Artery Foramen Research Articles

Preventing Iatrogenic Injury to the Vertebral Artery in Cervical Spine Surgery: A Case Report.

Iatrogenic injury of the vertebral artery (VA) in the context of surgical approaches to the cervical spine has been cited as a relatively rare complication with varying degrees of severity ranging from minimal injury to acute stroke and can result in death. It is estimated that ~50% of cases of iatrogenic VA injury are preventable after postoperatively examining a patient's preoperative imaging. Despite the low incidence (<2%) of this complication, the potential associated morbidity demonstrates the importance of meticulous preoperative vascular consideration. This study aims to emphasize the importance of preoperative vascular screening during anterior cervical access planning visualized through a rare case of a VA aberration in a 47-year-old patient. This study is designed as a retrospective case report. A thorough chart review was performed for this subject including all prior medical records, imaging studies, imaging reports, operative notes, and communication records. The subject was found to have a history of patent foramen ovale (PFO) repair with a rare vertebral artery malformation described as a medial transposition of the vertebral artery out of the vertebral artery foramen between C3 and C4. Consideration of cervical anatomy, particularly the VA, is key to minimizing the risk of adverse surgical outcomes in both anterior and posterior approaches to the cervical spine. During the workup process, the patient's primary care providers, radiologists, and surgeons should be aware of the potential variations of the VA with particular attention given to the course of the VA on MR imaging. Level IV.

C2 Vertebra: An Enigma for Young Spine/Neurosurgeons

Abstract Background Instrumentation of C2 vertebra is considered the most difficult for young neurosurgeons and trainees due to its complex anatomical structures, variety of surgical approaches and techniques, and proximity to important neurovascular structures. Key points from a surgical perspective for midline posterior approach is described in the era of neuroradiological advancements. Method Computed tomography angiographies (CTAs) of a total of 92 patients were evaluated with special attention to the key findings for insertion of screws for craniovertebral junction (CVJ) fixations. All these patients were operated though midline posterior approach in past 4 years. Results CTAs included various CVJ disorders, which included traumatic (n = 14), congenital (n = 55), and rheumatoid arthritis (n = 2) patients. Established landmarks for screw insertion sites do not prove safe for congenital anomalous CVJ conditions. Instead of highlighting screw insertion entry points, part of the corridor, which is relevant, should be stressed up on. Conclusion Midpoint of portion of bone segment medial to vertebral artery foramen should be the focus, which is important for pars interarticularis (and transarticular) and pedicle screws. A laminar screw should cross the midpoint of the lamina on each side.

Imaging study on the safety of axial pedicle screw placement by the position of the screw trajectory tip on the anteroposterior and lateral radiographs

Objective: To propose a method to judge the safety of axial pedicle screw placement based on the position of the tip of the screw trajectory on the anteroposterior and lateral X-ray radiographs. Methods: The cervical CT data of 40 patients admitted to the Second Affiliated Hospital of Wenzhou Medical University from December 2020 to December 2021 were selected, including 24 males and 16 females, with a mean age of (47.6±13.2) years. Based on the three-dimensional model reconstruction of Mimics software and its function of X-ray, the transmission of the axial pedicle screw and its anteroposterior and lateral films was simulated. The position of the tip of the simulated screw trajectory was divided into 5 regions (regions Ⅰ-Ⅴ) from the inside to the outside on the anteroposterior virtual radiographs, and the upper and lower regions (regions a, b) on the lateral virtual radiographs. By adjusting the direction of the screw, the tip of the screw was located in the corresponding 10 regions (80 screws in each area) on the virtual projections of the anteroposterior and lateral virtual radiographs respectively, and its accuracy was analyzed by CT to determine whether each screw penetrated the medial wall of the pedicle or vertebral artery foramen. The anteroposterior and lateral X-rays and postoperative CT data of 34 patients who underwent axial pedicle screw placement (67 axial pedicle screws were placed in total) from January 2014 to December 2021 were collected, including 18 males and 16 females, with a mean age of (45.8±14.1) years. The position of the tip of the screw trajectory on the anteroposterior and lateral films was divided in the same way. The number of screws in the corresponding 10 positions was counted, and CT analysis was used to determine whether each screw penetrated the medial wall of the axial pedicle or the vertebral artery foreman. Results: The results of the imaging simulation screw placement study showed that the perforation rate of the vertebral artery foramen in region Ⅳ and Ⅴ was 75.0% (120/160) and 100% (160/160), respectively, while the perforation rate of the medial wall of the axial pedicle in the region Ⅰ was 85.6%(137/160). The failure rate in regions Ⅱ and Ⅲ was relatively lower, and the performance of simulated screws located in the region a was better than those in region b. The perforation rates of the medial wall in regions (a-Ⅱ) and (a-Ⅲ) was 7.5% (6/80) and 0 (0/80), respectively, and the perforation rates of the vertebral foramen was 0 (0/80) and 21.3% (17/80), respectively. The retrospective imaging study also showed a higher rate of placement failure in regions Ⅰ, Ⅳ and Ⅴ, and relatively lower in regions Ⅱ and Ⅲ. There were total of 15 screws in region a-Ⅱ and a-Ⅲ, and no destruction of the medial wall of the axial pedicle and the vertebral artery foreman occurred there. Conclusions: Regions a-Ⅱ and a-Ⅲ are the "safety areas" of the tip of the pedicle screw trajectory in the axial vertebra. By analyzing the tip of the pedicle screw trajectory on the anteroposterior and lateral radiographs, the operator can determine the reasonable trajectory of axial pedicle screw placement, prevent the injury of the cervical spinal cord and vertebral artery, and reduce the risk of operation.

