Pedicle screw fixation is the state-of-the-art in spinal deformity correction.1–3 However, in severely rotated scoliotic spine with small diameters and asymmetrical shape of the pedicles and vertebrae, pedicle screw placement is challenging.4–7 The proximity of nearby neurovascular structures, including the great vessels and the spinal cord, increases the risk for serious complications because of malpositioned pedicle screws.8 In 2 recent reviews, the rate of screw misplacement based on postoperative computed tomography (CT) scans averaged at about 15%,9,10 with the majority of studies presenting even higher values for the thoracic spine (up to 50%).10–12 Implant-associated neurological deficits leading to reoperation occurred in up to 7% of the patients.13 Furthermore, malposition with cortical breach, extrapedicular position, or pedicle fracture may cause a biomechanical drawback14–16 with screw pullout during the reduction maneuver or secondary loss of correction due to screw loosening.8,9
Navigation systems that provide intraoperative assistance have been shown to improve pedicle screw insertion accuracy and safety in many clinical and cadaveric studies. The superiority of navigated surgery was most obvious when applied to abnormal spinal anatomy and structures. The best results have been reported for 3-dimensional (3D) fluoroscopy-based navigation with regard to complete intrapedicular screw placement (reviewed here).10,17 Intraoperative navigation is considered to have several disadvantages. Intraoperative navigation is typically associated with enormous economical and biological costs (often prohibitive) and space requirements in the operating room. The intraoperative navigation procedure is fault-prone and time consuming, especially during the learning curve.18 For these reasons the acquisition of an image-guided navigation system is often practical only for specialized high-volume centers, which perform >150 complex cases a year with a well-rehearsed team.19
Image-based drill templates have been applied as a relatively simple and effective alternative to improve the accuracy of screw placement in spinal surgery.20–25 However, the optimal design for patient-specific templates seems to be a crucial factor for successful use.26 Regarding such tools, spinal deformity surgery studies are rare23 and reports about clinical experiences and narratives on the challenges are also rare. Therefore, we present a new patient-matched instrument and pedicle screw placement guide (Fig. 1) for challenging deformities of the spine including complex curvatures and abnormal spine anatomies. In patients with severe scoliosis we evaluated safety, feasibility, and accuracy. In addition, we report on the intraoperative experiences, challenges, and provide initial recommendations to facilitate and manage the procedure.
MATERIALS AND METHODS
In 4 patients with idiopathic or secondary scoliosis, we used the new vertebral body-specific, patient-matched instrument and pedicle screw guides for surgical instrument guidance and pedicle screw insertion to apply posterior correction. Two experienced surgeons (M.P., C.L.) performed the surgeries at 2 centers specializing in spinal deformity. Patient-specific characteristics, classification of scoliosis, and deformity-specific parameters are presented in Table 1.
Screw Placement Template Design
The patient-matched instrument and pedicle screw guide is a rigid body to guide surgical instruments along specific trajectories. To guarantee the stability of the guide on the vertebral body during the surgical intervention, the device’s shape has to perfectly match the posterior bony structure of the vertebrae and to allow fixing all 6 degrees of freedom. After several iterative redesigns based on cadaveric studies, an optimal design to guarantee stability, screw position accuracy, a simple and reproducible process for positioning the guide on the vertebrae, and minimal invasiveness was developed. The final guide design was based on the following considerations:
- Modeling each vertebra separately, and creating guides for each vertebra should be used avoiding any potential placement failures caused by intervertebral movement during insertion or resulting from differences between the supine (CT-scan) and prone (surgery) position of the patient.
- Performing each surgical step from identifying and breaching the vertebral pedicle to screw placement through the guide should be possible without negatively impacting the surgical goals common in traditional open procedures.
- Superior/inferior and medial/lateral distances between the contact points have to be calculated to offer the highest potential stability of the guide, as well as to maximize the tactile feedback during the drill guide attachment step.
