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. 2016 Mar 30;8(1):3–10. doi: 10.1111/os.12216

History of Retractor Technologies for Percutaneous Pedicle Screw Fixation Systems

Ralph J Mobbs 1,2,3,✉, Kevin Phan 1,2,3
PMCID: PMC6584086  PMID: 27028375

Abstract

Minimally invasive techniques aimed at minimizing surgery‐associated risk and morbidity of spinal surgery have increased in popularity in recent years. Their potential advantages include reduced length of hospital stay, blood loss, and requirement for post‐operative analgesia and earlier return to work. One such minimally invasive technique is the use of percutaneous pedicle screw fixation, which is paramount for promoting rigid and stable constructs and fusion in the context of trauma, tumors, deformity and degenerative disease. Percutaneous pedicle screw insertion can be an intimidating prospect for surgeons who have only been trained in open techniques. One of the ongoing challenges of this percutaneous system is to provide the surgeon with adequate access to the pedicle entry anatomy and adequate tactile or visual feedback concerning the position and anatomy of the rod and set‐screw construct. This review article discusses the history and evolution of percutaneous pedicle screw retractor technologies and outlines the advances over the last decade in the rapidly expanding field of minimal access surgery for posterior pedicle screw based spinal stabilization. As indications for percutaneous pedicle screw techniques expand, the nuances of the minimally invasive surgery techniques and associated technologies will also multiply. It is important that experienced surgeons have access to tools that can improve access with a greater degree of ease, simplicity and safety. We here discuss the technical challenges of percutaneous pedicle screw retractor technologies and a variety of systems with a focus on the pros and cons of various retractor systems.

Keywords: Minimally invasive surgery, Percutaneous pedicle screw, Spine surgery, Technologies

Introduction

Minimally invasive techniques for spinal surgery have increased in popularity because of their potential advantages, which include reduced length of hospital stay, blood loss, and requirements for post‐operative analgesia and earlier return to work.1, 2, 3 The muscle retraction inherently required for the standard insertion of pedicle screws is associated with significant morbidity (Fig. 1), including muscle denervation and atrophy, devascularisation and increased bleeding and infections.4, 5, 6, 7

Figure 1.

Figure 1

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Advantages of percutaneous pedicle fixation. (A) Normal anatomy demonstrating the depth and angulation of the pedicle, (B) muscle retraction with traditional open surgery requires wide paraspinal muscle dissection and (C) because the approach angle matches the pedicle angle, tubular retractors with percutaneous pedicle screws require minimal paraspinal retraction.

The concept of percutaneous targeting and access to the lumbosacral pedicles was first described in 1977 by Magerl et al. 8 The initial report described the percutaneous placement of a spinal internal fixator to identify symptomatic levels of discogenic pain via immobilization prior to fusion being performed. The technique did not provide the desired results and was therefore abandoned.9 Dick et al. described stabilization of the lower thoracic and lumbar spine with external percutaneous screw fixation.10 Mathews and Long first described and performed an entirely percutaneous lumbar pedicle fixation technique in which they used plates as the longitudinal fixation connectors.11, 12

Foley et al. described the first clinical series of a percutaneous technique for fusion of degenerative lumbar pathologies, using a unique fixation system (Sextant; Medtronic, Minneapolis, MN, USA).13 A unique rod insertion device was developed that linked the screw extension sleeves, allowing for pre‐contoured rods to be inserted through a small stab wound and placed in a standard submuscular position with minimal manipulation and essentially no muscle dissection; this procedure did not require direct visual feedback. This landmark paper also described the use of fluoroscopic image guidance for screw trajectory and placement (Fig. 2).14, 15 Foley et al. styled an existing multiaxial lumbar pedicle screw system, modifying it such that screws could be placed percutaneously using an extension sleeve/tower that allowed for remote manipulation of polyaxial screw heads and remote engagement of the screw locking mechanism.13, 15 Nowadays, pedicle screw instrumentation enables a rigid construct to promote stability and fusion for numerous spinal pathologies, including trauma, tumors, deformity and degenerative disease of the spine.16, 17

Figure 2.

