Biomechanical comparison of alternative densities of pedicle screws for the treatment of adolescent idiopathic scoliosis

Efficacy and Safety of Low-Density Pedicle Screw versus High-Density Screw in Lenke I Scoliosis: A Systematic Review and Meta-Analysis

OBJECTIVE

To evaluate the efficacy and safety of low-density versus high-density pedicle screw in patients with Lenke I adolescent idiopathic scoliosis through systematic review and meta-analysis.

METHODS

A comprehensive literature search was conducted in PubMed, Web of Science, and Embase databases. Studies comparing low-density and high-density pedicle screw in Lenke I adolescent idiopathic scoliosis were included. Two authors independently selected studies, assessed risk of bias, and extracted data. Meta-analysis was performed using systematic review software.

RESULTS

The meta-analysis included 11 studies comprising 697 patients (397 in low-density group and 300 in high-density group). No significant differences were found between low-density and high-density groups in terms of blood loss, operative time, complication rates, or revision rates. Radiographic outcomes, including major Cobb angle, curve correction, thoracic kyphosis, and coronal and sagittal balance, were also similar between the groups. However, low-density pedicle screw was associated with significantly lower costs.

CONCLUSIONS

This meta-analysis suggests that low-density pedicle screw can achieve similar clinical and radiographic outcomes compared with high-density constructs in patients with Lenke I adolescent idiopathic scoliosis, while potentially reducing costs, making it a more cost-effective option without compromising patient outcomes.

Biomechanical Computational Study of Pedicle Screw Position and Density in Adolescent Idiopathic Scoliosis Instrumentation

STUDY DESIGN

Computer simulation of adolescent idiopathic scoliosis instrumentation.

OBJECTIVE

To test the hypothesis that different screw densities would result in different apical vertebral rotation (AVR) corrections and bone-screw forces in adolescent idiopathic scoliosis instrumentation.

SUMMARY OF BACKGROUND DATA

The "Minimize Implants Maximize Outcomes" Clinical Trial revealed that the use of more versus fewer screws resulted in similar coronal plane correction for Lenke 1A curves. However, the biomechanical impact of screw density on transverse plane correction is still unclear. Further investigation is needed to determine if and how transverse plane correction is correlated with screw density.

PATIENTS AND METHODS

We simulated apical vertebral derotation after segmental translation using patient-specific computer models of 30 patients from the "Minimize Implants Maximize Outcomes" Trial. For each case, 10 alternative screw patterns were tested with overall densities ranging between 1.2 and 2 screws per level fused, and local density at the 3 apical levels ranging between 0.7 and 2 (total: 600 simulations). Main thoracic (MT) Cobb angle, thoracic kyphosis (TK), AVR, and bone-screw forces were computed and compared.

RESULTS

The presenting MT (62 ± 11°; range: 45° to 86°), TK (27 ± 20°; -5° to 81°), and AVR (14±7°; -2° to 25°) were corrected through segmental translation to 22 ± 7° (10° to 41°), 26 ± 5° (18° to 45°), and 14 ± 7° (-4° to 26°). After apical vertebral derotation, they became 16 ± 8° (1° to 41°), 24 ± 4° (13° to 40°), and 4 ± 5° (-12° to 18°). There was no significant difference in MT among screw patterns; higher screw density had lower bone-screw forces ( P < 0.05). The apical vertebral derotation maneuver reduced AVR by an average of 70%, positively correlated with apical screw density ( r = 0.825, P < 0.05). There was no significant difference in TK.

CONCLUSION

Screw density had no significant effect on 3-dimensional correction through the primary segmental translation maneuver. Transverse plane correction through subsequent apical vertebral derotation was positively correlated with screw density at the apical levels ( r = 0.825, P < 0.05). Bone-screw forces were negatively correlated with overall screw density ( P < 0.05).

Intermittent pedicle screw application provides better kyphosis restoration in adolescent idiopathic scoliosis for Lenke type 1 and type 2 curves

PURPOSE

There is still no consensus on the optimum pedicle screw density required for the desired thoracic kyphosis restoration in adolescent idiopathic surgery (AIS). The aim of this study to evaluate the effect of pedicle screw density on thoracic kyphosis restoration in AIS surgery.

METHODS

The data of 106 patients from two centers that operated for Lenke type 1 and 2 AIS were retrospectively reviewed. Two groups were constituted according to the pedicle screw density: intermittent pedicle screw constructs (IPSC) (n = 52 patients) and consecutive pedicle screw construct (CPSC) (n = 54 patients) groups. The preoperative and at least 24-month follow-up radiographs and SRS-22 scores were evaluated. The Cobb angle of the main and concomitant curves in the coronal plane and the sagittal plane were measured and compared.

RESULTS

The mean follow-up period for the IPSC and CPSC groups was 72.3 ± 37.2 and 62.9 ± 28.8 months, respectively. In the SRS-22 questionnaire, there was no significant difference between the two groups in terms of self-image/appearance domain scores (p = 0.466), but better results were obtained in the IPSC group in terms of treatment satisfaction domain scores (p = 0.010) and better thoracic kyphosis restoration was achieved in IPSC group radiologically for Lenke type 1 curves with - 81.4 ± 81.4% in the IPSC group and 6.8 ± 83.8% in the CPSC group (p < 0.001).

CONCLUSION

It was considered that better thoracic kyphosis restoration could be achieved with the less lordotic effect of IPSC in Lenke type 1 curves. Although the current situation had a significant impact on radiological outcomes, its effect on SRS-22 scores was limited.

Surgical outcomes of severe spinal deformities exceeding 100° or treated by vertebral column resection (VCR). Does implant density matter?: an observational study of deformity groupings

STUDY DESIGN

Prospective multicenter international observational study.

OBJECTIVE

To investigate the effect of implant density on clinical outcomes in complex pediatric spine reconstruction. Implant density in spine deformity surgery has been a subject of much debate with some authors advocating higher density for better correction. Few studies have looked at the effect of implant density on severe curves > 100 deg or treated with vertebral column resection (VCR).

METHODS

250/311 pts with 2-year f/u enrolled in the FOX pediatric database from 17 international sites were queried for the impact of implant density and surgical outcomes. Patients were grouped into three implant density categories for comparative analysis Group 1 (density ≤ 1), Group 2 (1 < density < 1.5) and Group 3 (density; 1.5-2).

RESULTS

250 pts: 47 (Grp1)/99 (Grp2) /104 (Grp3); Pre-op age and etiology and curve types were similar in all groups, but body mass index (BMI) was higher in Grp3. Grps 1 and 2 had significantly higher sagittal deformity angular ratio (S-DAR) compared to Grp 3 (p < 0.001). Pre-op Halo Gravity Traction (HGT) was used in 55.3%/44.4%/31.7%, p = 0.017; Grp1/Grp2/Grp3, respectively. Average duration of surgery (min) was higher in Grp3 relative to Grp1 only: 352.5/456.5/515.0, p = 0.0029. Blood loss was similar in all Grps. Rate of VCR, PSO and SPO was similar in all Grps. Pre-op Coronal Cobb avg 96.1/83.6/88.6, p = 0.2342, attained similar correction after HGT (24.6%/27.2%/23.2%, p = 0.4864. Coronal Cobb corrections at 2-year follow-up (FU) were (37.1%/40.3%/53.5%, p = 0.0004). Pre-op sagittal Cobb was (105.4/101.9/75.9, p < 0.01.), achieved similar %correction in HGT (19.1%/22.3%/22.5%, p = 0.6851) and at 2-year FU (39.6%/41.4%/29.8%, p = 0.1916). After adjusting for C-DAR, S-DAR, pre-op coronal and sagittal Cobb, etiology, curve types, age, BMI and number of rods in multivariate analysis, the odds of developing post-operative implant complication was 11 times greater in group 1 compared to group 3 (OR = 11.17,95% CI 2.34-53.32). There was significant improvement in SRS scores in all Grps at 2-year FU.

