Ferromagnetic Dissection: A Comparison to Electrosurgery

Transcription

1 Ferromagnetic Dissection: A Comparison to Electrosurgery Written by Kim Manwaring, M.D., Chief Medical Officer, Domain Surgical, Inc. Compared to electrosurgery, ferromagnetic dissection achieves superior hemostasis with less tissue injury and postoperative edema. The effect is achieved by pure thermal conduction and no passage of RF energy into the tissues. Summary Ferromagnetic (FM) dissection utilizing the FMwand is a new energy modality to achieve the surgical goals of tissue dissection, hemostasis, and ablation. In contrast to electrosurgery, its effects are achieved by purely thermal conduction, i.e., there is no electrical current pathway in the tissue. Unique properties of the tissue interface are seen, including much reduced depth of thermal injury and no beyond the tip effects, high hemostatic effect on first pass, minimal drag or tissue distortion, and self-cleaning. Investigational postoperative MRI imaging of such surgical incisions shows less edema and reduced early scar formation. As such, this represents improved therapeutic outcome, particularly where critical margins against neurovascular tissues are encountered. Background Conventional surgical dissection can be achieved by cold blade with resulting excellent healing, but poor hemostasis. Various energy modalities have been developed to improve bleeding control while maintaining reasonable dissection characteristics. The mainstay for more than 80 years has been monopolar electrosurgery, employing an active dissecting electrode and ground pad. Tissue sealing while cutting is 1

2 achieved due to heating in the tissue interface and path of radiofrequency (RF) current in the typical range of 300 khz 1 MHz, 10 to 60 watts. A bipolar modality constraining current path between forceps tines is the preferential tool for handling vessels or tissue dissection adjacent to critical structures. Well known limitations or frustrations familiar to surgeons include the neuromuscular stimulatory effect, usually requiring muscle relaxants in addition to anesthesia, the need for blended or a high voltage coagulative waveform to achieve hemostasis in highly perfused tissues with associated charring or sparking, tissue build-up or sticking, moderate tissue injury with reactive edema and postoperative seroma, and occasional beyond the tip injury in the RF pathway. Alternative energy modalities have been developed to address some of these limitations or optimize a desired effect. These include various refined RF tips and frequencies, ultrasound, and laser. While RF frictionless dissection can be achieved with a sharp emitting surface and a more pure sinusoidal waveform, this effect offsets benefits of hemostasis. Ultrasound methods create heat by direct frictional contact upon or in tissue and are typically limited by the transducer size and linkage. Laser incising and heating relates both to the depth of light absorption and the frequency of the light and tissue pigment absorption, but is frequently limited by constraints of laser instrumentation use and speed of effect. An idealized dissection tool could be anticipated to show certain characteristics depending on the delicacy of the underlying tissue types. These include 1) superficial tissue hemostatic effect without deeper tissue injury, a sealing or searing without deep char or desiccation; 2) frictionless cutting without distortion or sticking; 3) on demand rapid onset and offset of effect as various tissue planes and types as well as vascular structures are encountered; and 4) easy cleaning to maintain the pace of surgery. Toward these goals a new method has been developed based on a purely thermal conductive effect, that is with no energy pathway into the tissues of RF, vibrational heat, or light absorption. Underlying Technology of Ferromagnetic Dissection: The ferromagnetic heating effect of the FMwand is based upon an inductive magnetic field effect of a current on a microscopically deposited alloy layer of ferromagnetic (FM) material on the incising or sealing tool. The tip in contact with tissue is configured as a loop, forming an electrical pathway of very high frequency (VHF range) current from the generator to the instrument handpiece and back. No RF energy passes into the tissue. However, the instrument tip has been coated in the area where heating is to be precisely achieved and controlled. The alloy consists microscopically of randomly oriented magnetic domains. As RF passes through the loop, a magnetic field perpendicular to the current induces alignment and re-alignment of the domains at the VHF frequency. While this inductive heating can be thought of as a frictional effect within the alloy, it is technically created by eddy currents and hysteresis loss as a consequence of the changing magnetic field. There is no magnetic induced effect in the tissue, only the alloy. The very thin layer of alloy responds with almost instant heating and, with de-keying of the generator, very rapid cool down to ambient tissue temperature. The peak heat when activated in air is approximately 550 C. Since all therapeutic effects of vascular sealing, tissue incision, ablation or vaporization, and sculpting are achievable below that temperature ceiling, the desired mode of function can be easily controlled. Further, non-sticking during incision and self-cleaning by vaporization of any debris are characteristics of the heated alloy surface. The two images below show the FM tip and a diagrammatic representation of randomly oriented magnetic domains of the alloy in the non-active mode, and the re-aligned magnetic domains as RF passes through the loop. 2