Brazilian Permian dvinosaurs (Amphibia, Temnospondyli): revised description and phylogeny

ABSTRACT Recently collected temnospondyl fossils from the Cisuralian Pedra de Fogo Formation (north-eastern Brazil) indicate a diverse assemblage of aquatic tetrapods, including the dvinosaurs Timonya anneae and Procuhy nazariensis. Here we present revised diagnoses for these species and detailed descriptions of their holotypes. Timonya anneae is distinguished from all other dvinosaurs by several cranial characters including: combined width of both parietals less than interorbital width; presence of a groove-like internal carotid artery foramen; and presence of an ossified opisthotic. In the mandible, T. anneae presents two parasymphyseal fossae on each side of the mandibular symphysis to accommodate palatal fangs and a well-developed postglenoid area. Procuhy nazariensis, mostly preserved as a mold of the skull table and mandible, is diagnosed by the presence of the anteriormost extension of the squamosal posterior to parietal midlength; pineal foramen posterior to the midlength of the parietal; supratemporal exposed on the posterior border of the skull table; and presence of postglenoid process of the surangular separated from the retroarticular process of the articular by the mandibular sulcus. A new inclusive phylogeny of Temnospondyli indicates that Dvinosauria consists of the Trimerorhachidae (including Procuhy as the sister-taxon of Trimerorhachis) and the ‘short-snouted’ dvinosaurs, with Timonya as an early-diverging representative of the latter clade. The Pedra de Fogo dvinosaurs show close relationships with Cisuralian taxa from the North American southwest, reinforcing close paleogeographic connections between these regions in the late Paleozoic, but represent endemic taxa, corroborating the pattern suggested by the plant fossil record.

Volar and Dorsal Blood Supply to the Lunate: A Cadaveric Study.

Although various studies have proposed vascular and mechanical factors, the etiology of Kienböck disease is unknown. Kienböck theorized that lunatomalacia resulted from traumatic disruption of blood supply and bony nutrition to the lunate. Extraosseous supply to the lunate, as far as volar or dorsal vessels are concerned, is still controversial. This study evaluated the extraosseous nutrient foramina from the dorsal and volar aspects of lunate specimens. A total of 913 specimens from the Hamann-Todd Osteological Collection in Cleveland, Ohio, were examined. The nutrient artery foramina on left and right lunate specimens were examined from dorsal and volar aspects. The number of nutrient artery foramen was tabulated. Age, sex, and race data were collected. Specimens were divided into groups according to the number of nutrient artery foramina, and the dorsal and volar foramina were compared. The average number of foramina on the dorsal aspect of the lunate (1.71) was greater than the volar aspect (1.64), except in specimens younger than 35 years. A greater number of specimens had 3 or more foramina on the dorsal side compared with the volar aspect. Based on this study, there was significant contribution of dorsal arterial vessels to the blood supply of lunate specimens older than 35 years, which contrasts with findings in earlier studies. The disruption of dorsal intercarpal and radiocarpal ligaments leading to the disruption of the dorsal arterial arches may contribute to vascular insufficiency of the lunate and should be evaluated further in the etiopathogenesis of Kienböck disease. [Orthopedics. 2021;44(2):e243-e247.].

Lengthening the nasoseptal flap pedicle with extended dissection into the pterygopalatine fossa.

