|Year : 2022 | Volume
| Issue : 2 | Page : 115-120
Implant density and curve correction in scoliosis surgery using a three-dimensional-based correction strategy
Bobby Kin-Wah Ng, Victor Illescas, Wai-Wang Chau
Department of Orthopaedics and Traumatology, Chinese University of Hong Kong, Hong Kong
|Date of Submission||11-Jan-2022|
|Date of Acceptance||30-Oct-2022|
|Date of Web Publication||30-Dec-2022|
Dr. Bobby Kin-Wah Ng
Department of Orthopaedics and Traumatology, Chinese University of Hong Kong
Source of Support: None, Conflict of Interest: None
Introduction: The growing trend toward the use of pedicle screws for the operative treatment of patients with idiopathic scoliosis is to provide a three-dimensional (3D) deformity correction using a three-column fixation was observed. Reports have variable recommendations regarding the implant density as well as the configuration of the pedicle screws. This study re-evaluated implant density and curve correction currently based on the 3D correction strategy by comparing it to side-bending correction (SBC). Materials and Methods: Seventy-six adolescent idiopathic scoliosis (AIS) patients who had undergone posterior spinal fusion from 2017 to 2019 visited our specialized center were recruited. Demographic variables and radiological measurements were collected. Patients filled out the Scoliosis Research Society (SRS-22) questionnaire from a mobile device, of which the SRS-22 was digitally adopted using mobile technology and cloud computation. Results: In the 76 AIS patients, 28 (37%) were rigid curves and 48 (63%) were flexible curves. Of the 28 rigid curves (SBC <30%), 13 (46%) patients had low pedicle screw density (PSD), while 15 (54%) had high PSD. Of the 48 flexible curves, 26 (55%) patients had low PSD, while 22 (45%) patients had high PSD. SBC index for the high PSD group (172) is almost the same compared to the low PSD group (174). Conclusions: Using high or low PSD makes the same amount of spinal correction for this group and additional screws do not make significant improvement on spinal correction. Higher screw density instrumentation is associated with the same amount of correction rate, whether in rigid or flexible curves, leading us to postulate that scoliosis correction relates more to intrinsic curve flexibility rather than instrument density.
Keywords: Adolescent idiopathic scoliosis, children, pedicle screw density, posterior spinal fusion, three-dimensional correction method
|How to cite this article:|
Ng BK, Illescas V, Chau WW. Implant density and curve correction in scoliosis surgery using a three-dimensional-based correction strategy. J Orthop Traumatol Rehabil 2022;14:115-20
|How to cite this URL:|
Ng BK, Illescas V, Chau WW. Implant density and curve correction in scoliosis surgery using a three-dimensional-based correction strategy. J Orthop Traumatol Rehabil [serial online] 2022 [cited 2023 Apr 1];14:115-20. Available from: https://www.jotr.in/text.asp?2022/14/2/115/365828
| Introduction|| |
Adolescent idiopathic scoliosis (AIS) is a three-dimensional (3D) deformity. There is a growing trend toward the use of pedicle screws for the operative treatment of patients with idiopathic scoliosis to provide a 3D deformity correction using a three-column fixation. Multiple studies have reported pedicle screws as superior to posterior hook and wire or hybrid constructs.,,,
Wide variability exists in the number of screws used for corrective scoliosis surgery. Authors have variable recommendations regarding the implant density as well as the configuration of the pedicle screws. High numbers of pedicle screws are increasingly used, but there is limited evidence to support this as best practice. A systemic review of implant density and curve correction in scoliosis showed limited evidence to support high-density instrumentation. Curve flexibility was also lacking in several studies. A study by Bharucha et al. reported no correlation between implant density and curve flexibility to curve correction. While the study by Sanders et al. stated that the number of fixation points within the curve and for each vertebral body was larger for curves with greater correction compared to the bending films. The purpose of this study is to re-evaluate implant density and curve correction currently based on the 3D correction strategy by comparing it to side-bending correction (SBC). We hypothesize that reducing the number of pedicle screws in scoliosis corrective surgery does not affect the deformity correction and even lowers the overall load on spinal instrumentation.
| Materials and Methods|| |
To evaluate the impact of screw density on curve rigidity and curve correction, numerical, and statistical analyses detailed in the following section were performed.
We reviewed 76 consecutive AIS patients who had undergone Posterior Spinal Fusion from 2014 to 2017. We included patients with AIS of any Lenke Classification. The exclusion criteria involve patients with neuromuscular conditions, congenital scoliosis, posttraumatic, and postinfectious scoliosis.