- Bone/guide contact points have to be positioned in areas that are typically exposed during a standard open surgical approach for deformity surgery.
- The surface of the bone/guide contact areas should be as small as possible to avoid damage to healthy soft tissue.
- The guide should offer visual verification of the correct attachment before proceeding with the intervention.
Preoperative Planning and Manufacturing of the Pedicle Screw Placement Templates
The preoperative planning was based on anterior/posterior and lateral whole spine upright standing radiographs, on lateral bending radiographs, and on a multislice-spiral CT of the thoracic and lumbar spine. CT-specific parameters are chosen to ensure that the accuracy as well as the low-dose radiation exposure for the patient during the scan is guaranteed. To calculate the effective radiation dose for the patient during the CT scan the following formula was used: effective dose (mSv)=k×dose-length product (conversion factor k=0.015 mSv/mGy cm).27
Each surgeon defined the segments and pedicles that were to be instrumented using radiographs. Planning of the dimensions (length and diameter) of the pedicle screws was based on the CT scans derived from the low-dose protocol.
On the basis of the CT scan the 3D model of each requested vertebra was virtually reconstructed according to the patient-specific anatomy using the software Mimics (Materialise, Leuven, Belgium) version 13.1. For each vertebra the screw entry point, screw alignment, diameter, and length have been 3 dimensionally planned (Fig. 2, software Solidworks; Dassault Systèmes, Vélizy-Villacoublay, France; version 2011 SP 5.0) by the surgeon in direct cooperation with the guide manufacturing company (Medacta International, Lugano, Switzerland).
During planning the surgeon was entitled to modify the following surgical parameters on the basis of his preference:
- Pedicle screw size (length, diameter).
- Left and right screw inclination in the sagittal plane.
- Left and right screw convergence in the transverse plane.
- Horizontal and vertical shift of the left and right screw entry point on the coronal plane.
- Screw tip distance from the anterior cortex and the endplate.
- Relation of the minimal cross-section of the pedicle and the screw caliber (the planning protocol included a warning in case of a screw diameter exceeding the minimal cross-sectional diameter of the pedicle).
All screws were planned completely inside the pedicle if possible. If this was not possible due to small and/or asymmetric pedicles, the screw was planned with the smallest available diameter (4.5 mm) in the laterally based in-out-in approach.4
On the basis of the approved 3D-planning of the implant position and the CT data of the vertebral body, positioning guides (MySpine Placement Tool, Medacta International) and the corresponding model of the vertebra were designed using the software Solidworks. In addition, to avoid a mix-up and confusion during surgery, the patient code, the vertebral segment, and the information about the screw sizes chosen were permanently printed in relief-manner on each guide and model of the vertebra. Subsequently, an STL file was created for each model of the vertebra and guide and transmitted to the selective laser sintering machine (P395, EOS e-Manufacturing Solutions, München, Germany) for production. The guides were manufactured out of medical grade polyamide (PA2200, EOS e-Manufacturing solutions) and supplied either already γ-sterilized or had to be steam pressure sterilized similarly to other implants. The complete planning process from the CT scan until the guides arrive in the hospital takes about 1 week.
To achieve maximum stability and optimal screw entry points, the MySpine Placement Tool profiles were specifically and differently designed for thoracic and lumbar segments. For thoracic vertebrae the guides’ shape was designed with contact points at the spinous process, both laminae, and transverse processes. At the lumbar spine, the upper articular processes instead of the laminae were chosen. According to the individual anatomy, single contact points could be selectively omitted using a specialized algorithm, assuring accuracy of placement and optimizing minimal tissue disruption and good tactile feedback when “docked” properly.