Releasing the nasoseptal flap (NSF) pedicle from the sphenopalatine artery (SPA) foramen may considerably improve flap reach and surface area. Our objectives were quantify increases in pedicle length and NSF reach through extended pedicle dissection into the pterygopalatine fossa (PPF) through cadaveric dissections and present clinical applications. Anatomical study and retrospective clinical cohort study. Twelve cadaveric dissections were performed. Following standard NSF harvest, the distance from the anterior edge of the flap to the anterior nasal spine while pulling the flap anteriorly was measured. As dissection into the SPA foramen and PPF continued, similar interval measurements were completed in four stages after release from the SPA foramen, release of the internal maxillary artery (IMAX), and transection of the descending palatine artery (DPA). The extended pedicle dissection technique was performed in seven consecutive patients for a variety of different pathologies. The mean length of the NSF from the anterior nasal spine and maximum flap reach were 1.91 ± 0.40 cm/9.3 ± 0.39 cm following standard harvest, 2.52 ± 0.61 cm/9.75±1.06 cm following SPA foramen release, 4.93 ± 0.89 cm/12.16 ± 0.54 cm following full IMAX dissection, and 6.18 ± 0.68 cm/13.41 ± 0.75 cm following DPA transection. No flap dehiscence or necrosis was observed in all seven surgical patients. Extended pedicle dissection of the NSF to the SPA/IMAX markedly improves the potential length and reach of the flap. This technique may provide a feasible option for reconstruction of large anterior skull base and craniocervical junction defects. Seven successful cases are presented here, but further studies with larger series are warranted to validate findings in a clinical setting. 4 Laryngoscope, 130:18-24, 2020.

Individualized transoral anterior surgery for complex craniocervical junction disorder with assistance of 3D printing technology

Objective To evaluate the value of 3D printing technology in transoral anterior surgery for complex craniocervical junction disorder. Methods From December 2009 to December 2013, 48 cases of atlantoaxial dislocation associated with complex craniocervical malformation were operated with the assistance of 3D printing technology. There were 19 males and 29 females, with mean 35 years of age (range, 6 to 59 years). Preoperative complications included 28 cases of atlas assimilation, 19 cases of os odontoideum, 17 cases of C2-3 fusion, 1 case of C2-5 fusion, 1 case of C1 posterior arch disconnection, and 1 case of C1 posterior arch aplasia. All cases had symptoms of spinal compression such as numbness and weakness of the extremity, standing or walking unsteady, etc. Preopoerative cervical MRI examinations showed the enlargement of anterior atlas-dens interval (ADI) or atlantoaxial dislocation. All cases' CT data in Dicom form were input into the Simpleware software to reconstruct 3D image, and then the image in STL form was imported to the 3D printer to make a fast prototyping model for the operation with same size as the cases' cervical spine. Based on above works, all cases underwent transoral anterior reduction plate (TARP) fixation in the craniocervical junction zone, including combined anterior-posterior surgery in 1 case. ADI was measured to evaluate the reduction of atlantoaxial dislocation, cervical-medullary angle (CMA) was measured to evaluate the improvement of compression on the medulla, and Japanese Orthopedic Association (JOA) scoring system was used to evaluate the improvement of spinal function. Results All cases went through successful surgery. Mean operation time was (145±45)min and mean blood loss was (53±15)ml. A total of 96 atlas lateral mass screws, 62 reverse axis pedicle screws and 36 axis vertebral body screws, 1 posterior axis pedicle screw and 1 axis translaminar screw were used. Postoperative CT scans showed all screw trajectories were good except that 4 reverse axis pedicle screws violated into the vertebrae artery foramen due to large outward inclination angle. No neurologic defect was found in all cases. Excellent rate of screw placement was 98%. Postoperative CT reconstruction images showed significant atlantoaxial reduction. ADI improved from (8.7±4.9)mm preoperatively to (2.7±1.9)mm postoperatively (P<0.05). CMA changed from preoperative (123±17)° to postoperative (159±18)° (P<0.05). All cases had obvious improvement of spinal compression symptoms. JOA scores improved from (9.5±1.7)points preoperatively to (15.7±0.9)points at 3 months postoperatively (P<0.05). All cases achieved bone union at a follow-up of 10-58 months. Conclusion 3D printing technology is helpful to the operation plan and manipulation for atlantoaxial dislocation associated with complex craniocervical deformities, which may improve the accuracy and safety during the procedure. Key words: Atlanto-axial joint; Dislocations; Imaging, three-dimensional; Computer-assisted design

Posterior C2 Instrumentation in the Management of Trauma: Accuracy and Risks Associated with Four Techniques