Figure 2

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Medtronic Sextant‐2 System. Percutaneous pedicle screw sleeves/towers with separate stab incision site for rod insertion (Medtronic). This fixation system provides pedicle screw fixation with small incisions and minimal soft tissue dissection. Each screw is placed through its own incision and a separate incision is required for placement of rods into the screws.

The instrumentation now available has no limitations on the number of levels instrumented and can be used to perform lumbar and thoracic fixation by placing rods that are straight, curved or pre‐bent to the desired shape. Mono‐axial screws can be used with this new instrumentation, allowing for powerful reduction maneuvers, including distraction and compression.18 A major issue with percutaneous fixation is progressive loosening when fusion is not performed concurrently. This fragility can be addressed by performing additional interbody bone grafting procedures from an anterior or lateral approach, or with a transforaminal lumbar interbody fusion approach as part of the percutaneous screw technique. And finally, new minimally‐invasive approaches to the spine that can be used in combination with percutaneous fixation to allow nerve decompression and intersomatic fusion have been developed.14

Percutaneous pedicle screw insertion can be an intimidating prospect for surgeons who have only been trained in open techniques.19 One of the ongoing challenges of any percutaneous system is to provide the surgeon with adequate access to the pedicle entry anatomy and adequate tactile or visual feedback concerning the position and anatomy of the rod and set‐screw construct.

In addition to technologies such as intraoperative CT, navigation and robotics to reduce surgeon exposure to radiation during percutaneous spinal procedures, technologies to provide the surgeon safe access to the tulip of the pedicle screws throughout these operations has also evolved over the last 15 years. The goal of this paper is to categorize, summarize and discuss the progression of these retractor technologies over this time.

Indications for Percutaneous Pedicle Screw Fixation

Indications for percutaneous pedicle screw fixation include (i) degenerative spine disorders; (ii) trauma; (iii) spinal neoplasia; (iv) infection); (v) obesity; and (vi) revision surgery.

Minimally invasive techniques have revolutionized degenerative spine surgery over the past several decades, particularly in cases of disc herniation, spinal stenosis decompression and fusion procedures. Percutaneous pedicle screw fixation as a component of posterior instrumentation for minimally invasive fusion may be indicated for mechanical lower back‐pain and grade I and II spondylolisthesis‐associated radicular pain. Minimally invasive percutaneous fixation is also an option for revision surgery. Open approaches may be more appropriate for higher grade spondylolistheses and in more challenging cases.

In cases of traumatic injuries from high velocity, high‐energy impacts, surgical strategies may include decompression, reduction, anterior column support if appropriate, restoration of posterior tension band and fusion to prevent spinal deformity. In selected cases of thoracic spine fractures, posterior minimally invasive percutaneous pedicle screw/rod fixation may be applied. This may allow stand‐alone fixation of stable burst or flexion distraction injuries. The use of the percutaneous approach may be beneficial in enabling early postoperative mobilization and preventing secondary injury.

In cases of metastatic spinal disease, better functional outcomes may be achieved by surgical decompression and stabilization. Benefits of surgery include improved survival and reduced requirements for corticosteroids and analgesia. To further extend these benefits, the use of minimally invasive and percutaneous pedicle screw technologies may reduce incision sizes and operation duration, which translates into reduced wound complications and improved quality of life for these patients. Percutaneous pedicle screw fixation may also be indicated in patients whom the surgical team deems high risk but who require intervention for progressive kyphosis and vertebral body collapse caused by vertebral osteomyelitis and infection.

Minimally invasive percutaneous pedicle screw fixation may also be indicated in obese patients, in whom traditional open approaches may be difficult. For example, the use of tubular retraction systems for percutaneous pedicle screw fixation means that similar sized incisions are required for all patients, whereas open approaches would require longer incisions to access the deeper spine in obese patients.

Definitions of Percutaneous Retractor, Extension Sleeve and Tower

There are no standardized published definitions that name or define the various aspects of a percutaneous retractor system or its components. Despite the numerous systems currently available, the authors offer the following definitions (Fig. 3):

  1. Percutaneous retractor sleeve/tower: an apparatus that attaches and provides direct access via a minimalist corridor from outside to the patient to the pedicle screw tulip, so that surgical interactions, including rods, screws and other devices, can be introduced, fixation performed and reduction maneuvers achieved.