The thoracic and lumbar levels to be instrumented were exposed by a longitudinal midline incision. The anatomic landmarks were prepared carefully; soft tissue was removed as necessary. After identifying the most superior segment planned for placing pedicle screws fluoroscopically, the correct fitting between the vertebra’s plastic model and the MySpine tool was checked to verify the contact surfaces and the pedicle screw entry points. To facilitate the identification of the correct entry points, “sham” pedicle screw heads had been modeled on the replicated artificial vertebral body (Fig. 1). The MySpine tool was placed on the corresponding vertebra of the patient. “Sleeves” were created to accommodate the shafts of the most common instruments used to identify and prepare vertebral pedicles for screw placement. With the well-docked guide, both pedicle awls with accompanying sleeves were inserted into the guide to the calculated entry point. On each side separately, the pedicle and vertebral body were then checked for cortical breach with a pedicle probe through the sleeve directed by the guide while the pedicle awl was still inserted contralaterally. This procedure sequence provided maximum docking stability and avoided slippage of the guiding tool. Finally, the polyaxial pedicle screw (M.U.S.T., Medacta International) was inserted as planned through the MySpine guide and with a sleeve surrounded standard pedicle screw driver while keeping the guide fixed using previously inserted contralateral pedicle awl and sleeve within the guide (Fig. 3). The screwdriver remained connected to the screw for tool coupling reasons until the contralateral side’s screw was implanted in a similar manner.
Consecutively, all segments planned were instrumented in the same manner. Intraoperatively, after all screws were implanted they were scanned fluoroscopically for malpositioning in the true posterioranterior pedicle, and lateral views. In case of suspected screw misplacement, the pedicle screw position was corrected using the MySpine tool under fluoroscopic control. Reduction of the deformity was performed by derotation of anatomically prebended rods after posterior liberation by multiple osteotomies and partial resection of the costotransversal joints at the convexity of the major thoracic curve.
Perioperatively, all implant-associated and non–implant-associated complications were recorded. Screw misplacement before repositioning and screw pullout during the reduction maneuver was closely monitored and registered. In addition, we analyzed all intraoperative challenges, such as, issues regarding the handling of the positioning guides, stability of guides during pedicle preparation and screw placement, and any challenges that occurred during surgery. The management/solution of each problem was noted.
The postoperative evaluation was based on a clinical examination, on whole spine upright standing radiographs in lateral and anterior/posterior views, and a low-dose CT. The radiographs were evaluated for correction of the major curve(s) in the coronal plane (postoperative Cobb-angle and percentage correction in relation to the preoperative values) and the correction of the rotational deformity at the apex of the major curve(s) (postoperative rotation-angle and percentage of correction in relation to the preoperative values). On the basis of the postoperative CT, the accuracy of the screw positioning was analyzed using the most common 4-tiered classification according to Mirza et al28: Grade 0—no cortical breach; Grade 1—cortical perforation with protrusion of the screw ≤2 mm; Grade 2—cortical perforation ≤4 mm; Grade 3—with perforation >4 mm. Pedicle screws that were preoperatively planned in the laterally based in-out-in approach were graded 0, if they were positioned as planned fitting to the medial wall of the pedicle. In addition, the direction of the pedicle cortical breach was determined (medial, lateral, cranial, caudal), as well as cortical breaches of the vertebral body. Furthermore, the fractional ratio of screw diameter and minimal pedicle diameter was calculated for all screws (screw/pedicle). All other implant-associated complications, for example, loosening, breakage, or loss of reduction after mobilization as well as clinical complications and complaints (eg, infection, neurological deficits) were recorded.
All radiologic measurements and gradings have been performed blinded and independently by 2 spinal surgeons (P.S., R.C.). A radiologist specialized in spinal imaging was used to adjudicate if the grading for cortical breaching differed. The interobserver variability for grading of cortical breaching was tested using κ statistics. Cobb angles and rotational measurements are reported as mean values of both measurements.