CONCLUSION

Although higher implant density was observed to be associated with greater curve correction and lower rates of post-operative implant-related complication and revision in heterogeneous case groups, the results may not imply causality of implant density on the outcomes in severe pediatric spine reconstruction.

Computational modelling of the scoliotic spine: A literature review

Scoliosis is a deformity of the spine that in severe cases requires surgical treatment. There is still disagreement among clinicians as to what the aim of such treatment is as well as the optimal surgical technique. Numerical models can aid clinical decision-making by estimating the outcome of a given surgical intervention. This paper provided some background information on the modelling of the healthy spine and a review of the literature on scoliotic spine models, their validation, and their application. An overview of the methods and techniques used to construct scoliotic finite element and multibody models was given as well as the boundary conditions used in the simulations. The current limitations of the models were discussed as well as how such limitations are addressed in non-scoliotic spine models. Finally, future directions for the numerical modelling of scoliosis were addressed.

Does Implant Density Impact Three-Dimensional Deformity Correction in Adolescent Idiopathic Scoliosis with Lenke 1 and 2 Curves Treated by Posterior Spinal Fusion without Ponte Osteotomies?

Study Design

Retrospective cohort study.

Purpose

To determine whether implant density impact three-dimensional deformity correction in posterior spinal fusion (PSF) without Ponte osteotomies (POs) for patients with Lenke 1 and 2 adolescent idiopathic scoliosis (AIS).

Overview of Literature

Currently, the optimal pedicle screw (PS) density for flexible moderate-sized thoracic AIS curve correction is still controversial. There are limited data regarding the impact of implant density on three-dimensional correction in PSF without the use of PO for thoracic AIS surgery.

Methods

A database of patients with AIS with Lenke 1 and 2 curves treated with PSF without PO and instrumented with PSs and ≥2-year follow-up was reviewed. The preoperative, immediate, and final follow-up postoperative radiographs were analyzed. The correlation between PS density and the following factors were determined: major curve correction (MCC), correction index (CI; MCC/curve flexibility), kyphosis angle change, and rib index (RI) correction. Then, patients were divided into low-density (LD) and high-density (HD) groups according to mean PS density for the entire cohort (1.5 PS per level). Demographics and radiographic and clinical outcomes were compared between groups.

Results

The study included 99 patients with Lenke 1 and 23 patients with Lenke 2 AIS. The average MCC was 67.2%. There was no correlation between screw density and these parameters: MCC (r=0.10, p=0.26), CI (r=0.16, p=0.07), change in T2–T12 kyphosis angle (r=−0.13, p=0.14), and RI correction (r=−0.09, p=0.37). Demographic and preoperative radiographic parameters were similar between the LD and HD groups. At the latest follow-up, there were no differences between the two groups in regard to MCC, CI, change in T2–T12 kyphosis angle, RI correction, and Scoliosis Research Society-30 scores (all p>0.05).

Conclusions

This study revealed no significant correlation between screw density and curve correction in any planes. HD construct may not provide better deformity correction in patients with flexible and moderate thoracic AIS undergoing PSF without PO.

How do spine instrumentation parameters influence the 3D correction of thoracic adolescent idiopathic scoliosis? A patient-specific biomechanical study

BACKGROUND

Patient-specific models promises to support the surgical decision-making process, particularly in adolescent idiopathic scoliosis. The present computational biomechanical study investigates how specific instrumentation parameters impact 3D deformity correction in thoracic scoliosis.

METHODS

1080 instrumentation simulations of a representative patient were run. The independent instrumentation parameters were: screw pattern, upper and lower instrumented vertebrae, rod curvature and rod stiffness. ANOVA and correlation analyses analyzed how the instrumentation parameters influenced the 3D correction.

FINDINGS

Coronal plane correction was affected by the lower instrumented vertebra and rod stiffness (explaining 84% and 11%, respectively, of its overall variance). The sagittal profile was controlled by rod curvature and the upper vertebra (56% and 36%). The transverse plane vertebral rotation was influenced by lower, upper instrumented vertebra and screw pattern (35%, 32% and 19%). The Cobb angle correction was strongly correlated with the number of fused vertebrae, particularly when grouped by the upper instrumented vertebra (r = -0.91) and rod stiffness (r = -0.73). Thoracic kyphosis was strongly correlated with the number of fused vertebrae grouped by rod curvature (r = 0.84). Apical vertebral rotation was moderately correlated with the number of fused vertebrae grouped by upper/lower instrumented vertebra (r = 0.55/0.58), although variations were minimal.

INTERPRETATION

Instrumenting the last vertebra touching the central sacral vertical line improves 3D correction. A trade-off between a more cranial vs. caudal upper instrumented vertebra, respectively beneficial for coronal/sagittal vs. transverse plane correction, is required. High rod stiffness, differential rod contouring, and screw pattern were effective for coronal correction, thoracic kyphosis, and axial vertebral derotation, respectively.

Influence of implant density and flexibility index on curve correction after scoliosis surgery

BACKGROUND

Numerous studies proved that all pedicle screw constructs produce the best outcome in the surgical treatment of patients with scoliosis. However, the optimal amount and distribution of screws has not yet been defined. In recent studies on idiopathic scoliosis, the correlation between curve correction and implant density has been discussed with diversifying results.

PURPOSE

The aim of this study was to detect a possible correlation of sagittal and coronal curve correction in dependence of metal load and flexibility index.

MATERIALS AND METHODS

Twenty-six patients were included in this study with surgical correction by one surgeon between January 2014 and December 2017. Clinical data and radiographic images (preoperative, postoperative and one-year follow-up) were retrospectively analyzed and metal load, flexibility index, correction rate and correction index were consequently calculated. The Pearson correlation analysis was used for metal load-correction index and metal load-correction rate, correction of kyphosis-metal load and correction of lordosis-metal load. According to the mean metal load of 88%, patients were divided into two subgroups-a low-density group of 12 patients and a high-density group of 14 patients. Clinical and radiographic features were examined by an independent two-sided t-test.

RESULTS

Eight patients were male, 18 patients female. Ten suffered from neuromuscular and 16 from idiopathic scoliosis. Mean age was 17.1 years. Correction rate directly postoperative was 70.43%, at the follow-up 67.90%. Mean correction index directly postoperative was 3.40 and at the follow-up 3.23. Pearson correlation of metal load-correction index directly postoperative was - 0.188 and one year postoperative - 0.189. The correlation between metal load-correction rate immediately after the surgery was 0.324 and at the follow-up 0.285. Correlation for correction of kyphosis-metal load postoperative was - 0.120 and one year later - 0.178. Pearson coefficient of lordosis-metal load directly after the surgical intervention was - 0.214 and at the follow-up - 0.220. Dependency of flexibility index and correction rate showed a positive trend (Pearson flexibility-correction rate direct 0.616; flexibility index-correction rate follow-up 0.516). A statistically significant difference between the high- and the low-density group was detected in the correction rate directly postoperative (p = 0.047).

CONCLUSION

With an implant density over 70%, satisfactory surgical treatment can be achieved in idiopathic and neuromuscular scoliosis. No statistical significance between the high-density (88-100%) and the low-density (73-87%) group could be verified in curve correction, ICU stay and complications.