  2. Fixation point: the point of attachment of the retractor sleeve/tower to the tulip of the pedicle screw. Systems either have a removable sleeve/tower (Medtronic Longitude, Medtronic; Nuvasive Precept, San Diego, CA, USA) or a fixed tower with a break‐off tab for removal of the sleeve/tower (ES‐2, Stryker).

  3. Transition point: following insertion of a percutaneous pedicle screw, the transition point is the skin level that divides the sleeve/tower into the zones above and below.

  4. Zone above skin: the area surrounding the sleeve/tower that is above the skin following insertion of a percutaneous pedicle screw.

  5. Zone below skin: the area surrounding the sleeve/tower that is below the skin following insertion of a percutaneous pedicle screw.

  6. Retractor impact: this important concept defines the volume lost around the retractor sleeve/tower and the impact on surgeon hand movement and potential clash of instruments and ergonomics for the surgeon.

  7. View space: another important concept, this relates to the surgeon's capacity to look down the sleeve/tower to visualize what is happening at the rod/tulip interface. Most systems have no view space (the tulip cannot be seen because the tower is enclosed); however, other systems have an open sleeve/tower so that the surgeon retains some feeling of open surgery can directly visualize the rod/tulip interface.

Figure 3.

Figure 3

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Definitions of percutaneous retractor sleeve/tower and regional zones/spaces. The percutaneous retractor sleeve/tower attaches and provides direct access via a minimalist corridor from outside to the patient to the pedicle screw tulip. Following insertion of the percutaneous pedicle screw, the transition point is the skin, which divides the sleeve/tower into zones above and below the skin. The fixation point is the point of attachment of the retractor sleeve/tower to the tulip of the pedicle screw.

Types of Tower/Retractor Sleeve

There are no published standardized definitions that name or define the various categories and types of percutaneous retractor sleeve/tower. The authors offer the following definitions (Fig. 4):

  1. Standard: this represents the starting point of all retractor sleeves/towers—a simple tower that attaches to the pedicle tulip. Multiple systems have numerous technical details to assist with reduction and deformity correction.

  2. Floppy: this style of retractor has a soft, malleable sleeve/tower. Its advantages include an excellent view space so that the surgeon can visualize the tulip, providing a feeling of open surgery. Its disadvantages include a loss of reduction capacity: no reduction thread is built into the floppy design.

  3. Flat: this retractor sleeve/tower presents nothing above the skin, the ends of the retractor being stuck to or adherent to the skin. The view space is greater and retractor impact minimized. No systems currently utilize this approach.

  4. Reduction: this design incorporates an extended internal thread so that the set screw can capture the rod and gradually reduce a deformity. Many current systems now incorporate this design feature.

Figure 4.

Figure 4

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The four types of retractors sleeves/towers: (A) standard: a simple tower that attaches to the pedicle tulip; (B) floppy/malleable: this has a soft and malleable sleeve/tower that provides the surgeon with an excellent view space; (C) flat: this presents nothing above the skin, both ends of the retractor being stuck/adherent to the skin and (D) reduction: this design incorporates an extended internal thread so that the set screw can capture the rod and gradually reduce a deformity.

Review of Commercially Available Systems

Numerous commercial percutaneous pedicle screw systems are currently available. First generation designs include Medtronic Sextant (Medtronic), Stryker MANTIS (Minimal Access Non‐Traumatic Insertion System; Stryker, Kalamazoo, MI, USA) and Viper (Depuy, Raynham, MA, USA). These systems permit safe and reproducible minimally invasive access to the posterior lumbar and thoracic spine for pedicle cannulation. Second generation percutaneous pedicle screw systems include ES‐2 (Stryker) Viper 2 (Depuy, Johnson & Johnson, Boston, MA, USA), Precept (NuVasive), and Longitude (Medtronic) systems. These allow for percutaneous fixation over multiple levels and for management of more complex pathologies and deformities via a posterior percutaneous approach. In addition, novel designs have included K2M flexible retraction sleeves (Leesburg, VA, USA).