Introduction Posterior instrumentation of C2 is important in treating occipitocervical, atlanto-axial, and subaxial spinal injuries. The variable C2 anatomy can make instrumentation challenging and prone to potentially severe and even life-threatening complications. The goal of this study was to (1) evaluate a large series of posterior C2 screws to determine accuracy as assessed by computed tomography (CT) scan, (2) assess dimensions of “safe bony windows” with CT, and (3) assess the perioperative complication rate related to errant screw placement. We hypothesize that the evolution of a greater variety of options for C2 fixation and the ability to tailor these techniques to patients' specific anatomy has resulted in decreased complications and increased accuracy. Material and Methods A retrospective review of a single Level I trauma center spine database was assessed to identify all patients with operative management for traumatic conditions with C2 instrumentation between December 2002 and September 2008. Radiographic analysis included evaluation of pre- and postoperative CT scans to quantify the patients' bony anatomy as well as to classify the accuracy of C2 screw placement. All C2 screws were graded using the following definitions: Type I: Screw threads completely within the bony cortex. Type II: Less than half the diameter of the screw violates the surrounding cortex. Type III: Clear violation of the transverse foramen or spinal canal. Results A total of 169 patients (99 males and 70 females) underwent posterior C2 screw fixation for traumatic conditions of the spine, including 23 atlanto-occipital dissociations, 16 unstable C1 ring injuries, 79 dens fractures, 9 Hangman fractures, 8 other C2 fractures, 3 C2 hyperextension injuries, 4 C2–C3 flexion–distraction injuries, 14 subaxial injuries, and 19 odontoid nonunions. Average CT measurements of pedicle height, axial width, and laminar width were 8.1 mm (standard deviation 2.1 mm), 5.8 mm (1.9 mm), and 5.7 mm (1.5 mm), respectively, with males having larger pedicle height ( p &lt; 0.001), pedicle width ( p &lt; 0.001), and laminar width ( p &lt; 0.022). A total of 324 screws underwent CT analysis and chart review. A total of 169 pedicle, 85 trans-articular, 50 laminar, and 20 short pars screws were placed with 80.6, 84.4, 96, and 94.4% accuracy rates (Grade I vs. II and III), respectively. Overall 85% of the screws were rated as being in the ideal location. There were no cases of known vertebral artery injury at the time of surgery. Two pedicle screws were unacceptably placed (Grade III). Both patients had screws encroaching on the vertebral artery foramen: one patient had a vertebral artery occlusion and the other had a Grade I dissection documented on CT angiogram without clinical signs of stroke or emboli as measured on transcranial Doppler. No neurological injuries resulted from instrumentation. Conclusion Our findings demonstrate a lower than previously reported incidence of complications associated with posterior C2 screw placement. In the management of 169 patients with 324 screws, only 2 patients had vascular injuries, neither of whom had any sequelae. The multiple techniques of posterior C2 fixation that are currently available allow for unprecedented flexibility in determining which technique is best suited for a given patient's anatomy.

English

BACKGROUND: The 1st cervical vertebra is named as atlas. It differs from all the other vertebrae in having no body and no spine. The atlas consists of two bulky lateral masses, connected to each other in front by a short anterior arch, and behind by a long curved posterior arch it thus forms a ring of bone. The posterior arch forms three-fifths of the atlantal ring. At the root of the arch the superior surface of posterior arch bears a wide groove for the vertebral artery, venous plexus and the C1 nerve immediately behind the lateral mass. In few cases this groove is converted into the completely or partially posterior/lateral vertebral artery foramen. AIM: This study was carried out to know the variations in posterior vertebral artery groove in atlas vertebra for clinical and surgical purpose. MATERIALS AND METHODS: The present study is based on the observation of the 300 macerated Human atlas vertebrae of North Indian present in the Anthropology Museum of Department of Anatomy, GSVM Medical College, Kanpur. The atlases were randomly selected the age and sex were not taken into consideration. RESULTS: We observed the complete posterior vertebral artery foramen in 8% specimen (2.33% bilateral and 5.67% unilateral). In 3.33% it was found on left side and 2.33% on right side. Incidence of incomplete posterior vertebral artery foramen was observed in 9.33% (1.33% bilateral and 8% unilateral). In 5% it was found on left side and 3% on right side. The bilateral presence of complete or incomplete lateral vertebral artery foramen was nil. The unilateral presence of complete or incomplete lateral vertebral artery foramen was found equal in frequency i.e. 0.67%. The unilateral presence of complete lateral vertebral artery foramen on either side was found in 0.33% specimen whereas the incomplete lateral vertebral artery foramen was found only on left side (0.67%). CONCLUSION: The presence of complete/incomplete posterior or lateral vertebral artery foramen may reduce the area of the space for the vertebral artery passing through the vertebral artery groove and may compress the vertebral artery which may result into various types of neurological symptomatology.

