Dual pitch screw design provides equivalent fixation to upsized screw diameter in revision pedicle screw instrumentation: a cadaveric biomechanical study

Background ContextThere are situations that require the replacement of pedicle screws. They are often exchanged when loose or broken or to accommodate a different sized rod or pedicle screw system. Traditionally, pedicle screws are replaced by up-sizing the core diameter until an interference fit is obtained. However, this method carries a risk of pedicle screw breach.PurposeTo determine if dual pitch screws, with cancellous pitch in the vertebral body and cortical pitch throughout the pedicle, allows for in-line screw revision without upsizing screw diameter.Study DesignCadaveric biomechanical StudyPatient SampleNot applicableOutcome MeasuresNot applicableMethodsPedicle screws were tested in the lumbar vertebrae from eleven cadavers. Standard pitch 5.5 mm screws were inserted and loaded using a "break-in" protocol. Screws were removed and replaced with one of four screw types: 5.5 mm Standard Pitch, 5.5 mm Dual Pitch, 6.0 mm Standard Pitch, or 6.0 mm Dual Pitch. Failure testing was done using a stepwise increasing cyclic loading protocol for 100 cycles at each increasing load level. The loading consisted of a combined axial and bending load simulating the load seen by the most inferior screw.ResultsFailure was consistent, with the tip of the screw displacing inferiorly into the vertebral body while simultaneously pulling out. Failure strength was lowest in the 5.5mm Standard (135.8±29.4N) followed by 6.0mm Standard (141.8±38.6N), 5.5mm Dual (158.1±53.8N), and 6.0mm Dual (173.6±52.1N, p=.023). There was no difference in the failure strength between the 5.5mm Dual and 6.0mm Standard. Lumbar level (p=.701) and donor spine (p=.062) were not associated with failure strength.ConclusionsAfter pedicle screw removal, screws with a larger core diameter or with a dual pitch have similar failure strengths. Dual pitch screws may allow for in-line revision of screws without upsizing screw diameter, minimizing the risk of pedicle breach or fracture.Clinical SignificanceDual pitch screws, with cancellous pitch in the vertebral body and cortical pitch through the pedicle, allows for in-line revision of pedicle screws without upsizing screw diameter; reducing the risk of pedicle breach or fracture when exchanging screws.     

Characteristics of immediate and fatigue strength of a dual-threaded pedicle screw in cadaveric spines

Background contextNovel dual-threaded screws are configured with overlapping (doubled) threads only in the proximal shaft to improve proximal cortical fixation.PurposeTests were run to determine whether dual-threaded pedicle screws improve pullout resistance and increase fatigue endurance compared with standard pedicle screws.Study design/settingIn vitro strength and fatigue tests were performed in human cadaveric vertebrae and in polyurethane foam test blocks.Patient sampleSeventeen cadaveric lumbar vertebrae (14 pedicles) and 40 test sites in foam blocks were tested.Outcome measuresMeasures for comparison between standard and dual-threaded screws were bone mineral density (BMD), screw insertion torque, ultimate pullout force, peak load at cyclic failure, and pedicular side of first cyclic failure.MethodsFor each vertebral sample, dual-threaded screws were inserted in one pedicle and single-threaded screws were inserted in the opposite pedicle while recording insertion torque. In seven vertebrae, axial pullout tests were performed. In 10 vertebrae, orthogonal loads were cycled at increasing peak values until toggle exceeded threshold for failure. Insertion torque and pullout force were also recorded for screws placed in foam blocks representing healthy or osteoporotic bone porosity.ResultsIn bone, screw insertion torque was 183% greater with dual-threaded than with standard screws (p<.001). Standard screws pulled out at 93% of the force required to pull out dual-threaded screws (p=.42). Of 10 screws, five reached toggle failure first on the standard screw side, two screws failed first on the dual-threaded side, and three screws failed on both sides during the same round of cycling. In the high-porosity foam, screw insertion torque was 60% greater with the dual-threaded screw than with the standard screw (p=.005), but 14% less with the low-porosity foam (p=.07). Pullout force was 19% less with the dual-threaded screw than with the standard screw in the high-porosity foam (p=.115), but 6% greater with the dual-threaded screw in the low-porosity foam (p=.156).ConclusionsAlthough dual-threaded screws required higher insertion torque than standard screws in bone and low density foam, dual-threaded and standard pedicle screws exhibited equivalent axial pullout and cyclic fatigue endurance. Unlike single-threaded screws, the mechanical performance of dual-threaded screws in bone was relatively independent of BMD. In foam, the mechanical performance of both types of screws was highly dependent on porosity.     

