The Effect of Arthrodesis, Implant Stiffness, and Time on the Canine Lumbar Spine

A comparative finite element analysis of artificial intervertebral disc replacement and pedicle screw fixation of the lumbar spine

Titanium alloy-based Pedicle screw-rod fusion is a very common technique to provide higher fusion regularity than other methods. In recent times, Carbon-fibre-reinforced (CFR)-PEEK rod is used to better reduce the fusion rate. Alternatively, total disc replacement (TDR) is also very common for the non-fusion treatment method for degenerative disc disease (DDD). This study aims to investigate flexibility (ROM), stability, stress condition in implant, implant adjacent bone of the implanted lumbar spine during different physiological movements and loading environments. The finite element (FE) intact model of the lumbar spine (L2-L5) with two-level pedicle screw-rod fusion at L3-L4-L5 and two-level artificial disc replacement was developed. CFR-PEEK was taken for rod for semi-rigid fusion. UHMWPE was taken as core part of the artificial disc. The FE models were simulated under 6, 8 and 10 Nm moments in left right lateral bending, flexion and extension movements. The total ROM was reduced for two-level pedicle screw fixation and increased for the artificial disc replacement model during flexion extension compared to the intact spine. The total ROM was reduced by around 54% and 25% for two-level fixation and increased by 30% and 19.5% for artificial disc replacement spine, under flexion-extension and left-right lateral bending respectively. For screw fixation, the ROM increased by 15% and 18% reduced by 4.5% and 14% for disc replacement at the adjacent segments for flexion-extension and left-right lateral bending.

Biomaterials in Spinal Implants: A Review

The aim to find the perfect biomaterial for spinal implant has been the focus of spinal research since the 1800s. Spinal surgery and the devices used therein have undergone a constant evolution in order to meet the needs of surgeons who have continued to further understand the biomechanical principles of spinal stability and have improved as new technologies and materials are available for production use. The perfect biomaterial would be one that is biologically inert/compatible, has a Young’s modulus similar to that of the bone where it is implanted, high tensile strength, stiffness, fatigue strength, and low artifacts on imaging. Today, the materials that have been most commonly used include stainless steel, titanium, cobalt chrome, nitinol (a nickel titanium alloy), tantalum, and polyetheretherketone in rods, screws, cages, and plates. Current advancements such as 3-dimensional printing, the ProDisc-L and ProDisc-C, the ApiFix, and the Mobi-C which all aim to improve range of motion, reduce pain, and improve patient satisfaction. Spine surgeons should remain vigilant regarding the current literature and technological advancements in spinal materials and procedures. The progression of spinal implant materials for cages, rods, screws, and plates with advantages and disadvantages for each material will be discussed.

Fractures After Removal of Spinal Instrumentation: Revisiting the Stress-Shielding Effect of Instrumentation in Spine Fusion

BACKGROUND

Many practicing spine surgeons believe that instrumentation can be removed during revision surgery in successful posterolateral or anterior spinal fusions, confirmed by computed tomography and intraoperative exploration. The stress-shielding effect of spinal instrumentation was well described in the late 1980s and 1990s but has not received recent attention. Despite the paucity of recent literature, concepts underlying the biology and biomechanics of the spinal fusion mass remain particularly salient given the increasing incidence of revision spinal fusion surgery. The aim of this study was to highlight a potential complication of instrumentation removal owing to stress shielding of instrumentation on the spinal column and fusion mass.

METHODS

A retrospective review was performed, and a small case series was described.

RESULTS

In 3 cases, despite apparent solid fusion demonstrated on preoperative computed tomography and confirmed by intraoperative exploration, new fractures developed after removal of instrumentation. In these cases, fracture occurred at the transition zone between the newly rigid instrumented area and previous fusion. This highlights the relative weakness of the fusion and may be explained by the stress-shielding effect of instrumentation within the fusion mass.

CONCLUSIONS

Spinal instrumentation revision requires careful consideration, and routine implant removal should not be performed. The presence of a solid fusion on computed tomography and/or at intraoperative exploration may not justify implant removal in these cases. In cases of extension of a fusion, use of a bridging connection to the new implants should be considered. The cases presented demonstrate the consequences of the stress-shielding effect of implants on the spine and fusion mass.

A study to compare the efficacy of polyether ether ketone rod device with titanium devices in posterior spinal fusion in a canine model

BACKGROUND

The benefits of posterior lumbar fusion surgery with orthotopic paraspinal muscle-pediculated bone flaps are well established. However, the problem of non-union due to mechanical support is not completely resolved. The aim of the study was to compare the efficacy of polyether ether ketone (PEEK) rod device with conventional titanium devices in the posterior lumbar fusion surgery with orthotopic paraspinal muscle-pediculated bone flaps.

