Computed tomography assessment of lateral pedicle wall perforation by free-hand subaxial cervical pedicle screw placement

Purpose

To present the technique of free-hand subaxial cervical pedicle screw (CPS) placement without using intra-operative navigating devices, and to investigate the crucial factors for safe placement and avoidance of lateral pedicle wall perforation, by measuring and classifying perforations with postoperative computed tomography (CT) scan.

Summary of background data

The placement of CPS has generally been considered as technically demanding and associated with considerable lateral wall perforation rate. For surgeons without access to navigation systems, experience of safe free-hand technique for subaxial CPS placement is especially valuable.

Materials and methods

A total of 214 consecutive traumatic or degenerative patients with 1,024 CPS placement using the free-hand technique were enrolled. In the operative process, the lateral mass surface was decorticated. Then a small curette was used to identify the pedicle entrance by touching the cortical bone of the medial pedicle wall. It was crucial to keep the transverse angle and make appropriate adjustment with guidance of the resistance of the thick medial cortical bone. The hand drill should be redirected once soft tissue breach was palpated by a slim ball-tip prober. With proper trajectory, tapping, repeated palpation, the 26–30 mm screw could be placed. After the procedure, the transverse angle of CPS trajectory was measured, and perforation of the lateral wall was classified by CT scan: grade 1, perforation of pedicle wall by screw placement, with the external edge of screw deviating out of the lateral pedicle wall equal to or less than 2 mm and grade 2, critical perforation of pedicle wall by screw placement, large than 2 mm.

Results

A total of 129 screws (12.64 %) were demonstrated as lateral pedicle wall perforation, of which 101 screws (9.86 %) were classified as grade 1, whereas 28 screws (2.73 %) as grade 2. Among the segments involved, C3 showed an obviously higher perforating rate than other (P < 0.05). The difference between the anatomical pedicle transverse angle and the screw trajectory angle was higher in patients of grade 2 perforation than the others. In the 28 screws of grade 2 perforation verified by axial CT, 26 screws had been palpated as abnormal during operation. However, only 19 out of the 101 screws of grade 1 perforation had shown palpation alarming signs during operation. The average follow-up was 36.8 months (range 5–65 months). There was no symptom and sign of neurovascular injuries. Two screws (0.20 %) were broken, and one screw (0.10 %) loosen.

Conclusion

Placement of screw through a correct trajectory may lead to grade 1 perforation, which suggests transversal expansion and breakage of the thinner lateral cortex, probably caused by mismatching of the diameter of 3.5 mm screws and the tiny cancellous bone cavity of pedicle. Grade 1 perforation is deemed as relatively safe to the vertebral artery. Grade 2 perforation means obvious deviation of the trajectory angle of hand drill, which directly penetrates into the transverse foramen, and the risk of vertebral artery injury (VAI) or development of thrombi caused by the irregular blood flow would be much greater compared to grade 1 perforation. Moreover, there are two crucial maneuvers for increasing accuracy of screw placement: identifying the precise entry point using a curette or hand drill to touch the true entrance of the canal after decortication, and guiding CPS trajectory on axial plane by the resistant of thick medial wall.     

The correlation between cervical range of motion and misplacement of cervical pedicle screws during cervical posterior spinal fixation surgery using a CT-based navigation system

Purpose

The aim of this study was to analyze the correlation between cervical range of motion and cervical pedicle screw (CPS) misplacement in cervical posterior spinal fusion surgery using a CT-based navigation system.

Methods

A total of 46 consecutive patients with cervical posterior spinal fusion surgery using CPSs were evaluated retrospectively. We analyzed the cervical range of motion (ROM) and the misplacement of CPSs that were placed using either separate or single-time multilevel registration with a CT-based navigation system to determine the optimum registration procedure. The screw-inserted vertebra was indicated as Registered vertebra-Pedicle Screw inserted vertebra (Re-PS) = 0, 1, 2, or 3 depending on its distance (level) from the registered vertebra. Grades 0 (no perforation) and 1 (perforations <2 mm) were categorized as “no misplacement.” Grades 2 (perforations ≧2 mm but < 4 mm) and 3 (perforations ≧ 4mm) were categorized as “misplacement.” We analyzed the correlations between CPS misplacement and Re-PS, and between CPS misplacement and preoperative cervical ROM.