The Intraosseous and Extraosseous Vascular Supply of the Fifth Metatarsal

Osteotomies of the fifth metatarsal may disrupt the nutrient artery and result in nonunion. The location of the nutrient artery foramen relative to the location of common osteotomies has not been described. The goal of this study was to describe the vascular supply of the proximal fifth metatarsal, including the artery of origin of the nutrient artery and the location of the nutrient artery foramen. Fifty-six adult cadaver specimens were amputated below the knee. The anterior tibial, posterior tibial, and peroneal arteries were injected with India ink and Ward's Blue Latex. The specimens were frozen for 48 hours and then thawed to room temperature. The soft tissues were débrided with sodium hypochlorite, and the extraosseous vascularity was recorded. The fifth metatarsal was then removed and the intraosseous vascular anatomy elucidated using a modified Spälteholz technique. The dorsalis pedis, posterior tibial, and peroneal arteries branch in predictable patterns to supply the fifth metatarsal. The nutrient artery arose from the fourth plantar metatarsal artery in 100% of specimens and inserted into the plantar medial diaphysis in 83% of specimens. The nutrient artery foramen was an average of 26.8 mm (range, 19-40) from the medial aspect of the base of the fifth metatarsal. When an operative approach to the fifth metatarsal is planned, care should be taken to avoid stripping the bone on the plantar and medial aspects. Osteotomies placed within the proximal 40 mm of the bone carry a risk of disrupting the nutrient artery, resulting in possible nonunion.

Posterior C2 Instrumentation

Retrospective review of a single tertiary care spine database to identify patients with C2 instrumentation between January 2001 and September 2008. (1) Evaluate a large series of posterior C2 screws to determine accuracy by computed tomography (CT) scan, (2) assess dimensions of "safe bony windows" with CT, and (3) assess perioperative complication rate related to errant screw placement. The variable C2 anatomy can make instrumentation challenging and prone to potentially severe complications. New techniques have expanded available options. Clinical data were obtained from the medical record. Radiographic analyses included preoperative and postoperative CT scans to quantify the patients' bone and to classify accuracy of instrumentation. Screws were graded using the following definitions: Seven hundred and thirty-six screws in 383 patients were identified. Fifty-five patients were excluded because of lack of data leaving 328 patients (188 male patients, 140 female patients) with 633 screws. Three hundred and thirty-nine pedicle, 154 transarticular, 63 laminar, and 77 short pars screws were placed, and of the 509 screws with postoperative CT scans, accuracy rates (Types I and II) were 98.8%, 98.5%, 100%, and 94.6%, respectively. Eight were unacceptably placed: two medially and six encroaching on the vertebral artery foramen. One patient had a vertebral artery occlusion and another had a dissection. There were no neurologic injuries. Mean CT measurements of pedicle height, axial width, and laminar width were 8.1, 5.8, and 5.7 mm respectively, with males having significantly larger pedicle height (P<0.001), pedicle width (P<0.001), and laminar width (P<0.022). We show a lower than previously reported incidence of complications associated with posterior C2 screw placement. The multiple techniques of posterior C2 fixation available allow for flexibility in determining ideal technique.

Initial experiences with the Amplatzer Vascular Plug 4 for permanent occlusion of the internal carotid artery in the skull base in patients with head and neck tumors

As the first report in the literature, we present our initial experiences with the latest generation's Amplatzer Vascular Plug 4 (AVP4) as a device for permanent occlusion of the internal carotid artery in its course in the skull base. Permanent occlusion of the internal carotid artery in the skull base was performed in four patients after administration of an intravenous heparin bolus (three patients preoperatively to facilitate radical surgery of head and neck tumors, one patient with a palliative intent to reduce tumor arrosion bleeding risk). In all patients, the deployment of the AVP4 in the artery's course in the skull base was feasible. In two patients, placement was possible in the cavernous segment of the internal carotid artery, and in the other two patients, more proximal in the artery's foramen lacerum and petrous segment, respectively. Occlusion was accomplished in one patient with one AVP4 (occlusion time 25min), in two patients with two AVP4 (occlusion time 20min in each case), and in one patient with one AVP4, three additional fibered coils and two Gelfoam pledgets (occlusion time more than 2h). Complications did not occur in any of the four patients. The AVP4 may be a promising device for permanent occlusion of the internal carotid artery, allowing in its latest generation, navigability as distal as the cavernous segment in some, but not in all cases. Difficulties may still occur in very tortuous vessel anatomy.

