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

A Biomechanical Analysis of Instrumentation Constructs During Vertebral Column Resection: Stability When You Need It!

STUDY DESIGN

Biomechanical testing.

OBJECTIVE

Investigate the optimal construct for stabilization of the spine during vertebral column resection (VCR).

BACKGROUND

VCR is a powerful technique for achieving correction in severe cases of spinal deformity. However, this also creates an unstable spine, which requires stable fixation to prevent iatrogenic neurological injury. It is common practice to place a temporary unilateral rod configuration to achieve this stability during surgery but no study to date has investigated the optimal construct configuration.

MATERIALS AND METHODS

A unilateral VCR model representing an acute 50° kyphotic deformity with a standardized 30 mm resection was created. Three conditions underwent testing: (1) Rod material and diameter, (2) Rod configuration, and (3) Number of fixation points. Six unique samples were tested in each group in both flexion and extension. Before testing, a 10N preload and underwent cyclical testing in flexion/extension. System stiffness was calculated and compared across groups.

RESULTS

Assessment of rod size and composition using a single screw construct (2 total screws) demonstrated that for titanium rods, increasing rod size significantly increased the construct stiffness ( P = 0.001). Although cobalt-chromium (Co-Cr) rods were significantly stiffer than the corresponding sized titanium rods, there was no significant difference between rod diameters for Co-Cr ( P = 0.98). However, when tested using a dual screw (4 total screws) construct, these constructs were significantly stiffer than the corresponding single screw constructs ( P < 0.0001). Of the various rod configurations, the dual rod demonstrated the greatest stiffness (34.8 ± 2.1 N/mm; P < 0.0001).

CONCLUSION

Surgical construct stiffness during a VCR is multifactorial. Larger rod diameter, increased number of fixation points, stiffer rod material, and increased number of rods across the resection site increase the construct stiffness. With minimal points of fixation using Co-Cr rods, increasing rod diameter does not impart greater construct stiffness unless additional fixation points are included.

Construct-construct "rail technique" decreases screw strain during spinal deformity corrective maneuvers: a mechanical analysis

PURPOSE

To compare screw strains adjacent to a simulated spinal osteotomy between segmental compression (SC) and cantilever bending (CB) to SC and CB performed over a construct-to-construct lateral accessory rod ("rail").

METHODS

10 PCF foam blocks were instrumented with 6 polyaxial pedicle screws, each with a linear strain gage. SC and CB were performed over a traditional construct (midline rods) or over a construct-to-construct lateral accessory rod. Real-time screw strains were collected and peak strains were reported and compared between corrective techniques.

RESULTS

Strains in screws closest to the osteotomy were significantly less during "rail" compression compared to traditional SC. Maximum screw strains were significantly lower during "rail" SC (p < .001) and CB (p = 0.003) compared to traditional SC and CB, respectively. Total screw strain was more evenly distributed over all 6 screws during "rail" compression and CB compared to traditional techniques, which concentrated strain at individual screws adjacent to the osteotomy.

CONCLUSIONS

Performing SC and CB across an accessory construct-to-construct lateral ("rail") rod resulted in significantly lower strain on individual pedicle screws adjacent to a simulated spinal osteotomy compared to traditional SC and CB. As such, the "rail" may lessen risk of screw pull-out and screw plow during maneuvers to correct spinal deformities. Future work focused on building upon this controlled study in cadaveric specimens will be important to validate these findings in more clinically relevant scenarios.

Multirod posterior occipitocervical instrumentation constructs: a biomechanical analysis and initial case series of 10 patients with complex craniocervical pathology

BACKGROUND CONTEXT

Stabilization of the occipitocervical (OC) junction with posterior instrumentation plays a vital role in addressing a spectrum of pathologies. Due to limited bone surfaces of the occiput and C1 lamina, achieving union across the OC junction is challenging.

PURPOSE

To explore the biomechanics and a clinical series of patients treated with multirod constructs across the OC junction using a novel occipital plate with single- and dual-headed, modular tulip heads.

STUDY DESIGN/SETTING

Biomechanical analysis and retrospective case series.

PATIENT SAMPLE

Adults at a single institution who underwent posterior cervical multirod constructs across the OC junction.

OUTCOME MEASURES

OC-C4 range of motion (ROM), maximum von Mises stress on the rods, and adjacent segment ROMs and intradiscal parameters. Patient demographics, revision operations, rod breakages, wound complications.

