Free-Hand Pedicle Screw Placement Using a Ball Tip Widening Technique for Small Pedicles

Abstract

The purpose of this study was to assess the accuracy and validity of the “ball tip widening technique.” We designed a retrospective analysis of 212 pedicle screws inserted in correction surgery for pediatric scoliosis, using posterior spinal instrumentation and fusion with pedicle screws at Seoul National University Bundang Hospital between July and August of 2021. The accuracy of pedicle screw placement was then classified using intraoperative computed tomography (CT; O-arm). Intraoperative CT revealed that 86.8% of pedicle screws (184 / 212) were placed in pedicle. The ball tip widening technique introduced in this study can enhance the accuracy of screw insertion into small pedicles. Furthermore, this technique may also be applied to lumbar surgery for degenerative conditions in the future.

Surgery for pediatric scoliosis that causes 3-dimensional (3D) spinal deformity must aim to stabilize progression of the pathological curve. Posterior spinal instrumentation and fusion with pedicle screws is the standard treatment for curve correction, and it has been established that pedicle screw placement improves 3D deformities in adolescent idiopathic scoliosis and adult spinal deformity.

However, previous studies assessing the accuracy of small pedicle screw placement have shown that C-arm fluoroscope-guided screw placement for extremely small thoracic pedicles in scoliosis cases had a violation rate of up to 7.9%, computed tomography (CT)-guided surgical navigation for small thoracic pedicles showed a violation rate of up to 3.4%, and robot-assisted pedicle screw placement for adolescent idiopathic scoliosis showed a violation rate of up to 7.2%.¹⁾ Thus, herein, we introduce a method for pedicle screw placement for pediatric patients using a free-hand technique with a ball-tip probe. The purpose of this study was to assess the accuracy and validity of the “ball tip widening technique.”

TECHNIQUE

The Institutional Review Board of Seoul National University Bundang Hospital approved this study’s protocol (IRB No. B-2411-938-102) and waived the requirement for informed consent owing to the retrospective nature of this study. We included a total of 12 patients who underwent correction surgery for pediatric scoliosis with posterior spinal instrumentation and fusion using pedicle screws at Seoul National University Bundang Hospital between July and August of 2021.

Surgical Technique

It is essential for surgeons to observe the accurate size of the thoracic or lumbar pedicles on radiographs before surgical procedures. Pedicle screw placement was conducted using the free-hand technique previously described by Kim et al.,²⁾ with the use of a ball-tip probe.

(1) Exposure and determination of the entry point: an important factor for successful pedicle screw placement is obtaining an adequate view of the posterior component. Thus, the spine is exposed to the tips of the transverse processes bilaterally, and the facet joints are thoroughly cleaned of any soft tissue. (2) Gearshift probing: a pedicle finder is engaged just posterior to the pedicle isthmus (Fig. 1A). (3) Sequenced ball-tip widening: an intraosseous passage is made using the first ball-tip probe, which is the most elastic and thinnest (Fig. 1B). When performing this technique, the surgeon should rely on fingertip sensation, noting any resistance from the cancellous bone and stopping upon encountering the dense anterior wall of the vertebral body. The elasticity of the first ball tip enables safe passage through the intraosseous surface without violating the pedicle wall. Next, the passage is widened along the penetrated passage with the second (Fig. 1C) and third (Fig. 1D) ball-tip probes, in that order. The second probe is stiffer and thicker than the first probe, and the third probe is most stiff and thick among the 3 probes, which were all invented by the author (HJK). After widening with the 3 ball-tip probes in sequence, the pedicle finder is finally used to widen the passage (Fig. 1E). (4) Palpation of the pedicle wall and measurement of pedicle length: next, the pedicle walls (medial, lateral, superior, and inferior) and anterior wall of the vertebral body are examined for any violation, and the pedicle length is measured for screw fixation. (5) Screw placement: the screw is then inserted into the widened intraosseous passage with gentle maneuvering.

Fig. 1. (A) Decortication of the entry point using a smaller pedicle finder, engaging just before the isthmus portion. (B) Making a cancellous passage with the first ball-tip probe that has a 1-mm diameter ball tip and the shaft diameter is less than 1 mm with cone shape and maximum flexibility. (C) Widening the passage with the second ball-tip probe that has a 1.5-mm diameter ball tip and the shaft diameter is less than 1.5 mm with cone shape and medium flexibility. (D) Widening the passage with the third ball-tip probe that has a 2-mm diameter ball tip and the shaft diameter is less than 2 mm with cone shape and lowest flexibility. (E) Final widening with a larger pedicle finder. This procedure was selectively done when the pedicle diameter was larger than 4 mm.

Assessment

The screw placement is then assessed for accuracy using intraoperative CT, O-arm, and electromyography stimulations, which are appropriate assessments for intraosseous screw fixations and prevention of neurovascular complications. The accuracy of pedicle screw placement is then classified into 4 groups using the central axis of the pedicle screw: fully contained within the pedicle (in pedicle), perforation with up to 2 mm of misplacement (grade A), 2–4 mm (grade B), and perforation with greater than 4 mm of misplacement (grade C).³⁾ Since a screw larger than the pedicle cannot be fully contained, the pedicle screw must be used as a central axis for assessing accuracy. In addition, preoperative pedicle diameters obtained by CT and the diameter of the pedicle screw were measured to differentiate the parameters of pedicles in other studies from those used in this study for small pedicles.