METHODS

This was a randomized controlled study with an experimental animal model. Thirty-two mongrel dogs were randomly divided into two groups-control group (n = 16), which received the titanium device and the treatment group (n = 16), which received PEEK rods. The animals were sacrificed 8 or 16 weeks after surgery. Lumbar spines of dogs in both groups were removed, harvested, and assessed for radiographic, biomechanical, and histological changes.

RESULTS

Results in the current study indicated that there was no significant difference in the lumbar spine of the control and treatment groups in terms of radiographic, manual palpation, and gross examination. However, certain parameters of biomechanical testing showed significant differences (p < 0.05) in stiffness and displacement, revealing a better fusion (treatment group showed decreased stiffness with decreased displacement) of the bone graft. Similarly, the histological analysis also revealed a significant fusion mass in both treatment and control groups (p < 0.05).

CONCLUSIONS

These findings revealed that fixation using PEEK connecting rod could improve the union of the bone graft in the posterior lumbar spine fusion surgery compared with that of the titanium rod fixation.

A Uniquely Shaped Rod Improves Curve Correction in Surgical Treatment of Adolescent Idiopathic Scoliosis

STUDY DESIGN

A retrospective cohort study.

OBJECTIVE

The aim of this study is to determine the initial curve correction in patients surgically treated for adolescent idiopathic scoliosis (AIS) using either beam-like rods (BRs) or traditional circular rods (CRs).

SUMMARY OF BACKGROUND DATA

Posterior fusion using all pedicle screw instrumentation has become the standard for the surgical treatment of AIS. Traditionally, the rod is circular in the cross-sectional plane. Recent biomechanical studies suggest that a beam-like structure of the rod may enhance the stiffness of the construct and thereby possibly improve curve correction.

METHODS

Two consecutive series of patients surgically treated for AIS between May 2011 and May 2015 were included in the study. Patients were all treated with an ultralow profile all-pedicle screw implant system. In the first series, conventional 5.5 mm CoCr CR were used, and in the second series, 5.5 mm CoCr BR were used. Antero-posterior and lateral radiographs preoperatively and within seven days after surgery were used to measure the correction obtained.

RESULTS

The first 60 patients were operated with CR and the subsequent 69 with BR. There was no statistical difference in age, gender, preoperative curve magnitude, Lenke type, or number of levels instrumented (P = 0.451). Major curve correction was significantly better in the BR group than in the CR group (66 vs. 57%) (P < 0.001). We found no difference in preoperative flexibility, secondary curve correction, sagittal balance, or coronal balance (P > 0.058). A postoperative decrease in thoracic kyphosis was seen with no significant difference between groups. Median T5-T12 change was -7° versus -3° for BR and CR, respectively (P = 0.051).

CONCLUSION

A BR design results in a significantly better curve correction than conventional rods, but the difference is moderate and the clinical value is uncertain.

LEVEL OF EVIDENCE

3.

Flexible Stabilisation of the Degenerative Lumbar Spine Using PEEK Rods

Posterior lumbar interbody fusion using cages, titanium rods, and pedicle screws is considered today as the gold standard of surgical treatment of lumbar degenerative disease and has produced satisfying long-term fusion rates. However this rigid material could change the physiological distribution of load at the instrumental and adjacent segments, a main cause of implant failure and adjacent segment disease, responsible for a high rate of further surgery in the following years. More recently, semirigid instrumentation systems using rods made of polyetheretherketone (PEEK) have been introduced. This clinical study of 21 patients focuses on the clinical and radiological outcomes of patients with lumbar degenerative disease treated with Initial VEOS PEEK^®-Optima system (Innov'Spine, France) composed of rods made from PEEK-OPTIMA^® polymer (Invibio Biomaterial Solutions, UK) without arthrodesis. With an average follow-up of 2 years and half, the chances of reoperation were significantly reduced (4.8%), quality of life was improved (ODI = 16%), and the adjacent disc was preserved in more than 70% of cases. Based on these results, combined with the biomechanical and clinical data already published, PEEK rods systems can be considered as a safe and effective alternative solution to rigid ones.

Biomechanical stability according to different configurations of screws and rods

STUDY DESIGN

Comparison of biomechanical strength according to 2 different configurations of screws and rods.

OBJECTIVE

To compare the biomechanical strength of different configurations of screws and rods composed of the same material and of the same size.

SUMMARY OF BACKGROUND DATA

Many complications related to instrumentation have been reported. The incidence of metallic failure would differ according to the materials and configurations of the assembly of the screws and rods used. However, to our knowledge, the biomechanical effects of implant assembly rods and screws with different configurations and different contours have not been reported.

