Unilateral Lumbar Spondylolysis on Radiography and MRI: Emphasis on Morphologic Differences According to Involved Segment

Transcription

1 Musculoskeletal Imaging Original Research Park et al. Radiography and MRI of Unilateral Lumbar Spondylolysis Musculoskeletal Imaging Original Research Ji Seon Park 1 Sung Kyoung Moon 1 Wook Jin 2 Kyung Nam Ryu 1 Park JS, Moon SK, Jin W, Ryu KN Keywords: MRI, pars interarticularis defect, pediatric imaging, radiography, spondylolysis, sports medicine, trauma DOI: /JR Received pril 20, 2009; accepted after revision June 17, This research was supported by a grant from the Kyung Hee University Research Fund (grant KHU ). 1 Department of Radiology, Kyung Hee University Medical Center, 1, Heoki-dong, Dongdaemun-ku, Seoul , Korea. ddress correspondence to J. S. Park (gdluck@hitel.net). 2 Department of Radiology, East-West Neo Medical Center, College of Medicine, Kyung Hee University, Seoul, Korea. CME This article is available for CME credit. See for more information. JR 2010; 194: X/10/ merican Roentgen Ray Society Unilateral Lumbar Spondylolysis on Radiography and MRI: Emphasis on Morphologic Differences ccording to Involved Segment OJECTIVE. The objective of our study was to retrospectively compare the radiography and MRI findings of unilateral spondylolysis in the upper lumbar segment and in the lower lumbar segment and to consider how these radiologic findings can be applied in the diagnosis of unilateral spondylolysis. MTERILS ND METHODS. Thirty patients with unilateral lumbar spondylolysis were categorized into one of two groups according to the lumbar levels involved with pars interarticularis defects: group (L1, L2, and L3) or group (L4 and L5). On radiographs, we evaluated contour bulging of the affected pars interarticularis, reactive sclerosis in the contralateral pedicle, anterolisthesis of the involved vertebra, and deviation of the spinous process. On MRI, we assessed pseudoarticulation of the pars interarticularis defect, uneven distribution of posterior epidural fat, the interspinous distance between adjacent segments, facet and disk degeneration in adjacent segments, and other anomalous changes. RESULTS. mong the 63 patients with unilateral spondylolysis, the upper lumbar segment was involved in 29 and the lower lumbar segment, in 34. Group often displayed contour bulging of the affected pars interarticularis, reactive sclerosis of the contralateral pedicle, and contralateral deviation of the spinous process, all of which were easily detectable on radiography. Group frequently showed anterolisthesis, pseudoarticulation of the pars interarticularis defect, adjacent facet disk degeneration, and other anomalous changes that were well observed on MRI. CONCLUSION. Unilateral lumbar spondylolysis displayed radiologic differences in morphology of the isthmic defect itself and in ancillary findings of the adjacent structures based on the segment involved. Recognition of different ancillary features of unilateral spondylolysis with the use of a feasible diagnostic tool can be helpful for the diagnosis of cases in which a direct sign of isthmic defect is equivocal. U nilateral spondylolysis, a unilateral osseous defect or fracture in the pars interarticularis, is a rare and unique entity that accounts for 3 33% of all cases of spondylolysis [1 3], which occurs in approximately 3 7% of the general population [4 9]. Spondylolysis presents predominantly in the lower lumbar segment, where the concentration of mechanical stress is high. The frequency of lower lumbar spondylolysis at L5, L4, and L3 is approximately 67%, 15 30%, and 1 2%, respectively [7 10]. Involvement of L1 and L2 is rarely encountered [5, 11 13]. Consequently, the condition of combined unilateral and upper lumbar spondylolysis is rarer than either condition alone. Radiologically, the direct sign of unilateral spondylolysis is a unilateral radiolucent area through the pars interarticularis, giving the appearance of a lucent collar on the neck of the so-called scottie dog, that is well shown on an oblique view. When radiologic diagnosis is solely based on the detection of this direct sign, however, unilateral spondylolysis is frequently missed because of suboptimal radiographs, lack of clinical suspicion, and partial volume artifacts on cross-sectional images [3, 14]. In previous radiologic studies, investigators documented various ancillary signs of unilateral spondylolysis including contralateral reactive sclerosis, asymmetric wedging of the posterior vertebral angle, asymmetric posterior neural arch, increased sagittal diameter of the spinal canal, anterolisthesis, marrow changes of the pars interarticularis defects, degenerative changes of the facet joints and disks, and other anomalies such as spina bifi- JR:194, January

