal L5–S1 junction. Important muscles, such as the psoas major, quadratus, interspinous, and intertransversus, attached to the L5 vertebra in a situation with an L4–L5 lumbosacral junction probably, lose their effectiveness in harnessing stability at lower back. In this study, the L4–L5 facets, in sacralization, present almost normal surface areas and angulation. This junction possibly is subject to a trajectory of load that is more axial in nature. The orientation of these facets allows normal range of motion. Angulation of facets at this region perhaps does not need to be as coronal as observed in normal or lumbarized lumbosacral junction. Asymmetry (tropism) is frequently encountered in the facets, especially where load bearing at the L5–S1 junctions is shared by accessory lumbosacral articulations.30 Unilateral L5–S1 accessory articulations are seen to be affected with increased asymmetry in surface area of the facets.To conclude, it is important to realize that transitions at the lumbosacral junction are associated with changes in the angulation of the facet joints and influence the overall morphologies at these joints. Alteration in morphology at the L5–S1 junction in general and at the facet joints in particular may be associated with abnormal patterns of load transfer leading to low back pain.Key Points * L5–S1 transitions are associated with structural alterations at the lumbosacral junction. * Facets are smaller and more coronally oriented in L5–S1 transitional states. * Changes related to sacralization are minimal. * Asymmetry in facet morphology can be detected in all types of L5–S1 junction.References1. 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Asymmetry in facet morphology can be detected with or without transitions.Facet Dimensions, Orientation, and Symmetry at L5–S1 Junction in Lumbosacral Transitional StatesMahato, Niladri Kumar MBBS, MS, DNBAnatomy936