INTRODUCTION
Degenerative flat back (DFB) refers to a spinal sagittal imbalance due to decreased lumbar lordosis caused by degenerative changes and various disabilities in daily living associated with stooped posture [
1,
2]. Although DFB is very rare in Western countries, it is a common spinal deformity in Asian countries. The cause of this ethnical difference is explained by different lifestyles, such as sitting on floor or working while squatting. Most patients reveal four cardinal and characteristic clinical symptoms: stooping while walking, inability to lift heavy objects in front, difficulty in climbing slopes, and the need to support oneself with the elbow, resulting in formation of a hard corn on the extensor surface of the elbow [
3]. Although the exact pathophysiology of DFB has not yet been confirmed, extensive degeneration and weakness of lumbar extensor muscles are thought to be responsible for that condition in most patients [
2,
4].
Because radiographic examinations reveal characteristic sagittal deviation of spinopelvic alignment, diagnosis of DFB is usually made on the basis of clinical and radiographic findings [
3]. Decreased lumbar lordosis causes anterior displacement of the center of gravity, which leads to spinopelvic angular changes and further affects standing posture and gait characteristics [
5,
6]. The studies regarding spinopelvic imbalance in the patients with DFB have focused mainly on static parameters measured by simple radiography [
3,
7,
8,
9]. However, radiography has the limitation that it can only reveal the static posture, and cannot evaluate the dynamic status, such as ambulation.
However, daily activities or functional aspects of patients are more related to dynamic status of the spinopelvic segment than static posture measured by radiography. Treatment outcomes or patient satisfaction is also influenced by dynamical parameters rather than static parameters. Thus, it is assumed that assessment of dynamic parameters of DFB can provide clinically useful data about patients' functional status and treatment outcomes. Three-dimensional (3D) motion analysis has been a useful tool to analyze biomechanical parameters in various conditions including spinal problems [
10,
11,
12,
13]. It is hypothesized that 3D motion analysis can provide specific characteristics of spinopelvic and lower limb joint motion in patients with DFB, and also reveal which improvement would occur following surgeries in a quantitative and objective way. Previous studies investigated dynamic parameters in the patients with DFB but they did not assess dynamic parameters of spinal segments and did not perform motion analysis test after corrective surgeries, so they could not evaluate the improvement acquired by surgical treatments [
5,
14].
The purposes of this study were (1) to evaluate the static spinopelvic alignment on a plain radiography and the dynamic spinal status and lower limb kinematics by 3D motion analysis of the patients with DFB compared with normal control; (2) to evaluate the improvement of static and dynamic parameters after corrective fusion surgeries in comparison with those of pretreatment; (3) to assess the correlation between the changes of static and dynamic parameter improvements after corrective surgeries; and (4) to compare the amount of the improvement of static and dynamic parameters between the successful and unsuccessful surgical outcome groups.
DISCUSSION
Patients with DFB show sagittal imbalance due to decreased lumbar lordosis and have functional impairments related to daily activities including ambulation. The assessment of the dynamic parameters is more associated with patients' functional aspects. It is assumed that 3D motion analysis can evaluate spinopelvic and lower limb kinematics in patients with DFB during ambulation and can identify the functional improvement obtained by surgery.
Our study indicated that surgical outcomes were more closely related to be the dynamic parameters, such as dynamic lumbar lordosis. Lumbar lordosis plays an important role in the sagittal alignment and balance so that a successful correction of the lumbar lordosis is critical to the functional improvement or patient satisfaction [
8]. Although more significant improvement of the dynamic lumbar lordosis was found in the successful group than in the unsuccessful group, no significant difference was found in the static parameters including lumbar lordosis between the two groups. This result was explained by the fact that the dynamic parameters during ambulation were more related to daily functional activities, and as a result, more influential factor to patient satisfaction. Therefore, the improvement of the dynamic parameters measured by the 3D analysis was more important in predicting the surgical outcomes than angular measurements on plain radiography.
Studies have related surgical outcomes to motion analysis. One reported that the posterior pelvic tilt among the dynamic parameters at pre-operation was important in predicting good surgical results after corrective surgeries in the patients with DFB, while none of the radiographic static parameters of the spine indicated any clue to surgical results [
5]. Another study reported that the posterior pelvic tilt angle was significantly related to successful outcomes of corrective osteotomy, whereas preoperative static pelvic tilt angle was not [
14]. However, these studies did not perform postoperative motion analysis therefore they could not compare the amount of improvement of the dynamic parameters achieved by surgeries between the successful and unsuccessful groups. As well, they did not evaluate the dynamic parameters of the spinal segments.
