Biomechanical study of rod stress after pedicle subtraction osteotomy versus anterior column reconstruction: A finite element study

Comparison of Revision Techniques for Rod Fracture after Adult Spinal Deformity Surgery: Rod Replacement Alone or Coupled with Lateral Lumbar Interbody Fusions or Accessory Rods

Background: Rod fracture (RF) is the most common cause of revision in adult spinal deformity (ASD) surgery, and various treatment strategies for preventing RF are reported in the literature. This retrospective study, involving 139 ASD patients (aged ≥65 years and a minimum 2-year follow-up) who underwent long-segment fixation from T10 to sacrum with pedicle subtraction osteotomy (PSO), analyzed long-term results, including radiographical parameters and the incidence of recurrent RF (re-RF), to determine the most effective revision method for preventing RF. Methods: Patients were classified into three groups according to the revision method performed for RF: simple rod replacement (RR group, n = 17), lateral lumbar interbody fusion around the PSO site (RR + LLIF group, n = 8), and accessory rod insertion (RR + AR group, n = 22). Baseline characteristics and radiographical and clinical parameters were analyzed. Results: RF occurred in 47 patients (34%) at an average of 28 months following primary deformity correction. Re-RF occurred in six patients (13%) at an average of 37 months. Re-RF occurred most commonly in the RR group (p = 0.048). Every re-RF in the RR group occurred at the PSO site; none occurred in the RR + LLIF group, and one in the RR + AR group occurred near the L4-5. After both primary deformity correction and revision surgery, spinopelvic parameters had shown favorable results, and clinical outcomes had improved in all three groups without significant intergroup differences. Conclusions: Accessory rod insertion or an additional LLIF around the PSO site seems to provide greater strength and stability to the previously fused segments than a simple rod replacement, which demonstrates the need for additional support in revision surgery for RF after a PSO.

Effects of Minimally Invasive Lateral Lumbar Interbody Fusion with Accessory Rod Technique on Rod Fracture in Adult Spinal Deformity Surgery: Analysis of 239 Patients

STUDY DESIGN

A retrospective study.

OBJECTIVES

To analyze factors associated with rod fracture (RF) in adult spinal deformity (ASD), and to assess whether the accessory rod (AR) technique can reduce RF occurrence in deformity correction in the setting of minimally invasive lateral lumbar interbody fusion (LLIF).

SUMMARY OF BACKGROUND DATA

Instrumentation failure is the most common reason for revision surgery in ASD. Several RF reduction methods have been introduced. However, there are insufficient studies on postoperative RF after deformity correction using minimally invasive LLIF.

MATERIALS AND METHODS

This study included 239 patients (average age 71.4 y and a minimum 2-year follow-up) with ASD who underwent long-segment fusion from T10 to sacrum with sacropelvic fixation. Patients were classified into the non-RF group and the RF group. After logistic regression analysis of the risk factors for RF, subgroup analyses were performed: pedicle subtraction osteotomy (PSO) with two-rod (P2 group) versus PSO with two-rod and AR (P4 group), and LLIF with two-rod (L2 group) versus LLIF with two-rod and AR (L4 group).

RESULTS

RF occurred in 50 patients (21%) at an average of 25 months. RF occurred more frequently in patients who underwent PSO than in those who underwent LLIF ( P =0.002), and the use of the AR technique was significantly higher in the non-RF group ( P <0.05).Following logistic regression analysis, preoperative PI-LL mismatch, PSO, and the AR technique were associated with RF. In subgroup analyses, RF incidence was 65% (24/37 cases) of the P2 group, 8% (4/51 cases) of the P4 group, and 21% (22/105 cases) of the L2 group. In the L4 group, there was no RF.

CONCLUSION

Minimally invasive multilevel LLIF with the AR technique is capable of as much LL correction as conventional PSO and appears to be an effective method for reducing RF.

Anterior Column Release: With Great Lordosis Comes Great Risk of Complications-A Case Series

STUDY DESIGN

Retrospective review.

OBJECTIVE

We sought to characterize complications associated with anterior column release (ACR).

SUMMARY OF BACKGROUND DATA

Correction of positive sagittal imbalance was traditionally completed with anterior column grafts or posterior osteotomies. ACR is a minimally invasive technique for addressing sagittal plane deformity by restoring lumbar lordosis.

METHODS

We conducted a retrospective review of consecutive patients who underwent ACR in a prospectively kept database at a tertiary care academic center from January 2012 to December 2018. The prespecified complications were hardware failure (rod fracture, hardware loosening, or screw fracture), proximal junctional kyphosis, ipsilateral thigh numbness, ipsilateral femoral nerve weakness, arterial injury requiring blood transfusion, bowel injury, and abdominal pseudohernia.

RESULTS

Thirty-eight patients were identified. Thirty-five patients had ACR at L3-4, 1 had ACR at L4-5, and 1 patient had ACR at L2-3 and L3-4. Eighteen patients (47.4%) had one of the prespecified complications (10 patients had multiple). Ten patients developed hardware failure (26.3%); 8 patients (21.1%) had rod fracture, 4 (10.5%) had screw fracture, and 1 (2.6%) had screw loosening. At discharge, rates of ipsilateral thigh numbness (37.8%) and hip flexor (37.8%)/quadriceps weakness (29.7%) were the highest. At follow-up, 6 patients (16.2%) had ipsilateral anterolateral thigh numbness, 5 (13.5%) suffered from ipsilateral hip flexion weakness, and 3 patients (5.4%) from ipsilateral quadriceps weakness. Arterial injury occurred in 1 patient (2.7%). Abdominal pseudohernia occurred in 1 patient (2.7%). There were no bowel injuries observed.

CONCLUSIONS

ACR is associated with a higher than initially anticipated risk of neurological complications, hardware failure, and proximal junctional kyphosis.

Modified S1 Pedicle Subtraction Osteotomy

STUDY DESIGN

Surgical technique video.

OBJECTIVE

To report a surgical technique to revise patients with previous fusions at L4-S1 leading to an iatrogenic flat back and sagittal imbalance using L5-S1 transforaminal interbody fusion combined with a small S1 corner osteotomy.