Iso-C3D navigation assisted pedicle screw placement in deformities of the cervical and thoracic spine

Background:Pedicle screw instrumentation of the deformed cervical and thoracic spine is challenging to even the most experienced surgeon and associated with increased incidence of screw misplacement. Iso-C3D based navigation has been reported to improve the accuracy of pedicle screw placement, however, there are very few studies assessing its efficacy in the presence of deformity. We conducted a study to evaluate the accuracy of Iso-C3D based navigation in pedicle screw fixation in the deformed cervical and thoracic spine.Materials and Methods:We inserted 98 cervical pedicle screws (18 patients) and 242 thoracic pedicle screws (17 patients) using Iso-C3D based navigation for deformities of spine due to scoliosis, ankylosing spondylitis, post traumatic and degenerative disorders. Two independent observers determined and graded the accuracy of screw placement from postoperative computed tomography (CT) scans.Results:Postoperative CT scans of the cervical spine showed 90.8% perfectly placed screws with 7 (7%) grade I pedicle breaches, 2 (2%) grade II pedicle breaches and one anterior cortex penetration (< 2mm). Five lateral pedicle breaches violated the vertebral artery foramen and three medial pedicle breaches penetrated the spinal canal; however, no patient had any neurovascular complications. In the thoracic spine there were 92.2% perfectly placed screws with only six (2%) grade II pedicle breaches, eight (3%) grade I pedicle breaches and five screws (2%) penetrating the anterior or lateral cortex. No neuro-vascular complications were encountered.Conclusion:Iso-C3D based navigation improves the accuracy of pedicle screw placement in deformities of the cervical and thoracic spine. The low incidence of pedicle breach implies increased safety for the patient.

Anatomic Determination of Optimal Entry Point and Direction For C1 Lateral Mass Screw Placement

Anatomic study of C1 osteology using computerized tomography. To define the anatomy of the C1 lateral mass and make recommendations for optimal entry point and screw placement at C1. C1 lateral mass screw fixation is a reliable biomechanical technique that gives equivalent stability to that of Magerl transarticular screw fixation combined with posterior wiring for C1-C2 fusion. Use of a lateral mass screw allows alternative stabilization constructs to the transarticular technique when C2 vertebral artery anatomy is unfavorable. Because the vertebral artery travels lateral to the lateral mass, then crosses medially over the C1 neural arch, it is at risk during instrumentation. Medially, the cord and canal contents are at risk. While the anatomy of the C1 vertebra and lateral mass is well known, specific definition of ideal entry points, screw pathway direction, and dimensions of screws requires further clarification to enable a clinically safe surgical technique. Fifty consecutive patients underwent computerized tomography scans of their cervical spine. Using calibrated scans, measurements were taken to give the average dimensions of the C1 vertebra with a view for insertion of lateral mass screws beneath the posterior arch. The range of anatomic dimensions was examined to assess risk of vertebral artery damage in this population. The average length of screw within the lateral mass is 17.9 mm with 21.5 mm of screw posterior to the lateral mass, necessary to allow rod placement posteriorly. The safest entry point was directly beneath the medial edge of the posterior arch/lamina where it joins the lateral mass. The ideal direction of screw angulation in the sagittal plane was parallel to the posterior arch of C1. In the medial lateral plane, direct anterior placement could be used, but the lateral mass will tolerate 20 degrees of medial angulation from this starting point. The average distance between the vertebral artery foramen laterally and the screw pathway was 8.8 mm using these landmarks, and 5.8 mm from the medial aspect of the lateral mass. The range of anatomic variation was such that 9 lateral masses had a vertebral artery foramen to screw distance of only 3 mm. The vertebral artery was not at risk when these anatomic landmarks were used. C1 lateral mass screws are best placed beneath the posterior arch, parallel with the arch in the sagittal plan. The entry point is the medial border of the neural arch at its junction with the lateral mass. Straight ahead screw direction is safe in the axial plane, but up to 20 degrees of medial angulation will increase the safety margin from the vertebral artery foramen, and this technique avoids vertebral artery damage and optimizes lateral mass screw purchase. We suggest that this is the preferred method of entry into the lateral mass of C1.

Undetected Vertebral Artery Groove and Foramen Violations During C1 Lateral Mass and C2 Pedicle Screw Placement

Prospective consecutive case series. To identify the incidence of undetected vertebral artery groove and foramen violations and arterial injuries during placement of C1 lateral mass and C2 pedicle screws. Although insertion of C1 lateral mass and C2 pedicle screws has become increasingly common, we are unaware of any report that has specifically examined postoperative CT scans with CT angiography to determine the incidence of undetected neurovascular violations during insertion of these screws. Twenty-three consecutive patients were examined after upper cervical screw fixation was performed under fluoroscopic guidance. In all patients, intraoperative clinical findings, fluoroscopic images, and postoperative radiographs were reviewed. Thin-cut CT scans and CT angiography with multiplanar and three-dimensional reconstruction were used to detect cortical breaches and arterial injuries. A total of 85 screws, including 39 C1 lateral mass, 39 C2 pedicle, and 7 C2 laminar screws were inserted. No apparent neurologic or arterial injuries were noted. However, CT scans revealed that 8 C2 pedicle screws (21%) resulted in vertebral artery groove violations. None resulted in arterial injury. Two C1 screws (5%) had errant trajectories. One breached the medial wall of the lateral mass and the other breached the vertebral artery foramen, causing arterial occlusion as seen on CT angiography. None of the violations were detected intraoperatively or on postoperative plain radiographs with the exception of 1 C1 medial breach observed on a postoperative open-mouth view. Although vertebral artery groove and foramen violations were relatively common on postoperative CT scans, none were detected intraoperatively or on postoperative radiographs. Most alarmingly, 1 vertebral artery injury was undetected intraoperatively. Since a vertebral injury can be potentially catastrophic, this study emphasizes the inherent dangers associated with placement of C1 and C2 screws and underscores the importance of careful postoperative evaluation and surgical technique when inserting these screws.

