Treatment of neuromuscular scoliosis with posterior-only pedicle screw fixation

Background To determine whether posterior-only approach using pedicle screws in neuromuscular scoliosis population adequately addresses the correction of scoliosis and maintains the correction over time. Methods Between 2003 and 2006, 26 consecutive patients (7 cerebral palsy, 10 Duchenne muscular dystrophy, 5 spinal muscular atrophy and 4 others) with neuromuscular scoliosis underwent posterior pedicle screw fixation for the deformity. Preoperative, immediate postoperative and final follow-up Cobb's angle and pelvic obliquity were analyzed on radiographs. The average age of the patients was 17.5 years (range, 8–44 years) and the average follow-up was 25 months (18–52 months). Results Average Cobb's angle was 78.53° before surgery, 30.70° after surgery (60.9% correction), and 33.06° at final follow-up (57.9% correction) showing significant correction (p < 0.0001). There were 9 patients with curves more than 90° showed an average pre-operative, post operative and final follow up Cobb's angle 105.67°, 52.33° (50.47% correction) and 53.33° (49.53% correction) respectively and 17 patients with curve less than 90° showed average per operative, post operative and final follow up Cobb's angle 64.18, 19.24(70% correction) and 21.41(66.64 correction); which suggests statistically no significant difference in both groups (p = 0.1284). 7 patients underwent Posterior vertebral column resection due to the presence of a rigid curve. The average spinal-pelvic obliquity was 16.27° before surgery, 8.96° after surgery, and 9.27° at final follow-up exhibited significant correction (p < 0.0001). There was 1 poliomyelitis patient who had power grade 3 in lower limbs pre-operatively, developed grade 2 power post-operatively and gradually improved to the pre-operative stage. There was 1 case of deep wound infection and no case of pseud-arthrosis, instrument failures or mortality. Conclusion Results indicate that in patients with neuromuscular scoliosis, acceptable amounts of curve correction can be achieved and maintained with posterior-only pedicle screw instrumentation without anterior release procedure.


Background
The prevalence of severe spinal deformity in patients with neuromuscular disorders is estimated between 50% and 80% [1][2][3]. The progression of untreated neuromuscular spinal deformities can cause aggravation of pain [4][5][6], decreased sitting balance [6][7][8][9][10], pressure sores, psychological problems (in patients without mental retardation), compromised pulmonary functions [11,12] and increased mortality [13]. Surgical management has been a reliable option for these patients since introduction of spinal instrumentation by Harrington and subsequent advances by others; most notably by Luque and Cotrel-Dubousset [14,15]. The use of hooks in the thoracic spine has been considered as a gold standard for the treatment of neuromuscular scoliosis. There has been a movement toward the use of thoracic pedicle screws in deformity surgery, based on the reports regarding clinical advantages of pedicle screw fixation in the lumbar spine in terms of enhanced correction and stabilization, when compared with a hook construct [5,16,17]. Uses of Luque rods or unit rod instrumentation have their own disadvantages such as loosening of wires, cutting out of wires, loss of fixation and loss of correction over time and in addition, delayed sitting and ambulation in post-operative phase. Controversy persists, which patient requires anterior release and/or fusion in combination with posterior instrumentation and arthrodesis to improve curve correction.
Treatment of the neuromuscular scoliosis with posterioronly pedicle screw instrumentation is a recent concept, which obviates the need for additional anterior release, and thus risk to life. Main purpose of this study was to evaluate the outcome of single-stage posterior-only pedicle screw instrumentation and fusion for the definitive management of neuromuscular scoliosis. Specific emphasis was kept on correction of scoliosis and maintenance of that correction over time.