BACKGROUND

This is a case of a woman (51 y old) with a history of multiple lumbar surgeries, severe back pain, sagittal imbalance, and loss of lordosis.

METHODS

We describe a feasible revision technique in a complex patient with the goal of attaining optimal distribution of lumbar lordosis and sagittal balance through a modified S1 pedicle subtraction osteotomy, and the use of an interbody cage to enhance the fusion rate and facilitate closure of the 3-column osteotomy.

RESULTS

The preoperative patient lordosis angle of 31 degrees at L1-L4 and 16 degrees at L4-S1 became 12 degrees at L1-L4 and 44 degrees at L4-S1 postoperatively.

CONCLUSION

The combination of L5-S1 transforaminal interbody fusion and S1 corner osteotomy is a feasible technique for the restoration of lumbar lordosis in patients with previous fusion and consequent loss of lordosis.

Posterior-based Osteotomies for Deformity Correction

Posterior-based osteotomies are crucial to the restoration of lordosis in adult spinal deformity. Posterior-column osteotomies are suited for patients with an unfused anterior column and non-focal sagittal deformity requiring modest correction in lordosis. When performed on multiple levels, posterior-column osteotomy may provide significant harmonious correction in patients who require more extensive correction. Pedicle subtraction osteotomy and vertebral column resection are appropriate for patients with a fused anterior column and more severe deformity, particularly focal and/or multiplanar deformity. The power of pedicle subtraction osteotomy and vertebral column resection to provide greater correction and to address multiplanar deformity comes at the cost of higher complication rates than posterior-column osteotomy.

Lumbar Disc Degeneration Affects the Risk of Rod Fracture Following PSO; A Finite Element Study

STUDY DESIGN

Finite element (FE) study.

OBJECTIVE

Pedicle subtraction osteotomy (PSO) is a surgical method to correct sagittal plane deformities. In this study, we aimed to investigate the biomechanical effects of lumbar disc degeneration on the instrumentation following PSO and assess the effects of using interbody spacers adjacent to the PSO level in a long instrumented spinal construct.

METHODS

A spinopelvic model (T10-pelvis) with PSO at the L3 level was used to generate 3 different simplified grades of degenerated lumbar discs (mild (Pfirrmann grade III), moderate (Pfirrmann grade IV), and severe (Pfirrmann grade V)). Instrumentation included eighteen pedicle screws and bilateral primary rods. To investigate the effect of interbody spacers, the model with normal disc height was modified to accommodate 2 interbody spacers adjacent to the PSO level through a lateral approach. For the models, the rods' stress distribution, PSO site force values, and the spine range of motion (ROM) were recorded.

RESULTS

The mildly, moderately, and severely degenerated models indicated approximately 10%, 26%, and 40% decrease in flexion/extension motion, respectively. Supplementing the instrumented spinopelvic PSO model using interbody spacers reduced the ROM by 22%, 21%, 4%, and 11% in flexion, extension, lateral bending, and axial rotation, respectively. The FE results illustrated lower von Mises stress on the rods and higher forces at the PSO site at higher degeneration grades and while using the interbody spacers.

CONCLUSIONS

Larger and less degenerated discs adjacent to the PSO site may warrant consideration for interbody cage instrumentation to decrease the risk of rod fracture and PSO site non-union.

Anterior Column Realignment: Adult Sagittal Deformity Treatment Through Minimally Invasive Surgery

This article focuses on the treatment of sagittal spinal deformity using a minimally invasive technique, anterior column realignment. Traditional methods to address sagittal spine deformity have been associated with high morbidity, long operative times, and excessive blood loss. This technique uses a minimally invasive lateral retroperitoneal approach to release the anterior longitudinal ligament and apply a hyperlordotic implant for interbody fusion to restore lumbar lordosis and sagittal alignment.

Does the anterior column realignment technique influences the stresses on posterior instrumentation in sagittal imbalance correction? A biomechanical, finite-element analysis of L5-S1 ALIF and L3-4 lateral ACR

STUDY DESIGN

Biomechanical finite-element study.

OBJECTIVE

To directly compare the biomechanical effects of two different techniques for sagittal plane correction of adult spine deformity based on the anterior longitudinal ligament (ALL) resection and use of hyperlordotic cages, namely, the anterior column realignment (ACR) in L3-4, and ALIF in L5-S1 in terms of primary stability and rod stresses using finite-element models.

METHODS

A finite-element model of the thoracolumbar spine was used to perform the analysis. Starting from this "intact" model, three further models were constructed through the insertion of spinal instrumentation, i.e., pedicle screws, rods and cages: 1) posterior instrumentation between T9 and S1 (referred to as "T9-S1"); 2) posterior instrumentation T9-S1 + Hyperlordotic (26°) ALIF cage in L5-S1 ("ALIF"); 3) posterior instrumentation T9-S1 + Hyperlordotic (30°) ACR cage in L3-4 ("ACR"). These models were studied by simulations applying, alternately, a pure moment of 7.5 Nm between the three planes of motion (flexion, extension, lateral bending, and bilateral axial rotation), uniformly distributed over the upper surface of the T9 thoracic vertebra. A total of 24 simulations were performed (6 per models).

RESULTS

All models presented a significant reduced ROM when compared to the intact model; the ROM reduction was higher both at L3-4 in the ACR model and at L5-S1 in the ALIF model. At L3-4, the ACR model had, in all cases, the lowest maximum values of Von Mises stresses on the rods, especially in flexion-extension. At L4-5, the ALIF model had the lowest stresses during flexion-extension and axial rotation, while the ACR model had the lowest stresses during lateral bending. At L5-S1, the ALIF model had, in all cases, the lowest stresses on the rods.

CONCLUSIONS

This finite-element study showed how both ACR at L3-4 and ALIF-ACR at L5-S1 are effective in restoring lumbar lordosis (LL), stabilizing the spine and reducing stress on posterior rods at the index level when compared to a simple fixation model. Interestingly, ALIF-ACR reduces rod stress even at L4-5 in flexion-extension and axial rotation, possibly due to a better distribution of LL, especially on the lower arch, while ACR reduces the stress at L4-5 in lateral bending, possibly thanks to the larger footprint of the cage that increases the area of contact with the lateral side of the endplates.

