Kim et al. found that the CSVA was a more comprehensive index better than C7 SVA cause the latter one significantly correlated with the ODI, SRS total score, pain and function sub-scores, but was not significantly correlated with SRS self-image, satisfaction, and mental health scores, and compared to C7 SVA, the CSVA demonstrated a significant correlation with the ODI, SRS total score, pain, self-image, function, satisfaction and mental health sub-scores [7]. And in our study, CSVA was more sensitive when the standing position changed.
The spine provides the structural support of the body and transfer the weigh of the body to the lower extremities connecting through the pelvis. In order to maintaining the whole-body balance, enough lordosis was needed to balance the kyphosis and then the horizontal gaze could be achieved [17]. However, some postoperative complications such as PJF [11 − 13] and rod breakage [14] still remains remind us unknown biomechanical issues following surgery are still there. During the process of evaluating the postoperative results by radiographic parameters limited to the spine-pelvic area [3–6, 18, 19], such as C7 SVA and PT, the total body sagittal alignment from the skull to the ankle joint was ignored which may influence the patient-reported outcomes.
In our clinical practice, some patients showed relatively poor improvement of clinical scores although we have assessed improvement in spinal sagittal balance for them with C7 SVA after surgical correction. The C7 SVA, which means a plumb line from the 7th cervical vertebra to the sacrum, is limited to the evaluation of thoracic and lumbar spine instead of the whole spine include the cervical, and let alone the lower limbs, so it is not sufficient in evaluating global balance of the patient [20–22]. Not only the spine itself, but also the pelvis and lower limbs would be changed to compensate when spinal imbalance occurs. The spinopelvic movement at the hip joint are rotational actions about the hip center, which determined by both pelvic retroversion and backward femoral inclination. The knee flexion follows to attain full body sagittal balance after maximum hip compensation achieved, and so does the ankle joints. If the spine, hip joints, knee joints, and ankle joints are considered as a linear chain, the knee joints are the most active part of the three factors besides the spine. We speculated that the reason is the hip joints are fixed in the pelvis and the movement of the ankle joints are restricted by the ground.
Kim et al. [7] suggested that the distance from the cranial sagittal vertical axis to the ankle joint (CrSVA-A) is needed as a radiographic parameter to predict the widest range of patient-reported outcomes, cause compared to C7 SVA, which is significantly associated with the Oswestry Disability Index (ODI) and only three of the SRS sub-scores (pain, function, and total score), the CrSVA-A (Global SVA) linking the head to the ankle joint showed strong correlation with the SRS satisfaction sub-score after a retrospective radiographic and clinical analysis for 108 in adult spinal deformity(ASD) patients. Hey et al. [23] considered that the changes with age is likely produced by relaxed postural tendencies, the latter happening before the former. Based on these conclusions, we think that using just radiographic parameters from part of the body might not be enough to fully encompass clinical outcomes, so in predicting the postoperative efficacy of adult patients with spinal deformity, the use of spinal-pelvic factors alone is not enough, the head and lower limbs should also be considered. The occurrence of PJK/PJF and rod failure, evoke us a mechanical problem caused by standing posture may increase the risk of mechanical complications. Different standing postures showed different spine profile, and further affected the angle of the pedicle screws, the arc of the connecting rods and the shear force of the entire implant system. Although we are not sure by now about the impact of standing posture on postoperative complications, we believe that in the process of deformity correction, a nature and comfortable upright position should be given the equal attention as the standard upright position.
There are still weaknesses to our study. First, the ethnic applicability can be a limitation. Second, pelvic morphology is known to be different between sexes, taking the closely relationship between lumbar morphology and pelvic morphology into account, potential bias due to pelvic morphological effects between genders should be considered.
Although the findings of this current study in young, healthy adults cannot be directly applied to ASD patients until a further study showing the reproducibility of these concepts in ASD patients can be produced, standard upright position and natural and comfortable upright position are equally important and should be focused on before performing a surgical plan for ASD patients.