2 Park et al. da [1, 3, 11, 14 16]. These findings were seen mainly in patients with unilateral spondylolysis with lower lumbar segment involvement. In a previous report, investigators described spondylolysis in the upper lumbar spine as possessing a high frequency of unilateral involvement (52.9%) and atypical pars interarticularis defects (36%) that are vertical at the inferolateral part and horizontal at the superomedial part [17]. Recently, we became concerned about the radiologic differences of unilateral spondylolysis according to the involved lumbar segment. To our knowledge, however, the morphologic differences of unilateral lumbar spondylolysis related to their level have not been reported in the radiology literature. Thus, we retrospectively compared and analyzed the radiologic findings of unilateral spondylolysis in the upper lumbar segment and in the lower lumbar segment on radiography and MRI. dditionally, we considered how these imaging findings can be applied for the accurate diagnosis of unilateral spondylolysis. Materials and Methods Patients The ethics committees of our institution did not require patient approval or informed patient consent for this retrospective study. We retrospectively reviewed the medical records of patients diagnosed between March 2002 and May 2009 with unilateral spondylolysis on the basis of both radiography and MRI in our institutions. We searched the radiologic databases on the PCS at our institutions using the term unilateral spondylolysis and identified the records of 63 patients (mean age, 41.5 years; age range, 5 82 years): 29 females (mean age, 48.5 years) and 34 males (mean age, 35.6 years). These patients were categorized into one of two groups according to the lumbar levels involved with the pars interarticularis defects. Group consisted of patients with upper lumbar segment defects at L1, L2, and L3, and group included those with lower lumbar segment defects at L4 and L5. Fig year-old man with right spondylolysis of L1. and, nteroposterior () and right oblique () radiographs show broadening of adjacent surfaces along right pars interarticularis defect of L1 (arrows), reactive sclerosis of contralateral pedicle, and slight deviation of inferior margin of L1 spinous process (open arrow, ) toward contralateral side. Imaging Radiography consisted of the following four lumbar spine images: anteroposterior, lateral, and both oblique views. MRI was performed on a 1.5-T scanner using a dedicated lumbar spine coil and a standard protocol for lumbar spine MRI. oth T1-weighted spinecho images (TR range/te, /10) and T2-weighted fast spin-echo images (TR range/ TE range, 3,000 3,500/ ) were obtained with the lumbosacral spine in the sagittal and axial planes. Typical imaging parameters were as follows: field of view, mm; matrix size, ; section thickness, 4 mm; intersection gap, 0.4 mm; and echo-train length, 2 3. The mean interval period between radiography and MRI was 12.3 days, with a range of days. Imaging nalysis Two experienced musculoskeletal radiologists, with 21 and 14 years experience, respectively, retrospectively reviewed all the available radiographs and MR images; all interpretations were decided by consensus. On radiographs, the reviewers assessed for the presence of the following: contour bulging of the affected pars interarticularis, reactive sclerosis and hypertrophy in the contralateral pedicle, anterolisthesis of the involved vertebra, and deviation of the spinous process and its direction. Contour bulging of the affected pars interarticularis was defined as marginal broadening along the course of the pars interarticularis defect or outbulging of the affected pars interarticularis (Fig. 1). nterolisthesis was considered present when the anterior translation was 1 mm or greater of the affected vertebra with respect to its inferior vertebra. Deviation of the spinous process was evaluated on the anteroposterior view, and its direction was classified as either ipsilateral or contralateral depending on which way the caudal end of the spinous process of the affected vertebra deviated from the midline. On MRI, we assessed six variables: pseudoarticulation of the pars interarticularis defect, uneven distribution of posterior epidural fat, the interspinous distance between adjacent segments, adjacent facet degeneration, adjacent disk lesions, and other anomalous changes of affected vertebrae. Pseudoarticulation of the pars interarticularis defect was defined as a jointlike formation between the adjacent surfaces of the pars interarticularis defect; on paramidline sagittal MR images, this finding appeared as a narrow gap with marginal irregularities of the adjacent surfaces with or without marrow changes of the adjacent surfaces or fluid and not as a thin line with low signal intensity. The distribution of posterior epidural fat was evaluated on axial or sagittal T1-weighted MR images. n uneven distribution was defined as the asymmetric distribution of posterior epidural fat seen on axial images of the affected