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

PURPOSE

To assess biomechanics of a lumbar PSO stabilized with different multi-rod constructs (4-, 5-, 6-rods) using satellite and accessory rods.

METHODS

A 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.

RESULTS

Compared 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.

CONCLUSIONS

In 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.

Systematic Review of Traumatic Thoracic Spondyloptosis and Presentation of a Novel Approach for Management With Quad Rod Construct

BACKGROUND

Traumatic thoracic spondyloptosis (TTS) is a rare but devastating spinal injury often secondary to high-impact trauma. TTS is typically managed with surgical fusion and stabilization.

OBJECTIVE

To evaluate current surgical management of TTS while presenting a novel surgical technique for reduction and fusion.

METHODS

We performed a systematic review of surgical management of TTS using Preferred Reporting Items for Systematic Reviews and Meta-Analyses of Individual Participant Data guidelines with 2 independent reviewers. We identified patient demographics, level of spondyloptosis, American Spinal Injury Association grade, level of spinal fusion, surgical approach, type of construct used, and reduction of fracture.

RESULTS

Seventeen articles with 37 patients with TTS managed surgically were identified. The male:female ratio was 31:6. The average age was 33 years (±15). Motorized injury including motor vehicle accident, road traffic accident, and motor vehicle collision (16 patients, 43%) and fall including fall from height, stairs, train, or standing (16 patients, 43%) were equivalent. The middle (15 patients 40%) and lower (18 patients, 49%) thoracic regions were similar for the level of spondyloptosis. Thirty-four patients (92%) were American Spinal Injury Association A. Thirty-six patients (97.3%) underwent posterior only surgery and 1 (2.7%) underwent a combined posterior-anterior approach. There were 29 (78%) dual rod constructs and 8 (22%) dual rod with connectors or crosslinks. Complete reduction was obtained in 24 (65%) patients, incomplete in 11 (30%), and 2 (5%) patients were not reported. Two of our patients underwent novel quad rod reconstruction with complete reduction.

CONCLUSION

Surgical management of TTS is typically posterior only with complete fracture reduction. We have presented a novel quad rod approach for reduction of TTS.

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

Objective

According 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.

Methods

Patients 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.

Results

The 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.

Conclusion

The satellite rod technique is also recommended for patients who undergo PSO osteotomy to correct ankylosing spondylitis kyphosis deformities.

Influence of double rods and interbody cages on range of motion and rod stress after spinopelvic instrumentation: a finite element study

PURPOSE

To compare instrumentation configurations consisting of bilateral single or double rods and additional interbody cages (IBCs) at different levels in terms of Range of Motion (ROM) and distribution of von Mises stress in rods.

METHODS

A previously validated L1-pelvis finite element model was used and instrumented with configurations consisting of single or double bilateral rods and IBCs at multiple levels. Pure moments of 7.5 N.m were applied to L1 in main directions in addition to a follower load of 280 N. Global, segmental ROM and distribution of von Mises stress in rods were studied.

RESULTS

All configurations reduced segmental and global ROM from 50 to 100% compared to the intact spine. Addition of IBCs slightly increased ROM at levels adjacent to the IBC placement. The simple rod configuration presented the highest von Mises stress (457 MPa) in principal rods at L5-S1 in flexion. Doubling rods and IBC placement reduced this value and shifted the location of maximum von Mises stress to other regions. Among studied configurations, double rods with IBCs at all levels (L2-S1) showed the lowest ROM. Maximal von Mises stresses in secondary rods were lower in comparison to main rods.

CONCLUSIONS

Double rods and IBCs reduced global and segmental ROM as well as von Mises stress in rods. The results suggest a possible benefit in using both strategies to minimize pseudarthrosis and instrumentation failure. However, increased ROM in adjacent levels and the shift of maximal von Mises stress to adjacent areas might cause complications elsewhere.

Are the Arbeitsgemeinschaft Für Osteosynthesefragen (AO) Principles for Long Bone Fractures Applicable to 3-Column Osteotomy to Reduce Rod Fracture Rates?

OBJECTIVE

The aim was to determine whether applying Arbeitsgemeinschaft für Osteosynthesefragen (AO) principles for external fixation of long bone fracture to patients with a 3-column osteotomy (3CO) would be associated with reduced rod fracture (RF) rates.

SUMMARY OF BACKGROUND DATA

AO dictate principles to follow when fixating long bone fractures: (1) decrease bone-rod distance; (2) increase the number of connecting rods; (3) increase the diameter of rods; (4) increase the working length of screws; (5) use multiaxial fixation. We hypothesized that applying these principles to patients undergoing a 3CO reduces the rate of RF.

METHODS

Patients were categorized as having RF versus no rod fracture (non-RF). Details on location and type of instrumentation were collected. Dedicated software was used to calculate the distance between osteotomy site and adjacent pedicle screws, angle between screws and the distance between the osteotomy site and rod. Classic sagittal spinopelvic parameters were evaluated.

RESULTS

The study included 170 patients (34=RF, 136=non-RF). There was no difference in age (P=0.224), sagittal vertical axis correction (P=0.287), or lumbar lordosis correction (P=0.36). There was no difference in number of screws cephalad (P=0.62) or caudal (P=0.31) to 3CO site. There was a lower rate of RF for patients with >2 rods versus 2 rods (P<0.001). Patients with multiplanar rod fixation had a lower rod fracture rate (P=0.01). For patients with only 2 rods (N=68), the non-RF cohort had adjacent screws that trended to have less angulation to each other (P=0.06) and adjacent screws that had a larger working length (P=0.03).

CONCLUSIONS

A portion of AO principles can be applied to 3CO to reduce RF rates. Placing more rods around a 3CO site, placing rods in multiple planes, and placing adjacent screws with a larger working length around the 3CO site is associated with lower RF rates.

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

STUDY DESIGN

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.

OBJECTIVE

Compare PSO constructs with different rod configurations and adjacent-level interbody support.

