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Robotic-guided and percutaneous pedicle screw placement are emerging technologies. We here report a retrospective cohort analysis comparing conventional open to open robotic-guided and percutaneous robotic-guided pedicle screw placement. 112 patient records and CT scans were analyzed concerning the intraoperative and perioperative course. 35 patients underwent percutaneous, 20 open robotic-guided and 57 open conventional pedicle screw placement. 94.5% of robot-assisted and 91.4% of conventionally placed screws were found to be accurate. Percutaneous robotic and open robotic-guided subgroups did not differ obviously. Average X-ray exposure per screw was 34 s in robotic-guided compared to 77 s in conventional cases. Subgroup analysis indicates that percutaneously operated patients required less opioids, had a shorter hospitalization and lower rate of adverse events in the perioperative period. The use of robotic guidance significantly increased accuracy of screw positioning while reducing the X-ray exposure. Patients seem to have a better perioperative course following percutaneous procedures.
Pedicular screw placement is a standard procedure for spinal fusion in the lumbar and thoracic spine. In recent years image guidance has increased the accuracy of pedicular screw placement [1–3]. Recent developments include a spinous process-mounted miniature robot, the SpineAssist™ (Mazor Robotics, Caesarea, Israel). Pechlivanis and colleagues  demonstrated the high accuracy of this robotic system. Nevertheless, with regard to the great variability of pedicle screw deviation rates given in literature (1–18% [5–7]), it remains difficult for the surgeon to estimate the benefit of image guidance without a control group.
Likewise, a number of techniques for percutaneous pedicle screw placement have been introduced, mostly with favorable initial results [8–12]. However, no clinical study directly comparing the outcome of percutaneous and open midline approaches has yet been published in the English literature.
We here present a single institution retrospective analysis of perioperative results following lumbar and thoracic pedicle screw insertion using three different techniques.
112 consecutive patients who underwent pedicular screw placement between January 2006 and August 2009 were analyzed regardless of the indication for spinal fusion.
Pedicle screw placement was performed using three alternative techniques: patients received pedicle screws either using conventional 2-D fluoroscopic guidance (BV Endura, Philips, Hamburg, Germany) via a midline incision (termed “conventional”), or using an robotic-guided technique (SpineAssist™ Mazor Robotics, Caesarea, Israel). These robotic-guided procedures were either performed via a midline incision (open robotic-guided), or via a percutaneous approach. For percutaneous surgery separate 2 cm incisions for each screw and a 3–4 cm midline incision, to attach the robot to the spinous process by a clamp, were performed (percutaneous robotic-guided, Fig. 1).
In percutaneous cases additional PLIF- and TLIF-cage implantation and decompression of the spinal channel by laminotomy or laminectomy could be performed via the 3–4 cm midline incision when indicated (Fig. 2).
A pre-operative planning-CT was performed in all cases (16-slice MSCT-scanner, Aquilion™, Toshiba Medical Systems, Neuss, Germany). In robotic-guided cases 3-D reconstructions of this CT were used to plan the screw position using the SpineAssist™ planning-software. Registration of the SpineAssist™ robot was performed by matching the pre-operative CT scan with two intraoperative fluoroscopies (AP- and 60° oblique images; BV Endura, Philips, Hamburg, Germany, see  for further details).
Six surgeons performed operations; all of them applied conventional and robotic-guided techniques. In 2006 all patients were operated conventionally, while robotic guidance was introduced in 2007 and implants for percutaneous surgery were introduced 2008. Following the introduction of the new techniques robotic-guided percutaneous surgery was preferred if the implants and instruments were available. The perioperative management, however, was not changed during this period.
Four types of pedicle screws were used: Tango™ (Ulrich medical, Ulm, Germany) and TRSH 3D™ (Medtronic, Tolochenanz, Switzerland) in conventional procedures, Titan Expertise™ (Peter Brehm, Weisendorf, Germany) in robotic-guided and Icon™ (Blackstone Medical, Laichingen, Germany) in percutaneous procedures. Screw diameter varied from 4.5 to 7.5 mm.