The importance of curve severity, type and instrumentation strategy in the surgical correction of adolescent idiopathic scoliosis: an in silico clinical trial on 64 cases

Adolescent idiopathic scoliosis is a three-dimensional deformity of the spine which is frequently corrected with the implantation of instrumentation with generally good or excellent clinical results; mechanical post-operative complications such as implant loosening and breakage are however relatively frequent. The rate of complications is associated with a lack of consensus about the surgical decision-making process; choices about the instrumentation length, the anchoring implants and the degree of correction are indeed mostly based on personal views and previous experience of the surgeon. In this work, we performed an in silico clinical trial on a large number of subjects in order to clarify which factors have the highest importance in determining the risk of complications by quantitatively analysing the mechanical stresses and loads in the instrumentation after the correction maneuvers. The results of the simulations highlighted the fundamental role of the curve severity, also in its three-dimensional aspect, and of the instrumentation strategy, whereas the length of the fixation had a lower importance.

In silico patient-specific optimization of correction strategies for thoracic adolescent idiopathic scoliosis

BACKGROUND

With modelling and simulation (or in silico) techniques, patient-specific optimization algorithms represent promising tools to support the surgical decision-making process, particularly in 3D correction of adolescent idiopathic scoliosis, where the best intraoperative instrumentation strategy and the correction goals are debated.

METHODS

1080 biomechanical intraoperative simulations of a representative pediatric thoracic curve were run according to a full-factorial design approach. Widely accepted instrumentation configurations (5 screw patterns, 4 upper and 3 lower instrumented vertebrae, 6 rod curvatures and 3 rod stiffnesses) were analyzed, assuming concave rod rotation and en bloc derotation as main correction maneuvers. Results in terms of 3D correction and mobility were rated using an objective function for thoracic scoliosis also including surgeon-dependent correction objectives. An extensive sensitivity analysis on correction objectives was performed.

FINDINGS

Multiple optimal strategies were identified, depending on the selected correction objective. They provided significantly better coronal (67% vs. 55%) correction, using comparable instrumented levels (9.9 ± 1.6 vs. 10.7 ± 2.1), screw patterns and significantly higher implant density (1.6 ± 0.3 vs. 1.4 ± 0.2 screws/vertebra) compared to worst ones. Optimal strategies typically included the neutral and the last touching vertebrae in the construct and high stiffness (CoCr, 6 mm) differentially/highly contoured rods.

INTERPRETATION

The computerized algorithm determined the best instrumentation parameters to achieve optimal correction for the considered thoracic case. Multiple clinically equivalent strategies may be used, as supported by the variety of considered correction objectives. The current approach could be translated to any scoliotic curves, including surgeon preferences in terms of instrumentation parameters, intraoperative correction maneuvers and correction objectives.

Correction objectives have higher impact than screw pattern and density on the optimal 3D correction of thoracic AIS: a biomechanical study

STUDY DESIGN

Assessment of screw pattern, implant density (ID), and optimization of 3D correction through computer-based biomechanical models.

OBJECTIVE

To investigate how screw pattern and ID affect intraoperative 3D correction of thoracic curves in adolescent idiopathic scoliosis, and how different correction objectives impact the optimal screw pattern. Screw pattern, ID, correction objectives and surgical strategies for posterior fusion of AIS are highly variable among experienced surgeons. The "optimal" instrumentation remains not well defined.

METHODS

10 patient-specific multibody models of representative adolescent idiopathic scoliosis Lenke 1A cases were built and used to compare alternative virtual correction surgeries. Five screw patterns and IDs (average: 1.6 screws/instrumented level, range: 1.2-2) were simulated, considering concave rod rotation, en bloc derotation, and compression/distraction as primary correction maneuvers. 3D correction descriptors were quantified in the coronal, sagittal and transverse planes. An objective function weighting the contribution of intraoperative 3D correction and mobility allowed rating of the outcomes of the virtual surgeries. Based on surgeon-dependent correction objectives, the optimal result among the simulated constructs was identified.

RESULTS

Low-density (ID ≤ 1.4) constructs provided equivalent 3D correction compared to higher (ID ≥ 1.8) densities (average differences ranging between 2° and 3°). The optimal screw pattern varied from case to case, falling within the low-density screw category in 14% of considered scenarios, 73% in the mid-density (1.4 < ID < 1.8) and 13% in the high-density. The optimal screw pattern was unique in five cases; multiple optima were found in other cases depending on the considered correction objectives.

CONCLUSIONS

Low-density screw patterns provided equivalent intraoperative 3D correction to higher-density patterns. Simulated surgeon's choice of correction objectives had the greatest impact on the selection of the optimal construct for 3D correction, while screw density and ID had a limited impact.

LEVEL OF EVIDENCE

N/A.

A finite element analysis of different pedicle screw placement strategies for treatment of Lenke 1 adolescent idiopathic scoliosis: which is better?

OBJECTIVE

To simulate the process of curve correction with different strategies of pedicle screws placement, and then explore the ideal strategy. Materials and methods: A three-dimensional model was constructed using CT data from a Lenke 1 AIS patient for finite element analysis. Five screw-placement strategies were designed, including collocation of consecutive, interval and alternate screws instrumentation in convex or concave side. The whole surgical procedures of screw-placement strategies were simulated. And their influences in the curve correction as well as the biomechanics of the implants and implant-bone interface were analyzed. Results: The densities of pedicle screws were 100%, 79%, 64%, 57% and 50%, respectively. During curve correction, interval and alternate screws instrumentation demonstrated higher screws stress, while the higher rods and screw-rod interface was observed in consecutive screws instrumentation. When it referred to the interaction force between screws and vertebrae, a lower stress was observed in in consecutive screws instrumentation. After the fixation of rod at convex side, the maximum stress on screws in each strategy was observed in interval screws instrumentation. The higher stress of rods was observed in alternate screws instrumentation in both sides. However, the correction rate of each strategy was similar. Conclusion: There was little distinction on the curve correction rate in different screw-placement strategies. The screw-placement strategy with interval and alternate screws instrumentation has better biomechanics properties than others, except for higher maximum interaction force between screws and vertebrae.

Periapical-dropout Screws Strategy For 3-Dimensional Correction of Lenke 1 Adolescent Idiopathic Scoliosis in Patients Treated by Posterior Spinal Fusion

STUDY DESIGN

This was a single-center, retrospective study.

OBJECTIVE

The objective of this study was to compare periapical-dropout screws strategy (PDSS) with traditional-multilevel pedicle screws strategy (TMSS) for 3-plane correction of Lenke 1 adolescent idiopathic scoliosis deformity.

SUMMARY OF BACKGROUND DATA

There are limited data in 3-plane correction and the optimal pedicle screw (PS) configuration for Lenke 1 adolescent idiopathic scoliosis surgery.

MATERIALS AND METHODS

Sixty-one consecutive patients with Lenke 1 curves (range: 50-80 degrees), undergoing single-stage posterior spinal fusion with PS fixation, were included. Patients with a minimum follow-up of 1 year were divided into 2 groups according to PS strategy. The PDSS group included 33 patients with PS placement bilaterally at both ends and apex of the construct. The TMSS group included 28 patients with conventional PS placement. Baseline, immediate, and last follow-up demographic, radiographic, and clinical outcomes were analyzed. Radiographic outcomes were assessed in axial (using rib index and apical vertebral rotation using Raimondi ruler and Upasani methods), coronal, and sagittal planes. The implant costs were also evaluated.