SUMMARY OF BACKGROUND DATA

Supplemental rods and anterior column support enhance biomechanical performance.

METHODS

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.

RESULTS

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).

CONCLUSION

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.

How do additional rods reduce loads on the primary rods in adult spinal instrumentation with pedicle subtraction osteotomy?

BACKGROUND

Additional auxiliary rods have been used in spinal instrumentation across pedicle subtraction osteotomy to reduce stresses in the primary rods. The auxiliary rods can be connected through dual-rod-screws, fixed-angle multi-rod connectors or variable-angle multi-rod connectors. The objective was to assess rod bending in conventional bilateral-rod construct vs. constructs with auxiliary rods.

METHODS

Computer models of two adult patients were developed to evaluate bending loads across a pedicle subtraction site in a control construct with bilateral rods vs. constructs with auxiliary rods bilaterally or unilaterally connected to the primary rods through either dual-rod-screws, fixed-angle multi-rod connectors, or variable-angle multi-rod connectors. Postoperative rod bending loads were computed and compared.

FINDINGS

Normalizing loads on the primary rods in the multi-rod constructs to the control construct, primary rod loads in multi-rod constructs were 17% to 48% lower than the control construct. Constructs with bilateral auxiliary rods through dual-rod-screws, fixed-angle multi-rod connectors, or variable-angle multi-rod connectors could result in similar primary rod bending loads. Bending loads on the auxiliary rods were higher or lower than those on the primary rods depending on how their curvatures matched the primary rods, and how they were locked onto the primary rods.

INTERPRETATION

Auxiliary rods noticeably reduced the bending loads on the primary rods compared with a standard bilateral-rod construct. Loads in the auxiliary rods were higher or lower than those in the primary rods depending on how their curvatures matched the primary rods, and how they were locked onto the primary rods.

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.

Finite analysis of stability between modified articular fusion technique, posterior lumbar interbody fusion and posteriorlateral lumbar fusion

Background

It is not clear whether modified facet fusion (MFF) is biomechanically different from traditional fusion techniques such as posterior lateral lumbar fusion (PLF) and posterior lumbar interbody fusion (PLIF).

Methods

In this study, a healthy adult Chinese male volunteer was selected to perform 3D reconstruction of CT image data and simulate the successful fusion of L4–5 MFF, PLF and PLIF, respectively. The motion range of L4–5 segments of the model was simulated under 6 working conditions, including forward flexion, extension, lateral flexion and rotation under normal physiological conditions, and the stability of the three fusion procedures in the pathological segments of the lumbar spine was compared.

Results

There was no difference in range of motion between MFF model and PLF or PLIF model (P < 0.05). Also, the stiffness of the PLFand the MFF model were comparable (P > 0.05), but were smaller than the PLIF model (P < 0.05).

Conclusions

MFF provides reliable stability at the lumbar fixation fusion level and does not differ significantly from PLF and PLIF in terms of range of motion.

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

OBJECTIVE

The 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.

METHODS

A 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.

RESULTS

All 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.

CONCLUSION

Our 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.

Reduced occurrence of primary rod fracture after adult spinal deformity surgery with accessory supplemental rods: retrospective analysis of 114 patients with minimum 2-year follow-up

OBJECTIVE

Rod fracture (RF) after adult spinal deformity (ASD) surgery is reported in approximately 6.8%-33% of patients and is associated with loss of deformity correction and higher reoperation rates. The authors' objective was to determine the effect of accessory supplemental rod (ASR) placement on postoperative occurrence of primary RF after ASD surgery.

METHODS

This retrospective analysis examined patients who underwent ASD surgery between 2014 and 2017 by the senior authors. Inclusion criteria were age > 18 years, ≥ 5 instrumented levels including sacropelvic fixation, and diagnosis of ASD, which was defined as the presence of pelvic tilt ≥ 25°, sagittal vertical axis ≥ 5 cm, thoracic kyphosis ≥ 60°, coronal Cobb angle ≥ 20°, or pelvic incidence to lumbar lordosis mismatch ≥ 10°. The primary focus was patients with a minimum 2-year follow-up.

RESULTS

Of 148 patients who otherwise met the inclusion criteria, 114 (77.0%) achieved minimum 2-year follow-up and were included (68.4% were women, mean age 67.9 years, average body mass index 30.4 kg/m2). Sixty-two (54.4%) patients were treated with traditional dual-rod construct (DRC), and 52 (45.6%) were treated with ASR. Overall, the mean number of levels fused was 11.7, 79.8% of patients underwent Smith-Petersen osteotomy (SPO), 19.3% underwent pedicle subtraction osteotomy (PSO), and 66.7% underwent transforaminal lumbar interbody fusion (TLIF). Significantly more patients in the DRC cohort underwent SPO (88.7% of the DRC cohort vs 69.2% of the ASR cohort, p = 0.010) and TLIF (77.4% of the DRC cohort vs 53.8% of the ASR cohort, p = 0.0001). Patients treated with ASR had greater baseline sagittal malalignment (12.0 vs 8.6 cm, p = 0.014) than patients treated with DRC, and more patients in the ASR cohort underwent PSO (40.3% vs 1.6%, p < 0.0001). Among the 114 patients who completed follow-up, postoperative occurrence of RF was reported in 16 (14.0%) patients, with mean ± SD time to RF of 27.5 ± 11.8 months. There was significantly greater occurrence of RF among patients who underwent DRC compared with those who underwent ASR (21.0% vs 5.8%, p = 0.012) at comparable mean follow-up (38.4 vs 34.9 months, p = 0.072). Multivariate analysis demonstrated that ASR had a significant protective effect against RF (OR 0.231, 95% CI 0.051-0.770, p = 0.029).

CONCLUSIONS

This study demonstrated a statistically significant decrease in the occurrence of RF among ASD patients treated with ASR, despite greater baseline deformity and higher rate of PSO. These findings suggest that ASR placement may provide benefit to patients who undergo ASD surgery.

