Pedicle Cortex Research Articles

Study of pedicle screw placement assisted by intraoperative three-dimensional navigation in lumbar vertebrae with axial rotation

To evaluate the effect of axial rotation of lumbar vertebrae on the accuracy of pedicle screw placement using the traditional method, as well as to assess the value of intraoperative three-dimensional (3D) navigation in improving the accuracy. Sixteen lumbar simulation models at different degrees of axial rotation (0°, 5°, 10° and 20°), with every four assigned with the same degree, were equally divided into two groups (traditional method group and intraoperative 3D navigation group). Random placement of pedicle screws was carried out, followed by CT scan postoperatively. Then the outer pedicle cortex contours were depicted from reconstructed sectional pedicle images using Photoshop. The accuracy of pedicle screw placement was evaluated by determining the interrelationship between screw trajectory and pedicle cortex (qualitative) and measuring the shortest distance from pedicle screw axis to outer cortex of the pedicle (quantitative). Eighty pedicle screws were implanted respectively in each group. In traditional method group, statistical difference existed in the accuracy of pedicle screw placement at different axial rotational degrees (P < 0.05). With degrees increasing, the accuracy declined. The accuracy of intraoperative 3D navigation group was higher than traditional method group in vertebrae with axial rotation (P < 0.01). In qualitative evaluation, the accuracy of the two methods had statistical difference when the degree was 20°, and in quantitative evaluation, statistical difference existed in 5°, 10° and 20° of vertebral axial rotation. Screw malposition can be caused by vertebral axial rotation in lumbar spine using traditional method. Accuracy of pedicle screw placement declines with the increase of axial rotational degrees. However, the accuracy can be improved by using intraoperative 3D navigation.

Improving accuracy and reducing radiation exposure in minimally invasive lumbar interbody fusion

The authors assessed the accuracy of placement of lumbar transpedicular screws by using a computer-assisted, imaged-guided, minimally invasive technique with continuous electromyography (EMG) monitoring. This was a consecutive case series with prospective assessment of procedural accuracy. Forty-seven consecutive patients underwent minimally invasive lumbar interbody fusion and placement of pedicle screws (PSs). A computer-assisted image guidance system involving CT-based images was used to guide screw placement, while EMG continuously monitored the lumbar nerve roots at the operated levels with a 5-mA stimulus applied through the pedicle access needle. All patients underwent CT scanning to determine accuracy of PS placement. All episodes of adjusted screw trajectory based on positive EMG responses were recorded. Pedicle screw misplacement was defined as breach of the pedicle cortex by the screw of more than 2 mm. Two hundred twelve PSs were inserted in 47 patients. The screw misplacement rate was 4.7%. One patient experienced new postoperative radiculopathy resulting from a sacral screw that was too long, with lumbosacral trunk impingement. The trajectory of the pedicle access needle was altered intraoperatively on 20 occasions (9.4% of the PSs) based on positive EMG responses, suggesting that nerve root impingement may have resulted from these screws had the EMG monitoring not been used. The combination of computer-assisted navigation combined with continuous EMG monitoring during pedicle cannulation results in a low rate of PS misplacement, with avoidance of screw positions that might cause neural injury. Furthermore, this technique allows reduction of the radiation exposure for the surgical team without compromising the accuracy of screw placement.

Accuracy of pedicle screw placement in the lumbosacral spine using conventional technique: computed tomography postoperative assessment in 102 consecutive patients