vertebral level or, compared with other vertebral levels, the interposition of posterior epidural fat between the posterior margin of the thecal sac and the anterior margin of the spinous process of the affected vertebra on sagittal images (Figs. 2F, 3E, and 4E). The interspinous distance, which was defined as the distance between the midpoint of the anterior margin of the spinous process of the affected vertebra and that of the vertebra above, was measured at the midsagittal image (Fig. 2E). The interspinous distance of the affected vertebra was compared with those of adjacent levels to evaluate whether the spinous process of the affected vertebra was displaced downward or not. Normally, lordotic lumbar curvature has decreased interspinous distances proceeding caudally. The interspinous distance was considered increased when the interspinous distance of the affected vertebra was greater than that of its contiguous upper segment. The presence of disk lesions, including degeneration, bulging, and herniation, and of facet degeneration was evaluated above and below levels adjacent to the affected vertebra. nomalous changes of the affected vertebra included spina bifida, pediculolysis, and fibrous ankylosis of contralateral pars interarticularis defects. Statistical nalysis The difference in age between the two patient groups was tested for significance using the Mann-Whitney test. The difference in the side of the pars interarticularis defects between the two groups was tested for significance using the Fisher s exact test. 208 JR:194, January 2010

3 Radiography and MRI of Unilateral Lumbar Spondylolysis Fig year-old girl with left spondylolysis of L3. and, nteroposterior () and left oblique () radiographs show broadening of adjacent surfaces along left pars interarticularis defect of L3 (black arrows), reactive sclerosis of contralateral pedicle, and slight deviation of inferior margin of spinous process (L3) toward contralateral side (white arrow, ). C, Right parasagittal T1-weighted image reveals hypertrophy and sclerosis of contralateral pedicle of L3 (circle). D, Left parasagittal T1-weighted image shows pars interarticularis defect with low-signal line on L3 (arrows). djacent facet joints and disks are preserved. E, Midsagittal T1-weighted image shows increased interspinous distance (ISD) between affected vertebra and vertebra above compared with that of contiguous upper segment (u-isd), suggesting downward displacement of affected spinous process. Lumbar alignment is preserved without anterolisthesis. F, On axial T1-weighted image at level of L3 L4 disk, posterior epidural fat is slightly thicker on intact right side (arrow) than involved side, indicating uneven distribution of posterior epidural fat at level of affected vertebra. total of 10 radiologic findings on radiography and MRI were compared between the two groups using a chi-square test. p value of less than 0.05 indicated a significant statistical difference. Results mong the 63 patients, pars interarticularis defects involving an upper lumbar segment (group ) were seen in 29 patients (mean age, 33.9 years; age range, 5 69 years) and those involving a lower lumbar segment (group ) were present in 34 patients (mean age, 48.0 years; age range, years). Group was younger than group with statistical significance (p = 0.008). In group, spondylolysis occurred at L1 in seven patients, L2 in 16 patients, and L3 in six patients. In group, spondylolysis was found at L4 in five patients and L5 in 29 patients. Of the 63 pars interarticularis defects, right-sided defects were noted in 37 patients (20 patients from group and 17 patients from group ) and left-sided defects were seen in 26 patients (nine patients from group and 17 patients from group ). There was no significant difference between the two groups based on the side of the pars interarticularis defect (p = 0.128). Radiography The radiography findings of the study population are shown in Table 1. Contour bulging of the affected pars interarticularis Marginal broadening along the course of the pars interarticularis defect or out- TLE 1: Radiographic Findings of Unilateral Lumbar Spondylolysis ccording to Involved Segment Radiographic Findings No. (%) of Patients Group a (n = 29) Group b (n = 34) Total (n = 63) Contour bulging of involved pars interarticularis 25 (86.2) 0 (0) 25 (39.7) Reactive sclerosis of contralateral pedicle 27 (93.1) 22 (64.7) 49 (77.8) nterolisthesis 1 (3.4) 14 (41.2) 15 (23.8) Deviated spinous process 26 (89.7) 17 (50) 43 (68.3) Contralateral deviation 24 (92.3) 12 (70.6) 36 (83.7) a Upper lumbar segment: L1, L2, and L3. b Lower lumbar segment: L4 and L5. C E D F JR:194, January

