Satellite Rod Research Articles

Evaluating the biomechanical effects of pedicle subtraction osteotomy at different lumbar levels: a finite element investigation

BACKGROUNDPedicle subtraction osteotomy (PSO) is effective for correcting spinal malalignment but is associated with high complication rates. The biomechanical effect of different PSO levels remains unclear, and no finite element (FE) analysis has compared L2-, L3-, L4-, and L5-PSOs. PURPOSETo assess the effects of PSO level on the spine's global range of motion, stresses on posterior instrumentation, load sharing with the anterior column, and proximal junctional stresses. STUDY DESIGNA computational biomechanical analysis. METHODSA validated 3D spinopelvic FE model (T10-Pelvis) was used to perform PSOs at L2, L3, L4 and L5. Each model was instrumented with a 4-rod configuration (primary rods + in-line satellite rods) from T11-Pelvis. Simulation included a 2-step analysis; (1) applying 300 N to thoracic, 400 N to lumbar, and 400 N to sacrum, and (2) applying a 7.5 Nm moment to the top endplate of the T10 vertebral body. Acetabulum surfaces were fixed in all degrees of freedom. The range of motion, spinopelvic parameters (lumbar lordosis [LL], sacral slope [SS], pelvic incidence [PI], and pelvic tilt [PT]), PSO force, and von Mises stresses were measured. All models were compared with the L3-PSO model and percentage differences were captured. RESULTSCompared to the intact alignment: LL increased by 48%, 45%, 59%, and 56% in the L2-, L3-, L4-, and L5-PSO models; SS increased by 25%, 15%, and 11% while PT decreased by 76%, 53%, and 45% in L2-, L3-, and L4-PSOs (SS and PT approximated intact model in L5-PSO); Lumbar osteotomy did not affect the PI. Compared to L3-PSO: L2-, L4-, and L5-PSOs showed up to 32%, 34%, and 34% lower global ROM. The least T10-T11 ROM was observed in L5-PSO. The left and right SIJ ROM were approximately similar in each model. Amongst all, the L5-PSO model showed the least ROM at the SIJ. Compared to L3-PSO, the L2-, L4-, and L5-PSO models showed up to 67%, 61%, and 78% reduced stresses at the UIV, respectively. Minimum stress at UIV+ was observed in the L3-PSO model. The L2-and L3-PSOs showed the maximum PSO force. The L5-PSO model showed the lowest stresses on the primary rods in all motions. CONCLUSIONOur FE investigation indicates that L5-PSO results in the greatest lumbar lordosis and lowest global, SIJ, and T10-T11 ROMs and stresses on the primary rods, suggesting potential mechanical benefits in reducing the risk of rod breakage. However, L4- and L5-PSOs led to the least force across the osteotomy site, which may increase the risk of pseudarthrosis. These findings provide biomechanical insights that may inform surgical planning, though further clinical investigation is essential to determine the optimal PSO level and validate these results. CLINICAL SIGNIFICANCEUnderstanding the biomechanical impact of PSO level is crucial for optimizing surgical outcomes and minimizing the risks of postoperative complications.

Application of Multi-Rod Constructs for the Revision of Thoracolumbar Fractures.

The revision procedure for failure of internal fixation after thoracolumbar fracture is controversial. Combined anterior and posterior surgery is associated with higher risk more intraoperative bleeding and tissue damage. The success rate of simple anterior surgery needs further confirmation, and posterior surgery lacks stability of internal fixation. This study evaluates the feasibility and surgical effect of multi-rod constructs in the revision of thoracolumbar fractures. Eleven patients with thoracolumbar fractures who underwent previous construct failure and were treated with revision and internal fixation with the multi-rod technique from March 2017 to September 2018 were analyzed. The original internal fixation was removed and replaced in the medial insertion of satellite rods and bone graft. The average follow-up time was 15.97 months. The intraoperation blood loss, the time of the operation, activation and discharge and the rate of rod fracture were calculated. The sagittal Cobb angle before revision, after revision and at the last follow-up were compared. The clinical effect was evaluated by visual analogue scale (VAS) and Oswestry Disability Index questionnaire (ODI). The average operation time was 107 min, the intraoperative blood loss was 131.81 mL, the active time was 1.59 days, and the discharge time was 10.89 days. No rod fractured again during the follow-up period. The paired t-test was used to compare the Cobb angle, VAS score, and ODI before and after surgery. There was significant difference in the sagittal Cobb angle between the pre-revision and the posterior sagittal position (p = 0.000), and no significant difference was found between post-revision and last follow-up (p = 0.551). VAS and ODI were greatly improved at the last follow-up. The literature on revision of thoracolumbar fractures is insufficient and comprises varying opinions. This paper proposes a new treatment option. The application of the multi-rod constructs in the revision of thoracolumbar fractures is safe, simple, and effective and might provide guidance for future clinical work.