RESULTS

There were no differences in demographic, preoperative radiographic parameters and levels fused. The number of PSs per level fused was significantly lower in the PDSS group (1.3 vs. 1.4; P=0.0002). At last follow-up, major Cobb correction averaged 79% for the PDSS group and 69.5% for the TMSS group (P=0.001). T2-T12 kyphosis angle changes were 1 degree in the PDSS group and -2.5 degrees in the TMSS group (P=0.35). Rib index correction was 28.2% for the PDSS group and 17.7% for the TMSS group (P=0.02). Upasani grade apical vertebral rotation was significantly better in the PDSS group (0.7 vs. 1.4; P=0.0001). Clinical outcomes evaluated by Scoliosis Research Society-30 scores were similar in both groups. Total implant costs were significantly lower in the PDSS group ($16,852 vs. $18,926; P<0.001).

CONCLUSION

The PDSS construct provides better deformity correction in all 3 planes and helps decrease implant costs compared with the TMSS construct. Thus, the PDSS construct can be considered as a rational strategy and cost-effective technique when treating moderate Lenke 1 curves with posterior spinal fusion.

Biomechanical Comparison of the Load-Sharing Capacity of High and Low Implant Density Constructs With Three Types of Pedicle Screws for the Instrumentation of Adolescent Idiopathic Scoliosis

STUDY DESIGN

Biomechanical numerical simulation analysis of implant design and density in adolescent idiopathic scoliosis posterior instrumentation.

OBJECTIVES

To evaluate the combined effect of pedicle screw design and density on deformity correction and construct load-sharing capacity.

SUMMARY OF BACKGROUND DATA

Screw density is an area of popular study because of the impact of cost and potential patient morbidity of higher-density constructs. Using fewer screws raises concern about reduced correction and greater forces on each screw.

METHODS

Personalized spinal numerical models were created for five patients. The correction techniques from five spine surgeons using both a high- and a low-density implant pattern (2 vs. 1.4 ± 0.22 screws/level) with uniaxial, multiaxial, and favored angle screws were simulated. The predicted correction and forces sustained by the implants were compared. The postoperative load-sharing capacity of a high- and a low-density construct, with or without crosslinks, was compared by simulating daily activities motions.

RESULTS

The major coronal curve correction was similar with high- and low-density constructs (73% ± 10% vs. 72% ± 10%; p > .05) but was higher when using uniaxial (77% ± 8%) compared to multiaxial (69% ± 11%) and favored angle screws (71% ± 10%; p = .009). High- and low-density constructs sustained similar intraoperative peak forces (305 ± 61 N vs. 301 ± 73 N; p = .23) regardless of screw design (all p > .05). Multiaxial and favored angle screws reduced the peak axial force by 23% and 38% compared to uniaxial screws (p = .007). The high-density construct reduced the postoperative loads sustained by each implant by 31% (p = .006). Crosslinks had no effect on load sharing (p = .23).

CONCLUSION

High- and low-density implant patterns achieved similar coronal correction with equivalent capacity to share corrective forces regardless of the screw design. Increased degrees of freedom of the screw head reduces the capacity to correct coronal deformity but generates lower bone-screw forces. The reduced number of screws increased the postoperative forces sustained by each screw, but its effect on potential complications requires further investigations.

LEVEL OF EVIDENCE

Level 4.

Significant variability in surgeons' preferred correction maneuvers and instrumentation strategies when planning adolescent idiopathic scoliosis surgery

BACKGROUND

Increased implant number is thought to provide better control on the scoliotic spine, but there is limited scientific evidence of improved deformity correction and surgical outcomes with high-density constructs. The objective is to assess key anchor points used by experienced spinal deformity surgeons and to evaluate the effect of implant density pattern on correction techniques.

METHODS

Seventeen experienced spine surgeons reviewed five Lenke 1 adolescent idiopathic scoliosis cases and provided their preferred posterior correction technique (implant pattern, correction maneuvers, and implants used for their execution) and an alternative technique with the minimal implant density they felt would be acceptable (170 surgical plans total). Additionally, for each case, they selected acceptable screw patterns for surgery from seven published implant configurations. Variability in the surgeons' plans was assessed, including instrumentation and correction strategies.

RESULTS

The preferred correction plan involved an average of 1.65 implants/vertebra, with 88% of the available anchor points at the apex ± 1 vertebra used for the execution of correction maneuvers and only 43% of possible anchor points used proximal and distal to the apical area. The minimal density that surgeons found acceptable was 1.24 implants/vertebra. The minimal density plan involved more in situ rod contouring (53 vs. 41%), fewer vertebral derotation maneuvers (82 vs. 96%), and fewer implants used for compression/distraction maneuvers (1.18 and 1.42 respectively) (p < 0.05). Implant placement at alternate levels or dropout of convex implants above and below the apical area was most frequently considered acceptable (> 70% agreement).

CONCLUSIONS

Implant position and number affect surgeons correction maneuvers selection. For low implant density constructs, dropout in the convexity and particularly in the periapical region is accepted by surgeons, with minor influence on planned correction maneuvers. Thus, preoperative implant planning must take into account which anchor points are needed for desired correction maneuvers.

Implant Density at the Apex Is More Important Than Overall Implant Density for 3D Correction in Thoracic Adolescent Idiopathic Scoliosis Using Rod Derotation and En Bloc Vertebral Derotation Technique

STUDY DESIGN

Biomechanical analysis of 3D correction and bone-screw forces through numerical simulations of scoliosis instrumentation with different pedicle screw patterns.

OBJECTIVE

To analyze the effect of different screw densities and distributions on 3D correction and bone-screw forces in adolescent idiopathic scoliosis (AIS) instrumentation.

SUMMARY OF BACKGROUND DATA

Instrumentation constructs with various numbers of pedicle screws and patterns have been proposed for thoracic AIS instrumentation. However, systematic biomechanical studies have not yet been completed on the appropriate screw patterns for optimal 3D correction.

METHODS

Patient-specific biomechanical models of the spine were created for 10 AIS cases (Lenke 1). For each case, surgical instrumentation patterns were computationally simulated using respectively a reference screw pattern (two screws per level fused) and six alternative screw patterns with fewer screws. Simulated surgical maneuvers and model definition were unchanged between simulations except the number and distribution of screws. 3D correction and bone-screw forces were compared.

RESULTS

A total of 140 posterior instrumentations were computationally simulated. Mean corrections in the coronal and sagittal planes with alternative screw patterns were within 4° to the reference pattern. Increasing screw density in the apical region from one to two screws per level improved percent apical vertebral rotation (AVR) correction (r = 0.887, P < 0.05). Average bone-screw force associated with the reference screw pattern was 243N ± 54N and those with the alternative screw patterns were 11% to 48% lower.

CONCLUSION

Compared with the reference maximal screw density pattern, alternative screw patterns allowed similar corrections in the coronal and sagittal planes. AVR correction was strongly correlated with screw density in the apical region; AVR correction varied significantly with screw patterns of the same overall screw density when an en bloc vertebral derotation technique was simulated. High screw density tended to overconstrain the instrumented spine and resulted in higher forces at the bone-screw interface.

LEVEL OF EVIDENCE

N/A.

The Adult Deformity Surgery Complexity Index (ADSCI): a valid tool to quantify the complexity of posterior adult spinal deformity surgery and predict postoperative complications

BACKGROUND CONTEXT

In 2008, Mirza et al. designed and validated the first and only index capable of quantifying the complexity of spine surgery. However, this index is not fully applicable to adult spinal deformity (ASD) surgery as it does not include the surgical techniques most commonly used and most strongly associated with perioperative complications in patients with ASD.

