The natural history of spondylolisthesis is not fully understood, a fact that complicates treatment decisions. The few natural history studies in existence are relatively small and are almost impossible to reproduce in this day and age. However, there are many reports in the literature documenting the intermediate and long-term outcomes of treatment for spondylolisthesis [2–7]. One must have some understanding of the natural history in order to verify the effectiveness of any intervention, surgical or otherwise.

One of the largest and longest natural history studies for spondylolisthesis was published by Beutler et al. in 2003. In the 1950s, 500 asymptomatic elementary school children from a single school system in Pennsylvania were screened radiographically for spondylolysis and spondylolisthesis. From this cohort, 30 subjects were identified and tracked for over 45 years from the time of diagnosis (either in childhood or early adulthood) until study completion. In this population, all the slips were low grade at presentation (Meyerding 1 or 2) and no slip progressed to a high grade (Meyerding 3–5) [2]. While some slips showed progression over time, the rate of slip progression slowed with each decade from 7 % over the first decade down to 2 % in the fourth decade. The presence of back pain or radicular leg symptoms was not associated with the slip percentage. In fact, the incidence and severity of back pain in this population mirrored the general population and there was no statistical association between pain and other factors including age at presentation, slip percentage, and lumbar index.

An early study out of Rochester, MN identified 114 children and adolescents (9–19 years of age) with spondylolysis and had variable follow-up of 1–9 years [1]. 85 % of their patients had grade 1 or 2 slips, and in these patients 82 % complained of back pain, 5 % presented with complaints about a postural change (from a parent), and 13 % had an incidental diagnosis (typically from a pre-employment radiograph). None of these patients treated without surgery had progression of their slip angle.

Nachamson and Frennered published their average 7-year follow-up of 47 patients diagnosed with spondylolysis or spondylolisthesis before the age of 16 in 1991 [4]. They found that only 4 % of patients with low-grade spondylolisthesis progressed (there were no high-grade slips in their cohort), none progressed to a Meyerding 3 or 4 slip, and there was no correlation between progression with age, sex, slip angle, lumbar index, or disc height. Of those treated non-operatively, 83 % continued normal activities of daily living without restriction. They also found that at last follow-up pain scores did not differ significantly when compared to age matched norms. Interestingly surgically treated patients had more progression of slip angle at last follow-up than non-surgical patients, a finding mirrored in other studies [8]. Nachamson concluded that low-grade spondylolisthesis has a benign course.

While there are no natural history studies for untreated high-grade spondylolisthesis (Meyerding grade 3 and 4 slips), there are several outcome studies in the literature, although the populations tend to be small as these slips are much less common. Harris and Weinstein compared long-term outcomes of non-operative and operative management of Meyerding grade 3–4 spondylolisthesis in young patients (age 10–25 at diagnosis) [9]. Of their 11 patients treated non-operatively, after an average 18-year follow-up, 36 % were symptom free, 55 % had mild symptoms, and only one had significant symptoms. Symptoms were associated with the presence of scoliosis, lateral listhesis, tight hamstrings, limited spinal motion, progression of the spondylolisthesis, obesity, weak abdominal muscles, and positive neurological findings. Symptoms were not associated with the severity of the slip. All of the patients led an active life, 45 % worked as manual laborers, and only one required minor adjustments in their lifestyle.

Specifically in the younger pediatric and adolescent age group (skeletally immature patients), those patients with high-grade spondylolisthesis seem to be much less likely to have successful outcomes on long-term follow-up with conservative management. Pizzutillo et al. retrospectively followed 82 adolescent patients (aged 6–21 years) with spondylolisthesis for 1–14.3 years [10]. Twelve of these patients had a high-grade 3 or 4 slippage, and only one improved with non-operative treatment. The rest (92 %) required operative treatment to control symptoms. However, 67 % of patients with grade 1 or 2 spondylolisthesis had significant improvement with non-operative treatments. It seems that in a younger population, a high-grade slip at presentation combined with an increased risk for progression with continued growth, spondylolisthesis is much more likely to become more symptomatic and requires surgical intervention to effectively treat symptoms.

