Minimally Invasive Treatment for Thoracolumbar Burst Fracture Using Sagittal Alignment Screws and A Trauma Reduction Device

Summary

The sagittal adjusting screw (SAS) system has been widely used for thoracolumbar (TL) spinal trauma. Percutaneous options and specialized trauma reduction devices are also available for the SAS system. We describe a technique for reducing TL burst fractures using an SAS system and a newly introduced trauma reduction device.

Abstract

Thoracolumbar (TL) burst fracture is one of the most common indications for minimally invasive percutaneous pedicle screw fixation. Although the indication for surgical treatment of neurologically intact TL fractures remains under debate, studies have demonstrated that posttraumatic malalignment may lead to a deterioration in the patient's quality of life. For burst fractures with malalignment or fragments in the spinal canal, a reduction technique using ligamentotaxis is commonly used to improve long-term outcomes.

The sagittal adjusting screw (SAS) system is a monoaxial screw system with a fixed head and concave sliding saddle that allows lordotic sliding of the rod in the sagittal plane after screw insertion. SAS also has a percutaneous option and has been used for TL spine fractures. Notably, the SAS only allows motion on the sagittal plane, allowing both secure fixation and angular reduction. The SAS has certain advantages over the conventional Schanz screw system or normal mono-/multiaxial pedicle screws for TL spine fracture treatment. In addition, specialized trauma reduction devices are available for the SAS system. In this video protocol, we discuss the indication for the SAS system in TL burst fracture and describe a technique of TL burst fracture reduction and fixation using the SAS system. Additionally, we describe our recent case series with radiological evaluation, including regional kyphotic angle and percent loss of anterior vertebral body height, to evaluate the newly introduced trauma reduction device.

Introduction

Thoracolumbar (TL) burst fractures are relatively common, occurring in approximately 20% of all vertebral fractures¹, and are characterized by retropulsion of the fractured middle column fragments into the spinal canal. Although the management of TL burst fractures has been extensively studied, the indication for surgical treatment remains under debate. Previous studies have reported that long-term functional outcomes may not differ substantially between operative and nonoperative treatment for neurologically intact TL burst fractures¹. Nevertheless, these comparative studies were launched decades ago, making their results not applicable in this era when surgical techniques have substantially improved since then.

Recent advances in surgical techniques may have changed the surgical indications. Recently, minimally invasive approaches for TL injuries have become popular, and good results have been reported. Although their sample sizes are small, studies about the emerging novel techniques and implants support surgical treatment for neurologically intact TL fractures¹.

Generally, surgical treatment of neurologically intact TL fractures aims to ensure good alignment correction, reduction of fracture fragments with ligamentotaxis, and secure fixation^2,3. In patients with neurological symptoms, direct or indirect decompression of the neural structure is added to these aims. There are variations in the surgical techniques used for posterior fixation of TL burst fractures, such as surgery with and without fusion, the use of vertebroplasty, different numbers of fixation levels, and the use of additional screws on the fractured vertebrae.

Non-fusion fixation using percutaneous pedicle screws (PPSs) is commonly performed because it is less invasive and allows segmental motion after implant removal. The advantages of PPS over conventional open techniques are not limited to smaller skin incisions; PPS is associated with less paraspinal muscle damage compared to an open approach⁴. Moreover, a human cadaver study reported that PPS reduces the risk of medial branch nerve injury, which leads to multifidus muscle denervation⁵. Currently, a TL fracture is among the most common indications for minimally invasive surgery using PPSs, and a minimally invasive PPS system without fusion is reportedly associated with improved surgical outcomes in patients with TL burst fractures^2,3.

Reduction techniques are also a topic of interest. Since the 1980s, the Schanz screw system has been used for TL burst fracture treatment; this system allows good angular correction, vertebral height reduction, and reduction of retropulsion fragments by ligamentotaxis. Good results have been reported with the Schanz screw system⁶. However, the original Schanz system appears unsuitable for percutaneous use because the connectors are too bulky and use a side-loading system.

In contrast, the designated PPS system, which was developed in the 2000s, has been widely used for various conditions⁷. Although it is associated with less invasiveness to back muscles, one drawback of conventional multiaxial screws (MAS), compared with the Schanz system, is that screws have less angular stability due to a mobile screw head. Monoaxial screws, which have better angular stability than MAS, are also available as PPS options. However, it is often technically demanding to align all screw heads in the same plane as rods percutaneously without any angulation of the screws to avoid excessive mechanical stress on the screws or bones. Additionally, creating lordosis after screw insertion is almost impossible when monoaxial screws are used.