Accuracy of fluoroscopically-assisted pedicle screw placement: analysis of 1,218 screws in 198 patients

Background contextWe retrospectively analyzed a total of 1,218 pedicle screws for accuracy, with postoperative computed tomography (CT), in 198 patients who were operated on between March 2004 and September 2012.PurposeTo determine the incidence of screw misplacement in patients who received a transpedicular screw fixation, with intraoperative fluoroscopy in the lateral and lateral with anteroposterior (AP) positions. The results are compared between the two groups.Study designRetrospective comparative study of accuracy of pedicle screw placement in thoracic and lumbar spine.Patient sampleThe sample consists of 198 consecutive patients who underwent transpedicular screw fixation.Outcome measuresAccuracy of screw placement was evaluated by postoperative CT scan. Misplacement was defined in cases where more than 25% of the screw size was residing outside the pedicle.MethodsThe indications for hardware placement, radiologic studies, patient demographics, and reoperation rates were recorded. Five hundred twenty-eight screws (Group A, n=81) were inserted into the vertebral body with the assistance of lateral fluoroscopy only, whereas 690 screws (Group B, n=117) were inserted with the assistance of lateral fluoroscopy, and the final positions of the screws were checked with AP fluoroscopy.ResultsA total of 1,218 screws were analyzed, with 962 screws placed at the lumbosacral region and 256 screws at the thoracic region. According to the postoperative CT scan, 27 screws (2.2%) were identified as breaching the pedicle. Nineteen of them (3.6%) were in Group A, whereas 8 (1.16%) were in Group B. The rate of pedicle breaches was significantly different between Group A and B (p=.0052). In Group A, the lateral violation of the pedicle was seen in 10 screws (1.9%), whereas medial violation was seen in 9 screws (1.7%). In Group B, the lateral violation of the pedicle was seen in six screws (0.87%), whereas medial violation was seen in two screws (0.29%). The medial and lateral penetration of screws were significantly different between Groups A and B (p<.05). A pedicle breach occurred in 21 patients, and 15 of them underwent a revision surgery to correct the misplaced screw. Of these patients, 11 (13.6%) were in Group A, and 4 (3.4%) were in Group B (p=.0335).ConclusionsIn this study, we evaluated and clarified the diagnostic value of intraoperative fluoroscopy in both the lateral and AP imaging that have not yet been evaluated in any comparative study. We concluded that the intraoperative use of fluoroscopy, especially in the AP position, significantly decreases the risk of screw misplacement and the results are comparable with other advanced techniques.     

The effect of pedicle screw redirection after lateral wall breach—a biomechanical study using human lumbar vertebrae

Background contextCurrently, pedicle screw segmental fixation of the spine is considered a standard of care for a number of conditions. Most surgeons employ a free-hand technique using various intraoperative modalities to improve pedicle screw accuracy. Despite continued improvements in technique, pedicle breach remains a frequent occurrence. Once a breach is detected intraoperatively, the most common corrective maneuver is to medially redirect the pedicle screw into the pedicle. To our knowledge, the biomechanical impact of medially redirecting a pedicle screw after a lateral pedicle breach has not been examined.PurposeTo compare the fixation strength of perfectly placed pedicle screws to the fixation strength of pedicle screws that were correctly placed after having been redirected (RD) following a lateral pedicle breach.Study design/settingA biomechanical study using human lumbar vertebrae.MethodsTen fresh human lumbar vertebrae were isolated from five donors. Each vertebra was instrumented with a monoaxial pedicle screw into each pedicle using two different techniques. On one side, a perfect center-center (CC) screw path was created using direct visualization and fluoroscopy. A 6.0-mm-diameter cannulated tap and a pedicle probe were used to develop the pedicle for the 7.0-mm-diameter by 45-mm-long cannulated pedicle screw, which was placed using a digital torque driver. On the contralateral side, an intentional lateral pedicle wall breach was created at the pedicle-vertebral body junction using a guide wire, a 6.0-mm-diameter cannulated tap, and a pedicle probe. This path was then redirected into a CC position, developed, and instrumented with a 7.0-mm-diameter by 45-mm-long cannulated pedicle screw: the RD screw. For each pedicle screw, we assessed four outcome measures: maximal torque, seating torque, screw loosening, and post-loosening axial pullout. Screw loosening and axial pullout were assessed using an MTS machine.ResultsThe biomechanical cost of a lateral pedicle breach and the requirement to redirect the pedicle screw are as follows: an overall drop of 28% (p<.002) in maximal insertion torque and 25% (p<.049) in seating torque, a drop of 25% (p<.040) in resistance to screw loosening, and a drop in axial pullout force of 11% (p<.047).ConclusionsCompared with a CC lumbar pedicle screw, an RD lumbar pedicle screw placed after a lateral wall breach is significantly weaker in terms of maximal insertional torque, seating torque, screw loosening force, and axial pullout strength. These significant decreases in biomechanical properties are clearly important when RD pedicle screws are placed at the cephalad or caudal end of a long construct. In this situation, augmentation of the RD screw is an option.     

Electromyographic thresholds after thoracic screw stimulation depend on the distance of the screw from the spinal cord and not on pedicle cortex integrity

