Mastering Orthopedic Techniques Spine Surgery Rajesh Malhotra, Bhavuk Garg
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Evolution of Spinal Instrumentation1

Bhavuk Garg,
Rajesh Malhotra
 
Introduction
Treatment of spinal conditions dates back to ancient times. New approaches to the spinal column and the use of spinal fixation have vastly improved our ability to treat spinal pathology. Treatment of spinal conditions has improved due to the development of advanced surgical techniques and improved spinal instrumentation. These advances allow surgeons to help their patients maximize their quality of life while striving to minimize the potential for complications.
Over the last several decades there has been a rapid expansion in the availability of implants for use in the spine. The impetus behind these advancements is a combination of the ingenuity of surgeons in the drive for better results, a better understanding of bone healing, better biomechanical understanding and testing, and improved surgical techniques: The indications for surgery remain the most critical component in patient outcome and must be strictly defined before the optimum implant can be selected on the basis of advantages and potential risks; Many new systems and devices have been created or modified over the last several decades.1 Their applications and results continue to evolve, with the earliest methods found in the Edwin Smith papyrus, which dates as far back as 1550 BC.2
The evolution of operative spine surgery was rather slow. Surgical intervention in a living patient was first proposed by Paulus of Aegina who lived in the seventh century. Paulus recommended that a fracture associated with paralysis should be treated by removal of bone fragments that cause a neurological deficit. It has been documented that in addition to using traction devices, Paulus used a red hot iron during spinal interventions.3 Most of the surgical techniques in the field of spine started developing since 1867, when concept of antisepsis was understood.2 The first laminectomy was performed in the United States in 1829 when Dr Alban Gilpin Smith (Fig. 1.1) removed a fractured spine bone to treat a patient with progressive leg weakness. This patient reportedly recovered and improved neurologically. Later in 1888, Dr Smith successfully removed a spinal tumor that was causing neurologic compression and was able to perform more involved surgeries to correct vertebral bones damaged by tuberculosis infections. Because tuberculosis was so common in the United States at the time, most spinal surgeries were performed for this reason. However, as time progressed, surgery also began to be used for other conditions including spinal deformities, fractures, and tumors.4,5
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Figure 1.1: Alban Gilpin Smith treating a case of spine fracture
With the application of spinal instrumentation, the following goals are expected to be reached.6 Implants should maintain correction after deformity surgery to degrees unobtainable with casting techniques. Unstable spinal segments, resulting from trauma, metabolic bone disease, degeneration, or neoplastic processes, are instrumented to stabilize the bony canal and prevent neurologic damage and deformity. Solid immobilization may enhance bony fusion. “Early surgical stabilization facilitates rehabilitation,” thereby avoiding the detrimental effects of recumbency. Certain spinal instrumentation may free the high-risk, neurologically impaired patient from external immobilization.6
The evolution of spinal instrumentation clearly parallels the recognition and achievement of these goals. Although no single type of instrumentation can universally be applied to every pathologic finding, advancements have allowed the spinal surgeon to stabilize the spine from the occiput to the sacrum securely, safely, and predictably. Some designs have focused on anatomical areas and are used across a wide spectrum of spinal diseases while other designs have been customized for a specific spinal problem.1
 
Historical Outline
The main aim of most of the spinal surgeries is spinal fusion. Spinal fusion was initially performed by placing bone graft along the bones of the spine and fusing the spine “in situ”. That is fusing the spine without an attempt of correcting spinal alignment. The earliest fusion procedures were performed without the use of instrumentation. In order to support the spine and avoid motion while the fusion was healing, patients were placed in casts, traction or braces after their surgeries. This technique required prolonged periods of bed rest and immobility ranging from 6 months to 1 year while patients were in casts or traction and ultimately led to very high rates of pseudarthrosis. Russell Hibbs performed the first spinal fusion for scoliosis in 1914. The pseudarthrosis rate of initial spinal fusion surgeries performed by Dr Hibbs was approximately 60%.5 In 1891; Berthold Earnest Hadra7,8 was credited to first reported use of spinal instrumentation, who accomplished dorsal stabilization of a cervical fracture–dislocation, using silver wires for facilitation of spinal fusion. In 1909, Fritz Lang7 affixed rigid celluloid rods to either side of the spinous processes by using silk thread and steel wires to stabilize the spine. Lang discovered that internal fixation induced timelier healing than immobilization therapy alone. The early instrumentation systems functioned as an “internal splint” which held the spine in position until the surgically applied bone graft developed into a fusion mass. Over the years, many different types of techniques and 3instrumentation have been developed to help correct spinal deformities and facilitate fusion. All instrumentation systems apply stabilizing or corrective forces on spinal segments. The points of fixation anterior, posterior, or transpedicular-define their fundamental differences and nomenclature.
 
