Introduction

For many years, percutaneous pedicle screws (PPS) have been used in minimally invasive spine surgery, such as minimal invasive transforaminal lumbar interbody fusion, as well as in percutaneous reduction and fixation for thoracolumbar fracture. The placement of the PPS plays a critical role in minimal spine surgery [1]. PPS have many advantages. Muscle can be protected from extensive damage [2]; postoperative pain, blood loss, and hospital stay time are greatly reduced [3–5]. The infection rate is also lower with PPS [5–7]. However, the fluoroscopy time required for PPS placement is extensively long for both surgeons and patients. According to many reports, the radiation exposure was much higher in closed reduction and fixation with PPS than with conventional open reduction [3, 8]. Extensive radiation exposure may cause skin injury, induce cancer, and lead to hereditary effects [9]; thus, reduction of X-ray exposure for both surgeons and patients is currently an important goal [9, 10].

With the conventional percutaneous procedure, the position and depth of PPS are repeatedly checked, and so the fluoroscopy time is extensive. We present here a new method for safe placement of PPS that allows precise control of the screw depth. The procedure involves no radiation exposure for the surgeon and very low-dose radiation for the patient. With this method, there is a reduction in total radiation exposure and operation time. This modified operation may be readily accepted by spine surgeons. The following is a detailed report about the method for PPS placement and controlling the screw depth.

Methods

The operation was performed according to the standard procedure with PPS. Before the screw placement procedure, the position and angle of the K-wire were confirmed. After tapping the pedicle, we placed the PPS on the entry site; the angle of the screws was 10°–15° from the vertical. We determined the depth of the PPS by counting the numbers of screw rotations. The distance between screw threads can be measured for a particular screw; thus, full rotation of the screw signifies that the screw has advanced into the pedicle the distance between screw threads.

To fully insert the screw into the pedicle, the number of full rotations has to equal the number of threads in the PPS. For example, the 45-mm PPS of FuLe Company (Beijing, China) has 17 threads (Fig. 1). If the surgeon intends to insert the entire screw into the pedicle, it is necessary to count the number of screw rotations when using the screwdriver. After 17 full rotations, the screw will be fully placed in the pedicle. By counting the number of screw rotations, the depth of the screw in the pedicle may be precisely controlled. Two full rotations are equal to about 5-mm advancement of the screw into the pedicle.

Fig. 1
figure 1

The percutaneous pedicle screw (FuLe Company). The 40-mm screw thread has 15 circles. To fully insert the screw into the pedicle demands 15 full rotations. Every three rotations, the screw advances 8 mm in the pedicle. With this method, we can precisely control the screw depth by counting the rotations. The 45-mm screw has 17 circles, and so 17 full rotations are needed to fully insert the screw into the pedicle

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In the fractured vertebra, the PPS usually has to be placed a little higher than in other vertebrae. For 45 mm screw, the screw will be precisely 5 mm higher than with the other pedicles after 15 rotations or 2.5 mm higher after 16 rotations. Using this method, the screw depth may be confirmed by counting the number of rotations. There is no need to use the C-arm to confirm repeatedly the position and depth of the screws. Once all six screws have been inserted, their position may be checked with the C-arm (Fig. 2). The connection rod is then inserted and reduction can be performed (Fig. 3).

Fig. 2
figure 2

a, b The G-arm anteroposterior (AP) and lateral image after all six percutaneous pedicle screws were inserted into the pedicle. The screws were at exactly same depth in the same vertebra. The screws in the fractured vertebra were 2.5 mm (one full rotation) higher than the screws the superior and inferior vertebrae. The AP view shows that all the screws were in a good position

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Fig. 3
figure 3

a, b The G-arm AP and lateral image after reduction of the fracture

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Results

From 2013 to 2016, we used this technique to insert PPS for 58 thoracic-lumbar fracture patients. In those operations, 348 screws were inserted by the authors (XL and ZWZ). No neurological symptoms were found in any of the patients. We checked the position of the PPS using X-ray or computed tomography, and no misplacement of the screws was evident. With this procedure, there is no radiation exposure for the surgeon, and it is very low for the patient in the screw placement procedure.

Discussion

We used a novel method to confirm the depth of PPS that does not require repeated checking with the C-arm. In all, 58 patients underwent an operation using this method; the screw depth could be precisely determined.

To the best of our knowledge, this is the first time for this method to be reported. With other techniques for placing PPS, the screw depth is repeatedly checked using the C-arm or with radiological methods. This does not allow precise control of the screw depth, and it increases the fluoroscopy time for both surgeons and patients. With our procedure, there is no need to check the position of the screws until all the PPS have been placed in the pedicle. When checking the position of the screws, surgeons and other staff could do so behind a lead wall, with no radiation exposure. Only one or two photographs need to be taken with the C-arm to confirm the position of the screws; thus, the fluoroscopy exposure for patients is much lower. This method offers greater protection for both surgeons and patients, the fluoroscopy time could be reduced. The average fluoroscopy time was 7 (5–11) s in traditional method and 2 (1–4) s in the new method with our group in the placement of screws.

For better reduction, PPS in the fractured vertebra need to be 3–5 mm higher than with other screws. The conventional radiological method demands repeated use of the C-arm, and the positioning of the screws is not precise. Our new method offers a better way to achieve that positioning by rotating half or a full rotation fewer of the screw than the screws put in other pedicles. The reduction of the fracture in our patients was good. Having screws in the pedicle with exactly the same depth on either side may provide better biomechanical support for the spine.

The screws used in this study were PPS made by FuLe Company. Other companies produce PPS with different designs. The screws we used may differ in their operation from other PPS, such as the Viper screws of Johnson & Johnson Company, America. In this regard, the structure of different PPS demands careful study: the distance between the screw threads may vary with different kinds of PPS.

For each rotation, the advance distance constant, c, in the pedicle is determined by this equation: c = L/N; where L is the length of the pedicle screw and N signifies the number of screw threads. Generally, PPS are intended to be inserted fully into the pedicle. In such situations, the number of threads can be counted. As the number of complete rotations is counted, the screws are fully inserted into the pedicle. If the surgeon does not plan to insert the screw fully into the pedicle, r is the distance of the screw outside the pedicle. The number of rotations is then equal to the total number of rotations minus r/c. Using these equations, our method can be applied for many types of PPS. However, it is not a suitable technique for screws with differential pitch.

The method of determining the screw depth by counting the rotations is a safe, effective way for placement of PPS. In the screw placement procedure, there is no radiation exposure for surgeons and other staff in the operation room and much lower fluoroscopy exposure for patients.