Background contextPresent studies concerning the safety and reliability of neurophysiological monitoring during thoracic pedicle screw placement remain inconclusive, and therefore, universally validated threshold levels that confirm osseous breakage of the instrumented pedicles have not been properly established.PurposeThe objective of this work was to analyze whether electromyographic (EMG) thresholds, after stimulation of the thoracic pedicle screw, depend on the distance between the neural structures and the screws. The modifier effect of different interposed tissues between a breached pedicle and neural structures was also investigated.Study designThis experimental study uses a domestic pig model.MethodsElectromyographic thresholds were recorded after the stimulation of 18 thoracic pedicle screws that had been inserted into five experimental animals using varying distances between each screw and the spinal cord (8 and 2 mm). Electromyographic thresholds were also registered after the medial pedicle cortex was broken and after different biological tissues were interposed (blood, muscle, fat, and bone) between the screw and the spinal cord.ResultsMean EMG thresholds increased to 14.1±5.5 mA for screws with pedicle cortex integrity that were placed 8 mm away from the dural sac. After the medial pedicle cortex was broken and without varying the distance of the screw to the dural sac, the mean EMG thresholds were not appreciably changed (13.6±6.3 mA). After repositioning the screw at a distance of 2 mm from the spinal cord and after medial cortical breach of the pedicle, the mean threshold significantly slowed to 7.4±3.4 mA (p<.001). When the screw was placed in contact with the spinal dural sac, even lower EMG thresholds were registered (4.9±1.9, p<.001). Medial pedicle cortex rupture and the interposition of different biological tissues in experimental animals did not alter the stimulation thresholds of the thoracic pedicle screws.ConclusionsIn the experimental animals, the observed electrical impedance depended on the distance of screws from the neural structures and not on the integrity of the pedicle cortex. The screw-triggered EMG technique did not reliably discriminate the presence or absence of bone integrity after pedicle screw placement. The response intensity was not related to the type of interposed tissue.     

Robotic and navigated pedicle screws are safer and more accurate than fluoroscopic freehand screws: a systematic review and meta-analysis

BACKGROUND CONTEXTNavigated and robotic pedicle screw placement systems have been developed to improve the accuracy of screw placement. However, the literature comparing the safety and accuracy of robotic and navigated screw placement with fluoroscopic freehand screw placement in thoracolumbar spine surgery has been limited.PURPOSETo perform a systematic review and meta-analysis of randomized control trials that compared the accuracy and safety profiles of robotic and navigated pedicle screws with fluoroscopic freehand pedicle screws.STUDY DESIGN/SETTINGSystematic review and meta-analysisPATIENT SAMPLEOnly randomized controlled trials comparing robotic-assisted or navigated pedicle screws placement with freehand pedicle screw placement in the thoracolumbar spine were included.OUTCOME MEASURESOdds ratio (OR) estimates for screw accuracy according to the Gertzbein-Robbins scale and relative risk (RR) for various surgical complications.METHODSWe systematically searched PubMed and EMBASE for English-language studies from inception through April 7, 2022, including references of eligible articles. The search was conducted according to PRISMA guidelines. Two reviewers conducted a full abstraction of all data, and one reviewer verified accuracy. Information was extracted on study design, quality, bias, participants, and risk estimates. Data and estimates were pooled using the Mantel-Haenszel method for random-effects meta-analysis.RESULTSA total of 14 papers encompassing 12 randomized controlled trials were identified (n=892 patients, 4,046 screws). The pooled analysis demonstrated that robotic and navigated pedicle screw placement techniques were associated with higher odds of screw accuracy (OR 2.66, 95% CI 1.24–5.72, p=.01). Robotic and navigated screw placement was associated with a lower risk of facet joint violations (RR 0.09, 95% CI 0.02–0.38, p<.01) and major complications (RR 0.31, 95% CI 0.11–0.84, p=.02). There were no observed differences between groups in nerve root injury (RR 0.50, 95% CI 0.11–2.30, p=.37), or return to operating room for screw revision (RR 0.28, 95% CI 0.07–1.13, p=.07).CONCLUSIONSThese estimates suggest that robotic and navigated screw placement techniques are associated with higher odds of screw accuracy and superior safety profile compared with fluoroscopic freehand techniques. Additional randomized controlled trials will be needed to further validate these findings.     

The biomechanical effect of pedicle screw hubbing on pullout resistance in the thoracic spine

Background contextThe biomechanical fixation strength afforded by pedicle screws has been strongly correlated with bone mineral density. It has been postulated that “hubbing” the head of the pedicle screw against the dorsal laminar cortex provides a load-sharing effect, thereby limiting cephalocaudad toggling and improving the pullout resistance of the pedicle screw.PurposeTo evaluate the pullout strength (POS) of monoaxial hubbed pedicle screws versus standard fixation in the thoracic spine.Study designBiomechanical investigation.MethodsTwenty-two human cadaveric thoracic vertebrae were acquired and dual-energy X-ray absorptiometry scanned. Osteoporotic (n=16) and normal (n=6) specimens were instrumented with a 5.0×35-mm pedicle screw on one side in a standard fashion. In the contralateral pedicle, 5.0×30-mm screw was inserted with hubbing of the screw into the dorsal lamina. A difference in screw length was used to achieve equivalent depth of insertion. After 2,000 cycles of cephalocaudad toggling, screws were pulled out with the tensile force oriented to the midline of the spine and peak POS measured in newtons (N). Four additional specimens were subjected to microcomputed tomography (micro-CT) analysis to evaluate internal pedicle architecture after screw insertion.ResultsHubbed screws resulted in significantly lower POS (290.5±142.4 N) compared with standard pedicle screws (511.5±242.8 N; p=.00). This finding was evident in both normal and osteoporotic vertebrae based on independent subgroup post hoc analyses (p<.05). As a result of hubbing, half of the specimens fractured through the lamina or superior articular facet (SAF). No fractures occurred on the control side. There was no difference in mean POS for hubbed screws with and without fracture; however, further micro-CT analysis revealed the presence of internal fracture propagation for those specimens that did not have any external signs of failure.ConclusionsHubbing pedicle screws results in significantly decreased POS compared with conventional pedicle screws. Hubbing predisposes toward iatrogenic fracture of the dorsal lamina, transverse process, or SAF during insertion.     