Anterior Cervical Spine Instrumentation
The era of anterior instrumentation began after Robinson and Smith9 popularized the anteromedial approach to cervical disk disease in the 1950s. The simple plating systems that evolved from appendicular stabilization were fraught with loosening, back out, and other, and devastating soft tissue consequences.10 Dedicated anterior cervical plating systems were first described in the 1970s. Cervical instrumentation continues to evolve with new disk replacement systems, dynamic and low-profile anterior plates, cervical cages, and resorbable implants. Although promising, some of these newer technologies have been implemented without evidence of added benefit.
Marked anatomic and biomechanical differences between the occipitocervical and the subaxial spine result in very different implants. In contrast to posterior cervical implants, all anterior implants are constrained by limited purchase sites, sagittal profile concerns, and the limited extensibility of most surgical approaches. Anterior fixation points are limited to the vertebral body and endplates.10
In upper cervical spine, anterior instrumentation is usually limited to stand alone screw stabilization of unstable odontoid fractures (Fig. 1.2). Mostly this is a 4 mm cannulated cancellous screw of 40 mm length. Other anterior screw fixation techniques into axis and occiput have also been described in literature. However, currently for all practical purposes, these techniques are only used in those conditions where, posterior techniques are contraindicated.10
 
ANTERIOR CERVICAL PLATING
The anatomy of the cervical spine, because of the presence of the vertebral artery laterally necessitates direct anterior application of anterior instrumentation. The use of anterior instrumentation in the cervical spine has become more popular with the advent of several simple, efficient, and effective plating systems. The plates used initially in the cervical spine were plates used for extremity trauma; however there was a high incidence of screw loosening.
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Figure 1.2: Stand-alone screw fixation for odontoid fracture
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Figure 1.3: Orozco plate
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Figures 1.4A and B: Caspar plate
Screw loosening or backing out endangered the nearby viscera, including the carotid artery, esophagus, and internal jugular vein.6
To avoid this problem, plates and screws were designed to have bicortical purchase for maximal stability.1 These designs provided no fixation of the plate to the screw. Early examples of these first-generation plates include the AO-Orozco plate (Fig. 1.3) and Caspar plates (Figs 1.4A and B). Earlier Caspar plate11 had parallel screw slots, allowing settling, making the caspar, the first axially dynamic plate. At that time, settling was considered undesirable, and the plate was modified by replacing half of its slots with round holes. Although the initial results of anterior cervical plating using bicortical screw purchase were encouraging,12 many concerns remained. Risk of penetration into the canal and subsequent spinal cord injury was very worrisome. Also, postoperative complications because of screw backout leading to construct failure as well as airway and swallowing difficulties were well documented.13,14
Second-generation cervical plating systems added rigidity and a locking mechanism. Many of these plates forced medial screw convergence to resist pullout through a triangulation effect. This locking mechanism allowed unicortical fixation, reducing the risk of penetration into the spinal canal.10 Some of these plates were AO cervical spine locking plate (Figs 1.5A and B), the orion plate (Figs 1.7A to C) and the peak polyaxial plate. The AO cervical spine locking plate consists of a hollow outer screw that expands when a second screw is inserted into it (Figs 1.6A and B). Clinically, this new generation plates proved superior and soon rigid plates with unicortical purchase became the standard.15,16
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Figures 1.5A and B: AO cervical spine locking plate
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Figures 1.6A and B: Locking mechanism of AO-CSLP
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Figures 1.7A to C: ORION plate
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Figures 1.8A and B: (A) Atlantis plate; (B) SLIM-LOC plate
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Figures 1.9A and B: SC– ACUFIX plate
However, because of the rigid nature of this construct, problem of stress shielding was soon realized. As we know that graft shortens with time and rigidity of plate does not allow dynamization, pseudarthrosis soon became a concern.1
The most recent generation of plate designs have been introduced that allow for dynamic compression with time allowing for biological or mechanical settling of the graft while maintaining rigidity facilitating load sharing and avoiding stress shielding of the graft promoting the biology of bone healing. Some of the examples include atlantis plate (Fig. 1.8A), C-TEC plate, slimloc plate (Fig. 1.8B), SC acufix plate (Figs 1.9A and B), etc.1
Recently, bioabsorbable cervical plates have been introduced,17 aiming to reduce or eliminate complications such as implant migration and failure, imaging degradation, and stress shielding of the fusion mass. However, there are concerns regarding the quality of fixation and the possibility of chronic inflammatory problems.18
 
