1 History of Minimally Invasive Spine Surgery

Since the earliest recorded history, a variety of pathologies have ravaged the spine. The first concrete delineation of operative treatment of the spinal column was proposed by Paulus of Aegina in the seventh century.¹ He advocated and conducted direct removal of osseous tissue at the site of pathology. Since then, a wide variety of approaches and techniques for operative treatment of the spine have been developed.

The concept of minimally invasive surgery can be traced back to William S. Halsted, who noted that outcome could be optimized when certain surgical principles were applied. Before this, it was believed that speed was the critical determinant in patient outcome. Halsted, although a quick surgeon himself, stressed the principles of minimizing tissue disruption, meticulous hemostasis, and proper closing of anatomic layers as essential to optimizing patient outcome. These principles were among the first that stressed techniques that made surgery less invasive and are still the basic fundamentals that underlie what is known today as minimally invasive surgery, or MIS.

In more recent times, economic pressures from the competitive health care climate as well as a desire to optimize patient satisfaction have also helped in spurring developments throughout multiple surgical disciplines that reduce operative time and complications, minimize intraoperative tissue trauma, diminish the length of hospital stay, lessen postoperative use of narcotics, and enhance postoperative outcome. With initial experiences of successful early ambulation after surgery and the advantages of outpatient surgery to the patient, hospital, and physician, the trend toward minimally invasive surgical procedures has progressed rapidly.^2,3,4,5,6

1.2 General Advancements

Central to the evolution of the field of minimally invasive spine surgery has been the development of three essential techniques: (1) endoscopy, (2) microsurgery, and (3) image guidance. Although it may seem that the technical advancements that have facilitated the development of minimally invasive procedures are new, in fact many of these innovations were conceived more than 100 years ago.

The evolution of the endoscope has its roots in 1806 when Philipp Bozzini developed an endoscopic device to delineate the exploration of various body cavities.⁷ In 1853, Desormeaux implemented the use of a lens and an alcohol-based fluid on the endoscope for direct-light focus and increased intensity, respectively.^8,9 The first endoscopic procedure was performed by Bevan in 1868 to address esophageal pathology.⁸ Later, Nitze¹⁰ developed a cystoscope that consisted of a working channel, illumination, and an optical lens for reflection. In 1902, the first laparoscopic surgery was performed,¹¹ and in 1911, Lespinasse, a urologist working in Chicago, performed the first ever cranial neuroendoscopic procedure when he fulgurated the choroids plexus in two infants with hydrocephalus.¹² In 1931, Burman¹³ first reported on the use of endoscopy for the spine, which he termed myeloscopy, and in 1938, Pool¹⁴ reported on his experience with myeloscopy to inspect the nerve roots and spinal cord in cases of disc herniation, hypertrophied ligamentum flavum, benign and malignant neoplasms, and arachnoiditis. As the years progressed, the endoscope was improved and used for various diagnostic and therapeutic purposes. Since then, there has been an explosion of endoscopic instruments that have made a large variety of endoscopic procedures possible. In 1997, Foley and Smith¹⁵ attached an endoscope to a tubular retractor system, and the concept of microendoscopic discectomy, or MED, was born.

Although tubular retractors have been used for quite some time in cranial neurosurgery,¹⁶ their introduction to spine surgery would help usher in a new era in minimally invasive spine surgery. In 1998, the METRx System (Medtronic Sofamor Danek) was introduced, which used the same tubular retractors as MED but allowed the use of either the endoscope or a microscope.

The development of the operating microscope has an equally interesting history and has been meticulously detailed elsewhere.¹⁷ Although the exact origin of the microscope is uncertain, it is believed to have been invented around 1600 in Holland.^18,19 Anton van Leeuwenhoek, a Dutch scientist from the 1600s, advanced the field of microscopy by improving considerably on the microscopes of his time and making countless microscopic observations.¹⁹ In the 1800s, Carl Zeiss, a German machinist, began building microscopes and was able to standardize lens production, allowing mass production of microscopes, and he also introduced the first binocular microscope.¹⁷ In 1921, Carl Nylen,²⁰ a Swedish otolaryngologist, was the first person to use a microscope in surgery, treating a case of chronic otitis media. Over the next several decades, many clinicians in the fields of ophthalmology and otolaryngology continued to advance the field of microsurgery and the development of the operating microscope.