In the 4 patients a total of 76 pedicle screws were implanted, 56 thoracic and 20 lumbar. The L5 screws in patient 2 could not be inserted using the positioning guide, because placement was blocked by L4 screw heads at the contact points. Therefore, these 2 screws were implanted with intraoperative fluoroscopic assistance. Two pedicle screws (2/76; 2.6%) were assessed to be misplaced (κ=0.97) on the intraoperative radiographic control with cranial perforation of the vertebral body. They were reinserted after further soft tissue preparation for a more stable docking of the MySpine guide tool with additional fluoroscopic control in the lateral view.
Screw placement accuracy results are shown in Table 2. A total of 84% were graded 0, and 96.1% were graded 0 or 1 (Fig. 4). All lumbar screws were graded 0. A total of 14 thoracic pedicles to be instrumented were smaller in diameter than 4.5 mm, and therefore these screws were planned in the laterally based in-out-in approach (Fig. 5). Six of these were graded 0. The remaining 8 represented 66% of all misplaced screws (8/12). All grade 2 malpositioned screws had been placed more laterally than planned. None of the breaches reported in the present study resulted in either neurological deficits or other morbidities. Two screws in Th11 of patient 3 were positioned in a more caudo-cranial direction than planned but without cortical breach of the vertebra or pedicle (Fig. 6). The mean (±single SD) fractional screw/pedicle diameter ratio was 97.7%±13% (Fig. 7).
We did not observe screw pullout during the reduction maneuver or any further perioperative or postoperative implant-associated or non–implant-associated complication. The clinical results and the correction of the deformities were excellent in all 4 patients (Table 1) without a loss of correction after postoperative patient mobilization.
The preoperative radiation exposure by means of the CT scan was 73.9 mGy cm for 14 vertebrae, which equates a mean effective dose of 1.1 mSv.
The following challenges occurred during surgery:
- The most stable and preferred positioning/docking of the guides/placement tools at the most superior spinal segment was facilitated with an elongation of the skin incision of approximately 4 cm.
- Expected precision for screw insertion by the placement tool is achieved with careful exposure and preparation of the vertebral contact points. The spinous processes and the laminae contact points were very important to docking and stability of the guides.
- On the convex side of the deformity, especially at the thoracic apex, the head of the ribs should be resected to allow correct positioning.
- The length of the instruments (screw driver and awl) generates considerable leverage. This may cause a slippage or tilting of the placement tool, leading to a malpositioning of the screw. Proper docking and fixation of the guide, facilitated by a surgical assistant is extremely helpful, especially during insertion of the awl on the initial operative side, or if only 1 side is to be instrumented. If pedicle screws are to be placed bilaterally, an instrument (such as an awl) should remain in the contralateral channel of the placement guide until the initial screw has been inserted.
- If repositioning of a pedicle screw is necessary, it can be difficult to perform with the vertebral placement guide because the awl tends to favor the initial entry point and trajectory of the pilot hole. Therefore, we recommend inserting the awl with a mild “overcorrection” when perforating the first cortical wall on the basis of fluoroscopic control either in posterioranterior view (in the case of medial or lateral malpositioning) or in lateral view (in the case of cranial or caudal misplacement). After the first 5 mm of the awl is introduced into the pedicle on its new trajectory, the placement tool can then be attached for added control of pedicle screw implantation.
- The exterior cortical wall at the lumbar screw entry points is very strong in most patients. We suggest partially resecting the cortical bone providing a flatter entry point to avoid slippage of the awl. This resection has to be carried out carefully preserving most of the facet joint, because this is a reference point for positioning of the placement tool. Alternatively, we recommend to use a drill instead of the awl.
In this pilot study we demonstrated the feasibility, safety, and accuracy of pedicle screw placement in patients with scoliosis using a new custom-made positioning device. Furthermore, we provide practical tips for its clinical application and the avoidance or management of possible intraoperative challenges.
Promising results have been reported in several cadaveric and some clinical feasibility studies for the use of drill templates in spinal surgery especially for cervical vertebrae.20–22,24,25,29–32 We achieved acceptable clinical results with this new technology and surgical technique, but observed several clinical challenges not previously published.