Connecting the S2 alar-iliac screw head to the satellite rod for surgical correction of degenerative sagittal imbalance.

The S2AI screw technique has several advantages over the conventional iliac screw fixation technique. However, connecting the S2AI screw head to the main rod is difficult due to its medial entry point. We introduce a new technique for connecting the S2AI screw head to a satellite rod and compare it with the conventional method of connecting the S2AI screw to the main rod. Seventy-four patients who underwent S2AI fixation for degenerative sagittal imbalance and were followed up for ≥ 2years were included. All the patients underwent long fusion from T9 or T10 to the pelvis. The S2AI screw head was connected to the satellite rod (SS group) in 43 patients and the main rod (SM group) in 31 patients. In the SS group, the satellite rod was placed medial to the main rod and connected by the S2AI screw and domino connectors. In the SM group, the main rod was connected directly to the S2AI screw head and supported by accessory rods. Radiographic and clinical outcomes were evaluated in both groups. There were no significant differences in postoperative complications, including proximal junctional failure, proximal junctional kyphosis, rod breakage, screw loosening, wound problems, and infection between the two groups. Furthermore, the correction power of sagittal deformity and clinical results in the SS group were comparable to those in the SM group. Connecting the S2AI screw to the satellite rod is a convenient method comparable to the conventional S2AI connection method in terms of radiological and clinical outcomes.

Use of supplemental rod constructs in adult spinal deformity surgery

Operative treatment for adult spinal deformity (ASD) commonly involves long posterior instrumented fusions with primary rods spanning from the base of the construct to the upper instrumented vertebra. Over the past decade, additional supplemental rods have been increasingly utilized to bolster the primary instrumentation and mitigate risk of primary rod fracture/pseudarthrosis at areas of high biomechanical stress (e.g., 3-column osteotomy [3CO], multiple posterior column osteotomies [PCOs], lumbosacral junction). Supplemental rods for 3CO include satellite rods (4-rod technique with 2 deeply recessed short rods independently attached to pedicle screws across the 3CO), accessory rods (attached to primary rods via side-to-side connectors), and delta rods (accessory rods contoured only at the proximal and distal attachments to primary rods). Utilizing more than 4 rods across a 3CO may increase posterior construct stability; however, diminished load transfer to the anterior vertebral column may increase risk of nonunion and instrumentation failure. Similar supplemental rod constructs can be utilized to support multiple PCOs and/or the lumbosacral junction. We generally recommend using bilateral accessory rods for a total of 4 rods to support the lumbosacral junction (2 accessory rods and 2 primary rods). The novel “kickstand rod” can help facilitate coronal correction and/or function as an accessory rod distally anchored to an independent iliac screw; appropriate nomenclature is “iliac accessory rod” in cases without true kickstand distraction. In this narrative review, we aim to (1) provide a brief historical overview of supplemental rod constructs, (2) describe current indications for supplemental rods, and (3) report our results from a longitudinal analysis (2008–2020) of supplemental rod constructs used by International Spine Study Group (ISSG) surgeons.

Assessing Alignment Using GAP Score and Complications for Pedicle Subtraction Osteotomy Revision Surgeries for Sagittal Deformity in Previously Fused Spines Using a Satellite Rod Technique

Pedicle subtraction osteotomy (PSO) is a surgical technique to restore lumbar lordosis in patients with rigid sagittal deformity. Herein, we report on outcomes of a surgical technique utilizing a 4-rod PSO with satellite rods. A retrospective study was performed for a cohort of patients who underwent 4-rod PSO revision surgery for sagittal deformity. Procedures were performed by one surgeon at 2 different academic centers between 2004-2018. Alignment outcomes and complications specifically for revision surgeries in previously fused spines were assessed. 40 patients underwent PSO with the satellite rod technique (n= 29 at L3, 72.5%; n= 7 at L4, 17.5%). Mean PSO angle was 28.7 ± 7.6°. Two patients (5%) had rod fracture necessitating revision surgery at 32 and 34months. Three patients (7.5%) developed proximal junctional kyphosis (PJK), but none required revision. No patients developed pseudoarthrosis. Mean preoperative sagittal vertical axis (SVA) was 13.5 ± 7.3cm, which decreased to 4.8 ± 7.3cm (P= 0.002) postoperatively. Mean preoperative pelvic incidence-lumbar lordosis (PI-LL) mismatch improved from 35.9 ± 16.6° preoperatively to 11.8 ± 14.6° postoperatively (P < 0.001). Mean preoperative pelvic tilt (PT) improved from 34.7 ± 9.8° preoperatively to 30.1 ± 9.0° (P= 0.026) postoperatively. Mean global alignment and proportion (GAP) score improved from 10 ± 2 preoperatively to 8 ± 2 postoperatively (P < 0.001), which was still considered severely disproportioned. Rod failure rate after PSO with the 4-rod satellite configuration was low. SVA, PI-LL mismatch, and PT significantly improved for these previously-fused patients. Mean GAP score also improved, but was still considered severely disproportioned, likely due to the inability of PSO to correct low LL or PT in previously-fused spines.

