Spondylolytic spondylolisthesis: various imaging features and natural courses
The purpose of this paper is to review typical and atypical imaging features of spondylolytic spondyolisthesis and natural courses. Imaging features of the typical pars defect in early phase include bone marrow edema of the isthmus on MR imaging with progression of bone defect on plain radiography or CT. There are various defect types (pedicular cleft, retroisthmic cleft) and locations (upper lumbar spine and unilateral lesions). Natural courses also vary, with different patterns of progression and stabilization. Clinical correlation of the imaging and clinical features is often difficult.
KeywordsSpondylolysis Spondylolisthesis Spondylolytic spondylolisthesis Ithmic spondylolysis Retroisthmic cleft Pedicular (retrosomatic) cleft
Spondylolytic spondylolisthesis is one of the major causes of low back pain in adolescents. Spondylolysis is defined as a bone defect of the posterior element of the vertebra. The isthmus (or pars interarticularis), which serves as a junction between the pedicle and the articular processes, is most commonly affected bilaterally. The fifth lumbar vertebra is involved in 95 % of cases and the incidence decreases cephalad. Generally the defect is bridged by fibrous, cartilaginous or osseous tissue, and osseous fusion represents healing . Spondylolisthesis is referred to as a displacement of the vertebra and there are two types, one associated with spondylolysis and the other with degeneration of facet joints and/or intervertebral disc. Many patients with spondylolysis remain asymptomatic and certain cases become symptomatic, with low back pain and, less commonly, radicular pain. Spondylolisthesis can also be asymptomatic, but may cause low back pain, spinal instability, and radiculopathy, which requires conservative and/or surgical treatment.
The incidence of lumbar spondylolysis has been reported to be 8 % in the adult population , and it was variable among races and sexes. Sakai et al.  reported that the prevalence of lumbar spondylolysis among the Japanese population between 20 and 29 years of age, found by multidetector CT, was 5.9 %, with a male to female ratio of 2:1. Spondylolysis has never been seen in newborn babies: it is seen only after walking. The upright posture and bipedal position play a role in the development of spondylolysis . The prevalence of lumbar spondylolisthesis was reported to be 20.7 % in the adult population, and spondylolysis without spondylolisthesis was relatively uncommon and was only seen at the lower lumbar level .
Although bone defects of typical isthmic spondylolysis run obliquely through the pars interarticularis, and are well depicted on oblique lumbar radiography, atypical isthmic defects have been reported in several papers [6, 7]. The pathomechanism and incidence of atypical isthmic defects is not well known, and the radiographic appearance of atypical isthmic spondylolysis has not been well recognized among radiologists. Other atypical spondylolyses included various types, and two variations have been reported in the literature [8, 9, 10]. Unilateral spondylolysis accounts for 3–33 % of all cases of spondylolysis and has been estimated to be approximately 3–7 % of the general population . It may be associated with certain athletic activities, such as cricket, ballet, and gymnastics, and the association with facet tropism has been investigated . The involvement of the upper lumbar spine has been rarely noted, and clinical aspects and radiological features of spondylolysis of the lumbar spine have been reported in the literature [7, 13, 14, 15].
The aim of this article is to present various imaging features of spondylolitic spondylolisthesis, including early and late changes of typical isthmic spondylolysis and atypical types of spondylolysis and/or spondylolitic spondylolisthesis in addition to their clinical courses.