Biomechanics of open versus minimally invasive deformity correction:​ comparison of stability and rod strain between pedicle subtraction osteotomy and anterior column realignment

OBJECTIVE

Anterior column realignment (ACR) is a new minimally invasive approach for deformity correction that achieves a degree of lordosis similar to that obtained with pedicle subtraction osteotomy (PSO). This study compared the biomechanical profiles of ACR with PSO using range of motion (ROM) and posterior rod strain (RS) to gain insight into the ACR technique and the necessary surgical strategies to optimize longevity and stability.

METHODS

An in vitro biomechanical study using standard flexibility testing (7.5 Nm) was performed on 14 human cadaveric specimens, separated into 2 groups similar in age, sex, bone mineral density, and intact ROM. For group 1 (n = 7, instrumented L1-S1), a 30° ACR was performed at L3-4. For group 2 (n = 7, instrumented T12-S1), a 30° L3 PSO was performed. Specimens were subjected to nondestructive loads in flexion, extension, axial rotation, lateral bending, and compression. Conditions tested were 1) intact, 2) pedicle screw with 2 rods (PSR), 3) ACR or PSO with 2 rods (+2R), and 4) ACR or PSO with 4 rods (+4R). Primary outcome measures of interest were ROM stability and posterior RS at L3-4.

RESULTS

No difference was observed between groups in lumbar lordosis (p = 0.83) or focal angular lordosis at L3-4 (p = 0.75). No differences in stability were observed between ACR+2R and PSO+2R (p ≥ 0.06);​ however, ACR+2R was significantly less stable than PSR in flexion and extension (p ≤ 0.02), whereas PSO+2R was less stable than PSR only in extension (p = 0.04). ACR+4R was more stable than ACR+2R in flexion, extension, left axial rotation, and compression (p ≤ 0.02). PSO+4R was more stable than PSO+2R only in extension (p = 0.04). Both ACR+2R and PSO+2R resulted in significant increases in RS in flexion and extension compared with PSR (p ≤ 0.032). RS in flexion and extension decreased significantly for ACR+4R versus ACR+2R and for PSO+4R versus PSO+2R (p ≤ 0.047). PSO+2R yielded lower RS than ACR+2R in compression (p = 0.03). No differences existed in RS between ACR+4R and PSO+4R (p ≥ 0.05).

CONCLUSIONS

Although ACR appeared to be slightly more destabilizing than PSO using traditional 2R fixation, both techniques resulted in significant increases in posterior RS. The 4R technique increased stability in ACR and decreased RS in both ACR and PSO but may be more beneficial in ACR. Longer-term clinical studies are needed to appropriately identify the durability of the ACR technique in deformity correction.

Development of a flexible instrumented lumbar spine finite element model and comparison with in-vitro experiments

Surgical corrections of degenerative lumbar scoliosis and sagittal malalignment are associated with significant complications, such as rod fractures and pseudarthrosis, particularly in the lumbosacral junction. Finite element studies can provide relevant insights to improve performance of spinal implants. The aim of the present study was to present the development of non-instrumented and instrumented Finite Element Models (FEMs) of the lumbopelvic spine and to compare numerical results with experimental data available in the literature. The lumbo-pelvic spine FEM was based on a CT-scan from an asymptomatic volunteer representing the 50th percentile male. In a first step a calibration of mechanical properties was performed in order to obtain a quantitative agreement between numerical results and experimental data for defect stages of spinal segments. Then, FEM results were compared in terms of range of motion and strains in rods to in-vitro experimental data from the literature for flexible non-instrumented and instrumented lumbar spines. Numerical results from the calibration process were consistent with experimental data, especially in flexion. A positive agreement was obtained between FEM and experimental results for the lumbar and sacroiliac segments. Instrumented FEMs predicted the same trends as experimental in-vitro studies. The instrumentation configuration consisting of double rods and an interbody cage at L5-S1 maximally reduced range of motion and strains in main rods and thus had the lowest risk of pseudarthrosis and rod fracture. The developed FEMs were found to be consistent with published experimental results; therefore they can be used for further post-operative complication investigations.

Rod-Screw Constructs Composed of Dissimilar Metals Do Not Affect Complication Rates in Posterior Fusion Surgery Performed for Adult Spinal Deformity

STUDY DESIGN

This was a retrospective cohort study.

OBJECTIVE

The objective of this study was to compare implant-related complications between mixed-metal and same-metal rod-screw constructs in patients who underwent posterior fusion for adult spinal deformity.

SUMMARY OF BACKGROUND DATA

Contact between dissimilar metals is discouraged due to potential for galvanic corrosion, increasing the risk for metal toxicity, infection, and implant failure. In spine surgery, titanium (Ti) screws are most commonly used, but Ti rods are notch sensitive and likely more susceptible to fracture after contouring for deformity constructs. Cobalt chrome (CC) and stainless steel (SS) rods may be suitable alternatives. No studies have yet evaluated implant-related complications among mixed-metal constructs (SS or CC rods with Ti screws).

METHODS

Adults with spinal deformity who underwent at least 5-level thoracic and/or lumbar posterior fusion or 3-column osteotomy between January 2013 and May 2015 were reviewed, excluding neuromuscular deformity, tumor, acute trauma or infection. Implant-related complications included pseudarthrosis, proximal junctional kyphosis, hardware failure (rod fracture, screw pullout or haloing), symptomatic hardware, and infection.

RESULTS

A total of 61 cases met inclusion criteria: 24 patients received Ti rods with Ti screws (Ti-Ti, 39%), 31 SS rods (SS-Ti, 51%), and 6 CC rods (CC-Ti, 9.8%). Median follow-up was 37-42 months for all groups. Because of the limited number of cases, the CC-Ti group was not included in statistical analyses. There were no differences between Ti-Ti and SS-Ti groups with regard to age, body mass index, or smokers. Implant-related complications did not differ between the Ti-Ti and SS-Ti groups (P=0.080). Among the Ti-Ti group, there were 15 implant-related complications (63%). In the SS-Ti group, there were 12 implant-related complications (39%). There were 3 implant-related complications in the CC-Ti group (50%).

CONCLUSION

We found no evidence that combining Ti screws with SS rods increases the risk for implant-related complications.

