BONESCALPEL™ ULTRASONIC BONE DISSECTOR: APPLICATIONS IN SPINE SURGERY AND SURGICAL TECHNIQUE GUIDE Peyman Pakzaban, MD, FAANS Houston MicroNeurosurgery - Houston, TX Abstract The Misonix BoneScalpel is a novel ultrasonic surgical device that cuts bone and spares soft tissues. This relative selectivity for bone ablation makes BoneScalpel ideally suited for spine applications where bone must be cut adjacent to dura and neural structures. Extensive clinical experience of this report is to describe BoneScalpel’s mechanism of action and the basis for its tissue selectivity, review the expanding clinical experience with BoneScalpel (including the author’s personal experience), and provide a few recommendations and recipes for en bloc bone removal with this revolutionary device. Printed by Misonix, Inc. | May 2013 Introduction Mechanism of Action The advent of ultrasonic bone dissection is as Ultrasound is a wave of mechanical energy propagated through a medium such as air, water, or pneumatic drill was several decades ago. Power drills liberated spine surgeons from the slow, repetitive, is typically above 20,000 oscillations per second fatigue inducing, and occasionally dangerous maneuvers that are characteristic of manually hence the name ultrasound. In surgical applications, operated rongeurs. Now ultrasonic dissection with this ultrasonic energy is transferred from a blade to BoneScalpel empowers the surgeon to cut bone with tissue molecules, which begin to vibrate in response. an accuracy and safety that surpasses that of the Whether tissue molecules can tolerate this energy power drill. transfer or be destroyed by it depends on the density The greater accuracy of BoneScalpel is a result of the back-and-forth m icro-m otion of B oneS calpel’s the low ultrasonic range. thin blade as opposed to the rotary macro-motion of a The BoneScalpel assembly consists of an ultrasonic afforded by a drill. In addition, BoneScalpel has two generator/irrigation console that connects to a attributes that provide greater safety. First, elimination hand-piece bearing a disposable cutting tip (Fig. of rotary motion avoids many of the risks associated 1). The cutting tip oscillates back and forth a very with the drill, such as slipping off the cutting surface small distance at rate of 22,500 times per second (a and entrapping important soft tissues. Second, BoneScalpel cuts bone better than soft tissue. This comes in two main varieties (additional ones are being tissue selectivity, which may seem counter-intuitive at developed): the blade and the shaver tip (Fig. 2). The blade behaves like an ultrasonic micro-osteotome the surgeon is routinely faced with the task of cutting en bone adjacent to dura. bloc removal of large pieces of bone. The shaver tip behaves like a non-rotating burr to selectively ablate bone in a small area. The integrated irrigation feature helps remove bone debris and cool the cutting tip. The BoneScalpel blade’s mechanism of action is best understood by analogy to an osteotome (Fig. 3). When an osteotome is struck by a mallet, the energy that is transmitted down the shaft of the osteotome is focused along its narrow tip. This focused energy is then transferred from the tip to a very narrow band of bone, which disintegrates in response, thus creating the leading edge of a cleavage plane in bone. Much like an osteotome, the blade of BoneScalpel Flat side edges moves forward (and backward) (Fig. 4). However, the amplitude of this movement is much smaller than that Central irrigation channel of an osteotome (35-300 microns), thus transferring only a small amount of energy to bone with each Safety stop at blade end moves back and forth to impact the bone (22.5 kHz) Jet nozzle expels irrigant compensates for the small energy of each individual impact, thus resulting in a large transfer of energy Beveled, blunt cutting edge to bone at the point of contact. Again, this energy disintegrates a narrow sliver of bone and develops a cleavage plane. Figure 1. BoneScalpel console and handpiece 2 En bloc Bone Precise Bone Dissection Ablation Figure 2. Figure 3. Osteotome mechanism of action (left) is used for en bloc bone removal while the shaver tip (right) is used for precise bone ablation. s (max) s (0) s (min) Frequency = 22, 500 cycles / sec Am plitude = 35–300 µm Figure 4. Back-and-forth micro-movement of BoneScalpel blade occurs at 22,500 cycles per second. 