Posterior fixation in conjunction with bone grafting is the standard technique to achieve bony fusion in lumbar spinal surgery. Pedicle screw fixation in the lumbar spine is the most commonly used method of posterior lumbar fusion.1,2 This technique provides three-column support of the vertebrae, contributing to the biomechanical strength of the construct and the efficacy of the procedure.1,2 However, open pedicle screw fixation often necessitates wide posterior exposure and dissection, with soft-tissue disruption of the facet joint. Major neurologic and cardiovascular complications can occur because of the anatomic location and the duration of the procedure. In addition, potential complications related to misplaced screws (2.4%), late-onset discomfort (23% to 24%), or nonunion must be considered.3 The safety and accuracy of pedicle screw fixation in patients undergoing revision surgery has been improved with the use of the freehand technique, advanced imaging, and perioperative fluoroscopic navigation.4-7 Despite these advances, the technical difficulty and precarious anatomic location of the procedure has spurred interest in new techniques and constructs.8,9 Alternative posterior fixation techniques have been developed with the goals of minimizing soft-tissue dissection, disruption of the adjacent facet joint capsule, neurologic risk, and implant cost.
These techniques offer results comparable to those of standard pedicle screw systems.10 These less rigid facet screw fixation systems were designed to promote biologic fusion by allowing implant-to-host load sharing and increased micromotion.11 However, the decreased rigidity of these constructs can result Kushagra Verma, MD, MS Anthony Boniello, MD Jeffrey Rihn, MD From the Department of Orthopaedic Surgery, University of California, San Francisco, CA (Dr. Verma), the Department of Orthopaedic Surgery, Drexel University College of Medicine, Philadelphia, PA (Dr. Boniello), and the Rothman Institute, Thomas Jefferson University Hospital, Philadelphia, PA (Dr. Rhin). Dr. Rihn or an immediate family member serves as a paid consultant to Pfizer; has received research or institutional support from DePuy Synthes; and serves as a board member, owner, officer, or committee member of the North American Spine Society. Neither of the following authors nor any immediate family member has received anything of value from or has stock or stock options held in a commercial company or institution related directly or indirectly to the subject of this article: Dr. Verma and Dr. Boniello. J Am Acad Orthop Surg 2016;24: 357-364 http://dx.doi.org/10.5435/ JAAOS-D-14-00378 Copyright 2016 by the American Academy of Orthopaedic Surgeons. June 2016, Vol 24, No 6 357 Copyright ª the American Academy of Orthopaedic Surgeons. Unauthorized reproduction of this article is prohibited. in excessive motion, which can lead to nonunion and early hardware failure. These alternative techniques have also been explored as adjuncts to anterior fixation. Here, we review screw fixation options available for treating lumbar instability, degeneration, and deformity. We focus on common and emerging screw fixation techniques in the lumbar spine.
Knowledge of the benefits and limitations of these alternative options is of particular importance in the treatment of patients with complex conditions or multiple comorbidities in whom pedicle screw implantation would carry an unacceptably high risk of complications. Terminology Highlighted here are alternatives to the standard open pedicle screw fixation method, with a particular focus on techniques designed to minimize soft-tissue disruption and surgical time. Because of a lack of universally accepted terminology in the spine literature and in practice, terms such as percutaneous and minimally invasive are often used interchangeably, leading to confusion. For example, many spinal surgeons perform standard open fusion techniques that are termed minimally invasive. To minimize confusion, we detail percutaneous alternatives to standard fixation techniques12-15 and avoid using the term minimally invasive. Posterior Fixation Techniques Pedicle Screw Fixation The paraspinal approach to the lumbar spine was first described by Watkins16 in 1959. A transmuscular paraspinal approach was described by Wiltse17 in 1973. The Wiltse approach was originally used for spinal fusion in patients with spondylolisthesis and later for removal of far lateral disk protrusions, for pedicle screw fixation, and for decompression of the spinal canal.18 As noted, pedicle screw fixation, the most commonly used method of posterior lumbar fusion, provides three-column support of the vertebrae, contributing to the biomechanical strength of the construct and the efficacy of the procedure.1,2 However, open pedicle screw fixation often necessitates wide posterior exposure, longitudinal separation of the sacrospinalis muscles between the multifidus and longissimus parts, and dissection with soft-tissue disruption of the facet joint.17 A percutaneous alternative to open pedicle screw fixation has been described.