In 1957, Theodore Kurze performed the first microneurosurgery procedure,¹⁷ the removal of a seventh nerve schwannoma, and in 1962 he published the first series on microneurosurgery.²¹ In 1958, Raymond Donaghy established the first microsurgery research laboratory in Burlington, Vermont, and in 1960 Jacobson, a vascular surgeon who had worked earlier with Donaghy, teamed with the Zeiss corporation to develop the first two-person microscope to allow the assistant to actually assist.¹⁷ Pool and Colton²² published the first account of the use of a microscope for intracranial aneurysm surgery in 1966. Mahmut Gazi Yasargil spent a year in Donaghy’s laboratory in 1966, returning then to Zurich to establish his own microneurosurgery program.¹⁷ Over the next several decades, the work of Yasargil and many others demonstrated the invaluable assistance that use of the microscope provided to the performance of surgery of the central nervous system. Over time, the techniques that were developed for intracranial surgery would be adapted to surgery of the spine.

More recently, image guidance has played a major role in advancing minimally invasive spine surgery. The development of the magnetic resonance imaging (MRI) scan has allowed the very precise location and characterization of spinal pathologies, allowing surgeons to attack the problem in the most direct manner. The only difficulty has been finding a way to more precisely guide the surgeon to the intraoperative pathology based on preoperative imaging. For the past several decades, intraoperative fluoroscopy has been utilized to confirm levels and help ensure proper localization for the surgeon. This provided for the ability of more frequent imaging than standard flat plate radiography, with images being obtained in real time, even in a continuous manner to closely monitor the placement of hardware. In more recent years, the O-arm (Medtronic Navigation) and Airo (Brainlab) have allowed more complete spinal imaging equivalent to a computed tomography (CT) scan to be done in real time by the surgeon in the operating room. Frequently, this imaging is paired with a surgical guidance system to assist in the placement of spinal hardware, such as the Brainlab Spinal Navigation system (Brainlab) or the StealthStation S7 (Medtronic Navigation). As helpful as these technologies have been, one major drawback has been the increase in radiation exposure as a result of their use, to both the patient and the operative team. In 2004, Mazor Robotics introduced its SpineAssist robotic spine surgery platform, with its second-generation platform, the Renaissance Guidance System, later launched in November 2011. This system continues to be the only robotic spine surgery platform Food and Drug Administration (FDA) approved for robotic guidance to the cervical, thoracic, and lumbar spine. This system uses two fluoroscopic images obtained with fiducial markers in place to register to the patient’s preoperative CT scan in a fashion similar to the technique used by the Cyberknife Robotic Radiosurgery System (Accuray). The Renaissance robot is then mounted to the patient’s spine and guides the placement of the spinal hardware without the need for repeated and excessive fluoroscopic images being taken. To date, there have been an estimated 50,000 or more screws placed with this system with no reported incidents of permanent nerve injury.

1.3 Advancements in Cervical, Thoracic, and Lumbar Surgical Spine Techniques

Although the development of endoscopic and microsurgical techniques is applicable to all regions of the spine, the history of the cervical, thoracic, and lumbar areas is sufficiently distinct to warrant discussing these topics separately. The focus will largely be on the treatment of disc herniations and spondylosis, because much of the progress in the field of minimally invasive spine surgery was made while treating these degenerative diseases due to the large number of patients with such diagnoses compared to other ailments. However, these advancements were quickly adapted to treat virtually all other conditions that involve the spine.

