Introduction

The term spondylolysis refers to a defect in the pars interarticularis, which can occur either unilaterally or bilaterally. Spondylolysis is most common in the lumbar spine, with L5 more frequently involved than L4, and other levels involved much less frequently. It may rarely occur at multiple contiguous levels. Approximately 80% of cases involve bilateral pars defects, which can result in the functional separation of the posterior column from the anterior and middle columns. This can result in an anterior translation of the cephalad vertebral body on the caudal level and is termed spondylolisthesis. ¹ Spondylolysis is one of the common causes of spondylolisthesis in the pediatric population. These two entities are the most frequent causes of low back pain in pediatric patients. This chapter discusses pediatric spondylolysis and low-grade spondylolisthesis (LGS).

Epidemiology

Spondylolysis and spondylolisthesis are two of the most common identifiable causes of low back pain in children and adolescents. The true incidence of either pathology is difficult to assess, as both are often asymptomatic. The reported incidence of spondylolysis is 4 to 5% by age 6, and increases to 6% by age 18. ¹ The male-to-female ratio is 2:1, but females with spondylolysis are more likely to develop spondylolisthesis. ² The incidence of spondylolysis is highest in adolescent athletes with low back pain, where rates as high 47% ³ to 50% ⁴ have been reported. Compared with the general pediatric population, a fourfold increase in incidence of spondylolysis has been reported in gymnasts and American football linemen. ⁵ Spondylolisthesis is more common at L5-S1 in both children and adolescents. ¹ Although spondylolysis is more common at L5 than at L4, patients with L4 spondylolysis are more frequently symptomatic. ⁶

Pathophysiology

The junction of the relatively mobile lumbar spine and the relatively immobile sacrum results in the L5-S1 joints and L5 pars absorbing more stress than any other level of the spine. ⁷ With standing posture and weight bearing, the L5-S1 disk resists compression, and shear forces are resisted by the posterior bonyligamentous complex. The junction of these two anatomic regions is the pars, which is thus exposed to both shear and compressive forces. Incomplete or delayed ossification of the pars, combined with excessive physiological strain can result in a pars stress reaction, which is the precursor to spondylolysis. A pars stress reaction is defined as sclerosis of the pars without a definite radiographic gap. With repetitive mechanical stress the sclerotic bone can fracture, resulting in spondylolysis. ¹

Spondylolysis has not been reported in nonambulatory patients, ⁸ suggesting that upright posture and ambulation are integral to its development. Spondylolysis is most common in adolescent athletes involved with gymnastics and diving and in adolescent ballet dancers; it is also common in athletes who play contact sports such as American football. Cadaveric biomechanical studies have shown that excessive flexion and extension produce the highest stress on the pars. ⁷ Of these actions, repetitive hyperextension or the “nutcracker” mechanism is believed to be the most responsible for the development of spondylolysis. ⁹ More recent studies that evaluated spinopelvic alignment have shown that patients with a large pelvic incidence more frequently have spondylolysis, suggesting that sheer forces also play a major role in the development of spondylolysis.

The development of spondylolisthesis is multifactorial; disk degeneration, ligamentous laxity, genetic predisposition, spina bifida, facet joint incompetency, and spondylolysis can all cause spondylolisthesis, either individually or in combination. Spondylolisthesis may also be iatrogenic. The commonality these factors all share is a functional failure of the posterior bone and ligamentous elements, and this loss of the posterior tension band enables anterior translation of the superior vertebra on the inferior vertebra. In the pediatric population, spondylolysis is the most common cause of spondylolisthesis, attributing to 14 to 21% of cases in previously reported series. ¹ , ¹⁰ In patients followed over a 45-year period, Beutler and colleagues ² observed that those with unilateral spondylolysis typically did not develop spondylolisthesis.

Classification of Spondylolisthesis

There are several classification systems for spondylolisthesis. Both the Wiltse-Newman and the newer Machetti-Bartolozzi systems use etiologic criteria as the basis for classification, whereas the Meyerding, Newman, and DeWald system describes the degree of translation.

The Wiltse-Newman is the oldest and the most commonly used classification of spondylolisthesis ( Table 12.1 ). Type I, dysplastic, results from structural abnormalities of the L5-S1 facet joints such as hypoplasia, or malorientation and sacral deficiency. Type II, isthmic, results from defects of the pars interarticularis. Type II is further subcategorized as follows: type IIA, pars lysis; type IIB, pars elongation; and type IIC, acute fracture. Type III results from degenerative disease. Type IV results from traumatic fracture of the posterior elements other than the pars. Type V is due to pathological destruction of the posterior elements.

Machetti and Bartolozzi subsequently proposed a classification that differentiates developmental and acquired types ( Table 12.2 ). ¹¹ In this classification, Wiltse I and II are grouped together as developmental. Degenerative, pathological, and traumatic causes are grouped into an acquired category.

Lastly, the Meyerding classification is a radiographic classification ( Table 12.3 ). ¹¹

The importance of spinopelvic parameters and global sagittal alignment, specifically with respect to treatment decisions in spondylolisthesis, has been demonstrated in the literature. ⁹ , ¹¹ – ¹³ The previously discussed classification systems do not include these measurements of spinopelvic alignment or balance.

In 2006, Mac-Thiong and colleagues ¹¹ proposed a new classification system based on the degree of slippage, the degree of dysplasia, and the sagittal spinopelvic balance. They described nine types and proposed tentative treatment guidelines based on the degree of severity.

More recently, the Spinal Deformity Study Group (SDSG) proposed a simplified classifica tion system for L5-S1 spondylolisthesis ( Table 12.4). ¹² It is based on the degree of listhesis, the pelvis type, and the spinopelvic balance. Based on these three parameters, the SDSG created six subtypes. The degree of slippage is either low grade (< 50%) or high grade (≥ 50%). The spinopelvic measurements of pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS), and C7 plumb line (C7PL) are used to classify the pelvis as balanced, retroverted with a balanced spine, or retroverted with an unbalanced spine. Low-grade spondylolistheses is one of three types: type I, low PI (< 45 degrees); type II, normal PI (45 to 60 degrees); and type III, high PI (> 60 degrees).

Higher grade slips similarly fit into three types: type IV, balanced pelvis; type V, retroverted pelvis with a balanced spine; and type VI, retroverted pelvis with an unbalanced spine. The spine is considered “balanced” if the C7PL falls on or behind the femoral heads, and unbalanced if it falls anterior to the femoral heads.

Utilization of this classification system for higher grade spondylolisthesis has been shown to improve surgical decision making. ¹²