In our limited study, the low incidence of radiographically observed screw malpositioning can be explained by inaccurate docking/coupling of the guide to the vertebrae. Most often, this was assumed to result from slippage or tilting of the device while breaching the pedicle, and/or creating pilot holes, etc. In our opinion, cranial or caudal deviation from the planned screw direction is a problem of solidly docking the guides to the laminae, especially in severely rotated and asymmetrical vertebrae. Therefore, accurate preparation of the contact points and an assured bone contact of the guiding tool are important aspects of the surgical procedure to accomplish for good clinical and radiographic success.
One possibility of enhancing the stability of the docking would be an enlargement of the bearing area, especially on the lateral lamina. Such a development of the device may simplify the basic positioning procedure and enhance rotational stability. Lu et al23 presented excellent results with 100% grade 0 and 1 screw positions using a full bearing template shape for thoracic instrumentation in patients with scoliosis. Surprisingly, this study group did not report about any issues regarding the preparation and positioning of the guide, although it might be considerably more difficult and time dependent than with the device presented here. Ferrari et al26 recently suggested several criteria for an optimal design of a feasible drilling or screw placement template for spinal surgery. The high stiffness of the guide used in the present study allowed performing all surgical steps of pedicle screw placement, including the insertion of the screw itself, without the need for removal of the tool, whereas in all other studies only drilling of the pedicle was guided. Therefore, we believe that this newly designed guidance template fulfills all criteria published by Ferrari et al.26 We believe these features create an important advancement in patient-matched spine technologies compared with any previous templates for thoracic and lumbar pedicle screw placement for patients suffering from significant spine deformity, such as scoliosis, presented or published previously.
On the basis of our findings, further advancement of the MySpine placement tool could be achieved by adding a handle for the assistant to the primary surgeon on the positioning guides and by shortening of the instruments to minimize the lever arm. For repositioning of pedicle screws we recommend the development of a threading device, which can be used in combination with the placement tool/guide.
Despite the issues mentioned, we found high accuracy and precision using the new instrument and pedicle screw guides with 84% of the screws completely contained in the pedicle without perforation. In a recent systematic review of 26 prospective clinical studies regarding thoracic pedicle screw placement in general, the grade 0 rates for free-hand placement were reported to range from 69% to 94%, from 25% to 85% with fluoroscopy, and from 89% to 100% with CT navigation.33 Therefore, despite the high grade of deformity in our patients, the results of the present study lie in the upper region for non-navigated screw placement. With respect to the mentioned rates for CT navigation our method appears to be superior to all other implantation techniques presented/published. Importantly, Rivkin and Yokom18 reported a learning curve of 15–30 cases to achieve an accuracy of 94%, with 17% misplaced screws for the first 15 and 14% for the first 30 surgeries. Regarding this and with respect to the very short learning curve, and with room for further improvement of the device as we described, it is our belief that this technology will be easily adopted and utilized to deliver pedicle screws with confidence and efficiency in challenging deformity cases. In contrast to various challenges and pitfalls reported for image-guided navigation,18,19 the use of the new placement tool presented here seems to be much easier and simple.
Lateral and medial pedicle wall breaches have been reported in several studies.34 Nevertheless, the acceptability of such breaches is controversially debated in the literature, because the breaches only rarely cause a clinical complication. A medial wall penetration ≤2 mm has been shown to be well tolerated and these screws (grade 1) can be considered to be positioned in the so-defined “safe zone.”11,35 Taking this into account, >96% of the screws in our series were “safely positioned,” which is a rate similar to that reported for image-guided navigation. However, at the thoracic apical level on the concave side the epidural space has routinely been reported to be <1 mm, which means there is no real “safe zone” here.36–39 For this reason we planned our pedicle screws in this region as lateral as possible, especially for the very small pedicles in the laterally based in-out-in approach. Therefore, applying our planning and positioning technique, we were able to avoid medial breaches also for these pedicles, which is an important finding of our study.
