Spondylolisthesis is defined as a condition where there is a translational displacement in the upper vertebral body compared to the lower vertebral body. Since the year 1782, after the gynecologist Herbiniaux [1] first described it, there have been many efforts to classify the condition based on clinical symptoms and anatomical form.

In 1976, Wiltse et al. [2] classified five types of congenital, isthmic, degenerative, traumatic, and pathologic spondylolisthesis. In 1989, Wiltse and Rothman [3] separated the post-surgical type from the pathologic type producing 6 different classifications for spondylolisthesis, which is the common form used today.

The central and lateral recess type of spinal stenosis which was led by facet hypertrophy, protrusion of disc, and hypertrophy of the ligament flavum had been commonly shown in spondylolisthesis. Also, this is related to degeneration closely. The isthmic type and the degenerative type are the most frequent types, but there were few articles for degeneration and related factors in adjacent segments and lesion segments of isthmic and degenerative spondylolisthesis.

We assessed the degree of degeneration and the associated factors through simple plain radiographs and magnetic resonance imaging (MRI) of the lesion segments and the adjacent superior and inferior segments of isthmic and degenerative spondylolisthesis.

There were 46 cases for the isthmus type, 16 cases being men. The average age was 53.6 years (range, 19 to 77 years) and there were 29 cases (63%) for L5-S1, 15 cases for L4-5 and 2 cases for L3-4. For the degenerative type there were 49 cases, out of which 34 were women. There were 34 cases (69.4%) for L4-L5, 3 cases for L3-4 and 12 cases for L5-S1, L4-L5 being the most common region (Table 1). Furthermore, intraobserver agreement was almost perfect at 92% (k:0.83).

In 1989, Wiltse and Rothman [3] have separated the post-surgical type from the pathologic type producing 6 different classifications for spondylolisthesis, which is the common form used today. The isthmic type and the degenerative type are the most frequent types and they differ in cause, developmental mechanism, natural development and treatment. The isthmus type is defined as a condition where there is fibrous loss in the isthmus of the posterior arch resulting in forward protrusion of the upper vertebral body and the anterior surface of the vertebral body being separated from the neural arch. Its main mechanism is thought to be repetitive with periodic anterior and posterior bending, especially, stress fracture due to extension [9]. For this type, there is a high incidence rate in men and it occurs frequently between the L5 and S1. In the case of adults, there is instability in the lesion, resulting in degenerative change, in turn causing nerve compression leading to severe pain and neurological symptoms, therefore, requiring surgical treatment. On the other hand, it was discovered by Newman and Stone [10] that the degenerative type is caused by degenerative change and instability of the lumbar causing hypertrophy of the bones and soft tissue, and as a result, causes back pain and neurological symptoms [11,12]. For this type, there is a high incidence rate in women and it occurs frequently between the L4 and L5. In most cases, symptoms can be alleviated through conservative treatment; however, if the condition is unresponsive to conservative treatment and pain persists surgical treatment is required [12]. The isthmic and the degenerative type of spondylolisthesis have many different features in terms of causes, pathogenesis, and natural history. However both types show progressive degeneration according to age. There are few studies concerning the degenerative features of the two types of spondylolisthesis. In this study, we assessed the degree of degeneration and the associated factors by analyzing simple plain radiographs and MRI of the lesion segment, adjacent superior and inferior segments of isthmic and degenerative spondylolisthesis.

In this study, there were 46 cases for the isthmus type, 16 of which were men and there were 29 cases (63%) for L5-S1, being the most frequent segment of exposure. For the degenerative type there were 46 cases, out of which 34 were women and there were 34 cases (69.4%) for L4-L5. The reason for the abundance of women in both types is thought to be because there were more women (69.4%) involved in the research.

The authors used the Meyerding method to measure the translational displacement. In the isthmus type and the degenerative type, altogether, there were 71 cases of stage 1, 22 cases for stage 2, as most cases were in stages 1 or 2. Previous studies have reported an increase of segmental instability in moderate to severe disc and posterior facet degeneration which improve when degeneration progresses [13]. Fujiwara et al. [14] reported segmental movement to be increased up to the fourth degree of disc degeneration and improve in the advanced fifth degree.

