The surgical correction of thoracic kyphosis (TK) in adolescent idiopathic scoliosis (AIS) patients has become an area of interest for spinal deformity surgeons. Some concern is based on reciprocal relationships between TK and lumbar lordosis that have been seen with spinal instrumentation.1 In addition, reciprocal relationships have been described for cervical lordosis.2,3 This lack of normal sagittal spinal alignment may be linked to future disability and degeneration.
The use of pedicle screws for the correction of spinal deformity is commonplace today. The type and number of anchors has been a matter of debate. Clements et al4 has reported that increasing the amount of pedicle screws results in thoracic flattening. Similarly Liu et al5 has found that both rod stiffness as well as implant density has an effect on the overall TK correction for the patient.2 From our literature review, the only study that addressed anchor type has been a comparison of hybrid hook and screw implants as compared with entirely pedicle screw–based constructs.5 This study argued that hybrid constructs performed better than all screw constructs at TK correction.
To our knowledge, there is no study that specifically examined the overall effect of screw type on the ability to achieve the restoration of normal TK. The advent of uniplanar screws allows for freedom of motion in the sagittal plane while allowing surgeons to derotate patients in the axial plane during a scoliosis correction. This would potentially allow for freedom of motion in the sagittal plane during correction with potentially less flattening of the thoracic spine.
The purpose of this study was to compare the role of uniplanar and fixed pedicle screws in correction of thoracic alignment in the sagittal plane in the treatment of AIS.
This was a single center retrospective review of patients from a single, pediatric spine surgeon. Patients were identified through surgical case logs. Inclusion criteria were surgical treatment of AIS during the study time period. Patient medical records as well as archived preoperative and postoperative x-rays were reviewed for the first erect film and the final follow-up (2 y) film.
The medical record review recorded patient demographics as well as past medical and surgical history. Patients were included if they received surgical treatment of AIS and excluded if their diagnosis included a condition potentially causative of a spinal deformity or if they had previous spinal surgery or trauma. Only single stage posterior pedicle screw–based constructs utilizing 6.35-mm stainless steel rods were included in the study. The operative record was reviewed for osteotomies and to confirm the type of screw implants utilized (uniplanar screws compared with fixed screws).
Patient x-rays were reviewed to assess sagittal alignment, curve type, Risser stage, coronal and sagittal curve magnitude, curve flexibility, fusion levels, type and location of instrumentation, and curve magnitude at the initial postoperative visit and at final follow-up.
TK was measured on all radiographs in an attempt to identify a relationship between the 2 variables. In this study, TK was defined as the angle between either T2 or T5 and T12. The thoracic coronal deformity was measured for all curves and compared to determine if there was a difference in coronal Cobb correction across groups. End vertebra was selected by the traditional Cobb method for this measurement. All radiographs were measured by the same reviewer (S.A.B.) for consistency and verified by the other authors.
Cases were grouped by the type of screw implants used. Consecutive patients with AIS treated by posterior spinal fusion in the years 2004 to 2006 with fixed screw and those treated in 2008 with uniplanar screws were included in the study. The sagittal profile of the 2 groups of patients who underwent posterior fusion for AIS was compared. One group had spinal fusions that utilized uniplanar screws (n=16) as bone anchors, and the second group had spinal fusions using the fixed screws (n=20). Uniplanar screws have the ability to be adjusted in the sagittal plane before they are secured to the rod; whereas, fixed screws have a tulip that is a fixed component of the screw.
Surgical groups were also compared with examine for grouping differences such as differences in curve type and curve magnitude. Categorical variables were compared utilizing a χ2 test. Mean values such as curve magnitude before and after surgery were compared utilizing an ANOVA test. All statistical analyses were conducted using SPSS version 22 (IBM Corp., Armonk, NY), and the level of significance was set at P<0.05 for all analyses.
