This study was motivated by the need to find ways to maintain the optimal sagittal balance in patients who underwent long-segment fixation surgery for ASD. The premise was that GMSE, among various postoperative physical exercises, would increase the gluteal muscle volume and assist in maintaining the restored sagittal balance. The quality of postoperative rehabilitation has a significant influence on reducing subsequent disability, recurring injury, and the additional use of health care services27. While there have been significant advances in deformity correction for patients with ASD with an increase in the number of surgical procedures over the past decades, the optimal rehabilitation program during the postoperative period remains unclear. The findings of this study suggest an effective exercise protocol for patients after surgical treatment for ASD, contributing to the improvement in surgical outcomes.
Gluteal muscle and rehabilitation
Humans are the only vertebrates that maintain an upright, totally vertical, bipedal position14, and the gluteus muscle plays a critical role in bipedalism28. The gluteus maximus muscle, in particular, is known as the significant muscle that allows numerous daily activities as well as the maintenance of the upright position28,29. Thus, the strengthening exercise of the gluteus muscle was predicted to assist in maintaining sagittal balance and preventing sagittal decompensation after ASD surgery. Many studies have reported the importance of rehabilitation of the gluteus muscle, but the main focus of these studies has been on lower extremity disorders15, and there have been no guidelines describing the need for, or recommended content of postoperative gluteal muscle rehabilitation after spinal surgeries, including deformity correction in ASD.
The known strengthening exercises for the gluteus muscle include lunge, bridging, squats, deadlifts, clams, leg presses, and step-ups. Reiman et al.23 reported that, during rehabilitative exercises, a very high-level activation on the electromyography (EMG) was observed for the gluteus maximus muscle in the forward step-up and for the gluteus medius muscle in single-limb squat and side-bridge to neutral spine position. Neto et al.30 also found that the step-up exercise and its variations presented the highest levels of gluteus maximus muscle activation. However, these studies were limited to healthy individuals, and their application to elderly patients with ASD following long-segment fixation from T10 to S1, with degeneration and atrophy in the paraspinal muscle, remains unclear. This study developed a GMSE protocol for elderly patients with ASD through the modification of the forward step-up and squat exercises that induced a high-level activation of the gluteus muscle in the two aforementioned studies.
Step-up exercise is related to the stabilization of the hip and knee in the upward and downward movement, while it extends the hip joint and maintains the pelvis level controlling excessive femur adduction and medial rotation30. Through the modification of the step-up exercise, the single leg stance exercise and the walking high knee exercise were applied to patients after ASD surgery. In the single-leg stance exercise, the patient is instructed to move as if to climb a stair from a static posture by lifting the knee on one side at 90° flexion to the position of the hip joint on the other side while maintaining the hip extension on the opposite side to maximally preserve the upright position and avoid swaying to the sides. In the walking high knee exercise, the single-leg stance exercise is maintained with the addition of a dynamic component in which the patient is guided to perform a slow walk. Both exercises were designed to determine the effects of the step-up exercise with maximal preservation of the neutral position.
A squat is a complex movement exercise involving the ankle, knee, and hip joints. The gluteus maximus muscle is activated throughout the squat exercise, and notably, the highest activation was reported based on EMG analysis for 90–60° hip flexion in the ascent phase31, while another study reported a higher EMG activity of the gluteus medius muscle during the squat exercise including wall squat and mini squat compared to other exercises32. Thus, noting the importance of squats in the GMSE, a forward wall squat exercise was developed for patients with ASD who are older and lack strength in the lower extremity. In the forward wall squat exercise, the patient is guided to hold the rail or the edge of the desk with both hands while maximally preserving the neutral position of the spine. The feet are positioned approximately 15° towards the exterior from the midline so as to maintain the hip in a state of slight external rotation. The inter-pedal distance on the coronal plane is based on the shoulder width to maintain the hip in a state of slight abduction. Next, the knee joint is flexed at 90°, while the patient applies force to the hip to stand up. McCaw et al.33 reported that, compared to the narrow stance, the wide stance of 140% shoulder width resulted in higher EMG activity of the gluteus maximus muscle. Thus, considering the physical state of patients with ASD, the exercise involved separating the feet distanced based on shoulder width.