Results

Our analysis included 196 screws in patients having a mean age of 53.2 years (range 5–84 years). Level of insertion relative to registration was Re-PS = 0 in 129 screws, Re-PS = 1 in 53, Re-PS = 2 in 10 and Re-PS = 3 in 4. The misplacement rates were 12.2 % (24 screws) overall, 6.2 % in Re-PS = 0, 22.6 % in Re-PS = 1, 20 % in Re-PS = 2, and 50 % in Re-PS = 3. The rate of CPS misplacement increased significantly with a Re-PS = 1 and a Re-PS = 2 and 3 compared to a Re-PS = 0. There was a significant difference in the cervical ROM in each grade and both misplacement groups: 1.8 in Grade 0, 2.3 in Grade 1, 7.8 in Grade 2, 12.9 in Grade 3, 11 in the misplacement group and 1.9 in the no misplacement group.

Conclusions

The precision of CPS placement in CT-based navigation surgery was evaluated. The misplacement rate in single-time multilevel registration increased to 23.4 % compared to 6.2 % for separate registration. As the distance increased between the registered level and the level of CPS insertion, the preoperative cervical ROM and the rate of CPS misplacement significantly increased. Thus, the rate of misplacement of CPSs is reduced when performing separate registration. Furthermore, when there is greater preoperative cervical ROM, separate registration would likely improve the safety and accuracy of CPS insertion.     

Perforations and angulations of 324 cervical medial cortical pedicle screws: a possible guide to avoid lateral perforations with use of pedicle screws in lower cervical spine

Background Context

More than half of the perforations reported with usage of cervical pedicle screws (CPS) are lateral perforations, endangering the vertebral artery. The medial cortical pedicle screw (MCPS) technique with partial drilling of the medial cortex shifts the trajectory of pedicle screws medially, decreasing the lateral perforations.

A comparison of feasibility and safety of percutaneous fluoroscopic guided thoracic pedicle screws between Europeans and Asians: is there any difference?

Purpose

To directly compare the safety of fluoroscopic guided percutaneous thoracic pedicle screw placement between Caucasians and Asians.

Methods

This was a retrospective computerized tomography (CT) evaluation study of 880 fluoroscopic guided percutaneous pedicle screws. 440 screws were inserted in 73 European patients and 440 screws were inserted in 75 Asian patients. Screw perforations were classified into Grade 0: no violation; Grade 1: <2 mm perforation; Grade 2: 2–4 mm perforation; and Grade 3: >4 mm perforation. For anterior perforations, the pedicle perforations were classified into Grade 0: no violation, Grade 1: <4 mm perforation; Grade 2: 4–6 mm perforation; and Grade 3: >6 mm perforation.

Results

The inter-rater reliability was adequate with a kappa value of 0.83. The mean age of the study group was 58.3 ± 15.6 years. The indications for surgery were tumor (70.3 %), infection (18.2 %), trauma (6.8 %), osteoporotic fracture (2.7 %) and degenerative diseases (2.0 %). The overall screw perforation rate was 9.7 %, in Europeans 9.1 % and in Asians 10.2 % (p > 0.05). Grade 1 perforation rate was 8.4 %, Grade 2 was 1.2 % and Grade 3 was 0.1 % with no difference in the grade of perforations between Europeans and Asians (p > 0.05). The perforation rate was the highest in T1 (33.3 %), followed by T6 (14.5 %) and T4 (14.0 %). Majority of perforations occurred medially (43.5 %), followed by laterally (25.9 %), and anteriorly (23.5 %). There was no statistical significant difference (p > 0.05) in the perforation rates between right-sided pedicle screws and left-sided pedicle screws (R: 10.0 %, L: 9.3 %).

Conclusions

There were no statistical significant differences in the overall perforation rates, grades of perforations, direction of perforations for implantation of percutaneous thoracic pedicle screws insertion using fluoroscopic guidance between Europeans and Asians. The safety profile for this technique was comparable to the current reported perforation rates for conventional open pedicle screw technique.