The horizontal translation and segment angle were measured in flexion and extension radiograph images. Segment horizontal translation was defined as the difference of translation in flexion and extension views and was recorded in units of mm. Angular segment motion was defined as the difference in the angle of the line drawn along the upper margin of each vertebral body in motion views. Cases where there were horizontal transpositions greater than 3 mm and angular movements greater than 10° were assumed to be unstable and eventually needed surgical treatment. In this study, the numbers of cases defined as more advanced degeneration, fifth degree, was shown frequently in the isthmus type. This may propose that restabilization is progressed in the isthmic type, but the actual measured horizontal translation and segmental motion represented that there was no statistical significance between the two types of spondylolisthesis. This indicates that there may be other factors contributing to the restabilization of the segment.

There were 47 cases where the patient underwent operation and among them there were 33 cases (70.2%) where the patients had instability. The rest were cases which the patients did not respond to conservative treatment. For the isthmus type and the degenerative type there were 23 cases (48.9%) and 24 cases (51.1%), respectively.

The classification by Pfirrmann was used on Disk degeneration while stages 3 or more was defined as degenerated. There was insufficient research done using MRI on disk degeneration within the lesion segment and the segments adjacent to the lesion of spondylolisthesis patients. Overall, for the disk degeneration, there was no difference in degenerative change between the two groups. Disc degeneration in the isthmus type had a significantly high occurrence in the index segment, where the lesion is located, and relatively low occurrence in the lower segment. This result is consistent in that advanced disc degeneration, fifth degree, was also most commonly observed in the index segment (p=0.034). For the degenerative type, disc degeneration was more common in the upper segment, but overall degeneration was similar in all segments. These differences did not have demonstrate statistical significance (p>0.05).

As the vertebral disks age, they undergo degenerative change. The loss of fluid and elasticity and the decrease in height leads to instability of the vertebra which eventually causes translation of the vertebral body [15-17]. Disc degeneration occurs prior to posterior facet joint degeneration, and this loss of disc height and change of mechanical properties can lead to posterior facet joint degeneration and low back pain through increasing load to the facet joints [18-20].

Modic et al. [7] classified spondylolisthesis into type I (low intensity in T1-weighted images, high intensity in T-2 weighted images), type II (high intensity in both T1 and T2-weighted images) and type III (low intensity in both T1 and T2-weighted images) depending on the existence of change in the endplates and the bone marrow. Type I indicates active and ongoing degenerative process and type II represents a more stable and chronic degenerative form than type I. Type III is thought to be associated with subchondral bone sclerosis. Another research has associated type I and occurrences in the L5-S1 region with patients' symptoms [21]. In this research, type II was the most abundant form in both the isthmus type and the degenerative type. There were more changes in the isthmus type than the degenerative type, specifically, in the index segment where the lesion is located. As advanced disc degeneration is more commonly observed in the index segment, we can conclude that the lesion site for the isthmic type is more stable than the upper and lower segment of the lesion.

A HIZ lesion is thought to be radial and concentric tears of the posterior annulus [22]. It occasionally appears as mucoid material associated with fluid or granulation tissue, outer annulus with ruptured neovasculature or substances formed under a PLL (posterior longitudinal ligament) complex [23]. There is much controversy in the clinical significance of HIZ lesions. Aprill and Bogduk [22] have stated there is a significant relationship between the existence of HIZ and annular tear, however, Ricketson et al. [24] have mentioned there is no association between HIZ and concordant pain.

SN is defined as a localized irregular shape in the vertebral endplates encasing the disk or disk herniation into the vertebral body. Despite that the function of SN is not well known, it is thought that it plays a role in the degeneration of the disk [25-28]. Also, SN is more prevalent in males because of the higher physical loading [29], and this is associated with its higher occurrence in males with the isthmus type.

This research has the limitation of being a retrospective study solely on outpatients with back pain and did not include patients without symptoms. So, patients of the isthmic type had an advanced degeneration respectively compared to general patients of the isthmic type. Other limitations are that the number of patients involved was relatively scarce and only radiological aspects were considered. Furthermore, as the height of disks was calculated on sagittal images of MRI, it could be different with height of disks on standing radiographs. Also, both patients that had surgery and patients that had conservative treatment were included, therefore, in the future a greater effort needs to be put forth towards finding associations with clinical results and to compare with adjacent segmental instability in post-surgical patients.

In the two most common forms of spondylolithesis, the isthmic type and the degenerative type, radiologic analysis showed no difference in the extent of translation and instability. Also, it showed more occurrences of the most advanced disc degeneration, fifth degree, SN and Modic change in the lesion segment of the isthmus type. Lastly, HIZ lesions occurred frequently in the segment superior to the lesion segment of the isthmic type compared to the degenerative type. Judging from these findings, we can consider that stabilization of the lesion segment may occur more commonly in the isthmic type of spondylolisthesis than in the degenerative type.