ANTERIOR CERVICAL CAGES1,10
Along with evolution of plate designs, cervical cages have evolved as well. These have evolved, as the need for structural support was understood with increasing complexity of anterior cervical reconstruction. Advantages include limited violations of the adjacent vertebrae, no donor site problems, no need for allograft and its associated risks of disease transmission, and no mass problems potentially associated with plate placement.
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Figure 1.10: Cage with in-built screw system
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Figure 1.11: Titanium cervical interbody cages
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Figure 1.12: PEEK cervical cage
Initially metallic mesh cages were used. Now, there are separate and special interbody devices are for anterior cervical discectomy and fusion, corpectomy defect, expandable cages and cages with in built screw system (Fig. 1.10). Cage materials include machined allograft, titanium (Fig. 1.11), polyetheretherketone (PEEK) (Fig. 1.12), carbon fiber, and trabecular metal. A lot of design philosophies are available with cervical cages; however, no one design is superior to another. Carbon or PEEK cages are more preferred as they are radiolucent and allow better assessment of fusion on postoperative radiographs.
 
CERVICAL TOTAL DISK REPLACEMENT
Cervical artificial disk replacement, also known as total disk arthroplasty, has been proposed as an alternative to anterior cervical discectomy and fusion for the treatment of symptomatic cervical degenerative disk disease. In the late 1950s, Fernstrom19 implanted the first disk prosthesis into the cervical and lumbar spines of humans. The prosthesis consisted solely of a steel ball placed within the annulus fibrosus after the nucleus pulposus had been removed.
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Figure 1.13: Bryan cervical disk
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Figure 1.14: Prodisk-C
Predictably, after a short period of symptom relief, the prosthesis ultimately failed secondary to subsidence of the implant. Since then, more complex prostheses have been designed for disk arthroplasty.
Although various materials such as cobalt-chromium alloy, titanium, and high-molecular-weight polyethylene have been tried in cervical TDR systems, optimal biomaterials have not been identified. Some systems rely on a midline keel to achieve fixation; others use spikes or endplate texturing. Keels may risk sagittal split fractures, especially in multilevel implantations.20 These devices also vary in terms of the constraint they place on normal segmental motion.21 Some examples are Bryan disk (Fig. 1.13), Prestige disk, prodisk (Fig. 1.14), etc.
 
Posterior Cervical Instrumentation
As mentioned earlier, stabilization of a cervical fracture dislocation by Hadra (Fig. 1.15)8 is first reported use of spinal instrumentation in cervical spine. For the next 100 years, cervical instrumentation remained limited to various posterior wiring techniques, of which Rogers' technique22 (Intersegmental wires passed through spinous processes) and Brooks' technique23 (wires passed around lamina) were the most frequently employed.
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Figure 1.15: Dr Berthold Earnest Hadra
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In the 1980s, wiring patterns began to include corticocancellous bone struts for added extension stiffness. Later on rigid methods of fixation through lateral mass screws and pedicle screws were introduced.10
As with anterior cervical instrumentation, evolution of posterior cervical instrumentation can also be described separately for occipitocervical spine and subaxial spine.
 