     

Does intraoperative navigation improve the accuracy of pedicle screw placement in the apical region of dystrophic scoliosis secondary to neurofibromatosis type I: comparison between O-arm navigation and free-hand technique

Purpose

To assess the accuracy of O-arm-navigation-based pedicle screw insertion in dystrophic scoliosis secondary to NF-1 and compare it with free-hand pedicle screw insertion technique.

Methods

32 patients with dystrophic NF-1-associated scoliosis were divided into two groups. A total of 92 pedicle screws were implanted in apical region (two vertebrae above and below the apex each) in 13 patients using O-arm-based navigation (O-arm group), and 121 screws were implanted in 19 patients using free-hand technique (free-hand group). The postoperative CT images were reviewed and analyzed for pedicle violation. The screw penetration was divided into four grades: grade 0 (ideal placement), grade 1 (penetration <2 mm), grade 2 (penetration between 2 and 4 mm), and grade 3 (penetration >4 mm).

Results

The accuracy rate of pedicle screw placement (grade 0, 1) was significantly higher in the O-arm group (79 %, 73/92) compared to 67 % (81/121) of the free-hand group (P = 0.045). Meanwhile, a significantly lower prevalence of grade 2–3 perforation was observed in the O-arm group (21 vs. 33 %, P < 0.05), and the incidence of medial perforation was significantly minimized by using O-arm navigation compared to free-hand technique (2 vs. 15 %, P < 0.01). Moreover, the implant density in apical region was significantly elevated by using O-arm navigation (58 vs. 42 %, P < 0.001).

Conclusion

We reported 79 % accuracy of O-arm-based pedicle screw placement in dystrophic NF-1-associated scoliosis. O-arm navigation system does facilitate pedicle screw insertion in dystrophic NF-1-associated scoliosis, demonstrating superiorities in the safety and accuracy of pedicle screw placement in comparison with free-hand technique.

     

Prospective comparative study between straight and curved probe for pedicle screw insertion

Purpose

To evaluate difference in accuracy of pedicle screw insertion in thoracic and lumbosacral spine using a straight pedicle probe vis-à-vis a curved one.

Methods

Prospective, comparative, non-randomized, single-blind study. Straight and curved pedicle probes used on opposite sides of same vertebra in patients undergoing thoracolumbar pedicle screw fixation for various indications. Postoperative blinded evaluation for pedicle breaches done with a CT scan. Pedicle breaches graded as grade 0: no breach, grade 1: <2 mm, grade 2: 2–4 mm and grade 3: >4 mm breach.

Results

After appropriate statistical power analysis, 300 screws inserted in 59 patients from T4–L5 levels. No significant differences noted between the two probes in terms of screw length [two-tailed p = 0.16]; grade 0 screws [two-tailed p = 0.49] or screws with grade 2/3 breaches [two-tailed  p = 0.68]. With the right-hand-dominant operating-surgeon standing to left of patient during surgery, no difference noted between the two probes for either the right or left-side pedicle screw insertion [two-tailed p = 1]. Repeating these tests in the subset of thoracic pedicle screws too, revealed no significant difference.

Conclusions

No significant difference in outcome of pedicle screw insertion with either a straight or a curved pedicle probe.     

Optimal cervical screw insertion angle determined by means of computed tomography scans pre- and postoperatively

Background Context

Cervical pedicle screw (CPS) insertion is technically demanding and carries a risk of serious neurovascular complications when screws perforate. To avoid such serious risks, we currently perform CPS insertion using a computed tomography (CT)-guided navigation system. However, there remains a low probability of screw perforation during CPS insertion that is affected by factors such as CPS insertion angle and anatomical pedicle transverse angle (PTA).

Accuracy of 3D fluoroscopy-navigated anterior transpedicular screw insertion in the cervical spine: an experimental study

Purpose

The technique of pedicle screw stabilization is finding increasing popularity for use in the cervical spine. Implementing anterior transpedicular screws (ATPS) in cervical spine offers theoretical advantages compared to posterior stabilization. The goal of the current study was the development of a new setting for navigated insertion of ATPS, combining the advantage of reduced invasiveness of an anterior approach with the technical advantages of navigation.