Satellite Rod Fixation around Rod-fracture Area in Revision Surgery after Three-column Osteotomy for Severe Kyphoscoliosis.

Three-column osteotomy (3CO) is considered valuable and increasingly utilized in the surgical treatment of severe spine deformity while associated with high implant-related complications and revision risks. This study aims to evaluate the feasibility and clinical outcomes of satellite rod fixation used around the rod-fracture area in revision surgery due to rod fracture after 3CO. Twenty-five patients applying satellite rod fixation in revision surgery from August 2012 to May 2016 were retrospectively reviewed as the SR group. Patients undergoing revision surgery with traditional strategy after 3CO due to rod-fracture were selected as the TR group. Cobb angle, distance between C7 plumb line and center sacral vertical line (C7 PL-CSVL), global kyphosis (GK) and sagittal vertical axis (SVA) were assessed. Patients were required to fulfill the Scoliosis Research Society-22 questionnaire (SRS-22) at pre-revision and the last follow-up. The paired t test was used to analyze the difference among pre-revision, post-revision and last follow-up. There was no statistical difference in terms of age, gender, body mass index (BMI), fusion level at 1st surgery, and follow-up period between SR and TR group (all P > 0.05). The operation time (1.5 ± 0.7 h vs 3.2 ± 0.9 h, P < 0.001) and intraoperative blood loss (178 ± 51 mL vs 324 ± 96 mL, P < 0.001) were significantly higher in the TR group. Patients in both groups obtained obvious deformity correction after revision surgery. For patients in SR group, the coronal Cobb angle significantly improved from 27.9 ± 21.5° at pre-revision to 21.8 ± 16.6° at post-revision (P = 0.034). The C7 PL-CSVL decreased from 22.6 ± 14.3 mm to 21.3 ± 10.9 mm (P = 0.719). Similarly, improvement was attained in post-revision GK (25.8 ± 17.0° vs 20.2 ± 15.1°, P = 0.061). SVA was corrected from 35.6 ± 33.9 mm to 30.8 ± 24.3 mm after revision (P = 0.182). At the last follow-up, no significant correction loss was observed in both coronal and sagittal parameters (all P > 0.05). All patients responded to the SRS-22 questionnaire and all the domains showed improvements in different levels. As compared to the TR group, the SR group had significantly better pain and management satisfaction scores (all P < 0.05). Additionally, there was no reoccurrence of implant failure during follow-up and all patients achieved solid bony fusion in SR group. Satellite rod fixation around rod-fracture area is indicated for patients in the requirement of revision surgeries due to rod fracture after 3CO. Compared with traditional revision strategies, revision surgery with satellite rods, if patients are selected adequately, is a simpler procedure with less intraoperative blood loss and shorter operating time.

Sequential correction using satellite rod for the treatment of severe rigid spinal deformity: a retrospective study of 19 cases

ObjectivesThe purpose of this study was to evaluate the effectiveness of sequential correction using satellite rod in patients with severe rigid spinal deformity undergoing posterior-only PVCR.Methods19 patients with severe rigid spinal deformity who underwent PVCR at our center from January 2014 to December 2019 were reviewed. Radiographic measurements, including major coronal Cobb angle, kyphotic curve angle, coronal and sagittal balance were measured. Clinical results were noted, including the SRS-22 questionnaire, the Oswestry Disability Index score, and complications.ResultsTotal 19 patients were followed at least 2 years. The mean coronal Cobb angle decreased from 122.7° ± 13.17° to 57.89° ± 8.65° postoperatively, and to 58.42° ± 8.98° at final follow-up. Correction rate is 52.8%. The kyphotic curve angle improved from 102.2° ± 17.05° preoperatively to 39.68° ± 13.67° postoperatively, and to 37.74° ± 12.14° at final follow-up. Correction rate is 61.2%. Compared to preoperative results, apex vertebral translation, ODI and SRS-22 were significantly improved at the final follow-up.ConclusionsFor patients with severe rigid spinal deformities, sequential correction with an auxiliary satellite rod can effectively reduce surgical difficulty and improve correction rate.