Both groups were comparable demographically, medically, in relation to the curve characteristics and the surgical treatment. (Table 1) The comparison of radiographic parameters by anchor type is illustrated in Table 2. The immediate postcorrection sagittal curvature measurements were greater using the uniplanar than the fixed screws, 36 versus 26 degrees (P=0.003). This difference was also seen at the final follow-up with greater kyphosis demonstrated in the uniplanar screw group at 41 degrees compared with 29 degrees in the fixed screw group. This difference was statistically significant (P=0.006). There was no difference seen across groups with respect to correction of the coronal thoracic deformity. The average first erect coronal Cobb measurements for the uniplanar group and fixed screw group were 16 degrees compared with 18 degrees (P=0.5). This remained the case at the final follow-up with an average coronal thoracic deformity for uniplanar group and fixed screw group of 18.6 degrees compared with 19 degrees, respectively (P=0.9).
In a subanalysis comparing both groups (fixed vs. uniplanar screws) in only the Lenke 1 and Lenke 2 curve population, a similar result was found. (Table 3) The immediate postcorrection sagittal curvature measurements were greater using the uniplanar than the fixed screws, 32 versus 23 degrees (P=0.02). This difference was also seen at the final follow-up with greater kyphosis demonstrated in the uniplanar screw group at 44 degrees compared with 28 degrees in the fixed screw group. This difference was statistically significant (P=0.004). There was no difference seen across groups with respect to correction of the coronal thoracic deformity. The average first erect coronal Cobb measurements for the uniplanar group and fixed screw group were identical at 16 degrees (P=0.9). This remained the case at the final follow-up with an average coronal thoracic deformity for uniplanar group and fixed screw group of 16 degrees compared with 19 degrees (P=0.9).
The surgical treatment for AIS aims to achieve 3-dimensional correction of the deformity with a balanced spine, ultimately arresting the deformity progression.1,2,5 The conventional focus of AIS surgery has been the correction of the major curve in the coronal plane. The normal TK has been described in the range of 20 to 40 degrees.6 In patients with AIS, thoracic hypokyphosis is a common characteristic of the curve morphology. In addition, with the widespread use of pedicle fixation for AIS correction, it is apparent that the coronal correction is inversely proportional to the TK; thus, leading to a“thoracic flat back.”1,2,5–9 The suboptimal achievement of the TK following AIS surgery could contribute to the development of adjacent segment disease. It has been demonstrated that the preservation of TK is critical to maintain lumbar lordosis in AIS surgery.10 Suboptimal kyphosis restoration also causes loss of compensatory lumbar lordosis, which could contribute to positive sagittal balance and accelerated disk generation. Early symptomatic spinal degeneration can result in pain, disability, and suboptimal quality of life.1,2,5,8,10–13 Hence, the enthusiasm to improve the sagittal alignment has increased, partly due to the awareness about the flat back phenomenon that can occur following pedicle screw constructs. In general, the focus has moved beyond coronal correction toward global correction. With this background, the surgical restoration of TK in AIS patients has gained renewed attention. Thus, the reason for change in the type of screw was for kyphosis correction and better control over the sagittal profile with uniplanar screws. However, the long-term outcomes, of patients with all types of screw constructs, remain to be seen and may present unforeseen challenges.
Clements et al4 investigated the effect of implant density on the changes in sagittal contour preoperatively to postoperatively. They found higher implant density to be related to decreased kyphosis in the thoracic spine. Our study did not facilitate a way to analyze this fact. During the time these patients underwent surgery, our construct density was >90% which was at the highest end of the correlation reported in that study which compared construct densities that varied from below 20% up to 100%. The preponderance of constructs in that study was below 80%. Further, we lacked an adequate amount of variation in our construct density to examine this variable.
Lui and colleagues studied the effects of rod stiffness and implant density on coronal and sagittal plane correction in patients with main thoracic curve AIS. The study concluded that both rods with high and low stiffness combined with high or low screw density could provide effective correction in the coronal plane. With respect to the sagittal alignment, rods with high stiffness along with high screw density on the concave side could provide better outcomes with respect to TK restoration.2,5 Our study did not vary in terms of rod character; and therefore, a comparison on this factor was not possible. All of the rods used were 6.35-mm stainless steel.