Methods

20 screws were implanted in levels C3 to C6 of four cervical spine models (SAWBONES^® Cervical Vertebrae with Anterior Ligament) with the use of 3D fluoroscopy navigation system [Arcadis Orbic 3D, Siemens and VectorVision fluoro 3D trauma software (BrainLAB)]. The accuracy of inserted screws was analyzed according to postoperative CT scans and following the modified Gertzbein and Robbins classification.

Results

20 anterior pedicle screws were placed in four human cervical spine models. Of these, eight screws were placed in C3, two screws in C4, six screws in C5, and four screws in C6. 16 of 20 screws (80 %) reached a grade 1 level of accuracy according to the modified Gertzbein and Robbins Classification. Three screws (15 %) were grade 2, and one screw (5 %) was grade 3. Grade 4 and 5 positions were not evident. Summing grades 1 and 2 together as “good” positions, 95 % of the screws achieved this level. Only a single screw did not fulfill these criteria.

Conclusion

The setting introduced in this study for navigated insertion of ATPS into cervical spine bone models is well implemented and shows excellent results, with an accuracy of 95 % (Gertzbein and Robbins grade 2 or better). Thus, this preliminary study represents a prelude to larger studies with larger case numbers on human specimens.

     

Pedicle screw diameter selection for safe insertion in the thoracic spine

Objective

Many thoracic pedicles are too small for the safe acceptance of a transpedicular screw. However, few studies have so far reported on the methods to select a proper pedicle screw size and to confirm the morphologic changes for such a small thoracic spine pedicle. The objective of this work was to determine the potential limits of a pedicle screw diameter for transpedicular screw placement in the thoracic spine.

Methods

T2–T9 vertebrae from eleven patients that underwent posterior thoracic instrumentation with the use of fluoroscopically assisted insertion method were analyzed. The outcome measures were the pedicle widths, the gap between the outer pedicle width and the selected pedicle screw diameter, and the penetration length of the pedicle screws using computed tomography. The screws were distributed into two groups according to the pedicle width and screw diameter, and the screw perforation rate of the two groups was compared. The relationships of the gap and the distance of the screw penetration were compared and investigated in regard to the pedicle screw diameter selection.

Results

A total of 16 screws demonstrated a smaller diameter than the inner pedicle widths, while 22 screws had a larger diameter than the inner pedicle widths. One screw (6.3%) perforated the pedicle cortex in the smaller screw group, and twelve screws (54.5%) perforated the pedicle cortex in the larger screw group (P?=?0.006). A linear regression analysis in the larger screw group revealed that when the gap was less than 0.5?mm, a risk of a pedicle wall violation was observed.

Conclusions

When the screws with a larger diameter than the inner pedicle width are selected, the screw perforation rate increases. Therefore, the size of the screw diameter must be at least 0.5?mm less than the outer pedicle width to ensure safe transpedicular screw placement.     

Zonal differences in risk and pattern of pedicle screw perforations in adolescent idiopathic scoliosis (AIS): a computerized tomography (CT) review of 1986 screws

Purpose

To evaluate the zonal differences in risk and pattern of pedicle screw perforations in adolescent idiopathic scoliosis (AIS) patients.

Methods

The scoliosis curves were divided into eight zones. CT scans were used to assess perforations: Grade 0, Grade 1(< 2 mm), Grade 2(2–4 mm) and Grade 3(> 4 mm). Anterior perforations were classified into Grade 0, Grade 1(< 4 mm), Grade 2(4–6 mm) and Grade 3(> 6 mm). Grade 2 and 3 (except lateral grade 2 and 3 perforation over thoracic vertebrae) were considered as ‘critical perforations’.

