Accuracy Of Screw Insertion Research Articles

Computer tomography assessment of pedicle screw placement in lumbar and sacral spine: comparison between free-hand and O-arm based navigation techniques

Transpedicular screw fixation has been accepted worldwide since Harrington et al. first placed pedicle screws through the isthmus. In vivo and in vitro studies indicated that pedicle screw insertion accuracy could be significantly improved with image-assisted systems compared with conventional approaches. The O-arm is a new generation intraoperative imaging system designed without compromise to address the needs of a modern OR like no other system currently available. The aim of our study was to check the accuracy of O-arm based and S7-navigated pedicle screw implants in comparison to free-hand technique described by Roy-Camille at the lumbar and sacral spine using CT scans. The material of this study was divided into two groups, free-hand group (group I) (30 patients; 152 screws) and O-arm group (37 patients; 187 screws). The patients were operated upon from January to September 2009. Screw implantation was performed during PLIF or TLIF mainly for spondylolisthesis, osteochondritis and post-laminectomy syndrome. The accuracy rate in our work was 94.1% in the free-hand group compared to 99% in the O-arm navigated group. Thus it was concluded that free-hand technique will only be safe and accurate when it is in the hands of an experienced surgeon and the accuracy of screw placement with O-arm can reach 100%.

Comparison of the clinical accuracy of cervical (C2-C7) pedicle screw insertion assisted by fluoroscopy, computed tomography-based navigation, and intraoperative three-dimensional C-arm navigation.

The complicated anatomy of the cervical spine and the variation among pedicles reduces the accuracy and increases the risk of neurovascular complications associated with screw implantation in this region. In this study, we compared the accuracy of cervical (C2-C7) pedicle screw fixation assisted by X-ray fluoroscopy, computed tomography (CT)-based navigation, or intraoperative three-dimensional (3D) C-arm navigation. This prospective cohort study was performed in 82 consecutive patients who underwent cervical pedicle screw fixation. The accuracy of screw insertion was assessed by postoperative CT scan with 3D reconstruction. The accuracy of screw insertion was assessed as: excellent (screw completely within pedicle); acceptable (≤ 1 mm screw outside pedicle cortex); poor (> 1 mm screw outside pedicle cortex). A total of 145 screws were inserted in 24 patients who underwent C-arm fluoroscopy. Of these, 96 screws (66.2%) were excellent, 37 (25.5%) were acceptable, and 12 (8.3%) were poor. One hundred and fifty-nine screws were inserted in 29 patients in the CT-based navigation group. Among these, 141 (88.7%) were excellent, 14 (8.8%) were acceptable, and 4 (2.5%) were poor. A total of 140 screws were inserted in 29 patients in the intraoperative 3D C-arm navigation group, of which 127 (90.7%) were excellent, and 13 (9.3%) were acceptable. No severe or permanent neurovascular complications associated with screw insertion were observed in any patient. CT-based and intraoperative 3D C-arm navigation were similarly accurate, and were both significantly more accurate than C-arm fluoroscopy for guiding cervical pedicle screw fixation. They were able to accurately guide the angle and depth of screw placement using visual 3D images. These two techniques are therefore preferable for high-risk cervical pedicle screw fixation. The ease and convenience of intraoperative 3D C-arm navigation suggests that it may replace virtual-fluoroscopy and CT-based navigation systems in future clinical applications.

Accuracy of percutaneous lumbar pedicle screw placement using the oblique or “owl's-eye” view and novel guidance technology

This study was conducted to assess the in vivo safety and accuracy of percutaneous lumbar pedicle screw placement using the owl's-eye view of the pedicle axis and a new guidance technology system that facilitates orientation of the C-arm into the appropriate fluoroscopic view and the pedicle cannulation tool in the corresponding trajectory. A total of 326 percutaneous pedicle screws were placed from L-3 to S-1 in 85 consecutive adult patients. Placement was performed using simple coaxial imaging of the pedicle with the owl's-eye fluoroscopic view. NeuroVision, a new guidance system using accelerometer technology, helped align the C-arm trajectory into the owl's-eye view and the cannulation tool in the same trajectory. Postoperative fine-cut CT scans were acquired to assess screw position. Medical records were reviewed for complications. Five of 326 screws breached a pedicle cortex—all breaches were less than 2 mm—for an accuracy rate of 98.47%. Five screws violated an adjacent facet joint. All were at the S-1 superior facet and included in a fusion. No screw violated an adjacent mobile facet or disc space. There were no cases of new or worsening neurological symptoms or deficits for an overall clinical accuracy of 100%. The owl's-eye technique of coaxial pedicle imaging with the C-arm fluoroscopy, facilitated by NeuroVision, is a safe and accurate means by which to place percutaneous pedicle screws for degenerative conditions of the lumbar spine. This is the largest series reported to use the oblique or owl's-eye projection for percutaneous pedicle screw insertion. The accuracy of percutaneous screw insertion with this technique meets or exceeds that of other reported clinical series or techniques.