To overcome these issues, the sagittal adjusting screw (SAS) system was introduced in 2013 and was first aimed to correct spinal deformity secondary to fractures. The SAS is a monoaxial screw system with a fixed head that resembles a common head-loading pedicle screw; it has a concave sliding saddle that allows lordotic sliding of the rod in the sagittal plane after screw insertion. Since then, the use of SAS has expanded among acute TL burst fractures^8,9 as well as elective procedures including correction of spondylolisthesis¹⁰. The SAS only allows motion on the sagittal plane, allowing both secure fixation and angular reduction, which is usually impossible after screw insertion using conventional monoaxial screws.

One biomechanical study demonstrated that the SAS system has better force-displacement properties and fatigue test results than other head-loading PPSs, equivalent to the Schanz screw system. Trauma reduction devices can be attached to the extension towers of the SAS system and, therefore, both distraction and angulation forces can be applied at the instrumented levels using such a device⁹. Good results have also been reported for such trauma reduction devices used for TL fractures⁸.

The aim of this protocol is to describe the technique for alignment correction and fracture reduction methods using the SAS system in TL burst fractures and discuss the advantages of the SAS system. In the following protocol, PPS placement and fracture reduction using the SAS system and trauma reduction device are outlined. We also show representative cases demonstrating the results of this procedure.

Protocol

This protocol was approved by the Institutional Review Board of Showa University Hospital (No.2023-017-A) and conducted in accordance with the ethical principles outlined in the Declaration of Helsinki. The protocol follows the guidelines of our institution's human research ethics committee. As this representative case series is retrospective in nature, informed consent from each patient was waived in accordance with the institutional review board policy. Informed consent was obtained from patients presented in the figures and video of this protocol.

1. Indication of this protocol and preoperative planning

1. Before the surgery, ensure the indication for the use of the SAS system and trauma reduction device. As mentioned earlier, there are controversies regarding surgical indications of TL burst fractures. Discuss the potential risks and benefits of surgery with the patient before surgery based on the best available evidence, even if it is limited.
   NOTE: In case of limited evidence, we believe it is acceptable to rely on the clinician's personal experience, expert opinion, and method of treatment that is taught and practiced among practitioners in one's specific region, linguistic group, and local professional community¹¹.
   1. Select the following indication criteria for the posterior percutaneous short fixation procedure: skeletal maturity, absence of severe osteopenia, no evidence of PLL injury, and meeting at least one of the following criteria: thoracolumbar injury classification and severity (TLICS)¹² score ≥ 4, any Arbeitsgemeinschaft Osteosynthesefragen (AO) classification¹³ type B1 and B2 fractures with intact PLL, local kyphotic deformity > 15°, coronal deformity > 10°, or inability to tolerate conservative treatment due to other medical/psychological/socioeconomic issues (Table 1).
   2. Select the following exclusion criteria: severe anterior segmental collapse and multiple consecutive fractures.
2. Evaluate the patient's bone status to determine the use of the trauma reduction device.
   NOTE: Avoid using the device in patients with moderate-to-severe osteoporosis for preventing iatrogenic fracture. Additionally, do not use the reduction device for patients with a ruptured posterior longitudinal ligament (PLL) because a reduction of the retropulsion fragment with ligamentotaxis is not possible.
   1. If time allows, obtain an MRI for the diagnosis of PLL¹⁴ and other posterior ligamentous complex injury, even if the patient is neurologically intact. Use CT to evaluate the presence of "reverse cortical sign"-the posterior wall fragment rotating 180° with the cortical surface facing anteriorly and the cancellous surface facing posteriorly in the canal-indicating PLL rupture and contraindication of ligamentotaxis.¹⁵​
3. Ensure that the length and diameter of the SAS are measured in advance using an appropriate imaging platform, and use these images to simulate the trajectory of each screw. Draw lines anteriorly, extending from the medial borders of the bilateral pedicles. Check the location of the intersection bilaterally between the abovementioned line and the ideal pedicle screw trajectory.
4. Perform surgery as early as reasonably possible.
   NOTE: A study reported that the quality of TL fracture reduction with ligamentotaxis is better when performed earlier¹⁶.

2. Equipment and patient positioning

NOTE: The procedure requires intraoperative fluoroscopy (or a navigation system).