3 Tissue Selectivity When performing the third step, the BoneScalpel’s relative selectivity for bone cutting provides a good The relative selectivity of BoneScalpel for bone cutting margin of safety, allowing the surgeon to contact has to do with the relative rigidity of bone compared to the underlying dura. However, it is important for the soft tissues (Fig. 5). When the blade of BoneScalpel surgeon to avoid the following pitfalls. First, one must comes in contact with rigid bone, the bone does not not plunge into the dura. As with any other surgical bend, deform, or move away from the tip. As a result, tool, such plunging may cut the dura and result in a large amount of energy is transferred to a small neural injury. Second, one should not linger over amount of bone at the point of contact, resulting the dura so as to avoid excessive heat development in destruction of that bone. In contrast, soft tissue and a thermal lesion. Once the inner cortex is penetrated, the blade is withdrawn and moved to longitudinal ligament, and dura) can bend, deform, an adjacent location. Third, one should avoid using move away, and vibrate upon contact with the blade, this device when dura is likely to be adherent to the thus dampening the energy transfer and protecting the inner bone cortex (e.g. in presence of epidural scar tissue from destruction. It is important to note that this selectivity is not these settings, the dura is at risk, since it cannot move away from the blade of BoneScalpel after the latter en bloc penetrates the inner cortex. Furthermore, of BoneScalpel in spine surgery depends on even if the bone is cut uneventfully, elevating it from developm ent of a tactile “feel” for penetrating the the underlying adherent dura is likely to result in dural inner cortex of bone. After this penetration occurs, laceration. Alternatively, one can cut a slice of bone the blade should come in contact with underlying adjacent to the region of epidural scarring, dissect the tissues for a limited time with limited pressure. adherent dura from the undersurface of the adjacent bone, cut another slice of bone in the dissected area Bone Cutting Technique and repeat these steps until the desired amount of bone is removed. The analogy to a micro-osteotome whose blade moves back and forth will help the surgeon understand has a surprisingly short learning curve. When teaching (axial) pressure rather than side-to-side (lateral) following recommendations: movements. In the author’s experience, a useful strategy for cutting bicortical bone consists of the following 3 steps: practicing on a bone specimen. It is important not only to develop a feel for when the inner cortex is 1. Lateral movement with little axial pressure to score penetrated, but also to familiarize oneself with the the outer cortex of bone to be cut (Fig. 6A). 2. Axial pressure and liberal lateral sweeps to cut through the cancellous mid-portion of the bone (Fig. Palpate with BoneScalpel off. If unsure of whether 6B). the inner bone cortex has been penetrated, momentarily stop the bone scalpel and “palpate” 3. Controlled cyclical forward/backward movement the residual bone with the BoneScalpel blade. with short lateral sweeps to penetrate the inner bone cortex (Fig. 6C). This step primarily involves Plan the cuts to be made. Unlike manual rongeurs the use of controlled axial (downward) pressure. or power drills, BoneScalpel removes bone en bloc. It is, therefore, imperative that one plans in advance Once the surgeon palpates the intended breach of the inner cortex, he withdraws the blade th e e n d o f th is a rtic le , s e v e ra l c u ttin g “re c ip e s ” fo r slightly, moves slightly to one side and repeats the various spine projects are provided. cannot visualize the underlying soft tissues through the thin trough that is created and must rely on it may appear counter-intuitive, it is often more tactile feedback. If unsure of having penetrated removing a whole lumber lamina) into two or three the cortex, the surgeon can momentarily stop the ultrasonic action, palpate the inner cortex with the of the thicker or deeper portions of the bone to be BoneScalpel blade, and then resume cutting. cut and will facilitate elevation of cut bone blocks from the underlying ligaments. 4 Figure 5. BoneScalpel’s tissue selectivity. A B C Figure 6. soft cancellous bone, and C. Repeatedly penetrating the inner cortex with controlled axial pressure. 5 Comparison with Other Bone Removal Clinical Reports Technologies An expanding body of literature describes BoneScalpel’s attributes are compared to other bone the successful use of BoneScalpel in clinical cutting tools in some detail in Table 1. It is important to applications ranging from laminoplasty to harvest recognize that these technologies are not competing but complementary. Just as power drills and Kerrison surgery. Before reviewing these clinical reports, it is rongeurs are used side-by-side to remove bone appropriate to begin with an experimental study in an and ligament in the same operation, BoneScalpel animal model. is fast becoming another indispensible tool in the Sanborn et al. at University of Pennsylvania compared spine surgeon’s toolbox to tackle surgical tasks not laminectomies using hand instruments and cutting burrs to those performed with BoneScalpel in a in comparison to high-speed drills, BoneScalpel offers several distinguishing features.