The goal of this study was to determine the incidence of screw misplacement and complications in a group of 102 patients who underwent transpedicle screw fixation in the lumbosacral spine with conventional open technique and intraoperative fluoroscopy. The results are compared with published data. Cases involving 102 consecutive patients (424 inserted screws) were reviewed. Surgery was performed in all cases by the same surgeon's team, using the same implant, and all results were assessed by means of a specific CT protocol. The screw position was assessed by the authors and an independent observer. Screw position was classified as correct when the screw was completely surrounded by the pedicle cortex, as "cortical encroachment" (questionable violation) if the pedicle cortex could not be visualized, and as "frank penetration" when the screw was outside the pedicular boundaries. Frank penetration was further subdivided as minor (when the edge of the screw thread was up to 2.0 mm outside the pedicle cortex), moderate (2.1-4 mm), and severe (> 4 mm). The incidence of intra- and postoperative complications not related to screw position as well as hardware failures were also registered, with a minimum follow-up duration of 8 months. The rate of frank pedicle screw misplacement was 5%. The rate of minimal or questionable pedicle wall violation was 2.8%. Among the frank misplacements, 6 were classified as minor, 12 as moderate, and 3 as severe penetration. Two patients (2%) had radicular pain and neurological deficits (inferomedial and inferolateral minor misplacement at L-4 and L-5, respectively), and 5 patients (4.9%) complained only of radicular pain. At the follow-up examination all patients had completely recovered their neurological function and radicular pain was resolved in all cases. The complications not related to screw malposition were 2 pedicle fractures (2% of patients), 1 nerve root injury (1%), and 1 dural laceration (1%). Five patients (4.8%) had postoperative anemia and required transfusions. Superficial or deep wound infection was noted in 3 patients (2.9%). Late hardware failure occurred in 2 patients (2%). One patient developed adjacent segmental instability and required additional surgery to extend the fusion. Our rates of screw misplacement and complications compare favorably with the lowest rates of the series in which conventional technique was used and are close to the rates reported for image-guided methods. The risk of malpositioning may be reduced with careful preoperative surgical planning, accurate knowledge of the spinal anatomy, surgical experience, and correct indication for conventional surgery. The conventional technique still remains a practical, safe, and effective surgical method for lumbosacral fixation.

Free-hand thoracic pedicle screws placed by neurosurgery residents: a CT analysis

Free-hand thoracic pedicle screw placement is becoming more prevalent within neurosurgery residency training programs. This technique implements anatomic landmarks and tactile palpation without fluoroscopy or navigation to place thoracic pedicle screws. Because this technique is performed by surgeons in training, we wished to analyze the rate at which these screws were properly placed by residents by retrospectively reviewing the accuracy of resident-placed free-hand thoracic pedicle screws using computed tomography imaging. A total of 268 resident-placed thoracic pedicle screws was analyzed using axial computed tomography by an independent attending neuroradiologist. Eighty-five percent of the screws were completely within the pedicle and that 15% of the screws violated the pedicle cortex. The majority of the breaches were lateral breaches between 2 and 4 mm (46%). There was no clinical evidence of neurovascular injury or injury to the esophagus. There were no re-operations for screw replacement. We concluded that under appropriate supervision, neurosurgery residents can safely place free-hand thoracic pedicle screws with an acceptable breach rate.

P159. EMG Thresholds after Thoracic Screw Stimulation are Dependent of Distance from Screw to Spinal Cord, But No of the Pedicle Cortex Integrity: Experimental and Clinic Support

P159. EMG Thresholds after Thoracic Screw Stimulation are Dependent of Distance from Screw to Spinal Cord, But No of the Pedicle Cortex Integrity: Experimental and Clinic Support

Image-based drill templates for cervical pedicle screw placement

Pedicle screws (PSs) in the cervical spine can provide fixation superior to current fixation methods; however, their implementation is challenging due to the small size of the cervical vertebrae. In this study, the authors explored and evaluated the use of patient-specific, rapid prototype drill templates for PS placement in the cervical spine. The goal of the study was to assess the accuracy of a drill template in the placement of cervical PSs at a single vertebral level. Volumetric CT scanning was performed in 4 cadaveric cervical spines. Using computer software, the authors constructed drill templates with a predefined trajectory designed to match the posterior surface of vertebrae C3-7. Twenty physical templates were created from the computer models using a rapid prototyping machine. The drill templates were used to guide pilot-hole drilling, and CT scanning was performed to assess the accuracy of the pilot holes. Pedicle screws (3.5-mm diameter) were placed in the pilot holes. The spines were then dissected to separate the vertebrae, and the trajectory of the screws was visually inspected. The feasibility of this patient-specific, rapid prototyping technique was demonstrated. Imaging and visual inspection of pilot holes and cervical PSs revealed that the majority of trajectories did not violate the pedicle cortex (19 [95%] of 20). The single cortical violation was judged as noncritical and would probably not have resulted in injury to a patient. The potential of drill templates to aid in the placement of cervical PSs is promising. The authors' methods appear to provide an accurate technique and trajectory for PS placement in the cervical spine.