METHODS

A validated occiput-cervical finite element (FE) model was used to simulate OC-C4 cervical fixation under multidirectional pure moment loading. A total of 4 rod configurations were simulated: (A) 2-rod-Ti (4.0 mm titanium rods); (B) 2-rod-CoCr (3.5 mm cobalt chrome rods); (C) 3-rods (4.0 mm titanium rods); (D) 4-rods (4.0 mm titanium rods). The aforementioned measures were compared. A retrospective analysis was also performed of adults at a single institution who underwent posterior cervical multirod constructs across the OC junction.

RESULTS

Biomechanically, lowest primary rod stresses were observed for 3- and 4-rod constructs. Compared to 2-rod-Ti (121.8 MPa), 2-rod-CoCr showed a 43.2% stress increase in the rods, while 3- and 4-rods experienced rod stress reductions of 20% and 23.2%, respectively. No appreciable differences in OC-C4 ROM, C4-5 ROM, and C4-5 discal stresses were found between multirod and 2-rod constructs. Maximum occipital and C4 screw stresses were decreased in multirod constructs compared to 2-rods, with least stresses noted in the 4-rod construct. Maximum plate stresses were slightly increased in the 4-rod construct compared to 2- and 3-rod fixation, though the forces were largely similar among the constructs. Ten patients (average age 66.4±10.6 years; 8 males) were assessed clinically. Nine of the ten operations were for primary stabilization of pathological fractures and associated craniocervical and/or atlantoaxial instability using 4-rods across the OC junction. At an average follow-up time of 1.58±0.5 years (range, 1-2.3 years), there were no instrumentation failures, no adjacent segment failures, and no wound complications.

CONCLUSIONS

In this proof-of-concept investigation, multiple rods (3- and 4-rods) across the OC junction using a novel occipital plate with single- and dual-headed, modular tulips was safe and effective in stabilizing the OC junction. Accompanying FE analysis demonstrated that multirod constructs decreased primary rod stresses and had lower stresses on occipital and C4 screws compared to 2-rod constructs, while occipital plate stresses were largely similar. Additional clinical studies are needed to confirm these findings and to determine the ultimate utility of multirod constructs across the OC junction.

Use of Supplemental Rod Constructs in Adult Spinal Deformity Surgery: A Review

Adult spinal deformity comprises a heterogeneous group of disorders that primarily affects older patients and can have a significant negative affect on health-related quality of life. Operative treatment for adult spinal deformity typically entails posterior instrumented fusions that have demonstrated the potential to significantly improve health-related quality of life outcomes. However, until fusion is achieved, the instrumentation providing structural support is subject to repetitive cyclical loading that disproportionately fatigues high-stress areas and can result in instrumentation failure. Despite considerable advances in surgical fixation techniques and technology, pseudarthrosis with subsequent implant failure still poses a challenge for surgeons and continues to be 1 of the most common complications, leading to revision surgery. The addition of supplemental rods to primary constructs has gained widespread popularity to mitigate implant failure. Theoretically, more rods will add stiffness, stability, and decreased surface strain, which will provide longer instrumentation lifespan to allow for osseous fusion. There is significant heterogeneity in these constructs, and different types of supplemental rods (eg, satellite, accessory, delta rods, "kickstand rod," and "iliac accessory rod") can be used independently or in combination to further increase strength. However, the use of supplemental rods may increase the rate of proximal junctional kyphosis/failure and paradoxically diminish anterior column fusion rates. Hence, indications and optimal configurations are still a matter of debate. The aim of this narrative review is to provide an overview of the supplemental rod constructs described in the literature and focus on the current evidence supporting their indications and potential impact.

Instrumentation Failure in Adult Spinal Deformity Patients

In recent years, advances in the surgical treatment of adult spinal deformity (ASD) have led to improved outcomes. Although these advances have helped drive the development of deformity surgery to meet the rising volume of patients seeking surgical treatment, many challenges have yet to be solved. Instrumentation failure remains one of the most common major complications following deformity surgery, associated with significant morbidity due to elevated re-operation rates among those experiencing mechanical complications. The two most frequently encountered subtypes of instrumentation failure are rod fracture (RF) and proximal junctional kyphosis/proximal junctional failure (PJK/PJF). While RF and PJK/PJF are both modes of instrumentation failure, they are two distinct entities with different clinical implications and treatment strategies. Considering that RF and PJK/PJF continue to represent a major challenge for patients with ASD and deformity surgeons alike, this review aims to discuss the incidence, risk factors, clinical impact, treatment strategies, preventive measures, and future research directions for each of these substantial complications.