Solid and hollow pedicle screws affect the electrical resistance: A potential source of error with stimulus-evoked electromyography

Background:

Although stimulus evoked electromyography (EMG) is commonly used to confirm the accuracy of pedicle screw placement. There are no studies to differentiate between solid screws and hollow screws to the electrical resistance of pedicle screws. We speculate that the electrical resistance of the solid and hollow pedicle screws may be different and then a potential source of error with stimulus-evoked EMG may happen.

Materials and Methods:

Resistance measurements were obtained from 12 pedicle screw varieties (6 screws of each manufacturer) across the screw shank based on known constant current and measured voltage. The voltage was measured 5 times at each site.

Results:

Resistance of all solid screws ranged from 0.084 Ω to 0.151 Ω (mean =0.118 ± 0.024 Ω) and hollow screws ranged from 0.148 Ω to 0.402 Ω (mean = 0.285 ± 0.081 Ω). There was a significant difference of resistance between the solid screws and hollow screws (P < 0.05). The screw with the largest diameter no matter solid screws or hollow screws had lower resistance than screws with other diameters. No matter in solid screws group or hollow screws group, there were significant differences (P < 0.05) between the 5.0 mm screws and 6.0 mm screws, 6.0 mm screws and 7.0 mm screws, 5.0 mm screws and 7.0 mm screws, 4.5 mm screws and 5.5 mm screws, 5.5 mm screws and 6.5 mm screws, 4.5 mm screws and 6.5 mm screws. The resistance of hollow screws was much larger than the solid screws in the same diameter group (P < 0.05).

Conclusions:

Hollow pedicle screws have the potential for high electrical resistance compared to the solid pedicle screws and therefore may affect the EMG response during stimulus-evoked EMG testing in pedicle screw fixation especially in minimally invasive percutaneous pedical screw fixation surgery.     

Risk of vertebral artery injury: comparison between C1–C2 transarticular and C2 pedicle screws

Background contextTo our knowledge, no large series comparing the risk of vertebral artery injury by C1–C2 transarticular screw versus C2 pedicle screw have been published. In addition, no comparative studies have been performed on those with a high-riding vertebral artery and/or a narrow pedicle who are thought to be at higher risk than those with normal anatomy.PurposeTo compare the risk of vertebral artery injury by C1–C2 transarticular screw versus C2 pedicle screw in an overall patient population and subsets of patients with a high-riding vertebral artery and a narrow pedicle using computed tomography (CT) scan images and three-dimensional (3D) screw trajectory software.Study designRadiographic analysis using CT scans.Patient sampleComputed tomography scans of 269 consecutive patients, for a total of 538 potential screw insertion sites for each type of screw.Outcome measuresCortical perforation into the vertebral artery groove of C2 by a screw.MethodsWe simulated the placement of 4.0 mm transarticular and pedicle screws using 1-mm-sliced CT scans and 3D screw trajectory software. We then compared the frequency of C2 vertebral artery groove violation by the two different fixation methods. This was done in the overall patient population, in the subset of those with a high-riding vertebral artery (defined as an isthmus height ≤5 mm or internal height ≤2 mm on sagittal images) and with a narrow pedicle (defined as a pedicle width ≤4 mm on axial images).ResultsThere were 78 high-riding vertebral arteries (14.5%) and 51 narrow pedicles (9.5%). Most (82%) of the narrow pedicles had a concurrent high-riding vertebral artery, whereas only 54% of the high-riding vertebral arteries had a concurrent narrow pedicle. Overall, 9.5% of transarticular and 8.0% of pedicle screws violated the C2 vertebral artery groove without a significant difference between the two types of screws (p=.17). Among those with a high-riding vertebral artery, vertebral artery groove violation was significantly lower (p=.02) with pedicle (49%) than with transarticular (63%) screws. Among those with a narrow pedicle, vertebral artery groove violation was high in both groups (71% with transarticular and 76% with pedicle screws) but without a significant difference between the two groups (p=.55).ConclusionsOverall, neither technique has more inherent anatomic risk of vertebral artery injury. However, in the presence of a high-riding vertebral artery, placement of a pedicle screw is significantly safer than the placement of a transarticular screw. Narrow pedicles, which might be anticipated to lead to higher risk for a pedicle screw than a transarticular screw, did not result in a significant difference because most patients (82%) with narrow pedicles had a concurrent high-riding vertebral artery that also increased the risk with a transarticular screw. Except in case of a high-riding vertebral artery, our results suggest that the surgeon can opt for either technique and expect similar anatomic risks of vertebral artery injury.     

Freehand technique for C2 pedicle and pars screw placement: is it safe?

Background Context

During placement of C2 pedicle and pars screws, intraoperative fluoroscopy is used so that neurovascular complications can be avoided, and screws can be placed in the proper position. However, this method is time consuming and increases radiation exposure. Furthermore, it does not guarantee a completely safe and accurate screw placement.