METHODS

Biomechanical testing was conducted to compare top tightening (TT) screw-rod configuration with side tightening (ST) screw-rod configuration. All tests were conducted using a hydraulic all-purpose testing machine. All data were acquired at a rate of 10 Hz. Both screw systems used spinal rods of 6 mm diameter and were made of TiAl4V ELI material. Among 5 types of tests, 3 were conducted on the basis of American Society for Testing and Materials (ASTM) F 1798 to 97 and F1717-10. The other 2 tests were conducted for comparing the characteristics between TT and ST pedicle screws according to modified methods from ASTM F 1717-10 and ASTM F 1798-97. All results including axial gripping capacity and yield forces were obtained using the same methods on the basis of the mentioned ASTM standards.

RESULTS

In the axial gripping capacity test, the mean axial gripping capacity of the TT screw-rod configuration was 3332 ± 118 N and that of ST was 2222 ± 147 N in straight rods (P = 0.019). In 15-degree contoured rods, TT was 2988 ± 199 N and ST was 2116 ± 423 N (P = 0.014). In 30-degree contoured rods, TT was 2227 ± 408 N and ST was 1814 ± 285 N (P = 0.009). In the pulling-out test, the pulling-out force of ST was 8695 ± 1616 N and that of TT was 6106 ± 195 N (P = 0.014). In the rod-pushing test, the failure force of ST was 4131 ± 205 N and that of TT was 5639 ± 105 N. In the compressive fatigue test, the maximum load was 145 N in ST and 119 N in TT. In the cycle fatigue test, the fatigue strength of ST was higher than that of TT. In the rod-pushing test, the failure force of ST was 4131 ± 205 N and that of TT was 5639 ± 105 N (P=0.046).

CONCLUSIONS

Two different configurations of rod-screw systems found statistically significant differences with axial gripping, pulling out, and fatigue failures. ST constructs improved fixation stability over TT constructs. It was concluded that ST configuration may reduce complications related to implantation.

Rods in spinal surgery: a review of the literature

BACKGROUND CONTEXT

Spinal instrumentation has been used for more than five decades. Since the introduction of the Harrington rod in 1962, new rod materials and concepts have been developed. Rigid rod fixation has achieved higher fusion rates than previous methods. Recently, semirigid rod fixation devices have been used for both dynamic stabilization and fusion fixation. Memory rods, which have an interesting ability to return to their pre-bent shape when the temperature increases, are expected to be used for scoliosis correction.

PURPOSE

To review the previous literature regarding biofunctionality and biocompatibility of rods in spinal surgery.

CONCLUSION

The properties of each type of rod need to be taken into consideration when performing spinal instrumentation surgery.

Posterior spinal fusion using pedicle screws

Few clinical studies have reported polyetheretherketone (PEEK) rod pedicle screw spinal instrumentation systems (CD-Horizon Legacy PEEK rods; Medtronic, Minneapolis, Minnesota). This article describes a clinical series of 52 patients who underwent posterior spinal fusion using the PEEK Rod System between 2007 and 2010. Of the 52 patients, 25 had degenerative disk disease, 10 had lateral recess stenosis, 6 had degenerative spondylolisthesis, 6 had lumbar spine vertebral fracture, 4 had combined lateral recess stenosis and degenerative spondylolisthesis, and 1 had an L5 giant cell tumor. Ten patients had 1-segment fusion, 29 had 2-segment fusion, and 13 had 3-segment fusion. Mean follow-up was 3 years (range, 1.5-4 years); no patient was lost to follow-up. Clinical evaluation was performed using the Oswestry Disability Index and a low back and leg visual analog pain scale. Imaging evaluation of fusion was performed with standard and dynamic radiographs. Complications were recorded. Mean Oswestry Disability Index scores improved from 76% preoperatively (range, 52%-90%) to 48% at 6 weeks postoperatively, and to 34%, 28%, and 30% at 3, 6, and 12 months postoperatively, respectively. Mean low back and leg pain improved from 8 and 9 points preoperatively, respectively, to 6 and 5 points immediately postoperatively, respectively, and to 2 points each thereafter. Imaging union of the arthrodesis was observed in 50 (96%) patients by 1-year follow-up. Two patients sustained screw breakage: 1 had painful loss of sagittal alignment of the lumbar spine and underwent revision spinal surgery with pedicle screws and titanium rods and the other had superficial wound infection and was treated with wound dressing changes and antibiotics for 6 weeks. No adjacent segment degeneration was observed in any patient until the time of this writing.