Screw position was assessed on CT scans obtained 1–4 days after surgery. Sagittal and axial reconstructions were analyzed by an investigator blinded to the insertion technique. The assessment was repeated 11 times using a millimeter scale; the relative numbers given in % represent the average values of these measurements. Screws entirely in the bone were graded 0, grade 1 described an encroachment of the cortical bone, grade 2 a deviation of less than 3 mm, grade 3 deviations from 3 to 6 mm and grade 4 for deviations of more than 6 mm (Fig. 3).
Pre- and postoperative CT scans were performed routinely. The total intraoperative X-ray exposure was recorded and divided by the number of screws placed. For robotic-guided cases this included the registration of the SpineAssist™.
Intraoperative adverse events, operating time, postoperative hospitalization, postoperative administration of opioids, adverse events and reoperations were assessed. Analysis was based on routine-examinations directly postoperative, before discharge and about 12 weeks after surgery. All information was extracted from the patients’ charts and CT scans.
Patients with spondylodiscitis were excluded from analysis hospitalization and infection rates, because their routine treatment included 3 weeks of i.v. antibiotics in hospital and they had infections prior to surgery.
Patients who were on opioids at hospital admittance were excluded from the analysis of postoperative opioid administration. Patients with incompletely documented data had likewise to be excluded from the concerned analysis (Table 1).
Statistical significance was tested by student t test when a Gaussian distribution was expected; in samples for which a normal distribution could not be assumed the Wilcoxon test was used instead. For unquantified data (nominal scale) the χ2 test was used. Significance level was set α = 0.05.
Average patient age was 63.1 years, gender ratio (m/f) was 52/60. Average BMI was 26.8. These values did not significantly differ between the 3 (sub)groups (Table 2).
A total of 536 pedicle screws were placed in 112 patients. 286 screws in 57 patients were implanted conventionally and 250 (55 patients) in robotic-guided procedures. 94 of the robotic-guided procedure were performed via a midline incision (20 patients), while 156 screws (35 patients) underwent percutaneous procedures.
94.5% of robotic-guided and 91.5% of conventionally placed screws were graded 0 or 1. Mild deviations (grade 2) were observed in 4.3% of robotic-guided and 5.0% of conventionally placed screws. Moderate (grade 3) and severe deviations (grade 4) were observed in 0.7 and 0.4% of robotic-guided and 1.9 and 1.6% of conventionally placed screws (Table 3). This was statistically significant regardless of whether grade 0 and grade 1 or exclusively grade 0 was rated as correct placement (Table 4). Results for percutaneous and open robotic-guided procedure did not differ obviously. Percutaneous procedures were significantly more accurate if only grade 0 was considered correct (p values, robotic-guided/conventional 0.00001; percutaneous/conventional 0.001, open robot-guided/convent. 0.00001; percut./convent., 0.001), while open robotic-guided procedures were superior if grade 0 and 1 were analyzed together (Table 3).
During robotic-guided procedures the average X-ray exposure was 34 s compared to 77 s in conventional cases. Average X-ray exposure for percutaneous robotic-guided screws was lower than X-ray exposure during open robotic-guided procedures (27 s compared to 43 s). The differences between robot-assisted (both subgroups together and separately) and conventional procedures were statistically significant, the difference between percutaneous and open robot-assisted procedures was not statistically significant (p values, robotic-guided/convent. 0.0001; percut./convent. 0.001, open robot-guided/convent. 0.023).
The duration of surgery documented in the patients’ records always included all surgical steps from positioning to wound closure (including registration of the SpineAssist™, PLIF- or TLIF-cages implantation, laminectomy, etc.). This operation time was divided by the number of screws placed. Average time per screw was found to be 59.1 min in robotic-guided (57.0 min in percutaneous and 65.2 min in open robotic-guided procedures) and 52.9 min in conventional procedures. These differences were not statistically significant.