比格犬颈前路涂层螺钉的生物力学稳定性研究

目的 了解钛涂层、羟基磷灰石(hydroxyapatite,HA)涂层和钛+HA复合涂层螺钉置入体内后早期的生物力学稳定性。方法 选取成年比格犬32只平均分为4组:普通螺钉对照组(对照组)、HA涂层螺钉组(HA组)、钛涂层螺钉组(钛组)、钛+HA复合涂层螺钉组(复合组)。每组4只用该组相应螺钉固定于C_2~7作为无应力螺钉测试组(无应力组),共测试32枚螺钉,其中16枚螺钉作旋出扭矩测试,16枚螺钉作拔出力测试。记录旋出扭矩峰值、拔出力峰值和拔出能量。每组另4只作C₄/C₅椎间融合后采用该组螺钉+钛板固定于C_4,5椎体上作为应力螺钉测试组(应力组)。每组16枚螺钉作抗拔出力测试。记录螺钉的拔出力峰值和拔出能量。结果 在应力及非应力条件下,各组螺钉旋出扭矩峰值、拔出力峰值、断裂能量比较, HA组>复合组>钛组>对照组。结论 螺钉表面涂层能提高螺钉-骨组织界面的结合力。HA涂层螺钉的生物力学稳定性最好,钛涂层+HA复合涂层其次,提示HA的生物活性在置入早期可能对螺钉的生物力学稳定性起重要作用。     

Pullout resistance of thoracic extrapedicular screws used as a salvage procedure.

BACKGROUND CONTEXT: Extrapedicular screws are placed more laterally than intrapedicular screws and pass through the transverse process or rib head before entering the vertebral body. These screws are sometimes placed to salvage failed pedicle screws, but the change in pullout resistance of extrapedicular screws after salvage has not been quantified. PURPOSE: To quantify the pullout resistance of thoracic extrapedicular screws compared with intrapedicular screws and the pullout resistance of newly inserted screws compared with extrapedicular screws used as salvage for failed intrapedicular screws. STUDY DESIGN: In vitro paired comparison of screw pullout resistance in isolated thoracic vertebrae. METHODS: Tapered monoaxial pedicle screws were inserted in the left or right pedicle of 11 human cadaveric thoracic vertebrae. An extrapedicular screw was inserted on the contralateral side. Both screws were pulled out axially at 0.5 mm/s using a servohydraulic test frame while applied load was recorded. Then a fresh extrapedicular screw was inserted as a salvage screw on the intrapedicular screw side and pulled out. RESULTS: In uncompromised vertebrae, the pullout strength of extrapedicular screws was 80+/-32% of that of intrapedicular screws (p=.073, repeated-measures one-way analysis of variance/Tukey). Salvage screws restored pullout strength to 65+/-30% of that of intrapedicular screws (p=.003). CONCLUSIONS: Extrapedicular screws provided comparable but slightly lower pullout resistance to intrapedicular screws in uncompromised vertebrae. They are therefore a feasible salvage technique when a compromised pedicle precludes reinsertion of an intrapedicular screw, but the salvage screw is significantly weaker than the original screw.     

Tapping insertional torque allows prediction for better pedicle screw fixation and optimal screw size selection

Background contextThere is currently no reliable technique for intraoperative assessment of pedicle screw fixation strength and optimal screw size. Several studies have evaluated pedicle screw insertional torque (IT) and its direct correlation with pullout strength. However, there is limited clinical application with pedicle screw IT as it must be measured during screw placement and rarely causes the spine surgeon to change screw size. To date, no study has evaluated tapping IT, which precedes screw insertion, and its ability to predict pedicle screw pullout strength.PurposeThe objective of this study was to investigate tapping IT and its ability to predict pedicle screw pullout strength and optimal screw size.Study designIn vitro human cadaveric biomechanical analysis.MethodsTwenty fresh-frozen human cadaveric thoracic vertebral levels were prepared and dual-energy radiographic absorptiometry scanned for bone mineral density (BMD). All specimens were osteoporotic with a mean BMD of 0.60±0.07 g/cm². Five specimens (n=10) were used to perform a pilot study, as there were no previously established values for optimal tapping IT. Each pedicle during the pilot study was measured using a digital caliper as well as computed tomography measurements, and the optimal screw size was determined to be equal to or the first size smaller than the pedicle diameter. The optimal tap size was then selected as the tap diameter 1 mm smaller than the optimal screw size. During optimal tap size insertion, all peak tapping IT values were found to be between 2 in-lbs and 3 in-lbs. Therefore, the threshold tapping IT value for optimal pedicle screw and tap size was determined to be 2.5 in-lbs, and a comparison tapping IT value of 1.5 in-lbs was selected. Next, 15 test specimens (n=30) were measured with digital calipers, probed, tapped, and instrumented using a paired comparison between the two threshold tapping IT values (Group 1: 1.5 in-lbs; Group 2: 2.5 in-lbs), randomly assigned to the left or right pedicle on each specimen. Each pedicle was incrementally tapped to increasing size (3.75, 4.00, 4.50, and 5.50 mm) until the threshold value was reached based on the assigned group. Pedicle screw size was determined by adding 1 mm to the tap size that crossed the threshold torque value. Torque measurements were recorded with each revolution during tap and pedicle screw insertion. Each specimen was then individually potted and pedicle screws pulled out “in-line” with the screw axis at a rate of 0.25 mm/sec. Peak pullout strength (POS) was measured in Newtons (N).ResultsThe peak tapping IT was significantly increased (50%) in Group 2 (3.23±0.65 in-lbs) compared with Group 1 (2.15±0.56 in-lbs) (p=.0005). The peak screw IT was also significantly increased (19%) in Group 2 (8.99±2.27 in-lbs) compared with Group 1 (7.52±2.96 in-lbs) (p=.02). The pedicle screw pullout strength was also significantly increased (23%) in Group 2 (877.9±235.2 N) compared with Group 1 (712.3±223.1 N) (p=.017). The mean pedicle screw diameter was significantly increased in Group 2 (5.70±1.05 mm) compared with Group 1 (5.00±0.80 mm) (p=.0002). There was also an increased rate of optimal pedicle screw size selection in Group 2 with 9 of 15 (60%) pedicle screws compared with Group 1 with 4 of 15 (26.7%) pedicle screws within 1 mm of the measured pedicle width. There was a moderate correlation for tapping IT with both screw IT (r=0.54; p=.002) and pedicle screw POS (r=0.55; p=.002).ConclusionsOur findings suggest that tapping IT directly correlates with pedicle screw IT, pedicle screw pullout strength, and optimal pedicle screw size. Therefore, tapping IT may be used during thoracic pedicle screw instrumentation as an adjunct to preoperative imaging and clinical experience to maximize fixation strength and optimize pedicle “fit and fill” with the largest screw possible. However, further prospective, in vivo studies are necessary to evaluate the intraoperative use of tapping IT to predict screw loosening/complications.     