Percutaneous implantation avoids wide disruption of the paraspinal muscles because tubular retractors are used to spread the musculature over the screw under fluoroscopic visualization.19 Placement of the pedicle screws is generally accomplished with the use of cannulated screws over a small guidewire, although a technique that does not require Kirschner wires has been successfully used.20 Placement of the rod or longitudinal connector depends on the system used. Generally, the rod is sequentially persuaded into each screw head and secured with a cap. This technique becomes increasingly difficult with multilevel fusions because of the highly complex constructs used. Preoperative spinal imaging (ie, MRI and CT) for the evaluation of pedicle size and orientation is of paramount importance for proper placement because the normal anatomic landmarks for screw placement are not exposed. Despite these difficulties, surgical constructs spanning multiple levels can be successfully used in select patients.19 He et al21 reported marked postoperative improvement from preoperative health-related quality of life scores (ie, Oswestry Disability Index and visual analog scale scores) in patients with spondylolisthesis, lumbar intervertebral disk herniation, and lumbar spinal canal stenosis. However, this technique has less potential for correction compared with open techniques.22 Misplacement of screws is also a concern because this technique limits the tactile and visual cues available to the surgeon. The authors of one study found a low screw misplacement rate of 6.6% and concluded that percutaneous insertion of pedicle screws in the lumbar spine is a safe and reliable technique.23 Cortical Screw Fixation Santoni et al24 proposed a cortical screw technique as a potential modification of the standard anatomic pedicle screw trajectory.
Compared with standard pedicle screws, the cortical screw system has a more medial starting point, and it follows a caudal-to-cephalad and medial-to-lateral trajectory (Figure 1). The cortical screw is firmly secured by cortical bone from the pedicle and is anchored within the pars interarticularis. Unlike pedicle screw fixation, cortical screw fixation requires interconnecting rods fastened with top-locking connectors.24,25 Although the cortical screw technique required a shorter screw (mean, 29 mm) with a smaller diameter (mean, 4.66 mm), Santoni et al24 reported a trend (P = 0.08) toward greater pullout strength than was observed with the pedicle screw technique and attributed this finding to the two cortices of purchase. Of note, 20% of the cortical screws had a breach of the medial wall of the pedicle on plain radiographs and gross examination, raising concerns for the potential of neurologic injury. Emerging Techniques for Posterior Fixation of the Lumbar Spine 358 Journal of the American Academy of Orthopaedic Surgeons Copyright ª the American Academy of Orthopaedic Surgeons. Unauthorized reproduction of this article is prohibited. A biomechanical cadaver study to assess the segmental range of motion of this technique was recently published by Perez-Orribo et al.25 Cortical (4.5 mm) and pedicle (6.5 mm) screws were analyzed in an intact spine in conjunction with transforaminal lumbar interbody fusion (TLIF) and in association with direct lateral interbody fusion. The authors of the study found that, in conjunction with an intact spine or a nonstabilizing TLIF, a standard pedicle screw system had slightly greater stiffness in axial rotation and lateral bending than did cortical screw systems.
However, cortical screw systems offered greater rigidity in flexion and extension. In conjunction with direct lateral interbody fusion, no substantial differences were found in range of motion between pedicle and cortical screw systems. Kasukawa et al26 recently evaluated the cortical approach in comparison with open pedicle and cortical screw techniques for TLIF. The open cortical screw approach resulted in less blood loss and a shorter surgical time than both the standard open pedicle screw approach and the percutaneous cortical technique. Cortical screw placement also resulted in rates of bone union, maintenance of lordotic angles, and accuracy of pedicle screw position that were similar to those of standard or percutaneous pedicle screw fixation methods.26 In regard to patients’ perceived outcomes, Mizuno et al27 reported improvements in function, with a mean gain in Japanese Orthopaedic Association scores of 10.6 and few complications at a mean 15-month follow-up (range, 3 to 26 months). The authors of the study concluded that this technique is effective and may be applicable for the treatment of lumbar spondylolisthesis in combination with midline decompression and insertion of an interbody graft.27 Cortical screw fixation improves pullout strength in osteoporotic bone and has the advantage of less soft-tissue dissection than pedicle screw systems require. However, the technical difficulty of a more medial starting point and the potential for neurologic injury resulting from violation of the medial pedicle wall may limit the clinical utility of this technique.