Preoperative standing, side-bend, and postoperative Cobb angle were measured. Curve correction was determined by the SBC index (SBCI) using the formula: 100 × (postoperative correction/SBC) where postoperative correction is: 100 × ([preoperative Cobb angle − postoperative Cobb angle]/preoperative Cobb angle) and where SBC is: 100 × ([preoperative Cobb angle − side-bending Cobb angle]/preoperative Cobb angle). Our study considers a curve as rigid when the SBC is at or <30% and flexible if SBC is more than 30%.,
Implant density was calculated as the percentage of the implant relative to the number of available implant sites within the measured Cobb angle. Pedicle screw density (PSD) for each patient is derived from: using the formula 100 × (pedicle screw/[level ×2]). The Low PSD group is defined as implant density at or below 70% of fused segments. High PSD is defined as implant density above 70% of fused segments. This is the first screw instrumentation study to use percentage to quantify density. We chose 70% as the threshold based on the systematic review of Larson et al. and the study by Bharucha et al. where their reports showed implant density below the mean number of screws per level for the entire cohort and studies reviewed is less than or equal to 1.3 ± 0.14 screws per level. The curve correction is expressed as operative correction with the formula as described above.
Three-dimensional correction method in the posterior instrumented fusion of scoliosis patient by all pedicle screw constructs [Figure 1]. The preoperative assessment determines the fusion level. This is determined by the curve type and the intended derotation and counter-derotation forces [Figure 2]. The left upper thoracic and left lumbar curves are used to counteract the right thoracic curve in the axial plane. In the sagittal plane, the thoracic kyphosis bend of the rod is used to restore thoracic kyphosis from a lordosis and push down the left lumbar and left upper thoracic kyphosis. The placement of the pedicle screw is done by navigation-guided technique, which allows the best trajectory for safety as well as to construct the derotation force by placing the pedicle screw in the most rotated position so that when derotated, it results in axial derotation of the vertebra. On the convexity of the curve, monoaxial screws are placed so that direct derotation by pushing force can be made. On the concave side, polyaxial screws or extended reduction screws are used as the correction force is a traction force.
|Figure 2: Segmental correction with derotation using extended tubes and slotted clamps|
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The anchorage pattern is planned out so that there are one or more pedicle screws above and below the apical vertebra or disc. For the proximal thoracic curve, such as T2 to T5 with T3, 4 as the apical vertebra or disc, then T2, and T4 would be instrumented. If the curve is longer or larger, then T3 and T5 can also be instrumented. For the main right thoracic curve, such as T5 to T11 with apex at T8, 9 the instrumented vertebrae would be at or above apex T5 or T6, T7, T8 and at or below apex T9, T11, and T12. The left lumbar curve instrumentation would be T12/L1 for the Lenke Type I and II curve from the mid-sacral bisected vertebra to one or two levels above. If the thoracic curve extends to T11, then T12-L1 would form a link couple. If the lower thoracic curve extended to T12, then L1-L2 are the lower lumbar vertebrae connected to form a linked unit. For Lenke type 3 and 4 cases, all Cobb level lumbar vertebrae would be instrumented to the lowest instrumented vertebra at the sacral bisector.
After placement of the pedicle screws, the correction maneuver involves first an intra-segment derotation by aligning the pedicle screws closest to or at the apex with that further away from the apex with a special instrument consisting of extended tubes connecting to the pedicle screws and these tubes are then held with a slotted frame to keep it in the same plane above and below the apex. We called it intra-segment derotation.
The left upper thoracic curve is first reduced with the 3D correction from the convexity derotating the hump from the left side anteriorly, coronal and sagittal correction by direct maneuver. A temporary rod is then placed on the right segment for temporary immobilization. The left side rod is then removed.
For the lumbar curve, when it is required to instrument down to L3, 4, 5 segments, above and below apex segmental intra-segment derotation and coupling is done with the extended tubes and fixed with short rods, coronal and sagittal plane correction is made with coronal and in situ benders. These two linked couple is then temporarily fixed with a slotted frame. A temporary rod is then placed on the right lumbar curve for temporary fixation. The left-side reduction tubes and rods are then removed, leaving them free for final definitive rod fixation.