Safety and accuracy of robot-assisted placement of pedicle screws compared to conventional free-hand technique: a systematic review and meta-analysis

BACKGROUND CONTEXTThe introduction and integration of robot technology into modern spine surgery provides surgeons with millimeter accuracy for pedicle screw placement. Coupled with computer-based navigation platforms, robot-assisted spine surgery utilizes augmented reality to potentially improve the safety profile of instrumentation.PURPOSEIn this study, the authors seek to determine the safety and efficacy of robotic-assisted pedicle screw placement compared to conventional free-hand (FH) technique.STUDY DESIGN/SETTINGWe conducted a systematic review of the electronic databases using different MeSH terms from 1980 to 2020.OUTCOME MEASURESThe present study measures pedicle screw accuracy, complication rates, proximal-facet joint violation, intraoperative radiation time, radiation dosage, and length of surgery.RESULTSA total of 1,525 patients (7,379 pedicle screws) from 19 studies with 777 patients (51.0% with 3,684 pedicle screws) in the robotic-assisted group were included. Perfect pedicle screw accuracy, as categorized by Gerztbein-Robbin Grade A, was significantly superior with robotic-assisted surgery compared to FH-technique (Odds ratio [OR]: 1.68, 95% confidence interval [CI]: 1.20–2.35; p=.003). Similarly, clinically acceptable pedicle screw accuracy (Grade A+B) was significantly higher with robotic-assisted surgery versus FH-technique (OR: 1.54, 95% CI: 1.01–2.37; p=.05). Furthermore, the complication rates and proximal-facet joint violation were 69% (OR: 0.31, 95% CI: 0.20–0.48; p<.00001) and 92% less likely (OR: 0.08, 95% CI: 0.03–0.20; p<.00001) with robotic-assisted surgery versus FH-group. Robotic-assisted pedicle screw implantation significantly reduced intraoperative radiation time (MD: ?5.30, 95% CI: ?6.83–3.76; p<.00001) and radiation dosage (MD: ?3.70, 95% CI: ?4.80–2.60; p<.00001) compared to the conventional FH-group. However, the length of surgery was significantly higher with robotic-assisted surgery (MD: 22.70, 95% CI: 6.57–38.83; p=.006) compared to the FH-group.CONCLUSIONThis meta-analysis corroborates the accuracy of robot-assisted pedicle screw placement.     

Ultrasound-based navigated pedicle screw insertion without intraoperative radiation: feasibility study on porcine cadavers

