18.1 Normal Development of Spine

The development of the spine involves longitudinal and axial growth that not only protects the neurological function but also maintains physiological motion. The aim is to have a well-balanced spine that allows achievement of normal everyday activities.

Our early ancestors walked with a bent-hip, bent-knee (BH-BK) gait similar to chimpanzees (Fig. 18‑1). This led to a poor upright posture, which in turn meant higher and more costly energy expenditure. This is borne out by the fact that the BH-BK gait throws the center of gravity of the body anterior to the hip joints. This generates an equal and opposite ground reaction force vector with an increased flexion moment arm over the hip joints. These forces need to be counteracted to maintain posture and result in increased energy expenditure.

The “normal” sagittal balance develops from a completely kyphotic spine, which develops into the compensatory lordosis curvature in the cervical and lumbar spine balanced by the kyphosis in the thoracic spine. This results in placement of the head up over the pelvis.

At birth, the T1-S1 segment measures about 20 cm with a gain in standing height approximately 25 cm during the first year of life and around 12.5 cm during the second year. ¹ As mentioned earlier, the spine at this stage is hyperkyphotic with lumbar lordosis (LL) only appearing as the child begins to acquire the upright position initially by sitting (6 to 12 months) and later by a bipedal stance (around one to two years). ² The thoracic kyphosis (TK) develops to balance the LL as the child grows. ^{3 ,​ 4}

The neonatal pelvis has no stance-related locomotion demands as the child is predominantly supine at this stage and the sacrum is vertical. ⁵ As the child begins to sit up and later stand up, axial weight is transferred to the sacrum leading to its increasing horizontalization. ^{6 ,​ 7}

Modern man therefore stands with an upright posture, which means the center of gravity falls between the points of contact with the ground (i.e., the feet). The ground reaction force generated is closer to the hip and knee joints and causes a much smaller flexion moment arm. The energy expenditure of maintaining this posture is much more economical. This progression from the quadrupedal to bipedal posture in humans can be compared to the transition from crawling to walking in the toddler. In both situations, spinopelvic balance is essential for cost-neutral energy expenditure. This is achieved with the development of lordotic curves in the cephalad and caudal regions of the spine and a kyphotic curve in between. Alignment is maintained with adequate muscle tone, diskoligamentous tension, and bony articulation. In the sagittal plane, these segments are interdependent and as a unit articulate with the pelvis and lower limbs to give an equilibrium that maintains an upright posture. In the child, this equilibrium can be disturbed by neurological conditions that affect muscle tone or those that affect alignment such as spondylolisthesis and scoliosis. Iatrogenic causes from treatment with growing rods and their complications is a well-known cause as well.

18.2 Fundamental Parameters in Spinal Alignment

Duval-Beaupère et al described pelvic incidence (PI) as a fundamental pelvic anatomic parameter that is specific and constant for each individual and determines pelvic orientation as well as LL. This principle is somewhat different in the child. ^{3 ,​ 8}

As in the adult population, adaptive changes occur in the pelvis and lower limbs in children with sagittal imbalance. Similarly, PI describes the morphology of the pelvis; pelvic tilt (PT) and sacral slope (SS) describe the orientation of the pelvis in relation to the bicoxofemoral axis and the vertical plane. Both PT and SS are positional parameters that change in relation to the orientation of the sacrum. Unlike PI in the adult, in the skeletally immature, PI evolves throughout growth.

Many studies have highlighted the relationship of the spine and pelvis in standing balance in normal adults and children particularly through the effect of LL. ^{2 ,​ 4} Schwab et al ⁹ described the gravitational line to remain fairly constant with age; however, the degree of TK associated with age would shift the plumb line anteriorly with a compensatory retroversion of the pelvis, increasing the PT to keep the gravitational line constant and maintain adequate sagittal balance.

It is thought that there is a trimodal age distribution of sagittal plane deformities. In the teenage years, sagittal plane deformities are usually secondary to Scheuermann’s kyphosis. The second group occurs in the 40- to 50-year age range, commonly a result of inflammatory disorders such as ankylosing spondylitis, and the last group is in the over 60 range, where the commonest problem is degenerative arthritis of the spine. The center of gravity line in the standing position lies just in front of the thoracic spine. There is therefore a natural tendency for the upper trunk to move forward but this is counterbalanced by the lordotic lumbar spine. The integrity of the intervertebral disks is important in maintaining this profile. In pathological states, there is collapse of the disk height, which leads to a loss of the normal sagittal curves and a straighter profile that is not biomechanically efficient. This is similarly found in physiological aging of the spine. ¹⁰