Does number of rods matter? 4-, 5-, and 6-rods across a lumbar pedicle subtraction osteotomy: a finite element analysis

PurposeTo assess biomechanics of a lumbar PSO stabilized with different multi-rod constructs (4-, 5-, 6-rods) using satellite and accessory rods.MethodsA validated spinopelvic finite element model with a L3 PSO was used to evaluate the following constructs: 2 primary rods T10-pelvis (“Control”), two satellite rods (4-rod), two satellite rods + one accessory rod (5-rod), or two satellite rods + two accessory rods (6-rod). Data recorded included: ROM T10-S1 and L2-L4, von Mises stresses on primary, satellite, and accessory rods, factor of safety yield stress, and force across the PSO surfaces. Percent differences relative to Control were calculated.ResultsCompared to Control, 4-rods increased PSO flexion and extension. Lower PSO ROMs were observed for 5- and 6-rods compared to 4-rods. However, 4-rod (348.6 N) and 5-rod (343.2 N) showed higher PSO forces than 2-rods (336 N) and 6-rods had lower PSO forces (324.2 N). 5- and 6-rods led to the lowest rod von Mises stresses across the PSO. 6-rod had the maximum factor of safety on the primary rods.ConclusionsIn this finite element analysis, 4-rods reduced stresses on primary rods across a lumbar PSO. Although increased rigidity afforded by 5- and 6-rods decreased rod stresses, it resulted in less load transfer to the anterior vertebral column (particularly for 6-rod), which may not be favorable for the healing of the anterior column. A balance between the construct’s rigidity and anterior load sharing is essential.

183. The ideal satellite rod configuration spanning a lumbar pedicle subtraction osteotomy: a finite element analysis

183. The ideal satellite rod configuration spanning a lumbar pedicle subtraction osteotomy: a finite element analysis

P24. Is there a benefit to increasing number of rods (4-, 5-, and 6-) across a lumbar PSO? A finite element analysis

P24. Is there a benefit to increasing number of rods (4-, 5-, and 6-) across a lumbar PSO? A finite element analysis

Clinical Results of Utilizing the Satellite Rod Technique in Treating Ankylosing Spondylitis Kyphosis

ObjectiveAccording to the literature, there are no clinical reports documenting the use of the satellite rod technique in the treatment of ankylosing spondylitis kyphosis. The purpose of this retrospective study was to compare the clinical outcome of patients with ankylosing spondylitis kyphosis who adopted satellite rods versus those who did not.MethodsPatients with ankylosing spondylitis kyphosis who underwent one or two‐level pedicle subtraction osteotomy (PSO) were reviewed, and total of 119 patients (112 males and seven females, average age 39.89 ± 6.61 years) were eligible and included in this present study. Anterior–posterior and lateral full‐length spine X‐ray films were performed preoperatively and at the two‐year follow‐up visit. Global kyphosis (GK), lumbar lordosis (LL), thoracolumbar kyphosis (TLK), thoracic kyphosis (TK), and osteotomy angle (OA) were measured. The complications of every group of patients were collected. Pre‐ and postoperative health‐related quality of life instruments, including the Bath Ankylosing Spondylitis Functional Index (Basfi) and Scoliosis Research Society outcomes instrument‐22 (SRS‐22), were recorded. The patients were divided into three groups based on features of their osteotomy including PSO levels and whether the satellite rod technique was applied. Patients who underwent one‐level PSO without the satellite rod technique were categorized in the one‐level group. Patients who underwent one‐level PSO with the satellite rod technique were classified in the satellite rod group. Patients who underwent two‐level PSO without the satellite rod technique were included in the two‐level group. The paired sample t test was used to compare pre‐ and postoperative parameters. One‐way ANOVA was performed for multiple group comparisons.ResultsThe average follow‐up time is 29.31 ± 3.66 months. The patients' GK were significantly improved from 46.84 ± 20.37 degree to 3.31 ± 15.09 degree. OS achieved through each osteotomy segment of one‐level group (39.78 ± 12.29 degree) and satellite rods group (42.23 ± 9.82 degree), was larger than that of two‐level group (34.73 ± 7.54 and 28.85 ± 7.26 degree). There was no significant difference between the one‐level group and the satellite rod group in achieving the OS. Thirteen patients experienced different complications (10.92%). Three patients experienced rod fracture in the one‐level group. There was no rod fracture or screw failure in the satellite rod group or the two‐level group.ConclusionThe satellite rod technique is also recommended for patients who undergo PSO osteotomy to correct ankylosing spondylitis kyphosis deformities.

Biomechanical comparison of multi-rod constructs by satellite rod configurations (in-line vs. lateral) and screw types (monoaxial vs. polyaxial) spanning a lumbar pedicle subtraction osteotomy (PSO): is there an optimal configuration?