BACKGROUNDNavigation systems for spinal fusion surgery rely on intraoperative computed tomography (CT) or fluoroscopy imaging. Both expose patient, surgeons and operating room staff to significant amounts of radiation. Alternative methods involving intraoperative ultrasound (iUS) imaging have recently shown promise for image-to-patient registration. Yet, the feasibility and safety of iUS navigation in spinal fusion have not been demonstrated.PURPOSETo evaluate the accuracy of pedicle screw insertion in lumbar and thoracolumbar spinal fusion using a fully automated iUS navigation system.STUDY DESIGNProspective porcine cadaver study.METHODSFive porcine cadavers were used to instrument the lumbar and thoracolumbar spine using posterior open surgery. During the procedure, iUS images were acquired and used to establish automatic registration between the anatomy and preoperative CT images. Navigation was performed with the preoperative CT using tracked instruments. The accuracy of the system was measured as the distance of manually collected points to the preoperative CT vertebral surface and compared against fiducial-based registration. A postoperative CT was acquired, and screw placements were manually verified. We report breach rates, as well as axial and sagittal screw deviations.RESULTSA total of 56 screws were inserted (5.50 mm diameter n=50, and 6.50 mm diameter n=6). Fifty-two screws were inserted safely without breach. Four screws (7.14%) presented a medial breach with an average deviation of 1.35±0.37 mm (all <2 mm). Two breaches were caused by 6.50 mm diameter screws, and two by 5.50 mm screws. For vertebrae instrumented with 5.50 mm screws, the average axial diameter of the pedicle was 9.29 mm leaving a 1.89 mm margin in the left and right pedicle. For vertebrae instrumented with 6.50 mm screws, the average axial diameter of the pedicle was 8.99 mm leaving a 1.24 mm error margin in the left and right pedicle. The average distance to the vertebral surface was 0.96 mm using iUS registration and 0.97 mm using fiducial-based registration.CONCLUSIONSWe successfully implanted all pedicle screws in the thoracolumbar spine using the ultrasound-based navigation system. All breaches recorded were minor (<2 mm) and the breach rate (7.14%) was comparable to existing literature. More investigation is needed to evaluate consistency, reproducibility, and performance in surgical context.CLINICAL SIGNIFICANCEIntraoperative US-based navigation is feasible and practical for pedicle screw insertion in a porcine model. It might be used as a low-cost and radiation-free alternative to intraoperative CT and fluoroscopy in the future.     

Transsacral screw fixation for high-grade spondylolisthesis

Background contextSymptomatic high-grade spondylolisthesis (Meyerding III–V) is usually treated by surgery. Recent literature shows that in situ fusion is better than reduction of the slip and fusion in high-grade spondylolisthesis. Furthermore, the outcome is improved if circumferential fusion is performed in severe spondylolisthesis. We have performed a new technique of circumferential fusion in high-grade spondylolisthesis using two transsacral hollow modular anchorage (HMA) screws supplemented with pedicle screw fixation and posterolateral fusion.PurposeThe aim of the study is to analyze the results of circumferential fusion using transsacral HMA screws supplemented with posterolateral fusion and pedicle screw fixation.Study designRetrospective study.Patient sampleTwelve patients with high-grade spondylolisthesis were reviewed.Outcome measuresOutcome was measured using short form 36 (SF-36) and the ability to return to work at the most recent follow-up.MethodsAll patients had interbody fusion using transsacral HMA screws filled with cancellous bone graft and supplemented with pedicle screw instrumentation and posterolateral fusion.ResultsThe male to female ratio was 2:1 with a mean age of 31 years (range 13–54 years). Eleven of 12 patients had disappearance of leg pain. There were no neurological complications in any of them. Circumferential fusion was achieved in all of them at a mean follow-up of 21 months. The average physical function score improved from 22.50±10.34 to 57.50±17.39 (p=.001, 95% confidence interval [CI] ?44.48 to ?25.52), whereas the average pain score improved from 22.22±13.40 to 61.11±15.35 (p=.001, 95% CI ?51.12 to ?26.66).ConclusionsHMA screws avoid the complications associated with autologous cortical fibular strut graft and also are useful to promote interbody fusion, as the hollowness in the screw can be filled with cancellous bone graft that helps in better fusion. Supplementary pedicle screw fixation is necessary to protect the HMA screws, and they together give a stable construct that can achieve a circumferential fusion in high-grade spondylolisthesis.     

Comparison between the accuracy of percutaneous and open pedicle screw fixations in lumbosacral fusion

Background contextIn pedicle screw fixation, accurate insertion is essential to avoid neurological injury or weak stability. The percutaneous pedicle screw system was developed for minimally invasive spine surgery, and its safety has already been reported. However, the accuracy of percutaneous pedicle screw fixation (PPF) has not been compared with that of the open system to date.PurposeTo compare the accuracy of PPF with that of open pedicle screw fixation (open PF) and to investigate the risk factors associated with pedicle wall penetration.Study design/settingA retrospective case series.Patient sampleThe study group included 237 patients who underwent posterior pedicle screw fixation between January 2008 and October 2010 at a single institute with a total of 1,056 pedicle screw fixations completed. One hundred and twenty-six patients with 558 screws underwent open PF and 111 patients with 498 screws underwent PPF.Outcome measuresPostoperative computerized tomography, including sagittal and coronal reformatted images.MethodsConsecutive surgeries with either conventional open PF or PPF for anterior lumbar interbody fusion or transforaminal lumbar interbody fusion were performed. The open pedicle screw employed was from the WSH system (Winova, Seoul, Korea), and the two percutaneous pedicle screw systems were the Sextant (Medtronics, Minneapolis, MN, USA) and the Viper systems (DePuy Spine, Raynham, MA, USA). Computed tomography images were evaluated to determine pedicle wall penetration after operation. Severity was classified as mild (<3 mm), moderate (3–6 mm), and severe (≥6 mm), and the direction was assessed as medial, lateral, inferior, and superior.ResultsPedicle wall penetration occurred in 75 patients (13.4%) in the open PF group and 71 patients (14.3%) in the PPF group and was not statistically different between the groups (p=.695). Assessment of the severity of the pedicle wall penetration revealed that minor penetration was the most common (open PF group, 9.7%; PPF group, 10.6%), although the distribution of the degree of severity was not statistically different between the groups (p=.863). A relatively higher incidence of lateral penetration was observed in the open PF group (66.7% vs. 43.7%), whereas medial, superior, and inferior penetrations were higher in the PPF group (p=.033). Other parameters such as age, sex, surgical method, and surgeon factors did not influence the penetration rate, but bone mineral densitometry negatively correlated with the penetration.ConclusionsPedicle wall penetration during screw fixation was not different between the open PF and PPF groups. The lateral, paraspinal, muscle-splitting approach seems to lessen medial wall penetration, especially in the S1 vertebra. Distribution of the direction of penetration differs between the groups, with lateral wall penetration being more prominent in the open PF group. Careful placement of pedicle screws is necessary for a stronger construct because of the high incidence of penetration.     