Compared with normal controls, the DFB group with decreased lumbar lordosis showed lordotic/flat thoracic angle and posteriorly tilted pelvis resulting in reduced SS, which was considered as a compensation mechanism in order to prevent the excessive sagittal malalignment or anterior translation of the body axis [
3,
7]. These compensatory changes could be reversed by corrective surgery of lumbar kyphosis [
9]. Our study showed compatible results to these previous literatures and additionally, we identified those compensatory changes at the adjacent segments and reversible corrections after surgery in terms of not only static parameters but also dynamic parameters (
Tables 1,
2).
DFB also leads to the change of the lower limb kinematics in addition to the spinopelvic kinematics. Increased knee flexion and ankle extension (dorsiflexion) associated with DFB have been reported [
7,
18,
19]. Our study also revealed more increased hip flexion, knee flexion, and ankle dorsiflexion angles in the DFB group than in the control group. Considering their old age, it was doubtful that the increased lower limb angles of DFB group might be the result of hip and knee osteoarthritis. But, the increased lower limb flexion angles at pre-operation were spontaneously reduced along with the improvement of the spinal angles after lumbar corrective surgeries. Therefore, the larger lower limb flexion angle was a secondary phenomenon resulted from lumbar kyphosis rather than from the joints problems of the lower limbs. It was assumed that the excessive anterior translation of the body center related to the stooped posture in the DFB group would be prevented by such compensatory mechanisms of the lower limb as increased hip and knee flexion, ankle dorsiflexion, which contributed to the posterior pelvic tilt [
6,
18].
To our knowledge, there has been no published study evaluating the correlation between the changes of improvement of the static and dynamic parameters achieved by surgeries in the patients with DFB. The significant relationship was mainly found between the dynamic and static parameters of the spinal segments. However, no significant relationship was revealed between dynamic spine versus static pelvic, dynamic pelvic versus static spine, and dynamic pelvic versus static pelvic parameters. The correlation between the static and dynamic parameters was relatively weak, and therefore, static angular changes measured on X-ray after surgeries had some limitation in predicting the degree of the improvement in the dynamic parameters. Considering that the dynamic parameters were more related to the functional status, the static parameters were not appropriate for the prediction of the functional outcomes after surgeries.
A successful result was defined on the basis of the MacNab criterion of patient subjective satisfaction. Although most of angular deviations of DFB were significantly corrected by corrective fusion surgeries in general, considerable proportion of the patients (36.2%) was not satisfied with surgical outcomes. There were two possible explanations. First, we established a relatively strict standard in judging successful and unsuccessful group. Only the excellent and good responders were included as the successful group. Second, our study identified that the dynamic lumbar lordosis was the only parameter that was related to the patients' subjective satisfaction among various parameters. Thus, those who did not show enough improvement of the dynamic lumbar lordosis were not satisfied with surgical results, in spite of general improvement of other parameters. This suggested that successful acquisition of enough dynamic lumbar lordosis was the most important factor which determined the success of corrective surgeries in terms of the patients' satisfaction and should be considered as the main goal of corrective surgery.
This study has several limitations. First, we conducted the motion analysis only in terms of ambulation. The tests about other daily activities regarding sit to stand, trunk flexion, or pick up object can provide more useful information related to the patients' functional aspects. Second, the motion analysis was done only within 6 months after the surgery. A 1- or 2-year follow-up study could assess the change of patients' functional status in terms of long-term surgical outcomes.
In conclusion, the patients with DFB showed increased TK and decreased SS in the static parameters and increased posterior pelvic tilt, hip flexion, and knee flexion angles in the dynamic parameters, which could be reversed by lumbar corrective surgeries. The correlation between the static and dynamic parameters was mainly found in the spinal parameters. Surgical outcomes in terms of the patients' satisfaction were more related to the improvement of the dynamic parameters rather than the improvement of the static parameters. Therefore, 3D motion analysis was clinically useful in the evaluation of the patients with DFB in that it allowed for the assessment of the dynamic parameters of the spinopelvic and lower limb segments, which was related to the daily function or treatment outcomes.