PurposeMulti-rod constructs are used commonly to stabilize pedicle subtraction osteotomies (PSO). This study aimed to evaluate biomechanical properties of different satellite rod configurations and effects of screw-type spanning a PSO.MethodsA validated 3D spinopelvic finite element model with a L3 PSO (30°) was used to evaluate 5 models: (1) Control (T10–pelvis + 2 rods); (2) lateral satellite rods connected via offsets to monoaxial screws (LatSat-Mono) or (3) polyaxial screws (LatSat-Poly); (4) in-line satellite rods connected to monoaxial screws (InSat-Mono) or (4) polyaxial screws (InSat-Poly). Global and PSO range of motions (ROM) were recorded. Rods’ von Mises stresses and PSO forces were recorded and the percent differences from Control were calculated.ResultsAll satellite rods (save InSat-Mono) increased PSO ROM and decreased primary rods’ von Mises stresses at the PSO. Lateral rods increased PSO forces (LatSat-Mono:347.1 N; LatSat-Poly:348.6 N; Control:336 N) and had relatively lower stresses, while in-line rods decreased PSO forces (InSat-Mono:280.1 N; InSat-Poly:330.7 N) and had relatively higher stresses. Relative to polyaxial screws, monoaxial screws further decreased PSO ROM, increased satellite rods’ stresses, and decreased PSO forces for in-line rods, but did not change PSO forces for lateral rods.ConclusionMulti-rod constructs using in-line and lateral satellite rods across a PSO reduced primary rods' stresses. Subtle differences in biomechanics suggest lateral satellite rods, irrespective of screw type, increase PSO forces and lower rod stresses compared to in-line satellite rods, which had a high degree of posterior instrumentation stress shielding and lower PSO forces. Clinical studies are warranted to determine if these findings influence clinical outcomes.

Failure of Posterior Lower Lumbar/Lumbosacral Hemi-Vertebra Resection: An Analysis of Reasons and Revision Strategies.

ObjectiveTo investigate the causes of failed primary surgery and the revision strategies for congenital scoliosis (CS) patients with lower lumbar/lumbosacral (LL/LS) hemi‐vertebra (HV).MethodsFifteen CS patients with LL/LS HV (seven females and eight males) with a mean age of 20.4 ± 10.4 years undergoing revision surgery in our center were retrospectively reviewed. The radiographic parameters including Cobb angle, distance between C7 plumb line and center sacral vertical line (C7PL‐CSVL), thoracic kyphosis (TK), lumbar lordosis (LL) and sagittal vertical axis (SVA) were assessed at pre‐revision, post‐revision and the last follow‐up. The causes of failure in primary operation, and radiographic and clinical outcomes of revision procedures were analyzed.ResultsThe revision rate of patients undergoing LL/LS HV resection and correction surgery was 11.4%. The average time interval between primary surgery and revision surgery was 18.2 ± 10.6 months. The operation duration and estimated blood loss of revision surgery were 194 ± 56 min and 326 ± 74 ml, respectively. Reasons for failed primary operations were as follows: internal fixation fracture in 10 cases, curve progression in two cases, implant loose in two cases and post‐operative coronal imbalance in one case. The post‐revision Cobb angle was significantly improved from 29.9° ± 8.3° to 18.7° ± 6.7° (P < 0.001) with a correction rate of 37.5% ± 12.6%. At the final follow‐up, the average Cobb angle was 18.9° ± 6.2° and the correction was well maintained (P = 0.788). The C7PL‐CSVL at pre‐revision, post‐revision and at last follow‐up were 23.2 ± 9.3 mm, 14.8 ± 4.8 mm and 14.9 ± 5.4 mm, respectively. Significant improvements (P = 0.004) were observed after revision surgery and there was no evident loss of correction (P = 0.703). There was no significant difference in TK, LL and SVA before and after revision surgery (all P > 0.05). At the last follow‐up, no significant correction loss of above coronal and sagittal parameters were observed (all P > 0.05). The revision methods were individualized according to the primary surgical procedures and the reasons for revision. The recommended revision strategies include incision of pseudarthrosis with sufficient bone graft, fixation of satellite rods, thorough residual HV excision, prolonged fusion to S2 and transforaminal lumbar interbody fusion at lumbosacral region. Solid bony fusion and no implant‐related complication were detected during the follow‐up.ConclusionsThe causes of revision surgery for patients with congenital scoliosis (CS) due to lumbosacral HV were verified and implant failure with pseudarthrosis was the main reason for failed primary operation.

The lexicon of multirod constructs in adult spinal deformity: a concise description of when, why, and how.