1.3.1 Cervical Surgical Spine Techniques

The first report of anterior spinal cord compression from posterior protrusion of a herniated disc was by Key in 1838.²³ Walton and Paul,²⁴ in 1905, reported on a patient with an apparent herniated disc that they operated on for a presumed cervical spinal cord tumor. Mixter and Barr,²⁵ in their landmark 1934 paper, included four patients with cervical disc herniations. Initial description of a lateral cervical disc herniation with nerve root compression (vs. a central herniation and compression of the cord) was made by Stookey, in 1928,²⁶ and lateral cervical disc herniation with nerve root compression resulting in neck pain and radiating arm pain was further detailed by Semmes and Murphy in 1943.²⁷

Initially, the posterior approach was used exclusively to approach cervical disc disease, with a transdural approach taken to the disc herniation, as first described by Elsberg.²⁸ In 1928, Stookey²⁶ introduced a less invasive hemilaminectomy to address cervical herniations, and Semmes and Murphy²⁷ advanced the minimally invasive approach to these lesions by operating through a smaller laminotomy. Frykholm,²⁹ Spurling and Segerberg,³⁰ and Scoville³¹ further advanced this procedure to the keyhole foraminotomy that is still in use today²⁹ ( Fig. 1.1).

Fig. 1.1 (a,b) Drawings showing the extent of bony removal for posterior laminoforaminotomy for lateral cervical disc herniation. (Reproduced with permission from Frykholm.²⁹)

Although the posterior approach had been used with success for several decades, patients all too frequently had new postoperative neurologic deficits, particularly when they had central herniations that caused spinal cord compression. To address these shortcomings, Robinson and Smith, in 1955,³² made their initial report of the anterior approach to the cervical spine to treat disc herniations. This allowed access to the primary pathology without the need to manipulate the intervening neural elements. Their report was followed by Cloward’s in 1958,³³ detailing experience and describing specific equipment designed for the procedure. After a gradual period of acceptance, the anterior approach became the procedure of choice for most spine surgeons. In 1975, Hankinson and Wilson³⁴ were the first to report on the use of the microscope for spine surgery when they reported on the series of 51 patients in whom microsurgical anterior cervical discectomies were performed.

Although the anterior approach was a significant advancement in cervical spine surgery, clinicians continued to strive to make this procedure as minimally invasive as possible. Originally, it was common practice for patients to spend prolonged periods of time in rigid cervical collars. During the 1970s, the concept of internal fixation to aid in cervical fusion was introduced, and in the 1980s the Caspar anterior cervical plating system was introduced.^35,36 This soon led to the development of numerous other types of anterior cervical plating systems. As plates have evolved over the years, clinicians have gradually stopped using collars routinely after single-level fusions and allowed patients’ more rapid return to normal activities. Use of allograft for cervical fusion was popularized throughout the 1990s, which allowed patients to avoid the morbidity of an iliac crest autograft.³⁷

Even with the freedom that these new internal fixation devices and fusion options have provided, there have been some who have continued to worry about the loss of a cervical motion segment and the possibility of progression of adjacent-level spondylosis when a fusion is performed. Anterior cervical decompressions done without fusions were originally promoted as a solution to this potential problem; however, even these procedures have shown a high rate of spontaneous fusion and ultimate loss of a motion segment.^34,38 In an effort to address this concern, Snyder and Bernhardt, in 1989,³⁹ building on the work of Verbiest,⁴⁰ developed the anterior cervical foraminotomy, which was further refined by Jho in 1996.⁴¹ This allowed decompression of the affected nerve root but maintained preservation of the motion segment. Jho⁴² went on to illustrate how this technique could be used to perform the central canal decompression required to treat patients with myelopathy. However, the inherent risks to the vertebral artery, nerve roots, and spinal cord when utilizing this approach elevate the technical demand of these procedures and have precluded this technique’s widespread adoption.

In more recent times, the application of endoscopy and other new techniques to spine surgery has led to an explosion in minimally invasive procedures for the cervical spine. The MED system, or one of its variants, has been used for minimally invasive procedures to attack pathology from both the anterior and posterior cervical approaches with great promise, particularly with posterior approaches, for which these new techniques seem to offer a significant advantage over the traditional open procedure.^{43,44,45,46,47,48,49,50} Although these procedures are essentially identical to their open-procedure predecessors, the use of the tubular retractor system, in particular, has made possible significant advances toward the ultimate goal of making cervical spine surgery as minimally invasive as possible.