Boulay et al ¹¹ found that PI can increase with growth in childhood or adolescence, reaching a constant value in adulthood. Mangione et al ¹² showed that PI tended to increase linearly during childhood after the acquisition of walking, but this article did not specifically document the influence of age on PI during adolescence. In another article, Descamps et al ¹³ showed that PI was relatively stable before 10 years old and then increased significantly during adolescence until reaching its maximum value in adulthood. However, the authors did not evaluate the influence of age on the PI with any correlation studies. In a prospective study, Mac-Thiong et al ¹⁴ looked at the sagittal alignment of the spine and pelvis and its change during growth in a normal pediatric population. Lateral standing radiographs in 180 patients were evaluated for TK, LL, SS, PT, and PI. They concluded that PI tended to increase with age from 4 to 18 years. PT and LL increased with age as well but SS was not significantly influenced by age after the onset of walking. There were also no differences between males or females in the study group.

The developing spine with the newly acquired bipedal status and locomotion requires the constant adaptation of the morphology and orientation of the pelvis to align the spine in adequate balance to cope with daily demands. Adequate alignment ensures minimum energy expenditure. ^{15 ,​ 16}

Mac-Thiong et al advocated two hypotheses to explain the observed changes in PI. First, the PT seeks to align the center of gravity optimally over the lower limb axis. It does this by keeping the sacral plate behind the hip axis. Because of the increase in body weight during growth and risk of anterior displacement of the center of gravity, the sacral plate is pushed further backward by an increase in PT. Second, with the onset of standing and locomotion, the sacral plate becomes more vertical (i.e., the sacrum becomes more horizontal). This leads to an increase in SS. ¹⁴ Geometrically, PI is the sum of SS and PT; hence, any increase in either PT or SS will inevitably lead to an increased PI. Both LL and TK increase with age. ² LL plays an important role in sagittal balance. ^{8 ,​ 17} Adequate lordosis is required to prevent anterior displacement of the center of gravity. The TK balances LL and any changes reflect the evolving status of LL. The development of the respiratory system or thoracic vertebra may also play a role that contradicts the previous hypothesis. This is because of differences observed in TK among patients with adolescent idiopathic curves. ¹⁸

Body mass index has been shown to have a strong correlation with PI and LL. ¹¹ This stems from the remodeling effect on the sacrum, which may continue into the early 20s.

18.3 Pediatric Spondylolisthesis

The link between pelvic parameters and spondylolisthesis has been well described in the literature. ^{19 ,​ 20 ,​ 21 ,​ 22} Therefore, it is pertinent that the management of spondylolisthesis requires a clear understanding of spinopelvic parameters in the local and global assessment of balance in the treatment algorithm.

18.4 Causative Factors in the Development of Pediatric Spondylolisthesis

The primary forces acting across the lumbosacral joint include axial loading, forward flexion, and truncal rotation. ²³ Sengupta ²⁴ explained that an increased shear force across the lumbosacral disk may explain the association of a large PI and SS with developmental spondylolisthesis, which is a result, primarily, of the loss of the posterior restraint and absence of anterior support allowing the shear forces to propagate the slip. Doming or anterior lipping of the sacral endplate were also noted in patients with high-grade developmental spondylolisthesis.

Spondylolisthesis has a prevalence of about 6% in the general population. ^{25 ,​ 26} The prevalence of high-grade spondylolisthesis is unknown. A study by the Spinal Deformity Study Group (SDSG) compared 240 cases with spondylolisthesis to 160 normal, asymptomatic young adults. They found that PI, SS, PT, and LL were significantly greater while TK was significantly lower in the cases with developmental spondylolisthesis when compared to the normal population. They also showed that pelvic anatomy has a direct influence on the development of spondylolisthesis. ²² Shear forces are then generated across the lumbosacral disk in the presence of increased PI and SS in developmental spondylolisthesis.

Historically, the two commonly used classifications were the Wiltse and Meyerding classifications. ^{27 ,​ 28} The Wiltse classification is a morphological description of five major types with I (dysplastic) and II (A, stress fracture; B, pars elongation; and C, acute fracture) being commoner in children and adolescents. The Meyerding classification describes slippage of one vertebra relative to another in the form of quadrants. There are five grades with III (50%–75%), IV (75%–100%), and V (>100%) being of most significance in childhood. An all-encompassing classification that incorporates slippage grade, PI, and overall spinopelvic balance has been devised by the SDSG ^{29 ,​ 30 ,​ 31} (Table 18‑1).

The SDSG classification describes low-grade slips (1–3) through pelvic PI and high-grade slips (4–6) through PI as well as overall spinopelvic balance. This classification informs decision making about what type of surgery to perform.