Posterior lumbar fusion by peek rods in degenerative spine: preliminary report on 30 cases

PURPOSE

Pre-curved peek rods to support posterior lumbar fusion have been available in the market since 4 years. Potential advantages using this new technology are increased load sharing on the anterior column promoting interbody fusion, reduced stress on bone-screw interface decreasing the rate of screw mobilization and, in the long term, reduced incidence of adjacent level disc degeneration.

METHODS

The authors retrospectively reviewed 30 cases in which posterior fusion was supported by peek rods, analyzing early complications, rate of fusion and clinical outcome.

RESULTS

At an average follow-up of 18 months, both clinical and radiographic results were satisfactory with only one case requiring surgical revision for a mechanical complication.

CONCLUSIONS

The semi-rigid systems can now be considered a viable option in the lumbar degenerative disease, although clinical evaluations are necessary in the longer term.

Analysis of instrumentation/fusion survivorship without reoperation after primary posterior multiple anchor instrumentation and arthrodesis for idiopathic scoliosis

BACKGROUND

During the past 25 years, spinal instrumentation systems and surgical techniques used to treat idiopathic scoliosis have evolved, achieving fewer patient restrictions during arthrodesis healing, shorter constructs, and better correction. The purposes of this retrospective comparative study were to determine the survivorship of the implant/fusion without reoperation and the risk factors influencing such survival.

METHODS

From 1989 through 2002, 208 consecutive patients (index patient included, age 10-20 years) underwent primary posterior instrumentation and arthrodesis with the same multiple anchor implant system by one surgeon, a co-designer of the system. Two hundred seven were followed for more than 2 years; reoperation status was available for them at an average follow-up of 8.3 years. Twenty-one independent demographic, deformity, instrumentation, and process variables possibly influencing the need for reoperation were studied by comparing the reoperated group with the unreoperated group.

RESULTS

Nineteen patients (9.2%) had reoperation; 16 (7.7%) were for indications related to posterior spine instrumentation. Survival of the implant/fusion without reoperation for spine instrumentation-related indications was 96% (95% confidence interval [CI], 93.2-98.7%) at 5 years, 91.6% (95% CI, 86.9-96.3%) at 10 years, 87.1% (95% CI, 79.5-94.6%) at 15 years, and 73.7% (95% CI, 48.6-98.6%) at 16 years, when the number at risk was nine. Reoperation need was significantly influenced by two implant variables: transverse connector design (p=.0012) and the lower instrumented vertebra anchors used (p=.0004). At 9 years, the longest interval allowing comparison, survival of the implant/fusion without reoperation for these two variables was 100% (six subjects at risk) compared to 82% (95% CI, 74.2-90.3%) with 59 patients still at risk for reoperation for those who did not have them, p=.0014.

CONCLUSIONS

The most stable lower instrumented vertebra anchor configuration, bilateral pedicle screws, and the stronger transverse connector design, closed drop entry, provided the best survival of the implant/fusion without reoperation with this system and the techniques used at 9-year follow-up. We hope that this post-market study using survivorship techniques will be a guide for studies of other spinal implants.

Biomechanical evaluation and comparison of polyetheretherketone rod system to traditional titanium rod fixation

BACKGROUND CONTEXT

Polyetheretherketone (PEEK) has been increasingly used as a biomaterial for spinal implants. PEEK lumbar fusion rods have recently become available for use in posterior lumbar fusion procedures.

PURPOSE

To compare Polyetheretherketone Rod System to traditional titanium rod fixation in a cadaveric model and provide mechanical test data for the PEEK system.

STUDY DESIGN

Biomechanical testing.

METHODS

Cadaveric biomechanical testing was conducted to compare Expedium 5.5 mm PEEK rods to titanium rods of equivalent diameter. Biomaterials testing was performed to determine static and dynamic performance of Expedium 5.5 mm PEEK rods with 6% BaSo4 in compressive bending and torsion.

RESULTS

Cadaveric testing demonstrated that PEEK rods can significantly reduce the range of motion of a destabilized segment. The testing showed no significant difference in the stability provided by PEEK and titanium rods in posterolateral fusion (PLF) or posterior lumbar interbody fusion (PLIF) constructs. PEEK static compressive bending tests showed 67 degrees displacement without fracture of the rod. Torsion testing showed 30 degrees of rotation without yield or plastic deformation. Dynamic compression testing revealed two fatigue runouts at 23 degrees.

CONCLUSIONS

PEEK rods provide comparable stability to titanium rods of equivalent diameter in cadaveric testing. Mechanical testing suggests PEEK rods can withstand far beyond the angular displacements suggested by cadaveric testing and that of normal physiologic range of motion. Potential advantages to PEEK rods include better anterior column load sharing, reduced stress at bone-to-screw interface, and reduced computed tomography and magnetic resonance imaging scatter and artifact.