Classification of spondylolysis modified by Wiltse
Type 1: Congenital (three subtypes)
1-A: Spina bifida of L5 or sacrum, scarce development of articular processes, and axial (horizontal) orientation of the facets
1-B: The facets may be sagitally oriented and not buttress the anterior slip of L5 over the sacrum or L4 over L5
1-C: Other congenital anomalies of the spine
Type 2: Isthmic (two subtypes)
2-A (Lytic): breakage in the pars articularis
2-B (Elongation of the pars articularis without separation): it may finally break and separate
Type 3: Degenerative
Type 4: Traumatic
Classification system by Marchetti and Bartolozzi
Primary category: Acquired
Primary category: Developmental
The pathogenesis of spondylolisthesis is still unclear, and two theories, traumatic and congenital, are known . The traumatic theory is based on a recent history of trauma in a patient, and high incidence of isthmic lyses in athletes, such as gymnasts and weight lifters. Ward et al. described an inadequate increase in the transverse interfacetal distance progressing caudally from L4 to S1, compared with normal control subjects, was found in the individuals with spondylolysis of L5. The inferior articular process of L4 and the superior articular process of S1 contact the same cross-sectional position of the L5 pars interarticularis in accordance with the reduction in the transverse interfacetal distance, so excessive pressure was applied on the isthmus due to pinching between the inferior articular process of the segment above and the superior articular process of the segment below, thus leading to bony resorption and bone weakness, finally resulting in lysis . In the analysis of cadaveric skeletons with L5 spondylolysis, Masharawi et al. noted that greater degrees of coronal orientation, facet tropism and symmetry of the lower lumbar facets was strongly associated with spondylolysis, and with increased contact surface area of the opposing articular processes because of coronally oriented facet joints: they postulated that more loading stress was being applied to the isthmus during flexion and extension.
In the congenital or developmental theory, the lysis is thought to be the result of the normal mechanical forces of the erect position toward the dysplastic isthmus and surrounding bone structures . This is also supported by the high incidence of spondylolisthesis observed in certain ethnic groups and the heredity of spondylolisthesis in some families. It might be reasonable that repetitive activities or micro trauma to the congenitally weak isthmus and surrounding bone lesion leads to the pars lysis . Disruption of the posterior elements caused by isthmic lysis or congenital dysplasia can weaken their tension-band effect and lead to anterior migration of the vertebral body, resulting in spondylolisthesis . Isthmic spondylolysis can diminish the stability of the posterior elements of the lumbar vertebrae and lead to spondylolisthesis during adulthood . Mihara et al.  described bilateral spondylolyses as increasing the intervertebral mobility at the involved vertebral segment and at the vertebral segment above the involved segment. This result supports the pathomechanism of spondylolysis development to spondylolisthesis. The progression of spondylolysis to spondylolisthesis most commonly occurs during adolescents younger than 16 years of age, and decreases with advanced age .
The mechanism of the slippage of the lumbar spine in skeletally immature subjects is still being debated. Farfan et al. documented the growth zone near to the vertebral endplate: the intervertebral disk was the weakest portion during growth, and pediatric spondylolisthesis may occur after epiphyseal separation . Sairyo et al. proposed a pathomechanism for pediatric spondylolisthesis. Their pathomechanism was as follows: spondylolysis changes the spinal kinematics, leading to stress concentration at the growth plate during motion, and continuous stress loading to the growth plate can lead to fracture at the vertebral body, resulting in spondylolisthesis . Spino-pelvic parameters, including the sacral slope, pelvic tilt, and pelvic incidence, measured by lateral radiographs of the lumbosacral spine, may also have an important role in the development of spondylolisthesis due to spondylolysis. A high pelvic incidence is a predisposing factor for L5 spondylolisthesis because high pelvic incidence indicates a high sacral slope and/or pelvic tilt, and may increase lumbar lordosis, which contributes to high shear stress to the L5 isthmus, resulting in lysis .
In adult patients, it is widely accepted that the development of vertebral slippage in the lumbar spine is related to disc degeneration, which may cause conversion from stable spondylolisthesis to unstable vertebral spondylolisthesis . It was demonstrated by the studies of McGregor et al. and Axelsson et al. [30, 31, 32] that a spondylolysis does not contribute to progression of instability or hypermobility of the lumbar spine, or increased progression of disc degeneration below the level of the spondylolytic vertebra.