4 Park et al. bulging of the affected pars interarticularis or nearby lamina (Figs. 1 and 2 2) was noted only in group (25/29, 86.2%) (p < 0.001). Reactive sclerosis and hypertrophy in the contralateral pedicle Reactive sclerosis and hypertrophy in the contralateral pedicle were more common in group (27/29, 93.1%) than group (22/34, 64.7%) (Figs. 1, 2, 3, and 5). This difference was statistically significant (p = 0.007). nterolisthesis of the involved vertebra nterolisthesis of the involved vertebra (Fig. 3) was present more often in group (14/34, 41.2%) than in group (1/29, 3.4%). This difference between the two groups was statistically significant (p < 0.001). Deviation of the spinous process and its direction Deviation of the spinous process (Figs. 1, 2, and 5) was more common in group (26/29, 89.7%) than group (17/34, 50%), and the difference was statistically significant (p = 0.001). ll but two patients in group showed the deviation of the spinous process toward the contralateral side (24/26, E 92.3%). In group, the direction of the deviated spinous process was contralateral in 12 patients (12/17, 70.6%) and ipsilateral in five (5/17, 29.4%). The prevalence of contralateral deviation was not statistically different between the two groups, although it was more predominant in group than group (92.3% vs 70.6%, respectively; p = 0.093). MRI The MRI findings of the study population are shown in Table 2. Pseudoarticulation of the pars interarticularis defect Pseudoarticulation of the pars interarticularis defect (Figs. 3C 3D, 4C 4D, and 5C 5D) was present more often in group (31/34, 91.2%) than in group (4/29, 13.8%) (p < 0.001). In the group patients without pseudoarticulation, the presence of a pars interarticularis defect was suggested by two or three contiguous parasagittal MR images because one parasagittal MR image failed to directly visualize the pars interarticularis defect. C Uneven distribution of posterior epidural fat Uneven distribution of posterior epidural fat (Figs. 2F, 3E, and 4E) was common in both group (22/29, 75.9%) and group (24/34, 70.6%). This difference between the two groups was not statistically significant (p = 0.638). Increased interspinous distance between adjacent segments n increased interspinous distance between adjacent segments (Fig. 2E) was detected in 25 patients of group (25/29, 86.2%) and two patients of group (2/34, 5.9%). This difference between the two groups was statistically significant (p < 0.001). Facet degeneration in adjacent segments Facet joint degeneration (Fig. 4E) was present more often in group (21/34, 61.8%) than in group (1/29, 3.4%) (p < 0.001). Disk lesions in adjacent segments Disk lesions in adjacent segments (Figs. 3E, 4D, and 5D) were noted more often in group (28/34, 82.4%) than in group (4/29, 13.8%) (p < 0.001). Other anomalous changes of the posterior elements Combined anomalous changes in Fig year-old man with right spondylolysis of L5., On anteroposterior radiograph, reactive hypertrophy with sclerosis of left pedicle and spina bifida occulta (arrow) are observed at L5, even though right isthmic defect is unclear., Subtle anterolisthesis of L5 (lines) is noted on lateral radiograph. C and D, Right parasagittal T1-weighted (C) and T2-weighted (D) images show gap with irregular margins and fibrovascular marrow changes of adjacent surfaces at right L5 pars interarticularis (arrows). E, Midsagittal T2-weighted image shows slight anterolisthesis of L5 on S1 and disk degeneration with bulging at adjacent L4 L5 level. dditionally, distance between anterior margin of spinous process and posterior wall of thecal sac (arrows) is slightly separated, indicating posterior epidural fat deposit. D 210 JR:194, January 2010

5 Radiography and MRI of Unilateral Lumbar Spondylolysis the posterior elements of the affected vertebrae (Figs. 5E and 5F) were detected in nine patients of group (9/34, 26.5%), including spina bifida occulta with an asymmetric neural arch in six patients, contralateral pediculolysis in two patients, and contra lateral fibrous ankylosis with contralateral pediculolysis in one patient. No anomalous changes were identified in group. The difference between the two groups was statistically significant (p = 0.003). Discussion Unilateral spondylolysis, a unilateral defect of the pars interarticularis that results in an asymmetric posterior arch, is thought to be caused primarily by an acquired stress fracture after progressive elongation of the isthmic portions secondary to chronic, repetitive, sports-related injuries that mainly develop in young age groups. Cricket, with its asymmetric rotatory movements, throwing sports, athletics with lumbar extension and rotation, parachute jumping with asymmetric axial compressive force, and gymnastics are known to be the most commonly related sports [18]. Fig year-old woman with right spondylolysis of L4., nteroposterior radiograph shows left L4 L5 facet degeneration without visualization of pars interarticularis defect, reactive sclerosis, or deviation of spinous process., Right oblique radiograph reveals bone defect with acute narrowing of its opposing portions (arrowheads) at