Biomechanical analysis of cervicothoracic junction osteotomy in cadaveric model of ankylosing spondylitis: effect of rod material and diameter

Optimizing cervicothoracic junction biomechanics after C7 pedicle subtraction osteotomy: a cadaveric study of stability and rod strain

OBJECTIVE

To compare biomechanical stability and rod strain among uniform rod (UR), tapered rod (TR), and uniform rod plus accessory rod (UR+AR) constructs in a human cadaveric C7 pedicle subtraction osteotomy (PSO) model of cervical deformity correction.

METHODS

Fourteen human cadaveric C2-T4 specimens were divided into 2 statistically equivalent groups. Specimens were instrumented from C2 to T3, and a 25° PSO was performed at C7. Group 1 was instrumented with 3.5-5.5-mm titanium TRs, and group 2 received 4.0-mm titanium URs. The UR group was also tested with lateral 4.0-mm titanium ARs (UR+AR) at C5-T2. All conditions were tested with 2.0 Nm pure moment and 70 N compressive load. Intervertebral range of motion (ROM) and posterior rod strain (pRS) were measured at C2-C3, T2-T3, and the PSO level. Statistical comparisons used one-way analysis of variance.

RESULTS

ROM was significantly reduced in the TR versus UR construct for right axial rotation (p=0.04) at the PSO level; ROM with TR was significantly greater than with UR and UR+AR in compression (p≤0.02). At the PSO level, pRS was significantly greater in TR than in UR+AR in flexion, extension, and right axial rotation (p≤0.02). At T2/3, pRS was higher in UR than TR in left axial rotation (p=0.003).

CONCLUSIONS

C7 PSO is highly destabilizing. Maximal rod strain was concentrated across the PSO and the cranial fixation site. TR provided higher stability than UR in one direction of movement; however, UR+AR provided the greatest reduction of pRS.

Indications for early revision surgery for material failure in spinal instrumentation: experience at a level 1 center for spinal surgery - a single-center study

Posterior instrumentation is an established treatment for a range of spinal disorders. Material failure is not uncommon, and the indications for a revision are very heterogeneous. This study aimed to evaluate the indications and timing for early revision spinal surgery due to material failure.In this retrospective, single-center cohort study, patients underwent spinal posterior instrumentation between January 2017 and July 2019. They were followed up at 3, 12, and 18 months postoperatively. The time of onset of material failure which led to revision surgery was analyzed. In addition, the relationship between the indications for revision surgery and independent variables was examined using a multivariate logistic regression model.A total of one hundred thirty-five patients were enrolled. Radiolucent zones were found in 30 patients (20%) after 3 months, whereas 48 patients (31%) had radiolucent zones after 12 months. Revision surgery was performed in 13 patients (8.5%). The peak time for revision due to instability was within the first four months of the primary surgery. Multivariate analysis revealed that location, pathology, ASA score, and smoking had no significant impact on the indication for revision surgery, and neither did BMI (P = .042). Non-fusion (P = .007) and radiolucent zones (P = .004), in combination with increased pain (P = .006), were predictors for revision.Our data show that the peak time for early revision of material failure after posterior instrumentation was within the first 4 months of primary surgery. The abnormalities (e.g., radiolucent zones) surrounding the screws without fusion, including persistence of pain, were predictors for revision surgery.

Strategies for prevention of rod fracture in adult spinal deformity: cobalt chrome rod, accessory rod technique, and lateral lumbar interbody fusion

OBJECTIVE

Restoring the proper sagittal alignment in adult spinal deformity (ASD) can improve radiological and clinical outcomes, but pseudarthrosis including rod fracture (RF) is a common problematic complication. The purpose of this study was to analyze the methods for reducing the incidence of RF in deformity correction of ASD.