OCCIPITOCERVICAL INSTRUMENTATION
Managing traumatic injuries in this area is burdened by the fact that this is a very unstable region where an extremely mobile cervical spine transitions into a very rigid occiput. Because of challenge to approach from the front and, due to unique bony anatomy, this region is difficult to stabilize with standard anterior instrumentation. Thus, posterior route is most commonly employed. Posterior instrumentation is also extremely challenging because of the very unique and diverse anatomy available for instrumentation in these region.24
Initial onlay fusion and simple wire techniques required periods of traction and long-term bed rest followed by a halo.25 Sonntag and Dickman26 described a rod-wire technique for occipitocervical fusion in which a contoured Steinmann pin is used in a U shape such that the closed end of the U rests against the suboccipital bone. Other rod-wire techniques (Fig. 1.16) include the Ohio Medical Instruments loop with lateral mass linkages, the Hartshill-Ransford loop, custom-formed Luque rods, and other combinations.2733 Rod and wire constructs were more stable but continued to have difficulty controlling axial loads due to the rods pistoning through the sublaminar wires.
An inverted hook technique as described by Faure et al34 involves the insertion of inverted hooks through a burr hole in the occipital bone attached to hooks placed in caudal lamina. The hooks were then connected via contoured rods. However, hook and rod constructs were fraught with complications of the sublaminar hook within the cervical spinal canal.
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Figure 1.16: Rod-wire construct for occipitocervical fusion
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Figure 1.17: Screw plate construct for occipitocervical fusion
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Figures 1.18A and B: Modern screw-rod system for occipitocervical fusion
Screw-plate techniques have been well described in the literature and have many supporting proponents and systems.35 Such techniques include the use of AO plates and inverted Y-shaped plates (Fig. 1.17) secured to C1–C2 with transarticular screws and to the suboccipital bone with paramedian screws. Plate and screw constructs were the first truly stable types of fixation but depended on fixed hole-hole distances in the plate, which made proper insertion of the screws difficult. These devices also failed to have a rigid connection of the screw to the plate. Modification of this technique may include use of lateral mass screws below C1–C2 to include additional caudal levels in the construct. Modern screw-rod devices (Figs 1.18A and B) allow independent insertion of the screw anchors as well as stable connection to the longitudinal rod.24
 
Atlanto-axial Instrumentation
Historically, Gallie's (Fig. 1.19) technique36 has been described which involves passing of sublaminar wires in C1 and wires around spinous process in C2. Brooks23 modified this technique (Fig. 1.20) by placing a fashioned bone block between C1 arch and C2 lamina.
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Figure 1.19: Gallie's technique
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Figure 1.20: Brook's technique
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Figure 1.21: Halifax clamps
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Figures 1.22A and B: Magerl transarticular screw fixation technique
In 1980s, Halifax (Fig. 1.21) clamps37 were is use, however soon went into disrepute as they were prone to arch fracture, implant slippage, and difficulties with graft placement.38,39 In 1987, Magerl et al40 described a rigid C1-2 transarticular screw fixation technique (Figs 1.22A and B). These screws offer greater rigidity, especially in rotation, and better maintenance of reduction than wired fusions. In 2001, Harms41 published his technique for C1 lateral mass fixation combined with C2 pedicle screws in atlanto-axial instability. If C2 fixation alone is required, options are true pedicle, pars, and intralaminar screws.
 
SUBAXIAL CERVICAL INSTRUMENTATION
The first recorded posterior cervical spine instrumentation dates back to 1891 when Dr Berthold Earnest Hadra8 performed an internal operative spine immobilization by wiring together the spinous processes of the sixth and seventh cervical vertebrae. Rogers22 initially described the interspinous wiring technique for the treatment of trauma-induced cervical instability in 1942. This technique involves drilling holes into the spinous processes and passing wires through them.
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Figure 1.23: Bohlman triple wiring technique
The Bohlman42 triple-wire fixation technique (Fig. 1.23) was a modification of the Rogers technique to stabilize a single or multilevel site of segmental instability. The interspinous wiring method is similar to that of the Rogers technique in that wires are passed through drilled-out holes in the spinous processes. Additional wires are then passed through the holes and are threaded through corticocancellous iliac crest bone grafts. The two wires are tightened to secure the bone grafts against the decorticated spinous processes and laminae on each side. The smaller subaxial canal increases the risk associated with sublaminar wiring below C2. Sublaminar and spinous process wiring techniques cannot be used if posterior elements are deficient or fractured. Facet wiring provides an additional alternative for cervical spine fusion (Figs 1.24A and B). This method was introduced in 1977 by Callahan et al.43
Tucker44 first described the use of interlaminar clamps in 1975. This technique requires the presence of intact laminae at the fusion level and may increase the risk of neurological injury by contributing to canal stenosis due to sublaminar hooks.
Rigid subaxial instrumentation options include hooks and lateral mass, pedicle, and transarticular screws. In the late 1980s, Roy-Camille et al45 introduced the concept of using a lateral mass screw-and-plate system to stabilize the cervical spine. Lateral mass screws and rods were introduced because the lateral mass plating system (Fig. 1.25) cannot accommodate complex spinal abnormalities such as those occurring in severe degenerative spondylosis or trauma. Some of these constructs are the cervifix system (Synthes), the vertex system (Medtronic) (Fig. 1.26), and the summit system (DePuy).
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Figure 1.24: Callahan facet wiring
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Figure 1.25: Lateral mass screws and plate system
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Figure 1.26: Vertex system (Medtronics)
Hook-rod devices such as Harrington instrumentation have been applied to the cervical spine in certain cases. The benefits of distraction, compression, and rigidity provided by these devices must offset the disadvantages of possible hook encroachment in the canal and their bulkiness.46
In 1994, Abumi et al47 introduced and used cervical pedicle screws in a novel method of posterior cervical instrumentation. The superior stability, fixation, and resistance to screw pullout provided by this technique, compared with the lateral mass plating system, has been demonstrated in animal models and in human cadavers.48
Various longitudinal members are available for posterior cervical spine surgery. Titanium rods 3.0 to 4.0 mm in diameter are the most frequently employed. Stainless steel implants may be preferable when durability and deformity correction are sought. Various specialized cervical cross-links are available. Cross-links should also be considered for increased torsional stability, to reconstruct axial bursting injuries, in longer constructs, and in osteoporotic bone.10
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Figure 1.27: Laminoplasty miniplate
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Figure 1.28: Paul Harrington, the man with “golden rod”
A wide variety of implants support the many described laminoplasty techniques. Most common are sutures, spacers, precut bone grafts, and miniplates (Fig. 1.27) for the commonly used “open door” technique. Complications of laminoplasty plates are uncommon.10
 
