Degenerative disc diseases (DDD) is a spectrum of diseases, which may present as disc herniation, spinal stenosis, spondylolisthesis, facet joint arthropathy, or their combination. Recent studies suggest that sagittal balance is important in DDD in which it closely associated with the patient quality of life (QOL)[1, 2]. T1 tilt, thoracic kyphosis (TK), lumbar lordosis (LL), sacrum slope (SS), pelvic tilt (PT), pelvic incidence (PI) and sagittal vertical axis (SVA) have been identified as important spinopelvic parameters in maintaining spinal sagittal balance, and proven to be essential and effective referremce in spinal fusion surgery. Many studies have reported a significant loss of LL in patients with DDD, especially after spinal fusion surgery, resulting in a compensative increase in PT[4–6]. The increase of PT represents the pelvis is in a backward rotation, which is a kind of compensatory mechanism of sagittal balance. The backward rotation of the pelvis can continue to a certain extent, the femoral head is forward result from the increasing tilt of the pelvis, meanwhile, the sacrum and the spine are backward. This causes the C7 plumb line to stay behind the perpendicular line passing through the middle of the femoral head, and the gravity line to fall between the feet. As a result, the full body is balanced, but it is a kind of uneconomic compensatory balance ,in which the posterior spinal muscle can be used as a tension band, trying to recover some lumbar lordosis. Moreover, this is a process of energy consumption, pain occurs quickly, explaining a certain degree of low back pain.
Schwab et al. find a correlation between a high PT (threshold 20°) and a poor health-related QOL, and between a low PT and a good clinical outcome. So PT < 20° means the pelvis is in a state of balance. However, a high absolute value of PT does not necessarily represent increased retroversion, PT > 20° can also be an anatomic trait and simply reflect a large PI, in a similar way, the PT threshold may need to be lower than 20° for patients with a smaller PI. Futher more, the normal range of those spinopelvic parameters is so wide due to individual differences and measurement errors that it is challenging to differentiate a compensatory from a normal state of balance by using a single radiographic parameter alone. In the present study, we designed several composite parameters including maximal lumbar lordosis-maximal thoracic kyphosis (maxLL-maxTK), sacral slope-pelvic tilt( SS-PT) and sacral slope/pelvic tilt (SS/PT) to reduce the measurement errors and improve the accuracy and also attempt to find their diagnostic value in distinguishing compensatory balance from normal balance.
DDD resulting in loss of lumbar lordosis, can lead to sagittal plane deformities.The loss of lumbar lordosis can be considered as the initiating event of sagittal imbalance. Because of the potential mechanism of compensation, just like pelvic retroflexion, thoracolumbar hyperextension, knee flexion, ankle flexion, and finally cervical extension, the patients with DDD present with a compensatory sagittal balance. This not only influence the indication for spinal surgery, but also conceal the severity of the disease and mislead the treatment strategy. The recognition of compensatory sagittal balance has some clinical significance for the diagnosis and treatment of DDD. Nevertheless, few parameters have been used in the assessment of a state of compensatory sagittal balance, and comparison of these parameters between DDD patients with compensatory sagittal balance and asymptomatic adults without DDD has not been extensively explored. The purpose of this study was to explore possible correlations between the composite radiological parameters maxLL-maxTK, SS-PT, SS/PT and the diagnosis of DDD patients with compensatory sagittal balance, and identify which parameters could be used to discriminate DDD patients with compensatory sagittal balance from asymptomatic adults.