right L4 pars interarticularis. C and D, Right parasagittal T1-weighted (C) and T2-weighted (D) images show gap with irregular margins of opposing surfaces at right L4 pars interarticularis (arrows). L4 is slipped anteriorly on L5. Note severe disk degeneration and bulging at levels of L4 L5 and L5 S1, resulting in ipsilateral neural foraminal stenosis. E, On axial T1-weighted image, facet degeneration with subluxation at left L4 L5 facet is apparent. Uneven distribution of posterior epidural fat (arrow) is also noted. Other causes of unilateral spondylolysis to consider are a hereditary predisposition, such as an asymmetric developmental deficiency of the posterior arch [3], and unilateral healing of bilateral spondylolysis, accounting for approximately 10 15% of unilateral spondylolysis cases [7, 10, 14, 19]. Patients with unilateral spondylolysis may be asymptomatic or may experience ambiguous symptoms, such as lower back pain, in the early stages that may result in a delay of E diagnosis [2]. Significant lower back pain or radiculopathy eventually develops in approximately 25% of lumbar spondylolysis patients [20]. These symptoms initially begin as a radiographically occult lesion without cortical disruption and, with repetitive injury, progress to a pars interarticularis defect that is visible on radiography. The natural history of spondylolysis can vary over time, including healing with fibrous ankylosis, nonunion, unstable posterior arches, compensatory hy- TLE 2: MRI Findings of Unilateral Lumbar Spondylolysis ccording to Involved Segment MRI Findings No. (%) of Patients Group a (n = 29) Group b (n = 34) Total (n = 63) Pseudoarticulation of pars interarticularis defect 4 (13.8) 31 (91.2) 35 (55.6) Uneven distribution of posterior epidural fat 22 (75.9) 24 (70.6) 46 (73) Increased interspinous distance 25 (86.2) 2 (5.9) 27 (42.9) Facet degeneration in adjacent vertebrae 1 (3.4) 21 (61.8) 22 (34.9) Disk lesions in adjacent vertebrae 4 (13.8) 28 (82.4) 32 (50.8) Other anomaly of posterior elements 0 (0) 9 (26.5) 9 (14.3) a Upper lumbar segment: L1, L2, and L3. b Lower lumbar segment: L4 and L5. C D JR:194, January

6 Park et al. E pertrophy of the contralateral pedicle, stress fracture of the contralateral posterior element, degenerative changes of adjacent disk lesions, spondylolisthesis, and more [1, 14]. Fujii et al. [21] reported that the stage of pars interarticularis defects and involved spinal levels are the predominant factors influencing the bony union in pars interarticularis defects, along with the site of the defect in the neural arch, the degrees of lumbar lordosis and inclination, the presence of spondylolisthesis of the affected vertebra, and the condition of the contralateral pars interarticularis. The clinical outcome of lumbar spondylolysis depends on appropriate treatment accompanied by early diagnosis, which is especially important for affected children and adolescents participating in sports and at an increased risk of developing subsequent vertebral changes [6, 22 24]. The incidence of unilateral lumbar spondylolysis has been estimated at approximately F C Fig year-old woman with right spondylolysis of L4, contralateral pediculolysis, and contralateral fibrous ankylosis of previous pars interarticularis defect., nteroposterior radiograph shows suspicious finding of radiolucent line at right pars interarticularis of L4 (thin arrows) and hypertrophy of contralateral pedicle. Slight deviation of spinous process toward contralateral side (thick arrow) is seen., Right oblique radiograph shows radiolucent defect at right L4 pars interarticularis (arrowheads), with narrowing of opposing surfaces at defect site. C and D, Right parasagittal T1-weighted (C) and T2- weighted (D) images show gap with irregular margins and fatty marrow changes of adjacent surfaces at right L4 pars interarticularis (arrows). E and F, Contiguous left parasagittal T1-weighted images reveal bony defect at right L4 pedicle (white arrows) and fibrous ankylosis of left L4 pars interarticularis defect (black arrows, E). less than 2% in the general population, with an overwhelming predominance of pars interarticularis defects at L5 [1 6, 24, 25]. However, several authors of anthropologic and radiologic reports [1, 3] have suggested that the frequency of unilateral spondylolysis has been underestimated because of radiologically and clinically missed lesions. In addition to the clinically missed lesions described, the radiologically missed lesions of unilateral spondylolysis are thought to be due to suboptimal D 212 JR:194, January 2010