Thoracic Pedicle Screws

Experimental design using cadaveric computerized tomography (CT) scans and a computer-assisted image guidance system to compare various thoracic pedicle screw start points and trajectories. To compare described thoracic pedicle screw start points and trajectories to determine which allows strictly intrapedicular screw placement with the most margin of error. Thoracic pedicle screws are being used in a variety of spinal conditions to include fracture, tumor, and deformity. Optimal thoracic pedicle screw start points have received increasing attention in the literature. Optimal thoracic pedicle trajectory is still undetermined. Using fine cut CT scans of 3 cadaveric male specimens (aged 65-70 years) loaded onto a computer-assisted image guidance system, 966 pedicle screws, were virtually inserted. The effective pedicle diameter (EPD) and maximum insertional arc (MIA) was assessed using 3 different trajectories and start points: (1) straight ahead, (2) straight forward, and (3) anatomic. EPD was measured by placing a maximum-sized virtual screw, using a specific trajectory, without cortical violation of the pedicle and/or the vertebral body. The MIA was assessed by measurement of the angle formed by the most superiorly and inferiorly directed 0.1-mm virtual screw through a given start point without violation of the pedicle cortex and obtaining at least 50% vertebral body purchase. Mean EPD in the sagittal plane was 7.6 +/- 0.3 (SEM) mm for the straight forward trajectory and 9.1 +/- 0.3 (SEM) mm for the anatomic trajectory, a 20% increase (P < 0.0005). Mean EPD in the axial plane was 4.1 +/- 0.2 (SEM) mm for the straight ahead trajectory and 5.0 +/- 0.2 (SEM) mm for the anatomic trajectory, a 22% increase (P < 0.0005). EPD was found to be statistically different based on the trajectory used for placement in both the axial and sagittal planes in the upper (T1-T4), middle (T5-T8), and lower (T9-T12) thoracic spine. Mean MIA in the sagittal plane was 18.7 +/- 1.1 (SEM) for straight ahead start points, 25.8 degrees +/- 0.8 degrees (SEM) for straight forward start points, and 30.2 degrees +/- 0.8 degrees (SEM) for anatomic start points, a 38% increase (P < 0.0005) in MIA compared with straight ahead and a 17% increase (P < 0.0005) in MIA compared with straight forward. Mean MIA in the axial plane was 17.8 degrees +/- 0.6 degrees (SEM) for straight ahead and anatomic start points, and 18.6 degrees +/- 0.6 degrees (SEM) for straight forward start points. This difference was not statistically significant (P = 0.086). MIA was found to be statistically different based on start points used in the sagittal, but not the axial plane, in the upper, middle, and lower thoracic spine. EPD and MIA are trajectory (EPD) and start point (MIA) dependent. In the axial plane, anatomic EPD was greater than straight ahead EPD. In the sagittal plane, anatomic EPD was greater than straight forward EPD. Using anatomic start points in the sagittal plane, a greater MIA is achievable. These data suggest that in the diminutive thoracic pedicle or when a larger screw is needed, an anatomic trajectory using anatomic start points may allow a larger bone channel for intrapedicular placement of instrumentation.

