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.


Introduction
Electrocautery and spinal implants are principal tools in the spine surgeon's armamentarium. These tools are utilized in complex surgeries to treat a multitude of spinal conditions and to ameliorate patients' disabilities [1,2].
Technological advancements in biomechanics and biomaterials have revolutionized the sphere of spinal instrumentation, enabling surgeons to widen the indications for corrective surgery, and broadened the array of surgical techniques. In this context, spinal fusion was first introduced by Hibbs and Peltier to stabilize a spine affected by tuberculosis [3,4]. Albee used the tibia as graft material for spine stabilization [5]. King [6] and Lange [7] attempted internal fixation [1,4]. Harrington [8] introduced an innovative rod distraction system to correct coronal deformity [4], and Cotrel and Dubousset metamorphosed the profession with their segmental instrumentation for simultaneous correction of coronal and sagittal planes, thereby facilitating a three-dimensional (3D) approach to treat a 3D spine deformity [9]. These surgeries demand sophisticated technical aptitude and hence require meticulous preoperative planning. Consequently, one of the major preoperative decisions is choosing the correct deformity-specific, pathology-appropriate, spinal fusion construct [1].
Spinal implants must fulfil many criteria to be considered safe and efficient. Qualities such as biocompatibility, osseointegration, high strength, low Young's modulus, high corrosion, and wear resistance are pivotal to implants' long-term performance. Because no current material can fulfil all the clinical and biomedical requirements, material scientists have endeavored to enhance these qualities utilizing many surface modification techniques [10]. These processes include mechanical treatment, sol-gel application, thermal spraying, chemical and electrochemical treatment, micro-arc oxidation, laser surface modification, friction stir processing, and ion application [11,12].
Heating techniques and plasmochemical techniques occupy an important role in the sphere of biomedical engineering and materials science. Heat treatment is utilized to increase the fatigue strength of an alloy and to orchestrate an optimal balance of the material's ductility, machinability, and stability [10]. Plasma implementation is non-toxic because it is free of solvents [13]. Implementing techniques such as plasma assisted microwave chemical vapor deposition, plasma etching, plasma nitriding, and deposition of a-DLC (amorphous diamond-like carbon) layers inoculated with nitrogen and silicon have been shown to have a significant impact on the microstructure and surface characteristics of alloys [13].
Accurate and appropriate surface modification techniques diversify and optimize the clinical use of alloys in surgical fields. However, spine surgeries are overburdened with post-operative complications [2], including implant-related failure and rod fracture (RF) [1]. The overall complication rates of surgical corrections of spine-deformity are reported to be 30% within two-years after surgery [14][15][16][17][18]. An important source of complications remains the intrinsic limitation of the robustness of spinal implants [15,[19][20][21][22][23][24]. In a prospective multicenter assessment of risk factors for early RF following adult spinal deformity (ASD) surgery with one-year follow-up, Smith et al. [16] identified that RF occurred in as many as 9% of ASD patients, and up to 22% in patients who underwent pedicle subtraction osteotomy (PSO), a powerful technique for the correction of sagittal spino-pelvic malalignment. In another retrospective study, Smith et al. [23] found that most RF occurred within one year of index surgery.
In the literature to date, several risk factors for RF have been analyzed. They could be patient-related-e.g., higher Body Mass Index (BMI), patient age, history of spine surgery, development of pseudarthrosis, and greater baseline sagittal spinopelvic malalignment (Sagittal Vertical Axis (SVA), Pelvic Tilt (PT), and Pelvic Incidence-Lumbar Lordosis (PI-LL) mismatch), and a greater need for sagittal correction-or implant-related (implant material, or in situ rod contouring in the context of PSO) [16,21,23,24]. Notably, the arthroplasty literature provided a clue to an underlying confounder camouflaged behind the subtleties of RF: the thermal damage of electrocautery to the microstructure of implants. Huber et al. [25] were the first to document a case of shaft breakage of a hip endoprosthesis secondary to contact with an electrocautery device. Subsequently, Konrads et al. [26] reported on four similar cases which occurred after revision surgery. These authors argued that the thermal microstructural disruption ensued from an intra-operative electric arc seen post-contact of the electrosurgical electrode with the implant; this could have eventually led to breakage of the implant at the contact point. In addition, Yuan et al. [27] examined 1859 explanted hip implants and concluded that "iatrogenic arc melting" due to electrocautery was responsible for the color changes and corrosion on the surface of the studied implants. Furthermore, via fatigue testing, metallurgic analysis, and electron microscopy, Sonntag et al. [28] delineated a microstructural change in the Ti6A14V base material with high-frequency electrosurgery on hip endoprosthesis, resulting in a decreased load-to-failure when the electrocautery tip was applied to high-load areas of the stem during revision surgery.
Clinically speaking, it is difficult to avoid contact between the electrocautery tip and the implant during spine surgery. Particularly in cases of revision surgery, these electrical arcs could be deemed common.
To the best of our knowledge, this is the first biomechanical study that addresses this topic in spine surgery. In concordance with the American Standard for Testing and Materials, our objective was to assess the impact of electrocautery contact on the fatigue life of titanium (Ti) and cobalt-chrome (CoCr) spinal implants.