METHODS

The authors retrospectively selected 178 consecutive patients (mean age 70.8 years) with lumbar degenerative kyphosis (LDK) who underwent deformity correction with a minimum 2-year follow-up. Patients were classified into the non-RF group (n = 131) and the RF group (n = 47). For predicting the crucial factors of RF, patient factors, radiographic parameters, and surgical factors were analyzed.

RESULTS

The overall incidence of RF was 26% (47/178 cases), occurring in 42% (42/100 cases) of pedicle subtraction osteotomy (PSO), 7% (5/67 cases) of lateral lumbar interbody fusion (LLIF) with posterior column osteotomy, 18% (23/129 cases) of cobalt chrome rods, 49% (24/49 cases) of titanium alloy rods, 6% (2/36 cases) placed with the accessory rod technique, and 32% (45/142 cases) placed with the 2-rod technique. There were no significant differences in the incidence of RF regarding patient factors between two groups. While both groups showed severe sagittal imbalance before operation, lumbar lordosis (LL) was more kyphotic and pelvic incidence (PI) minus LL (PI-LL) mismatch was greater in the RF group (p < 0.05). Postoperatively, while LL and PI-LL did not show significant differences between the two groups, LL and sagittal vertical axis correction were greater in the RF group (p < 0.05). Nonetheless, at the last follow-up, the two groups did not show significant differences in radiographic parameters except thoracolumbar junctional angles. As for surgical factors, use of the cobalt chrome rod and the accessory rod technique was significantly greater in the non-RF group (p < 0.05). As for the correction method, PSO was associated with more RFs than the other correction methods, including LLIF (p < 0.05). By logistic regression analysis, PSO, preoperative PI-LL mismatch, and the accessory rod technique were crucial factors for RF.

CONCLUSIONS

Greater preoperative sagittal spinopelvic malalignment including preoperative PI-LL mismatch was the crucial risk factor for RF in LDK patients 65 years or older. For restoring and maintaining sagittal alignment, use of the cobalt chrome rod, accessory rod technique, or LLIF was shown to be effective for reducing RF in ASD surgery.

The Effect of Rod Pattern, Outrigger, and Multiple Screw-Rod Constructs for Surgical Stabilization of the 3-Column Destabilized Cervical Spine - A Biomechanical Analysis and Introduction of a Novel Technique

Objective

Anterior-only reconstructions for cervical multilevel corpectomies are prone to fail under continuous mechanical loading. This study sought to define the mechanical characteristics of different constructs in reducing a range of motion (ROM) of the 3-column destabilized cervical spine, including posterior cobalt-chromium (CoCr)-rods, outrigger-rods (OGR), and a novel triple rod construct using lamina screws (6S3R). The clinical implications of biomechanical findings are discussed in depth from the perspective of the challenges surgeons face cervical deformity correction.

Methods

Three-column deficient cervical spinal models were produced based on reconstructed computed tomography scans. The corpectomy defect between C3 and C7 end-level vertebrae was restored with anterior titanium (Ti) mesh-cage. The ROM was evaluated in a customized 6-degree of freedom spine tester. Tests were performed with different rod materials (Ti vs. CoCr), varying diameter rods (3.5 mm vs. 4.0 mm), with and without anterior plating, and using different construct patterns: bilateral rod fixation (standard-group), OGR-group, and 6S3R-Group. Construct stability was expressed in changes and differences of ROM (°).

Results

The largest reduction of ROM was noticed in the 6S3R-group compared to the standard- and the OGR-group. All differences observed were emphasized with an increasing number of corpectomy levels and if anterior plating was not added. For all simulated 1-, 2-, and 3-level corpectomy constructs, the OGR-group revealed decreased ROM for all motion directions compared to the standard-group. An increase of construct stiffness was also recorded for increased rod diameter (4.0 mm) and stiffer rod material (CoCr), though these effects lacked behind the more advanced construct pattern.

Conclusion

A novel reconstructive technique, the 6S3R-construct, was shown to outperform all other constructs and might resemble a new standard of reference for advanced posterior fixation.

