Surgical Techniques and Instrumentation in MISS

12 Surgical Techniques and Instrumentation in MISS

Robert Nicholas Hernandez, Sertac Kirnaz, and Michael S. Virk

Summary

This chapter highlights an essential part of MISS, Surgical Techniques and Instrumentation. Due to the small working corridor of MISS and the proximity to critical anatomical structures, instruments should be able to facilitate the field of view of the surgeon and not obstruct any of the relevant anatomy. To achieve this, MISS often utilizes smaller instruments that are often curved or bayoneted to further increase the available viewing angle; to provide examples, the following instruments/ scenarios are discussed: visualization, retractors, power drills, bone scalpel, endoscopic techniques, and CSF leak repair.

Keywords: CSF leak endoscopy instruments laminotomy minimally invasive spine surgery tubular retractors ULBD

12.1 Introduction

The surgical principles of minimally invasive spine surgery (MISS) revolve around minimizing the operative footprint while achieving the same goals as traditional open approaches. Specifically, minimizing tissue disruption and instability, achieving bilateral decompression via a unilateral approach, and achieving indirect neural element decompression have driven the evolution and advancement of MISS techniques and instrumentation. As the field advances to better achieve the goals of MISS, instruments that facilitate safe and efficient procedures through narrow, highly targeted access corridors inherent in MISS approaches are mandatory. The design of MISS instruments, therefore, must abide by the following principles:

•Provide variable extension in order to reach the target.

•Offer bayoneted or curved design that does not obscure the surgeon’s line-of-sight.

•Present more precision and specialized tissue interfaces with variable angles or degrees of manipulation.

•Offer enhanced ergonomics.

With these characteristics in mind, this chapter will discuss the instrumentation required for the performance of MISS techniques.

12.2 Bayoneted or Curved Instrumentation

Bayoneted or curved designs allow the surgeon to comfortably hold instruments without obstructing the surgeon’s view as he or she operates down the operative corridor. It is a fundamental design principle of MISS. Bayoneted instruments also offer extension to allow the surgeon’s hands to work above the operative field while at the same time reaching the target, which can be located several centimeters deep. This is particularly necessary in obese patients, in whom minimally invasive procedures have proven to be advantageous over open techniques,1 as the depth of tubular retractor can reach 10 cm or greater. Lateral approaches also require extended length instruments. The basic set of extended, bayoneted, or curved instruments includes (Fig. 12.1 and Fig. 12.2):

•Suction tips and suction retractors.

•Bipolar and monopolar electrocautery (the extended monopolar base can be bent slightly to create a bent or curved-like instrument).

•High-speed power drills.

•45° and 90° Kerrison rongeurs.

•Microinstruments such as ball-tip probes, nerve hooks, curettes, microdissectors, etc.

Fig. 12.1 Bayoneted microinstruments.

Fig. 12.2 Bayoneted Kerrison rongeurs.

12.3 Retractors

12.3.1 Tubular Retractors

The advent of tubular retractors propelled MISS and allowed for marked proliferation. Since the introduction by Foley and Smith in 1997,² tubular retractors have been applied for the treatment of disease at all levels of the spinal column. Tubular retractor systems consist of a table clamp, a rigid arm, a series of dilating tubes of sequentially enlarging diameter, and the tubular retractors (Fig. 12.3). The tubular retractors come in various depths and diameters, often ranging from 15 to 24 mm in diameter and 40 to 100 mm in depth. Typically, we use a 15-mm retractor for lumbar microdiscectomy and cervical foraminotomy, an 18-mm retractor for laminectomies at all levels, and a 21-mm retractor for transforaminal lumbar interbody fusion. After sequential dilation of the soft tissue and musculature, the depth to the target is identified and the appropriate tubular retractor is passed over the dilators and secured in position by the rigid table arm. In this manner, the retractor is fixed in place but can be “wanded” or redirected in any direction to achieve additional visualization and exposure outside the fixed diameter of the tubular retractor. This “wanding” technique is critical to achieving the goals of MISS when using tubular retractors. It permits visualization of multiple regions beyond the initial docking site through a single corridor. See also Chapter 5 in this book.³

Fig. 12.3 Tubular retractor system.

12.4 Drills

12.4.1 Power Drills

High-speed power drills used in MISS include curved and extended drill shafts and bits that attach to the same power sources used in open surgeries. This allows the drill to reach the target while affording the surgeon line-of-sight and ergonomic use. In our practice, we use a 15-cm curved drill shaft. In addition, self-irrigating drills are also available.

Two drill bits lend themselves well to use in MISS. The 3-mm fluted matchstick drill bit is a side-cutting burr that has a blunt tip. This drill bit is particularly useful for minimally invasive laminectomy; the blunt tip can safely rest atop the ligamentum flavum while the lamina is drilled away in a side-cutting fashion (Fig. 12.4). In this manner, the ligamentum flavum can be entirely exposed and acts as a protective layer from injury to the underlying dura.⁴

Fig. 12.4 Curved matchstick drill bit.

The diamond burr is another drill bit useful in MISS. This drill bit can be used in a side-cutting manner, similar to the matchstick; also it can drill with gentle downward pressure. With the appropriate amount of pressure, the diamond burr will drill away bone but not ligamentum flavum, rather pushing the soft tissue away. Thus, similar to the matchstick bit, the ligamentum flavum can be exposed and kept intact to protect the underlying dura. One advantage of the diamond burr is that due to the heat it gives off, bony bleeding is minimized through thermal coagulation. However, continuous irrigation is required to avoid thermal injury to adjacent neural structures.

While extended drill shafts are necessary for tubular MISS, the long distance between the surgeon’s hand and the tip of the drill can result in poor drill control. To improve control, the drill should be held with the hand resting on the patient to stabilize the hand and the shaft of the drill resting on the edge of the tubular retractor to stabilize the tip of the drill. Only very small movements of the hand are required.

12.4.2 Bone Scalpel

The ultrasonic bone scalpel has been used in a variety of open spine surgeries and has demonstrated decreased blood loss compared with traditional high-speed drills⁵^,⁶ with similar rates of incidental durotomy.⁷ Bone scalpels on thin, curved, extended shafts are available for MISS (Fig. 12.5). While still too bulky for 16- to 18-mm tubular retractors, they are compatible with expandable and mini-open retractors. The potential advantages of decreased blood loss, decreased heat generation when operating near neural structures, and decreased mechanical injury to neural tissue⁸^,⁹ are attractive.