Reply to Letter to the Editor by Pandher and Boparai (MS 08-00749)

Dear Sirs: I appreciate your interest and comments regarding the article by Li et al. [1]. I received numerous comments about this paper. I and the other authors of that article believe more discussion and critical review will aid progress. In response to your concerns, first, delaying mobilization is against our principles. Our postoperative protocol is based on mobilizing the patient as early as possible. Approximately 80% of our patients with Frankel E and D spinal injuries walked within 6 to 8 hours after surgery, and the remaining 20% after 8 to 24 hours. We encourage patients to walk as soon and as much as possible. We also believe that most burst fractures without neural deficits will not need any surgery if patients can stay in bed for 3 months to allow bone and disc healing. We believe short-segment fixation with TpBA augmentation is the gold standard for one burst fracture. We believe long-segment fixation is unnecessary for most burst fractures and we do not use it. Therefore, we would be unable to follow your suggestion to compare results of long-segment fixation with TpBA augmentation. Ideally, a single vertebral body problem should be treated over the lesion site alone; we believe fusion involving an intact vertebra is not logical and unnecessary and should be avoided if possible. I predict the trend to treat burst fractures will be long-segment fixation, then short-segment fixation with anterior support, and finally only vertebral reconstruction. With practice, blood loss and operating time can be limited. After acquiring experience with the TpBA technique, inserting one TpBA is no more than one pedicle screw. We used dynamic views to check the spine, including stability of the TpBA and status of adjacent discs or vertebrae. Finally, the failure in one of our patients occurred during the early learning curve and the pedicle cortex problem was neglected at primary surgery. At revision surgery we noted the inferior cortex of the pedicle had been violated. More than 5000 TpBA vertebroplasties have now been performed at our hospital and such problems with pedicle rupture essentially have been eliminated.

Cortex of the pedicle of the vertebral arch. Part I: deformation characteristics during screw insertion

Elastic deformation has been proposed as a mechanism by which vertebral pedicles can maintain pullout strength when conical screws are backed out from full insertion. The response to the insertion technique may influence both the extent of deformation and the risk of acute fracture during screw placement. The aim of this study was to determine the deformation characteristics of the lumbar pedicle cortex during screw placement. Lumbar pedicles with linear strain gauges attached at the lateral and medial cortices were instrumented using 7.5-mm pedicle screws with or without preconditioning by insertion and removal of 6.5-mm screws. The strains and elastic recoveries of the medial and lateral cortices were determined. Mean medial wall strains tended to be lower than mean lateral wall strains when the 6.5-mm and 7.5-mm screw data were pooled (p = 0.07). After the screws had been removed, 71 to 79% of the deformation at the lateral cortex and 70 to 96% of the deformation at the medial cortex recovered. When inserted first, the 7.5-mm screw caused more plastic deformation at the cortex than it did when inserted after the 6.5-mm screw. Occasional idiosyncratic strain patterns were observed. No gross fracture was observed during screw placement. Screw insertion generated plastic deformation at the pedicle cortex even though the screw did not directly contact the cortex. The lateral and medial cortices responded differently to screw insertion. The technique of screw insertion affected the deformation behavior of the lumbar pedicles. With myriad options for screw selection and placement available, further study is needed before optimal placement parameters can be verified.

Cortex of the pedicle of the vertebral arch. Part II: microstructure

Although the gross anatomy of the pedicle in the human spine has been investigated in great detail, knowledge of the microanatomy of trabecular and cortical structures of the pedicle is limited. An understanding of the mechanical properties and structure of the pedicle bone is essential for improving the quality of pedicle screw placement. To enhance this understanding, the authors examined human cadaveric lumbar vertebrae. In this study, the authors obtained seven human cadaveric lumbar vertebrae. The lateral and medial cortices of these pedicle specimens were sectioned and embedded in polymethylmethacrylate. Cross-sectional slices of cortex were obtained from each specimen and imaged with the aid of a high-resolution light microscope. Assessments of osteonal orientation, determinations of relative dimensions, and histomorphometric studies were performed. The cortex of the pedicle in each human lumbar vertebra had an osteonal structure with haversian canals laid down mainly in the anteroposterior (longitudinal) direction. The organization of osteons across the transverse cross-section was not homogeneous. The layer of lamellar bone that typically envelops cortical bone structures (such as in long bones) was not observed, and the lateral cortex was significantly thinner than the medial cortex (p < 0.05). The cortical bone surrounding the pedicle differed from bone in other anatomical regions such as the anterior vertebral body and femur. The osteonal orientation and lack of a lamellar sheath may account for the unique deformation characteristics of the pedicle cortex seen during pedicle screw placement.