Results

1986 screws in 137 patients were analyzed. The overall perforation rate was 8.4% after exclusion of the lateral perforation. The highest medial perforation rate was at the transitional proximal thoracic (PT)/main thoracic (MT) zone (6.9%), followed by concave lumbar (6.7%) and convex main thoracic (MT) zone (6.1%). The overall critical medial perforation rate was 0.9%. 33.3% occurred at convex MT and 22.2% occurred at transitional PT/MT zone. There were 39 anterior perforations (overall perforation rate of 2.0%). 43.6% occurred at transitional PT/MT zone, whereas 23.1% occurred at concave PT zone. The overall critical anterior perforation rate was 0.6%. 5/12 (41.7%) critical perforations occurred at concave PT zone, whereas four perforations occurred at the transitional PT/MT zone. There were only two symptomatic left medial grade 2 perforations (0.1%) resulting radiculopathy, occurring at the transitional main thoracic (MT)/Lumbar (L) zone.

Conclusion

Overall pedicle perforation rate was 8.4%. Highest rate of critical medial perforation was at the convex MT zone and the transitional PT/MT zone, whereas highest rate of critical anterior perforation was at the concave PT zone and the transitional PT/MT zone. The rate of symptomatic perforations was 0.1%.

     

How Does a Novel Monoplanar Pedicle Screw Perform Biomechanically Relative to Monoaxial and Polyaxial Designs?

Background

Minimally invasive spinal fusions frequently require placement of pedicle screws through small incisions with limited visualization. Polyaxial pedicle screws are favored due to the difficulty of rod insertion with fixed monoaxial screws. Recently, a novel monoplanar screw became available that is mobile in the coronal plane to ease rod insertion but fixed in the sagittal plane to eliminate head slippage during flexion loads; however, the strength of this screw has not been established relative to other available screw designs.

Questions/purposes

We compared the static and dynamic load to failure in polyaxial, monoaxial, and monoplanar pedicle screws.

Methods

Six different manufacturers’ screws (42 total) were tested in three categories (polyaxial, n = 4; monoaxial, n = 1; monopolar, n = 1) utilizing titanium rods. An additional test was performed using cobalt-chromium rods with the monopolar screws only. Screws were embedded into polyethylene blocks and rods were attached using the manufacturers’ specifications. Static and dynamic testing was performed. Dynamic testing began at 80% of static yield strength at 1 Hz for 50,000 cycles.

Results

In static testing, monoaxial and monoplanar screws sustained higher loads than all polyaxial screw designs (range, 37%–425% higher; p < 0.001). The polyaxial screws failed at the head-screw interface, while the monoaxial and monoplanar screws failed by rod breakage in the static test. The dynamic loads to failure were greater with the monoplanar and monoaxial screws than with the polyaxial screws (range, 35%–560% higher; p < 0.001). With dynamic testing, polyaxial screws failed via screw-head slippage between 40% and 95% of static yield strength, while failures in monoaxial and monoplanar screws resulted from either screw shaft or rod breakage.

Conclusions

All polyaxial screws failed at the screw-head interface in static and dynamic testing and at lower values than monoaxial/monoplanar screw designs. Monoplanar and monoaxial screws failed at forces well above expected in vivo values; this was not the case for most polyaxial screws.

Clinical Relevance

Polyaxial screw heads slip on the screw shank at lower values than monoaxial or monoplanar screws, and this results in angular change between the rod and pedicle screw, which could cause loss of segmental lordosis. The novel monoplanar screw used in this study may combine ease of rod placement with sagittal plane strength.     

Accuracy of powered surgical instruments compared with manual instruments for pedicle screw insertion: Evaluation using o-arm-based navigation in scoliosis surgery

Background

Though powered surgical instruments for pedicle screw insertion combined with navigation have been developed to reduce time taken for spine surgery, clinical evidence demonstrating the safety and effectiveness of powered surgical instruments is limited. The goals of the present study were to compare the accuracy of powered instruments and manual instruments using O-arm-based navigation in surgery for scoliosis.

Are pedicle screw perforation rates influenced by distance from the reference frame in multilevel registration using a computed tomography-based navigation system in the setting of scoliosis?

Background Context

Pedicle screw fixation is commonly employed for the surgical correction of scoliosis but carries a risk of serious neurovascular or visceral structure events during screw insertion. To avoid these complications, we have been using a computed tomography (CT)-based navigation system during pedicle screw placement. As this could also prolong operation time, multilevel registration for pedicle screw insertion for posterior scoliosis surgery was developed to register three consecutive vertebrae in a single time with CT-based navigation. The reference frame was set either at the caudal end of three consecutive vertebrae or at one or two vertebrae inferior to the most caudal registered vertebra, and then pedicle screws were inserted into the three consecutive registered vertebrae and into the one or two adjacent vertebrae.