Impact of Spinal Correction Surgeries with Osteotomy and Pelvic Fixation in Patients with Kyphosis Due to Osteoporotic Vertebral Fractures

Study Design

Combination of retrospective and prospective study.

Purpose

We aimed to compare the clinical outcomes between local fixation surgery and spinopelvic fixation surgery for the treatment of kyphosis secondary to osteoporotic vertebral fractures with spinopelvic malalignment.

Overview of Literature

The clinical characteristics of patients with rigid kyphosis due to osteoporotic vertebral fracture differ from that of middle-aged patients with vertebral fractures in terms of bone fragility and presence of spinopelvic malalignment. Little is known about the surgical strategies for these deformities, most especially the extent of fusion of vertebra involved.

Methods

We analyzed 24 patients with vertebral osteotomy at the level of the fracture and spinal fixation without pelvic fixation (local group), and 22 patients with vertebral osteotomy and pelvic fixation (pelvic group). Radiographic parameters, the incidence of proximal junctional kyphosis (PJK), distal junctional kyphosis (DJK), rod fractures, and the Oswestry Disability Index (ODI) were compared between the two groups over a 2-year follow-up period.

Results

In the pelvic group, postoperative spinopelvic parameters significantly improved, with the improvements maintained. No remarkable changes in spinopelvic parameters were seen in the local group. The mean ODI scores 2 years after surgery were 45.3 and 33.0 in the local and pelvic group, respectively (p-value <0.05). There was no significant difference in the incidence of PJK in the local and pelvic groups, but there was a higher rate of DJK (41.7%) in the local group. In contrast, rod fractures were more common in the pelvic group (45.5%). Patients with DJK had higher ODI scores 2 years after surgery (52.0 in DJK patients vs. 34.8 in non-DJK patients; p-value <0.05).

Conclusions

For patients with rigid kyphosis due to osteoporotic vertebral fractures, better spinopelvic alignment and health-related quality of life can be achieved through extensive corrective surgery with pelvic fixation.

Advances in Spinal Fusion Strategies in Adult Deformity Surgery

BACKGROUND

As the frequency of adult deformity surgery (ADS) continues to increase, our understanding of techniques that enhance fusion must continue to evolve because pseudarthrosis can be a serious and costly event.

PURPOSES/QUESTIONS

We sought to conduct a review of the literature investigating techniques that can enhance outcomes of ADS.

METHODS

Two databases were searched for keywords such as "advances in spinal fusion," "new technology in adult spinal deformity," "interbody devices for adult spinal deformity," "adult spinal deformity rods," and "screw design in adult spinal deformity" to examine recent literature and trends in ADS.

RESULTS

We identified 45 articles for our review. Topics studied include the use of multiple rods, interbody fusion, distal fixation techniques, and bone morphogenetic protein or iliac crest bone graft.

CONCLUSIONS

Many recent innovations in treatments to enhance fusion in ADS have been studied, some more controversial than others. Further research into the efficacy of these techniques may increase fusion rates in ADS.

Long additional rod constructs can reduce the incidence of rod fractures following 3-column osteotomy with pelvic fixation in short term

STUDY DESIGN

A retrospective analysis of a prospectively collected consecutive case series of patients with adult spinal deformity who underwent 3-column osteotomy (3-CO) with pelvic fixation.

OBJECTIVES

The objectives were to verify the effect of additional rods and determine the optimal type of additional rod for preventing rod fracture (RF). RF is a frequent complication following 3-CO surgery in patients with adult spinal deformity. Two types of additional rod constructs were utilized to prevent RF.

METHODS

We evaluated data retrieved from a prospectively collected single-center database. Patients were classified into two groups depending on rod constructs: conventional 2-rod and 2-rod constructs with additional rods. The additional rod constructs were also stratified into two groups according to the location that they covered: only osteotomy site (short additional rod) or osteotomy site and lumbosacral junction (long additional rod).

RESULTS

A total of 48 patients (average age 67.6 years; follow-up rate: 90.6%; 2-rod: 25 cases and additional rod: 23 cases) were included. No significant difference was observed in age, fusion level, operation time, blood loss, and rod composition. The incidence of RF in the additional rods (36%) was significantly lower than that in the 2-rod constructs (68%). Comparing the additional rod constructs, RF occurred in 7 cases (64%) in short additional rod and 2 cases (17%) in long additional rod. RF occurred at the level below the additional rod construct in 6 of 7 RF cases with short additional rod.

CONCLUSIONS

Additional rods covering the osteotomy site and lumbosacral junction reduced the incidence of RF following 3-CO surgery with pelvic fixation in short term.

LEVEL OF EVIDENCE

3.

Minimally Invasive Anterior Longitudinal Ligament Release for Anterior Column Realignment

STUDY DESIGN

Review of the literature.

OBJECTIVES

Anterior column realignment (ACR) is a powerful but relatively new minimally invasive technique for deformity correction. The purpose of this study is to provide a literature review of the ACR surgical technique, reported outcomes, and future directions.

METHODS

A review of the literature was performed regarding the ACR technique. A review of patients at our single center who underwent ACR was performed, with illustrative cases selected to demonstrate basic and nuanced aspects of the technique.

RESULTS

Clinical and cadaveric studies report increases in segmental lordosis in the lumbar spine by 73%, approximately 10° to 33°, depending on the degree of posterior osteotomy and lordosis of the hyperlordosis interbody spacer. These corrections have been found to be associated with a similar risk profile compared with traditional surgical options, including a 30% to 43% risk of proximal junctional kyphosis in early studies.

CONCLUSIONS

ACR represents a powerful technique in the minimally invasive spinal surgeon's toolbox for treatment of complex adult spinal deformity. The technique is capable of significant sagittal plane correction; however, future research is necessary to ascertain the safety profile and long-term durability of ACR.

Optimizing biomechanics of anterior column realignment for minimally invasive deformity correction

BACKGROUND CONTEXT

Anterior column realignment (ACR) is a powerful but destabilizing minimally invasive technique for sagittal deformity correction. Optimal biomechanical design of the ACR construct is unknown.