Thoracic Pedicle Screw Insertion in Scoliosis Using Posteroanterior C-arm Rotation Method

Previous researches have emphasized the importance and difficulties in accurate thoracic pedicle screw insertion in scoliosis patients. However, there has been no report on accuracy of the insertion using posteroanterior C-arm fluoroscopy rotated to allow en face visualization of the pedicle in humans. This study aimed to evaluate the accuracy of the thoracic pedicle screw insertion technique using a C-arm fluoroscopy rotation method for the treatment of scoliosis. Between October 1997 and September 2005, 33 scoliosis patients who underwent surgical treatment with a total of 410 screws were analyzed. Eleven were male, 22 female and the mean age was 13.4 years. The mean preoperative Cobb angle was 59.7 degrees. Screws were inserted using the C-arm rotation method; screw positions were evaluated with postoperative computed tomography scans. The mean preoperative Cobb angle of 59.7 degrees was corrected to 18.9 degrees (range, 3 to 45 degrees) in the coronal plane (mean correction rate 68%). Postoperative computed tomography scans demonstrated 48 screws penetrated the medial (9 screws) or lateral (39 screws) pedicle cortex with a mean distance of 3.1 and 3.6 mm, respectively. No screws penetrated the inferior or superior cortex in the sagittal plane. Thoracic pedicle screw insertion in scoliosis patients using the posteroanterior C-arm rotation method allows en face visualization of both pedicles by rotating the C-arm to compensate for the rotational deformity, making it a practical, simple and safe method.

A Prospective Randomized Study of Unilateral Versus Bilateral Instrumented Posterolateral Lumbar Fusion in Degenerative Spondylolisthesis

Prospective randomized study on 82 patients with degenerative lumbar spondylolisthesis, having undergone posterolateral fusion with bilateral or unilateral instrumentation. To determine the effectiveness of unilateral pedicle instrumentation in clinical outcome and rate of union in comparison with the classic bilateral system. Instrumentation has proved to have advantages and disadvantages related to its rigidity. The use of less rigid systems applied to posterior lumbar fusions proved promising according to the results achieved in both experimental and clinical field. Eighty-two patients were randomized into 2 groups: Group 1 (n = 42) had had bilateral instrumentation, and Group 2 (n = 40) had only had unilateral instrumentation. One case from Group 1, L3-S1 dropped out; only fusions of 1 or 2 levels remained in the study. Length of time spent on operating, blood loss, blood transfusion, hospital stay, complications, clinical results measured by SF-36v2, and radiologic assessment of union and of loss of height of adjacent discs were analyzed and compared by means of chi2 test, t test, and Fisher exact test. Statistically, there was no significant difference between the 2 groups in relation to demographics, blood loss, need of transfusion, hospital stay, complications, clinical results, rate of union, and effect on adjacent discs. The operating time needed for Group 2 was significantly shorter in than the time needed for Group 1 (P < 0.001). In Group 1, 3 of 186 screws violated the pedicle cortex requiring reoperation because root irritation versus no complication on a total of 90 screws in Group 2. Unilateral instrumentation used for the treatment of degenerative lumbar spondylolisthesis is as effective as bilateral instrumentation when performed in addition to 1- or 2-level posterolateral fusion. The cost of this method is lower, saves time, and reduces possible risk inserting screws in only one side.