In intraoperative CT, 86.8% of pedicle screws (184 / 212) were fully contained within the pedicle (Fig. 2A). Screws assigned “grade A” (Fig. 2B) comprised 9.4% (20 / 212), “grade B” (Fig. 2C) comprised 3.8% (8 / 212), and “grade C” comprised 0% (0 / 212). The measurements were performed by 2 independent spine fellows who were otherwise not involved in this study. The kappa index among the 2 spine fellows was 0.767, indicating substantial agreement, and intraobserver agreement was 0.816, which is considered almost perfect agreement.

Fig. 2. (A) The central axis of the pedicle screw lies entirely within the pedicle (in pedicle). (B) The central axis of the pedicle screw is displaced medially by less than 2 mm (grade A). (C) The central axis of the pedicle screw is displaced medially by more than 2 mm (grade B).

DISCUSSION

This novel technique is safer and more accurate than conventional free-hand technique for pediatric patients, who tend to have small pedicles. For assessment of accuracy with the free-hand technique, most studies defined the acceptable position of a pedicle screw as our “in pedicle” or grade A criteria. According to this definition, the accuracy rates of previous studies ranged from 70.8% to 94.6% (Table 1),³,⁴,⁵,⁶⁾ lower than the 96.2% (204 / 212) achieved in our study. To the best of our knowledge, this is the first study for enhancing the efficacy of the free-hand technique for pediatric scoliosis patients who tend to have small pedicles.

Table 1. Comparison of Reported Accuracy Rates for Pedicle Screw Placement Using Free-Hand Techniques.

Study	Accuracy rate (%)	Accuracy criteria	Neurological complication
Gertzbein and Robbins3)	90.0	Medial violation up to 4 mm	2 / 40
Kim et al.2)	93.8	Central line of pedicle screw within pedicle outer cortex	0 / 394
Modi et al.5)	90.7	Medial violation up to 2 mm and lateral violation up to 4 mm	0 / 43
Sarlak et al.6)	70.8	Medial violation up to 2 mm and lateral violation up to 6 mm	0 / 19
Seo et al.11)	94.6	Pedicle screw axis within pedicle cortex	0 / 31
This study	96.2	Central line of pedicle screw within 2 mm violation	0 / 12

This technique must be considered with some precautions. First, tactile feedback while widening the pedicle diameter is essential. This is a sensitive procedure in which surgeons should feel the narrow and rough inner cortex of the pedicle very carefully, as if touching it with their own fingers. Second, understanding the characteristics of the ball tip (i.e., flexibility or rigidity) is also important, because if the ball tip is too flexible, it will be unable to penetrate; if it is too rigid, it can cause a pedicle breach. Thus, in many aspects, this technique is similar to the existing ball-tip technique introduced by Watanabe et al.⁷⁾ However, extremely small cancellous channel pedicles posed a limitation in their study. To complement the previously established ball-tip probe technique, we developed and applied firmer sequential ball-tip probes with the ability to widen the pedicle’s intraosseous diameter in this study.

New imaging guidance techniques require additional equipment such as fluoroscopy or intraoperative CT, ultimately increasing radiation exposure. In a study by Bratschitsch et al.,⁸⁾ radiation exposure to the surgeon during fluoroscopy was measured as 566 ± 560 μSv; 49 ± 19 μSv. Further, Peng et al.⁹⁾ reported that robot-assisted surgery required a longer operation time compared with that of free-hand spinal surgery, with a mean difference of 15.12 minutes. We surmise that expenses would also increase. However, in the present study, this technique ultimately only required additional ball tips, indicating that additional equipment was not needed, and suggesting efficiency and economic favorability.

We used this technique for pediatric patients initially as pediatric bone offers elasticity and plasticity. These characteristics of pediatric bone enable gradual widening of the pedicle without any breach of the pedicle. In this study, as we expected, even extremely small pedicles contained the pedicle screw accurately and completely. Furthermore, as osteoporotic bone also has elasticity,¹⁰⁾ we applied this technique in degenerative spinal surgery with a pedicle diameter of less than 3 mm, primarily in the upper lumbar pedicle, and found the pedicle screw to be in an accurate position postoperatively.

Several limitations of this study should be acknowledged. First, this study involved only pediatric patients and had a retrospective design. Now that this technique has been evaluated for safety, application to elderly patients and prospective studies may be designed. An additional limitation of this study was that the technique was performed by a single senior spine surgeon who designed it. This indicates that a learning curve must be considered. However, tactile feedback, the most important aspect of the technique, can become relatively familiar to a junior surgeon if they become experienced in free-hand technique and usage of a ball-tip probe. Lastly, this study did not compare the accuracy rate with that of conventional free-hand technique surgery. However, based on previous studies that showed the difficulty of working with small pedicles along with the results of this study, this technique will likely enhance the accuracy rate for placement of pedicle screws in small pedicles.

Working with small pedicle diameters is challenging for surgical pedicle screw placement but is important nonetheless. The ball tip widening technique introduced in this study can enhance the accuracy of screw insertion into small pedicles. Further, this technique may also be applied to lumbar surgery for degenerative conditions in the future.