Posterior Thoracolumbar Instrumentation
The first report of internal fixation of the spine is credited to BF Wilkins when he reduced a fracture dislocation of an infant at T12/L1 and fixed the two vertebrae together with a carbonized silver suture passed around the pedicles.49 In the 1940s, Don King first described the used of screw through the facet.50 Boucher modified this in 1959 directing the screw more medially down into the pedicle. This was the first description of a pedicle screw.51
The evolution of modern posterior spinal instrumentation began in the late 1950s with the development of the Harrington hook and rod system (Fig. 1.28).52 Stainless steel hooks and rods were applied to the concavity of the spine in distraction. Distraction hooks were originally placed under the laminae at the caudal and cephalad ends of the instrumentation. By distracting across the rod, surgeons were able to partially reduce spinal deformities (Figs 1.29A to D). Harrington's distraction instrumentation did address the frontal curve of the scoliosis pattern; however, the sagittal contour of the patient was often negatively influenced, particularly in the lumbar spine.
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Figures 1.29A to D: Harrington rod distraction system
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Figure 1.30: High complication rate was seen with Harrington rod system
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Figure 1.31: Luque sublaminar wiring system
The distraction forces of the Harrington instrumentation tended to decrease the amount of lumbar lordosis which led some patients to develop a “flat-back syndrome”. The system also fails to provide the necessary lordosis or rotational control at the thoracolumbar junction. Other complications of posterior Harrington rod instrumentation include wound infection (Fig. 1.30), hook dislodgment, hook–rod disengagement, and laminar fracture.1,5
Eduardo Luque53 introduced further improvement in spinal fixation through segmental attachments by combining rods and segmental sublaminar wires (Fig. 1.31). Segmental fixation with wires did improve correction of the frontal plane as well as allow for the maintenance of a physiologic sagittal contour; however, spinal deformities occur in three dimensions and none of these early techniques allowed for rotational correction during surgery. Of major concern with the Luque system is the risk of neural damage associated with the passage of sublaminar wires either at the time of placement or subsequent removal.3
A hybrid approach combing Harrington and Luque system was developed. With this “Harri-Luque” system (Figs 1.32A and B), standard Harrington distraction rods segmentally affixed with sublaminar wires are used. Similarly, Hartshill technique (Figs 1.33A and B) was introduced. A major drawback with these technique was an increased risk of hook encroachment into the spinal canal. Also the neurological risk of sublaminar wires was still there.6
In 1970, Roy-Camille of France described the first pedicle screw construct.54 In the 1984, a new system was introduced combining the rod and hook construct of Harrington, the advantages of segmental fixation of Luque, and the rigid fixation of the pedicle screw, named as Cotrell-Dubousset (CD) instrumentation (Figs 1.34A to E) system.55 This instrumentation system allowed for correction of the spine in the coronal, sagittal, and axial planes (rotation) during spinal reconstructions. This was a major technical advancement. However, it is a technically demanding system. Its major disadvantage was that it was a bulky implant. Next advancement in spinal instrumentation was the development of cross linking devices. Cross links are simple transverse implants that connect between rods that are placed on each side of the spine.
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Figures 1.32A and B: Harri-Luque system
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Figures 1.33A and B: Hartshill fixation
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Figures 1.34A to E: Components of CD system
These devices provide additional stability to spinal instrumentation. The TSRH implant system (Figs 1.35A to F) was the first to utilize cross-links and was developed at the Texas Scottish Right Hospital in 1983.5,6
Pedicle screw fixation is more rigid than previous hook, rod or wire implants and has therefore allowed for improved correction of spinal curvatures and higher fusion rates. Another benefit of pedicle screw implants is that they require fewer segments to be instrumented and fused during deformity correction.5 The biomechanical advantages of the pedicle screw were soon realized and multiple systems incorporating the pedicle screw with numerous technical variations have been developed. Some systems also combined pedicle screw fixation with hooks and wires. These systems used either plates or rods with pedicle screws. The original Steffee plates (Fig. 1.36) and the Danek plate and screw system are early examples of the plate and screw system and the Wiltse system and Edwards modular system are examples of rod and screw systems. However, the plate systems had difficulty, when applied for multilevel pathology as fixed hole distances in plates hampered safe pedicle screws placement.
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Figures 1.35A to F: Components of TSRH system
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Figure 1.36: Steffee plate
Ultimately rod-screw systems proved superior.1 Soon the polyaxial screws (Fig. 1.37) were also developed which also provided mediolateral freedom.
The development of transpedicular fixation has been a revolutionary means of stabilizing and effecting change in spinal alignment. Since the inception of transpedicular fixation more that 50 years ago, pedicle screw technology and its placement has advanced immensely and continues to evolve.
 
