Adolescent idiopathic scoliosis: evaluating perioperative back pain through a simultaneous morphological and biomechanical approach

Twenty (20) consecutive patients (15 females and 5 males) between the ages of 10 and 18 diagnosed with AIS and scheduled for a spinal fusion surgery were recruited at the Shriners Hospitals for Children - Canada. The mean age of the cohort was 14.9 ± 1.68 years, the average body mass index (BMI) was 20.66 ± 3.25 and the mean exposure time was 20.62 ± 2.93 s at the preoperative time point. Regarding to the curve type, there were (8) patients with Lenke 1, (5) Lenke 2, (2) Lenke 3, (1) Lenke 4, (2) Lenke 5, and (2) Lenke 6. Preoperative morphological results can be seen in Table 1. Patients were excluded of the study if their primary diagnosis was not AIS, could not take a standing radiograph, could not speak or understand French or English, had a cognitive disability preventing them from answering questionnaires, had previously undergone spinal or lower limb surgery or were diagnosed with neuromuscular impairment. Prior to study participation, written informed consent was obtained from all patients and if necessary, their parents. This project received ethical approval from McGill University (IRB# A08-M71-14B).

Due to the lack of literature on simultaneous assessments of balance and spinal morphology an evaluation bias between the two can be observed. Thus, we created the Global Biomechanical and Morphological Assessment (GBMA). This simultaneous evaluation is comprised of a three-dimensional (3D) modelling assessment of the scoliotic spine and a balance assessment using wireless sensor insoles. These sensor insoles captured live data of the patient’s balance during their standing biplanar radiographs. A Cartesian plane was placed on the floor of the radiographic cabin as to associate the location and orientation of the feet to the spinal morphology. After the acquisition, the spinal morphology was reconstructed in 3D for better evaluation. The morphological data was then analyzed in parallel with the postural balance data to find associations. Since all variables are captured at the same time in the same position, this technique eliminates the bias of two separate evaluations of the balance and the spinal morphology while also providing a visual representation of their associations (as seen in the article by St-Georges et al. submitted to Scientific Reports). The GBMA was used at every time point during this study.

Patients underwent simultaneous two-dimensional head-to-toe posterior-anterior (coronal) and lateral (sagittal) radiographs using the EOS-Imaging system (EOS-Imaging®, Paris, France) as part of the standard of care. This was done at every time point: preoperatively, 6 weeks and 6 months postsurgical. Patients were asked to place their hands on the wall in front of them to expose the spine without the obstruction of the humerus in the sagittal plane. This position also reduced the potential of a second exposure. Patients were asked to keep their eyes open and keep a normal breathing cycle not to disturb the image. Once the acquisition complete, all 60 spinal radiographs were reconstructed in three-dimension (3D) through the SterEOS software version 1.6.4.7977 (EOS-Imaging®, Paris, France) by trained professionals.

Radiological parameters extracted from the SterEOS software were: Cobb angles from the Proximal Thoracic (PT), Mid Thoracic (MT) and Thoracolumbar/Lumbar (TL/L) curves; the rotation in all three planes of these curves apical vertebrae (AVR); the intervertebral rotation in all planes of every vertebrae; the T1-T12 kyphosis; the L1-S1 lordosis; the pelvic incidence, axial rotation and tilt in the lateral and frontal planes; and the sacral slope. All of these variables were taken in the patient’s reference plane for time point comparison. The patient reference plane is a reconstruction based on all three-dimensional planes. It bases itself on the location of the acetabula inside the cabin to reconstruct the spine instead of the position of the patient, which changes from visit to visit [26]. This allows for better comparison through the perioperative period. Additional variables: torsion index of the main curve [27] and trunk shift were interpreted after the reconstruction. Visual description of the variables can be found in submitted work from St-Georges et al. 2019 to Scientific Reports.

The static standing balance assessment was conducted with Moticon© OpenGo sensor insoles (Moticon GmbH, Munchen, Germany) during the radiological capture. They were activated and then slipped under the patient’s feet in the cabin just prior to the radiographic exposure. The plantar recording was time matched with the radiograph acquisition to have a simultaneous picture of the effect of the abnormal spine on the biomechanics of the feet.

Every patient was sized for a pair of insoles. The patients were asked to stand comfortably with their feet pointing forward in the centre of the cabin. Once the patient entered the cabin and the insoles were slipped under their feet, their feet were marked upon the Cartesian plane. The patient’s feet were not moved nor reoriented to evaluate, as much as possible, their normal stance.