In general, designs have moved towards simplification of design and ease of surgeon handling, with additional instrumentation to provide the power to achieve deformity reduction (Fig. 5). Currently, there are no published studies that directly compare the commercially available percutaneous screw sleeve/tower systems. Choices are primarily dictated by surgeon preference and comfort with a particular system. Future percutaneous systems may include multiple retractor options within a single system.

Figure 5.

Figure 5

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Evolution of retractors from complex to simple. (A) Percutaneous retractors were initially overly complicated, fiddly and technically demanding. Bulky retractor towers inhibited surgeon hand movement and caused significant retractor impact around the operative field. (Centennial Spine, Las Vegas, NV, USA). (B) Later designs have focused on simplicity and ease of use (Stryker). The arrow indicates a navigation reference tower for percutaneous screw placement that is attached midline to a spinous process.

There are limited published data that discuss the outcomes of particular percutaneous systems. In 2012, Krüger et al. prospectively investigated 51 patients with thoracic and lumbar fractures who underwent percutaneous minimally invasive stabilization using the Sextant II Rod insertion system (Medtronic).20 The median operative time was 61 minutes and the median fluoroscopy time 132 seconds. One hundred and ninety‐seven of 204 screws were correctly placed and all fractures had evidence of bony union after 6 weeks. However, the authors reported that the multiaxial pedicle screws were not able to conserve the slight correction obtained perioperatively via positioning and longitudinal traction.20 In 2014, Wang et al. retrospectively reviewed 100 patients who underwent minimally invasive pedicle screw fixation using the Sextant system (Medtronic).21 They found that percutaneous screw fixation through the pedicle of the fractured vertebra was superior to a conventional open posterior short‐segment four pedicle screw fixation technique for correcting kyphotic deformities. There were significant improvements in visual analogue scale and Oswestry Disability Index scores, as well as sagittal Cobb angle, vertebral body angle and anterior height of the fractured vertebrae. Given the limited evidence available to date, there is a need for further studies of the safety and efficacy of percutaneous pedicle screw systems.22

Future Directions

There is no one single perfect system on the market today, each system providing pros and cons that favor various aspects of the percutaneous fixation technique. All systems are relatively easy to use for single level fixation and stabilization; however, difficulties arise with multi‐level constructs, difficult compression and distraction techniques, complex pedicle angulations such as L5S1 (Fig. 6) and when additional maneuvers such as cement augmentation are required.

Figure 6.

Figure 6

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Potential issues with retractor sleeves/towers: L5S1 screw head proximity. Flexible retraction sleeves minimize the problem of screw head proximity because it is easy to deflect the retractors out of the way of percutaneous pedicle screw placement. It is common for the surgeon to require only a single incision at L5S1 because the retraction sleeves are immediately adjacent at the skin edge.

The concept of “one system, many options” regarding retractor sleeve/towers is a likely direction that will evolve as surgeons demand a flexible percutaneous pedicle screw system to provide multiple fixation and sleeve/tower options to manage complex pathologies. One example is standard reduction pedicle screws spanning a fracture (Fig. 7) with the option of cement augmentation at one or more levels, for use in patients with osteoporosis or metastatic disease with pathological fractures (Fig. 8).

Figure 7.

Figure 7

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Complex fracture management using a variety of percutaneous retractor sleeve/tower options. Retractor sleeves/towers will likely evolve towards the concept of “one system, many options” in response to surgeons’ demands for a flexible percutaneous pedicle screw system that provides multiple fixation and sleeve/tower options for managing complex pathologies. An example is shown of standard reduction pedicle screws spanning a fracture.

Figure 8.

Figure 8

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Fracture management using percutaneous fixation with cement augmentation utilizing various retractor sleeves (ES‐2 and MANTIS Systems; Stryker). In patients with pathological fractures associated with osteoporosis or metastatic disease, standard reduction pedicle screws can be used to span the fracture, with the option of cement augmentation at one or more levels.