Navigation with Three-dimensional Rotational Radiographic Data for Transpedicular Percutaneous Needle Introduction: Feasibility and Comparison with Fluoroscopic Guidance

To investigate the feasibility of navigation with three-dimensional (3D) rotational radiographic data for transpedicular percutaneous needle introduction and to compare navigation with 3D rotational radiographic data with conventional fluoroscopic guidance. A navigation system was coupled to a 3D rotational radiographic imaging system. In a cadaver study, 60 biopsy needles were introduced into vertebral bodies with fluoroscopic guidance or navigation with 3D rotational radiographic data by two interventionalists with different levels of experience in percutaneous procedures. Radiation exposure, fluoroscopy and introduction times, and needle position were evaluated and compared. For 3D rotational radiographic navigation, the needle position was equivalent for both interventionalists (7 mm). For fluoroscopic guidance, the interventionalist with less experience in percutaneous procedures had significantly more pedicle cortex violations than the other interventionalist (eight vs one). Radiation exposure measured at the operators' hand was lower for 3D rotational radiography-guided needle introductions. Radiation exposure measured at the body wall of the cadaver was equivalent between modalities (9 mGy). Navigation with 3D rotational radiographic image data is feasible for transpedicular percutaneous needle introduction and has two advantages compared with fluoroscopic guidance: it poses less strict requirements on the expertise of the interventionalist, and it reduces radiation exposure to the interventionalist.

Intra-operative fluoroscopy for detecting pedicle screw violation in lumbar vertebrae by guided pin axial position

To develop an applicable and accurate intra-operative fluoroscopical angle to observe the placement of pedicle screw and pedicle violation. Lateral radiographs were taken on eighty lumbar pedicles in eight cadaveric vertebral specimens to measure the sagittal pedicle angles. The pedicles of the vertebral arch were scanned by computed tomography (CT) to measure the transverse pedicle angles. Guided pins and pedicle screws were inserted by awl into the pedicles through the holes drilled in the lumbar pedicles so as to establish 3 types of model: medial violation (n = 29), lateral violation (n = 25), and properly placed screw (n = 26) the anterior and posterior portions of the lumbar pedicle cortex could be identified by the fluoroscopy C-arm. Fluoroscopy with the aid of C-arm was taken on each pedicle in five different directions: lateral position; anteroposterior position; pedicle axial position; guided pin axial position; and pedicle screw axial position. The positions of guided pin and pedicle screw were evaluated. The gold standard was acquired by excising the vertebral plate and observing directly. Nine cases of medial violation were discovered and the accuracy rate was 33.75% by anteroposterior fluoroscopy. The accuracy rate was 95% by the pedicle axial fluoroscopes, including 4 cases of medial inclination but properly placed screws were mistaken for medial violation by pedicle axial fluoroscope. The accuracy rate was 100% by guided pin axial position fluoroscopy, including 29 cases of medial violations, 25 cases of lateral violation, and 26 cases of properly placed screws. The approach of intra-operative fluoroscopy of guided pin axial position is a reliable technology for detecting pedicle violation with the aid of C-arm. The incidence of flexible pedicle screw from human factors can be reduced.

The Clinical Risk of Vertebral Artery Injury From Cervical Pedicle Screws Inserted in Degenerative Vertebrae

Postoperative outcomes of cervical pedicle screw (CPS) placement were evaluated. The screws were inserted in degenerative vertebrae using anatomic landmarks. To evaluate the risks of this procedure caused by misplacements and subsequent complications. The CPS gives superior vertebral fixation but risks causing vertebral artery and spinal cord injury. However, few clinical reports have been published concerning CPS misplacement and subsequent complications. Therefore, the risk of this procedure is still unclear. There were 18 consecutive patients, who submitted to CPS fixation of degenerative vertebrae C2-C6, evaluated using computerized tomography (CT). In 5 of 9 patients in whom the screw critically violated the transverse foramen, viability of the vertebral artery was evaluated using multi-detector row CT angiography. There were no intraoperative vertebral artery or spinal cord injuries and no serious postoperative complications. A total of 86 screws were inserted, and 25 (29%) breached the pedicle cortex. Of them, 84% deviated laterally and violated the transverse foramen. There were 13 screws (15%) that deviated more than 2 mm. However, multi-detector row CT angiography showed the continuity of the vertebral artery in all cases. The deviation rate of CPS placement using anatomic landmarks was higher than anticipated, although no serious complications were encountered. Refinement of the technique (e.g., using a computer-assisted navigation system) is strongly recommended.

