Degenerative spine diseases and deformities are the most common indications for the stabilization and fusion of spinal segments. The pedicle screw–rod system is currently considered the gold standard for dorsal stabilization and fusion of the lumbar spine [1].

In particular, when treating multisegmental spinal deformities and segmental instabilities with spondylolisthesis, a mismatch between the rigid pedicle screws and the rod often occurs during surgery. Here, the spinal surgeon uses a special instrument at his/her disposal, the so-called rod persuader (RP, also called rod-insertion forceps), which presses the rod into the head of the pedicle screw by axially pulling it. Through targeted use, this technique enables not only the treatment of deformities but also the repositioning of a sliding vertebra to restore normal spine alignment. In everyday clinical practice, RPs are also increasingly used to fit the rod into the screw head. This technique does not require the removal of the rod again and manual shaping it outside the site, saving surgical time [2].

Pedicle screw loosening is the most common complication after dorsal instrumentation of the spine. A study reported a pedicle screw loosening rate of 15% in nonosteoporotic spines and up to 60% in osteoporotic vertebral bodies [3]. Consequences of pedicle screw loosening include painful nonunion, spinal instability, deformation, material dislocation up to perforation through the skin, and ultimately, the need for revision surgery [3].

In addition to bone quality, the anchoring stability of pedicle screws is also influenced by the screw design, screw thickness, insertion technique, insertion torque, and screw placement [3]. In contrast, increased axial tension, such as that occurring when using an RP, can result in screw loosening and even tearing out of the screw. This effect has been examined biomechanically in the thoracic spine without additive fusion surgery [2,4]. Despite the widespread use of RPs in everyday clinical and surgical practice, no biomechanical studies have examined the biomechanical consequences of using an RP in lumbar fusion surgery.

This biomechanical in vitro study aimed to examine the extent to which the use of RPs leads to additional mechanical stress on the pedicle screw–rod system and its influence on the bony anchoring of the primary pedicle screws. We hypothesized that the use of RPs increases the tension on the rod, thereby reducing the bony anchoring of the pedicle screw. As a secondary outcome, this study examined whether this technique leads to an outright extraction of the pedicle screws.

The study analyzed 10 fresh-frozen lumbar spines from body donors at the Medical Faculty of the Anatomical Institute. The lumbar spines were thawed before preparation and immediately sent for biomechanical analysis.

Ten vertebral bodies were prepared for the experiment, and one of them was not used because of a vertebral body fracture and pedicle fracture before preparation. Thus, only nine vertebral bodies were suitable for use in the experiment. They were divided into group 1 (n=9; persuader group) and group 2 (n=9; nonpersuader group). The mean patient age was 87±6.5 years, and there were more women (55.5%) than men. The average BMD in HU the studied vertebrae was 104±80.3 HU. The rod bending values at the beginning of the experiment (0 minutes) were 3,219±1,769 μm/m and 167±173 μm/m in groups 1 and 2, respectively (p<0.001) (Fig. 3). At the end of the experiment (30 minutes), the rod bending values reduced to 2,511±1,854 μm/m and 140±172 μm/m in groups 1 and 2, respectively (p<0.001) (Fig. 4). Furthermore, after the system release, rod bending persisted at 2,009±1,227 μm/m and 129±163 μm/m in groups 1 and 2, respectively (p<0.001) (Fig. 5). Significant differences were found between the control and test specimens at all measurement points (Table 1). The same general trends were observed with no apparent differences noted within those groups when further stratified based on BMD. After rod reduction, screw pullout with an average of 1.5±0.79 mm of dislocation was observed in group 1 (88.9%) compared with 0 mm of dislocation in group 2 (0%) after system release (Fig. 6).

Despite the routine use of a pedicle screw system for the treatment of spinal instability, complications reported are severe, including screw breakage, loosening, or pullout [6,7]. In this study, the results suggest that the use of an RP has devastating biomechanical consequences by significantly increasing pedicle screw loosening and pedicle detachment. Our results, obtained through continuous measurement of rod bending using strain gauges, demonstrated that RPs not only exert axial tension to the pedicle screw but also bend the rod toward the pedicle screw. Interestingly, rod bending decreased within the first 10 minutes after connection to the screw–rod system and then changed only imperceptibly thereafter until the system was released. This indicates that an elastic restoring force continues to exert an axial pull on the screw even after using an RP. In a closed bow system, as observed in this study, approximately 10% of rod bending was “lost” in the system. Furthermore, according to the law of conservation of matter, as postulated by Lomonosov [8], in a closed system, “matter is neither created nor destroyed, it only transforms”. In the present study, this implies that further repositioning of the vertebral body (with good screw hold) or a slow axial pullout of the screw (with poor screw hold, such as in osteoporotic bones) has occurred. Similar phenomena might be observed when using a rod pusher. However, if deformity correction is not necessary, a rod pusher can still be advantageous because the initial axial stress on the pedicle screw would be less.

