The role of sagittal balance parameters in the formation of kyphotic deformity in patients with ankylosing spondylitis

Objective. To establish a pathobiomechanical relationship between the initial parameters of sagittal balance and the formation of kyphotic deformity at the thoracic and lumbar levels in patients with ankylosing spondylitis.

Material and Methods. Data from a retrospective cohort study of 46 patients aged 18 to 70 years with axial form of ankylosing spondylitis and decompensated sagittal imbalance with loss of horizontal gaze were studied. All patients underwent digital teleradiography of the spine in a standing position with the calculation of a complex of spinopelvic parameters and assessment of functional status according to the Oswestry index (ODI).

Results. Analysis of sagittal balance parameters revealed that greater severity of kyphotic deformity is significantly associated with a greater value of the pelvic index (Pelvic Incidence -PI). A strong negative correlation was found between the ODI and the spino-sacral angle (SSA) at a mean value of the pelvic index (45°≤ PI <55°) and at its high index (PI ≥55). A critical link between quality of life and the gaze angle was also found at a high PI. For patients with a low PI, the main trigger of maladaptation was trunk translation, which correlates with the sacro-femoral distance. Combinations of a mean PI with a decrease in SSA ≤85° and a high PI with a decrease in SSA ≤117° were identified as reliable mathematical predictors of deterioration in functional capacity.

Conclusion. The formation of kyphotic deformity is determined by the baseline architectonics of the spinopelvic complex, and the proposed threshold values for the SSA angle allow for an objective prediction of the extent of patients’ functional capacity.

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