P122. The unique microstructure of pedicle cortex facilitates plastic deformation during screw insertion

P122. The unique microstructure of pedicle cortex facilitates plastic deformation during screw insertion

Design of Cervical Pedicle Locator and Three-Dimensional Location of Cervical Pedicle

To develop a high-accuracy, easy-manipulated, and low-cost cervical pedicle three-dimensional locator for guiding cervical pedicle screw placement. To improve the safety of cervical pedicle screw insertion, simplify its manipulation, and offer safety guarantees in clinical application. Because of the complicacy of the anatomic structure of cervical pedicle, clinical application of cervical pedicle screw remained hard to popularize. And recent advances on cervical pedicle location did not simplify the manipulation of procedure significantly. Without aid of locator, distance and angle errors were difficult to control. So far, the only instrument for the location of pedicle "computer-assisted image-guided surgical system" is too expensive for clinical use. Furthermore, it has not yet been documented on location instrument for insertion of cervical pedicle screws. The high-accuracy three-dimensional locator, made of stainless steel was designed to aid cervical pedicle nail placement. The cadavers were inserted nails under guidance of the locator that has been input the corresponding parameters collected from CT images. Results were evaluated by CT scans. A total of 90 nails were inserted into cervical pedicles, 90% of which were found right in the pedicles; 10% had pedicle cortex breaches. Parallel offset error of the locator in cross-sectional and lateral CT image that was 0.56 +/- 0.70 and 1.04 +/- 0.99; angular offset error was 1.69 +/- 2.41 and 6.54 +/- 7.08. Three-dimensional locator of cervical pedicle and its localization technique achieved good results for individuated screws insertion into cervical pedicles, having such advantages as easy manipulation, high accuracy, and low cost. The system presented safety guarantee to the application and advanced the prevalence of pedicle techniques. Its principium and design were demonstrated feasible.

Causes for pedicle screw misplacement in the thoracolumbar spine and countermeasures

目的 探讨胸腰椎椎弓根螺钉误置的原因及对策.方法1996～2002年对293例脊柱疾病患者施行椎弓根螺钉内固定术发生的螺钉误置情况进行回顾性总结、分析.结果293例共置入椎弓根螺钉1256枚,螺钉植入节段错误7例,占2.38%;上胸椎(T2-7)置钉113枚,有11枚螺钉穿破椎弓根皮质,占9.74%;下胸椎(T8-12)置钉261枚,有9枚螺钉穿破椎弓根皮质,占3.45%;腰骶椎置钉882枚,有16枚穿破椎弓根皮质,占1.81%,共计36枚螺钉,占2.87%;螺钉角异常65枚,占5.81%.结论椎弓根螺钉误置与操作技术、解剖学变异及脊柱病损因素密切相关;良好的手术技巧、术前影像资料的认真观测及术中必要的影像监控是准确置钉的关键。

