Percutaneous Pedicle Screw Fixation and MISS SI Joint Fusion

44 Percutaneous Pedicle Screw Fixation and MISS SI Joint Fusion

Greg Anderson

Summary

The appropiate radiological identification of the pedicles and sacroilliac joint is key in every lumbosacral instrumentation case. This chapter provides the reader with detailed images about how to perform a transpedicular and sacroiliac instrumentation in a safe and efficient way.

Keywords: SI joint fusion SI joint SI instrumentation percutaneous pedicle screws percutaneous technique pedicle screws

44.1 Introduction

Posterior instrumentation techniques performed via a standard open approach with a midline lumbar incision and subperiosteal muscle dissection are associated with iatrogenic soft-tissue and muscle injury. As early as 1984, Magerl described thoracic and lumbar percutaneous skeletal fixation.1 Atraumatic instrumentation techniques have now become established as reliable, and are regularly performed as the method of choice for many spine surgeons practicing minimally invasive spine surgery (MISS) for the treatment of a wide range of spinal disorders.

44.2 Indications

Pedicle screws are utilized to correct deformity, stabilize segmental instability, and support interbody implants. When compared to open techniques, minimally invasive spine procedures utilizing percutaneous pedicle screws require less soft tissue dissection, reduce postoperative pain, reduce intraoperative blood loss, and shorten hospitalization.²^,³^,⁴

44.3 Contraindications

The only absolute contraindications to the use of percutaneous pedicle screws are the presence of pedicles that are too small to accommodate the available implants or inadequate intraoperative images (usually fluoroscopy) to guide the procedure. Scenarios that can lead to problems with intraoperative imaging can include broken or poor-quality imaging equipment, severe obesity, overlying gastrointestinal (GI) contrast, or severe osteoporosis.

A relative contraindication may exist for the use of percutaneous fixation in a complex case by an inexperienced surgeon. The placement of percutaneous pedicle fixation has a well-defined learning curve and thus less experienced surgeons should apply this technique to simple cases initially. As the surgeon gains experience with percutaneous fixation, more challenging cases can be approached in this fashion. Factors that can make a case more challenging include: morbid obesity, significant spinal deformity, severe osteoporosis, major spinal instability, or a long surgical construct.

44.4 Technique

44.4.1 Step 1: Preoperative Planning

It is important to review the advanced imaging studies, including computed tomography (CT) and/or magnetic resonance imaging (MRI), prior to the procedure to understand the size and number of implants required for the procedure and the anatomy of the spine in the region of surgery. Additionally, it is useful to note the lumbosacral junction anatomy when performing lower lumbar surgery as lumbosacral dysmorphisms may exist that could impact the difficulty of surgery.

44.4.2 Step 2: Set up and Positioning

The key to successful minimally invasive spine techniques is maintaining high levels of precision and attention to detail in each step of the procedure, beginning with positioning. Placement of the chest pad along the sternum (just below the sternal notch) and the hip pads just below the anterior superior iliac spine allows the abdomen to be decompressed, minimizing epidural bleeding during surgery. The surgeon should also ensure that the patient is properly aligned on the bed, eliminating an iatrogenic rotation and/or side bending of the spine. Draping should be wide to allow appropriate trajectories for minimally invasive Wiltse approaches, especially in obese patients.

44.4.3 Step 3: Localization

After sterile prep and draping of the surgical site and C-arm, the C-arm is brought into the surgical field. The surgical levels are identified using reproducible fluoroscopic landmarks, such as the lumbosacral junction. In some cases, prior implants can be used as useful surgical landmarks. A spinal needle placed under fluoroscopic guidance can be used to define the ability of a proposed surgical incision to reach a particular region of the spinal anatomy prior to committing to a specific incision.

The authors find it useful to mark out the location of the pedicles on a “true” anteroposterior (AP) image of the vertebra. A true AP image is obtained by aligning the C-arm so that there is perfect overlap of the anterior and posterior margins of the superior end plate (end plate projects as a single radiolucent line). When using the true AP view, the pedicles will be located just caudal to the superior end plate and the spinous process will be centered between the pedicles. Subpar alignment of the vertebra on fluoroscopic images should not be accepted, as this can lead to the inaccurate placement of instrumentation (Fig. 44.1).

Fig. 44.1 Lateral (a) and anteroposterior (AP) (b) fluoroscopic images. On the lateral, perfect overlap of the superior and inferior end plates, creating an outline of the vertebral body and pedicles, is obtained. On the AP, the pedicles are at the upper and outer corners of the vertebra and symmetric in appearance. The superior and inferior end plates are overlapped and the spinous process is in midline. (Reproduced from Positioning. In: Vialle L, ed. AOSpine Masters Series, Volume 6: Thoracolumbar Spine Trauma. 1st Edition. New York: Thieme; 2015.)