PURPOSE

Evaluate the effect of ACR design on radiographic lordosis, range of motion (ROM) stability, and rod strain (RS) in a cadaveric model.

STUDY DESIGN/SETTING

Cadaveric biomechanical study.

PATIENT SAMPLE

Seven fresh-frozen lumbar spine cadaveric specimens (T12-sacrum) underwent ACR at L3-L4 with a 30° implant.

OUTCOME MEASURES

Primary outcome measure of interest was maximum segmental lordosis measured using lateral radiograph. Secondary outcomes were ROM stability and posterior RS at L3/4.

METHODS

Effect of grade 1 and grade 2 osteotomies with single-screw anterolateral fixation (1XLP) or 2-screw anterolateral fixation (2XLP) on lordosis was determined radiographically. Nondestructive flexibility tests were used to assess ROM and RS at L3-L4 in flexion, extension, lateral bending, and axial rotation. Conditions included (1) intact, (2) pedicle screw fixation and 2 rods (2R), (3) ACR+1XLP with 2R, (4) ACR+2XLP+2R, (5) ACR+1XLP with 4 rods (4R) (+4R), and (6) ACR+2XLP+4R.

RESULTS

Segmental lordosis was similar between ACR+1XLP and ACR+2XLP (p>.28). ACR+1XLP+2R was significantly less stable than all other conditions in flexion, extension, and axial rotation (p<.014); however, adding an extra screw improved stability to levels equal to 4R conditions (p>.36). Adding 4R to ACR+1XLP reduced RS in all directions of loading (p<.048), whereas adding a second screw did not (p>.12). There was no difference in strain between ACR+1XLP+4R and ACR+2XLP+4R (p>.55).

CONCLUSIONS

For maximum stability, ACR constructs should contain either fixation into both vertebral bodies (2XLP) or accessory rods (4R). 2XLP can be used without compromising the maximal achievable lordosis but does not provide the same RS reduction as 4R.

CLINICAL SIGNIFICANCE

ACR is a highly destabilizing technique that is increasingly being used for minimally invasive deformity correction. These biomechanical data will help clinicians optimize ACR construct design.

Biomechanical analysis of sagittal correction parameters for surgical instrumentation with pedicle subtraction osteotomy in adult spinal deformity

BACKGROUND

High complication rate has been documented in spinal instrumentation with pedicle subtraction osteotomy. The objective was to analyze the biomechanics of spinal instrumentation with pedicle subtraction osteotomy as functions of three instrumentation parameters.

METHODS

Patient-specific biomechanical computer models of 3 adult patients who were instrumented with pedicle subtraction osteotomy were used to simulate the actual instrumentations and alternative instrumentations derived by varying instrumentation parameters: pedicle subtraction osteotomy wedge angle and rod contouring angle, location of the osteotomy, and number of rods (2 vs. 4).

FINDINGS

A change of the sagittal correction by +7.5° (-7.5°) resulted in a change in the screw axial forces and rods bending loads around the osteotomy by +38% (-19%) and +28% (-11%), respectively. The bending moments in the rods were 31% lower at the osteotomy site when it was located at one level above, and 20% higher when it was located at a level below. Additional rods allowed the rod bending and screw axial loads to be reduced by 24% and 22%, respectively.

INTERPRETATION

The amount of sagittal correction was positively correlated with loads sustained by the screws and rods. Rods were subjected to higher bending loads at the pedicle subtraction osteotomy site when it was done at a lower level. A 4-rod construct is an effective way to reduce the risk of rod breakage by reducing the loads sustained by the rods.

[Treatment of osteoporotic vertebral fracture combined with moderate to severe spinal kyphosis by transintervertebral release, bone impaction grafting, and posterior column compressed-closing]

OBJECTIVE

To explore the safety and preliminary effectiveness of transintervertebral release, bone impaction grafting, and posterior column compressed-closing in the treatment of osteoporotic vertebral fracture combined with moderate to severe spinal kyphosis.

METHODS

The clinical data of 21 elderly patients with osteoporotic vertebral fracture combined with moderate to severe spinal kyphosis were retrospectively analyzed between March 2016 and November 2017. There were 1 male and 20 females, aged 55-75 years, with an average of 64.8 years. The disease duration was 8-24 months, with an average of 13.1 months. The bone density T value ranged from -3.4 to -2.1, with an average of -2.3. Lesion segments: T 11 in 2 cases, T 12 in 6 cases, L 1 in 8 cases, L 2 in 1 case, T 11, 12 in 1 case, T 12, L 1 in 2 cases, and T 12, L 2 in 1 case. Preoperative neurological function was classified according to the American Spinal Injury Association (ASIA): 5 cases of grade D and 16 cases of grade E. All patients underwent transintervertebral release, bone impaction grafting, and posterior column compressed-closing. The effectiveness was evaluated by visual analogue scale (VAS) score and Oswestry dysfunction index (ODI) score before operation, at 3 months after operation, and at last follow-up. The neurological function was assessed by ASIA at last follow-up. Local kyphosis Cobb angle (LKCA), thoracic kyphosis (TK), lumbar lordosis (LL), and sagittal vertebral axis (SVA) were measured on the X-ray films of the full-length lateral spine of the patient before operation, at 1 week after operation, and at last follow-up.

RESULTS

No complication such as fracture of internal fixator or nerve injury occurred. LKCA, TK, and SVA were significantly improved at 1 week after operation and at last follow-up ( P<0.05). There was no significant difference between at 1 week after operation and at last follow-up ( P>0.05). There was no significant difference in LL before and after operation ( F=3.013, P=0.057). The VAS and ODI scores were significantly improved at 3 months after operation and at last follow-up, and further improved at last follow-up when compared with the scores at 3 months after operation, showing significant differences between time points ( P<0.05). Five patients with ASIA grade D neurological function recovered to grade E at 6 months after operation.

CONCLUSION

Transintervertebral release, bone impaction grafting, and posterior column compressed-closing for treating osteoporotic vertebral fracture combined with moderate to severe spinal kyphosis has definite effectiveness, strong orthopaedic ability, and minimal trauma, which can effectively restore the sagittal balance of the spine, alleviate pain, and improve the patients' quality of life.