Usefulness of the Nutrient Foramen of Lamina for Insertion of Thoracic Pedicle Screws

A cadaveric study. To determine the utility of the nutrient foramen on the lamina in thoracic pedicle screw fixation. Recent studies have examined the portal of entry for pedicle screws. Unfortunately, there is no distinct anatomic marker, and the portal of entry usually depends on the surgeon's preference. This study investigated the nutrient artery foramen on the lamina from fourth to eighth thoracic vertebrae after partial laminectomy of 7 cadaveric spines. A 2-mm Steinmann pin was inserted as follows: vertical insertion on the right nutrient foramens and a 10-degree caudal on the left nutrient foramens. Radiographs and computed tomography were obtained and the relationship between the Steinmann pin and pedicle were analyzed. There were 44 nutrient artery foramens (63%, 44/70). Among them, 31 foramens (70%, 31/44) were on the right. On the lateral radiographs, the right pins were inserted parallel within the upper end plate and the left pins were inserted within the vertebral body in T4 and T5. For T6 to T8, the right pins were inserted below the upper end plate and the left pins were inserted below the center of the vertebral body. On the computed tomography axial scan, 17 degrees of medialization for T4 to T7 and 5 degrees for T8 was possible when the size of pedicle screw for human is considered. The nutrient artery foramen on the lamina of thoracic vertebrae is useful for inserting the thoracic pedicle screw.

Accuracy of pedicle screw insertion in the cervical spine for internal fixation using frameless stereotactic guidance

Although transpedicular fixation is a biomechanically superior technique, it is not routinely used in the cervical spine. The risk of neurovascular injury in this region is considered high because the diameter of cervical pedicles is very small and their angle of insertion into the vertebral body varies. This study was conducted to analyze the clinical accuracy of stereotactically guided transpedicular screw insertion into the cervical spine. Twenty-seven patients underwent posterior stabilization of the cervical spine for degenerative instability resulting from myelopathy, fracture/dislocation, tumor, rheumatoid arthritis, and pyogenic spondylitis. Fixation included 1-6 motion segments (mean 2.2 segments). Transpedicular screws (3.5-mm diameter) were placed using 1 of 2 computer-assisted guidance systems and lateral fluoroscopic control. The intraoperative mean deviation of frameless stereotaxy was < 1.9 mm for all procedures. No neurovascular complications resulted from screw insertion. Postoperative computed tomography (CT) scans revealed satisfactory positioning in 104 (90%) of 116 cervical pedicles and in all 12 thoracic pedicles. A noncritical lateral or inferior cortical breach was seen with 7 screws (6%). Critical malplacement (4%) was always lateral: 5 screws encroached into the vertebral artery foramen by 40-60% of its diameter; Doppler sonographic controls revealed no vascular compromise. Screw malplacement was mostly due to a small pedicle diameter that required a steep trajectory angle, which could not be achieved because of anatomical limitation in the exposure of the surgical field. Despite the use of frameless stereotaxy, there remains some risk of critical transpedicular screw malpositioning in the subaxial cervical spine. Results may be improved by the use of intraoperative CT scanning and navigated percutaneous screw insertion, which allow optimization of the transpedicular trajectory.

Description of the chimaerid jaw and its phylogenetic origins.