7 Radiography and MRI of Unilateral Lumbar Spondylolysis radiographs (i.e., overlapped soft-tissue shadows, improper patient positioning, inadequate setting of parameters, less widening of spinal canal related to less mobility of the affected vertebral body, and so on), a lack of concern by the radiologist, and the partial volume artifacts related to sclerosis of the pars interarticularis and coexisting facet degeneration on MRI [3, 14, 25]. In the current study, the ratio of upper to lower segments in unilateral spondylolysis was relatively high (29/63, 46%) compared with previous reports; however, it would not be appropriate to use our results to evaluate the incidence and preferential level of unilateral lumbar spondylolysis because our study included subjects with spondylolysis diagnosed based on only initial radiologic reports. Nevertheless, our results suggest that radiologic detection rates, including total incidence and segment predilection for lumbar spondylolysis, may differ according to the impressions of the radiologist and to the imaging technique. In this study, we found that it was easier to detect unilateral spondylolysis involving the upper lumbar segment than cases involving the lower lumbar segment on radiographs because of easier visualization of pars interarticularis defects without the disturbance of overlapping shadows and because of the greater frequency of accompanying ancillary findings. On the other hand, some cases of unilateral spondylolysis involving the lower lumbar segment were missed on radiography and were then identified by MRI, which was performed for the evaluation of accompanying degenerative changes of the adjacent facet or disk. Unilateral spondylolysis has displayed a right-sided predominance in previous cohort studies [1, 24], and our results are in agreement with those reports. These findings suggest some sort of relationship with handedness. The imaging techniques for the diagnosis of unilateral spondylolysis include radiography, scintigraphy, SPECT, CT, and MRI. lthough commonly used, radiography has limited efficacy for the diagnosis of unilateral spondylolysis for the reasons described earlier. The advent of modern imaging tools has facilitated the diagnosis and characterization of spondylolysis. Scintigraphy and SPECT have improved the diagnostic localization of spondylolysis by detecting increased metabolic activity of pars interarticularis defects in response to stress [11]. CT can provide the best visualization of unilateral spondylolysis, enabling clear delineation of the course, shape, and degree of union of pars interarticularis defects [25, 26]. In addition, CT depicts any changes of adjacent bony structures. lthough MRI can feasibly detect marrow changes surrounding pars interarticularis defects and accompanying disk lesions, it is inferior to CT for the direct visualization of pars interarticularis defects because of partial volume artifacts and various other factors such as motion artifacts related to back pain, no acquisition of contiguous axial images, and lowfield-strength units [14, 15, 27]. Radiologic findings of unilateral spondylolysis include the direct sign of a unilateral defect of the pars interarticularis and indirect signs such as reactive sclerosis of the contralateral pedicle, asymmetric posterior neural arches, deviation or step-off signs of the spinous process, widening of the spinal canal with anterolisthesis or posterior subluxation of the posterior element, marrow changes of the pars interarticularis defect, and degenerative changes of the facet joints and disks adjacent to the affected vertebra [1, 3, 11, 14 16, 25]. These various radiologic features simultaneously represent a combination of the causes, serial changes, and final results of unilateral lumbar spondylolysis, and it may be impossible to discriminate the cause and effect from images obtained at a single moment in time. In this study, we hypothesized that the radiologic manifestations of unilateral spondylolysis would differ according to the involved segment because its clinical outcome is known to be mainly affected by the level of the affected vertebra [21]. y comparing radiologic findings from unilateral spondylolysis of the upper and lower lumbar segments, we identified differences and similarities between the two groups. oth groups commonly displayed reactive sclerosis of the contralateral pedicle, uneven distribution of epidural fat, and deviation of the spinous process, although spinous deviation and reactive sclerosis were seen more frequently in the upper segment group with statistical significance. Reactive sclerosis or hypertrophy of the contralateral pedicle, a reflection of the stress response created by an unstable posterior arch, was present in most of our cases (49/63, 77.8%), which is in agreement with previous results [1, 3, 25]. The lower detection rate of contralateral sclerosis or hypertrophy in the lower segment than the upper segment may be partly attributed to suboptimal radiography. Uneven distribution of epidural fat, which was commonly seen on sagittal or axial MR images in our series (46/63, 73%), is believed to be one manifestation of an asymmetric posterior arch and may also be associated with isolated