Pedicle screw insertion accuracy with different assisted methods: a systematic review and meta-analysis of comparative studies.

Studies revealed that navigation systems that provided intraoperative assistance might improve pedicle screw insertion accuracy, and also implied that different systems provided different pedicle screw insertion accuracy. A systematic review and meta-analysis was conducted to focus on the pedicle screw insertion accuracy with or without the assistance of image-guided system, and the variance among the different navigation systems. Comparative studies were searched on pedicle screw insertion accuracy between conventional and navigated method, and among different navigation systems. A total of 43 papers, including 28 clinical, 14 cadaveric and 1 model studies, were included in the current study. For clinical articles, there were 3 randomized clinical trials, 4 prospective comparative studies and 21 retrospective comparative studies. The incidence of pedicle violation among computer tomography-based navigation method group was statistically significantly less than that observed among the conventional group (OR 95% CI, in vivo: 0.32-0.60; in vitro: 0.24-0.75 P<0.01). Two-dimensional fluoroscopy-based navigation system (OR 95% CI, in vivo: 0.27-0.48; in vitro: 0.43-0.88 P<0.01) and three-dimension fluoroscopy-based navigation system (OR 95% CI, in vivo: 0.09-0.38; in vitro: 0.09-0.36 P<0.01) also obtained significant reduced screw deviation rate over traditional methods. Between navigated approaches, statistically insignificant individual and pooled RR values were observed for all in vivo subgroups. Pooled estimate of in vitro studies show that computer tomography-based and three-dimension fluoroscopy-based navigation system provided more accurate pedicle screw insertion over two-dimension fluoroscopy-based navigation system. Our review showed that navigation provided a higher accuracy in the placement of pedicle screws compared with conventional methods. The superiority of navigation systems was obvious when they were applied to abnormal spinal structure. Although no strong in vivo evidence has detected significantly different pedicle screw placement accuracy among the three major navigation systems, meta-analysis revealed the variance in pedicle screw insertion accuracy with different navigation methods.

Accuracy and complications of transpedicular C2 screw placement without the use of spinal navigation

The objective of the study was to describe the technique, accuracy of placement and complications of transpedicular C2 screw fixation without spinal navigation. Patients treated by C2 pedicle screw fixations were identified from the surgical log book of the department. Clinical data were extracted retrospectively from the patients' charts. Pedicle screw placement accuracy was assessed on postoperative CT scans according to Gertzbein and Robbins (GRGr). A total of 27 patients were included in the study. The mean age of the patients was 56 +/- 22.0 years; 51.9% of them were female. As much as 17 patients suffered from trauma, 5 of degenerative disease, 3 of inflammations and 2 of metastatic disease. A total of 47 C2 transpedicular screw fixations were performed. The canulated screws were inserted under visual control following the preparation of the superior surface of the isthmus and of the medial surface of the pedicles of the C2. Intraoperative fluoroscopy was additionally used. The postoperative CT findings showed in 55.3% GRGr 1, in 27.7% GRGr 2, in 10.6% GRGr 3, and in 6.3% GRGr 4 pedicle screw insertion accuracy. Screw insertions GRGr 5 were not observed. Screw malpositioning (i.e., GRGr 3 and 4) was significantly associated with thin (<5 mm) pedicle diameters and with surgery for C2 fractures. In the three patients with screw insertions GRGr 4, postoperative angiographies were performed to exclude vertebral artery affections. In one of these three cases, the screw caused a clinically asymptomatic vertebral artery compression. Hardware failures did not occur. In one patient, postoperative pneumonia resulted in the death of the patient. Careful patient selection and surgical technique is necessary to avoid vertebral artery injury in C2 pedicle screw fixation without spinal navigation. A slight opening of the vertebral artery canal (Gertzbein and Robbins grade < or =3) does not seem to put the artery at risk. However, the high rate of misplaced screws when inserted without spinal navigation, despite the fact that no neurovascular injury occurred, supports the use of spinal navigation in C2 pedicle screw insertions.