Over the past several years, there has been an increased emphasis on motion preservation when performing spine surgery, in the hope of avoiding the development of adjacent segment degeneration following a spinal fusion. In 2007, Mummaneni et al⁵¹ first published their results with the PRESTIGE ST Cervical Disc System (Metronic Sofamor Danek) that allowed artificial disc motion preservation at the operated level in the hope that this would eliminate the appearance of adjacent segment degeneration, and since then many additional variants of the artificial disc have been developed and marketed. Initial studies have shown equivalency of the artificial disc to a standard anterior discectomy and fusion.^52,53,54 More recent studies appear to begin to show superiority of the artificial disc over fusion.^55,56 In the cervical spine, the ease of the standard anterior approach, which surgeons are already very familiar with, has helped make the acceptance of the cervical artificial disc considerably easier than its lumbar counterpart.

1.3.2 Thoracic Surgical Spine Techniques

One of the first proposals for surgical intervention for an ailment of the thoracic spine was made by Pott, in 1779,⁵⁷ when he recommended drainage of the tuberculum abscess in his work on paraplegia. Since then, patients with an assortment of ailments afflicting the thoracic spine have undergone a wide variety of surgical approaches in an attempt to minimize the morbidity of this procedure. Surgery for the anterior column of the thoracic spine poses certain challenges not seen in either the cervical or lumbar region. Operating on either the thoracic or cervical region means that the surgeon will have anatomic structures that complicate their anterior exposure. It was for this reason that early surgical approaches to the thoracic spine, like approaches to the cervical spine, were posterior in nature and resulted in the same relatively poor outcomes, prompting surgeons to seek alternate approaches. However, unlike the cervical spine, which is easy to approach anteriorly, the contents of the chest make anterior approaches challenging for the thoracic spine. Thus, when surgeons first began to devise alternate approaches to treat lesions of the thoracic spine, they needed to contend with the presence of the spinal cord posteriorly and the thoracic cavity contents anteriorly. This led to the development of a number of approaches that varied in how they addressed these two limitations, all with the intention of making surgery of this nature as minimally invasive as possible. The history of the treatment of thoracic disc herniations provides an excellent example of how surgery of the thoracic spine has evolved over the past century.

The first reported case of a herniated thoracic disc was made by Key in 1838.²³ In 1911, Middleton and Teacher⁵⁸ provided additional evidence that disc herniations occur in the thoracic spine. The first known case of operative treatment of a thoracic herniated disc was described in 1922 by Adsen, who performed disc removal after a laminectomy.⁵⁹ For much of the next several decades, the procedure of choice was a posterior approach, but the results were dismal.^59,60,61,62 The lateral extracavitary approach was first described by Menard, in 1894,⁶³ to treat Pott’s disease and was later adapted to treat disc herniations.⁶⁴ Cauchoix and Binet⁶⁵ reported, in 1957, on their initial experience with a transsternal approach to the upper thoracic spine. In 1969, Perot and Munro,⁶⁶ and separately Ransohoff et al,⁶⁷ adapting the transthoracic technique of Hodgson and Stock,⁶⁸ simultaneously reported on a transthoracic approach to treat thoracic disc herniations. In 1978, Patterson and Arbit⁶⁹ described the transpedicular approach in an attempt to avoid entering the thoracic cavity, and in 1998 Stillerman et al^70,71 introduced the slightly less invasive transfacet pedicle-sparing approach. In 1995, McCormick⁷² described the retropleural approach, which avoided entering the thoracic cavity and also avoided disruption of the facet joint. In 1997, Jho⁷³ introduced the posterior microendoscopic approach to treat thoracic disc herniations.