Pedicle screw fixation strength: pullout versus insertional torque

Background context Researchers studying early pedicle screw designs have suggested that pullout strength and insertional torque are correlated. For surgeons using pedicle screws, insertional torque is widely believed to be a good predictor of pullout strength and initial stability of the screw and construct. How appropriate is this assumption when applied to new screw and thread designs? Purpose This study investigated the correlation between insertional torque and pullout strength of three different pedicle screw designs, with different insertional torque characteristics. We hypothesized that a significant increase in insertional torque would indicate a commensurate increase in pullout strength. Study design Biomechanical analysis of instrumented vertebral specimens. Methods Calf lumbar vertebra were prepared and instrumented with one of three pedicle screws. Pilot hole preparation was standardized and coaxial orientation was confirmed by direct inspection. Screws did not penetrate the pedicle cortex or abut or penetrate the anterior vertebral cortex. Any specimen with pedicle wall breach was discarded. The pedicles were instrumented with one of three screws: 1) 7.5×40 mm conical, asymmetric progressive thread (Xia; Stryker Spine, Allendale, NJ), 2) 7.5×40 mm conical with traditional V-shaped thread (Osteonics, Stryker Spine, Allendale, NJ)) or 3) 6.5×40 mm cylindrical with V thread (Osteonics, cylindrical). Paired testing allowed individual screws to be directly compared with a contralateral “control.” Insertional torque and peak torque values were recorded for each rotation up to full insertion. Pullout testing was conducted at a rate of 1 mm/minute. Load-displacement data were recorded at 20 Hz. Stiffness was considered the slope of the most linear part of the curve before the yield point. Results Peak loads for 7.5 conical Xia screws measured 1,783±589.1 N compared with 1,943.0±625.8 N for 7.5 conical Osteonics screws and 1,641.0±356.7 N for 6.5 cylindrical Osteonics screws. The peak insertional torque values were 6.7±1.9 Nm (158% greater than control), 4.5±1.1 Nm (73% greater than control) and 2.6±0.7 Nm, respectively. Insertional torques for Xia screws were significantly greater than conical (p=.001) and cylindrical Osteonics screws (p<.0001), and insertional torques for Osteonics conical screws were significantly greater than those of cylindrical screws (p<.0001). Although pullout loads for the conical Osteonics screws were consistently higher than either the Xia or Osteonics cylindrical screws, the differences were not significant (p>.05). There was no significant correlation between pullout strength and insertional torque (p>.05). Conclusions This unexpected result is best explained by the progressively narrowing flutes of the Xia screw, which compact the trabeculae into a smaller volume as the screw nears full insertion. The trapezoidal threads also increase contact with the cortical surface area and compress trabeculae toward the cortex, thus creating greater friction and higher torque values. This increase in torque did not translate into a commensurate increase in pullout strength, where trabeculae fail in shear.

Transpedicular screwing of the seventh cervical vertebra: anatomical considerations and surgical technique.

The purpose of this study was first to assess the feasibility of C7 transpedicular screwing with a morphological study and secondly to evaluate the safety of such a surgical technique when guided only by posterior landmarks. Eighteen C7 vertebrae, harvested from fresh human cadavers, were included in this study. First the morphometry of C7 pedicle was performed on computed tomography with multiplanar reconstructions. Results of this quantitative anatomy were compared with the literature data. Secondly 30 pedicle screws, whose placement was guided only by anatomical features on the posterior face of the dorsal arch, were inserted in 15 C7 vertebrae. A second computed tomographic examination was done after the surgical procedure to check the screw placement in both planes. The average pedicular width was 6+/-1.2 mm and the average height was 5.8+/-1.1 mm. The pedicle angulation in the transverse plane was 33.3 degrees +/-6.6 degrees, the pedicle angulation in the sagittal plane was 4.3 degrees +/-4.5 degrees downward with reference to the lower endplate of C7. The average distance from the entry point of transpedicular screwing to the anterior cortex of the vertebral body was 29+/-3 mm. Concerning the safety of transpedicular screwing, 63% of screws were found entirely inside the pedicle without any violation of the pedicle cortex. Most of pedicle violations were observed in the transverse plane. No grade II violation of the pedicle was observed. Dimensions of the C7 pedicle are amply compatible with transpedicular fixation using 3.5 mm screws. Such a surgical technique seems to be an interesting option when posterior fixation of C7 is required. Nevertheless morphological guidelines appeared not to be sufficient to ensure safe transpedicular screwing. Laminoforaminotomy is strongly recommended, although it has not been evaluated in this study.