Accuracy of lower cervical pedicle screw placement with assistance of distinct navigation systems: a human cadaveric study

Purpose

Evaluate the accuracy of five different techniques for lower cervical pedicle screw placement.

Methods

Forty human cadaveric cervical spines were equally divided into five groups, and each group had eight specimens. Pedicle screws with dia. 3.5 mm were used. Group 1 was blind screw placement without any assistance; Group 2–5 was assisted by the X-ray fluoroscopy, virtual fluoroscopy navigation system, CT-based navigation system, and Iso-C 3D navigation system, respectively. Thereafter, cortical integrity of each pedicle was evaluated by anatomic dissection of the specimens.

Results

A total of 398 pedicle screws were inserted. In the Group 1–5, the average operation time per sample was 27 ± 3.0, 112 ± 10.3, 69 ± 6.4, 98 ± 11.0, and 91 ± 6.0 min, respectively. The outcome for excellent, fair and poor were 29 (36.3 %), 21 (26.2 %) and 30 (37.5 %) in Group 1; 35 (44.9 %), 29 (37.2 %) and 14 (17.9 %) in Group 2; 34 (42.5 %), 36 (45.0 %) and 10 (12.5 %) in Group 3; 70 (87.5 %), 10 (12.5 %) and 0 (0.0 %) in Group 4; 72 (90.0 %), 8 (10.0 %) and 0 (0.0 %) in Group 5.

Conclusions

Blind screw placement was surely unsafe. Lower cervical pedicle screw placement assisted by the CT-based navigation system or the Iso-C 3D navigation system significantly improved the accuracy compared to the fluoroscopy assistance and the virtual fluoroscopy navigation assistance.     

Biomechanical advantage of C1 pedicle screws over C1 lateral mass screws: a cadaveric study

Purpose

The established technique for posterior C1 screw placement is via the lateral mass. Use of C1 monocortical pedicle screws is an emerging technique which utilizes the bone of the posterior arch while avoiding the paravertebral venous plexus and the C2 nerve root. This study compared the relative biomechanical fixation strengths of C1 pedicle screws with C1 lateral mass screws.

Methods

Nine human C1 vertebrae were instrumented with one lateral mass screw and one pedicle screw. The specimens were subjected to sinusoidal, cyclic (0.5 Hz) fatigue loading. Peak compressive and tensile forces started from ±25 N and constantly increased by 0.05 N every cycle. Testing was stopped at 5 mm displacement. Cycles to failure, displacement, and initial and end stiffness were measured. Finally, CT scans were taken and the removal torque measured.

Results

The pedicle screw technique consistently and significantly outperformed the lateral mass technique in cycles to failure (1,083 ± 166 vs. 689 ± 240 cycles), initial stiffness (24.6 ± 3.9 vs. 19.9 ± 3.2 N/mm), end stiffness (16.6 ± 2.7 vs. 11.6 ± 3.6 N/mm) and removal torque (0.70 ± 0.78 vs. 0.13 ± 0.09 N m). Only 33 % of pedicle screws were loose after testing compared to 100 % of lateral mass screws.

Conclusions

C1 pedicle screws were able to withstand higher toggle forces than lateral mass screws while maintaining a higher stiffness throughout and after testing. From a biomechanical point of view, the clinical use of pedicle screws in C1 is a promising alternative to lateral mass screws.     

Combining pedicle screw stimulation with spinal navigation,a protocol to maximize the safety of neural elements and minimize radiation exposure in thoracolumbar spine instrumentation

Purpose

The O-arm-based navigation increases the accuracy of pedicle screw positioning and offers the possibility of performing a 3D scan before wound closure. However, repeating the 3D scan exposes the patient to additional radiation. We combined O-arm navigation with pedicle screw (PS) stimulation followed by a 3D scan to evaluate their accuracy and aimed for the creation of a protocol that maximizes the safety and minimizes radiation.