Spinal Fusion Constructs and Vertebrectomy Models
Twelve unused specimens of CoCr and Ti implants were included (Expedium Spine, DePuy, Raynham, MA, USA). All rods and pedicle screw constructs had the same diameter (5 mm) and were cut to a length of 100 mm. Monoaxial pedicle screws (Ti6-Al4-V, 5.5 mm × 7 mm × 30 mm) and locking screws (Ti6-Al4-V, 7 mm) were used to rigidly couple the rods on the vertebrectomy model according to manufacturer-specific instructions using original instruments. Models were composed of dual Polypoxymethylene (POM) with dimensions as described by the American Standard for Testing and Materials (ASTM) F1717 [29]. This standard provides a uniform approach to conduct fatigue testing on spinal implants. The two blocks comprising each vertebrectomy model simulate two adjacent vertebrae fixed by posterior-instrumentation. The standard distance between the two blocks was 56 mm.
Specimens were divided into four groups by rod material and application of monopolar-electrocautery device on the rods' surface (Table 1).

Electrocautery Contact
Electrocautery application (VIO-300, Erbe Elektromedizin, Tübingen, Germany) was performed by an experienced spine surgeon. Analogous settings used in the operation room of our institution were utilized (mode: swift-coag, 90 Watt, effect: 3, duration:~4 s). High-frequency monopolar-electrocautery was applied on the surface of each rod at three defined positions ( Figure 1). Under these conditions electric light arc was visible in all cases and discoloration of the rod surface was observed.

Dynamic Mechanical Testing
Models were mounted in a servo-hydraulic uniaxial testing apparatus (HCE, Bosch-Rexroth, Lohr a. Main, Germany) under dry conditions such that the rods were aligned in the direction of vertical force application. The upper and lower vertebrae blocks were free to rotate in order to account for any bending of the rods during dynamic testing. A multistep fatigue test was performed at a frequency of 12 Hz (Figure 2).

Outcome Measures
Primary outcomes estimated load-to-failure, total number of cycles to failure, and location of rod failure.

Statistical Analysis
One-way analysis of variance (ANOVA) with post-hoc-test correction of least statistical difference was used to compare continuous values of the outcome measures. All data are presented as mean values ± standard deviation and 95% confidence interval. Statistical tests were computed using statistical package (SPSS-v.24, IBM, Armonk, NY, USA). The threshold of statistical significance was set at 0.05. Tests started at an initial maximum sinusoidal load of 100 N (minimal load: 10 N). After one million cycles, the maximum load was increased by 50 N, and the load ratio R was kept constant at 0.1, until construct failure occurred. Based on the total number of steps and cycles completed, the estimated load-to-failure was calculated using this formula: where F −1 is the maximum force of the previous step before fracture, and n is the number of cycles in the step where the specimen failed.

Outcome Measures
Primary outcomes estimated load-to-failure, total number of cycles to failure, and location of rod failure.

Statistical Analysis
One-way analysis of variance (ANOVA) with post-hoc-test correction of least statistical difference was used to compare continuous values of the outcome measures. All data are presented as mean values ± standard deviation and 95% confidence interval. Statistical tests were computed using statistical package (SPSS-v.24, IBM, Armonk, NY, USA). The threshold of statistical significance was set at 0.05.

Results
Results of the biomechanical test are summarized in Tables 2 and 3.

Titanium Control Group (Ti-CG)
All specimens exhibited fatigue fracture of the rod at the rod-screw junction ( Figure 3).

Titanium Electrocautery Group (Ti-EG)
In two specimens (Ti-EG1 and Ti-EG3), failure sites corresponded to the sites of electrocautery application (Figure 4).    Ti-EG revealed a significantly lower load-to-failure than Ti-CG (17% decrease, p = 0.023). Intergroup comparison of cycles to failure revealed a significant mean decrease of almost 9 × 10 5 cycles (p = 0.03). ( Figure 5).      Notably, Ti-EG2 failed at the rod-screw junction similar to all of the Ti-CG rods.