The main thoracic curve correction is similarly done with the extended tubes placed at mono-axial screws above and below the apex. Intra-segment derotation is done, and a short rod with an appropriate kyphotic molding is then placed to maintain this intra-segment derotated position for the two segments. The coronal bender is placed in the middle of each of the connecting rods of each segment to produce a coronal correcting force and a set of in situ benders are used to create a thoracic kyphotic force. Once the correction is completed, a long-slotted frame is used to hold the whole construct stable. Usually, some rebound or loss of the correction is seen when the coronal and sagittal benders are removed.
The reduction of each segment has been made with the temporary fixation on the right side with two segmental rods and an external frame fixing the thoracic segment. The definitive left side rod is then measured contoured into normal physiological sagittal alignment with thoracic kyphosis and lumbar lordosis. This is placed into the top open pedicle screws from the proximal thoracic segment and then fit into all the pedicle screws. The reduction screws allow the partially rotated rod to be captured by all pedicle screws. The rod is then rotated into the normal sagittal profile. The gradual tightening of the reduction screw from the apex outward in a centrifugal direction allows the curve to be reduced to the contoured rod in normal sagittal profile. Often the rod is seen to deform into the curve and the thoracic contoured kyphosis is flattened by the curve. The spine now conforms to the definitive rod in normal physiological profile. When all the fixation had been completed, further fine-tuning of in situ correction is made with benders to maximize the correction, and further segmental compression distraction is made to level the lower instrumented vertebra to a horizontal level. The temporary fixation rods on the right side are all removed and a definitive rod with the same physiological sagittal profile is fixed to the right-side pedicle screws. Two cross-link connectors are measured and fixed to the rods to complete the fixation.
Data were recorded and analyzed comparing the group of patients who underwent corrective scoliosis surgery with high density (PSD >70%) and the group of patients with low density (PSD <70%) screw fixation. Evaluation of groups of patients with rigid (<30% SBC) and flexible curve (>30% SBC) were analyzed to curve correction. P < 0.05 was considered statistically significant.
| Results|| |
Of the 76 cases, there were 62 (81.6%) females and 14 (18.4%) males, with a mean age of 17.52. Mean Cobb angle of 68.6. The average side-bending Cobb angle is 45 and SBC is 35%. There were 35 (46%) patients who had high PSD, while 41 (54%) patients had low PSD. Average postoperative Cobb angle is 20° and postoperative correction is 71%. Average SBCI is 271. Average SBCI is higher for rigid curves (354%) than for flexible curves (173%). Our results show that increasing Cobb's angle is directly correlated with curve rigidity and indirectly correlated with postoperative correction [Figure 3] and [Figure 4].
|Figure 4: Postoperative percent correction versus side-bending correction|
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Of the 28 rigid curves (SBC < 30%), 13 (46%) patients had low PSD, while 15 (54%) had high PSD. The average for instrumentation for the high PSD group is 87%, while for low PSD group is 64%. The average SB correction for all rigid curve groups is 20%. The high PSD group, who had higher SB correction (22% vs. 17%), showed increased postoperative curve correction of 68% compared to the low PSD group at 61%, although the difference was not significant (P = 0.05). Despite the improved curved correction with higher PSD, we observe SBCI is lower for the high PSD group (348) compared to the low PSD group (359), showing that correction is not dependent on screw density but on curve flexibility [Table 1].
|Table 1: Side-bending correction Index (SBCI), post-operative percent correction for rigid and flexible curves|
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Of the 48 flexible curves, 26 (54%) patients had low PSD, while 22 (45%) patients had high PSD. Average instrumentation for high PSD group is 81%, while for low PSD group is 66%. Average SB correction for all flexible curve groups is 45%. The high PSD group, who had higher SB correction (47% vs. 44%), had a higher average curve correction of 76%, compared to the low PSD group at 73%, although the difference was not significant (P = 0.05). SBCI for the high PSD group (172) is almost the same compared to the low PSD group (174). This implies that using high or low PSD leads to the same amount of correction for this group, and additional screws does not lead to significant improvement [Table 1].
| Discussion|| |
This study gives a detailed assessment of thoracic scoliosis by pre- and post-operative measurement comparisons. By showing the complete set of data for all patients, this provides data on pedicle screw instrumentation density with correlation to intrinsic curve flexibility. Our analysis shows that, overall, increasing the number of pedicle screws improves curve correction. The amount of how much pedicle screws to place before it plateaus is thus analyzed [Table 2]. Increasing the length of fused levels is also statistically correlated to the improvement of curve correction [Table 2].
|Table 2: Linear regression analysis showing the relationship between curve correction with the number of pedicle screws, fused levels, and pedicle screw density|
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High efforts have been made to correct spine deformities. Major curve correction in the coronal plane has long been the focus for AIS surgery. As surgeons became more familiar with thoracic pedicle screw instrumentation, they have used higher implant density constructs. A study by Clements et al. observed a statistically significant and moderate relationship between main curve correction and implant density (r = 0.31, P < 0.001). Other biomechanical studies advocate that increased apical instrumentation is associated with more effective curve correction.
