The relationship between coronal curves and sagittal balance in patients with AIS is not well understood. Based on a large study population, we observed large variabilities in coronal and sagittal alignment. The variability in sagittal alignment is apparently independent of coronal curve type and magnitude. Conversely, the PI has a greater influence on sagittal spino-pelvic parameters. Depending on the PI, variations in LL, PT and SS are observed and appear positively correlated. However the TK is consistently hypokyphotic regardless of coronal curve magnitude or degree of PI. Thus coronal and sagittal plane changes should be considered independently and individualized per patient (Figures 2 and 3). Nevertheless, this study provided the general “norm” in which patients with certain coronal deformity patterns present with in the sagittal plane.
As AIS is a three-dimensional spinal deformity with vertebral rotation, it is common for patients to have a loss in TK due to thoracic structural curves and compensation for sagittal imbalance. The mean value of TK is comparable to other studies.(9, 18) Thoracolumbar/lumbar curves may have more vertebral rotation leading to increases in LL. These coupling relationships are independent from the coronal curve magnitude. Despite different severities of Cobb angle or curve type and location, the TK and LL remains constant. This may suggest that the coronal plane deformity has less influence on the sagittal alignment. It is similar to results from another report(29), albeit smaller sample of 192 subjects, specifically looking at small (<20°) thoracic curves with or without lumbar curves at an early stage of AIS. The authors observed much less TK in thoracic curves with lumbar curves (27.6° vs 41.9°) as compared to our respective findings (17.3° vs 19.4°). This difference is observed even in our single vs multiple structural curves. Our findings suggest that the sagittal profile variations are less pronounced as the curve size increases. The thoracic hypokyphosis is likely to deteriorate with anterior column growth.
PI is a fundamental component of the “pelvic vertebra” that governs what is acceptable sagittal balance.(7) With increases in LL while the PI remains constant, there is more PI-LL mismatch in thoracolumbar/lumbar curves.(30) Depending on the degree of PI, the entire panel of sagittal spino-pelvic parameters may be altered. The relationship between LL and PI is similar to reports in adults.(25) In the smaller PI group, the LL is comparably much larger. In contrast, the LL matches PI in the large PI group. This is a relationship independent from the major coronal curve deformity. Longitudinal follow-up of these different PI groups is warranted to identify what changes occur with growth. There are growth modulation processes unique to a paediatric population(31) before the PI becomes a static parameter in adults. It is apparent that the sagittal pelvic parameters influence the sagittal alignment more so than the coronal Cobb angle. Yet, the sagittal alignment may be altered by interventions made for coronal curve correction.(32) Hence, monitoring the sagittal alignment should not be neglected.
We expect these features to be a true representation of the curve patterns in AIS. It is unlikely for sagittal decompensation to occur and recruitment of compensatory mechanisms like pelvic retroversion is not observed. Pelvic retroversion is represented by increased PT which maintains the center of gravity over the femoral heads to achieve sagittal balance. The degree of PI is a determination on the possible compensation mechanisms.(26) Patients with larger PI have a larger capacity for pelvic retroversion but requires larger LL.(7) For these patients with AIS, the degree of tolerance appears to be quite high. We expect patients with PI-LL mismatch to have an increase in PT for compensation.(33) With a negative PI-LL mismatch, we expect significant forward bending of the whole sagittal spine to achieve balance. However, despite some patients with low PI presenting with large PI-LL mismatches, there are still no significant changes in pelvic orientation.
In patients with sagittal imbalance, thoracic hypokyphosis is an important compensatory mechanism to maintain balance. This may not explain the patterns observed in the AIS population. These patients are all adolescents who we presume to have normal back musculature. However, since thoracic hypokyphosis occurs in all cases regardless of coronal curve magnitude or PI, we expect this to be a characteristic of scoliosis deformity rather than compensated sagittal malalignment.(9, 18) Nevertheless, this presentation of cases is important because this group of patients will become adults who may develop adult spinal deformities in the future. Compensatory mechanisms in this background become limited due to the inherent thoracic hypokyphosis and early decompensation may occur as compared to de novo degenerative conditions. Nevertheless, the sagittal appearance of these patients with AIS will need reassessment during adulthood.
Despite establishing these relationships on radiographs, changes in quality of life scores appear to rely mostly on the coronal features. It is important to note firstly that although AIS is a common spine problem in adolescents, its effect on the quality of life of patients in general may not be very detrimental. Thus, the SRS-22r questionnaire scores are generally quite high in our study population. Physical aspects, including function and pain, generally have higher scores than the psychological aspects, including appearance and mental health. This shows that AIS causes less confidence in the self-perceived appearance and self-image of patients, even though it has some negative effects on function and pain scores.(2, 12) It seems that different sagittal alignments do not affect the quality of life, while the magnitude of coronal Cobb angle is the main influence of the scores. The greater the coronal Cobb angle, the lower the total score and various domain scores. Importantly, these differences in function and pain domains reached MCID for clinical significance as reported by Carreon et al.(21) The relationship observed between the SRS-22r domain scores and the coronal Cobb angle is compatible with other studies.(20, 34) Older patients seem to have a better self-confidence regarding their appearance, despite greater perceived pain. This may be a result of acceptance of the deformity and development of more chronic muscle imbalance and associated back pain. However, caution is needed when interpreting these minor correlations. The effect of age is likely spurious since there is minimal correlation between age and SRS-22r scores.
There are certain limitations in using two-dimensional radiographic images to examine the condition of patients with AIS. Errors in static images may occur especially with sagittal alignment measurements due to rotational deformities.(9) Hence assessment of the rotational profile is crucial to provide the missing link between coronal and sagittal alignment. The lack of variability with TK is likely a result of axial plane deformity associated with vertebral remodelling of the apical vertebrae.(35, 36) Longitudinal data is necessary to observe the changes that occur with growth.(37, 38) In addition, we have not included the global sagittal parameters which is important for understanding alignment effects on SRS-22r scores. Though, we do not expect global imbalance to be present in AIS as young patients have strong compensatory maneuvers and any hypokyphosis or mismatch between PI and LL should reflect this. We also observed that multiple structural curves had larger Cobb angles than single structural curves. This mainly reflects the problem of a cross-sectional study as it is possible for single structural curves to develop into multiple structural curves with age. Hence, we are unable to verify the importance of multiple curves without longitudinal follow-up. One additional parameter that should be studied in the future is the cervical alignment which as seen from our case examples appear mostly kyphotic.