Methods
Between 2003 and 2006, Twenty-six consecutive patients (17 male and 9 female) with progressive neuromuscular scoliosis underwent posterior only pedicle screw fixation and fusion by a single spine surgeon. We did not perform anterior surgery in any of these patients in the study. Seven patients had CP (cerebral palsy), 10 had DMD (Duchenne muscular dystrophy), 5 had SMA (Spinal muscular atrophy) and four (2 post poliomyelitis residual paralysis, 1 each multiple sclerosis and traumatic paraplegia) had other pathologies. Twelve, out of 26 patients, had right side curve and 14 had left side curve. Fifteen, out of the 26 patients, had apex at thoraco-lumbar junction. Fourteen patients had single curves and 12 had double curves. Pre-operatively, 18 patients were wheel chair bound while eight were in ambulatory status (walking independently or with the help of either crutches or walker).
Pre-operative planning of each case was based on a total of eight radiographs, i.e. antero-posterior and lateral radiographs in the sitting and supine positions, lateral bending view and in maximal flexion and extension views. Entire group underwent pulmonary function tests (PFT) preoperatively. We retrospectively reviewed medical records, operative records and sequential radiographs of all patients. Radiographic measurements were recorded in terms of number of curves, amount of preoperative flexibility, preoperative, post-operative and final follow-up Cobb's angle, pelvic obliquity, thoracic kyphosis and lumbar lordosis. Skeletal maturity was assessed by documenting the Risser's sign. Pelvic obliquity was measured as the angle between the line joining two iliac crests and horizontal line. Thoracic kyphosis was measured form upper end-plate of T4 to lower end-plate of T12 and lumbar lordosis was measured form upper end-plate of L1 to lower end-plate of L5. The results of pulmonary function tests, duration of anesthesia, operating time and amount of blood loss were obtained from inpatient charts while postoperative complications and improvement in sitting balance and parents' or care takers' satisfaction were documented from follow up sheets.

Operative procedure
All patients were operated in prone position with posterior-only approach. Spine was dissected, subperiosteally, up to the tip of the transverse processes at all levels. Pedicle screw inserted bilaterally with free hand technique at all preoperatively decided levels. Bilateral facetectomies was done at all levels,, including the apex, to facilitate the maximum rotational correction. Contouring of the rods were done manually with rod bender and mounted over pedicle screws bilaterally, with concave side being the first followed by the convex. Derotation maneuver was subsequently done, with or without in situ bending of rods, simultaneously on both sides. Rods were fixed by tightening of the caps over screws and posterior fusion achieved, after thorough decortications of posterior laminae, using bone grafts mixed with cancellous allograft. Wound was closed, over two-drainage tube, in layers. The entire patients underwent for radiogram and CT scan postoperatively, which were stored in our computerized PACS system with preoperative data. Posterior vertebral column resection (PVCR) was performed in those patients who had a stiff or rigid spine with a large curve (more than 90°). Pelvic fixation was done using poly-axial ilio-lumbar connectors developed by the authors in patients who had severe pelvic obliquity or contractures in lower extremities.
We have statistically analyzed the results of pre operative and postoperative corrections in Cobb's angle and pelvic obliquity using paired t-test. In addition, we have divided the study group in two categories depending upon the severity of curve: group 1 (curve < 90°) and group 2 (curve > 90°). The correction rate of Cobb's angle and pelvic obliquity between group1 and group 2 were compared using unpaired t-test. P value less than 0.05 was considered the significant for all the tests.

