Smaller Intervertebral Disc Volume and More Disc Degeneration after Spinal Distraction in Scoliotic Children

Abstract

In recent decades, magnetically controlled growing rods (MCGR) were established to treat progressive early-onset scoliosis. The aim of this investigation was to assess the effect of long-term MCGR with continuous distraction on intervertebral discs in scoliotic children. Magnetic resonance imaging (MRI) of 33 children with spinal muscular atrophy was analyzed by grading intervertebral disc degeneration (IDD) and measuring intervertebral disc volume. Cohort I (n = 17) were children who had continuous spinal distraction with MCGRs for 5.1 years and MRI before (av. age 8.1) and after (av. age 13.4) MCGR treatment. Cohort II (n = 16, av. age 13.7) were patients without prior surgical treatment. Lumbar intervertebral disc volume of cohort I did not change during 5.1 years of MCGR treatment, whereas disc volumes were significantly larger in age- and disease-matched children without prior treatment (cohort II). Cohort I showed more IDD after MCGR treatment in comparison to early MRI studies of the same patients and children without surgical treatment. MRI data showed a volume reduction and disc degeneration of lower thoracic and lumbar intervertebral discs in scoliotic children after continuous spinal distraction with MCGRs. These effects were confirmed in the same subjects before and after treatment as well as in surgically untreated controls.

Keywords: MRI; disc degeneration; growth-friendly implants; intervertebral disc; scoliosis; spinal muscular atrophy; volume.

Figure 1

Bilateral rib-to-pelvis fixation with magnetically controlled growing rods (MCGR) in a 5-year-old spinal muscular atrophy type II girl with progressive scoliosis. Posterior anterior (p.a.) (A) and lateral (B) sitting radiographs before the surgical intervention. Deformity correction (C/D) after surgical treatment. Implant exchange after maximal distraction after 3 years of treatment. 4-year follow-up (E/F) with second MCGRs. 6-year follow-up (G/H) with fully distracted rods.

Figure 2

Cohorts and analyzed intervertebral discs. Cohort I had MRI investigations before and after on average 5.1 years of MCGR treatment. Cohort II had an MRI investigation at an average age of 13.7 years thus matching the late MRI exam of cohort I (A). On MRI scans intervertebral discs between thoracic vertebra 4 and 5 (T4/5), 7 and 8 (T7/8) and 11 and 12 (T11/12) were analyzed (B) as well as all intervertebral discs in the lumbar region (C).

Figure 3

Measurements of intervertebral disc volume and intervertebral space dimensions. Intervertebral disc volume was determined by manual drawing of intervertebral disc circumference on each individual plane of the MRI scan (A) and subsequent software-based volume calculation (B). Intervertebral space dimensions were anterior intervertebral height (AIVH), posterior intervertebral height (PIVH), intervertebral disc length (IVDL) and height at the middle of IVDL. These parameters were measured on each individual plane of lateral MRI images (C) in order to use them for formula-based calculation of the intervertebral space volume (Figure 4).

Figure 4

Calculation of intervertebral space volume and correlation with intervertebral disc volume. Intervertebral space dimensions measured as described in Figure 3 were used to calculate the intervertebral space volume using the formula depicted in (A). First, the area A of the intervertebral space was calculated for each individual plane using the measured lengths as depicted in Figure 3 and the Trapezoidal rule. Again, using the Trapezoidal rule, the areas of all planes (A1, A2, A3, … Ax with x being the number of planes) were subsequently multiplied by the sheath thickness of MRI scans to gain the total intervertebral space volume V. Correlation of intervertebral disc volume (determined by drawing of circumferences and software-based calculation) and intervertebral space volume (determined by landmark measurements and calculation using formula) for 202 intervertebral discs (T4/5, T7/8, T11/12, L1/2-L5/S1) revealed a strong correlation of both methods (R² = 0.98) (B). On average, the intervertebral disc volume was larger by a factor of 1.18 than the intervertebral space volume, so that 1.18 (+/−0.11) may be applied as a correction factor to calculate the intervertebral disc volume from the intervertebral space volume, and a correction factor of 0.86 (+/−0.08) may be applied vice versa. (C). Given is mean +/− standard deviation (SD).

Figure 5

Intervertebral disc volume in children before and after 5.1 years of MCGR treatment. In cohort I, MRI measurements of children with spinal deformity were compared before and after 5.1 years of MCGR treatment (A). No significant changes in intervertebral disc volumes (B) and height of the intervertebral disc space (C) were seen after 5.1 years in the lower thoracic and lumbar spine. (B and C) Paired t-test; mean +/− SD; early (Cohort I before MCGR) and late (Cohort I after MCGR) time point: n = 14 (T4/5), n = 9 (T7/8), n = 12 (T11/12), n = 12 (L1/2), n = 12 (L2/3), n = 12 (L3/4), n = 13 (L4/5) and n = 13 (L5/S1); statistical level of significance p < 0.05 (*), p < 0.01 (**).

Figure 6

Intervertebral disc volume in age- and disease-matched children with and without MCGR. Comparing age- and disease-matched children with and without prior MCGR (A). Untreated children had significantly higher lumbar disc volumes in comparison to MCGR-treated children (B). Note the trend towards increased height of intervertebral disc space in untreated children (C). (B) and (C) Unpaired t-test; mean +/− SD; n = 17/14 (T4/5), 13/11 (T7/8), 15/10 (T11/12), 14/12 (L1/2), 15/11 (L2/3), 15/12 (L3/4), 16/14 (L4/5) and 17/14 (L5/S1) with prior MCGR (Cohort I after MCGR) and without prior MCGR (Cohort II) respectively; statistical level of significance p < 0.05 (*), p < 0.01 (**); for t-test results close to significance, p-values are given above the corresponding bars.

Figure 7

Intervertebral disc degeneration (IDD) measured with the Pfirrmann [13] score in 346 intervertebral discs. (A) Example images from the study population representing Pfirrmann scores I-V. (B) After an average of 5.1 years of MCGR treatment there was significantly more IDD in the same population. Age- and disease-matched patients without prior surgical treatment had significantly less IDD in comparison to pretreated individuals. Intervertebral discs were scored by five independent observers in a blinded manner and mean values from all observers are displayed. Cohort I (before MCGR) vs. Cohort I (after MCGR) Wilcoxon matched pairs test; Cohort I (after MCGR) vs. Cohort II Mann–Whitney test, n-numbers are given in the graph; included are n = 16/16/15 (T4/5), 14/14/15 (T7/8), 16/16/14 (T11/12), 13/13/16 (L1/2), 13/13/16 (L2/3), 13/13/16 (L3/4), 13/13/16 (L4/5), 13/13/16 (L5/S1), intervertebral discs from Cohort I (before MCGR), Cohort I (after MCGR) and Cohort II respectively; statistical level of significance p < 0.001 (***).