Image-guided pedicle screw insertion accuracy: a meta-analysis

Improved pedicle screw insertion accuracy has been reported with the assistance of computer tomography-based navigation. Studies also indicated that fluoroscopy-based navigation offers high accuracy and is comparable to CT-based assistance. However, different population characteristics and assessment methods resulted in inconsistent conclusions. We searched OVID, Springer, and MEDLINE databases to conduct a meta-analysis of the published literature specifically looking at accuracy of pedicle screw placement with different navigation methods. Subgroups and descriptive statistics were determined based on the subject type (in vivo or cadaveric), navigational method, and spinal level. A total number of 7,533 pedicle screws were summarised in our database with 6,721 screws accurately inserted into the pedicles (89.22%). Overall, the median placement accuracy for the in vivo CT-based navigation subgroup (90.76%) was higher than that with the use of two-dimensional (2D) fluoroscopy-based navigation (85.48%). We concluded that CT-based navigation could provide a higher accuracy in the placement of pedicle screws for all subgroups presented. In the lumbar level, 2D fluoroscopy-based navigation was comparable with CT-based navigation. Discrepancy between the two navigation types increased in the thoracic level for the in vivo populations, where there was less potential in the use of 2D fluoroscopy-based navigation than CT-based navigation.

Cooperative robotic assistant with drill‐by‐wire end‐effector for spinal fusion surgery

PurposeThe purpose of this paper is to present a surgical robot for spinal fusion and its control framework that provides higher operation accuracy, greater flexibility of robot position control, and improved ergonomics.Design/methodology/approachA human‐guided robot for the spinal fusion surgery has been developed with a dexterous end‐effector that is capable of high‐speed drilling for cortical layer gimleting and tele‐operated insertion of screws into the vertebrae. The end‐effector is position‐controlled by a five degrees‐of‐freedom robot body that has a kinematically closed structure to withstand strong reaction force occurring in the surgery. The robot also allows the surgeon to control cooperatively the position and orientation of the end‐effector in order to provide maximum flexibility in exploiting his or her expertise. Also incorporated for improved safety is a “drill‐by‐wire” mechanism wherein a screw is tele‐drilled by the surgeon in a mechanically decoupled master/slave system. Finally, a torque‐rendering algorithm that adds synthetic open‐loop high‐frequency components on feedback torque increases the realism of tele‐drilling in the screw‐by‐wire mechanism.FindingsExperimental results indicated that this assistive robot for spinal fusion performs drilling tasks within the static regulation errors less than 0.1 μm for position control and less than 0.05° for orientation control. The users of the tele‐drilling reported subjectively that they experienced torque feedback similar to that of direct screw insertion.Research limitations/implicationsAlthough the robotic surgery system itself has been developed, integration with surgery planning and tracking systems is ongoing. Thus, the screw insertion accuracy of a whole surgery system with the assistive robot is to be investigated in the near future.Originality/valueThe paper arguably pioneers the dexterous end‐effector appropriately designed for spinal fusion, the cooperative robot position‐control algorithm, the screw‐by‐wire mechanism for indirect screw insertion, and the torque‐rendering algorithm for more realistic torque feedback. In particular, the system has the potential of circumventing the screw‐loosening problem, a common defect in the conventional surgeon‐operated or robot‐assisted spinal fusion surgery.

Computer.assisted thermoplastic elustomer film localization system for screw insertion to fix posterior column of acetabulum

Objective To assess accuracy and efficacy of the mini-invasive screw insertion for fix-ation of the posterior column of acetabulum by the computer-assisted thermoplastic elastomer film localization system. Methods In a simulated surgical setup, 20 cannulated screws were placed into the posterior column of acetabulum in 10 corpses under the guidance of the computer-assisted thermoplastic elastomer film localization system. The positions of the screws were checked with CR and CT. Results All the 20 screws were in the safe area. Radiation exposure was totally avoided. Conclusion The system can pro-vide precise, effective and safe navigation for screw insertion to fix the posterior column of acetabulum, and markedly prevent the patient and the staff from exposure to the radiation. Key words: Surgery, computer-assisted; Thermoplastic elastomer film localization system; Acetabulum; Screws