CoCr Control Group (CoCr-CG) and Electrocautery Group (CoCr-EG)
In all tested constructs, the Ti pedicle screw failed. None of the CoCr-CG or CoCr-EG rods had fractured, exhibiting an increased load-to-failure in comparison to their Ti counterparts.
Intergroup differences were insignificant across all outcome measures. This implies that, in these tested constructs, the pedicle screw proved to be the weakest component under cyclic loading. In all specimens of these two groups, fatigue fracture transpired at the first thread of the titanium pedicle screw (Figures 7 and 8).

Discussion
The human body is an intricate biomechanical composite that exerts cyclic loading on spinal instrumentation, which could lead to their failure [21,30]. The predominant use of electrocautery in operating-rooms mandates spine surgeons and implant engineers to contemplate the potential

Discussion
The human body is an intricate biomechanical composite that exerts cyclic loading on spinal instrumentation, which could lead to their failure [21,30]. The predominant use of electrocautery in operating-rooms mandates spine surgeons and implant engineers to contemplate the potential

Discussion
The human body is an intricate biomechanical composite that exerts cyclic loading on spinal instrumentation, which could lead to their failure [21,30]. The predominant use of electrocautery in operating-rooms mandates spine surgeons and implant engineers to contemplate the potential negative impact of electrocautery on the long-term integrity of spinal fusion constructs. Thus, addressing differences between the mechanical properties of these materials and studying the thermal effect of electrocautery are essential to predict the in vivo behavior of implants.
The micro-architecture of Ti and CoCr materials dictates their biomechanical performance and fatigue life [21,22,31]. Instrumentation failure is thought to be caused by recurring loading which generates oscillating stresses that may be well below the yield stress of the material. Unfortunately, this process is unnoticeable, either by the patient during daily activities or by the surgeon throughout follow-up visits, until final construct failure develops [21,32,33].
In vivo loading on spinal implants has been previously measured and reported to be 250 N in the prone position, 400 N whilst walking, and up to 700 N during exercise [34][35][36][37]. Notably, Ti-EG constructs failed at a maximum load of 250 N. This underlines the potential detrimental impact of electrocautery on the safety of Ti constructs, even within daily physiological loading.

Thermal Damage and Notch Sensitivity: A Threat to Ti Biomechanical Integrity
Previous investigations have emphasized that the load-to-failure is contingent upon a material's resistance to surface alterations. Fatigue fractures almost always nucleate at the exact location of notches or "discontinuity of geometrical structure" on the material's surface [21,24,31,38,39]. This is particularly important when the site of the notch corresponds to areas where maximal loading is applied. Very small cracks relative to the dimensions of the micro-structure have been shown to cause failure faster than a long crack [40,41]. Therefore, the radius of the notch is, as a geometrical function, important [42]. Jang et al. demonstrated that thermal damage of laser etching on hip implants led to their early fracture [43]. Huber et al. [25] and Konrads et al. [26] reported a decrease in the fatigue life of Ti hip implants due to the thermal damage of electrocautery. In this context, Sonntag et al. [28] further demonstrated that this thermal damage caused a significant alteration of Ti microstructure. Ti6Al4V alloy has a bimodal microstructure consisting of a globular α-matrix phase, enveloped by the so-called Widmannstätten structure (α + β) [32,[44][45][46]. Thermal energy affects the internal kinetics of theses phases and triggers a cascade of diffusion processes, as well as transforming the dimensions of the α and β phases. Alteration of those dimensions correlates with their load-to-failure and susceptibility to internal crack initiation. It could even be regarded as an internal notch [33,[47][48][49].
In the light of these findings, the thermal energy applied on the tested Ti rods and the marks seen on their surface, which may be due to a corrosive attack [27] could set the stage for premature rod failure.

Pathogenesis of Ti Rod Fractures at the Rod-Screw Junction
Among all Ti-CG and one of the Ti-EG specimens, a crack nucleation site was visually detected in the proximity of the locking screw. Tightening of the screw leaves surface marks on the rods and could account for this failure (Figure 9). The presented results echo previously published findings: Lindsey et al. and Ngyuen et al. [21,22] detected an increased trend of CoCr rod failure at the same location of our tested constructs. Yamanaka et al. [40] described the same phenomenon on Ti rods. Dick et al. [31] suggested that surgeons should avoid severe tightening of the screws as this would lead to notch formation on the surface of Ti rods. Hence, a modification of the operating manual might be indicated to avoid severe tightening of the screw. This, however, requires specific testing and is a secondary outcome of this study.