PURPOSE

The objective of this study is to develop an "Adult Deformity Surgery Complexity Index" (ADSCI) to quantify objectively the complexity of the ASD posterior surgery.

STUDY DESIGN/SETTING

This is an expert consensus (Delphi process) and retrospective analysis of prospectively collected data using multiple regression models.

PATIENT SAMPLE

Patients were prospectively enrolled in two comparable multicenter ASD databases sharing the same inclusion criteria.

OUTCOME MEASURES

The ADSCI was internally and externally validated using R² for intraoperative bleeding and length of surgery. Receiver operating characteristics (ROC) and area under the curve (AUC) analysis was used to assess the sensitivity and specificity of ADSCI.

METHODS

The development and validation of ADSCI was based on the construction and comparison of three different tools: ADSCI score was constructed by three rounds of expert consensus (ADSCI-Delphi) and two multiple regression models (ADSCI-RM-Simple and ADSCI-RM-Mixed). Their predictive capability was compared by means of R² values in the same subrogated of surgical complexity as in the Mirza index validation: intraoperative bleeding and duration of surgery. Sensitivity and specificity were evaluated using ROC curves and AUC analysis. The external validity was also examined by evaluating its predictive capability in another multicenter ASD database of comparable patients in the United States.

RESULTS

At the time of the study, the database included 1,325 patients. Four hundred seventy-five patients were eligible for the study, having been treated surgically using a posterior approach only (52.2 years standard deviation 20; 77.7% women; 85.4% American Society of Anesthesiologists I/II). Fifty-one international experts participated in the Delphi consensus process. The surgical variables selected by consensus and included in the equation were divided into actions and factors. Actions selected were number of fused segments, decompressions, interbody fusions, and cemented levels; number and type of posterior osteotomies; and use of pelvic fixation. The factors included were implant density, revision surgery, and team experience. ADSCI-RM-Mixed (regression model with Delphi formula interactions) provided the best estimates and predictive value, well above Mirza's invasiveness index. The ADSCI-RM-Mixed, with greater AUCs (>0.70), was also the most sensitive and specific for both of the dependent variables studied and for complication prediction. ADSCI-RM-Mixed obtained also the highest R² value in the validation cohort in predicting blood loss (R²=0.34) and surgical time (R²=0.26) with effect sizes similar to those for the derivation cohort.

CONCLUSIONS

The ADSCI is the first tool to be specifically developed for the preoperative assessment of the complexity of ASD surgery. This study confirms its validity, specificity, and sensitivity, and shows that it has greater predictive capability than the more generic Mirza invasiveness index. The ADSCI should be useful for quantitatively estimating the increased risk associated with more invasive surgery and adjusting for surgical case-mix when making safety comparisons in ASDS.

Analysis of correlation between regional implant density and the correction rate in treatment of Lenke 1A and 1B adolescent idiopathic scoliosis with pedicle screws

A retrospective study.The optimal implant density in patients with Lenke type 1 adolescent idiopathic scoliosis (AIS) is undefined, and there is no study reporting the correlation between the partitional implant density and the correction outcome.To determine whether the implant density in structural and nonstructural regions would affect the coronal correction outcome of Lenke 1A and 1B AIS.Preoperative general data and postoperative follow-up data of Lenke 1A and 1B AIS patients who received posterior fusion with the pedicle screw system were analyzed. Correlations between the implant density in structural and nonstructural regions and the correction rate of coronal Cobb angle, as well as between the correction rate and loss of the coronal correction angle during a 2-year follow-up period were analyzed. According to the implant density, the patients were classified into 2 groups: structural region group (including A1 and A2), and nonstructural region group (including B1 and B2). Differences in related parameters between the 2 groups were compared statistically.Except for the mean implant density, there was no statistical difference in the other parameters between group A1 and A2. In group B1 and B2, the correction rate of the main thoracic (MT) curve was 63.0% and 71.6% (P = .022), and the loss of the correction angle was 2.1° and 4.2°, respectively (P < .01), showing a statistical difference in the correction rate and postoperative angle loss of the MT curve between group B1 and B2.The correction rate of the MT curve at the coronal plane and postoperative loss of the correction angle were not related to the implant density in structural regions but may be related to the implant density in nonstructural regions in the treatment of Lenke type 1A and 1B AIS with pedicle screw instrumentation.

Comparison of low density and high density pedicle screw instrumentation in Lenke 1 adolescent idiopathic scoliosis

BACKGROUND

The correlation between implant density and deformity correction has not yet led to a precise conclusion in adolescent idiopathic scoliosis (AIS). The aim of this study was to evaluate the effects of low density (LD) and high density (HD) pedicle screw instrumentation in terms of the clinical, radiological and Scoliosis Research Society (SRS)-22 outcomes in Lenke 1 AIS.

METHODS

We retrospectively reviewed 62 consecutive Lenke 1 AIS patients who underwent posterior spinal arthrodesis using all-pedicle screw instrumentation with a minimum follow-up of 24 months. The implant density was defined as the number of screws per spinal level fused. Patients were then divided into two groups according to the average implant density for the entire study. The LD group (n = 28) had fewer than 1.61 screws per level, while the HD group (n = 34) had more than 1.61 screws per level. The radiographs were analysed preoperatively, postoperatively and at final follow-up. The perioperative and SRS-22 outcomes were also assessed. Independent sample t tests were used between the two groups.

RESULTS

Comparisons between the two groups showed no significant differences in the correction of the main thoracic curve and thoracic kyphosis, blood transfusion, hospital stay, and SRS-22 scores. Compared with the HD group, there was a decreased operating time (278.4 vs. 331.0 min, p = 0.004) and decreased blood loss (823.6 vs. 1010.9 ml, p = 0.048), pedicle screws needed (15.1 vs. 19.6, p < 0.001), and implant costs ($10,191.0 vs. $13,577.3, p = 0.003) in the LD group.

CONCLUSIONS

Both low density and high density pedicle screw instrumentation achieved satisfactory deformity correction in Lenke 1 AIS patients. However, the operating time and blood loss were reduced, and the implant costs were decreased with the use of low screw density constructs.

A biomechanical investigation of different screw head designs for vertebral derotation in scoliosis surgery

BACKGROUND CONTEXT

The posterior pedicle screw-rod system, which is widely used to correct spinal deformities, achieves a good correction rate in the frontal and coronal planes but not in the axial plane. Direct vertebral derotation (DVD) was developed to correct axial plane deformities. However, the design of screw head and body connection, in terms of monoaxial, polyaxial, and uniplanar screw, may influence the efficiency of DVD.

PURPOSE

This study compared the efficiency of a newly designed uniplanar screw with that of monoaxial and polyaxial screws in the DVD maneuver.

STUDY DESIGN

A porcine spine model and monoaxial, polyaxial, and uniplanar screws were used to examine the biomechanics of the DVD maneuver.

METHODS

Six T7-T13 porcine thoracic spine segments were used as test specimens in this study. Pedicle screws were inserted in the left pedicles of the T9-T11 spinal segments and then connected with a rod. Three types of pedicle screws with different screw head designs (monoaxial, polyaxial, and uniplanar) were employed in this study. The material testing system (MTS) machine generated a rotational moment through the derotational tube on the T10 (apical body) pedicle screw, which simulated the motion applied during the surgical vertebral derotational procedure. The pedicle strain and the kinematics of the vertebral body and derotational tube were recorded to evaluate the derotational efficiency of different pedicle screw head designs.