Impact of Electrocautery on Fatigue Life of Spinal Fusion Constructs-An In Vitro Biomechanical Study

Instrumentation failure in the context of spine surgery is attributed to cyclic loading leading to formation of fatigue cracks, which later propagate and result in rod fracture. A biomechanical analysis of the potential impact of electrocautery on the fatigue life of spinal implants has not been previously performed. The aim of this study was to assess the fatigue life of titanium (Ti) and cobalt-chrome (CoCr) rod-screw constructs after being treated with electrocautery. Twelve spinal constructs with CoCr and Ti rods were examined. Specimens were divided into four groups by rod material (Ti and CoCr) and application of monopolar electrocautery on the rods’ surface (control-group and electrocautery-group). Electrocautery was applied on each rod at three locations, then constructs were cyclically tested. Outcome measures were load-to-failure, total number of cycles-to-failure, and location of rod failure. Ti-rods treated with electrocautery demonstrated a significantly decreased fatigue life compared to non-treated Ti-rods. Intergroup comparison of cycles-to-failure revealed a significant mean decrease of almost 9 × 10⁵ cycles (p = 0.03). No CoCr-rods failed in this experiment. Electrocautery application on the surface of Ti-rods significantly reduces their fatigue life. Surgeons should exercise caution when using electrocautery in the vicinity of Ti-rods to mitigate the risk of rod failure.

Biomechanical Evaluation of Cervicothoracic Junction Fusion Constructs

OBJECTIVE

We studied the effect of different cervicothoracic construct design variables on biomechanical stability in vitro.

METHODS

Six fresh-frozen human cadaveric spines (C5-T4) were used. After intact analysis, each specimen was destabilized and reconstructed, with all groups having 4.0-mm pedicle screws placed at T1-T3. The 2 hook-rod constructs included interlaminar hooks at C6 and C7, with either 3.5-mm or 4.0-mm rods (C6-T3). The 2 screw-rod constructs tested included lateral mass screws at C6 and C7, with either 3.5-mm or 4.0-mm rods (C6-T3). The 2 screw-connector-rod constructs tested included lateral mass screws at C6 and C7, with either 3.5-mm or 4.0-mm rods; 1 rod spanned C6-C7 with a connector to a second rod of the same size spanning T1-T3. Global (C6-T3) and intervertebral (C6-C7, C7-T1, T1-T2, and T2-T3) ranges of motion were compared for each construct.

RESULTS

In terms of global (C6-T3) stability, 3.5-mm versus 4.0-mm rod constructs were not significantly different, regardless of whether the construct was hook-rod, screw-rod, or screw-connector-rod. The hook-rod constructs provided less stability compared with the screw-rod and screw-connector-rod constructs in lateral bending (P < 0.04) and axial rotation (P < 0.001). The screw-rod constructs demonstrated a similar range of motion to that of the screw-connector-rod constructs, except for significantly less axial rotation at the C6-C7 level with 3.5-mm rods (P = 0.04).

CONCLUSIONS

We found that the rod diameter of a construct does not appear to significantly influence the biomechanical stability of subaxial constructs. The screw-rod construct resulted in certain biomechanical advantages compared with the screw-connector-rod construct, and both were significantly superior to the hook-rod construct.

Choice of Rods in Surgical Treatment of Adolescent Idiopathic Scoliosis: What Are the Clinical Implications of Biomechanical Properties? - A Review of the Literature

The surgical treatment of adolescent idiopathic scoliosis (AIS) involves 3-dimensional curve correction with multisegmental pedicle screws attached to contoured bilateral rods. The substantial corrective forces exert a high level of stress on the rods, and the ability of the rod to withstand these forces without undergoing permanent deformation relies on its biomechanical properties. These properties, in turn, are dependent on the material, diameter, and shape of the rod. The surgical treatment of AIS is characterized by the requirement for a special biomechanical profile that may differ substantially from what is needed for adult deformity surgery. This overview summarizes the current knowledge of rod biomechanics in frequently used rod constructs, with a particular focus on translational research between biomechanical studies and clinical applicability in AIS patients.