Beside risk reduction regarding serious neurovascular injury, accurate pedicle screw placement could contribute important benefits to the patient in terms of safety, superior biomechanical stability, and potentially improved bony fusion.40 With the placement guide pedicle screws can be inserted as preoperatively planned. In addition, for the laterally based in-out-in implanted pedicle screws a pullout strength of about 80% compared with a completely intrapedicular position was described before.5,41 Another important factor to optimize the pullout strength is the fractional ratio of screw/pedicle-diameter.14–16 However, our results with a mean ratio of 98% was significantly better than reported by Luther et al40 with 71% using the 3D FluoroNav technique, but the ratio is higher when the screw diameter exceeds the pedicle diameter in many cases. However, excluding the laterally based in-out-in approach planned screws, still in 83.9% an excellent screw/pedicle-diameter ratio of between 90% and 100% (97.8% within 80%–100%) could be achieved (Fig. 7), which is a result of planning every screw in a virtual 3D model.
The preoperative effective radiation dose for the patient during the CT scan can be considered as comparatively low. The calculated mean of 1.1 mSv is below the reported average annual background radiation in the US of 3.1 mSv when excluding medical radiation and meets the dose of 2 standard chest radiographs.42 The radiation exposure for the patient is also below that reported for the cone-beam CT (O-arm, Medtronic, Minneapolis). According to Lange et al43 the radiation dose for a navigated surgery on 17 vertebrae with the O-Arm system averages at 32.4 mSv for slim and at 80.9 mSv for obese patients, respectively.
Summarizing, the necessity for a device or method that helps to guide pedicle screw placement in deformity surgery, especially in cases with small pedicles, high-grade rotation, or asymmetry, has been demonstrated and is noncontroversial for most spinal surgeons.4,8–10,17,40 Although image-guided navigation fulfills most requirements and it is therefore rated beneficial from the majority of the surgeons, only 11% indicated routine use of intraoperative navigation in a recent worldwide survey.44 The most commonly cited barriers to adoption of intraoperative navigation are: high initial and ongoing costs, lack of trained support personnel, and a long learning curve to competency as the main rationale for nonutilization. It is our collective belief that the technology and procedure described in this study represents an option that would be adopted more readily.
The most important limitation of our study is the pilot study format with a small number of patients.
Second, we have presented the results of 2 senior spinal surgeons who are very experienced in scoliosis surgery. We cannot exclude the possibility that surgeons with less experience with the treatment of demanding deformity cases achieve as good results as presented here. Further clinical study with surgeons of varying degrees of experience in treating complex cases will help measure the true clinical utility and adoption potential of this new technology. Nevertheless, the aim of this pilot study was to prove the feasibility, safety, and accuracy of the new device to plan a prospective multicenter randomized trial, which compares the new technique to established pedicle screw positioning methods. Third, neuronavigation was not applied in the present study. Finally, it should be noted that the MySpine instrument and screw guide is a device that can simplify individual steps of the surgery, but as in all surgical scenarios, it relies on the competence, experience, and professional judgment of the operative surgeon to produce good clinical outcomes for patients.
The new custom-made, patient-matched instrument and pedicle screw placement guide is a feasible tool, which permits safe and accurate implantation of pedicle screws in patients with severe scoliosis. The preoperative planning process, specialized algorithms to create patient-matched vertebral models, and guides for treating asymmetric and severely rotated vertebrae with small pedicles deliver a multitude of efficiencies and economies that positively impact the surgeon, the patient, and the hospital team. Because of its easy applicability and the possibility to avoid the disadvantages of the image-guided navigation systems we initially recommend the use of the placement tool in spine centers with <150 complex cases a year.
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Keywords:© 2017 by Lippincott Williams & Wilkins, Inc.
pedicle screw guide; 3D-printing; positioning tool; navigation; scoliosis; deformity