3 AC Current Randomly Oriented Magnetic Domains Coating Magnetic Field liver, dura, brain, and spinal cord, and an animal model of vascular tumor, typifying the usual surgical approaches through such tissues. The studies were undertaken at the University of Utah, Barrow Neurological Institute, and Johns Hopkins Medical Institutes to gain a portfolio of effects for submission and characterization of the ferromagnetic effect. Most specific to ability to cut and coagulate living tissue by mainstay electrosurgical techniques, incision in liver was selected as challenging due to its highly vascularized nature. Liver also lends itself well to post-incisional histologic evaluation of depth of injury. Dorsal fascia and muscle incision was also undertaken to study post-surgical edema and healing by MRI imaging and necropsy. Brain cortical incision in pigs was similarly investigated to compare the acute effects of edema and collateral injury of traditional bipolar and suction corticotomy and with ferromagnetic incision into deep white matter. Ferromagnetic Coated Region of Tissue Dissecting Tip Investigations Due to the unique characteristics of a pure thermally conductive effect in a dissecting instrument, animal investigations were undertaken to discern the differences or benefits in handling various surgical tissues. These included comparative incisions among swine, rabbits, rats and goats using standard settings of electrosurgery, ultrasound blade, and laser. Tissue types included muscle, Liver Studies Comparative incisions were made with monopolar RF (Valleylab Force 2, 40/40 W Blend 2) in each of 3 lobes of the liver in 3 anesthetized rabbits. The ferromagnetic dissection loop was employed at a comparable 60 W setting (Domain Surgical FMwand). The performing veterinary surgeon subjectively graded incisions blinded to the author of this paper and to the reviewing pathologist for four characteristics: tissue drag, incision uniformity, hemostasis, and collateral damage. On a scale of 1 to 5 (where 1 was optimal, 5 unacceptable), statistical analysis yielded the following tabulation: 3

4 FMwand Valleylab Difference Scale 0.50 mm Tissue Drag statistically significant Hemostasis statistically non-significant Margin Uniformity statistically significant Collateral Damage statistically significant The pathologist concluded the FMwand incisions showed uniform margins of heat injury, the depth of which likely correlated with dwell time of incision. In contrast, the Valleylab monopolar incisions showed extreme collateral damage and extreme variability. Representative histologic slides of difference are shown below. Typical FMwand incisions in brain and liver are shown below with visible thin margins of thermal injury and sharp edges of viable tissue. Pig Brain Scale 0.08 mm Rabbit Liver Dorsal Fascia and Muscle Studies 10 rabbits underwent dorsal paraspinal fascia and muscle incision comparing monopolar coagulative and pure cut waveforms at 30 W to FMwand at 30 W and cold blade. MRI imaging was undertaken at 24 hours and 14 days. Tissue edema by T2 imaging was most extensive and severe about the electrosurgical incisions, particularly the coagulation waveform. This persisted, though diminished and with early retraction injury effect at 14 days. In contrast, FMwand incision edema was acutely much less. At 14 days the ferromagnetic incision appeared indistinguishable from cold blade. The images below are illustrative: 4