dorsal subluxation of the posterior elements, spina bifida, or a floating posterior arch [16]. This finding appeared to be well observed on axial images in the upper segment group, contrary to good visualization on sagittal images in the lower segment group. ased on previous studies [1, 3], a deviation of the spinous process on radiography may be related to the combined effect of asymmetry of the posterior elements by rotation, displacement, and hypoplasia if it occurs before complete growth maturity. We observed such findings in 43 of the 63 patients in our series, and the preference toward contralateral deviation that was noted, especially in the upper segment group, was in agreement with a previous report [1]. The relatively low incidence of spinous deviation and the heterogeneity of its direction in the lower lumbar segment appear to be due to variants of the lower lumbar spinous process itself or positional scoliosis. Increased interspinous distance, which refers to the downward tilting of the affected spinous process and is considered another manifestation of an asymmetric or floating posterior arch, was found in all but four patients in the upper lumbar segment group. In contrast, this finding was absent in the lower segment group except in two cases, both of which showed kyphotic curvature of the lumbar spine. We speculate that the narrow interspinous distances in the lower lumbar segment and the abrupt change of curvature at the lumbosacral junction may result in no detectable downward tilting. Contour bulging of the involved pars interarticularis, well delineated on anteroposterior and ipsilateral oblique views, was found only in the upper segment group, and it was useful for establishing the diagnosis of a unilateral pars interarticularis defect using radiography. In previous studies of upper lumbar spondylolysis, investigators described a specific configuration of the affected pars interarticularis that resembles a facet joint consisting of a vertical inferolateral portion and a horizontal superomedial portion [11, 17]. Our cases had a similar appearance. In all cases except four, contour bulging of the pars interarticularis seen on radiographs presented as a thin, low-signal line or could not be visualized on sagittal MR images. On the other hand, the JR:194, January

8 Park et al. affected pars interarticularis in the lower segment group looked narrower on radiographs. These defects appeared as a pseudoarticulation or a gap with irregular margins of opposing surfaces and were commonly accompanied by adjacent marrow changes on sagittal MR images, as described previously [14, 27], and may reflect the stress reaction. The authors of one anthropologic study explained that an affected pars interarticularis that is wider or narrower than the intact side may be the result of a bone reaction and resorption related to stress fracture and that the narrow pars interarticularis may be observed in cases in which a distinct gap has developed between opposing sides [1]. Our combined results from radiographs and MRI findings support these explanations. In addition, we think that pars interarticularis defects in the upper segment tended to show evidence of remodeling, whereas those in the lower segment tended to have degenerative changes secondary to biomechanical stress, slippage, and instability. The development of spondylolisthesis in patients with unilateral spondylolysis has been controversial. Merbs [1] described cases of unilateral spondylolysis accompanying spondylolisthesis, with a degree of slippage ranging from 3 to 8 mm, whereas eutler et al. [2] found that unilateral spondylolysis was not associated with spondylolisthesis or disability. In another study, investigators detailed an association of wedging and hypoplasia of L5 with spondylolysis that may create a radiographic appearance of spondylolisthesis even when true slippage is not present and reported that posterior wedging and spondylolisthesis coexist with degrees that are well correlated [14]. We observed spondylolisthesis of the affected vertebra only on lateral radiographs in 14 cases of the lower segment group and one case in the upper segment group. This difference may be related to the tendency of slippage in the lower lumbar segment due to increased biomechanical stress resulting from lordotic lumbar curvature and an erect position. mong the 15 cases showing spondylolisthesis on radiographs, 11 cases displayed anterior slippage on MRI. In the remaining four cases, the preservation of lumbar alignment on MRI but not on radiography could have been caused by instability or changes in the position of the patient during imaging. Overall, anterolisthesis on radiography may be helpful in identifying pars interarticularis defects. Facet degeneration and disk lesions in adjacent segments, which were predominant in the lower segment group in the current study, may represent