Anterior Thoracolumbar Instrumentation
During the 1950s, the development of sophisticated intraoperative monitoring techniques allowed for safer anterior spinal surgery.56 Hodgson and Stock57 popularized the use of the anterior approach for the treatment of Pott's disease.
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Figure 1.37: Polyaxial screws
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Figure 1.38: Dwyer system
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Figure 1.39: Zielke system
In 1969, Dwyer et al58 reported on the advantages of the anterior thoracic approach for the treatment of scoliosis. He emphasized the importance of compressing the convex side of scoliosis rather than distracting the concave side as in Harrington system. In this system, titanium screws were attached to vertebral bodies through staples. The screws were linked through a cable (Fig. 1.38). Unfortunately, the lumbar kyphosis was often reported postoperatively with this system.56
In 1976, Zielke59 reported on his modification of the Dwyer system, which consisted of replacing the flexible cables with a semi-rigid rod. This system also permitted derotation of curve through a temporary application of derotation outrigger device. Disadvantages of the system include the need to perform a retroperitoneal or thoracolumbar approach and the risk of damage to visceral, vascular and neural (sympathetic) structures. The Zielke instrumentation (Fig. 1.39), although more rigid than original Dwyer cable, did not appear to provide adequate stability resulting in unacceptable rates of pseudarthrosis and hardware failure.6
Dunn60 reported use of curved plate and staples connected to vertebrae with screws (Fig. 1.40). However, there were reports of vascular erosion with catastrophic results from these relatively high-profile systems; the Dunn device is no longer used.
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Figure 1.40: Dunn system
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Figure 1.41: Kostuik-Harrington system
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Figures 1.42A and B: KASS system
The anterior Kostuik-Harrington (Fig. 1.41) system61,62 uses Kostuik screws attached to a Harrington distraction and compression rod. This system provided adequate rigidity and stability to allow early mobilization of the patient and also possibly avoid the need for further posterior surgery. The advantages include its versatility, ease of application, and adaptability. Later reports showed higher incidence of hardware failure with this system.
The Kaneda (Figs 1.42A and B) system63 was developed in Saporo, Japan in 1979. Bicortical screws are placed through staples on the lateral aspect of the vertebral body. Threaded rods link the screws, and a distractor and/or the setting nuts can be used to correct any kyphotic deformity. A rigid cross-link can also be added to the instrumentation, further strengthening the construct.56 Several plate systems with nonlocking like Z plate (Fig. 1.43) and locking screw systems like AO locking plate (Figs 1.44A and B) have also been described.
A lot of anterior instrumentation systems with slight modifications are evolving. However, still long-term studies are awaited to label one as the best. Video-assisted thoracic surgery (VATS) has developed very rapidly recently for the treatment of spinal conditions. The application of thoracoscopy can be traced back to nearly one hundred years ago, when Dr Jacobaeus64 first reported his experiences in the diagnosis and treatment of pleural effusions by thoracoscope in 1909.
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Figure 1.43: Z plate
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Figures 1.44A and B: AO locking anterior thoracolumbar system
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The development of fibro-optic light transmission, the illumination and the image processing techniques, as well as the refinement of related spinal instrumentation made video-assisted thoracoscopy more easily and broadly applied to spinal conditions after the 1990s. Similarly, anterior laparoscopic and endoscopic techniques have also been described, but are not so popular.
 