Typical isthmic spondylolyses
With progression of spondylolyses, fracture of the isthmus can be recognized as a linear low signal intensity on T1- and T2-weighted images, surrounded by an area of bone marrow edema. The marginal sclerosis showing low signal intensity on both T1- and T2-weighted images may represent disease progression. Fracture healing may not occur before bone marrow edema . As fracture healing progresses, the fracture line disappears with marginal sclerosis. The signal intensity of the defect is variable: low on T1-weighted images, low to intermediate on T2-weighted images when the defect is composed of fibrous or fibrocartilaginous tissue, and low on both T1- and T2-weighted images when osseous union is noted. In the case of non-union, fibrous scar and/or fluid may be seen in the gap. The signal intensity of the gap may be inhomogeneous as the result of the mixed fibrous liquid components .
MR imaging classification of spondylolysis by Hollenberg et al. (from Ref. )
Grade 0: Normal marrow signal with intact cortical margin of the isthmus
Grade 1: Bone marrow edema with intact cortical margin of the isthmus with or without signal changes in adjacent pedicle or articular processes
Grade 2: Incomplete fracture showing bone marrow edema and cortical breakage incompletely running though isthmus
Grade 3: Complete fracture denoting a complete bone defect of isthmus with bone marrow edema
Grade 4: Fracture non-union presenting complete separation of isthmus without bone marrow edema
MR diagnosis of spondylolysis without spondylolisthesis can be difficult without ancillary findings. Ulmer et al.  found that a widened sagittal diameter of the spinal canal, wedging of the posterior aspect of the vertebral body at the level of spondylolysis, and bone marrow signal changes of the pedicle adjacent to the isthmic spondylolysis were useful in making a diagnosis of lumbar spondylolysis without or with spondylolisthesis. Epidural fat interposition between the dura mater and the L5 spinous process is known as an indirect sign on midsagittal MR images of the lumbar spine .
CT is believed to be the most useful modality in the evaluation of size and location of the isthmic fracture. Fujii et al. described the fracture of the isthmus as having three stages: early, progressive, and terminal . The early stage showed a narrow fissure through the isthmus with a sharp margin. The progressive stage was defined as a narrow fissure with a slightly rounded margin. The terminal stage had a wide and round margin and sclerosis. They also noted that a significantly greater proportion of defects achieved union in the early stage compared with progressive and terminal stages, and no bony union was seen in the terminal stage .
Atypical isthmic spondylolysis
The prevalence of atypical defects of pars is uncertain. Lee et al.  found nine atypical defects among 25 defects of the three upper lumbar vertebrae, and five atypical defects among 27 defects of pars from T12 to L6 were seen in the 29 vertebrae of an adult skeleton study . Lee et al. also reported the defect having the vertical inferolateral portion, and the horizontal superomedial portion was seen in the majority of the patients with atypical spondylolyses. These clefts were not found in the lumbar vertebrae of the 700 dissected fetuses and neonates in Wiltses’ study  and 500 neonates radiographed in Fredrickson’s study . It is suggested that atypical isthmic spondylolysis is not congenital but multifactorial, and that a pre-existing bony discontinuity in the superomedial or the inferolateral portion may lead to mechanical weakness of posterior elements of vertebrae and cortical breakage of the other site may occur by repeated shearing forces to the facets during extension and flexion.
Pedicular (retrosomatic) cleft
Other atypical distributions
Upper lumbar vertebrae
The imaging features of spondylolytic spondylolisthesis are various, depending on the stage, type and location. The typical isthmic spondylolysis can be diagnosed on plain radiography, and MR imaging, especially a fluid-sensitive T2-weighted image, is useful in the detection of bone marrow edema of the pars interarticularis at an early stage of spondylolysis, since fracture healing may not occur before the presence of bone marrow edema and a bony fusion can be noted by conservative treatment without surgical intervention. Atypical isthmic spondylolysis is rare and a bone defect having the vertical inferolateral portion and the horizontal superomedial portion is seen in the majority of the patients. Atypical locations of spondylolysis, such as retrosomatic and retroisthmic cleft are rarely seen. Unilateral spondylolysis is uncommon and it occurred in the lower lumbar spine and may be symptomatic. Spondylolysis in the upper lumbar spine is often unilateral and associated with congenital vertebral anomalies. The natural history is also various with different time courses of evolution and involution.
Conflict of interest
The authors declare that they have no conflict of interest.
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