Although the evolution of these approaches details significant advances in making surgery for thoracic disc herniations much less invasive, such approaches were still somewhat invasive. In an attempt to minimize operative morbidity, thoracoscopic video-assisted spine surgery was developed and first performed in the early 1990s simultaneously by Mack et al⁷⁴ in the United States and Rosenthal and Dickman⁸ in Europe. Initially, thoracoscopic spine procedures were implemented to treat disc herniations or for tumor biopsies. However, in the ensuing years, thoracoscopy was used to address scoliosis, anterior interbody fusion, drainage of disc space abscess, and vertebrectomy for tumor.^{75,76,77,78,79,80}

1.3.3 Lumbar Surgical Spine Techniques

In 1857, Virchow⁸¹ described traumatic intervertebral disc rupture, and in 1911 Middleton and Teacher⁵⁸ expanded on his first account when they reported on their autopsy findings of traumatic disc rupture. Also, in 1911, Goldthwaite⁸² speculated that lumbar disc herniations were responsible for sciatica. The first account of a lumbar laminectomy performed in the United States occurred in 1829 and was performed by Smith to treat progressive paresis stemming from a previous fracture.⁸³ Using this laminectomy approach, in 1908 Oppenheim and Krause,⁸⁴ and later Elsberg,²⁸ performed resection of what were, in all likelihood, herniated lumbar discs. In 1929, Dandy⁸⁵ reported the first definitive description of operative treatment of lumbar disc disease, recognizing its traumatic origin. In 1934, the influential description by Mixter and Barr²⁵ of neurologic deficits associated with a herniated lumbar disc and subsequent treatment by a lumbar discectomy via a laminectomy stressed the practicality of operative intervention to address such lumbar pathology. In 1938, Love⁸⁶ advanced the practice of lumbar disc surgery when he reported on his experience of removal of herniated discs through a minimally invasive interlaminar approach that typically required no bone removal. Adaptations of this technique form the basis for the lumbar microdiscectomy that is still in use today. In 1973, Scoville and Corkill⁸⁷ reported on their experience with early mobilization of the postoperative discectomy patient. Before this, patients were usually left in bed for days and had their activity severely restricted for weeks or months after a discectomy. Striving to promote intraoperative visual acuity and reduce tissue trauma, Caspar⁸⁸ and Yasargil⁸⁹ simultaneously introduced the use of an operating microscope and microsurgical techniques in 1977 for the treatment of lumbar disc disease. In addition, Caspar’s 1977 paper⁸⁸ described instruments and techniques that bear a striking resemblance to those used two decades later by Foley and Smith¹⁵ in their description of MED⁸⁸ ( Fig. 1.2). Williams⁹⁰ further supported and popularized microdiscectomy in the 1970s by advocating minimizing the skin incision and removing only the herniated disc fragment through an intralaminar window not unlike Love’s technique.

During the 1990s, an explosion in the number of endoscopic procedures and techniques occurred in numerous fields, and spine surgery was no exception. The use of an endoscope for lumbar discectomy was introduced by Mayer and Brock in 1993.⁹¹ A transforaminal endoscopic microdiscectomy was reported in the mid-1990s and provided direct visualization of the epidural space via entry under the pars and superior articular facet.⁹² In 1997, Foley and Smith¹⁵ illustrated the development and application of the MED system. Shortly thereafter, the METRx-MD system (Medtronic Sofamor Danek) was introduced and allowed surgeons to take advantage of the tubular retractors that could be used with either the endoscope or the microscope. Based on the success of these systems in treating herniated discs,⁹³ a minimally invasive technique for treatment of lumbar stenosis was developed, which avoids many of the consequences of the traditional laminectomy, including excessive scar tissue formation, disruption of the posterior bony elements, subsequent instability of the spine, and ensuing biomechanical alterations.⁹⁴

Fig. 1.2 Drawing of a retractor introduced by Caspar in 1977, with striking similarities to the technique of microendoscopic discectomy introduced by Foley and Smith in 1997. (Reproduced with permission from Caspar.⁸⁸)

In an attempt to further decrease tissue trauma and enhance postoperative outcome, various other minimally invasive surgical procedures have been developed to address lumbar disc herniation. In 1941, Jansen and Balls⁹⁵

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