1. Use an open Jackson table for the procedure. Place the patient in the prone position and extend the patient's legs slightly.
2. Make efforts to achieve normal spinal alignment when placing bolsters under the patient's chest and bilateral anterior superior iliac spine before surgery to make reduction and screw insertion easier. Check the lateral fluoroscopic view, and make sure the abdomen is free of compression.
3. When monitoring is used, ensure that all electrodes are securely placed. For average-risk surgeries, such as this procedure, place the recording electrodes with a belly-tendon montage in the vastus medialis, anterior tibialis, and adductor hallucis to monitor major lower extremity myotomes.
   NOTE: The use of intraoperative neuromonitoring is based on each institutional protocol. In our institution, transcranial stimulation motor evoked potential is used for average-risk surgeries, including this procedure.
4. Use fluoroscopy to position the fractured vertebra and pedicles of the planned instrumented levels, one level above and below, as well as the fractured level at a minimum, and two levels above and/or below; if necessary, mark them on the skin. The identification of the fractured level is usually easy; however, if it is unclear or there are multiple previously fractured vertebrae, count the number of vertebrae from the sacrum or first thoracic vertebra. Record the position of the image intensifier to ensure good anteroposterior (AP) and lateral views of the spine during surgery.
   NOTE: The endplates should be parallel to the image trajectory in the AP view. Keep in mind that the best angles may differ for each vertebra owing to local lordosis/kyphosis in the area.
5. Make sure there is enough clearance, at least 30-40 cm, between the patient's body and the image intensifier.
   NOTE: Each procedural step of the SAS system, such as needle insertion using a mallet, tapping, and screw placement, requires a certain clearance.

3. Skin incisions and needle insertion

1. After patient positioning, prepare the surgical site using an alcohol or iodine-based skin disinfectant. Subsequently, drape with a disposable surgical sheet set, and apply an iodophor-impregnated adhesive incision drape¹⁷.
2. Similar to the standard PPS insertion⁷, make an incision lateral to the skin marks of the pedicles. Place the incisions 1-1.5 cm laterally to the lateral margin of the pedicles in the lumbar spine as per standard technical notes for PPS.
3. Dissect the fascia longitudinally after the skin incision. Make the skin incision longer than that of a normal PPS (at least 2 cm) to prevent skin problems when using a trauma incision device. Make skin incisions relatively medial to the standard incisions in patients with a low body mass index and lateral to the standard incisions in obese patients.
   NOTE: The positions should be modified based on operated levels: 0-0.5 cm lateral to the pedicles in the middle thoracic spine. Rod insertion is easier with a longitudinal incision used as the insertion site than with transverse incisions. The long midline incision, which is not "percutaneous" but still has muscles attached to the lamina, is sometimes used for upper-middle thoracic spine fractures because the pedicles are too close to make separate incisions or in cases requiring direct decompression.
4. Position the needle at the intersection of the facet and transverse process after skin incisions and fascia dissection. Obtain an AP image and ensure that the vertebra is not rotated.
5. Gentle tapping with a mallet, insert the needle tip into the bone with fluoroscopic control. As the needle advances, ensure the tip of the needle approaches the medial margin of the pedicle on the AP image.
6. Obtain a lateral image when the needle is located close to the medial margin of the pedicle. Confirm that the needle tip is located anterior to the posterior wall of the vertebra and close to the intersection point described.
7. Repeat all steps bilaterally from steps 3.2 to 3.5 for as many instrumented levels as necessary. Ensure that the bilateral needles are placed in parallel, resembling a rectangle and not a trapezoid. Consider making the incision larger in case of facet degeneration and flattening caused by using a rongeur or surgical bar when placing needles; achieving a good position is difficult because of such a prominent bone.