RESULTS

The variances of the derotation for the monoaxial, polyaxial, and uniplanar screws were 2.22°±1.43°, 32.23°±2.26°, and 4.75°±1.60°, respectively; the derotation efficiency was 0.65, 0.51, and 0.12, respectively, when the torques of the spinal constructs reached 3 Nm. The rotational variance of the polyaxial screw was statistically greater than that of the monoaxial and uniplanar screws (p<.05). The maximum micro-strains of the pedicles for the monoaxial, polyaxial, and uniplanar screws were 1,067.45±550.35, 747.68±393.56, and 663.55±271.04, respectively, with no statistically significant differences (p>.05).

CONCLUSIONS

The screw head design played an important role in the efficiency and variance of the derotation during the DVD maneuver. The derotational efficiency of the newly designed uniplanar screw was closer to that of the monoaxial screw group than to that of the polyaxial screw group. The polyaxial screw was inferior to DVD owing to a derotational variance between the derotational tube and the apical body that was correlated with the range of motion of the screw head. In the present study, the pedicle strain was similar in all groups. However, the pedicle strain of the uniplanar screw group was lower than that of the monoaxial screw group and was similar to that of the polyaxial screw group when the angle of rotation of the apical body increased.

In vitro analysis of the segmental flexibility of the thoracic spine

Basic knowledge about the thoracic spinal flexibility is limited and to the authors' knowledge, no in vitro studies have examined the flexibility of every thoracic spinal segment under standardized experimental conditions using pure moments. In our in vitro study, 68 human thoracic functional spinal units including the costovertebral joints (at least n = 6 functional spinal units per segment from T1-T2 to T11-T12) were loaded with pure moments of ±7.5 Nm in flexion/extension, lateral bending, and axial rotation in a custom-built spine tester to analyze range of motion (ROM) and neutral zone (NZ). ROM and NZ showed symmetric motion behavior in all loading planes. In each loading direction, the segment T1-T2 exhibited the highest ROM. In flexion/extension, the whole thoracic region, with exception of T1-T2 (14°), had an average ROM between 6° and 8°. In lateral bending, the upper thoracic region (T1-T7) was, with an average ROM between 10° and 12°, more flexible than the lower thoracic region (T7-T12) with an average ROM between 8° and 9°. In axial rotation, the thoracic region offered the highest overall flexibility with an average ROM between 10° and 12° in the upper and middle thoracic spine (T1-T10) and between 7° and 8° in the lower thoracic spine (T10-T12), while a trend of continuous decrease of ROM could be observed in the lower thoracic region (T7-T12). Comparing these ROM values with those in literature, they agree that ROM is lowest in flexion/extension and highest in axial rotation, as well as decreasing in the lower segments in axial rotation. Differences were found in flexion/extension and lateral bending in the lower segments, where, in contrast to the literature, no increase of the ROM from superior to inferior segments was found. The data of this in vitro study could be used for the validation of numerical models and the design of further in vitro studies of the thoracic spine without the rib cage, the verification of animal models, as well as the interpretation of already published human in vitro data.

Correction Capability in the 3 Anatomic Planes of Different Pedicle Screw Designs in Scoliosis Instrumentation

STUDY DESIGN

Computer simulations to compare the correction capabilities of different pedicle screws in adolescent idiopathic scoliosis (AIS) instrumentations.

OBJECTIVE

To compare the correction and resulting bone-screw forces associated with different pedicle screws in scoliosis instrumentations.

SUMMARY OF BACKGROUND DATA

Pedicle screw fixation is widely used in surgical instrumentation for spinal deformity treatment. Screw design, correction philosophies, and surgical techniques are constantly evolving to achieve better control of the vertebrae and correction of the spinal deformity. Yet, there remains a lack of biomechanical studies that quantify the effects and advantages of different screw designs in terms of correction kinematics.

METHODS

The correction capabilities of fixed-angle, multiaxial, uniaxial, and saddle axial screws were kinematically analyzed, simulated, and compared. These simulations were based on the screw patterns and correction techniques proposed by 2 experienced surgeons for 2 AIS cases. Additional instrumentations were assessed to compare the correction and resulting bone-screw forces associated with each type of screw.

RESULTS

The fixed-angle, uniaxial and saddle axial screws had similar kinematic behavior and performed better than multiaxial screws in the coronal and transverse planes (8% and 30% greater simulated corrections, respectively). Uniaxial and multiaxial screws were less effective than fixed-angle and saddle axial screws in transmitting compression/distraction to the anterior spine because of their sagittal plane mobility between the screw head and shank. Only the saddle axial screws allow vertebra angle in the sagittal plane to be independently adjusted.

CONCLUSIONS

Pedicle screws of different designs performed differently for deformity corrections or for compensating screw placement variations in different anatomic planes. For a given AIS case, screw types should be determined based on the particular instrumentation objectives, the deformity's stiffness and characteristics so as to make the best of the screw designs.

Biomechanical effect of pedicle screw distribution in AIS instrumentation using a segmental translation technique: computer modeling and simulation

BACKGROUND

Efforts to select the appropriate number of implants in adolescent idiopathic scoliosis (AIS) instrumentation are hampered by a lack of biomechanical studies. The objective was to biomechanically evaluate screw density at different regions in the curve for AIS correction to test the hypothesis that alternative screw patterns do not compromise anticipated correction in AIS when using a segmental translation technique.

METHODS

Instrumentation simulations were computationally performed for 10 AIS cases. We simulated simultaneous concave and convex segmental translation for a reference screw pattern (bilateral polyaxial pedicle screws with dorsal height adjustability at every level fused) and four alternative patterns; screws were dropped respectively on convex or concave side at alternate levels or at the periapical levels (21 to 25% fewer screws). Predicted deformity correction and screw forces were compared.

RESULTS

Final simulated Cobb angle differences with the alternative screw patterns varied between 1° to 5° (39 simulations) and 8° (1 simulation) compared to the reference maximal density screw pattern. Thoracic kyphosis and apical vertebral rotation were within 2° of the reference screw pattern. Screw forces were 76 ± 43 N, 96 ± 58 N, 90 ± 54 N, 82 ± 33 N, and 79 ± 42 N, respectively, for the reference screw pattern and screw dropouts at convex alternate levels, concave alternate levels, convex periapical levels, and concave periapical levels. Bone-screw forces for the alternative patterns were higher than the reference pattern (p < 0.0003). There was no statistical bone-screw force difference between convex and concave alternate dropouts and between convex and concave periapical dropouts (p > 0.28). Alternate dropout screw forces were higher than periapical dropouts (p < 0.05).

CONCLUSIONS

Using a simultaneous segmental translation technique, deformity correction can be achieved with 23% fewer screws than maximal density screw pattern, but resulted in 25% higher bone-screw forces. Screw dropouts could be either on the convex side or on the concave side at alternate levels or at periapical levels. Periapical screw dropouts may more likely result in lower bone-screw force increase than alternate level screw dropouts.

