1. Introduction
Adult spinal deformity (ASD) causes lower back pain not only with activity but also with rest. Intermittent claudication and gait disturbance have been associated with spinal deformity [
1]. It has been reported that worsening sagittal plane alignment is more detrimental to quality of life for ASD patients compared with coronal plane malalignment [
2]. While improving sagittal alignment is important, ASD patients take time to adapt to the new posture after corrective surgery, and their ability to walk and balance are temporarily impaired [
3,
4]. According to gait analysis, preoperative ASD patients walk in a characteristic crouching posture due to spinal sagittal malalignment [
5]. After ASD surgery, patients’ gait and balance ability were comparable with those of the general elderly population [
6,
7]. Usually, gait analysis is generally performed with 3D motion analyzers [
6,
8] and force plates [
9,
10,
11]. The advantages of these methods are their high reliability and validity, and they aid in advanced analysis and providing a detailed understanding of functional impairment. The disadvantages of these methods are their cost-effectiveness, the complicated operation of the equipment, and the analysis process.
In recent years, gait sway evaluation using accelerometers (wearable sensors) has become a new method of gait evaluation, partially due to good cost-effectiveness [
12,
13,
14]. Accelerometers are easy to wear and have no limitations on measurement location, making them simple and practical tools in clinical practice [
15]. The root mean square (RMS) of trunk acceleration has been used as one of the indicators of gait sway, measured using accelerometers [
16]. This parameter constitutes a statistical measure of the magnitude of acceleration of X, Y, and Z axes (
Figure 1). RMS represents the degree of amplitude of the waveform, and a larger trunk acceleration RMS during gait indicates a greater gait sway. Since postural alignment changes after corrective spinal fusion surgery, walking sway is likely to change.
When ASD patients return to regular life after surgery, their risk of falling may increase without the care of medical staff. Previously, fall risk after ASD surgery has been reported [
17]. Therefore, the early evaluation of this risk is very important to prevent fall risk and fractures in ASD patients. Even so, investigations of early changes in gait sway before and after ASD surgery have not been reported. This study aimed to investigate changes in gait sway before and following ASD surgery, using accelerometers, and also to examine motor function related to postoperative gait sway.
6. Discussion
Adult spinal deformity (ASD) causes lower back pain and spinal imbalance in the elderly population [
1]. Surgical correction is necessary to prevent the progression of the deformity and to relieve complaints of pain [
25]. However, postoperatively, the elderly find it difficult to adapt to the new posture following surgery for ASD [
7]. Regaining spinal balance by physical therapy after surgery is a key to avoiding accidental falls due to spinal imbalance [
26]. Furthermore, the risk of a femoral neck fracture due to falls after ASD surgery has been reported [
17]. A significant factor among walking and balance tests in post-operative ASD patients was the timed up-and-go test (TUG), which correlated with the Oswestry disability index (ODI) after ASD surgery [
27,
28]. In recent years, many gait analyses have been performed using accelerometers to calculate RMS, a measure of gait sway in orthopaedic disorders [
29,
30,
31]. However, there have been no reports using accelerometers to investigate changes in RMS before and after ASD corrective surgery. In this study, we examined postoperative changes in RMS and related motor function.
Dubousset reported that an energy-efficient posture is to keep the head above the pelvis [
32]. In healthy individuals, the vertical shift of the center of gravity (RMS
V) is lower in the early phase of stance and higher in the middle phase of stance [
33]. Inverse pendulum theory is the basis of energy-efficient gait [
34]. In this study, RMS
V was significantly increased in the postoperative period. The preoperative gait of ASD patients is compensatory for increased SVA by placing the lower limb joints in flexion, reducing horizontal gaze and the shift of center of gravity during gait in a crouched posture [
35,
36]. This crouched posture results in a less energy-efficient gait [
32]. The improvement of spinal alignment after ASD surgery is thought to improve the compensatory crouching posture, resulting in an energy-efficient gait with improved RMS
V (
Figure 8). RMS
V was moderately negatively correlated with KE in our results (
r = −0.625). KE is believed to work by absorbing the shock associated with the lowering of the center of gravity during the initial contact with the ground while walking [
33]. RMS
V increases if the lowering of the center of gravity cannot be controlled [
37]. On the other hand, RMS
V had a strong positive correlation with TUG, which is an indicator of dynamic balance capacity (r = 0.77). Excessive RMS
V is associated with the risk of falling [
38].
RMS
ML was significantly improved postoperatively. In previous studies, the preoperative gait of ASD patients showed different stride lengths between the left and right sides and uneven floor-reaction forces compared with the general elderly population [
7]. Large values of preoperative Cobb angle and CSVL indicate coronal plane malalignment and lateral trunk shift. This might affect stability during the stance phase of gait, altering the stride and worsening RMS
ML. Although corrective fusion surgery emphasizes improving the sagittal alignment, improvement of coronal alignment also improves RMS
ML and stabilizes gait.
RMS
AP had a moderate negative correlation with postoperative HF and SVA-RR (r = −0.58 and −0.47). The Iliopsoas muscle (IPM), a representative muscle of HF, has an important role in hip flexion as well as spinal column support as a trunk muscle [
39] and contributes to maintaining upright posture [
40]. Before ASD surgery, the IPM was shortened due to the compensatory crouching posture caused by worsening SVA, and the trunk support function was considered to have been failing. The muscle force of the IPM decreases when the spine is upright and the hip is extended [
41]. The shortened IPM is considered to have been stretched after corrective fusion surgery, resulting in decreased muscle force, lower limb swing, and trunk posture retention during gait, and increased RMS
AP during gait (
Figure 11).
These results suggest that the improvement of muscle power of HF and KE may be a key to reduce RMS after ASD surgery. Previous reports emphasized that pre-surgical physiotherapy increased walking ability and lower extremity strength in patients with degenerative lumbar spine disorders compared with waiting-list controls. [
42]. ASD patients usually have muscle weakness before surgery due to less daily activity. Preoperative muscle exercise of lower extremities, especially HF and KE, is recommended. Furthermore, trunk muscle training for seniors was reported to be important because this exercise improves functional mobility (TUG) [
43]. With adequate muscle exercise, trunk muscle strength increased by 26% for extension and 23% for flexion at the 12-month postoperative follow-up [
44]. Corrective spinal fusion for ASD patients is relatively invasive surgery, and the trunk muscle of the patient may be damaged. Preoperative and postoperative muscle exercise are the keys to important improvements in the activities of daily living for ASD patients.
Limitations of this study included the small sample size and the inclusion of only females. However, our results are generalizable as ASD is more prevalent in females, as previous reported [
45]. Joint angles of the lower extremities in the ASD patients were not assessed. It is unclear from the data collected whether improved crouching posture is associated with improved energy expenditure and walking endurance. Furthermore, the long-term course of the study is unknown, as it is a comparison of preoperative and one-month postoperative results. Therefore, it is necessary to increase the sample size and determine the long-term extent to which improvement in crouch walking in ASD improves energy expenditure, walking endurance, and quality of life.