4. Sagittal adjusting screw insertion

1. Insert the guidewire through the cannulated hole of the needle after insertion. Ensure that the wire does not breach the anterior cortex.
   NOTE: The anterior cortex of the vertebral body does not always overlap with the anterior margin of the vertebra on the lateral fluoroscopic view due to bone morphology. Recheck the vertebra shape on axial CT images before needle insertion.
2. Use a 1 mm undersized cannulated tap for tapping. Assemble the tab extenders and cap to the SAS screws, and insert the screws into the vertebrae (Figure 1). Repeat all steps from steps 4.1 to 4.2 for as many instrumented levels as necessary.
   NOTE: Place all SAS screws parallel to the endplates in the sagittal plane to maximize the distraction force of the reduction device. Keep in mind that the risk of rod impingement on the facet joint, lamina, or spinous process will increase if the screw heads are located obliquely on the coronal plane.
3. If necessary, make modifications to the fixation at this step, such as intermediate screws (screws in the fractured vertebra) and additional screws on the two levels above/below in cases of severe deformity or impaired bone strength. Insert additional screws after fracture reduction as needed.
   NOTE: When inserting screws into the fractured vertebra, ensure that the instruments do not exceed the anterior wall of the vertebra. The fractured bone offers significantly less resistance, and sharp instruments may easily deviate anteriorly due to the fracture void.
   Normal multiaxial screws are usually used for intermediate screws or additional screws because rod connections may become difficult when the SAS is used for these levels. When using intermediate screws, ensure that there is no fracture in the pedicles.

5. Fracture reduction using a trauma reduction device

NOTE: This process has been described previously¹⁰.

1. Set the distractor pinion of the reduction device in a neutral position. Slide the distractor rack into the distractor pinion.
2. Place the inner guides on the extenders until they are fully seated. Ensure that the distractor module is placed in the posts of the extenders, and the distractor module is fully seated.
3. Place the lordosis modules in a neutral position. Ensure that the distraction key is turned to distract to obtain ligamentotaxis.
4. Stretch the posterior longitudinal ligament and bring the vertebral body segments back into the vertebral body from the spinal canal to restore the vertebral body height.
   NOTE: Increase the incision size to seat the device if the device pinches the surrounding skin. Ensure that adequate deep application of the device is impossible in morbidly obese patients. Normal distraction/compression devices for the PPS system can still be used in these patients.
5. Set the lordosis indicator to zero on the lordosis module so that the lordosis indicator slides along the rack and allows an estimation of the extent of lordosis that has been applied. Each marking on the lordosis module is 2.5°. Check the real-time alignment with fluoroscopy during this maneuver.
6. Squeeze the handles to restore lordosis and monitor the correction of lordosis under fluoroscopy. Verify the final correction. (Figure 2 and Figure 3).
   1. If deemed necessary, perform vertebroplasty using hydroxyapatite bone substitute or bone cement at this point. Place a guide needle, followed by a wire, dilator, and inserter, into the fractured vertebra via a transpedicular approach.
      NOTE: It is crucial to ensure that all instruments remain within the bone during the procedure, as breaching the vertebral cortex is more likely given the compromised integrity of the fractured bone. Intermediate screw placement is also possible after vertebroplasty. If bone cement is used, prepare the screws in advance to ensure their insertion before the bone cement has fully consolidated.

6. Rod connection

1. Measure the rod lengths using a rod ruler placed on the screw heads or a measure placed on the skin. Cut the rods at least 5 mm longer than the measurements to avoid the shortage of rod length; re-confirm the length with a measure before insertion. For fractures in or below the mid-thoracic level, insert the rods through the uppermost incision. For upper thoracic fractures, insert the rods from the lowermost incisions since inserting rods in the cranial to caudal direction is often difficult due to the kyphotic shape of the thoracic spine.
   NOTE: Rod bending is usually unnecessary at the thoracic or thoracolumbar junctional level. For the lumbar level, rods need to be bent, or use prebent rods to match the lumbar lordosis.
2. Ensure again that all the implants, rods, and screw heads are located under the fascial layer before closure to prevent compartment syndrome of back muscles.
3. Apply set screws to fix the rod in an appropriate position. If the set screw application is difficult, check the depth of the screws and adjust them adequately.
   NOTE: Ensure that the bilateral rods are placed parallel on the coronal plane to slide both rods appropriately during reduction.