INTERBODY CAGES
In the mid 20th century, a variety of interbody fusion methods have been published using both anterior and posterior interbody techniques. Cylindrically shaped corticocancellous dowels were first used for an anterior lumbar fusion in 1963 by Harmon65 and 1965 by Sacks.66 Ralph Cloward6769 pioneered the dowel technique beginning in 1953. While he utilized a posterior approach, his methods for disk removal, endplate preparation and grafting came to be used extensively. O'Brien70 devised a hybrid interbody graft using a biological fusion cage (femoral cortical allograft ring) packed with autogenous cancellous bone graft. However, all of them were plagued by high morbidity and poor stability leading to loss of correction, graft related complications, and unacceptably high nonunion rates.71
Three general structures1 for interbody devices had been developed. Horizontal cylinders based on previous dowel techniques, vertical rings similar to femoral cortical rings, and open boxes based on tricortical grafts were the most commonly used interbody devices.
During the mid-1970s, George Bagby (Fig. 1.45)71 et al treated “Wobbler syndrome”, a chronic cervical instability causing myelopathy in horses, by means of a smooth, stainless steel, fenestrated cylinder (Bagby Basket) placed through an anterior approach. Bagby proved that the intervertebral disk space could be permanently distracted and stabilized by means of this rigid, porous, hollow interbody spacer containing only morselized local bone graft without the need for additional fixation or additional bone graft. This stand-alone interbody fusion technique continued to evolve with material changes and the design of threaded cages to increase stability and decrease displacement rates. In the early 1980s, with the assistance of Dr Stephen Kuslich, the originally Bagby basket was modified into the BAK (Bagby and Kuslich) implant.72 This cylindrical titanium cage (Fig. 1.46) has threads to screw into the endplates, thereby stabilizing the device and allowing for increased fusion rate with a stand-alone anterior device.
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Figure 1.45: George Bagby
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Figure 1.46: BAK cage
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Figure 1.47: Carbon interbody radiolucent cage
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Figure 1.48: TLIF PEEK cage
Accompanying tools were developed that allowed for distraction of the disk space, establishment of lordosis, and simple and safe insertion of the device both anteriorly and posteriorly. Ray73 developed a similar titanium interbody fusion device, which was initially used in posterior lumbar interbody fusions (PLIFS), but expanded to include ALIF procedures.
Later on various vertical cages like Harms cage, Pyramesh cage, and Brantigan cage were developed. The advantage of these devices was their ability to maximize the area available for bony fusion. Because of their thin walled structure, marginal endplate fixation, and lack of mechanical stability, most surgeons recommend supplemental fixation.1 In 1991, Brantigan and Steffee74 designed a carbon fiber open box design posterior interbody cage. Carbon fiber (Fig. 1.47) is radiolucent allowing adequate assessment of the fusion on routine radiography. With the advent of TLIF techniques, a variety of TLIF cages (Fig. 1.48) have also been developed. Advances in interbody fusion techniques and technology continue to evolve. The last several years has seen the use of these devices many types of adult spinal surgery in addition to discogenic diseases.
 