Accuracy of Thoracic Vertebral Screw Insertion in Adolescent Idiopathic Scoliosis

A cross-sectional study with axial computed tomography (CT) to compare the accuracy of vertebral screw insertion in the thoracic spine for adolescent idiopathic scoliosis (AIS) between the thoracoscopic and the mini-open thoracotomy approach. To evaluate the safety of vertebral screw placement in anterior instrumentation for thoracic AIS. Thoracoscopic anterior instrumentation has been used with good results for AIS. It is technically demanding especially for the insertion of vertebral screws. The important issue of whether the screws inserted thoracoscopically is as accurate and safe as those inserted through thoracotomy approach has not been well studied. Thirty-one patients with thoracic AIS receiving thoracoscopic or mini-open thoracotomy anterior instrumentation were included in this study. They were divided into Group A and B, respectively. Postoperative sequential CT scanning on the thoracic vertebral screws was carried out. The relative position between screws and the spinal canal, the aorta, and the bicortical purchase were analyzed with CT images. The percentage of screws in good position was defined and further analyzed. Seventy-three and 162 thoracic vertebral screws were inserted in 10 patients in Group A and 21 patients in Group B, respectively. Eighty-nine percent of screw tips in Group A and 80.2% in Group B were distant from the aorta, 89.0% and 87.0% of screws achieved bicortical purchase in Group A and B, respectively. No significant difference was found in all thoracic levels including the upper thoracic, periapical, or lower thoracic vertebrae. Seventy-four percent and 66.7% of screws were in good positions in Group A and B, respectively and there was no statistically significant difference between the 2 groups. The vertebral screws inserted through thoracoscopic approach were as accurate as those inserted through a mini-open thoracotomy approach. The accuracy could be enhanced by using screws with smaller increments, with special attention to the possible migration of aorta with anterior spinal instrumentation.

Clinical accuracy of 3D fluoroscopy–assisted cervical pedicle screw insertion

Cervical pedicle screw (PS) misplacement leads to injury of the spinal cord, nerve root, and vertebral artery. Recently, several investigators reported on the usefulness of a spinal navigation system that improves the accuracy of PS insertion. In this study, the authors assessed the accuracy of cervical pedicle, lateral mass, and odontoid screw insertions placed using a 3D fluoroscopy navigation system, the Iso-C3D unit. In this prospective analysis of the authors' initial 50 cases of 3D fluoroscopy-assisted cervical screw insertion, the authors inserted 176 PSs, 58 lateral mass screws, and 5 odontoid screws into the C1-7 vertebrae. They placed screws using intraoperative acquisition of data by the isocentric C-arm fluoroscope and a computer navigation system. They obtained postoperative fine-cut CT scans in all patients and assessed the accuracy of screw insertion. A PS (>or= 3.5 mm) could be inserted into 24 (63%) of 38 pedicles at the level of C-3, 18 (53%) of 34 pedicles at C-4, 30 (65%) of 46 at C-5, 33 (80%) of 41 at C-6, and 43 (100%) of 43 at C-7. Of 176 PSs inserted into vertebrae between C-2 and C-7, 171 screws (97.2%) were classified as Grade 1 (no pedicle perforation), and 5 screws (2.8%) were classified as Grade 2 (screw perforation of the cortex by up to 2 mm). Clinically significant screw deviation in the present study was considered Grade 3 (screw perforation of the cortex by > 2 mm), and this occurred in 0% of the placements. In this study, the authors were able to correctly insert cervical PSs using the 3D fluoroscopy and navigation system.

Kirschner wire placement in scaphoid bones using fluoroscopic navigation: a cadaver study comparing conventional techniques with navigation

During scaphoid fixation, a pin guide is first inserted along the axis of the scaphoid, and then a cannulated screw is inserted around the pin guide. At least, the pinguide is removed. To verify the position of the pin guide, fluoroscopy is typically used, with the disadvantage of irradiation. Thus, it is impossible to visualize the pin guide in more than one view simultaneously. The goal of this study was to compare two pin guide placement techniques in scaphoid fixation: conventional (CF) vs. fluoroscopic navigation (FN). Eleven upper limbs of cadavers were divided into two groups. The CF group included four scaphoids which were to be fixed with pin guide. The FN group included seven scaphoids which were to be fixed with the same technique under FN. The accuracy of screw insertion in both groups does not differ. In the CF group, the X-ray exposure time is four times higher. The total duration of the surgical procedure is slightly higher in the FN group. We are of the opinion that FN could be applied in clinical practice and could offer significant benefits in the treatment of fixation of the scaphoid.