manifestations of degenerative changes that develop after the spondylolytic segment becomes unstable, just as a pseudoarticulation, with or without marrow changes, develops. However, the high prevalence of facet degeneration and disk lesions in the lower lumbar segment may be related in part to the preference of facet or disk lesions for the lower lumbar segment. The other anomalies of the affected vertebra that we observed consisted of spina bifida, pediculolysis, and contralateral fibrous ankylosis, which were seen only in the lower segment group. These anomalous findings may be caused by secondary stress phenomena or environmental factors before maturity rather than genetic problems [11, 17]. Of these findings, the association of spina bifida and spondylolysis in L5 has been frequently documented [1, 4]. The clinical significance of the coexistence of these lesions is that they may produce a free-floating posterior arch followed by impingement of the nerve root or nonunion [3, 24]. Even though spina bifida of L5 is a relatively common incidental finding on radiography in the general population, the coexistence of spondylolysis should still be considered. In summary, unilateral spondylolysis involving the upper segment tended to show remodeling or redistribution of the posterior elements such as contour bulging of the affected pars interarticularis, reactive sclerosis or hypertrophy of contralateral pedicle, and contralateral deviation and downward displacement of the spinous process, which allowed easy detection on radiography. In contrast, unilateral spondylolysis involving the lower segment was likely to display degenerative changes and instability such as pseudoarticulation of the pars interarticularis defect, facet degeneration and disk lesions in adjacent segments, and anterolisthesis, which were relatively well visible on MRI. These results indicate that most radiologic findings of unilateral spondylolysis may represent secondary structural changes related to the asymmetric biomechanical stress that impacts the lower segment more than the upper segment because of erect position and lordotic lumbar curvature. This idea is consistent with the previous suggestion that the clinical outcome of spondylolysis is affected mainly by the level of the affected vertebra [21]. dditionally, our results suggest that radiography has the advantage of easily identifying unilateral spondylolysis involving the upper segment, whereas MRI is better for establishing the presence of unilateral spondylolysis involving the lower segment. This study has several limitations. First, all cases of unilateral spondylolysis in this study were diagnosed on the basis of the initial radiologic reports alone, resulting in a selection bias. Consequently, we were not able to evaluate the incidence of unilateral spondylolysis and the ratio of upper to lower lumbar segment involvement. Second, the age at occurrence and duration of spondylolysis cannot be exactly defined because of ambiguous symptoms, so the correlation of these clinical factors with imaging findings could not be considered. Third, a general assessment and understanding of the affected vertebra and adjacent structures of the lumbar spine were limited because radiography and MRI were performed according to routine protocols. The use of CT could have provided more information and should be considered for future study. Finally, deviation of the spinous process or uneven distribution of posterior epidural fat often has a subtle appearance that may be affected by patient positioning and can be visible in the lumbar spine of the general population. In addition, facet or disk lesions and anomalous changes, such as spina bifida, have a preference for the lower lumbar segment. Thus, we think that these ancillary findings could never be diagnostic for unilateral spondylolysis but may be helpful to minimize missed unilateral spondylolysis, although we did not perform a comparison with a control group of individuals who did not have spondylolysis. In the current study, unilateral lumbar spondylolysis displayed radiologic differences according to the segment involved, with respect to both the morphology of the isthmic defect itself and ancillary findings of adjacent structures. These variations may result from the diverse biomechanical stresses applied to each lumbar segment with a pars interarticularis defect. Recognition of the different ancillary features of unilateral spondylolysis according to lumbar segment with the use of reliable diagnostic tools can aid diagnosis in cases in which direct signs of an isthmic defect are equivocal. References 1. Merbs CF. symmetrical spondylolysis. m J Phys nthropol 2002; 119: eutler WJ, Fredrickson E, Murtland, Sweeney C, Grant WD, aker D. The natural history of spondylolysis and spondylolisthesis: 45-year fol- 214 JR:194, January 2010

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