To date, the literature lacks sufficient research comparing percutaneous and open screw techniques and analyzing optimal screw insertion angles. Transfacet Screw Fixation Ipsilateral lumbar transfacet screw fixation was first described by King28 in 1948. In the original description, the screw was laterally placed through the ipsilateral lamina across the articulating joint. This placement was modified by Boucher et al29 in 1959 to help achieve fusion in the lumbosacral spine with the use of longer screws placed across the facet joints (Figure 2). Overall, the transfacet screw is directed “down and outward” (ie, caudally and laterally).11,30 In contrast to pedicle screws, transfacet screws do not span the three columns of the vertebrae. Therefore, many studies have attempted to quantify the strength and resistance to stress achieved with this technique compared with that achieved with other techniques and to elucidate the ideal role of transfacet screw fixation. In an anatomic and radiographic analysis, Su et al30 described anatomic considerations, the ideal screw length and trajectory, and the ideal starting point for fixation in the L3- S1 levels (Figures 3 and 4). The authors noted that the L2-L3 level was not amenable to ipsilateral transfacet screw placement because of the vertical orientation of the facet joint. For levels inferior to L2-L3, the screw should be directed toward the inferior half of the inferior segment with caudal angulation (measured relative to the intervertebral disk) of 26! at L3-L4, 29! at L4-L5, and 31! at L5-S1. In the axial plane, the screw should be laterally angulated 15! at L3-L4 and L4-L5 and 18! at L5-S1.30 The authors of numerous biomechanical studies have reported on the efficacy of transfacet screw Figure 1 Lateral (A), axial (B), and AP (C) radiographs demonstrating cortical and pedicle screw trajectories. Compared with the standard pedicle screw trajectory (dotted line), the cortical screw trajectory (dashed line) has a more medial starting point.
The cortical screw trajectory is caudal to cephalad in the sagittal plane and medial to lateral in the axial plane. (Reproduced with permission from PerezOrribo L, Kalb S, Reyes PM, Chang SW, Crawford NR: Biomechanics of lumbar cortical screw-rod fixation versus pedicle screw-rod fixation with and without interbody support. Spine (Phila Pa 1976) 2013;38:635-641.) Kushagra Verma, MD, MS, et al June 2016, Vol 24, No 6 359 Copyright ª the American Academy of Orthopaedic Surgeons. Unauthorized reproduction of this article is prohibited. fixation to augment anterior or lateral lumbar interbody fusion techniques with comparable results between the two techniques.31-33 The study by Volkman et al10 showed that posterior transfacet screw fixation improved stiffness of the construct in compression and extension after anterior lumbar interbody fusion (ALIF) in a cadaver model. However, it is unclear whether this added stiffness is sufficient to improve the rate of fusion. In a different biomechanical cadaver study, Mahar et al12 compared a percutaneous 4.5-mm transfacet screw system with a 5.5-mm pedicle screw system when each was used with an anterior interbody cage with compression load cells. The transfacet screw system allowed for more compression across the facet joints. The authors reported no differences between the fixation systems in stiffness or in load fluctuation with flexion, extension, lateral bending, and torsion of the L4-L5 specimens. Ferrara et al11 compared the relative efficacies of transfacet screw (4.5-mm · 40-mm lag screws) and pedicle screw (6.5-mm · 45-mm fully threaded screw with 6.5-mm titanium rod) fixation systems with short- and long-term repetitive cycling in a biomechanical cadaver study. In short-term testing, both screw systems showed equivalent stiffness and range of motion when used in conjunction with bilateral semicircular interbody spacers. Long-term testing showed a small amount of residual motion that remained constant in the testing period and was equivalent between groups. In a cadaver study of L4-L5 vertebrae, Agarwala et al34 showed that transfacet screws and bilateral pedicle screws had similar rigidity in flexion and extension.