Changes in foraminal area with anterior decompression versus keyhole foraminotomy in the cervical spine: a biomechanical investigation

OBJECTIVE

Anterior cervical discectomy and fusion (ACDF) with or without partial uncovertebral joint resection (UVR) and posterior keyhole foraminotomy are established operative procedures to treat cervical disc degeneration and radiculopathy. Studies have demonstrated reliable results with each procedure, but none have compared the change in neuroforaminal area between indirect and direct decompression techniques. The purpose of this study was to determine which cervical decompression method most consistently increases neuroforaminal area and how that area is affected by neck position.

METHODS

Eight human cervical functional spinal units (4 each of C5–6 and C6–7) underwent sequential decompression. Each level received the following surgical treatment: bilateral foraminotomy, ACDF, ACDF + partial UVR, and foraminotomy + ACDF. Multidirectional pure moment flexibility testing combined with 3D C-arm imaging was performed after each procedure to measure the minimum cross-sectional area of each foramen in 3 different neck positions: neutral, flexion, and extension.

RESULTS

Neuroforaminal area increased significantly with foraminotomy versus intact in all positions. These area measurements did not change in the ACDF group through flexion-extension. A significant decrease in area was observed for ACDF in extension (40 mm2) versus neutral (55 mm2). Foraminotomy + ACDF did not significantly increase area compared with foraminotomy in any position. The UVR procedure did not produce any changes in area through flexion-extension.

CONCLUSIONS

All procedures increased neuroforaminal area. Foraminotomy and foraminotomy + ACDF produced the greatest increase in area and also maintained the area in extension more than anterior-only procedures. The UVR procedure did not significantly alter the area compared with ACDF alone. With a stable cervical spine, foraminotomy may be preferable to directly decompress the neuroforamen; however, ACDF continues to play an important role for indirect decompression and decompression of more centrally located herniated discs. These findings pertain to bony stenosis of the neuroforamen and may not apply to soft disc herniation. The key points of this study are as follows. Both ACDF and foraminotomy increase the foraminal space. Foraminotomy was most successful in maintaining these increases during neck motion. Partial UVR was not a significant improvement over ACDF alone. Foraminotomy may be more efficient at decompressing the neuroforamen. Results should be taken into consideration only with stable spines.

Biomechanical Analysis of Bilateral Facet Joint Stabilization Using Bioderived Tendon for Posterior Cervical Spine Motion Reservation in Goats

OBJECTIVES

To investigate the biomechanical properties of a novel stabilization method for posterior cervical motion preservation using bioderived freeze-dried tendon.

METHODS

Experiments were conducted both in vitro and in vivo. For the in vitro group, 15 fresh-frozen goat spines (C1-C7) were randomly divided into 3 subgroups: intact (INT-vitro, n = 5), injury model (IM-vitro, n = 5), and bilateral facet joint stabilization (BFJS-vitro, n = 5) subgroups. For the in vivo group, 15 adult goats were randomly divided into 3 experimental subgroups: INT-vivo subgroup (n = 5), IM-vivo subgroup (n = 5), and BFJS-vivo subgroup (n = 5). Goats in the in vivo group were euthanized 12 weeks after surgery. Biomechanical tests were performed to evaluate range of motion. Histologic analysis was conducted to evaluate survival and reactions associated with the bioderived tendon.

RESULTS

Compared with the INT-vitro and INT-vivo subgroups, the flexion of IM-vitro and IM-vivo subgroups increased significantly, respectively (P < 0.05). The flexion of the BFJS-vitro and BFJS-vivo subgroups was significantly smaller than in the IM-vitro and IM-vivo subgroups, respectively (P < 0.05). Significant differences between the BFJS-vitro and BFJS-vivo subgroups were observed in flexion, lateral bending, and rotation (P < 0.05). Histologic evaluation demonstrated that fibers arranged regularly and stained homogeneously. New vessels in growth indicated that the bioderived tendon was survival and processed good regeneration.