Postoperative routine included administration of NSAIDs. Novaminsulfone was added on request. When this regimen failed, opioids were added. 45.45% of the robotic-guided (37.5% of percutaneous and 66.6% of open robotic-guided procedures), and 88.9% of conventionally operated patients who were not on opioids pre-operatively required postoperative administration of opioid analgesics (Table 5. Differences between robot-assisted and conventional and percutaneous robot-assisted and conventional (sub)groups were statistically significant (p = 0.0002 and 0.004).
Intraoperative adverse events (1 major hemorrhage, 6 dural tears) were observed in 4.7% of robotic-guided cases (6.1% of the percutaneous and 0% of the open robotic-guided cases) and 9.1% of conventional procedures.
Postoperative cerebrospinal fluid fistulas were not observed following robotic-guided procedures (0%), while 6.1% of the conventionally operated patients developed this complication. Postoperative infections occurred in 2.7% of robotic-guided (0% after percutaneous and 10.0% after open robotic-guided procedures), while it was observed in 10.7% of the conventional procedures (Table 5). Statistical analysis showed significance between robot-assisted and conventional procedures for intraoperative adverse events only (p = 0.04).
Routinely patients were scheduled to remain ten postoperative days in hospital. Average hospitalization was 10.6 days following robotic-guided fusion (10.1 days for percutaneous, 11.6 days for open robotic-guided procedures) and 14.6 days following conventional procedures (Table 5). Differences between the robot-assisted and conventional and percutaneously and conventional (sub)groups were statistically significant (p values, robot-guided/convent. 0.009; percut./convent. 0.012).
Revision of misplaced screws was performed in 8 cases; 1.0% robotic guided (0.3% percutaneous and 6.3% open robotic-guided) and 12.2% conventional procedures (Table 5). Secondary sutures (mostly under local anesthesia on the ward) had to be placed because of wound healing disorders/infections in 10 cases (0.6% percutaneous procedures, 12.6% open robotic-guided procedures and 12.2% conventional procedures). 1 case of dislocated PLIF cage had to be revised (open robotic-guided). No statistical significances were found.
Perioperative clinical outcome and accuracy of pedicle screw placement in cohorts of robotic-guided (open and percutaneous) and conventionally placed pedicle screws were assessed. We focused on intra- and perioperative outcome, because the advantage of image guidance should show during surgery, respectively, on a postoperative CT scan, while the effect of a minimally invasive approach will be strongest during wound healing. Long-term results (fusion rate, etc.) were not included as it can be expected that these will be similar for all pedicle screws regardless of the surgical approach applied.
Assessment of pedicle screw position in replicate determinations by a single investigator blinded to the insertion technique was performed in order to minimize the effect of investigator-dependent errors. Assessment was based on the evaluation scale proposed by Wiesner  and Schizas .
Many clinicians accept deviations up to 2 or 3 mm because minor deviations rarely become symptomatic. Lonstein et al.  report in their meta-analysis of 4,790 screws of 5.1% screws breaching the cortical bone. No more than 0.2% of these caused neurological symptoms. Nevertheless, from time to time surgeons will be confronted with remaining/new-onset symptoms in the presence of a minor screw deviation and face the dilemma whether to reoperate or not. Therefore, we here recorded all minor and major deviations.
Following the literature conventional screw placement results in a deviation rate of 1–18% [5–7], while other authors described a significant reduction of this deviation rate when image guidance is applied (2.7–3.9% [13–15]).
Presently, a great variety of systems offering spinal image guidance are under research, clinical evaluation or already in routine clinical use. Among the proposed techniques are electromagnetic navigation , intraoperative 3-D fluoroscopy and CT navigation [16, 18], and percutaneous reference frames  and robotic-guided surgery .
Pechlivanis and colleagues  report in their recent study using SpineAssist™ of 91.7% of screws entirely within the bone, while further 6.8% of the screws showed deviations of less than 2 mm. This corresponds to our results (94.6% grade 0 and 1 and 4.3% grade 2).