In axial rotation and lateral bending, transfacet screws offered less rigidity than pedicle screw systems when used in isolation. However, the differences were minimal when transfacet screws were used as an adjunct to anterior column support. In a similar cadaver study of lateral interbody arthrodesis, Molina et al33 found no differences in range of motion at the L2-L3 levels. Overall, these studies suggest that transfacet fixation is comparable to pedicle screw fixation when used in conjunction with an anterior or lateral interbody cage. Pedicle screw systems remain biomechanically superior when used in isolation. Kang et al13 described a CT-based percutaneous transfacet screw fixation technique in patients for adjunctive fixation after ALIF. The procedure was performed with the patient under conscious sedation, with an average duration of 43 minutes, resulting in successful fusion in all 17 patients. The authors concluded that percutaneous transfacet screw fixation safely improves construct stiffness in extension and axial rotation at one or two levels but is biomechanically inferior to pedicle screws when interbody support is also employed. In a prospective study, Voyadzis and Anaizi14 evaluated 10 patients undergoing percutaneous lumbar transfacet fixation in the lateral decubitus position as an adjunct to extreme lateral interbody fusion for degenerative disk disease and radiculopathy. Nine of 10 patients had good to excellent relief of preoperative back pain and leg pain, with mean improvement in visual analog scale scores of 8.0 for back pain and 7.1 for leg pain at a mean follow-up of 8.2 months. The authors of the study concluded that minimally invasive percutaneous transfacet screw fixation can be performed safely and effectively.14 Because pedicle screw systems offer greater purchase of bone, transfacet screw fixation is biomechanically inferior. However, the use of transfacet screws for posterior percutaneous fixation can lead to a reduction of surgical time and surgical morbidity in Figure 2 AP (A) and lateral (B) schematic illustrations demonstrating transfacet screw insertion at L4-L5.
The screw insertion points lie at the intersections of a horizontal line at the level of the inferior end plate of the vertebra above with vertical lines drawn at the medial aspect of the pedicles. (Reproduced with permission from Voyadzis JM, Anaizi AN: Minimally invasive lumbar transfacet screw fixation in the lateral decubitus position after extreme lateral interbody fusion: A technique and feasibility study. J Spinal Disord Tech 2013;26:98-106.) Emerging Techniques for Posterior Fixation of the Lumbar Spine 360 Journal of the American Academy of Orthopaedic Surgeons Copyright ª the American Academy of Orthopaedic Surgeons. Unauthorized reproduction of this article is prohibited. select patients.14 Used in conjunction with anterior fixation, transfacet fixation systems offer reduced surgical time with comparable strength. Translaminar Screw Fixation A translaminar lumbar approach, first described by Jacobs et al35 in 1989, uses screws inserted bilaterally in the spinous process of the upper vertebra, passing through the contralateral lamina across the facet joints, and ending at the transverse process of the lower vertebra36 (Figure 5). Translaminar screw fixation has been proposed for the treatment of segmental dysfunction, lumbar spinal stenosis with painful degenerative changes, and lumbar disk herniation with concomitant degenerative changes, as well as for segmental revision surgery after diskectomy.37 This technique is commonly used to augment anterior column support with additional posterior fixation via translaminar screws. Several biomechanical studies of translaminar screw fixation in conjunction with anterior cages have shown promising results.32,38 In a cadaver study, Phillips et al38 demonstrated that ALIF cages at L5-S1 had less angular motion and compressive preload under extension when used alone than when supplemented with translaminar screws. Additionally, the amount of angular motion was substantially reduced in extension with translaminar screw fixation compared with cages alone or the intact spine. The authors concluded that adjunctive translaminar facet screw fixation can enhance stability of the joint, especially when low compressive preloads are present and the cage alone would be unlikely to provide sufficient stability.38