Computer‐Assisted Surgery for Screw Insertion into the Distal Sesamoid Bone in Horses: An In Vitro Study

To compare the precision of computer-assisted surgery with a conventional technique (CV) using a special guiding device for screw insertion into the distal sesamoid bone in horses. In vitro experimental study. Cadaveric forelimb specimens. Insertion of a 3.5 mm cortex screw in lag fashion along the longitudinal axis of intact (non-fractured) distal sesamoid bones was evaluated in 2 groups (8 limbs each): CV and computer-assisted surgery (CAS). For CV, the screw was inserted using a special guiding device and fluoroscopy, whereas for CAS, the screw was inserted using computer-assisted navigation. The accuracy of screw placement was verified by radiography, computed tomography, and specimen dissection. Surgical precision was better in CAS compared with CV. CAS improves the accuracy of lateromedial screw insertion, in lag fashion, into the distal sesamoid bone. The CAS technique should be considered for improved accuracy of screw insertion in fractures of the distal sesamoid bone.

Reliability of direct probing isthmus of axial vertebrae to defining the screw path in atlas during Magerl technique

To evaluate the accuracy and reliability of atlantoaxial transarticular screw insertion (Magerl's technique) in atlas under lateral fluoroscopic monitoring without anteroposterior view. Seventy-five consecutive patients with atlantoaxial instability, 48 males and 27 females, aged 35.1, were treated by Magerl's technique. The screw path in atlas was achieved by direct probing the isthmus of axial vertebrae then identifying its position through the atlantoaxial joint correlation. Postoperative transoral X ray and CT film were used for analysis of the position of the 150 screws in the mass of atlas. Three areas were delimitated in and around the lateral mass of atlas: area A (inside the joint face), area: B (outside the joint face but still in lateral mass), and area C area (outside the lateral mass) so as to analyze the location of screws. Among the 150 screws 139 (92.7%) were located in area A, 8 in area B(1) and 3 in area B(2) (7.3%), and none in area C. All cases got atlantoaxial union 3 months after operation. The method of "direct probing" is reliable for identifying the path of screw in atlas.

Spinal Reconstruction Using a Cervical Pedicle Screw System

Although the cervical pedicle screw system is considered the most effective instrumentation for posterior cervical spine surgery, reports on clinical application are limited because of the unresolved and inherent risk of neurovascular complications. The purpose of this study is to retrospectively evaluate the accuracy of placement and clinical effectiveness of the cervical pedicle screw. The cases of 26 patients were investigated. The average followup was 20 months. All but one patient had simultaneous decompression and fusion of various extent. In these cases, screw placement, alignment of the fused segment, fusion rate, neurologic improvement, and complications were assessed. Using an image intensifier, 134 screws were inserted. Five screws (3.7%) completely perforated and 10 (7.4%) partially perforated. The complete perforations occurred in the first 10 patients. Except for two patients with metastasis, bony union was obtained in all patients without loss of correction. Neurologic impairment also was improved in all patients, and no complications associated with the cervical pedicle screw were seen. Clinical results of the cervical pedicle screw system were excellent with few complications. However, there is the possibility of screw perforation, which could cause neurovascular complications. Increased accuracy of screw insertion is needed to make this instrumentation safer.

Use of an aiming device in posterior atlantoaxial transarticular screw fixation. Technical note.

Posterior atlantoaxial transarticular screw fixation is an excellent procedure associated with high fusion rates. There is, however, the potential risk of vertebral artery (VA) injury. The authors designed a special aiming device that allows a cannulated screw to be inserted accurately in the most posterior part of the C1-2 joint via the most posterior and medial part of the isthmus of C-2; this screw pathway most safely avoids VA injury. The instruments include an aiming device and a flexible screw-inserting system. The tip of the aiming device is placed on the ridge of the C-2 isthmus just posterior to the atlantoaxial joint. The guide wire should then pass 1 mm below the device tip. The system consists of flexible guide wires, a drill, a tap, and a screwdriver, and the screw is inserted easily via a posterior approach in which the patient's back is not obstructive. Ten patients with atlantoaxial subluxation or osteoarthritis underwent surgery in which the device was used. In all cases, the screws were inserted safely without causing VA injury, although preoperative computerized tomography (CT) reconstructions revealed a high-risk high-riding unilateral VA in three patients. Postoperative CT reconstructions demonstrated that all screws but one were inserted as planned, and successfully cleared the vertebral groove. In conclusion, this newly designed device is practical and useful for the accurate insertion of screws, thus avoiding VA injury during atlantoaxial transarticular screw fixation.