Le Navéaux et al. found that certain low-density implant configurations, with predominant implant placement on the concave side, resulted in similar corrections compared to high density. In the studies of Bharucha et al., and Chen et al., and Gotfryd and Avanzi, they reported no direct relationship between curve severity, flexibility, and curve correction to implant density.,, Other studies report that low-density instrumentation still provides an adequate amount of correction and no correlation between implant density and the magnitude of the correction.,,, While in a study by Hwang et al., they reported placing skipped screw insertion on the supportive side of the curve, with good outcomes even at 5-year follow-up. A study by Li et al. reported interval pedicle screw placement constructs seem to be equally effective as consecutive constructs for facilitating curve correction leading to the conclusion that a more limited pedicle screw construct is equally as effective as a consecutive screw construct. Biomechanical data by Wang et al. simulating curve correction with variable implant density suggest that a minimum-density screw pattern may result in a comparable correction as found with a high-density construct. A study by Mac-Thiong et al. showed that increasing the PSD by more than 70% is not likely to result in a significantly greater correction in the coronal plane.
The present study supports the findings of previous studies and also includes a broad range of Lenke-type curves, large patient volume, with any amount of Cobb angulation. There were limited studies that included flexibility/stiffness to curve correction with implant density. The study by Hwang et al. and Li et al. only included flexible curves showing good results with low implant density used., Study by Sanders et al. included Lenke 1 curves. They had a similar goal of looking at curve correction relative to bending films to correct for the inherent differences in curve flexibility and found that increased fixation points in the curve lead to increased correction. No other study has grouped patients according to rigid or flexible curves with high or low implant density placed and analyzed to curve correction. Our results showed the flexible curves having increased amount of correction compared to the rigid group. No difference with correction was observed whether high or low pedicle density was used in both rigid and flexible curve groups [Figure 5]. Rigid curves showed higher SBCI compared to flexible curves signifying the amount of stiffness of this group [Table 2 and Figure 6].
|Figure 5: Illustrations of postoperative correction versus pedicle screw density for: (a) high PSD for rigid curve, (b) low PSD for rigid curve, (c) high PSD for flexible curve, and (d) low PSD for flexible curve. PSD: Pedicle screw density|
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|Figure 6: Illustrations of Side-Bending Correction Index versus Pedicle Screw Density for: (a) high PSD for rigid curve; (b) low PSD for rigid curve; (c) high PSD for flexible curve; and (d) low PSD for flexible curve. PSD: Pedicle screw density|
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Average SBCI for rigid curves is lower for the high PSD group (348) compared to the low PSD group (359). The fact that the high PSD group showed lower SBCI despite improved Postoperative correction tells us that correction is dependent on the intrinsic flexibility of the curve and not implant density [Table 2]. The average SBCI for the flexible group is almost the same as with the high PSD group (172 vs. 174). This implies that with the same amount of flexibility of curves within this group, adding more screw density is not correlated with improvement and that correction just plateaus out after additional screws are placed.
The statistical results might have been limited by the number of patients recruited in this study. Confounding factors existed, although statistical controlling of confounding factors was applied in trying to minimize the error effect.
| Conclusions|| |
Corrective scoliosis surgery using posterior spinal instrumented fusion has consistently provided good operative correction. In this study, with detailed analysis of curve rigidity and PSD, results showed there is no significant difference in putting high or low-density screw instrumentation for rigid curves as they showed similar correction. For flexible curves, there is no advantage of placing high PSD, as the correction plateaus out. Increasing screw density for the flexible curves seemed associated with lesser curve correction after the plateau. Higher screw density instrumentation is associated with the same amount of correction rate, whether in rigid or flexible curves, leading us to postulate that scoliosis correction relates more to intrinsic curve flexibility rather than instrument density. A randomized controlled study having the same demographics and considering body mass index, and bone density should be undertaken in the future.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
[Table 1], [Table 2]