In a clinical study analyzing 70 patients treated with translaminar facet screws for segmental degenerative disease, Marchesi et al39 found a 98.5% fusion rate at long-term follow-up (mean, 45 months; range, 24 to 74 months) and satisfactory results in 81% of patients. In a similar study of 173 patients, Grob and Humke37 reported a 94% fusion rate with a 5% revision rate and few reported complications (one patient had Figure 3 Schematic illustrations demonstrating translaminar screw fixation with screw placement through the contralateral lamina across the facet joint (A), transfacet screw fixation through the ipsilateral lamina across the facet joint (B), and pedicle screw fixation demonstrating disruption of a portion of the facet joint (C). Kushagra Verma, MD, MS, et al June 2016, Vol 24, No 6 361 Copyright ª the American Academy of Orthopaedic Surgeons. Unauthorized reproduction of this article is prohibited. persistent weakness, one patient had a dural tear, and three patients had nerve root irritation). Sasso et al15 described a percutaneous technique for translaminar screw placement in a cadaver model. The starting point, visualized on fluoroscopy, was the junction of the spinous process and the lamina, extending to the junction of the pars interarticularis and the pedicle. In the study, all 10 screws that were placed had acceptable purchase according to surgeon feedback and did not violate the spinal canal. However, five of the screws had violation of the laminar cortex on CT scan. Translaminar screw fixation of the lumbar spine has been shown to be a safe and effective technique for short-segment fusion of the lumbar spine.37,39 Additionally, translaminar screws can be used as an adjunct to anterior stabilization to augment anterior fusion. Adjunctive translaminar facet screw fixation can improve stability when threaded cages are used, particularly in the presence of low compressive preloads.
Posterior Fixation Adjunctive to Anterior Fusion For adjunctive posterior stabilization in patients who undergo anterior fusion, both transfacet and translaminar fixation constructs have been proposed because of the decreased surgical time necessary for implantation and the equivalent clinical outcomes of these techniques. As with Figure 4 Schematic illustrations demonstrating the pathway used for percutaneous transfacet screw fixation in the lumbar spine. A, Screw length and screw trajectory (u) necessary for transfacet screw fixation in the sagittal plane. Note the caudal angulation relative to a line drawn parallel to the intervertebral disk of the instrumented level. The path terminates in the inferior half of the pedicle at the pedicle-vertebral body junction. B, Screw length and screw trajectory (b2) necessary for transfacet screw fixation in the axial plane. Note the lateral angulation relative to the midline of the spinous process. C, The ideal starting point for transfacet screw fixation at the L3-S1 levels. IEP = inferior end plate, MedP = medial pedicle, SP = starting point Figure 5 Lateral (A) and AP (B) radiographs of translaminar screws used as augmentation of anterior interbody fusion. Emerging Techniques for Posterior Fixation of the Lumbar Spine 362 Journal of the American Academy of Orthopaedic Surgeons Copyright ª the American Academy of Orthopaedic Surgeons. Unauthorized reproduction of this article is prohibited. any screw fixation technique, the learning curve must be taken into account. In a biomechanical study, Harris et al40 examined axial rotation after pedicle screw fixation in combination with TLIF in cadaver models.
The authors found that TLIF with a cage did not increase overall spine flexibility but resulted in increased axial rotation at L4-L5. Adjunctive bilateral pedicle screws had the greatest decrease in range of motion, followed by unilateral pedicle screws and then a unilateral translaminar screw. In contrast, Razi et al41 found no differences in range of motion between transfacet and pedicle screws used in conjunction with a femoral ring allograft in a cadaver study. Eskander et al42 reported that both adjunctive pedicle and translaminar screw systems decreased flexion and, to a lesser degree, rotation in two-level ALIF procedures, compared with an intact spine. No difference was found in extension between instrumented and noninstrumented spines. Overall, pedicle screws tended to have increased stiffness compared with translaminar screws. In contrast, Zhan and Tian31 demonstrated that two-level posterior augmentation of ALIF cages improved stiffness compared with the intact spine or anterior instrumentation alone. The type of posterior fixation, pedicle versus translaminar screws, did not affect the outcome. Because of the decreased surgical time and less invasive approach, both transfacet and translaminar screw fixation constructs seem to be a safe and efficacious alternative to pedicle screw fixation. Neither procedure has been definitively proven to be superior to the other, with most studies reporting no difference between the two posterior systems in stiffness, range of motion, and adjacent disk pressure.31,40-42 Summary Pedicle screw fixation of the lumbar spine remains the most commonly employed technique for achieving rigid fixation from a posterior approach. However, certain patients, especially those who have a low risk of nonunion or who require adjunctive posterior fixation along with anterior column support, may benefit from shorter procedures and a limited posterior exposure.