The Advantages of 4-rod Construct over the 2-rod Techniques in Adult Spinal Deformity Patients who Underwent Pedicle Subtraction Osteotomy: A Multicenter Retrospective Comparative Study

OBJECTIVE

In this retrospective study we compared clinicoradiologic outcomes and complication profiles of the traditional 2-rod construct versus the 4-rod construct in patients with adult spinal deformity (ASD) who underwent pedicle subtraction osteotomy (PSO).

METHODS

We performed a retrospective review of 208 ASD patients at 2 referral centers who underwent lumbar PSO and long fusion from thoracic to the pelvis. Two different techniques, including the 4-rod construct and the traditional 2-rod technique, were used at the PSO level. Clinicoradiologic outcomes and complication profiles of the patients were documented and compared statistically between the groups.

RESULTS

The 4-rod construct was associated with statistically lower rates of rod fracture (44.8% vs. 26.4%, P < 0.01), pedicular screw loosening at the PSO level (25.3% vs. 14.0%, P = 0.04), and reoperation (49.4% vs. 33.9%, P = 0.02). Radiologically, the 4-rod construct was associated with higher degree of lumbar lordosis (LL) (-37.4°vs. -26.8°; P < 0.01) and improved pelvic tilt (PT) (-17.2° vs. -9.9°; P < 0.01) and sacral vertical axis (SVA) corrections (-211.5° vs. -192.2°; P = 0.04). Overall, the 4-rod construct was associated with improved quality of life (P = 0.04) and statistically lower Oswestry Disability Index score at 12 months postoperatively (P < 0.01).

CONCLUSIONS

Our results showed that the 4-rod construct was associated with statistically lower rates of rod fracture and pedicular screw loosening at the osteotomy level, higher degree of LL correction and improved PT and SVA than the 2-rod technique. The 4-rod construct was also associated with improved quality of life and Oswestry Disability Index and lower complication profiles.

Biomechanical evaluation of multi-rod constructs to stabilize an S1 pedicle subtraction osteotomy (PSO): a finite element analysis

PURPOSE

To develop and validate a finite element (FE) model of a sacral pedicle subtraction osteotomy (S1-PSO) and to compare biomechanical properties of various multi-rod configurations to stabilize S1-PSOs.

METHODS

A previously validated FE spinopelvic model was used to develop a 30° PSO at the sacrum. Five multi-rod techniques spanning the S1-PSO were made using 4 iliac screws and a variety of primary rods (PR) and accessory rods (AR; lateral: Lat-AR or medial: Med-AR). All constructs, except one, utilized a horizontal rod (HR) connecting the iliac bolts to which PRs and Med-ARs were connected. Lat-ARs were connected to proximal iliac bolts. The simulation was performed in two steps with the acetabula fixed. For each model, PSO ROM and maximum stress on the PRs, ARs, and HRs were recorded and compared. The maximum stress on the L5-S1 disc and the PSO forces were captured and compared.

RESULTS

Highest PSO ROMs were observed for 4-Rods (HR + 2 Med-AR). Constructs consisting of 5-Rods (HR + 2 Lat-ARs + 1 Med-AR) and 6-Rods (HR + 2 Lat-AR + 2 Med-AR) had the lowest PSO ROM. The least stress on the primary rods was seen with 6-Rods, followed by 5-Rods and 4-Rods (HR + 2 Lat-ARs). Lowest PSO forces and lowest L5-S1 disc stresses were observed for 4-Rod (Lat-AR), 5-Rod, and 6-Rod constructs, while 4-Rods (HR + Med-AR) had the highest.

CONCLUSION

In this first FE analysis of an S1-PSO, the 4-Rod construct (HR + Med-AR) created the least rigid environment and highest PSO forces anteriorly. While 5- and 6-Rods created the stiffest constructs and lowest stresses on the primary rods, it also jeopardized load transfer to the anterior column, which may not be favorable for healing anteriorly. A balance between the construct's rigidity and anterior load sharing is essential.