'Flat back deformity' is a disorder of the thoracolumbar spine that results in chronic back pain and forward inclination of the trunk. The term 'flat back' was originally described by Doherty, as an iatrogenic complication of wherein thoracolumbar scoliosis correction by Harrington-distraction instrumentation [1]. Currently, it generally means sagittal imbalance caused by various factors [2]. Normal sagittal alignment is determined by various shapes and sizes of thoracic kyphosis (TK) and lumbar lordosis (LL) by the position of head, trunk, and pelvis in relation to the center of gravity. Loss of LL due to various causes usually initiates and leads to global sagittal imbalance. In flat back deformity, stooping of the trunk occurs because the lumbar spine, which is naturally lordotic, loses spinal sagittal balance by abnormal decrease in LL [3]. Conservative approach in flat back deformity is inefficient and has uncertain effects. Therefore, it is important to restore abnormal LL to a normal balanced state by realignment surgical treatment [45]. There are various surgical approaches to treat flat back deformity, generally comprising the posterior only and the anterior and posterior combined approach [67]. Recent studies show that the anterior approach with interbody fusion at multiple levels combined with posterior reconstruction is effective in the restoration of LL, especially in lower lumbar levels [8]. After surgical correction of flat back deformity, LL can be well maintained throughout the follow-up period [9]. However, in some cases, correction loss may also occur during follow-up. Each individual has an inherent normal value of LL, which gradually decreases with age [10]. Thus, the target of surgery should focus on the restoration of LL to the original alignment. However, there is no definite criteria for the determination of the degree of restoration because each individual has a wide range of LL that varies from approximately 20° to 60°. Pelvic incidence (PI) is one of the pelvic parameters that is an independent anatomical parameter and unique to individual. It determines pelvic orientation and size of LL. There have been numerous attempts to predict LL in relation to pelvic parameters [111213141516]. Schwab et al. [15] suggested that LL should be within 9° of PI for realignment of spinal deformity. The objective of this study was to provide an ideal correction angle of LL in degenerative flat back deformity based on the relationship between LL and PI from radiological and clinical results.

Understanding the concept of sagittal spinal balance and compensatory mechanism is essential to planning surgical treatment of sagittal spinal deformity. Sagittal imbalance can be caused by decreased LL and increased TK, and can furthermore be influenced by the PI and flexion contracture of the hip and knee joints [3]. Basically, flat back deformity occurs as a result of decrease in LL and thus restoration of LL to the normal state is the main target of surgery. In the past, it was recommended to restore LL to normal range. However, since there is a wide normal range among individuals, some surgeons attempted to correct LL by approximately 20° more than TK. Kim et al. [20] analyzed postoperative sagittal balance following long adult spinal instrumentation and fusion. They reported that a sagittal Cobb angle difference between LL and TK of >20° is advisable to achieve optimal sagittal balance [20]. However, in flat back deformity, it is difficult to determine the accurate amount of correction with the method mentioned above, because TK usually decreases due to the compensatory action, as a result of loss of LL. Moreover, chin-brow vertical angle method is another means of predicting LL. However, it is usually used in gaining horizontal gaze in fixed global deformity such as ankylosing spondyltis, thus it is inappropriate for flexible types.

Recently, numerous studies emphasized the relevance of LL and corresponding influence of pelvic parameters [18212223], including pelvic incidence (PI), PT, sacral slope, and overhang (Fig. 1). PI is defined as the angle between the perpendicular line to the sacral end plate at its midpoint and the line connecting this point to the axis of the femoral heads [12]. Among the pelvic parameters, PI is the most critical factor because it is the only parameter that is unique to each patient and is independent of the position of the pelvis. In present study, preoperative, postoperative, and last follow-up PI of each individual showed no significant difference, suggestive of constant and unique features. Functionally, PI is closely related to optimal LL [232425]. There have been various attempts to predict LL using pelvic parameters. Legaye et al. [12] measured pelvic parameters in 49 adults free of vertebral disease and demonstrated a predictive equation for lordosis based on the parameters. Moreover, they suggested that a low value of PI implies low values of pelvic parameters and a flattened lordosis, whereas a high value implies well-tilted pelvic orientation and pronounced lordosis [12]. Vialle et al. [13] measured sagittal parameters of 300 asymptomatic volunteers and analyzed the relationships among them to predict maximal LL. Boulay et al. [11] evaluated 149 healthy adults to determine a lordosis predictive equation based on PI. They additionally used T9 tilt to predict LL and claimed that it increased reliability [11]. In 2010, Schwab et al. [15] analyzed radiographic values and ODI of 125 patients with spinal deformity and suggested that realignment objectives should include SVA<5 cm, PT<25°, and LL proportional to the PI (LL=PI±9°) [15]. In our study, patients who were under-corrected eventually had more loss of correction at the last follow-up (57.1%). We focused on the degree of OC of LL to restore normal sagittal alignment. We used Schwab's method (LL=PI±9°), which was relatively simple and tried to realign LL proportional to PI for prediction of target LL. Nevertheless, 14 of 49 patients (29%) were under-corrected (UC group; LL<PI-9°) on immediate postoperative radiographs. Patients in OC group had significantly better maintenance of sagittal balance (SVA≤5 cm) with less correction loss at the last follow-up (p<0.05). Thus it is critical to realign LL proportional to PI, within 9°, to get a satisfactory result (Fig. 2).

Each individual has unique size of PI and therefore we hypothesized that if a patient has low PI, it would be easier to get OC since less LL is required to be corrected, resulting in better prognosis. Lee et al. [16] reported that mean value of PI in healthy adults was 52.5°. In addition, Legaye et al. [12] indicated the mean value of PI as 51°. The mean PI of 49 patients included in this study was 55.6°. Therefore, we classified the patients into 2 groups-low PI group (PI<55°) and high PI group (PI≥55°). The low PI group showed a tendency of better OC (86.4%), as compared to high PI group (59.3%) although it was not statistically significant (p=0.057). It means that if a patient has high PI, more LL is required to be corrected in order to make LL proportional to PI, which results in more chances of under-correction and correction loss at the last follow-up. Therefore, we recommend for sufficient LL in relation to PI, and in high PI patients, any type of osteotomy should be performed if optimal LL is not achieved with ALIF (Fig. 3).

ODI improved at the last follow-up, as compared to the preoperative ODI in both OC and UC groups (p<0.05). However the mean ODI at the last follow-up of both groups showed no significant difference (p=0.975). This is because ODI not only reflects stooping symptoms due to sagittal imbalance but also reflects back and leg pain and other factors influencing daily living activities.

The limitations of the study were that it was a retrospective study with short follow-up period. Other factors influencing the outcome of surgery (e.g., age, osteoporosis, condition of hip and knee joint) should also be considered. Further study with a higher number of cases and longer follow-up will be needed to confirm our preliminary report.

Correction of LL proportional to PI (LL=PI±9°) will result in better sagittal balance with less correction loss in corrective surgery of flat back deformity. Loss of sagittal balance due to insufficient correction may often occur in patients with high PI (PI≥55°). Therefore, care should be taken to obtain sufficient LL proportional to PI.