Although transfacet and translaminar fixation methods are generally biomechanically inferior to pedicle screw fixation when used in isolation, these techniques may be useful as adjunctive posterior fixation. In contrast, early reports suggest that cortical screws are biomechanically stable as stand-alone fixation or as an adjunctive construct. Cortical screws have been shown to yield high fusion rates with few complications. When used in a percutaneous fashion, transfacet fixation and translaminar fixation are suitable alternatives to pedicle screws, minimizing soft-tissue dissection. The biomechanical strength of alternative fixation constructs and that of pedicle screw systems are similar when these methods are used as an adjunct to anterior or lateral interbody fusion cages. Further clinical studies elucidating the safety and clinical efficacy of these strategies, particularly ones that provide high-quality evidence and use validated outcome measures, are needed. Additionally, the use of open versus closed approaches for the alternative techniques requires further study. Particular emphasis must be placed on comparing clinical outcomes and the cost-effectiveness of these techniques. An understanding of these emerging screw fixation techniques may be helpful when counseling patients considering lumbar spine surgery.
References Evidence-based Medicine: Levels of evidence are described in the table of contents. In this article, references 14, 21, and 22 are level II studies. References 1, 6, 7, and 23 are level III studies. References 3-5, 8, 9, 13, 20, 26, 27, 35, 37, and 39 are level IV studies. References 2, 10-12, 15-19, 24, 25, 28-34, 36, 38, and 40-42 are level V expert opinion. References printed in bold type are those published within the past 5 years. 1. Boden SD: Overview of the biology of lumbar spine fusion and principles for selecting a bone graft substitute. Spine (Phila Pa 1976) 2002;27(16 suppl 1):S26-S31. 2. Vaccaro AR, Garfin SR: Pedicle-screw fixation in the lumbar spine. J Am Acad Orthop Surg 1995;3(5):263-274. 3. Lonstein JE, Denis F, Perra JH, Pinto MR, Smith MD, Winter RB: Complications associated with pedicle screws. J Bone Joint Surg Am 1999;81(11):1519-1528. 4. Kim YJ, Lenke LG, Bridwell KH, Cho YS, Riew KD: Free hand pedicle screw placement in the thoracic spine: Is it safe? Spine (Phila Pa 1976) 2004;29(3):333-342. 5. Kim YW, Lenke LG, Kim YJ, et al: Freehand pedicle screw placement during revision spinal surgery: Analysis of 552 screws. Spine (Phila Pa 1976) 2008;33(10): 1141-1148. 6. Su P, Zhang W, Peng Y, Liang A, Du K, Huang D: Use of computed tomographic reconstruction to establish the ideal entry point for pedicle screws in idiopathic scoliosis. Eur Spine J 2012;21(1):23-30. 7. Luther N, Iorgulescu JB, Geannette C, et al: Comparison of navigated versus nonnavigated pedicle screw placement in 260 patients and 1434 screws: Screw accuracy, screw size, and the complexity of surgery. J Spinal Disord Tech 2015;28(5): E298-E303. 8. Esses SI, Sachs BL, Dreyzin V: Complications associated with the technique of pedicle screw fixation: A selected survey of ABS members. Spine (Phila Pa 1976) 1993;18(15):2231-2239. 9. Blumenthal S, Gill K: Complications of the Wiltse pedicle screw fixation system. Spine (Phila Pa 1976) 1993;18(13):1867-1871.