Effect of gluteal muscle strengthening exercise and ASD
The radiographic measurements showed that, in both groups, radiographic improvements in SVA, PT, and TK were observed following lumbar lordosis correction and at the last follow-up, and at the one-year follow-up, the optimal sagittal alignment was seen to have been maintained in 91.5% of patients. However, the percentage of patients showing the optimal sagittal alignment in the exercise group was 97.8%, which was significantly higher than that in the control group (84.2%), while a significantly higher value was observed for the last follow-up SVA and SVA change in the control group. These results suggest that GMSE in combination with the basic postoperative rehabilitation protocol could be a method to prevent sagittal decompensation in patients with ASD after surgery. To corroborate the results in this study, the actual margin of increase in the gluteus muscle after the postoperative GMSE was quantitatively evaluated by measuring the gluteal muscle volume using CAD and 3D-CT data obtained immediately after surgery and at one year after surgery.
First, the 3D-CT data were applied to the CAD program, where the fat tissue was removed through the HU border settings to estimate the area of the pure muscle mass. Next, by estimating the area of the overall cross-sectional images rather than the area of a single image based on the reference point, the errors from the cutting position in a single image were minimized, and the volume that approximated the actual whole muscle was obtained. Based on the results of these accurate and reliable analyses, a significantly higher gluteus maximus volume was observed in the exercise group than in the control group at the one-year follow-up, while significantly higher gluteus maximus and medius volumes were observed at the one-year follow-up.
For patients with ASD, restoring sagittal balance is an important surgical goal, and its maintenance is also crucial3. However, pain levels may increase again with postoperative sagittal decompensation and mechanical complications such as pseudarthrosis and proximal junctional kyphosis3,34,35. Kim et al.35 reported that sagittal decompensation was observed in 23% of patients after ASD surgery, and extensor muscle weakness due to aging was identified as one of various causal factors. Inami et al.34 also reported that sagittal decompensation was observed in 15.4% of patients after ASD surgery, and a probable cause was inadequate muscle strength due to aging. Notably, we included and analyzed patients with a single etiology among those with ASD, namely LDK16. In patients with LDK, the paraspinal muscles, particularly the multifidus and erector spinae muscles, tend to decrease in mass to a greater extent as patients grow older36,37. Thus, for patients with ASD showing paraspinal muscle weakness and who are older, the gluteal muscle exercise protocol developed in this study is an adequately suitable exercise protocol after deformity correction surgery. It is anticipated that this protocol will improve surgical outcomes (Fig. 3).
This study had a few limitations. First, the one-year follow-up period was inadequate. Further studies should be performed with a longer follow-up period. Second, there may be problems with post-discharge adherence to rehabilitation programs, including GMSE. However, the deformity correction for patients with ASD at our hospital is performed for primarily for those patients who have a high desire for recovery to a normal life due to the deterioration of their basic life quality. We tried to increase adherence to rehabilitation by providing information and education to patients and their family members before surgery, and monitoring the exercise status at every outpatient visit at short intervals (3-month) after surgery. Using this approach, we were able to get satisfactory results. Third, the level of gluteal muscle activation as a result of exercise in this study was not identified. This should be addressed in future studies using an electromyogram. Despite these limitations, the significance of this study lies in its being the first to analyze the effects of a suitable exercise for patients after ASD surgery. Importantly, this study evaluated the positive effects of our novel protocol of GMSE and all muscle volume measurements were performed using a series of cross-sectional images rather than a single image. Furthermore, this study was able to produce reliable results by increasing the accuracy of gluteal muscle volume measurement using CAD.