A quantitative meta-analysis was conducted by Cao and colleagues on the studies reporting results of spinal surgery in AIS with instrumentation of pedicle screw or hybrid construct with the objective to evaluate the efficacy to restore TK. Hybrid constructs seem to be more powerful in restoring kyphosis than pedicle screws. Factors like rod stiffness, fashion of hybrid construct, in situ bending, and rod rotation maneuver could have an influence in restoration of the sagittal alignment.5,9 In contrast, a study by Suk et al14 indicated that segmental pedicle screw fixation was more effective than multiple hooks in restoring kyphosis in patients with hypokyphotic scoliosis and created kyphosis similar to that in patients without preoperative hypokyphosis.
Clement et al15 described the use of a technique involving simultaneous translation on 2 rods for AIS correction that achieved satisfactory restoration of TK. Similarly, Sudo et al16 described a simultaneous double-rod rotation technique for treating Lenke 1 AIS providing significant sagittal correction of the main thoracic curve while maintaining sagittal profiles and correcting coronal and axial deformities. In another study by Clement et al,17 they concluded that simultaneous translation on 2 rods provides a better correction of TK than the sequential approximation by cantilever reduction on patients with preoperative hypokyphosis. Sucato and colleagues compared the use of 3 surgical approaches in the treatment of AIS with the aim of restoring TK. They found anterior spinal fusion and instrumentation was the best method to restore TK when compared with posterior approaches using only hooks or a hybrid constructs.18 Imagama and colleagues described the surgical procedure and results for posterior correction and fusion with a hybrid approach using pedicle screws, hooks, and ultra-high molecular weight polyethylene tape with direct vertebral rotation for treatment of AIS. The study concluded that the technique is advantageous for kyphosis restoration compared with segmental screw fixation, and uses the minimum number of pedicle screws.19 Our present study did not involve the use of any hybrid fixation devices, special approaches, or correction techniques and did not evaluate these aspects.
Uniplanar pedicle screws have been a recent addition to the armamentarium used for AIS correction. Uniplanar screws allow for freedom of motion of the screw head in the sagittal plane. This feature can accommodate the rod curvature and eventually the sagittal spinal profile better than the fixed variant. In our study, the patient receiving uniplanar screws demonstrated an improvement in the TK, irrespective of their preoperative thoracic alignment being normo-kyphotic or hypokyphotic. Thus, the study suggests that uniplanar screws aid in maintaining TK.
Our study was limited by its small sample size as a single center study. Repeating the study across multiple sites prospectively would be ideal to determine if the choice of screws affects the postoperative sagittal contour. Because of our small sample size, subanalysis based on parameters such as Lenke curve type (only 13 thoracic curves in the fixed group and 7 in the uniplanar group) is limited and should be interpreted carefully. There was a statistical difference between patients treated with fixed versus uniplanar screws for the study as a whole and when Lenke 1 and 2 curve types were grouped together and examined as a data subset. This grouping was chosen because these curves are entirely based in the thoracic spine and tend to be the most hypokyphotic in our experience. Uniplanar screws seemed to offer an advantage in restoring a more normal kyphotic thoracic spine. We do note that by comparing in this manner, a small sample size results with only 13 cases in the fixed group compared with 7 in the uniplanar group. Identifying a larger sample size would aid in confirming this finding.
We selected examining the immediate postoperative (first erect) and final follow-up films as radiographic parameters. The final follow-up films demonstrated more TK than the immediate postoperative films. A similar change was not seen in the coronal Cobb values. We believe this is likely due to the transition to higher quality digital films for the 2-year follow-up imaging which allowed for better windowing and measurement of the cranial end vertebra; however, other possible explanations include measurement error or perhaps real changes in TK. We did not assess apical rotation in this study, either by postoperative computed tomographic scans or radiographic surrogates. Lastly, a selection bias and other unmeasured factors may have influenced the outcomes. A study design involving a larger sample size and a simultaneous randomization of the patients to the 2 groups would have greatly added to the power of this study.
We believe our study supports the potential benefit of uniplanar screws in the sagittal plane for idiopathic scoliosis. Clinically, this should be weighed against other factors such as implant cost, implant number, and surgeon comfort with the use of a particular implant. Further work in this area will facilitate scoliosis surgeons in selecting the proper implants for each type of deformity to provide the most natural spinal contour.
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