The maintenance of spinopelvic alignment is most important for adults with spinal deformity, as this is the primary determinant of life quality after corrective surgery. However, sagittal imbalance reportedly increases the probability of ASD after spinal fusion for LDD. LL is important for maintaining sagittal balance and upright posture. The most widely-used method of measuring LL is to measure the Cobb angle between the upper endplate of L1 and the upper endplate of S1 in the standing position. Currently, the relationships between LL and age, sex, and other factors are unclear; however, LL is positively correlated with lumbar spondylolisthesis and spondylolysis, and negatively correlated with LBP[10,15]. Failure to maintain normal LL may also increase the incidence of facet arthritis. If the LL is small, this increases the risk of sagittal imbalance after surgery and is a predictor of ASD, which is similar to our findings. Thus, restoration of the physiological curvature of the lumbar spine is very important in improving patient quality of life and preventing postoperative complications.
The spinopelvic balance plays an important role in LDD. Several formulas have been created to evaluate the ideal LL to be reestablished in lumbar fusion surgery in different populations. Based on Legaye’s formula in Korean patients, Lee et al. found that overcorrection of LL (postoperative LL angle > ideal LL) effectively maintains the optimal SVA in patients with degenerative lumbar kyphosis during a minimum 2-year follow-up. Considering the effect of age, Xu et al. determined the predictive formula for the ideal LL in Chinese adults as: LL = 0.508 × PI - 0.088 × age + 28.6. Therefore, the surgical reconstruction of the ideal LL must consider variables such as age and ethnicity. Given that the normal range of LL varies widely (18.5–72.3° using the Cobb method), it is difficult to estimate the normal/optimal LL angle for an individual.
There is not enough existing knowledge to accurately reconstruct the lordotic curvature. Our study attempted to explore the relationship between the △LL and the need for reoperation for ASD after lumbar fusion in patients of different ages. A △LL of > 10° was associated with an increased risk of ASD in patients > 60 years old (Figure 5), but not in patients ≤ 60 years old. This suggests that surgeons should not markedly change the LL angle in older adults. No previous study has investigated the effect of the postoperative change in LL on the prevention of ASD. However, the regional Cobb angle of L4-S1 is reportedly a crucial factor affecting the formation of LL, and a review of the data from 274 patients found that a postoperative L4–S1/L1–S1 lordosis ratio of < 50% increased the prevalence of ASD. Further studies are required to confirm the ideal correction of the L4–S1/L1–S1 lordosis ratio and △LL.
A recent study reported that the variables most related to severe disability (Oswestry Disability Index > 40) due to adult spinal deformity are a PT of > 22 °, SVA of > 47 mm, and PI-LL of > 11°. Based on age-specific Oswestry Disability Index values, a subsequent study revealed that the ideal spinopelvic alignment values for patients aged < 35 years are a PT of 10.9°, PI-LL of 10.5°, and SVA of 4.1 mm, while those for patients aged > 75 years are a PT of 28.5°, PI-LL of 16.7°, and SVA of 78.1 mm. PI-LL mismatch can also be used to predict the incidence of ASD after spinal fusion surgery. Rothenfluh et al. reported that patients with a PI-LL of ≥ 10° were 10 times more likely to undergo revision surgery than those with a PI-LL of < 10°. Sagittal imbalance after lumbar fusion may increase the incidences of postoperative complications and ASD. In the present study, patients > 60 years old with a PI-LL of > 20° had an increased incidence of ASD (Figure 3). However, a PI-LL of > 10° was associated with a high prevalence of ASD in patients ≤ 60 years old (Figure 4). Patients with a PI-LL of ≥ 10° experience greater shear stresses and compression forces at the intervertebral joints after lumbar fusion compared with those with a PI-LL of < 10°, which may indicate a poor natural history. Figure 6 highlights a patient with a PI–LL of < 10° and a change in lumbar lordosis of < 10° were significantly less likely to develop adjacent segment disease.
For every adult, the PI is fixed and is a reliable morphological parameter of the human body. The size of the PI-LL mismatch reveals the relative decrease in LL, resulting in the displacement of the gravity axis of the PI and the inhomogeneity of the sagittal alignment of the spine. When the sagittal plane of the spine is unbalanced, the body will instigate a series of compensatory mechanisms to maintain the balance of the sagittal plane. The first compensatory mechanism of the spine is overextension of the thoracic vertebrae, which reduces the thoracic kyphosis. The later compensation tends to manifest as retrodisplacement and posterior translation of the pelvis, along with flexion of the knees and ankles. Clinically, the trunk of older adults is pitched forward due to loss of LL, and so they can withstand degenerative sagittal imbalances. Thus, it may be counterproductive to fully return the spinal curvature to normal in older adults. Our current strategy is to determine the appropriate LL and PI-LL at the time of surgery to prevent ASD via long-term or short-term fusion. To obtain the optimal LL and PI-LL, surgeons should consider using methods such as appropriate hyper wedge cages and the bend screw-rod system that can meet normal physiological curve of the spine.
The present study had some limitations. (1) The data were obtained from cases of spinal surgery performed in a single institution. (2) The relationship between LL and quality of life was not assessed. However, as the assessment was based only on radiological measurements, the data were relatively objective. (3) The optimal LL angle varies in accordance with ethnicity, age, sex, and other variables. Our study cohort only represents a demographically homogenous group of Chinese patients.