CONCLUSIONS

Bilateral facet joint stabilization can significantly limit excessive flexion motion and maintain adequate stability. Furthermore, the preservation of extension motions without limiting lateral bending and rotation ideally simulates the features of the posterior ligamentous complex. This preserves the dynamic stability of the lower cervical spine.

Quantitative in vivo biocompatibility of new ultralow-nickel cobalt-chromium-molybdenum alloys

Nickel (Ni) eluted from metallic biomaterials is widely accepted as a major cause of allergies and inflammation. To improve the safety of cobalt-chromium-molybdenum (Co-Cr-Mo) alloy implants, new ultralow-Ni Co-Cr-Mo alloys with and without zirconium (Zr) have been developed, with Ni contents of less than 0.01%. In the present study, we investigated the biocompatibility of these new alloys in vivo by subcutaneously implanting pure Ni, conventional Co-Cr-Mo, ultralow-Ni Co-Cr-Mo, and ultralow-Ni Co-Cr-Mo with Zr wires into the dorsal sides of mice. After 3 and 7 days, tissues around the wire were excised, and inflammation; the expression of IL-1β, IL-6, and TNF-α; and Ni, Co, Cr, and Mo ion release were analyzed using histological analyses, qRT-PCR, and inductively coupled plasma mass spectrometry (ICP-MS), respectively. Significantly larger amounts of Ni eluted from pure Ni wires than from the other wires, and the degree of inflammation depended on the amount of eluted Ni. Although no significant differences in inflammatory reactions were identified among new alloys and conventional Co-Cr-Mo alloys in histological and qRT-PCR analyses, ICP-MS analysis revealed that Ni ion elution from ultralow-Ni Co-Cr-Mo alloys with and without Zr was significantly lower than from conventional Co-Cr-Mo alloys. Our study, suggests that the present ultralow-Ni Co-Cr-Mo alloys with and without Zr have greater safety and utility than conventional Co-Cr-Mo alloys. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1505-1513, 2016.

Biomechanical demands on posterior fusion instrumentation during lordosis restoration procedures

OBJECTIVE The goal of this study was to investigate the forces placed on posterior fusion instrumentation by 3 commonly used intraoperative techniques to restore lumbar lordosis: 1) cantilever bending; 2) in situ bending; and 3) compression and/or distraction of screws along posterior fusion rods. METHODS Five cadaveric torsos were instrumented with pedicle screws at the L1-5 levels. Specimens underwent each of the 3 lordosis restoration procedures. The pedicle screw pullout force was monitored in real time via strain gauges that were mounted unilaterally at each level. The degree of correction was noted through fluoroscopic imaging. The peak loads experienced on the screws during surgery, total demand on instrumentation, and resting loads after corrective maneuvers were measured. RESULTS A mean overall lordotic correction of 10.9 ± 4.7° was achieved. No statistically significant difference in lordotic correction was observed between restoration procedures. In situ bending imparted the largest loads intraoperatively with an average of 1060 ± 599.9 N, followed by compression/distraction (971 ± 534.1 N) and cantilever bending (705 ± 413.0 N). In situ bending produced the largest total demand and postoperative loads at L-1 (1879 ± 1064.1 and 487 ± 118.8 N, respectively), which were statistically higher than cantilever bending and compression/distraction (786 ± 272.1 and 138 ± 99.2 N, respectively). CONCLUSIONS In situ bending resulted in the highest mechanical demand on posterior lumbar instrumentation, as well as the largest postoperative loads at L-1. These results suggest that the forces generated with in situ bending indicate a greater chance of intraoperative instrumentation failure and postoperative proximal pedicle screw pullout when compared with cantilever bending and/or compression/distraction options. The results are aimed at optimizing correction and fusion strategies in lordosis restoration cases.

