Ultrasonic Bone Dissectors in Minimally Invasive Spine Surgery

17 Ultrasonic Bone Dissectors in Minimally Invasive Spine Surgery

Shrinivas M. Rohidas

17.1 Introduction

In the past two decades, spinal surgery has advanced greatly with the help of operating microscopes, high-speed drills, and endoscopes with high-definition (HD) cameras, and spinal surgery is entering a new era of minimalism with the help of the endoscope and HD camera. The angulation ability of the endoscope makes it easy to access the narrow corridors of the opposite side inside the spinal canal. However, when the endoscopic drill is used adjacent to soft tissues in narrow corridors, such as dura, nerve roots, spinal cord, and vessels, there is always some risk of damaging these important tissues by drilling, even under endoscopic magnified HD camera vision. The risks are high during removal of masses with hard consistency near delicate tissues. In addition, cooling fluid is required to protect soft tissues from heat injury, and endoscopic surgery may have to be interrupted frequently for irrigation and suction to clean the debris off the lens, as well as to clear fogging of the lens. Protection of the surrounding soft tissue using cotton is not recommended because of the risk of cotton tangling with the high-speed drill. The kicking movement is dangerous, particularly in deep corridors with delicate structures.

For several decades, ultrasonic aspirators have been effectively used to debulk large brain tumors and some spinal tumors.1,2,3 Now ultrasonic bone curets are available for skull base surgery and spinal surgery.^4,5 This chapter discusses the clinical application of ultrasonic bone dissectors in minimally invasive spine surgery using Endospine—Destandau’s technique. Its advantages and disadvantages in comparison to drills are included.

17.2 Clinical Material and Methods

The author has used the ultrasonic bone dissector/rasp/cutter, Sonoca 300, in conjunction with the Endospine system in surgery for spinal degenerative disorders. The surgical device is composed of a power supply unit with irrigation and suction, foot switch, handpieces, and various tips. The handpiece is bayonet-shaped, with irrigation and aspiration channels (Fig. 17.1).

The dissector handpiece weighs 80 g. It has a working length of 100 mm, the tip width is 4.5 mm, and the height is 3.5 mm. Irrigation is from the outside through a sheath, and aspiration is through the central channel, with a frequency of 35 kHz (Fig. 17.2, Fig. 17.3, Fig. 17.4).

The bone-cutting handpiece/cool knife has a working length of 100 mm, a frequency of 35 kHz, a cutting tip diameter of 3 mm, a cutting tip length of 7 mm, and a cutting width of 0.8 mm. Irrigation is from outside with no aspiration (Fig. 17.5, Fig. 17.6).

The rasp has a frequency of 35 kHz, with working length of 100 mm, and the rasp area is 4 mm². Irrigation is from the outside and inside with no aspiration. The dissector handpiece is reusable because it can be autoclaved, but the rasp and cutter tips are not reusable, because the tips become blunt after use on bone (Fig. 17.7, Fig. 17.8).

Fig. 17.1 Ultrasonic bone dissector developed by Dr. Rohidas with Soering.

Fig. 17.2 Ultrasonic bone dissector used with Endospine.

Fig. 17.3 Ultrasonic bone dissector used to undercut opposite lamina in lumbar spine.

Fig. 17.4 Serrations on the ultrasonic bone dissector tip.

Fig. 17.5 Cool knife/ultrasonic bone cutter/loop.

Fig. 17.6 Cool knife used with Endospine.

Fig. 17.7 Ultrasonic rasp.

Fig. 17.8 Rasp used with Endospine in the cervical spine.

The ultrasonic bone dissector/rasp/cutter is used to remove bone near dura, nerve root, and blood vessels in endoscopic bilateral decompression using a unilateral approach in the lumbar spine, posterior cervical diskectomy and canal decompression for stenosis, and endoscopic anterior cervical diskectomy.

17.3 Ultrasonic Basics

Ultrasonic movement is measured in Hertz. For example, 25 kHz is 25,000 Hz, equal to 25,000 cycles per second. One cycle is one Hz (Fig. 17.9). Less than 20 Hz is infrasound; 20 Hz to 20 kHz is the audible spectrum of sound. Ultrasound is 20 kHz to 20 MHz. Frequencies suitable for ultrasonic dissection are from 20 kHz to 60 kHz (Fig. 17.10).

The generator converts the main voltage into electric energy of the desired frequency. Then the converter transforms the electric energy into mechanical energy. The stack of piezoelectric quartzes transforms the electric energy of the generator into longitudinal, deformational, mechanical vibration of the sonotrode tip. The sonotrode is designed so that the entire system—converter, final mass, sonotrode—is in resonance. The stroke at the distal tip is 120 µm. The deformation movement is around 10 µm (Fig. 17.11).

Fig. 17.9 Basic physics of ultrasonic energy.

Fig. 17.10 Ultrasonic frequencies used for dissection.

Fig. 17.11 Diagram showing tip to-and-fro and deformation movement.

Three physical effects are used: cavitation, mechanical abrasion, and thermal effect.

17.3.1 Cavitation

Pressure fluctuations are caused by the vibrating sonotrode. Forward movement of the sonotrode tip displaces the surrounding liquid, while backward movement forces the liquid to return. The pressure of the surrounding liquid changes rapidly between high and low values. Due to the high speed and mass inertia of the surrounding medium, the volume cannot be fully compensated by returning liquid. During backward movement, the pressure drops so much that the liquid evaporates and small bubbles are formed. During forward movement, the pressure rises, and the bubbles collapse. The imploding bubbles transmit high-energy pulses into their immediate vicinity. So the cascade of events is as follows:

1. Small gas bubbles are formed.

2. The gas bubbles grow because the surrounding liquid evaporates.

3. The bubbles then reach maximum volume.

4. Then implosion starts by unilateral inversion.

5. The liquid perforates the bubble wall and generates a shock wave.

Abrasion

On a hard, rigid, firm surface, the oscillating movement of the sonotrode acts like a file. Bone tissue incapable of following the vibrating sonotrode is pulverized by friction and abrasion on the edges and tips. On soft tissue, no abrasion occurs because soft tissue is pushed away from the sonotrode tip due to the tissue’s elasticity. This is the basic principle of ultrasonic bone dissectors.

17.3.2 Thermal Effect

The vibrating sonotrode generates friction heat in the liquid and directly on the tissue concerned. The benefit of heat is its coagulation effect on soft tissue. For bone, local overheating is prevented by continuous irrigation of the tip.