5 MRI - 24 Hours After Incision with Monopolar Coag 1, Monopolar Cut 2, Scalpel 3, FMwand 4 Monopolar Coagulation - 30 Watts Rabbit Paraspinous Muscle Healing After 2 Weeks MRI - 2 Weeks After Incision with Monopolar Coag 1, Monopolar Cut 2, Scalpel 3, FMwand 4 Monopolar Coagulation - 30 Watts Rabbit Paraspinous Muscle Healing After 2 Weeks 5

6 FMwand - 30 Watts Rabbit Paraspinous Muscle Healing After 2 Weeks Brain Corticotomy Studies 3 pigs underwent bifrontoparietal craniotomy with 2 cm linear incisions front to back through cortex into white matter a depth of 8 mm. The first incision on one side was made by traditional bipolar vascular sealing cortical blanching, followed by cold blade incision, repeat bipolar sealing as needed, and suction dissection into white matter. The second, contralateral incision was made by the FMwand at an identical depth in one pass. MR imaging at 1.5 hours post procedure demonstrated about ½ the edema depth into brain tissue on T2 images when the ferromagnetic method was used. The comparative images in the coronal plane below were typical: 1 2 FMwand - 30 Watts Rabbit Paraspinous Muscle Healing After 2 Weeks MRI Hours After Incision with Bipolar/Suction 1 and FMwand 2 6

7 Discussion The FMwand creates an incision by a purely thermal effect with alloy surface temperature greater than 300 C. As there is no RF energy passed into the tissue toward a ground path or opposing tine, high uniformity with sharp margins of thin collateral injury are seen. There is almost no perceivable drag. However, if the surgeon chooses to dwell slowly, deeper thermal effect is seen. First pass hemostasis is excellent, even in highly vascularized tissue like liver, but shows no char or desiccation/retraction of the capsule. Edema and scar reaction are much less noticeable by acute and delayed MR imaging as well as histologic grading. Such incisions resemble those produced by a cold blade, but with hemostasis. Conclusions Unique effects are seen with ferromagnetic dissection compared to electrosurgery which supports conclusions of less collateral injury, shallower and more uniform margins of incisions, and less post-surgical edema and scar formation. Even in a highly vascular organ, first pass hemostasis appears effective with sealing, but avoiding deep char or adhesion. The tactile effect of minimal drag is especially impressive in fascial and muscular dissection where no muscular stimulation is seen. The lack of neuromuscular irritation should lend to improved functional monitoring during surgery as well as less use of muscle relaxants. MR imaging of tissues for edema and injury effect suggests that FMwand dissection may heal faster and with minimal seroma formation. One might expect diminished postoperative pain and faster recovery based on these findings as ferromagnetic dissection is employed in patients. References Jensen, CR: Comparative Electrosurgery Device Study Using Domain Surgical s FMwand Ferromagnetic Surgical System. White Paper. Domain Surgical, Inc., Salt Lake City, UT, August, Acknowledgements The author acknowledges the contributions of: Greg Burns, DVM, Ph.D., Associate Director of Research and Pathology in the Office of Comparative Medicine, University of Utah, Salt Lake City, UT. Steven S. Chin, M.D., Ph.D., Director of Neuropathology, University of Utah, Salt Lake City, UT. Mark Preul, M.D., Director of Neurosurgery Research Laboratory, Barrow Neurological Institute, Phoenix, AZ Joel MacDonald, M.D., Associate Professor of Neurosurgery, University of Utah, Salt Lake City, UT. George Jallo, M.D., Ph.D., Professor of Neurosurgery, Johns Hopkins Medical Institutions, Baltimore, MD. Contact For further specific information relating to the content of this paper, contact: Kim Manwaring, M.D., Chief Medical Officer, or Curtis Jensen, Director of Quality and Regulatory Affairs Domain Surgical, Inc South 2100 East, Salt Lake City, UT (801)

8 Domain Surgical, Inc South 2100 East Salt Lake City, UT (801) Domain Surgical, Inc. All rights reserved. LB-017 Rev. A 8