7. Wound closure and after-treatment

1. Irrigate the wounds with 500-1,000 mL of 0.5% diluted povidone-iodine solution^18,19. Subsequently, apply a layered suture with size 0 antibacterial absorbable poly-filament suture for fascial layer closure and 2-0 or 3-0 antibacterial absorbable poly-filament suture for the subcutaneous layer. Close the epidermal layer using a topical skin adhesive glue or thin adhesive bandage. Ensure again that all the implants, rods, and screw heads are located under the fascial layer before closure to prevent compartment syndrome of back muscles²⁰.
2. Use a soft brace for comfort and start ambulation as soon as possible.
   NOTE: Although there is no clear evidence to support the use of a brace after spine fracture surgery²¹, a Jewett type frame brace may be applied for patients who have certain risk factors of reduction loss, such as preexisting spinal malalignment or osteoporosis, based on the treating physician's discretion.
3. Use intravenous and oral acetaminophen as the first-line medication for pain management or nonsteroidal anti-inflammatory drugs (NSAIDs) to control severe pain and reduce opioid use. Discharge the patient after confirming they can ambulate safely outside the hospital.
   NOTE: Try to minimize the dosage of NSAIDs as they likely inhibit bone fusion after spinal fusion surgery²², although the effect of NSAIDs on bone healing after spinal fracture is still inconclusive²³. Moreover, minimize opioid use since opioids may inhibit bone healing²⁴ and are associated with various complications and higher readmission rates²⁵.
4. Perform radiological follow-up per institutional protocol. To follow this protocol, obtain AP and lateral radiographs immediately and at 1, 3, 6, and 9 months after the surgery. Additionally, obtain postoperative CT scans before implant removal, approximately 1 year after the surgery, to confirm bone union.
   1. For radiological assessment, use the regional kyphotic angle (RKA) and percent loss of anterior vertebral body height (%AVB) on lateral radiographs (Figure 1).
      1. Measure the RKA of the fractured segment as the angle between the superior endplate of the upper adjacent vertebra and the inferior endplate of the lower adjacent vertebrae²⁶.
      2. Calculate the %AVB as the anterior height of the fractured vertebra divided by the mean of the anterior height of the adjacent two vertebrae using the following formula:
         AVB = 100 {1- (a¹ + a²)/2 ×a⁰}
         Where the anterior vertebral body height of the fractured vertebra is marked as a⁰, and the anterior vertebral body heights of the upper and lower adjacent vertebrae are marked as a¹ and a², respectively²⁷.
         NOTE: We utilized %AVB because the RKA could be improved even if the fractured vertebra is not reduced, owing to the flexibility of the intervertebral disk.
5. Remove the implant approximately 1 year after the initial surgery to restore segmental motion and prevent long-term consequences associated with the fixed region.
   NOTE: Patients who had undergone implant removal had a better quality of life and less symptoms than those not undergoing implant removal²⁸.

Results

Patient selection
The records of patients with traumatic burst fractures who underwent spine surgery between January 1, 2022, and October 31, 2023, at our university hospital were retrospectively reviewed. Patients who underwent posterior fixation with SAS devices using a trauma reduction device and met the following criteria were included in the final analysis.

Data collection and classifications
Information on age, biological sex, and fractured l...

Discussion

In this video manuscript, we describe our posterior minimally invasive fixation procedure for TL burst fractures using the SAS system and a trauma reduction device. Our representative case series showed good correction of the local kyphotic deformity and vertebral morphology. In previous studies, the potential reduction of kyphosis is 5-9% using an open approach³⁰. Our data demonstrated results comparable to those of the open approach, even with a percutaneous approach. This is consistent with one...

Materials

Name	Company	Catalog Number	Comments
3M Ioban 2 antimicrobial incise drape	3M	MSDS_0832279_US_EN_RDS	Iodophor-impregnated adhesive incision drape
CDH Solera Longitude II  SAS	Medtronic		SAS system
CDH Solera Voyager FNS SAS	Medtronic		SAS system
DERMABOND ADVANCED Topical Skin Adhesive	Ethicon Inc.		Topical skin adhesive glue or thin adhesive bandage
INFINITT PACS	INFINITT Healthcare Co., Ltd.		Picture archiving and communication system
Jewett brace	(Various manufacturers)		Jewett type brace
Nforce Trauma	Medtronic		Trauma reduction device
OEC Elite	GE healthcare		Fluoroscopy/image intensifier/c-arm
Sacro-wide Dx	Alcare	368-0208-0223/5	Soft brace
Steri Strips Standard Skin Closure	3M	MSDS_1084623_US_EN_AIS	Thin adhesive bandage.
Trauma Instrument Set	Medtronic		Trauma reduction device
Vicryl Plus Antibacterial 0	Ethicon Inc.	VCP587H	Antibacterial absorbable poly-filament suture
Vicryl Plus Antibacterial 2-0	Ethicon Inc.	VCP453H	Antibacterial absorbable poly-filament suture
Vicryl Plus Antibacterial 3-0	Ethicon Inc.	VCP398H	Antibacterial absorbable poly-filament suture