A novel bone cement injector augments Chinese osteoporotic lumbar pedicle screw channel: a biomechanical investigation.

BACKGROUND: The study aimed to (1) create a series of pedicle injectors with different number of holes on the sheath especially for the Chinese elderly patients and (2) further investigate the effects of the injectors on the augmentation of pedicle screw among osteoporotic lumbar pedicle channel. METHODS: This study used the biomechanical test module of polyurethane (Pacific Research Laboratory Corp, USA) to simulate the mechanical properties of human osteoporotic cancellous bone. The bone cement injectors were invented based on anatomical parameters of lumbar pedicle in Chinese elderly patients. Mechanical test experiments were performed on the bone cement injectors according to the three groups, namely, a local augmentation group, a full-length augmentation group, and a control group. The local augmentation group included three subgroups including 4-hole group, 6-hole group, and 8-hole group. All holes were laterally placed. The full-length augmentation group was a straight-hole injector. The control group was defined that pedicle screws were inserted without any cement augmentation. Six screws were inserted in each group and the maximum insertion torque was recorded. After 24 h of injecting acrylic bone cement, routine X-ray and CT examinations were performed to evaluate the distribution of bone cement. The axial pull-out force of screws was tested with the help of the material testing system 858 (MTS-858) mechanical tester. RESULTS: The bone cement injectors were consisted of the sheaths and the steel rods and the sheaths had different number of lateral holes. The control group had the lowest maximum insertion torque as compared with the 4-hole, 6-hole, 8-hole, and straight pore groups (P < 0.01), but the differences between the 4-hole, 6-hole, 8-hole, and straight pore groups were no statistical significance. The control group had the lowest maximum axial pull-out force as compared with the other four groups (P < 0.01). Subgroup analysis showed the 8-hole group (161.35 ± 27.17 N) had the lower maximum axial pull-out force as compared with the 4-hole group (217.29 ± 49.68 N), 6-hole group (228.39 ± 57.83 N), and straight pore group (237.55 ± 35.96 N) (P < 0.01). Bone cement was mainly distributed in 1/3 of the distal end of the screw among the 4-hole group, in the middle 1/3 and distal end of the screw among the 6-hole group, in the proximal 1/3 of the screw among the 8-hole group, and along the long axis of the whole screw body in the straight pore group. It might indicate that the 8-hole and straight-hole groups were more vulnerable to spinal canal cement leakage. After pullout, bone cement was also closely connected with the screw without any looseness or fragmentation. CONCLUSIONS: The bone cement injectors with different number of holes can be used to augment the pedicle screw channel. The pedicle screw augmented by the 4-hole or 6-hole sheath may have similar effects to the straight pore injector. However, the 8-hole injector may result in relatively lower pull-out strength and the straight pore injector has the risks of cement leakage as well as cement solidarization near the screw head.

Treatment of basilar invagination with atlantoaxial dislocation: atlantoaxial joint distraction and fixation with transoral atlantoaxial reduction plate (TARP) without odontoidectomy.

PURPOSE: Although direct transoral decompression and one-stage posterior instrumentation can obtain satisfactory cord decompression for the treatment of basilar invagination with atlantoaxial dislocation, surgical injuries run high as combinative anterior-posterior approaches were necessary. Furthermore, the complications will rise notably when involvement of dens and/or clivus in the decompression necessitates relatively complicated surgical techniques. First initiated in 2005, transoral atlantoaxial reduction plate (TARP) works as an internal fixation for the treatment of basilar invagination with irreducible atlantoaxial dislocation. Therefore, this article aimed to describe several operative experiences about this approach, which has delivered successful decompression, fixation and fusion. METHODS: 21 consecutive patients with basilar invagination underwent the TARP operation. The pre- and postoperative medulla-cervical angles were measured and compared. The JOA scores of spinal cord function were calculated pre- and post-operatively. 20 cases (20/21) were followed up to average 12.5 months. RESULTS: Symptoms of all the 20 cases were relieved in different degrees. The postoperative imaging showed the odontoid processes obtained ideal reduction and the internal fixators were all in good position. The medulla-cervical angle was correctd from an average (± standard deviation) 128.7° + 11.9° (n = 20) before surgery to 156.5° + 8.1° (n = 20) after surgery (P < 0.01). The average preoperative and postoperative Japaneses Orthopedic Association scores were 11.25 (n = 20) and 15.9 (n = 20), respectively, indicating 76 % improvement. Screw-loosening was observed in one patient due to severe osteoporosis. After a revised operation with a TARP in another size, the neurological symptoms showed no obvious improvements. Then the treatment was terminated. CONCLUSIONS: The TARP operation and intra-operative traction could reduce the odontoid process superiorly migrating into the foramen magnum, directly ease the ventral compression of spinal cord, and fix the reduced atlantoaxial joints through a single transoral approach without the need of a posterior operation. In this stury, 21 patients were evaluated and 20 did well with TARP operation. The preliminary clinical result was satisfactory.

Classification and Surgical Treatment of Developmental Spinal Canal Stenosis at Atlas Level: A 15-Case Study.