Ideal starting point and trajectory for C2 pedicle screw placement: a 3D computed tomography analysis using perioperative measurements

Background contextC2 pedicle screws provide stable fixation for posterior cervical fusion. Placing C2 pedicle screws is fraught with risks, and a misplaced screw can result in cortical breach of the pedicle, resulting in injury to the vertebral artery or spinal cord.PurposeWe sought to identify a reproducible starting point and trajectory for C2 pedicle screw placement using three-dimensional (3D) computed tomography (CT) imaging. Our aims included identifying correct cephalad and mediolateral angles used for determining the most accurate trajectory through the C2 pedicle.Study designA radiographic analysis of the anatomy of the C2 pedicle using CT.Patient sampleA random sample of 34 cervical spine CT scans in patients without medical or surgical pathology of the cervical spine.Outcome measuresNormal anatomic measurements made in the axial and sagittal planes of the CT scans. Angles and measures in millimeters were recorded.MethodsThe C2 pedicles were evaluated using CT scanning with a 3D imaging application. The ideal trajectory through each pedicle was plotted. The mediolateral and cephalad angles were measured using the midline sagittal plane and the inferior vertebral body border as references. Other measurements made were the distances through the pedicle and vertebral bodies, and the surface distances along the laminae between the isthmus and the starting point of the chosen trajectories. Other measurements involving the height of the laminae were also made. The mean values, standard deviations, and intraobserver variations are presented.ResultsCT scans from 34 patients were reviewed. The sex of the patient did not predict angle measurements (p=.2038), so combined male and female patient measures are presented. The mean mediolateral angle measured was 29.2°, and the mean cephalad angle was 23.0°. The mean distance along the lamina surface between the isthmus and the starting point was 8.1 mm. The mean distance from the superior border of the lamina to the starting point was 5.7 mm. There were no statistically significant differences between the dataset collected in duplicate by the same observer (p=.74); as such, we present one data analysis on combined data from the two datasets collected.ConclusionIt is possible to determine an ideal trajectory through the C2 pedicle. These measurements may facilitate C2 pedicle screw fixation decreasing the risk of injury to the vertebral artery, spinal cord, or nerve roots. Delineating the individual anatomy in each case with imaging before surgery is recommended.     

Accuracy of upper thoracic pedicle screw placement using three-dimensional image guidance

Background contextPedicle screw malposition rates using conventional techniques have been reported to occur with a frequency of 6% to 41%. The upper thoracic spine (T1–T3) is a challenging area for pedicle screw placement secondary to the small size of the pedicles, the inability to visualize this area with lateral fluoroscopy, and significant consequences for malpositioned screws. We describe our experience placing 150 pedicle screws in the T1–T3 levels using three-dimensional (3D) image guidance.PurposeThe aim of this study was to assess the accuracy of 3D image guidance for placing pedicle screws in the first three thoracic vertebrae.Study designThe accuracy of pedicle screw placement in the first three thoracic vertebrae was evaluated using postoperative thin-section computed tomography (CT) scans of the cervicothoracic region.Patient sampleThirty-four patients who underwent cervicothoracic fusion were included.Outcome measuresRadiological investigation with CT scans was performed during the postoperative period.MethodsThirty-four consecutive patients underwent cervicothoracic instrumentation and fusion for a total of 150 pedicle screws placed in the first three thoracic vertebrae. All screws were placed using 3D image guidance. Medical records and postoperative imaging of the cervicothoracic junction for each patient were retrospectively reviewed. An independent radiologist reviewed the placement of the pedicle screws and assessed for pedicle breach. All cortical violations were reported as Grade 1, 0 to 2 mm; Grade 2, 2 to 4 mm; and Grade 3, greater than 4 mm.ResultsOverall, 140 (93.3%) out of 150 screws were contained solely in the desired pedicle. All 10 pedicle violations were Grade 1. The direction of pedicle violation included three medial, four inferior, two superior, and one minor anterolateral vertebral body. No complication occurred as a result of screw placement or the use of image guidance.ConclusionsUpper thoracic pedicle screw placement is technically demanding as a result of variable pedicle anatomy and difficulty with two-dimensional visualization. This study demonstrates the accuracy and reliability of 3D image guidance when placing pedicle screws in this region. Advantages of this technology in our practice include safe and accurate placement of spinal instrumentation with little to no radiation exposure to the surgeon and operating room staff.