The role of retractor sleeve/towers for use with cortical bone trajectory/Medio‐Latero‐Supero Technique (MLST) trajectory screws is yet to be explored and will likely revolve around the use of flexible retractor options (Fig. 9).23, 24 As yet, there are no published technical reports of such options and devices; however, with the expanding range of indications for cortical bone trajectory/MLST screws, similar minimally invasive surgery (MIS) options involving percutaneous sleeves/towers that provide access for pedicle screws using this technique will likely become available in the future.

Figure 9.

Figure 9

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Flexible retraction sleeves that can be laid flat to reduce retractor impact and facilitate surgeon hand movement around the operative field. These types of malleable/bendable sleeves are potentially useful for cortical bone trajectory/MLST screws.

Further directions and improvements in MIS pedicle screw systems are likely to revolve around the use of navigation systems and robotics for ease and safety with pedicle cannulation. Robotic systems will likely evolve to perform the dual function of accurate pedicle navigation and cannulation and screw placement simultaneously. Extension sleeves/towers will likely be attached to robotic arms to assist in deformity correction and precise compression and distraction maneuvers performed under precise navigated control by a surgeon or team who may not be present at the physical location of the surgical procedure. One example of this performance of remote surgery to fixate spinal fractures on a battlefield by a surgical team located elsewhere. It is likely that further designs will aid the surgeon in specific circumstances, such as screw insertion at junctional levels, which involves the issue of retractor collision. Because standard rigid towers are not suitable for such indications, malleable technologies may useful, such as the “Wings” retractor concept (Fig. 10).

Figure 10.

Figure 10

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Wings retractor concept. (A) A malleable retractor sleeve/tower provides the surgeon with ease of vision to the tulip rod/screw interface, (B) a benefit of malleable retractors is ease of packaging and storage and (C) issues of retractor collision, say at L5S1, are solved with malleable sleeves/towers.

Minimally invasive access options for percutaneous pedicle screws are likely still at an early stage of their evolution; many improvements are yet to be developed. This paper assists in defining the language and terminology for describing this expanding field of medical technologies.

Discussion

Over the past decade, the techniques for minimally invasive spinal stabilization and fusion have improved significantly. A key ingredient of percutaneous pedicle screw techniques is the access from the skin level to the pedicle screw tulip. The evolution of percutaneous sleeves/towers to provide this access has expanded, as have the pathologies being managed.

In addition to multiple techniques for placing interbody implants/grafts, there are various methods for mechanically fixating adjacent vertebrae to aid stabilization and progression of fusion. With increasing experience, indications for minimally invasive spinal fusion have expanded.25, 26 The current indications are similar to those for open surgery and strongly rely on the surgeon's experience with the procedure and ability to adapt various technologies. Most minimally invasive spinal techniques have steep learning curves and require different cognitive, psychomotor and technical skills.27, 28, 29, 30, 31 As access sleeves/towers become more user friendly with less fiddle‐factor, the surgeon will experience less stress and have a smoother surgical experience.

It is recommended that surgeons have adequate experience with open procedures before attempting minimally invasive techniques and that they practice on simple one level MIS procedures before attempting such surgery for complex pathologies such as multilevel deformity, tumor or trauma.27 Depending on the procedure, various patient characteristics and the surgeon's experience, MIS may take more time to perform than open surgery.16, 17 It is hoped that development of more streamlined access options will aid in the success of these surgical interventions.

As indications for percutaneous pedicle screw techniques expand, the nuances of the MIS technique and associated technologies will also expand. Improvements in retractor technologies are in relative infancy with numerous future developments likely. Navigation and robotics will combine with access sleeves/towers to make pedicle screw fixation safer and more reliable.

Conclusions

As the indications for MIS spinal fixation surgery evolves, so must the instruments and implants develop. With technological advancements, such as those developed for retractor technologies for percutaneous pedicle screws, it is expected that MIS fusion techniques will be increasingly implemented. This review article adds to the published work educating prospective surgeons on the nuances of access options for percutaneous pedicle screws and where the future of these technologies lies.

Disclosure: Dr. Mobbs has consulted on multiple percutaneous pedicle screw systems, previously worked as a consultant for K2M, is a design surgeon for the Stryker ES‐2 system and receives royalties. He holds intellectual property rights on various associated technologies for percutaneous systems. Dr. Phan has no conflicts to disclose.

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