Anatomical delineation of the holocephalan palatoquadrate has proven to be difficult and, so, has been an extensively debated topic as it relates to the evolutionary derivation of jaws, modes of jaw suspension, and the interrelationships of the hondrichthyes (Elasmobranchii and Holocephali). Embryological analyses of the chimaerid jaw and cranium are presented to provide an anatomical description of the palatoquadrate in modern chimaerids. The palatoquadrate fuses, anteriorly, to the nasal capsule early in development. This marks the first point of contact between the mandibular arch and cranium. Orbitonasal canal foramina delineate the dorsal palatoquadrate margin. The posteriormost margin is marked by fusion of the upper jaw with trabecular and parachordal cartilages in the region of the efferent eudobranchial artery foramen and by a suborbitally positioned basitrabecular cartilage. This basitrabecula generates a subocular shelf as it fuses medially to the parachordal cartilage and posteriorly to the postorbital wall and cranial otic process. The results of these analyses are related to morphological studies of Paleozoic chondrichthyan fishes, particularly the autodiastylic paraselachians that represent morphological intermediates to selachians and holocephalans. The paraselachian basitrabecular, which was mechanically fundamental to stabilizing the free autodiastylic upper jaw and a hyoid operculum, is shown to correlate with the suborbital basitrabecular of today's chimaerids. Further analyses of both extant and fossil data permit us to conclude that the primordial chondrichthyan palatoquadrate did not extend posteriorly to include a palatoquadrate-derived otic process. Rather, the posteriormost extent of this element is primitively found within the limits of the orbit and is demarcated by the highly conserved basitrabecular element. The collective analyses support autodiastyly as the ancestral condition from which all fundamental suspensorial states are derived. J. Morphol. 239:45-59, 1999. © 1999 Wiley-Liss, Inc.

Location of the Vertebral Artery Foramen on the Anterior Aspect of the Lower Cervical Spine by Computed Tomography

Axial computed tomography scans were performed on 14 cadaveric cervical spines to determine the location of the vertebral artery foramen on the anterior aspect of the lower cervical spine. The best scan through the vertebral artery foramen from each level at C3-C6 was selected. Measurements of the vertebral artery foramen included the foramen depth, foramen width, interforaminal distance, the distance from the anterior border of the transverse foramen to the anterior border of the transverse process, the distance from the posterior border of the transverse foramen to the posterior border of the lateral mass, and the distance from the medial border of the vertebral artery foramen to the lateral border of the vertebral body. The results show that the average transverse foramen width was 5.5 +/- 0.4 mm at C3, 5.7 +/- 1.0 mm at C4, 5.9 +/- 0.7 mm at C5, and 5.7 +/- 0.7 mm at C6. The average transverse foramen depth and average interforaminal distance gradually increased from C3 to C6. The distance from the anterior border of the vertebral artery foramen to the anterior border of the transverse process gradually increased from C3 (1.2 +/- 0.4 mm) to C6 (2.7 +/- 0.8 mm) as well. The average distance between the medial border of the transverse foramina and the lateral border of the vertebral body for C3-C6 ranged from 1.8 to 2.2 mm. The average distance between the anterior borders of the vertebral artery foramina and the anterior border of the vertebral body gradually decreased from C3 (8.4 +/- 1.4 mm) to C6 (7.0 +/- 1.6 mm). This study suggested that the lateral border of the vertebral body may be a reliable landmark during anterior cervical decompression. The vertebral artery foramen should be free of violation if vertebrectomy or subtotal vertebrectomy is performed medial to the lateral border of the vertebral body.

The location of the vertebral artery foramen and its relation to posterior lateral mass screw fixation.

This study evaluated the anatomic relationship between the vertebral artery foramen and the posterior midpoint of the cervical lateral mass using cervical spine specimens. To determine quantitatively the location of the vertebral artery foramens from C3 to C6 and their relationship to the posterior midpoints of the lateral masses. Anatomic studies of the cervical nerve root and facet relative to lateral mass screw placement have been addressed. It is necessary to know the correct location of the vertebral artery foramen during lateral mass screw placement to minimize the risk of injury to the vertebral artery. Forty-three cervical spines from C3 to C6 were directly evaluated for this study. Anatomic evaluation included the dimension of the vertebral artery foramen and its projection on the posterior aspect of the lateral mass. The vertical distance from the posterior midpoint of the lateral mass to the posterior border of the vertebral artery foramen, and the angle between the parasagittal plane and the line connecting the posterior midpoint of the lateral mass with the lateral limit of the vertebral artery foramen, were also measured. The vertical distances from the posterior midpoint of the lateral mass to the vertebral artery foramens at C3-C6 averaged from 9.3 to 12.2 mm for male and female specimens. The average angles medial to the sagittal plane, between the parasagittal plane and the line connecting the posterior midpoint of the lateral mass with the lateral limit of the vertebral artery foramen, from C3 to C5, were found to range from 6.0 degrees to 6.3 degrees for male specimens and from 5.3 degrees to 5.5 degrees for female specimens. At C6, the average angles lateral to the sagittal plane, between the parasagittal plane and the line connecting the posterior midpoint of the lateral mass with the lateral limit of the vertebral artery foramen, were 6.4 degrees for male specimens and 5.4 degrees for female specimens. The present study indicated that there is no risk of damaging the vertebral artery if a screw is directed perpendicular to the posterior aspect of the lateral mass at C3-C5 and 10 degrees lateral to the sagittal plane at C6 starting at the midpoint of the lateral mass.