Biomechanical in vitro comparison between anterior column realignment and pedicle subtraction osteotomy for severe sagittal imbalance correction

PURPOSE

To investigate the biomechanical effects of anterior column realignment (ACR) and pedicle subtraction osteotomy (PSO) on local lordosis correction, primary stability and rod strains.

METHODS

Seven cadaveric spine segments (T12-S1) underwent ACR at L1-L2. A stand-alone hyperlordotic cage was initially tested and then supplemented with posterior bilateral fixation. The same specimens already underwent a PSO at L4 stabilized by two rods, a supplemental central rod (three rods) and accessory rods (four rods) with and without adjacent interbody cages (La Barbera in Eur Spine J 27(9):2357-2366, 2018). In vitro flexibility tests were performed under pure moments in flexion/extension (FE), lateral bending (LB) and axial rotation (AR) to determine the range of motion (RoM), while measuring the rod strains with strain gauge rosettes.

RESULTS

Local lordosis correction with ACR (24.7° ± 3.7°) and PSO (25.1° ± 3.9°) was similar. Bilateral fixation significantly reduced the RoM (FE: 31%, LB: 2%, AR: 18%), providing a stability consistent with PSO constructs (p > 0.05); however, it demonstrates significantly higher rod strains compared to PSO constructs with lateral accessory rods and interbody cages in FE and AR (p < 0.05), while being comparable in FE or slightly higher in AR compared to PSO constructs with two and three rods.

CONCLUSION

Bilateral posterior fixation is highly recommended following ACR to provide adequate primary stability. However, primary rod strains in ACR were found comparable or higher than weak PSO construct associated with frequent rod failure; therefore, caution is recommended. These slides can be retrieved under Electronic Supplementary Material.

Retrospective Multicenter Assessment of Rod Fracture After Anterior Column Realignment in Minimally Invasive Adult Spinal Deformity Correction

BACKGROUND

Anterior column realignment (ACR) was developed as a minimally invasive method for treating sagittal imbalance. However, rod fracture (RF) rates associated with ACR are not known. Our objective was to assess the rate of and risk factors for RF following ACR in deformity correction surgery.

METHODS

We conducted a retrospective multicenter review of patients with adult spinal deformity (ASD) who underwent ACR for deformity correction. ASD was defined as coronal Cobb angle ≥20°, pelvic incidence-lumbar lordosis >10°, sagittal vertical axis ≥5 cm, pelvic tilt ≥25°, or thoracic kyphosis ≥60°. Inclusion criteria were ASD, age >18 years, use of ACR, and development of RF or full radiographs obtained at least 1 year after surgery that did not demonstrate RF.

RESULTS

Ninety patients were identified, with mean follow-up of 2.3 ± 1.4 years (age, 64.1 ± 9.4; 54 [60%] women). The most common ACR location was L3/4 (42 cases; 47%). Mean fusion length was 7.5 ± 3.6 levels. Four (4.4%) of 90 patients developed RF within 12 months of surgery. RF occurred adjacent to ACR in all cases; RF was not associated with focal correction (P = 0.49), rod material (P = 0.8), degree of correction (P > 0.07), or interbody at L5/S1 (P = 0.06). RF was associated with longer fusion constructs in univariate (P = 0.002) and multivariate (P = 0.03) analyses.

CONCLUSIONS

RF occurred in 4.4% of patients with ASD who underwent ACR with a minimum of 1-year follow-up. RF was not associated with focal correction but appears to be associated with global correction and extent of fixation.

S1 Pedicle Subtraction Osteotomy in Sagittal Balance Correction. A Feasibility Study on Human Cadaveric Specimens

BACKGROUND

A cadaveric feasibility study was carried out. Osteotomies to correct fixed sagittal imbalance are usually performed at L₃/ L₄.

OBJECTIVE

To investigate the feasibility of S₁ pedicle subtraction osteotomy to correct spinal deformity and spinopelvic parameters, achieving better results with more limited exposure. The data obtained will allow a fixation construct specific for this osteotomy to be designed.

METHODS

S₁ pedicle subtraction osteotomy was performed on 12 cadaveric specimens. Baseline and postprocedural computed tomography and biomechanical studies were performed. Data were analyzed with a fixation system SolidWorks model, and the redesigned fixation construct was described and analyzed with an ANSYS model.

RESULTS

S₁ pedicle subtraction osteotomy is technically feasible. The fixation can be achieved with L₄, L₅, and iliac screws connected with bars. The system can be reinforced with a polyetheretherketone cage placed anteriorly in the S₁ body osteotomy site, a cross-connecting bar, a double iliac screw, or an anterior interbody cage placed at the L₅-S₁ disc. The fixation strength is improved by angulating the iliac rod channel 10°, adding a semi-sphere to the locking screw contact surface and 2 fins to its saddle. The redesigned construct showed suitable stress and deformation levels, achieving the expected biomechanical requirements.

DISCUSSION

Compared with surgery on higher levels, S₁ pedicle subtraction osteotomy allows greater correction with shorter fixation, because the osteotomy is performed at a more caudal level, modifying the spinopelvic parameters.

CONCLUSIONS

S₁ pedicle subtraction osteotomy is technically feasible. Finite element analysis results indicate that it has appropriate biomechanical properties.

Approach Selection: Multiple Anterior Lumbar Interbody Fusion to Recreate Lumbar Lordosis Versus Pedicle Subtraction Osteotomy: When, Why, How?

Restoration of physiologic lumbar lordosis is a fundamental principle of spinal deformity surgery. Techniques using multilevel anterior lumbar interbody fusion or pedicle subtraction osteotomy (PSO) are described. Multilevel anterior lumbar interbody fusion provides a gradual multilevel correction and avoids the morbidity associated with PSO but necessitates familiarity with the anterior approach or an approach surgeon. PSO provides a large angular correction at a single level, requires only one approach, and allows for simultaneous multiplanar correction and open posterior decompression. This article provides guidance on the appropriate use of each technique for restoration of lumbar lordosis in patients with degenerative lumbar deformity.