Spinal Navigation in Cervical Fractures—A Preliminary Clinical Study on Judet-Osteosynthesis of the Axis

Objective: To evaluate the accuracy of CT-based and computer-aided screw insertion into the pedicles of the axis using the method of Judet.Materials and Methods: In two patients, four transpedicular implants of C2 were positioned using a computer-guided technique. One patient with iatrogenic destabüization of the posterior structures C3 and C4 and one patient with a hangman's fracture of the axis required pedicular fixation. In addition, intraoperative documentation of the additional time required for the navigation procedures was made. Finally, postoperative CTs of each patient provided further information about transpedicular implant localization.Results: Image-guided implantation of screws was possible in all scheduled pedicles of the axis. In the postoperative CT scans, none of the inserted screws perforated the medial or lateral pedicle. All screws were accurately positioned within the pedicles, and no anterior perforation of the screws into the vertebral foramen was observed.Conclusions: Our initial results using computer-aided implantation of Judet screws showed that these screws were inserted correctly. However, it is important to realize that movements can occur in the vertebral arch and lateral mass during the time between preoperative CT and surgery. This possibility for movement should be taken into account when procedures such as CT-based insertion of image-guided screws into patients with fractures of the isthmus C2 are performed, because damage to neurovascular structures can result. Therefore, this navigation system should only be used by experienced surgeons who can, if necessary, continue the intervention with more conventional techniques. In addition, it is essential that the surgeon have a complete understanding of the principles of the tracking systems to prevent possible misinterpretation of computer-generated information.

Planning and Insertion of Pedicle Screws with Computer Assistance

Transpedicular screw fixation relies on thorough knowledge of the pedicular anatomy and a reliable intraoperative technique. To enhance the safety and accuracy of screw insertion, computer-assisted systems have been introduced. Our in vitro study investigated the potential benefits of such a system for the preoperative planning and the intraoperative visualization. In part 1, the potentially possible range of screw paths (trajectories) through lumbar pedicles was analyzed. In part 2, the accuracy of actual pedicle-hole preparation with and without preoperative planning was assessed. It was shown that, especially in the lower lumbar regions, the possible range of trajectories is considerable, with inclinations of < or = 40 degrees in the transverse plane and a range of angulation in the sagittal plane of > 20 degrees. The computer assistance in preparation of 100 pedicle holes resulted in cortex perforation in only one case. Computer assistance therefore may be used as a valuable tool to minimize the risks of transpedicular screw insertion. It furthermore may assist in determining the desired screw orientation and length and transform this planning into the intraoperative pedicle-hole preparation.

Complexity of the thoracic spine pedicle anatomy

Transpedicular screw fixation provides rigid stabilization of the thoracolumbar spine. For accurate insertion of screws into the pedicles and to avoid pedicle cortex perforations, more precise knowledge of the anatomy of the pedicles is necessary. This study was designed to visualize graphically the surface anatomy and internal architecture of the pedicles of the thoracic spine. Fifteen vertebrae distributed equally among the upper, middle, and lower thoracic regions were used. For the purpose of mapping surface anatomy, each pedicle was cleaned, spray-painted white, and marked with more than 100 fine points. Using an optoelectronic digitizer, three-dimensional coordinates of the marked points and three additional points, representing a coordinate system, were digitized. A solid modeling computer program was used to create three-dimensional surface images of the pedicle. To obtain cross-sectional information, each pedicle was sectioned with a thin diamond-blade saw to obtain four slices, 1 mm in thickness and 0.5 mm apart. The pedicle slices were X-rayed and projected onto a digitizer. The internal and external contours were digitized and converted into graphs by a computer. The pedicles exhibited significant variability in their shape and orientation, not only from region to region within the thoracic spine, but also within the same region and even within the same pedicle. These variations are extremely significant in light of current techniques utilized in transpedicular screw fixation in the thoracic spine. Information documenting the three-dimensional complexity of pedicle anatomy should be valuable for surgeons and investigators interested in spinal instrumentation.