Tubular and Specular Retractors in MISS

6 Tubular and Specular Retractors in Miss

Christoph Wipplinger, Sara Lener, Sertac Kirnaz, Ahmed Abdelsalam, Roger Härtl, and Claudius Thomé

Summary

The choice of the retractor system has been controversial since the introduction of minimally invasive spinal surgery (MISS). The great variety of available retractor systems can be categorized into tubular and specular retractors. Both retractor systems have various advantages and shortcomings. The main differences between them are the extent of dissection required in order to insert the retractor and the surgeon’s visual field provided by the retractor.

Keywords: tubular retractor specular retractor minimally invasive spine surgery lumbar decompression lumbar disc herniation

While specular retractors require more extensive dissection than tubular retractors, the visual field is wider and expandable. However, tubular retractors cause far less soft-tissue damage and provide a more targeted approach. The choice of the retractor should, therefore, be based on the surgeon’s experience as well as on the spine pathology that needs to be addressed.

6.1 Introduction

Since minimally invasive spinal surgery (MISS) was introduced, the choice of the retractor system has been a controversial topic. One of the main goals of MISS is to reduce trauma to the paraspinal tissues and muscle groups.1 To achieve this, the retractor should be chosen carefully and adapted to the patient’s and surgeon’s needs.

Among the most frequently used systems, specular and tubular retractors are the most acclaimed. In 1991, Caspar et al implemented the use of a specular retractor for MISS discectomy procedures.² Later, in 1996, Smith and Foley performed the first microendoscopic lumbar discectomy through the so-called microendoscopic discectomy (MED) tubular retractor.³ Within the past few years, retractor systems have progressively developed in parallel with the increase and advancement of MISS procedures. For example, the Minimal Exposure Tubular Retractor (METRx; Medtronic Sofamor Danek, Memphis, TN) MD tubular retractor system, which was a modified version of the original MED tube, was developed and first described in 2002 and allowed the use of a surgical microscope.⁴ Since then, many different MISS retractor systems have emerged, involving both tubular and specular retractors.

The purpose of this chapter is to highlight the different features of tubular and specular retractor systems in MISS.

6.2 Tubular Retractor

The tubular retractor has revolutionized the practice of spinal surgery. While initially developed for endoscopically assisted surgery,³ it has now become an indispensable tool for MISS surgery. The tubular retractor allows the treatment of focal lesions in a targeted manner without significantly distorting the normal anatomy. Although the initial indication for tubular microsurgery was lumbar microdiscectomy, the versatility of the technique has enabled a large variety of procedures. Today, tubes are used not only for lumbar, but also for cervical and thoracic spine surgery, spinal fusion, and even complex deformity corrections.⁵

Multilevel pathologies can be treated through small incisions and ports, resulting in minimal tissue trauma. Recently, tubular anterior cervical spine approaches have emerged and may replace wide anterior exposures, which have been associated with considerable morbidity such as dysphagia and poor cosmetic results.⁶

There are multiple commercially available tubular retractor systems (Fig. 6.1) consisting of sequential dilators in diameter, all of which are marked in length to select the final appropriate retractor size for the case, several tubular retractors in lengths and diameters, a table clamp, and a holding arm. In practice, the diameter of the tube is dependent on pathology being treated. For example, microdiscectomies or cervical foraminotomies can be accomplished with 14-mm tubes, whereas fusion procedures may require tubes with diameters of up to 22 mm.

Fig. 6.1 Tubular retractors and sequential dilators (left image).

Sequential transmuscular dilation minimizes collateral tissue trauma and can better target the site of the pathology in comparison to open approaches. The insertion of a tubular retractor will merely split the paraspinal muscles, minimizing muscle trauma and facilitating rapid recovery after surgery. Tubular retractors are also less prone to cause a crush injury than self-retaining retractors. Furthermore, subperiosteal approaches are also more likely to lead to irreversible denervation of multifidus muscle during dissection.⁷

Pivoting of the tube allows for a larger exposure of the spine with minimal additional muscle dissection. The small size of the approach and the reduced tissue trauma have shown to minimize infection rates, especially in obese patients.⁸^,⁹ Subsequently, the reduced complication rates enable patients to recover faster, shortening hospital stays.¹⁰ Apart from the obvious benefits such as less tissue damage, benefits also include socioeconomic advantages, decreasing the economic burden.¹¹

The transmuscular approach minimizes dead space at the surgical site created by muscle detachment and extensive soft tissue retraction; thereby, reducing the risk of postoperative fluid collections such as seromas. The close approximation of tissues immediately after removal of the tube without a dead space also reduces the risk of cerebrospinal fluid (CSF) leaks. Small incidental durotomies may not require closure in tubular approaches; however, it is recommended.¹² Despite that, primary dural closure using a standard technique and surgical instruments can be challenging in this setting due to the limited surgical corridor through tubular retractor systems. If a watertight dural closure cannot be achieved, this may predispose patients to complications such as CSF fistula, pseudomeningocele, or wound infection.¹³

Tubular microsurgery has a more challenging learning curve than many open spinal techniques.10 Three-dimensional orientation in a deep narrow field can be cumbersome due to the limited exposure of the anatomy, which may even be distorted as a result of the underlying pathology. Therefore, meticulous knowledge of the anatomy and preparation are required in order to be able to target the site of the spinal pathology precisely. As a result, tubular procedures may require extensive radiation that is dependent on the surgeon’s experience, but can be overcome by using intraoperative navigation. A main advantage of tubular surgery compared with open or mini-open approaches is that no subperiostal muscle detachment is necessary. Furthermore, the use of tubular techniques in oblique lateral approaches, such as extraforaminal disc herniations or costotransversectomy trajectories, is advantageous as other approaches would require extensive open dissections.⁷^,¹⁴

6.2.1 Example: Tubular Lumbar Microdiscectomy Approach

A small incision is made approximately 1 to 1.5 cm from the midline over the corresponding herniated disc space and facet joint. A blunt probe is then passed perpendicularly through the incision until the bony surface of the inferior edge of the lamina is encountered. Then a lateral fluoroscopy image should be taken to confirm the dilator position over the lamina/medial facet joint. Once the position is satisfactory, dilators of increasing diameter are inserted with a twisting motion to avoid undue pressure and plunging (special attention should be taken when using plastic sterile covers to not introduce the wrap inside of the wound). The tubular size necessary for a discectomy is approximately 16 mm, depending on the surgeon’s experience. Fluoroscopy is then used to confirm that the dilator is positioned directly on the bone at the inferior edge of the lamina with no or minimal intervening soft tissue. The working channel is then secured with a rigid holding arm and directed slightly medially toward the lamina and spinous process (Fig. 6.2). At this point, a muscle layer remaining on the bone often has to be removed using the monopolar. The subsequent steps are carried in a similar fashion as a standard microdiscectomy.

Fig. 6.2 Utilization of tubular and specular retractors, (a) paramedian skin marks, (b) serial dilation left, (c) overview of the operative field through an 18-mm tubular retractor, (d) healed scars after multilevel tubular surgery, (e) midline skin mark, (f) size of the operative field through a specular retractor, (g)overview of the operative field through a specular retractor, and (h)closed wound after specular surgery.