In accordance with the literature , in our study the accuracy of image-guided pedicle screw placement was significantly higher than in conventional procedures (about 3.1%). In robotic-guided procedures the surgical approach did have any obvious influence on the accuracy of screw positions. This contrasts with the situation in conventional procedures, as discussed above [2, 3, 16].
Intraoperative X-ray exposure was significantly reduced by application of robotic guidance. Pre- and postoperative CT scans were performed routinely, so that the SpineAssist™ planning-CT did not cause additional X-ray exposure. Furthermore, the preoperative planning-CT did not affect the surgical team which, in contrast to the patient, faces the intraoperative radiation frequently.
Interestingly, we observed a decrease of intraoperative X-ray doses during percutaneous robotic-guided procedures compared to open robotic-guided procedures. The best explanation is, however, our learning curve with the SpineAssist™. The first robotic-guided procedures in 2007 were open robotic-guided and the decreased radiation during percutaneous procedures might reflect a growing confidence in the SpineAssist™ in 2008.
Duration of postoperative hospitalization, postoperative opioid administration (indicator of severe pain), infection rate and rate of screw revisions were better in the robotic-guided group. However, the percutaneous subgroup did still better, while the open robotic-guided subgroup resembled more the conventional cases. This indicates that the perioperative clinical course depends less on the application of image guidance than on the surgical approach (percutaneous versus open).
In literature many authors report good clinical results following percutaneous pedicle screw placement for various indications (trauma , spondylodiscitis  and spondylolisthesis [10, 11]). Accordingly, several studies measuring serum creatine kinase levels after surgery indicated less muscular trauma in percutaneous operations [21, 22]. Cadaver and animal studies comparing percutaneous and open midline approaches supported these findings [23, 24].
However, studies directly comparing clinical outcome after percutaneous and open pedicle screw placement in humans are not yet published in the English literature. Grass and colleagues report in a German journal “Der Unfallchirurg” of 57 patients with thoracolumbar fractures. EMG examination proved greater trauma in open procedures and blood loss was significantly higher. Two of the 35 patients operated percutaneously needed a revision. However, the study focuses on intraoperative issues and accuracy of screw placement, further clinical data is not given in this interesting article . Another study, which appeared in the Chinese language journal “Zhonghua Wai Ke Za Zhi” reports of a prospective study in 60 patients with thoracolumbar burst fractures, who had been treated with either percutaneous or open pedicular screw placement . The authors conclude that “pedicle screw fixation for thoracolumbar fracture has the advantage of less trauma, quicker recovery and better esthetic outcomes, however, it has the same results with the traditional open produce after 2 years of surgery”. Our study likewise indicates that clinical outcome following percutaneous procedures might be superior to that following an open midline approach.
The underlying pathologies were heterogeneous. Posterior stabilization with pedicle screws and rod systems was frequently combined with other surgical procedures. Therefore, the assessed parameters were not exclusively related to pedicle screw implantation, but may partly reflect additional surgical procedures. Some cases had to be excluded from the analysis of one or several parameters because of the underlying disease. This influences the quality of our data and the conclusions derived from it.
Besides this study spans from 2006 to 2009 and the robot-assisted (percutaneous) subgroup represents the most recently operated cohort. Patients were operated by six different surgeons and the data includes all their learning curves with percutaneous approaches, the SpineAssist™ and spinal fusion surgery in general. Additionally, no randomization was performed, so that an entry bias might further confound this study.
Robotic-guided pedicle screw implantation resulted in accurate placement in 94.5% compared to 91.4% in conventional screw placement. Duration of intraoperative radiation was significantly lower in robotic-guided compared to conventional procedures.
Postoperative administration of new opioid analgesics, duration of postoperative hospitalization and rates of adverse events seem to be lower in percutaneous robotic-guided procedures compared to open robotic-guided and conventional procedures.
Open Access This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
This study contains parts of a doctoral thesis by S. Baerwinkel.