Results
The average age at the time of operation was 17.5 years (range 8 -44 years) and the average follow-up was 25 months (range, 18 -52 months) (table 1). The average percentage of pre operative flexibility was 41% (11%-74%). Average preoperative, postoperative and final follow-up Cobb's angle, pelvic obliquity, thoracic kyphosis and lumbar lordosis are shown in table 2. The average correction rate in Cobb's angle was 60.9% post operatively and 57.9% at final follow up. Similarly, the average correction rate in pelvic obliquity was 44.92% and 43.02% postoperatively and at final follow-up respectively. There has been statistically significant correction achieved, in Cobb's angle (p < 0.0001, paired t-test) and pelvic obliquity (p < 0.0001, paired t-test), post operatively which are maintained at final follow-up. The difference in average preoperative, postoperative and final follow-up thoracic kyphosis did not reveal any significant difference (p = 0.74, ANOVA) while the difference in preoperative, postoperative and final follow-up lumbar lordosis showed significant improvement (p = 0.001, ANOVA).
There were 17 patients with curve less than 90° (group 1) (figure 1) and 9 with curve more than 90° (Group 2) (figure 2) with average pre operative Cobb's angle of 64.18°a nd 105.67° respectively. Average preoperative pelvic obliquity was 14.94° for group 1 and 18.78° for group 2. The average post operative correction in Cobb's angle was 70.02% and 50.31% for group I and group II respectively; which did not show significant difference (p = 0.1284) in correction between both groups. Similarly, the post operative correction in pelvic obliquity did not reveal any significant difference (p = 0.3239) in correction between both groups. Seven patients underwent PVCR from group 1. Their average pre-operative Cobb angle was 106.71°, with 24% flexibility, had an average post-operative Cobb's angle 55.86° showing 47.65% correction, which is similar to other patients from group 1.   14 years male with CP  Postoperatively all patients exhibited improvement in sitting balance. Two patients who were wheelchair bound preoperatively were able to walk with the help of walker postoperatively and one wheel chair bound patient was able to walk with the help of crutches (table 4). One SMA patient who was able to walk with the help of walker preoperatively was able to walk with the help of crutches postoperatively. None of the patient had deterioration in sitting balance at final follow-up. Parents or care-takers of all patients exhibited better personal and hygienic care postoperatively.

Complications
Deep wound infection was seen in one patient with paraplegia who had continuous bleeding from the operated site for which exploration of the wound revealed bleeding from an epidural vessel, which was cauterized. She developed wound dehiscence and deep infection and pus culture grew vancomycin resistant staphylococcus for which she received teicoplanin and regular dressings. One patient with poliomyelitis who had grade 3 power of the lower limbs pre-operatively, developed grade 2 power post-operatively but gradually improved to the pre-operative stage. He also had severe blood loss in the peri-operative period, went into DIC, and was in the ICU for 2 weeks. There was no mortality, pseudarthrosis or implant failurein the study.

