Pre- to post-surgical changes in dynamic sagittal balance and outcomes
From the 15 ASD patients (age 62 ± 10 years; females, n = 13; males, n = 2) for which we have complete post-surgical follow-up data, we compared changes in DSB before and after realignment surgery. Patient-reported outcomes all significantly improved on average following surgery, including 25% decrease in ODI (52–38, p = 0.03), 27% decrease in VAS (6.6–4.8, p = 0.05), and 34% increase in EQ5D utility (0.50–0.67, p = 0.03). In addition, radiographic SVA significantly improved by an average of 53% (96.2–45.6 mm, p < 0.001).
Many DSB measurements significantly changed following surgery (Tables 2 and 3). Peak SVA decreased (− 28%, p < 0.001), and time needed to rise to a stable standing position decreased (− 37%, p < 0.001). Biomechanical forces on the spine changed, including reduced torque on the lower lumbar spine (− 29%, p < 0.01) and, more specifically, reduced compressive and shear forces acting on the sacrum from altered dynamics of the trunk (max compression: − 23%, p = 0.02; max shear: − 19%; p = 0.03). In addition, peak spinal extensor muscle force decreased as well (− 30%, p < 0.01). Flexion at the hip was lower (− 28%, p < 0.001), as was the peak torque (− 33%, p < 0.001) and energy (− 65%, p < 0.01) required from the hip to complete the maneuver. Flexion at the knee was greater (+ 7%, p = 0.04), as was energy (+ 26%, p = 0.04).
Variables that did not significantly change following surgical intervention include standing or dynamic center of pressure, vertical and horizontal momentum, hip and knee maximum extension (at standing), and knee torque.
Comparing DSB between surgical patients and healthy controls
We compared both pre- and post-surgical DSB data from the ASD patients to 10 healthy control subjects (age 31 ± 10; females, n = 3; males, n = 7). Comparing DSB between our pre-surgical patient data and healthy controls, we found significant differences in some DSB variables that also showed significant change with surgery (Tables 2 and 3). A number of the DSB variables were significantly different from the control data prior to surgery (time: + 38%; peak SVA: + 101% (Fig. 2); lumbar torque: + 32%; spinal extensor muscle force: + 22%; hip torque: + 30%; hip energy: + 64%; hip flexion: − 31%, knee energy: + 16%). Following surgery, some of these variables improved by closing the gap in differences with the controls (time: 15%; peak SVA: + 30% (Fig. 2); hip flexion: − 16%), and others improved by becoming not significantly different than the controls (lumbar torque; spinal extensor muscle force; hip torque; hip energy).
Interestingly, several pre-surgical variables that were not different from the control data were different at the post-surgical time point, including vertical and horizontal momentum (− 30%, − 15%), maximum compression and shear force on the sacrum from the dynamics of the trunk (− 10%, − 15%), and maximum knee flexion (+ 13%).
Pre- to post-surgical changes in relative ratios of kinetic variables between hip and knee
Ratios for torque and energy between the hip and knee significantly reduced following surgery [torque: 52% to 33% (Fig. 2), p < 0.001; energy: 41% to 11%, p = 0.002]. Pre-surgical ratios were significantly higher than controls [torque: 52% to 34% (Fig. 2), p < 0.001; energy: 41% to 16%, p = 0.02], but post-surgical values did not significantly differ from controls.
PJK case study
One patient developed a PJK complication that required revision surgery. For this patient, we have data from three time points: (1) follow-up from prior surgery, (2) visit following the onset of PJK, and (3) follow-up visit after revision surgery. Standout DSB metrics as potential risk factors include peak SVA and relative ratios of hip and knee torque and energy. These values were higher than the controls prior to PJK, and following revision surgery, they more closely matched the control data (Fig. 3).