Interestingly, thoughts on the impact of disk degeneration at the level of the spondylolisthesis have been evolving. Many authors have supposed that the degenerative process of the disc occurring at the level of the spondylolisthesis, as well as the levels above, predisposes to progression of the slippage and over time may become a pain generator [11]. However, there has been recent speculation in the literature (supported by the natural history data) that progression of spondylolisthesis often does not occur, especially in those who have finished growing and have low grades of slip at presentation. With these cases, there may actually be a loss of mobility and inherent stabilization at that segment as the disc degenerates, therefore decreasing the risk of progression at that level. Seitsalo, a proponent of this concept, has published long-term studies showing minimal progression and/or stability of the disc at the level of the spondylolisthesis in children with low-grade spondylolisthesis over a 10 to 15-year follow-up [8, 12]. His data is supported by the natural history data of Beutler [2] as well as other outcome studies [4].

For spondylolysis to exist, by definition there must be some insufficiency or incompetency of the posterior elements. As indicated in the Wiltse classification scheme, the problem can be either traumatic (isthmic) or developmental (dysplastic) (it has never been noted congenitally). Of note, family history has been implicated in developing spondylolysis, and this condition occurs in 15–70 % of first degree relatives of individuals with the disorder [13]. The pars injury (spondylolysis) that occurs in the isthmic form of spondylolisthesis is always bilateral, as unilateral lesions have never been observed to progress to a slip [1, 2, 14]. There are clearly certain activities that place increased stress on the posterior elements of the spine and predispose to this problem including sports and dance activities that require repetitive hyperlordosis/extension of the lumbar spine or repetitive loading and unloading of the lumbar spine.

Several long-term outcome studies have shown that most slips demonstrate little, if any progression; 80–90 % of the ultimate slip percentage is present at the time of diagnosis and only 44 % of patients, athletes, and non-athletes alike, demonstrate any additional progression [14, 15, 16]. However, there are factors that seem to be associated with an increased risk of slip progression including increasing slip percentage (greater than 20–30 % slippage on presentation), skeletal immaturity, and the presence of a dysplastic (as opposed to isthmic) spondylolysis [17].

Furthermore, there is research to suggest several sagittal plane radiographic parameters, including the lumbar index, sacral inclination and slip angle, may be helpful in predicting which patients will progress, although findings are conflicting [18, 19]. The lumbar index looks at the relative wedging of lumbar vertebrae and is defined as the ratio of anterior vertebral height to posterior vertebral height. The sacral inclination is the angle formed between a line tangential to the posterior border of the sacrum and perpendicular to the floor, and thus measures of how vertical or horizontal the sacrum is in space. The slip angle measures the orientation of the slipped vertebra relative to the sacrum and is defined as the angle between the superior endplate of the slipped vertebra (usually L5) and a perpendicular drawn from a line parallel to the posterior border of S1. Individuals with increased sacral inclination and slip angles have increased sheer forces across their spondylolysis and can lead to worsening sagittal imbalance at the level of the slip [20]. The forward displacement of the lumbar vertebral body creates a lumbosacral kyphosis and displaces the center of gravity anteriorly in relation to the sacrum, thus producing a positive sagittal balance. The body compensates with hyperlordosis of the lumbar spine and thoracic hypokyphosis, which can initially help restore this balance. However it has been postulated that a larger lumbosacral kyphosis at diagnosis may constitute a risk factor for further progression due to the inability of these compensatory mechanisms to accommodate and maintain the center of gravity in the setting where a slip continues to progress [7, 19]. Pelvic incidence (PI), a non-modifiable measurement intrinsic to a pelvis (e.g., it does not change as the position of the pelvis changes in space), measures the relationship of the center of the hips to the sacrum with larger values indicating the hips are more anterior to the sacrum. It has been shown that individuals with spondylolisthesis have a higher PI than do individuals without slips [21].

Given the benign natural history of low grade spondylolisthesis, non-operative interventions should represent the mainstay of treatment. As the natural history studies have shown us, there is no role for treating asymptomatic low-grade spondylolisthesis. Even most high-grade lesions can initially be treated conservatively. Treatment options for low-grade spondylolisthesis in children and adolescents include pain medications, bracing, activity restriction, and therapeutic exercises. The treatment of spondylolisthesis focuses primarily on pain management, although prevention of further slippage may be a consideration as well [22]. While many cases of congenital (dysplastic) spondylolisthesis are asymptomatic [3, 22] and discovered incidentally, children and adolescents are often diagnosed with spondylolisthesis during a workup for pain [2]. The pain may be in the lower back, radicular (with symptoms radiating into the buttock, thigh, leg, or foot) or both [14]. The pain generator may be due to slippage, dynamic motion/instability, pars fracture/elongation, or from the intervertebral disc [3, 23, 24].