Discussion
Despite the magnitude of this surgery, successful outcome of an operation for spinal deformity, secondary to neuromuscular disease, is considered beneficial by most patients and/or their principal care providers [7,15,18,19]. Aims of the surgery for neuromuscular scoliosis are safe correction of deformity, to stop curve progression, to maintain or recreate sitting balance and to achieve a solid fusion of the balanced spine in the frontal and sagittal planes [20,21]. The average age of the patients in this study was 17.5 years. The increase in age results in increasing stiffness and rigidity of the curve and difficulty to achieve acceptable correction. The mean age in this study was much higher than majority of studies, where patients underwent earlier correction at around 12 years of age or earlier [20,22,23]. However we achieved acceptable correction in both Cobb's angle and pelvic obliquity over al period of 25 months without significant loss of postoperative correction.
We found that there were significant differences in PFT values between curves greater than 90° and less than 90°. Sussman noted an increased risk of postoperative pulmonary problems when the FVC was less than 35% [13,24]. In our study, 10 patients had FVC less than 35% and 6 of them needed postoperative ventilation, supporting Sussman's [25] findings. Postoperative PFT assessment is not being done as a routine basis in our patients. Post-operative PFT values would indicate whether there was any improvement of these parameters following surgery. Therefore, we think, this is the limitation of present study.
Average preoperative pelvic obliquity was 16° which decreased to 9° after the surgery, and remained same even at final follow-up. There is a growing controversy regarding the distal extent of fusion in patients with neuromuscular scoliosis ( figure 1). There was a general trend to include the pelvis in all cases of neuromuscular scoliosis to correct pelvic obliquity or to prevent its development [20,[26][27][28][29][30]. With all the problems described in the literature associated with pelvis fusion [9,31] few patients who had pelvic obliquity greater than 15° and other patients with pelvic obliquity less than 15° had severe lower extremity contractures were chosen for pelvic fusion [32]. Recently, Tsirikos, et al [33,34] challenged the long-term belief that fusion should be avoided in ambulatory patients with CP. In our experience, we have noticed that patients who have gross pelvic obliquity, do not exhibit any problem with sitting balance without much progression over a short term. Therefore presently we do pelvic fixation in patients with pelvic obliquity more than 15°( figure 2) or with severe lower extremity contractures.
Westerlund et al [35] reported 66% correction in Cobb's angle and 75% correction in pelvic obliquity in twenty-six neuromuscular scoliosis with posterior-only unit rod instrumentation. They did not perform any anterior procedure in their series and reported excellent results in immature spine. Boachie-Adjei, et al [4] in their study with 46 patients of neuromuscular scoliosis had an equal number of patients in both the spastic and flaccid group and concluded that corrections for scoliosis and pelvic obliquity were similar in both the groups. In present study, we have also found similar correction in both Cobb's angle and pelvic obliquity without any anterior procedure over a follow-up of 25 months. In addition, none of the patient displayed deterioration in thoracic kyphosis or lumbar lordosis at final follow-up and therefore we did not feel any need for anterior procedure for the correction. Improvements in thoracic kyphosis and lumbar lordosis resulted in to improved sitting balance. Various authors [1,2,36] have used combined anterior and posterior approaches to correct scoliosis, usually in the presence of a very large or stiff curve. Their curves were all less than 90° and they achieved correction rates from 41% to 71%, mostly by using Luque or Unit rod systems.
In the group with curve greater than 90° and the group who underwent PVCR, we observed curve correction of 50.31% and 47.65% respectively. To achieve more correction, we prefer PVCR [21] at another extra level. The pedicle screw system has an advantage of being a consolidated fixation including all three columns [5,16,17]. This greatly enhances the ability to simultaneously correct the three dimensional nature of these complex spinal deformities. Using the advantages of both pedicle screw and PVCR, better correction can be expected. Various studies have shown that posterior instrumentation with fusion alone is sufficient to correct and to maintain even larger and stiffer curves in neuromuscular scoliosis and it also prevents crankshaft phenomenon in skeletally immature patients [20,28,37,38]. Reports have also shown that the addition of an anterior procedure, whether staged or same-day, potentially contributes to the risk profile and morbidity in these patients which further supports our criteria of avoiding such measures unless clear benefit can be supported. Our results support that neither anterior release nor anterior arthrodesis is generally indicated to obtain acceptable curve correction in even severe cases.
There are various reports suggesting use of hooks in thoracic level. The main purpose of using hook was to avoid any neurological complications intraoperatively. However other reports [5,16,17] suggested that use of pedicle screw provides stronger purchase and better rotational correction in thoracic spine. Therefore we have used pedicle screws all levels in the subjects and did not notice any major neurological injury postoperatively. Our results are also comparable with the hooks or any other implants.
In present study, all patients underwent for posterior-only pedicle screw and fusion for neuromuscular scoliosis were consecutive and not randomized which is, we think, the limitation of study. If operations had been done in selected patients, results would have been better than this. However, majority of patients achieved acceptable correction in thoracic kyphosis and lumbar lordosis with improvement in sitting balance showed the success of treatment. In addition patients' parents or care takers have also reported better nursing care after operation.

Conclusion
In conclusion, our series demonstrates the efficacy of posterior-only spinal fusion using the pedicle screw fixation for the definitive management of neuromuscular scoliosis. Curves greater than 90 degrees or rigid curves may require additional PVCR at single level or if necessary, at second level to achieve better outcome. Appropriate care requires the availability of specialized personnel acting as a multidisciplinary team. Our results demonstrate that posterior-only pedicle screw fixation with PVCR if necessary is effective in obtaining and maintaining alignment in the neuromuscular scoliosis population. This technique may avoid those risks incumbent with the addition of an anterior approach.