CoCr Versus Ti Rods
CoCr exhibited a higher fatigue life than Ti, which is in line with previous investigations [23,50,51]. However, we are not aware of previous studies addressing spinal CoCr susceptibility to thermal modification of electrocautery. Uniquely, our tested constructs were not affected by this thermal modification as the pedicle screws failed. This could be explained by their superior rigidity,

CoCr Versus Ti Rods
CoCr exhibited a higher fatigue life than Ti, which is in line with previous investigations [23,50,51]. However, we are not aware of previous studies addressing spinal CoCr susceptibility to thermal modification of electrocautery. Uniquely, our tested constructs were not affected by this thermal modification as the pedicle screws failed. This could be explained by their superior rigidity, corrosion, and wear resistance [52]. This biomechanical superiority is also the reason for their implementation in demanding surgical techniques such as PSO. Paradoxically, in the context of PSO, CoCr constructs were associated with higher rates of RF. This could be due to the notch effect of in situ bending inherent within the PSOs, and the unusually high loading they need to withstand [16]. However, the superior performance of CoCr comes at the expense of greater stiffness at both ends of the construct, resulting in increased incidence of adjacent segment disease [53][54][55]. This could be attributed to the fact that Ti-6Al-4V alloy has a much lower Young's modulus (∼112 GPa) when compared to Co-Cr alloys (∼210 GPa) [56,57].
One of the strengths of our study could be its clinical implications. Patient expectations of surgical outcomes have reached unparalleled heights [58]. However, implant-related complications have significant drawbacks on post-operative Health-Related Quality of Life (HRQoL) and were associated with slower rates of improvement [1,23,59]. Moreover, the negative economic impact of instrumentation failure and post-operative complications on the health-care system in the context of spine surgery is high [60,61].
This study provides a biomechanical strongpoint by testing constructs under high sinusoidal loading levels (n = 8, each 10 6 cycles) in which rod diameter was uniform, which makes the results comparable. Previous biomechanical investigations have tested constructs with different rod diameters and were tested under lower load levels [21,22]. Higher loading levels would give more confidence to the conclusion of long-term fatigue endurance. Intuitively, the longer a spinal implant persists intact in the body, the more cyclic loading it has to withstand. Testing constructs under short-and long-term mechanical loading enabled the conclusion that CoCr rods, even under long-term cyclic loading (8 × 10 6 cycles), proved unaffected by thermal damage. Furthermore, constructs were examined according to a standard for spinal instrumentation testing [29]. The application of the ASTM F1717 standard enabled the delineation of electrocautery's impact on the mechanical properties of materials and efficiently eliminated potential confounding factors, such as implant design or donor variability in the context of cadaver studies.
On the other hand, intrinsic to in vitro biomechanical investigations, this study was conducted within a scaffold of limitations and simplifying premises that are important to discuss. First, severity of the electrocautery attack cannot be fully controlled during manual application which could lead to heterogeneity of thermal damage on the tested rods. This could explain why the Ti-EG specimens fractured at different locations. Second, the use of only one cautery mode precludes conclusions on other electrocautery modes. Third, in vivo clinical translation of the findings is impeded by the dry conditions of these experiments which were conducted at room temperature; fatigue life could differ when implants are at body temperature and surrounded by body fluids [39]. Fourth, the application of the ASTM F1717 sinusoidal testing environment represents a "worst case scenario"; constructs alone bore the burden of the axial loading which does not fully represent the in vivo state where the load is also shared by the spine and neighboring soft tissue. However, physiological stresses on implants could be as high as 700 N which permits us to draw our conclusions, as Ti rods failed in the range of 250 N. Fifth, tested constructs underwent only axial loading, resulting in a bending moment which mimics one modality of human movement, flexion-extension, and does not take into account lateral bending and axial rotation. Finally, we did not perform a power analysis prior to conducting this study to determine the minimum sample size required for the CI of 95%. Also, the wide CI is attributed to the small sample size. However, this limitation means that the impact of electrocautery could have been underestimated in our results. Subsequently, a larger sample size would enable calculation of a statistically significant hazard ratio to quantify the negative impact of thermal damage on the fatigue life of Ti rods. Future biomechanical studies with a larger sample size are merited to analyze different settings of coagulation and cutting modes, and to assess loadings exerted by bending and axial rotation of vertebrae. Randomized controlled studies are required to determine the absolute effect of electrocautery on spinal implants. Nevertheless, our findings disentangle a potential culprit underlying post-operative Ti rod fracture.

Conclusions
This biomechanical study showed that the impact of electrocautery on Ti rods could have significant clinical and biomechanical repercussions for patient safety and satisfaction, surgical training, and implant design. Ti rods exhibited decreased fatigue life and failed at the site of electrocautery application. Spine surgeons should exercise caution in the vicinity of spinal implants, especially during revisions. Similarly, it is also reasonable for manufacturers to devise strategies against thermal damage and explore methods to increase the long-term structural integrity of Ti rods.