LUMBAR DISK REPLACEMENT
A rudimentary lumbar disk replacement consisting of a single metallic ball was first implanted in the late 1950s.75 Kostuik76 developed a total disk replacement that rotated around an articulating hinge within the posterior third of the disk space. The next most significant reported step in total disk replacement appears to have been the development of devices with synthetic-on-synthetic articulating surfaces. This concept was born in the 1980s, initially with development of the SB Charite (Fig. 1.49) and the prodisk artificial total lumbar disk replacement.77
SB Charite device consisted of a sliding core of ultra high molecular weight polyethylene (UHMWPE) interposed between metallic end plates. The initial device (SB Charite' I) had small, shell like end plates made of steel that were actually smaller in diameter than the PE core itself. The second-generation implant (SB Charite' II) featured flat extensions on the left and right sides of the metal end plates. Although settling had been reduced, fatigue fractures of the steel end plates were common and led to early failures.
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Figure 1.49: SB Charite disk prosthesis
The third version (SB Charite' III) developed in 1987; features broadened, flat end plates end plate was manufactured from a cobalt-chromium-molybdenum (CoCrMo) alloy. To maximize osseous integration, the end plates were porous coated with titanium, and a layer of calcium phosphate was applied.77
Marnay developed a total disk replacement in the late 1980s currently called the prodisk. The implant relied on a single semiconstrained articulating interface between the polyethylene core fixed to the inferior end plate and a polished superior metallic end plate. A single midline sagittal fin is used to improve immediate bony fixation of the metallic end plates, as opposed to the six small teeth of the Charite.77
With these designs, concerns of PE induced osteolysis were raised. A lot of metal on metal disk designs like Maverick disk were introduced and more and more designs are still evolving. Also a lot of complex elastic designs with shock absorbing capacity like acroflex disk were also tried, but could not clear clinical trials. Also designs with nucleus replacement like PDN (prosthetic disk-nucleus) and Memory Coiling Spiral deigns have been described but are still under evaluation.77
 
Newer Concepts and Designs
Various new concepts are being introduced in the field of spine surgery. For young children with spinal deformity, numerous implant systems seek to limit curve progression without arresting axial spine growth. Physeal staples, which may be inserted thoracoscopically, are being used to halt growth selectively on the convex side of the deformity. A vertical expandable prosthetic titanium rib (VEPTR) (Fig. 1.50) provides an internal, nonrigid brace for spinal or thoracic cage deformity to allow further thoracic cage growth. More traditionally, “growing rods” have been to provide temporary, internal bracing of rapidly progressive curves in young children and to allow additional axial growth before a formal fusion is performed. Similarly, a lot of motion preserving implants like Dynesys system (Fig. 1.51), Interspinous spacers (Fig. 1.52) (Coflex, etc.) are also gaining popularity as an alternative to spinal fusion.
 
Minimally Invasive Spine Surgery
Minimally invasive spinal surgery is one of the recent developments in the long history of our intellectual and technologic evolution in understanding and treating spinal disease. Disk surgery has seen a lot of revolution in the MISS techniques. Microdiscec-tomy, percutaneous endoscopic discectomy, laser discectomy, arthroscopic discectomy, microendoscopic discectomy (MED), Intradiskal electrical thermocoagulation (IDET) are some of them.
23
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Figure 1.50: VEPTR system
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Figure 1.51: Dynesys system
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Figure 1.52: Co-flex interspinous spacer
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Figure 1.53: SEXTANT system (Medtronics)
Application of image guidance and navigation has also led to several percutaneous techniques like percutaneous pedicle screw fixation and minimally invasive TLIF, etc. Several minimally invasive interbody fusion techniques like AXIALIF, DLIF have also been described and gaining popularity. These minimally invasive techniques are also being applied to complex spinal pathologies like spondylolisthesis and scoliosis. SEXTANT (Fig. 1.53), Longitude, revolve are few of these systems. Vertebroplasty and Kyphoplasty have also revolutionized the treatment of osteoporotic compression fractures.
 
Summary
Various strong and user-friendly spinal instrumentations have been developed in recent years and continue to evolve. Current instrumentation systems and techniques allow spine surgeons to create biomechanically stable constructs through various approaches. As our experience improves, indications and techniques continue to evolve. On one 24hand, the explosion in the number and variety of implants available offers spinal surgeons choices previously not available. On the other hand, choosing a rational approach for patients becomes increasingly difficult. With the evolution of more sophisticated stabilization and fusion devices and motion-sparing techniques, the surgeon requires a clear understanding of spine biomechanics. One must not forget that functional outcome is frequently related more with patient selection, decompression, and patient comorbidities, rather than type of instrumentation.
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