Pedicle screws have become important tools in instrumentation of lumbar spine fusion, but the traditional open approach to place the screws is associated with significant patient morbidity. Therefore, much recent interest has focused on minimally invasive approaches for pedicle screw fixation. This chapter reviews those techniques, their complications, and their results.
The percutaneous technique has the advantage of less tissue trauma than the mini-open technique.
The mini-open technique has less dependence on fluoroscopy and allows access to the transverse process, pars, and facets for decortication and bone grafting.
If minimally invasive transforaminal lumbar interbody fusion is performed, the same approach may be used for pedicle screw placement.
Preoperative imaging is crucial to detect deformity, which would affect planning for safe screw trajectory.
The percutaneous approach is more technically challenging.
The mini-open technique allows direct visualization of the pedicle entry point but risks injury to the medial branch of the dorsal rami and multifidus tendon.
Pedicle screws remain an important adjunct for spine surgeons. Their use in complex reconstructions provides for immediate stability until fusion occurs. Placing pedicle screws as a minimally invasive technique is another important innovation, one that potentially spares the patient from sequelae that result from conventional open surgery.
Conventional open spinal approaches require extensive dissections to allow for the identification of anatomic landmarks for instrumentation and graft placement. Multiple studies have demonstrated that these approaches lead to paraspinal muscle atrophy and scarring with impaired spinal function.
Open surgery requires prolonged tissue retraction that is associated with increased contact pressures, tissue ischemia, decreased muscle density, and paraspinal electromyographic abnormalities. Gejo et al. studied 80 patients who underwent open lumbar surgery. They note that prolonged retraction time resulted in a decrease in trunk muscle strength. The study also correlates prolonged retraction times with an increased incidence of low back pain. In addition, Datta et al. demonstrate that muscle retraction time was proportional to pain and disability scores.
Minimally invasive surgery (MIS) techniques have the potential to decrease the amount of muscle injury by using tubular retractors and tissue dilators that produce less tissue ischemia. In addition, these techniques eliminate muscle stripping that may denervate the paraspinal musculature. Several postoperative comparisons between open and MIS approaches have supported this claim. Kim et al. have compared patients with open and MIS posterior fusion. The study demonstrates that there is a significant decrease in the multifidus muscle cross-sectional area (CSA) and a clinically significant decrease in trunk extension strength seen only in the patients with open posterior fusion. In a similar study, Stevens et al. compares the magnetic resonance images taken from patients who underwent posterior lumbar fusion by either a standard open or a minimally invasive approach. They report “striking visual differences in muscle edema” in the open posterolateral group of patients compared with the minimally invasive patient group. Hyun et al. retrospectively assessed a group of patients who underwent unilateral transforaminal lumbar interbody fusion (TLIF) with ipsilateral instrumented posterior spinal fusion via a standard open midline approach. An additional contralateral instrumented posterior spinal fusion was performed at the same level, using a paramedian intermuscular approach. After surgery, a significant decrease in the CSA of the multifidus on the side of the midline approach occurred. On the contralateral side where the paramedian approach was used, no reduction in the multifidus CSA was measured.
The aforementioned muscle and soft-tissue injury that results from open surgery have led to an increased focus to perform less tissue-disruptive spinal procedures. MIS procedures strive to follow several key concepts. First, the exposure should be limited to only the target surgical site, limiting the unnecessary resection of bone, tendons, and muscle. Through preoperative planning and use of intraoperative imaging, the most direct surgical approach to the pathology is identified. When possible, the surgeon should utilize intermuscular, internervous, and/or intervascular planes to develop the surgical corridor. Second, increased contact pressures that result from retraction must be avoided. Finally, surgeons should exploit the interbody space to achieve fusions. The interbody compartment not only provides for the best fusion environment because of natural axial compressive forces, but also constitutes an isolated tissue compartment. These strategies entail methods to preserve tissue vital to the patient long after a successful decompression or arthrodesis. The ultimate goal of any MIS spine surgery is to limit surgical morbidity while achieving outcome rates comparable with conventional open surgery.
Percutaneous minimally invasive pedicle screw placement depends on precise preoperative planning to safely apply MIS principles. When studied properly, preoperative imaging studies give the surgeon important information for efficient screw placement. From these images, pedicle diameters and screw length allow the surgeon to preselect the screws, thereby limiting intraoperative measuring. Moreover, anomalous anatomy is identified, thereby avoiding injury to vital structures. Minimally invasive placement of pedicle screws in patients with deformity or in those with revision surgery where a large fusion mass may overlie the starting point warrants meticulous evaluation of the studies to ensure a safe trajectory of the screw.
Finally, the surgeon must decide which technique to utilize for minimally invasive pedicle screw placement. The two most common techniques utilized are the mini-open technique and the percutaneous technique. The mini-open technique allows for exposure of the pedicle entry point for direct visualization. Unfortunately, this mini-open technique requires electrocautery dissection of the lateral aspect of the superior articular process where the medial branch of the dorsal rami and multifidus tendons are located. The percutaneous technique largely avoids injury to these structures but remains a greater technical challenge (unpublished data).
Percutaneous Pedicle Screws
The patient is placed prone on a radiolucent table. A perfect en face anteroposterior (AP) image is obtained by rotating the patient in relation to a fully upright C-arm. A Jamshidi needle is then placed through a small paramedian stab incision. If an MIS TLIF was performed, the same paramedian TLIF incision is used. The Jamshidi needle is docked using fluoroscopy on the lateral pedicle margin, midway between the superior and inferior aspects of the pedicle ( Fig. 44-1 A–D ). With subtle caudal and cephalad adjustments of the surgeon’s hand, the needle can be adjusted. Each time the needle is advanced, fluoroscopic images should be obtained. The needle is held with a ringed towel clamp when obtaining fluoroscopic images to avoid excessive radiation to the surgeon’s hand. Gentle taps advance the tip into bone. Generally, we start with the bevel laterally (tip medially). By doing so, the needle advances from lateral to medial across the pedicle (see Fig. 44-1 E–H ). The bevel is especially useful if the needle advances toward an undesired position. By simply rotating the tip, the needle can cut an improved trajectory across the pedicle. For example, if the needle is advancing too inferiorly, the tip can be rotated superiorly. Combined with gentle caudally directed force on the handle, the needle can be driven toward to a more favorable position.