Methods

Patients had pedicle screws insertion using O-arm spinal navigation, then had PS triggered electromyography (EMG), and finally a 3D scan to evaluate the accuracy of screw position.

Results

447 screws were inserted in 71 patients. In 10 patients, 11 screws needed repositioning. Comparing results of PS triggered EMG responses to the 3D scan, we found: (a) negative stimulation response with negative 3D scan findings, corresponding to 432 acceptable screw position (96.6 %) in 58 patients (81.7 %). In these cases, the redo 3D scan could be avoided. (b) Positive stimulation response with positive 3D scan findings, corresponding to 7 unacceptable screw position (1.5 %) in 6 patients (8.4 %). In these cases, PS stimulation detected malpositioned screws that would be missed without a redo 3D scan.

Conclusion

We propose a protocol of routinely performing PS stimulation after screw insertion using spinal navigation. In case of positive stimulation, a 3D scan must be performed to rule out a probable screw mal position (6 patients 8.4 %). However, in case of negative stimulation, redo 3D scan can be avoided in 81.7 % of patients.

     

Effect of pilot hole on biomechanical and in vivo pedicle screw–bone interface

Purpose

To experimentally study the influence of pilot hole diameter (smaller than or equal to the internal (core) diameter of the screw) on biomechanical (insertion torque and pullout strength) and histomorphometric parameters of screw–bone interface in the acute phase and 8 weeks after pedicle screw insertion.

Methods

Fifteen sheep were operated upon and pedicle screws inserted in the L1–L3 pedicles bilaterally. The pilot hole was smaller (2.0 mm) than the internal diameter (core) of the screw on the left side pedicle and equal (2.8 mm) to the internal diameter (core) of the screw on the right side pedicle. Ten animals were sacrificed immediately (five animals were assigned to pullout strength tests and five animals were used for histomorphometric bone–screw interface evaluation). Five animals were sacrificed 8 weeks after pedicle screw insertion for histomorphometric bone–screw interface evaluation.

Results

The insertion torque and pullout strength were significantly greater in pedicle screws inserted into pilot holes smaller than internal (core) diameter of the screw. Histomorphometric evaluation of bone–screw interface showed that the percentage of bone-implant contact, the area of bone inside the screw thread and the area of bone outside the screw thread were significantly higher for pilot holes smaller than the internal (core) diameter of the screw immediately after insertion and after 8 weeks.

Conclusion

A pilot diameter smaller than the internal (core) diameter of the screw improved the insertion torque and pullout strength immediately after screw insertion as well the pedicle screw–bone interface contact immediately and 8 weeks after screw placement in sheep with good bone mineral density.     

Cervical pedicle screw placement using intraoperative computed tomography imaging with a mobile scanner gantry

Purpose

A multi-detector computed tomography (CT) imaging system with a mobile scanner gantry in the operating room can provide intraoperative reconstructed images with a high resolution. We devised a technique for cervical pedicle screw (CPS) placement using the mobile CT system and evaluated the accuracy of this technique.

Methods

Forty-eight patients who underwent cervical fixation using CPSs were prospectively enrolled in this study. Initial pedicle probing was performed approximately to the depth of the posterior aspect of the vertebral body using fluoroscopic lateral view, and a marking pin was put in place. Intraoperative CT images were obtained to confirm whether the position of the marking pin was accurate. After adequate modification of the trajectory was performed, an appropriately sized CPS was inserted. The accuracy of the CPS was evaluated using postoperative reconstructed CT images, and compared with a historical control group of 22 patients (CPS insertion using only fluoroscopy).

Results

A total of 193 CPSs were inserted. Intraoperative CT images demonstrated that 12.4 % of the initial probings were not accurate, and modification of the trajectory was required. On postoperative CT, 92.7 % of the CPSs were found to be placed accurately: the accuracy was significantly higher than the control group (80.9 %). In the cases using intraoperative CT images, only 1.0 % of the screws were judged to show grade 2 screw misplacement; no neurovascular complications associated with screw placement were observed.

Conclusions

The technique of CPS placement using mobile CT was shown to be safe and effective in preventing catastrophic complications associated with CPS insertion.