The use of multirod constructs in the setting of adult spinal deformity (ASD) began to prevent rod fracture and pseudarthrosis near the site of pedicle subtraction osteotomies (PSOs) and 3-column osteotomies (3COs). However, there has been unclear and inconsistent nomenclature, both clinically and in the literature, for the various techniques of supplemental rod implantation. In this review the authors aim to provide the first succinct lexicon of multirod constructs available for the treatment of ASD, providing a universal nomenclature and definition for each type of supplementary rod. The primary rod of ASD constructs is the longest rod that typically spans from the bottom of the construct to the upper instrumented vertebrae. The secondary rod is shorter than the primary rod, but is connected directly to pedicle screws, albeit fewer of them, and connects to the primary rod via lateral connectors or cross-linkers. Satellite rods are a 4-rod technique in which 2 rods span only the site of a 3CO via pedicle screws at the levels above and below, and are not connected to the primary rod (hence the term "satellite"). Accessory rods are connected to the primary rods via side connectors and buttress the primary rod in areas of high rod strain, such as at a 3CO or the lumbosacral junction. Delta rods span the site of a 3CO, typically a PSO, and are not contoured to the newly restored lordosis of the spine, thus buttressing the primary rod above and below a 3CO. The kickstand rod itself functions as an additional means of restoring coronal balance and is secured to a newly placed iliac screw on the side of truncal shift and connected to the primary rod; distracting against the kickstand then helps to correct the concavity of a coronal curve. The use of multirod constructs has dramatically increased over the last several years in parallel with the increasing prevalence of ASD correction surgery. However, ambiguity persists both clinically and in the literature regarding the nomenclature of each supplemental rod. This nomenclature of supplemental rods should help unify the lexicon of multirod constructs and generalize their usage in a variety of scientific and clinical scenarios.

Multiple-rod constructs and use of bone morphogenetic protein-2 in relation to lower rod fracture rates in 141 patients with adult spinal deformity who underwent lumbar pedicle subtraction osteotomy.

The purpose of this study was to compare rod fracture (RF) rates among three types of rod constructs (RCs) following lumbar pedicle subtraction osteotomy (PSO) for adult spinal deformity (ASD). A retrospective review of consecutive patients with adult spinal deformity who were treated with lumbar PSO between 2007 and 2017 was performed. The minimum follow-up was 2 years. Three RCs were compared: standard (2 main rods), satellite (2 main rods with satellite rod), and nested (2 main rods and 2 short rods spanning osteotomy). Outcomes examined included RF rate, time to RF, pseudarthrosis, and reoperation. Multivariate analysis was used. A total of 141 patients were included 55 with standard, 23 with satellite, and 63 with nested RCs. The mean age was 65.2 years and 34.8% of patients were male. Radiographic preoperative and postoperative results were as follows: sagittal vertical axis (11.0 vs 3.9 cm), lumbar lordosis (28.5° vs 57.1°), pelvic tilt (30.6° vs 21.0°), pelvic incidence (61.5° vs 60.0°), distance between central sacral vertical line and C7 plumb line (2.2 vs 1.5 cm), and scoliosis (18.9° vs 11.3°). The average time to RF was 12.4 months. Overall RF, bilateral RF, pseudarthrosis, and reoperation rates were 22.7%, 5.0%, 20.6%, and 17.7%, respectively. Standard RCs had a significantly higher RF (36.4% vs 13.0% vs 14.3%, p = 0.008), bilateral RF (35.0% vs 0.0% vs 0.0%, p = 0.021), pseudarthrosis (34.5% vs 8.7% vs 12.7%, p = 0.004), and reoperation (30.9% vs 4.3% vs 11.1%, p = 0.004) rates. Satellite RCs (OR 0.21, p = 0.015), nested RCs (OR 0.24, p = 0.003), and bone morphogenetic protein-2 (OR 0.28, p = 0.005) were independently associated with lower odds of RF. Use of multiple rods in the satellite RC and nested RC groups was associated with lower rates of RF, pseudarthrosis, and reoperations following lumbar PSO. Bone morphogenetic protein-2 was associated with a reduction in RF rate as well.

Pedicle Subtraction Osteotomy Construct Optimization: A Cadaveric Study of Various Multirod and Interbody Configurations.