Because these differences were not observed when the rod was inserted without tension using an RP, the authors emphasized the importance of fitting the rod appropriately outside the surgical site using a rod bending device whenever possible. In deformity surgery, if screw pullout occurred after repositioning, implanting a pedicle screw with a larger screw diameter must be considered. Alternatively, for osteoporotic bones, cement augmentation of the pedicle screws before the repositioning maneuver can be performed to prevent material failure caused by poor bone quality. Recent research reveals a significant number of biomechanical studies evaluating the multiple variables affecting rod–pedicle screw system strength [5]. These factors include osteoporosis [9–13], pedicle morphology [10,14], pedicle screw size [15–18], screw insertion [19,20], pilot-hole tapping [21,22], insertional torque [21,23–26], and pedicle screw augmentation with cement or bone graft substitute [27].

Although the clinical consequences of decreased pedicle screw pullout strength reported in biomechanical studies need further elucidation, the significance of the apparent biomechanical effects of the rod reduction device on pedicle screw fixation, which are nearly universally used particularly during longitudinal construct assembly for posterior instrumentation, cannot be understated. Thus, our results suggest that the rod reduction maneuver causes weakening of the screw–bone interface with consequent rupture by forcing the pedicle screw to be pulled out from the osseous canal, causing fractures in the lumbar spine [5]. A threshold mismatch distance may have existed between the rod and pedicle screw construct that precludes catastrophic failure of the screw. Furthermore, depending on the complexity of the spine pathology, an RP could be used in situations with an important ventral–dorsal mismatch such as spondylolisthesis.

A limitation of this study is the evaluation of an isolated pedicle screw system. Although a comparison of all available commercial pedicle screw systems is untenable, the findings of this study could not be generalized to other screw systems. This issue could depict the failure rate between one screw type and another, requiring improvements in spondylodesis material and further investigations. In addition, cemented pedicle screws in osteoporotic specimens should be compared with pedicle screws with a larger diameter. Furthermore, as with mostly cadaveric biomechanical studies, the whole experiment under controlled conditions is limited by a small sample size and is dependent on the availability of specimens in the laboratory. Although each spine level was compared by pair, variations in the bone matrix quality and pedicle morphology may bias the results. Our laboratory setting enables the use of an X-ray device (fluoroscope) with direct visualization of the vertebral composition, allowing for optimal trajectories of pedicle screws and avoiding accidental fractures of the vertebral body along the axis of the pedicle screw during experiments.

Overall, our results may have significant application in the clinical setting. Although many spine surgeons promoted the rod reduction technique to be safe and effective, they may unknowingly compromise the biomechanical fixation strength of the manipulated pedicle screw. Therefore, close observation of the bone–screw interface during any reduction technique is necessary. Based on the results of our in vitro experience, many intraoperative screw pullout events may be unnoticed when using an RP. Despite the ease and convenience of rod reduction, our results point out that this technique biomechanically results in a poor pedicle screw pullout strength, increased failure of the screw–rod system, and subsequent instability in the operated spine segments. Nevertheless, the exact biomechanical mismatch of the RP through rod reduction on the overall system is poorly understood and was not assessed in this study. A detailed literature search did not yield clinical studies evaluating the abovementioned question. Pedicle screw loosening after using an RP may not be necessary after unilateral reduction in the middle part of a long screw–rod construct. However, screw loosening could be catastrophic with the use of an RP at the cranial or caudal portion of a long screw–rod construct or bilateral at the same level. Techniques to reduce the use of an RP should include rod contouring, using polyaxial screw heads, adjusting the screw depth, or redirecting the trajectory of the pedicle screws [2,20–26].

RP reduction increases pedicle screw pullout in the lumbar spine and typically results in failure of the spondylodesis material. Therefore, in cases where a rod does not fully fit within the pedicle screw head, the use of an RP should proceed with caution, particularly in the osteoporotic spine. Techniques for reducing the need for rod reduction to the pedicle screw heads may include further rod contouring, using polyaxial screw heads, adjusting the screw depth, or redirecting the pedicle screw trajectory. Ultimately, surgeons must be patient, and a detailed preoperative in intraoperative surgical planning is necessary.

Although many confounding factors are likely involved in patient outcomes after spinal surgery, most surgeons use rod reduction devices in the clinical setting. Clinical examinations of the effects of rod reduction on construct failure or pseudarthrosis rates are necessary.