Construct Type and Risk Factors for Pseudarthrosis at the Cervicothoracic Junction

STUDY DESIGN

Retrospective cohort.

OBJECTIVE

The primary goal is to compare the clinical results of 2 types of constructs commonly used at the cervicothoracic junction: small rods (3.2-mm/3.5-mm rods) or transitional constructs. The secondary goal is to perform a case-control study of risk factors for pseudarthrosis at the cervicothoracic junction.

SUMMARY OF BACKGROUND DATA

Various constructs have been used to stabilize across the cervicothoracic junction; however, no study to date has objectively compared their outcome. Our hypothesis was that both constructs would have similar fusion and complication rates.

METHODS

A retrospective review of a prospectively collected database revealed 135 patients with the aforementioned constructs and having followed up with imaging at 6 months, 12 months, and 24 months. Univariate analysis comparing the 2 different construct groups was performed. Multivariate analysis for risk factors of pseudarthrosis was also performed.

RESULTS

There were a total of 10 patients with pseudarthrosis at 2-year follow-up. There was no difference in pseudarthrosis rate between the small rods (7%) and transitional constructs (8.6%) (P = 0.99). The overall construct lengths were similar (5.8 levels in small rods, 6.7 levels in transitional construct). Blood loss was higher in transitional constructs (574 ± 69 mL) than in small rods (236 ± 53 mL) (P < 0.001). Transitional constructs also had longer operating times (249 min) than small rods (207 min) (P < 0.03). Overall complication rate was higher in the transitional constructs (P < 0.03). Tobacco use, corpectomy, lack of an anterior construct, and construct length were all risk factors for cervicothoracic junction pseudarthrosis in the multivariate analysis.

CONCLUSION

Overall pseudarthrosis rates were similar between small rods and transitional constructs. There was higher complications rate, blood loss, and operating time associated with transitional constructs. Pseudarthrosis risk factors at the cervicothoracic junction include tobacco use, corpectomy, lack of an anterior construct, and longer constructs.

LEVEL OF EVIDENCE

3.

Prospective multicenter assessment of risk factors for rod fracture following surgery for adult spinal deformity

OBJECT

Improved understanding of rod fracture (RF) following adult spinal deformity (ASD) surgery could prove valuable for surgical planning, patient counseling, and implant design. The objective of this study was to prospectively assess the rates of and risk factors for RF following surgery for ASD.

METHODS

This was a prospective, multicenter, consecutive series. Inclusion criteria were ASD, age > 18 years, ≥5 levels posterior instrumented fusion, baseline full-length standing spine radiographs, and either development of RF or full-length standing spine radiographs obtained at least 1 year after surgery that demonstrated lack of RF. ASD was defined as presence of at least one of the following: coronal Cobb angle ≥20°, sagittal vertical axis (SVA) ≥5 cm, pelvic tilt (PT) ≥25°, and thoracic kyphosis ≥60°.