Does higher screw density improve radiographic and clinical outcomes in adolescent idiopathic scoliosis? A systematic review and pooled analysis

OBJECTIVE The radiographic and clinical outcomes of low-density (LD) versus high-density (HD) screw constructs in patients with adolescent idiopathic scoliosis (AIS) treated with all-pedicle screw constructs are still controversial. A systematic review and pooled analysis were performed to compare radiographic, perioperative, and quality-of-life (QOL) outcomes and complications in patients with moderate AIS treated with LD or HD screw constructs. METHODS The MEDLINE, Embase, and Web of Science databases were searched for English-language articles addressing LD versus HD screw constructs in AIS patients treated with all-pedicle screw constructs. The division of LD and HD groups was based on relative screw density and screw techniques. This systematic analysis strictly followed the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines, and all articles included in the analysis met the criteria specified in the guidelines. Two reviewers independently assessed the quality of the studies using the Newcastle-Ottawa Scale. Date on radiographic, perioperative, and QOL outcomes and complications were extracted from the included studies. RESULTS Twelve studies, involving a total of 827 patients (480 treated with LD constructs, 347 with HD), were analyzed-1 randomized controlled trial, 1 quasi-randomized controlled trial, and 10 retrospective studies. The patients' age at surgery, preoperative Cobb angle of the major curve, amount of thoracic kyphosis, and major curve flexibility were reasonably distributed, and no statistically significant differences were found. Regarding the outcomes at most recent follow-up, there were no significant differences in the Cobb angle of the major curve (mean difference 0.96°, 95% CI -0.06° to 1.98°, p = 0.06, I² = 1%), major curve correction (mean difference -0.72%, 95% CI -2.96% to 1.52%, p = 0.53, I² = 0%), thoracic kyphosis (mean difference -1.67°, 95% CI -4.59° to 1.25°, p = 0.26, I² = 79%), complications (odds ratio [OR] 0.66, 95% CI 0.31-1.42, p = 0.29, I² = 0%), and QOL outcomes. Reduced operative time (mean difference -48.56 minutes, 95% CI -82.69 to -14.43 minutes, p = 0.005, I² = 87%), blood loss (mean difference -77.85 ml, 95% CI -153.10 to -2.60 ml, p = 0.04, I² = 0%), and hospital charges (mean difference -$5.92K, 95% CI -$6.59K to -$5.26K, p < 0.00001, I² = 0%) were found in the LD group, compared with the HD group. CONCLUSIONS LD and HD screw constructs are both associated with satisfactory radiographic and QOL outcomes with few complications. This study supports the use of LD screw constructs for the treatment of moderate AIS, because they resulted in reduced operative time, blood loss, and hospital charges while maintaining radiographic and QOL outcomes and complication rates similar to those achieved with HD screw constructs.

Comparison of Consecutive, Interval, and Skipped Pedicle Screw Techniques in Moderate Lenke Type 1 Adolescent Idiopathic Scoliosis

OBJECTIVE

To compare perioperative, radiographic, and Scoliosis Research Society-22 (SRS-22) outcomes of consecutive, interval, and skipped pedicle screw techniques in patients with moderate Lenke type 1 adolescent idiopathic scoliosis (AIS).

METHODS

We retrospectively reviewed 65 consecutive moderate Lenke type 1 AIS patients at a single institution using all-pedicle screw constructs, with a minimum of 2 years of follow-up. In the consecutive group (C group, n = 22), pedicle screws were instrumented at consecutive levels bilaterally. In the interval group (I group, n = 18), pedicle screws were placed at every level on the concave side while skipping levels on the convex side. In the skipped group (S group, n = 25), pedicle screws were instrumented by skipping levels bilaterally. Perioperative, radiographic, and SRS-22 measurements were analyzed with a 1-way analysis of variance.

RESULTS

No significant differences were found in length of hospital stay, fused levels, coronal correction, and SRS-22 scores among the 3 groups. Increased surgery time was found in the C group compared with the I and S groups (P = 0.001 and P = 0.005, respectively). Decreased blood loss and blood transfusions were found in the S group compared with the C group (P = 0.04 and P = 0.047, respectively). Decreased implant costs were found in the S group compared with the C and I groups (P < 0.001 and P = 0.03, respectively).

CONCLUSIONS

Consecutive, interval, and skipped pedicle screw techniques all provide satisfactory deformity correction and SRS-22 outcomes with few complications. With better perioperative outcomes, interval and skipped pedicle screw techniques are the more cost-effective options for patients with moderate Lenke type 1 AIS.

The effect of metal density in thoracic adolescent idiopathic scoliosis

PURPOSE

Determine impact of metal density on curve correction and costs in thoracic adolescent idiopathic scoliosis (AIS). Ascertain if increased metal density is required for larger or stiffer curves.

METHODS

Multicentre retrospective case series of patients with Lenke 1-2 AIS treated with single-stage posterior only surgery using a standardized surgical technique; constructs using >80 % screws with variable metal density. All cases had >2-year follow up. Outcomes measures included coronal and sagittal radiographic outcomes, metal density (number of instrumented pedicles vs total available), fusion length and cost.

RESULTS

106 cases included 94 female. 78 Lenke 1. Mean age 14 years (9-26). Mean main thoracic (MT) Cobb angle 63° corrected to 22° (66 %). No significant correlations were present between metal density and: (a) coronal curve correction rates of the MT (r = 0.13, p = 0.19); (b) lumbar curve frontal correction (r = -0.15, p = 0.12); (c) correction index in MT curve (r = -0.10, p = 0.32); and (d) correction index in lumbar curve (r = 0.11, p = 0.28). Metal density was not correlated with change in thoracic kyphosis (r = 0.22, p = 0.04) or lumbosacral lordosis (r = 0.27, p = 0.01). Longer fusions were associated with greater loss of thoracic kyphosis (r = -0.31, p = 0.003). Groups differing by preoperative curve size and stiffness had comparable corrections with similar metal density. The pedicle screw cost represented 21-29 % of overall cost of inpatient treatment depending on metal density.

CONCLUSIONS

Metal density affects cost but not the coronal and sagittal correction of thoracic AIS. Neither larger nor stiffer curves necessitate high metal density.

Implant distribution in surgically instrumented Lenke 1 adolescent idiopathic scoliosis: does it affect curve correction?

STUDY DESIGN

Retrospective review of prospective multicenter database of patients with adolescent idiopathic scoliosis who underwent posterior spinal fusion.

OBJECTIVE

To analyze implant distribution in surgically instrumented Lenke 1 patients and evaluate how it impacts curve correction.

SUMMARY OF BACKGROUND DATA

Although pedicle screw constructs have demonstrated successful surgical results, the optimal pedicle screw density and configuration remain unclear.

METHODS

A total of 279 patients with adolescent idiopathic scoliosis treated with pedicle screws were reviewed. Implant density was computed for each side of the instrumented segment, which was divided into 5 regions: distal and proximal ends (upper/lower instrumented vertebra +1 adjacent vertebra), apical region (apex ± 1 vertebra), and the 2 regions in between (upper/lower periapical). Centralized measurement of Cobb angle and thoracic kyphosis was performed on preoperative and at 1-year postoperative radiographs as well as percent curve flexibility.

RESULTS

The mean implant density was 1.66 implants per level fused (1.08 to 2) with greater available pedicles filled on the concavity (92%, 53%-100%) compared with the convex side (73%, 23%-100%, P < 0.01). The concave distal end region had the highest density with 99% of pedicles filled (P < 0.01), followed by the other concave regions and the convex distal end region (88%-94%) (P > 0.05). Other convex regions of the construct had less instrumentation, with only 54% to 78% of pedicles instrumented (P < 0.01). Implant density in the concave apical region (69%, 23%-100%) had a positive effect on curve correction (P = 0.002, R = 0.19).

CONCLUSION

Significant variability exists in implant distribution with the greatest variation on the convex side and lowest implant density used in the periapical convex regions. Only instrumentation at the concave side, particularly at the apical region, was associated with curve correction. This suggests that for a low implant density construct, the best regions for planned screw dropout may be in the periapical convexity.

