A good understanding of the principles of balance is vital to achieve optimal outcomes when treating spinal disorders. A complex interaction of the neuromotor system and muscular recruitment is necessary for ergonomic balance and deliberate displacement of the human body. Spinal alignment has to allow an individual to stand pain free with minimal muscular energy expenditure. This concept is reflected in the "Cone of Economy" principle by Jean Dubousset. Sagittal plane misalignment in spinal deformities challenges balance mechanisms used for maintenance of an upright posture. The current state of the art diagnostic work-up of spinal deformities is mainly a static 2D radiological evaluation in Scoliosis Research Society (SRS) free standing position with analysis of the spinopelvic parameters as described by Duval-Beaupmet and others.

Until now sagittal balance has been assessed by dropping a vertical plumb line from C7 vertebral body center and quantifying the distance of the sacral plate from this vertical (Sagittal Vertical Axis or SVA). Others measure the T1 spinopelvic inclination angle (T1-SPI). SVA, T1-SPI and pelvic tilt are correlated with self-reported disability and health related quality of life scores (HRQL) compared to age- and sex-related normal subjects.

Literature suggests a multifactorial etiology of impaired balance capacity with neurological or vestibular disease, muscular atrophy in mm erector spinae, increasing age, low back pain and history of spinal surgery. The occurrence of postoperative complications after spinal deformity correction like under-correction of sagittal misalignment, postoperative reciprocal changes in thoracic kyphosis, proximal junctional kyphosis and failure of instrumentation are possibly due to the current state-of-the art inadequate diagnostic work-up.

Investigators do not fully understand the roll of vision and exact strategy of recruitment of neuromuscular units (trunk, pelvis, lower limbs) in patients with sagittal plane misalignment during standing and walking. Several compensatory mechanisms in sagittal balance disorders are identified in the static situation. Intra-spinal mechanisms like hyperextension of lumbar discs, retrolisthesis of lumbar vertebrae, reduction of thoracic kyphosis, and pelvic back tilt and extra-spinal mechanisms like knee flessum and ankle extension are suggested to act as compensatory mechanisms. In literature, a strong correlation between the occurrence of knee flessum and lack of lumbar lordosis is seen. To understand these mechanisms a dynamic evaluation of individuals with spinal deformities is needed. Currently there is only very little research performed in the field of clinical balance tests and instrumented movement analysis in patients with spinal deformity. With regard to clinical balance tests the Fullerton Advanced Balance Scale (FAB scale) is presented as a reliable tool to predict wether or not higher-functioning older adults will fall. The FAB scale is a reliable and valid tool in Parkinson disease with minimal ceiling effect and shows promising results in detecting small balance disturbances.The use of these balance tests in patients suffering from spinal deformity with sagittal imbalance has not been validated in literature till now. Last but not least the use of instrumented movement analysis to examen gait in subjects with spinal deformity is unconventional. Subjects with fixed sagittal balance are reported to have a significant slower walking speed and poorer endurance score relative to age matched controls. An inadequate use of pelvic tilt during walking is also observed. Subjects with forward inclination of the trunk present with abnormal kinematics and kinetics of the lower limbs during walking compared to age- and sex-related normal subjects. When deformity exceeds the primary compensation mechanisms, additional mechanisms, such as crouch gait, are used to reorient the trunk to a more vertical position.

The currently used trunk model in movement analysis in UZ Leuven has been developed by Heyrman et al after the work of Leardini et al. Armand et al also considered the thorax not as one rigid segment and presented the use of an optimal marker placement set on the thorax for clinical gait analysis. However they did not include markers on the head. Heyrman et al showed in their study with cerebral palsy children (CP) that increased altered trunk movements during gait were related to a lower performance on the Trunk Control Measurement Scale (TCMS) in sitting, indicating the presence of an underlying trunk control deficit. These authors were thus able to show a correlation between a clinical postural test like the TCMS and trunk-and lower limb parameters during gait. However, they could not find a significant correlation between overall altered trunk movements and altered lower limb movements during gait in a CP population and concluded that observed thorax movements during gait, most likely are the resultant of both compensatory movements for lower limb deficits and an underlying trunk control deficit. The current concept of thinking is that in an adult spinal deformity population with sagittal plane misalignment the observed altered movements in the lower limbs during standing and gait are compensatory for the forward inclination of the trunk. The challenge for future studies is to further unravel the relation between trunk and lower limb movements, grouped into functional movement patterns. Moreover, additional information on trunk and lower limb kinetics and muscle activity (using dynamic electromyography (EMG)) will highly contribute to the understanding of this functional relationship, and will provide more in-depth insights into compensatory mechanisms of the trunk versus the lower limbs and vice versa.