RESULTS

Of 287 patients who otherwise met inclusion criteria, 200 (70%) either demonstrated RF or had radiographic imaging obtained at a minimum of 1 year after surgery showing lack of RF. The patients' mean age was 54.8 ± 15.8 years; 81% were women; 10% were smokers; the mean body mass index (BMI) was 27.1 ± 6.5; the mean number of levels fused was 12.0 ± 3.8; and 50 patients (25%) had a pedicle subtraction osteotomy (PSO). The rod material was cobalt chromium (CC) in 53%, stainless steel (SS), in 26%, or titanium alloy (TA) in 21% of cases; the rod diameters were 5.5 mm (in 68% of cases), 6.0 mm (in 13%), or 6.35 mm (in 19%). RF occurred in 18 cases (9.0%) at a mean of 14.7 months (range 3-27 months); patients without RF had a mean follow-up of 19 months (range 12-24 months). Patients with RF were older (62.3 vs 54.1 years, p = 0.036), had greater BMI (30.6 vs 26.7, p = 0.019), had greater baseline sagittal malalignment (SVA 11.8 vs 5.0 cm, p = 0.001; PT 29.1° vs 21.9°, p = 0.016; and pelvic incidence [PI]-lumbar lordosis [LL] mismatch 29.6° vs 12.0°, p = 0.002), and had greater sagittal alignment correction following surgery (SVA reduction by 9.6 vs 2.8 cm, p < 0.001; and PI-LL mismatch reduction by 26.3° vs 10.9°, p = 0.003). RF occurred in 22.0% of patients with PSO (10 of the 11 fractures occurred adjacent to the PSO level), with rates ranging from 10.0% to 31.6% across centers. CC rods were used in 68% of PSO cases, including all with RF. Smoking, levels fused, and rod diameter did not differ significantly between patients with and without RF (p > 0.05). In cases including a PSO, the rate of RF was significantly higher with CC rods than with TA or SS rods (33% vs 0%, p = 0.010). On multivariate analysis, only PSO was associated with RF (p = 0.001, OR 5.76, 95% CI 2.01-15.8).

CONCLUSIONS

Rod fracture occurred in 9.0% of ASD patients and in 22.0% of PSO patients with a minimum of 1-year follow-up. With further follow-up these rates would likely be even higher. There was a substantial range in the rate of RF with PSO across centers, suggesting potential variations in technique that warrant future investigation. Due to higher rates of RF with PSO, alternative instrumentation strategies should be considered for these cases.

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.

Postoperative magnetic resonance imaging artifact with cobalt-chromium versus titanium spinal instrumentation: presented at the 2013 Joint Spine Section Meeting. Clinical article

OBJECT

Cobalt-chromium alloy (CoCr) rods haves some preferred biomechanical properties over titanium rods for spinal fixation. The use of CoCr rods in spinal fusion is relatively new, and there is no study in the existing world literature assessing the artifact caused by these rods in patients undergoing postoperative MRI. The purpose of this study is to compare the amount of imaging artifact caused by these implants and to assess its impact on the visualization of neighboring neural structures.

METHODS

This study investigated MR images in patients who underwent implantation of thoracolumbar instrumentation using 5.5-mm-diameter CoCr rods between November 2009 and March 2011 and images obtained in a comparison group of patients who had 5.5-mm titanium rods implanted during the same time period. Axial measurements of the artifact created by the rods between the screw heads were compared between the groups. Two blinded board-certified radiologists performed the measurements independently. They scored the visualization of the spinal canal using a subjective scoring system of 1-3, with 1 representing very good visualization and 2 and 3 representing reduced (good or suboptimal, respectively) visualization as a result of rod-related artifact. All measurements and scores were independently provided for T1-weighted and T2-weighted fast spin echo sequences (1.5-T magnet, 5-mm slice thickness).

RESULTS

A total of 40 levels from the CoCr group (6 patients) and 30 levels from the titanium group (9 patients) were included in the analysis. Visualization of the canal at all levels was rated a score of 1 (very good) by both evaluators for both the CoCr and titanium groups. The average artifact on T1-weighted images measured 11.8 ± 1.8 mm for the CoCr group and 8.5 ± 1.2 mm for the titanium group (p < 0.01). The corresponding measurements on T2-weighted images were 11.0 ± 2.3 mm and 8.3 ± 1.7 mm (p < 0.01), respectively. In a mixed regression model, the mean artifact measurement for the CoCr group was, on average, 3.5 mm larger than for the control group. There was no significant difference between the measurements of the 2 evaluators (p = 0.99).

CONCLUSIONS

The artifact caused by CoCr rods is approximately 3.5 mm larger than that caused by titanium rods on axial T1- and T2-weighted MRI. However, artifact from either CoCr or titanium was not found to interfere with the evaluation of the spinal canal and surrounding neural elements.