Fourteen cadaveric specimens were separated into two groups: (1) L3 pedicle subtraction osteotomy (PSO) with transforaminal lumbar interbody fusion (TLIF) or (2) lateral lumbar interbody fusion (LLIF). A 2-rod configuration (2R) was compared with two supplemental rod configurations: 4-rod (4R) with accessory rods (ARs) using connectors or 4R with satellite rods (SRs) without connectors. Compare PSO constructs with different rod configurations and adjacent-level interbody support. Supplemental rods and anterior column support enhance biomechanical performance. Pure moments were applied in (1) intact, (2) pedicle screws and rods, (3) PSO + 2R, (4) 4R AR, and (5) 4R SR conditions. Primary and supplemental rods had strain gauges across the index level. Sacral screw bending moments and range of motion (ROM) were recorded. For TLIF, AR decreased ROM during flexion (P = 0.02) and extension (P < 0.001) versus 2R. For LLIF, AR and SR decreased motion versus 2R during left (AR: P = 0.03; SR: P = 0.04) and right (AR: P = 0.002; SR: P = 0.01) axial rotation. For LLIF, sacral screw strain increased with SR compared with AR in compression and right lateral bending (P ≤ 0.03). During lateral bending, rod strain increased with PSO+TLIF+SR versus PSO+LLIF+2R and PSO+LLIF+AR (P ≤ 0.02). For LLIF, SR configuration increased rod strain versus AR during flexion, extension, and lateral bending (P≤ 0.01); for TLIF, rod strain increased with SR versus AR during extension (P = 0.03). For LLIF, AR configuration increased posterior supplemental rod strain versus SR during flexion (P = 0.02) and lateral bending (P < 0.001). Both supplemental rod configurations reduced motion in both groups. Constructs with the SR configuration increased the primary rod strain and the sacral screw bending moment compared with AR constructs, which can share strain. Deep-seated SRs, which have become increasingly popular, may be more vulnerable to failure than ARs. LLIF provided more stability in sagittal plane. Protective effect of supplemental rods on rod strain was more effective with TLIF.Level of Evidence: NA.

Failed Primary Surgery in Congenital Scoliosis Caused by a Single Hemivertebra: Reasons and Revision Strategies.

ObjectiveTo analyze the factors causing failure of primary surgery in congenital scoliosis (CS) patients with single hemivertebra (SHV) undergoing posterior spinal fusion, and to elucidate the revision strategies.MethodsIn this retrospective study, a total of 32 CS patients secondary to SHV undergoing revision surgery from April 2010 to December 2017 due to failed primary surgery with more than 2 years follow‐up were reviewed. The reasons for failure of primary surgery and revision strategies were analyzed for each patient. The radiographic parameters including coronal Cobb angle, segmental kyphosis (SK), coronal balance (CB), and sagittal vertical axis (SVA) were compared between pre‐ and post‐revision. The complications during revision and follow‐up were recorded.ResultsThe mean age at revision surgery of the 32 CS patients was 15.8 ± 9.7 years and the average duration between primary and revision surgery was 31.0 ± 35.4 months. The reasons for failed primary surgery were severe post‐operative curve progression of focal scoliosis in 14 cases (43.8%), implant failure in 17 (53.1%) and trunk imbalance in 12 (37.5%). The candidate revision strategies included thorough resection of residual hemivertebra and adjacent discs, extending fusion levels, complete pseudarthrosis resection, massive bone graft, replacement of broken rods, satellite rod fixation, horizontalization of upper/lower instrumented vertebrae and rigid fusion of structural compensatory curves were performed individually. After revision surgery, the coronal Cobb angle, SK, CB and SVA showed significant improvement (P < 0.05) with no significant correction loss during follow‐up (P > 0.05). The intra‐operative complications included alarming changes of neurologic monitoring in three (9.4%) patients and dual tear in two, while rod fracture re‐occurred was detected in one patient at 18 months after revision.ConclusionsThe common reasons for failed primary surgery in CS patients with SHV undergoing posterior spinal fusion were severe post‐operative curve progression of focal scoliosis, implant failure and trunk imbalance. The revision strategies including thorough resection of residual hemivertebra and adjacent discs, extended fusion levels to structural curvature, complete pseudarthrosis resection, massive bone graft, replacement of broken internal fixation and horizontalization of upper/lower instrumented vertebrae should be individualized based on the causes of failed primary surgery.

A Comparative Biomechanical Analysis of Various Rod Configurations Following Anterior Column Realignment and Pedicle Subtraction Osteotomy.

ObjectiveThe objective of this study was to compare the biomechanical differences of different rod configurations following anterior column realignment (ACR) and pedicle subtraction osteotomy (PSO) for an optimal correction technique and rod configuration that would minimize the risk of rod failure. MethodsA validated spinopelvic (L1-pelvis) finite element model was used to simulate ACR at the L3–4 level. The ACR procedure was followed by dual-rod fixation, and for 4-rod constructs, either medial/lateral accessory rods (connected to primary rods) or satellite rods (directly connected to ACR level screws). The range of motion (ROM), maximum von Mises stress on the rods, and factor of safety (FOS) were calculated for the ACR models and compared to the existing literature of different PSO rod configurations. ResultsAll of the 4-rod ACR constructs showed a reduction in ROM and maximum von Mises stress compared to the dual-rod ACR construct. Additionally, all of the 4-rod ACR constructs showed greater percentage reduction in ROM and maximum von Mises stress compared to the PSO 4-rod configurations. The ACR satellite rod construct had the maximum stress reduction i.e., 47.3% compared to dual-rod construct and showed the highest FOS (4.76). These findings are consistent with existing literature that supports the use of satellite rods to reduce the occurrence of rod fracture. ConclusionOur findings suggest that the ACR satellite rod construct may be the most beneficial in reducing the risk of rod failure compared to all other PSO and ACR constructs.

