Most investigators favor the opinion that the neural arch defect develops in a congenitally weakened pars, most probably as a fatigue failure. The incidence in newborns has in repeat studies been found to be zero (2,3,4,5). In their 45-year follow-up of 30 subjects out of an initial cohort of 500 children, Beutler et al. found that 4.4% had uni- or bilateral pars defects of the lumbar spine at the age of 6 years and that another eight subjects developed a defect between the ages of 12 and 25 years (6). Only one of the women (n = 10) in this unique sample reported back problems in relation to childbearing, but only during one of her pregnancies. A single pain episode during childhood and adolescence was reported, by just one of the 30 subjects, and none reported any severe pain episodes during all the follow-up years. Although only recorded during the last years of the study, the health-related quality of life (SF-36) did not differ from that of age-matched controls (6).

It has been noted that when the lumbosacral spine has been subjected to loading, the pars region is quite resistant to tensile bending and shearing stresses but is likely to fail relatively early when loaded repetitively (submaximal loading = fatigue failure). In subjects
with a weakened pars region, the stresses caused during normal walking have been assumed to be enough to cause a fatigue failure. Whether the greatest stress on the pars occurs with the spine in flexion or in extension is still not settled.

The shape of the lumbar lordosis has long been related to the occurrence of a lysis or olisthesis. It has not been possible to confirm such a relation when the size of the lordosis was compared in children with spondylolisthesis and normal controls.

Although the size of the lordosis has not been found to strongly influence the risk for an isthmic type of olisthesis, a lot of interest has been focused on the sagittal alignment of the spine as a significant risk factor for a lysis and especially for a slip. For the lumbosacral region, parameters measurable on radiographs, like lumbosacral joint angle, sagittal rotation, sacral inclination and/or rounding, wedging of the vertebral body, and others, have not been shown to increase the risks for either the development of a lysis or the progression of a slip (7,8). In retrospective studies, factors influencing the risk for a progression have been slip percentage, high-grade olisthesis, and slip angle (9,10,11). Studies have also shown that the measuring error in determining the degree of slip is large, which adds to the difficulty of finding prognostic factors (9,10,11).

Recently it has been suggested that a more pronounced pelvic incidence appears to be a risk factor for the development of an olisthesis (12). This measure has been described as specific and constant for each individual and is also said to determine the orientation of the pelvis and to be essential for the shape of the lumbar lordosis. The final importance of this measure has to be validated in future prospective studies.

More general factors proposed as risks for the development or progression of a slip, like gender and rapid growth, have never been solidly established.

As stated in the beginning of this chapter, typical symptoms due to a spondylolisthesis are varying degrees of back pain and sciatica. While some with a light or moderate slip have no symptoms at all, others—usually with a larger slip—might exhibit increasing deformity and sometimes severe symptoms.

Whether the spondylolisthesis is of an isthmic or a degenerative type, there are two main pathomechanisms in the generation of pain. The first one is the slip, or the forward translation of the vertebra, itself. At a certain point, the translation compromises the available space for the nerve roots in the cauda to such an extent that direct compression of the nerve roots typically elicits symptoms and signs of neurogenic claudication. A secondary effect of an advanced slip might be a direct stretching of the nerve roots at the actual vertebral level. A third effect of the vertebral translation might be stretching of the back muscles due to the elongation that occurs as a result of the translation.

The compression of the nerve roots in the cauda is likely to cause symptoms when the compression due to translation creates a pressure among the roots that obstructs circulation. In spinal stenosis, the pressure among the nerve roots builds when the available space for the roots, the cross-sectional area of the cauda, is reduced below around 75 mm² (13,14). This so-called critical size (CS) was determined on the L3-L4 and L4-L5 levels and not on the L5-S1. It is likely that the CS on this level is less than on the more proximal vertebral levels. As far as known, no studies have tried to define the minimum available space in the spinal canal at the site of the olisthesis.

As early as 1929, Bohart suggested that certain radiographic findings, including spondylolysis and spondylolisthesis, indicated a frail spine (16). That suggestion initiated, especially in the United States, the use of preemployment roentgenographs of job applicants’ spines. Numerous studies involving tens of thousands of subjects have analyzed the prognostic effects of a lot of different radiographically visible changes in the human spine on the occurrence of work-related back problems. Spondylolysis and olisthesis irrespective of isthmic or degenerative have usually been two of the visible changes found in studies of that type.

A unique opportunity to evaluate the relevance of a lysis or olisthesis for the development of acute and/or more long-lasting back problems leading to back disability occurred when the x-ray archives at an occupational clinic became available (17). The films from 615 sets of spinal examinations were studied. The films consisted of 208 preemployment screening radiographs of longshoremen. Two hundred seven of the films were taken at the time of acute back injury claims by longshoremen whose employers did not use preemployment radiographic screening. Two hundred were selected among longshoremen disabled by back problems for more than half a year. Since most of the longshoremen were men, the study was restricted to men. All three groups were randomly selected from the files except 50 subjects in the group with long-lasting disability. To avoid skewed age distribution between this group and the other two groups, these 50 were instead randomly selected among subjects below the age of 30 years with long-term back disability.

The x-ray examinations of all participants were made during the same 9-month period. The examinations included anteroposterior and lateral projection of the lumbosacral spine, including spot views of the L5-S1 region.

All the roentgenograms were scrutinized for the presence or absence of:

The study showed that in spite of the selection used, those in the chronic disabled group tended to be somewhat older than those in the control and the acute groups. As can be seen in Table 5.1, there was no difference between the incidence of either lysis or olisthesis in the control, acute, or chronic groups. Since the same finding has been made in several other studies, it seems shown beyond most objections that neither lysis or olisthesis will increase the risk for either acute or long-lasting work-related back problems.

Instability resulting from motions either in the pars defect or possibly due to the degenerative processes in the intervertebral joint is, besides the nerve compression effects of the slip, a source of pain and symptoms in spondylolisthesis. Irrespective of the direct cause of the instability, its significance is still heavily debated. A lot of different methods have been used to study the intervertebral motions in both the isthmic and the degenerative types of spondylolisthesis. Examples of such methods used are flexion-extension and biplanar radiography, videofluoroscopy, and stereophotogrammetry.

When eight patients with isthmic spondylolysis or olisthesis were studied with roentgen stereophotogrammetry and their results were compared with those from a control group, no significant differences were found in the intervertebral mobility along the sagittal or the vertical axis of motion. The transverse translations were found to be negligible in both the groups (18,19).