A standardized nomenclature for cervical spine soft-tissue release and osteotomy for deformity correction

Object

Cervical spine osteotomies are powerful techniques to correct rigid cervical spine deformity. Many variations exist, however, and there is no current standardized system with which to describe and classify cervical osteotomies. This complicates the ability to compare outcomes across procedures and studies. The authors' objective was to establish a universal nomenclature for cervical spine osteotomies to provide a common language among spine surgeons.

Methods

A proposed nomenclature with 7 anatomical grades of increasing extent of bone/soft tissue resection and destabilization was designed. The highest grade of resection is termed the major osteotomy, and an approach modifier is used to denote the surgical approach(es), including anterior (A), posterior (P), anterior-posterior (AP), posterior-anterior (PA), anterior-posterior-anterior (APA), and posterior-anterior-posterior (PAP). For cases in which multiple grades of osteotomies were performed, the highest grade is termed the major osteotomy, and lower-grade osteotomies are termed minor osteotomies. The nomenclature was evaluated by 11 reviewers through 25 different radiographic clinical cases. The review was performed twice, separated by a minimum 1-week interval. Reliability was assessed using Fleiss kappa coefficients.

Results

The average intrarater reliability was classified as “almost perfect agreement” for the major osteotomy (0.89 [range 0.60–1.00]) and approach modifier (0.99 [0.95–1.00]); it was classified as “moderate agreement” for the minor osteotomy (0.73 [range 0.41–1.00]). The average interrater reliability for the 2 readings was the following: major osteotomy, 0.87 (“almost perfect agreement”); approach modifier, 0.99 (“almost perfect agreement”); and minor osteotomy, 0.55 (“moderate agreement”). Analysis of only major osteotomy plus approach modifier yielded a classification that was “almost perfect” with an average intrarater reliability of 0.90 (0.63–1.00) and an interrater reliability of 0.88 and 0.86 for the two reviews.

Conclusions

The proposed cervical spine osteotomy nomenclature provides the surgeon with a simple, standard description of the various cervical osteotomies. The reliability analysis demonstrated that this system is consistent and directly applicable. Future work will evaluate the relationship between this system and health-related quality of life metrics.

Assessment of symptomatic rod fracture after posterior instrumented fusion for adult spinal deformity

BACKGROUND

Improved understanding of rod fracture (RF) in adult spinal deformity could be valuable for implant design, surgical planning, and patient counseling.

OBJECTIVE

To evaluate symptomatic RF after posterior instrumented fusion for adult spinal deformity.

METHODS

A multicenter, retrospective review of RF in adult spinal deformity was performed. Inclusion criteria were spinal deformity, age older than 18 years, and more than 5 levels posterior instrumented fusion. Rod failures were divided into early (≤12 months) and late (>12 months).

RESULTS

Of 442 patients, 6.8% had symptomatic RF. RF rates were 8.6% for titanium alloy, 7.4% for stainless steel, and 2.7% for cobalt chromium. RF incidence after pedicle subtraction osteotomy (PSO) was 15.8%. Among patients with a PSO and RF, 89% had RF at or adjacent to the PSO. Mean time to early RF (63%) was 6.4 months (range, 2-12 months). Mean time to late RF (37%) was 31.8 months (range, 14-73 months). The majority of RFs after PSO (71%) were early (mean, 10 months). Among RF cases, mean sagittal vertical axis improved from preoperative (163 mm) to postoperative (76.9 mm) measures (P<.001); however, 16 had postoperative malalignment (sagittal vertical axis>50 mm; mean, 109 mm).

CONCLUSION

Symptomatic RF occurred in 6.8% of adult spinal deformity cases and in 15.8% of PSO patients. The rate of RF was lower with cobalt chromium than with titanium alloy or stainless steel. Early failure was most common after PSO and favored the PSO site, suggesting that RF may be caused by stress at the PSO site. Postoperative sagittal malalignment may increase the risk of RF.