Sequential correction using satellite rod for severe thoracic idiopathic scoliosis: an effective method to optimize deformity correction.

The aim of this paper was to compare the radiographic and clinical outcomes between the sequential correction (SC) technique and the traditional 2-rod correction (TC) technique in patients with severe thoracic idiopathic scoliosis (STIS) undergoing posterior-only correction surgery. Records of a consecutive series of STIS patients undergoing posterior-only correction surgery between October 2013 and October 2017 with more than 2 years of follow-up were reviewed. The radiographic parameters were assessed preoperatively, postoperatively, and at the last follow-up. Radiographic parameters, operative time, blood loss, and complications were compared between the two groups. A total of 33 patients were included in the SC group, and 21 patients were included in the TC group. There was no significant difference in age, sex, or deformity magnitude (93.6° ± 7.8° vs 89.8° ± 6.6°, p = 0.070) preoperatively between groups. The operation time was shorter in the SC group than in the TC group (251.5 ± 42.8 minutes vs 275.4 ± 39.8 minutes, p = 0.020), while both blood loss (1284.6 ± 483.3 vs 1398.0 ± 558.4 ml, p = 0.432) and number of fused levels (13.1 ± 2.8 vs 13.6 ± 2.4, p = 0.503) were similar between the groups. Compared with the TC group, patients in the SC group had a higher correction rate (55.8% ± 9.2% vs 45.7% ± 8.8%, p < 0.001), less coronal (1.1° ± 0.81° vs 2.9° ± 0.93°, p < 0.001) and sagittal (1.5° ± 0.96° vs 2.1° ± 0.64°, p = 0.015) correction loss at the 2-year follow-up, and a lower incidence of intraoperative pedicle screw pullout (14.3% vs 23.8%, p = 0.026). The SC technique could significantly and practically reduce the difficulty of rod installation with better deformity correction outcomes than the traditional TC technique. The SC technique was an effective alternative for patients with STIS.

Use of the dual construct lowers rod strains in flexion-extension and lateral bending compared to two-rod and two-rod satellite constructs in a cadaveric spine corpectomy model

BACKGROUND CONTEXTComplex spinal reconstructions involving corpectomies, or osteotomies, place spinal implants at extremely high stresses that can lead to pseudoarthrosis and ultimately to rod failure, resulting in revision surgery. Current clinical options to increase the biomechanical strength of a construct include increasing rod diameter, changing rod material, or placing an additional satellite/outrigger rod on a standard two rod construct. Fundamentally, all of these constructs still rely on two longitudinal rods across the reconstruction site and are therefore at risk for rod fracture and loss of alignment. Initially described in 2006, the Dual Construct was developed to address this limitation by utilizing four distinct mechanically independent rods, which allowed for the creation of two separate, and distinct, constructs within each patient. Although there is early clinical evidence to support its efficacy, this is the first biomechanical study to compare the Dual Construct to the two-rod and two-rod with satellite configurations in a cadaveric study. PURPOSETo assess the biomechanical impact of the Dual Construct technique to traditional two-rod and two-rod with satellite rod construct in a cadaveric model. STUDY DESIGN/SETTINGBiomechanical cadaveric study METHODSNine fresh-frozen human cadaveric spines (6 males, 3 females, 56 year +/- 9 years) from T9-pelvis were instrumented and tested utilizing all three configurations including two-rod construct, two-rod with satellite construct, and the Dual Construct technique. Biomechanical testing order of the various constructs was randomized to reduce potential effects of order bias. Strain gauges were placed in both the coronal and sagittal planes of the rods to track the strains during flexion-extension and lateral bending while undergoing range of motion testing. Testing was performed using pure-moment flexibility testing protocols. RESULTSIn flexion-extension, the resultant strain in the two-rod construct was an average 600±228 microstrain, the two-rod with satellite rod strain averaged 603±237 microstrain, and the Dual Construct averaged 403±149 microstrain. In lateral bending, the resultant strain in the two-rod construct was an average of 266±134 microstrain, the satellite rod strain was an average of 310±158 microstrain, and the Dual Construct averaged 118±51 microstrain. In both flexion extension and lateral bending, a significant reduction in strain was observed between the Dual Construct condition compared to both the two-rod and satellite configurations. No significant difference was found between the two-rod and two-rod with satellite rod configurations. CONCLUSIONSThe increase in load sharing significantly decreases the strain experienced across the Dual Construct compared to traditional two-rod and two-rod with satellite constructs. Global rod strains on primary rods cannot be reduced by simply increasing the number of satellite rods, but can only be reduce by increasing the actual number of primary rods.