The insoles measured nine different balance parameters through 13 sensors in each insole. All parameters were based on the percentage of pressure or the centre of pressure (CoP) of each individual foot. These variables are as follows: the mean and standard deviation of the anteroposterior and mediolateral CoP location (CoP AP and CoP ML respectively), where a positive result is seen in the anterior and medial plane based on the zero located approximately at the metatarsal-phalangeal joints and in the midline of the foot; the CoP velocity, measured in mm/s; the CoP trace length, measured in meters; the CoP anteroposterior and mediolateral bounding box, where the greater the measurement the greater the CoP distance travelled in said plane; The CoP sway area, which is the product of both planes bounding box measurements; the mean foot pressure; and the pressure distribution in the front, mid and back of the foot. Moticon© OpenGo software version 01.11.00_11072-929d380 was used to analyze the balance variables at every time point. (Moticon GmbH, Munchen, Germany).

Self- reported questionnaires were given to the patient after their morphological and biomechanical assessment at each time point. The questionnaires given to the patients were: the Scoliosis Research Society Questionnaire-30 (SRS-30), and a modified version of the Adolescent Pediatric Pain Tool (APPT) containing a back diagram for pain location and intensity. The latter was evaluated with a 0–10 numeric rating scale where 0 is no pain and 10 is the worst pain imaginable. Pain severity groups were then created: mild (0–3), moderate (4–6) and severe (7–10). The questionnaires were asked to be done without the help of their parents to avoid bias. If for any reason the patient would have questions, the research assistant was at their disposal to answer them. All questionnaires were reviewed after completion and entered into a database.

As seen in Table 2, all three (PT, MT, TL/L) Cobb angle means were significantly reduced whether 6 weeks or the 6 months after surgery compared to before (P < 0.001). The MT apical vertebra translation (P < 0.001), the TL/L apical vertebral rotation (6 weeks P = 0.029; 6 months P = 0.025), MT’s absolute torsion (6 weeks P < 0.001; 6 months P = 0.013) and the sacral slope (6 weeks P = 0.008; 6 months P = 0.002) also followed suit in their significant reduction at 6 weeks and 6 months postoperative. Others, MT apical vertebra rotation and L1-S1 lordosis were significantly reduced at 6 weeks but were no longer at 6 months postoperative. The T1-T12 kyphosis oppositely was significantly increased at 6 months but not at 6 weeks postoperative.

All but one mean balance parameter did not differ significantly throughout the perioperative period (Table 3). The left foot medial/lateral location of the CoP was significantly different at 6 weeks after surgery (P = 0.017). Although when compared to the 6 months postoperative time point, no difference was observed with the preoperative results. Therefore, since no significant difference in postural balance was observed between the preoperative and the 6 months postoperative time points, no significant associations could be made with the changes in spinal morphology.

Perioperative pain data can be observed in Tables 4 and 5. When looking at the 6 weeks postoperative data only, pelvic pain (P = 0.265), and the number of painful regions (P = 0.625) were not significantly different. All APPT worst pain, thoracic and lumbar pain severity, and painful regions were reduced significantly after 6 months. These finding were also corroborated with the results of the SRS-30 Pain score as they improved significantly (p = 0.004) at 6 months posterior spinal fusion (PSF). In contrast, pain in the neck area was significantly worse (P = 0.044). Self-reported pain was significantly reduced in the lumbar and pelvic region (P = 0.002 and P = 0.041 respectively) after surgery (Fig. 1).

Our study reports no significant difference in balance in the perioperative period in patients with AIS. This would suggest that the change of the spinal morphology would not affect the balance in this short postoperative period of time. This might be due to the lack of time for the body to fully adapt to the new posture. Patients were used to the way they stood before surgery and it possibly takes a longer period to adapt its balance to the new spinal morphology. Schimmel et al. [11], de Abreu et al. [28], and O’Beirne et al. [13] have also demonstrated that balance in AIS patients does not change with the modification of the spinal alignment up to a year after surgery. However, Valles et al. [10] have demonstrated that patients with AIS had a significantly better postural balance with their eyes closed, and significantly worst postural balance with their eyes opened. They also could significantly better regulate their balance with constant feedback during quiet standing at 1 year postoperatively [10]. In spite of the above results, we have found an association between the amount of pressure exerted on the right foot and right thoracic deformity preoperatively, but not postoperatively. Suggesting a relationship between the morphology and the balance before the surgical intervention that plays a lesser role postoperatively. Subsequent studies should consider including a longer follow-up period to confirm these results. The lack of significant associations could be explained by one of the leading theories regarding the disparity in balance of AIS patients. It theorizes that AIS patients’ decreased balance could be associated with an intrinsic impaired dynamic regulation of sensory-motor integration effecting the patient’s balance [1, 8, 29, 30]. Thus, suggesting that the morphology might not be the main root of the balance issues in these patients and why we were not able to see a change throughout the perioperative period.

These reported results, for the exception of an association between the severity of the pain and pressure in the back of the feet at 6 months post-surgery, also suggest that since the mean postural balance does not change throughout the perioperative period it has little effect on the pain felt by the patient. de Abreu et al. [28] evaluated pain in these patients using the SRS questionnaire and although they infer that the slight decrease in oscillation could be a reason for a decrease in pain they lacked to show any significant associations, thus aligning with our conclusions.