Using the true AP view, the authors will mark out each vertebral level in the construct prior to making any incisions. A K-wire is laid over the skin and adjusted until it is seen to bisect the pedicles of a particular level. Next, a transverse line is drawn over the midline of the pedicles. The authors prefer to write the level of each line and the sagittal angle of the C-arm neck to each pedicle line (i.e., L3, +10 degrees of sagittal plane angle on the true AP view). Next, vertical lines are drawn over the lateral boarders of each pedicle. Next, the optimal location of the skin incision(s) can be determined. Generally speaking, the authors use paramedian incisions, positioned about 2 cm lateral to the lateral boarder of the pedicle. In some situations, a single incision can be utilized to reach multiple pedicles due to the lordosis of the spine (Fig. 44.2). Obese patients will require a more lateral incision to reach the pedicle as their soft tissue envelope is thicker, creating a longer distance at the medical to lateral trajectory to reach the spine (Fig. 44.3). In contrast, a very thin patient or an upper lumbar instrumentation site (e.g., L1) may require a small distance (10 mm) between the incision and the lateral boarder of the pedicle.

Fig. 44.2 Artist’s depiction of spine superimposed on photograph of proposed skin incision demonstrating the proximity of the requisite anatomy relative to the entry point for percutaneous pedicle screw trajectory. (Reproduced from Phase I: Incision, Docking Minimal Access Ports and Pedicle Screw Placement. In: Tumialán L, ed. Minimally Invasive Spine Surgery: A Primer. 1st Edition. New York: Thieme; 2020.)

Fig. 44.3 The larger angles at L5 and S1 mandate a more lateral starting point to ensure the trajectory of the access port is parallel or nearly parallel to the pedicle angle. Therefore, at L4–5 and L5–S1, 4.0 cm is preferred, whereas 3.5 cm is preferred in patients with a lower body mass index. (Reproduced from Phase I: Incision, Docking Minimal Access Ports and Pedicle Screw Placement. In: Tumialán L, ed. Minimally Invasive Spine Surgery: A Primer. 1st Edition. New York: Thieme; 2020.)

44.4.4 Step 4: Approach

After making the skin incision, the two layers of fascia must be incised. Remember that when working lateral, you will encounter both the thicker lumbodorsal fascia as well as the thinner epimysium covering the erector spinae muscles. It is crucial to adequately release the fascia so that the implants can be placed in the correct trajectory without applying undue force. This may require undercutting of the fascia relative to the skin incision, which can be accomplished using curved scissors. It is also useful to use digital palpation to feel the facet joint and transverse process prior to placing the Jamshidi needle.

44.4.5 Step 5: Pedicle Preparation and Pedicle Screw Placement

With the C-arm aligned on the true AP view of a particular level, the Jamshidi needle can be docked at the base of the transverse process as medial as possible. The goal is to position the tip of the needle at the 9 o’clock (left side) and 3 o’clock (right side) position of the pedicle shadow as seen on the true AP view (Fig. 44.4). If the needle tip is malpositioned, it should be adjusted to achieve this position prior to penetrating the bony surface. After the tip has been properly positioned, the cortex of the bone is penetrated with a gentle tap of the mallet on the upper portion of the Jamshidi needle. Next, the position is rechecked fluoroscopically to ensure the Jamshidi needle remains on target to the pedicle. The needle should be held so that the tip is pointing toward the center of the pedicle. By adjusting the amount of angulation of the needle, the needle can be directed more or less aggressively directed toward the center of the pedicle. This allows the surgeon to “steer” the needle into position based on imaging taken during pedicle penetration. After the bone is initially penetrated, a mark (using a marking pen) is placed on the needle shaft about 20 mm above the skin edge. This is used to follow the depth of penetration of the needle into the bone. As the needle is inserted deeper into the pedicle, the surgeon will know that the tip of the needle has reached the pedicle–vertebral body junction when the needle shaft mark reaches the skin edge. At this point, a fluoroscopic image can be utilized to assess the position of the tip of the needle. The needle tip should still lie within the pedicle shadow about three-fourths of the distance from lateral to medial across the pedicle. Crossing the medial wall prior to the 20 mm of needle advancement would suggest a medial pedicle breach, and reposition of the needle should be immediately pursued. Although not required in each case, lateral C-arm imaging is useful to confirm the depth of needle penetration (Fig. 44.5).