LEVEL OF EVIDENCE

3.

Skipped versus consecutive pedicle screw constructs for correction of Lenke 1 curves

PURPOSE

Thoracic pedicle screws provide superior curve correction to hook and wire constructs in adolescent idiopathic scoliosis, while increasing cost. The number of implants required for best correction and outcome has not yet been determined.

METHODS

We retrospectively reviewed pre- and post-operative radiographs and self-reported outcome measures in an age- and curve-matched cohort of 40 patients with Lenke I AIS who underwent selective fusions between T3/4 and L1. Twenty patients were treated with thoracic pedicle screws at every level bilaterally (CON) and 20 patients with screws at every level on the concave side and skipped levels on the convex side of the curve (SKP). All patients had a minimum 2-year follow-up. Radiographs were assessed for coronal and sagittal curvatures, as well as thoracic torsion and vertebral rotation. Health-related quality of life was assessed using the SRS-22 instrument. Instrumentation cost data were collected for each case.

RESULTS

Postoperative follow-up averaged 28 months for the CON group and 29 months for the SKP group. No statistically significant differences were found between groups with respect to age and pre- and post-operative radiographic parameters. Both constructs provided acceptable correction of the main thoracic curves (66.9 vs. 66.6 %, CON group and SKP group, respectively; p = 0.92), and spontaneous correction of the proximal thoracic (41.5 vs. 41.1 %; p = 0.92) and thoracolumbar/lumbar curves (54.8 vs. 54.3 %; p = 0.92). No significant difference was found in postoperative SRS-22 scores (96 vs. 94.3; p = 0.34). The CON group cost for instrumentation was significantly higher than the SKP group ($19,500 vs. $13,300; p = 0.002). There was no statistically significant difference in operating room times between groups.

CONCLUSION

Both construct types provide excellent coronal correction and sagittal balance, with no significant differences in radiographic findings or clinical outcomes. A significant decrease in cost was found with use of skipped screw constructs.

CoCr rods provide better frontal correction of adolescent idiopathic scoliosis treated by all-pedicle screw fixation

PURPOSE

Pedicle screw fixation is considered biomechanically advantageous in adolescent idiopathic scoliosis (AIS) correction, because it uses as an anchor the pedicle, which is the hardest part of the vertebral body. The ability of the rod to correct and hold the correction is a key factor in the selection of rod material. The goal of this study was to compare the results obtained by stainless steel (SS) and cobalt-chromium (CoCr) rods materials for the treatment of AIS curves.

METHODS

Ninety patients were retrospectively included. Sixty-four patients (group 1) were operated on using CoCr rods. Twenty-six patients (group 2) were operated on using SS rods. All the patients were treated by the same surgeon using all-pedicle screw constructs.

RESULTS

In group 1, the correction was respectively 41.03° and 35.78° for main and secondary curves. In group 2, the correction was respectively 30.98° and 24.42° for main and secondary curves. Statistical analysis showed improved correction rates in patients operated with CoCr rods for main (P < 0.0001) and secondary (P = 0.0003) curves with a lower loss of correction at final follow-up. Regarding the sagittal profile, postoperative T4T12 thoracic kyphosis was 28.04° in CoCr group compared to 22.79° in SS group (P = 0. 0.0038).

DISCUSSION

The present study confirms the ability of the all-pedicle screw construct to reach the maximum coronal plane correction and prevent deformity progression while maintaining balance. CoCr rods have the ability to exert higher corrective forces on the spine with relatively small amounts of rod deformation. Our findings confirm that CoCr rods have the ability to produce higher correction rates in frontal plane compared to SS rods of the same diameter.

Are More Screws Better? A Systematic Review of Anchor Density and Curve Correction in Adolescent Idiopathic Scoliosis

STUDY DESIGN

Systematic review of clinical studies.

OBJECTIVES

To critically evaluate existing literature to determine whether increased anchor or implant density (screws, wires, and hooks per level fused) results in improved curve correction for adolescent idiopathic scoliosis (AIS) surgery.

SUMMARY OF BACKGROUND DATA

Wide variability exists in the number of screws used for AIS surgery. High numbers of pedicle screws are increasingly used, but there is limited evidence to support this as best practice.

METHODS

Online English-language databases were searched to identify articles addressing anchor density. Articles were reviewed for anchor type/number, radiographic measures, and patient-reported outcomes.

RESULTS

Of 196 references identified, 10 studies totaling 929 patients met the inclusion criteria. Reported mean anchor density varied from 1.06 to 2.0 implants per level fused. Mean percent coronal curve correction varied from 64% to 70%. Two studies (463 patients) analyzed hook, hybrid (combined hooks and screws), and screw constructs as a single cohort. Both found increased correction with high-density constructs (p = .01, p < .001), perhaps as a result of the hooks and hybrid constructs. Eight retrospective studies and 1 prospective randomized, controlled trial had predominantly screw constructs (466 patients). Increased anchor density was not associated with improved curve correction. The studies evaluating screw density are significantly underpowered to detect a difference in curve correction.

CONCLUSIONS

Wide heterogeneity in anchor density exists in the surgical treatment of AIS. Reports evaluating the effects of increased anchor density are mostly retrospective and significantly underpowered to detect a difference in curve correction and patient outcomes. At this time, there is insufficient evidence to show that anchor density affects clinical outcomes in AIS.

Biomechanical analysis of vertebral derotation techniques for the surgical correction of thoracic scoliosis. A numerical study through case simulations and a sensitivity analysis

STUDY DESIGN

Biomechanical analysis of vertebral derotation techniques for the surgical correction of thoracic scoliosis.

OBJECTIVE

To model and analyze vertebral derotation maneuvers biomechanically to maximize the tridimensional correction of scoliosis and minimize the implant-vertebra forces.

SUMMARY OF BACKGROUND DATA

Vertebral derotation techniques were recently developed to improve the correction of scoliotic deformities in the transverse plane. Those techniques consist in applying a combination of moments and forces using a vertebral derotation device, cohesively linked to the thoracic apical pedicle screws, to derotate the spine and the rib cage. However, many variations of the technique exist and the correction mechanisms are not fully understood to achieve an optimal correction of scoliosis.

METHODS

A biomechanical model was developed to simulate the instrumentation surgery numerically of 4 Lenke type 1 patients with scoliosis, instrumented using a vertebral derotation device and vertebral derotation maneuvers as major correction technique. Then, for each case, 32 additional instrumentation surgical procedures were simulated to better understand the biomechanics of the vertebral derotation technique, varying the implant type and density, the number of derotation levels, the vertebral derotation angle and the posteriorly oriented force applied during the maneuver.

RESULTS

On average, among 32 additional simulations, there was an important variability of the resulting apical vertebral rotation (15°) and the mean resultant implant-vertebra force (205 N) but little variability for the main thoracic Cobb angle (6°) and the thoracic kyphosis (4°). The implant type, the implant density and the vertebral derotation angle were the parameters that most influenced the correction of scoliosis. The correction in the coronal and transverse planes was improved using monoaxial pedicle screw density of 2 and a bilateral vertebral derotation maneuver on 3 levels at the apex of the thoracic curve, with an extra 15° applied on the vertebral derotation device. When reducing the implant density by 50%, it was possible to reduce the mean implant-vertebra forces while keeping a good correction.

CONCLUSION

Biomechanically, it is possible to significantly improve the correction of thoracic scoliotic deformities, particularly in the transverse plane, when using vertebral derotation maneuvers.