In accordance with previous literature, our results highlight a decrease in pain severity after PSF and they also identify spinal morphological variables associated with the pain felt throughout the perioperative period. This, in part, confirms our second hypothesis, for which the morphology does impact the pain felt. The decrease in the severity of pain compliments the works of authors such as Sieberg et al. [24], Merola et al. [30], Connelly et al. [31] and Mimura et al. [32] that identified a significant decrease in pain reported after PSF in AIS patients as early as 6 months and as late as 5 years.

Interestingly, we identified that self-reported pain prevalence (Table 5 and Fig. 1) and pain intensity (Table 4) in the neck area increased from the baseline to the post-operative assessments. One possible explanation to increased pain in the neck area is the surgical trauma to the upper thoracic spine and its musculature in patients in which fusion was extended to that area. Another hypothesis is a possible influence of reciprocal changes in cervical spinal alignment after surgical correction. Further larger studies should explore pain referred to the neck area after surgical correction in AIS.

When considering our independent time point results, they indicate moderate to strong correlations between the thoracic deformity and the pain felt in the thoracic area before surgery. Recent literature [19, 33], found similar results, associating severity and the location of the curve in the thoracic area to the pain reported in said area. In all cases, the association was made at the preoperative time point but does not seem to persist throughout the postoperative period. Future studies should explore this concept and validate these results with a greater cohort permitting them to subgroup more accurately.

At 6 weeks and 6 months post PSF, our results show an association between the residual curves of the spine and the pain felt. To our knowledge, this is the first time this has been reported. Although the fusion of the lumbar vertebrae is usually avoided unless necessary to permit the flexibility of the spine, results presented in this study suggest that not ensuring stability in the lumbar region by diminishing the lumbar translation and reducing the lumbar AVR, creating a greater difference between the severity of the residual lumbar and thoracic curves, could be detrimental to the pain felt in the early postsurgical period. This association is not observed at 6 months, thus suggesting that it might only be a result of acute postsurgical pain. This might explain why authors such as Mimura et al. have not observed these effects later on in the postoperative period. We hypothesize that this might be due to the sudden uneven adaptation of muscle activation in the lower back for support creating a strenuous pain. Future research should explore the difference in the early postsurgical report of pain and last vertebrae fused. The 6 months postoperative association indicates a relationship between residual thoracic deformity and pain severity. Suggesting, that if the initial deformity is not corrected properly, it could result in greater severity of pain and possibly in its chronicity. Future studies should validate these results with greater sized cohorts. Although, pain is multifactorial [34] and biomechanical and morphological factors account for only a portion of the explanation of pain reported [27], these results, when and if validated, could help improve the pain management of patients through the perioperative period. This could be done by rethinking the surgical planning, potentially making improvements to the lumbar or the thoracic correction but also by identifying patients at risk of more severe pain and adapting their management accordingly.

This study has limitations that should be taken in consideration. The small sample size restricts the power of the analysis and diminishes the possibility of subgrouping by type of curves. Therefore, the evaluation of the effect of different curvatures on spinal balance was restrained. Evaluating patients with a right thoracic curve and patients with both right thoracic and left lumbar curves as part of the same cohort with possibly different balance [35], could have affected the mean variables of balance measured. Future studies should research the effect of independent subgrouping of curves and their effect on balance throughout the perioperative period thus eliminating the aforementioned possible bias. As per hospital protocol, during the radiological acquisition, to not obstruct the image of the sagittal spine, patients were asked to put their hands on the wall, without standardizing their hand pressure. This could have affected the postural balance parameters of the patients [36], especially the CoP variables in the anteroposterior plane. Future studies should try to evaluate patients with an in-cabin posture that does not require the support of the hands [36]. Although the insoles used in this simultaneous evaluation have been showed to identify overall comparable balance data to a force plate in the article under review by Spine from St-Georges et al. 2020, they do lack specificity in CoP displacement in static standing evaluation [37‐39]. Therefore, the insoles might not have captured subtle differences in balance, possibly causing the lack of balance difference observed in the perioperative period. That said, these were able to be used simultaneously during radiological acquisition, future studies should consider their involvement in overall clinical analysis. These patients were followed up to 6 months post-surgically, it might not have been enough time to examine a change in postural balance. Future studies should gather data on a longer period of time to examine balance change through time due to the new spinal alignment. Future work should continue utilizing GBMA for its strength in assessing the morphology and plantar biomechanics simultaneously as to better understand their relationship as well as their possible effect on pain.

To our knowledge, this was the first study of its kind to evaluate simultaneous 3D standing biplanar radiographs at the same time as postural balance to understand pain. This methodological strength permits the assessment of the effect of the 3D morphology of the spine on balance without reducing the direct correspondence that comes with different evaluations. Which also present a stronger morphological and biomechanical profile to associate to the perioperative pain report of the patient.