STUDY DESIGN: A retrospective study. OBJECTIVE: Investigate the diagnosis and surgery strategy for treatment of development spinal canal stenosis (DSSA) at atlas plane based on computerized tography (CT) image characters. SUMMARY OF BACKGROUND DATA: The occurrence of spinal canal stenosis in the atlas plane is relatively rare compared with lower cervical. METHODS: Fifteen patients diagnosed with DSSA were included from 2014 to 2018. They are divided into four subgroups based on the character of CT images: group I (small size atlas), group II (hypertrophy of posterior arch of the atlas [PAA]), group III (incurved of PAA), and group IV (hypertrophy odontoid). RESULTS: There are type I 7, type II 3, type III 2, and group IV 3 in the 15 cases. All the patients received different surgery procedures respectively: (1) posterior arch osteotomy were performed for group I/III//IV without atlantoaxial dislocation, (2) posterior arch resect and replantation were performed for group II, (3) occipital cervical fixation and fusion were added to the patients with associated atlantoaxial dislocation (AAD), (4) a new method of odontoid reduce and atlantoaxial fixation by transoral approach were performed for group IV with associated AAD. All cases underwent surgery successfully which included posterior occipitocervical fixation (OCF) + posterior arch resection (PAR) eight cases, PAR four cases, posterior arch remodeling and re-implantation (PARR) two cases, and Dens remodeling + trans-oral anterior reduction and plate fixation (DR+TARP) one case without severe complications. All patients show different improvement in the symptoms. Japanese orthopaedic association score improved from 9.2 to 14.7 in 1 year follow-up. CONCLUSION: DSSA could be easily diagnosed and divided into four subgroups according to the character of CT image, corresponding surgery strategy could receive a fine clinical result.Level of Evidence: 4.

[An observation on risk of infection in treatment of craniovertebral junction disorders by transoral approach operation].

OBJECTIVE: To study the changes of bacterial flora after a series of preoperative oral disinfection and the postoperative recovery of patients with craniovertebral junction disorders who were treated with transoral approach operations. And to provide a theoretical basis for the prevention of postoperative complications such as infection. METHODS: The clinical data of 20 cases with craniovertebral junction disorders and treated with transoral approach operations between October 2009 and May 2010 were analyzed. There were 8 males and 12 females, aged 2-66 years (median, 34.5 years). According to the classification of American Spinal Injury Association (ASIA)，there were 4 cases of grade B, 8 of grade C, 6 of grade D, and 2 of grade E. The Japanese Orthopedic Association (JOA) score was 10.3±3.0. The mucosa samples of the posterior pharyngeal wall were sent for bacteria culture. These samples were collected by sterile cotton swabs at four crucial points including 3 days before operation/before gargling (T1), 3 days after continuous gargling by chlorhexidine acetate/after anesthesia intubation on the day of operation (T2), after intraoperative cleaning and washing of the mouth (T3), and after intraoperative iodophor immersion for 5-10 minutes (T4). The microflora was stained by means of smear and further counted after an investigation by microscope. The ASIA classification and the JOA scores were applied to evaluate the postoperative nerve function of the patients. A regular reexamination of cervical vertebra with X-ray film, CT, and MRI was conducted after operation to evaluate the reduction of atlantoaxial dislocation, internal fixation position, bone graft fusion, inflammatory lesion, and tumor resection in the craniovertebral junction. RESULTS: After a series of oral disinfection, the mucosa of the posterior pharyngeal wall of all the patients was in a sterile state, which was considered as type Ⅰ incision. All these 20 patients were treated with successful operations, without any intraoperative injury in vertebral artery and spinal cord, or any postoperative complications such as plate loosening, incision infection, or intracranial infection. All the patients were followed up 3-23 months, with an average of 5.15 months. The symptoms such as neck pain, limb numbness and weakness, neural symptoms, etc. were improved to different degrees after operation. The JOA score was improved to 13.4±1.9 at 3 months after operation, showing significant difference when compared with preoperative score ( t=8.677, P=0.000); and the atlantoaxial joints had been fused. At last follow-up, the ASIA grades were improved when compared with those before operation. CONCLUSION: It is safe and effective to cut the posterior pharyngeal muscle layer and implant internal fixation by means of transoral approach in the treatment of craniovertebral junction disorders.

Management of pediatric patients with irreducible atlantoaxial dislocation: transoral anterior release, reduction, and fixation.

OBJECTIVE: Although transoral atlantoaxial reduction plate (TARP) surgery has been confirmed to be safe and effective for adults who have irreducible atlantoaxial dislocation (IAAD) with or without basilar invagination or upper cervical revision surgery, it is rarely used to treat these disorders in children. The authors of this study aimed to report on the use of the anterior technique in treating pediatric IAAD. METHODS: In this retrospective study, the authors identified 8 consecutive patients with IAAD who had undergone surgical reduction at a single institution in the period between January 2011 and June 2104. The patients consisted of 5 males and 3 females. Three had os odontoideum, 2 had basilar invagination, and the other 3 experienced atlantoaxial rotatory fixed dislocation (AARFD). They were all treated using transoral anterior release, reduction, and fusion with the TARP. Preoperative and postoperative CT scans and MR images were obtained. American Spinal Injury Association (ASIA) Impairment Scale grades were determined. RESULTS: All symptoms were relieved in all 8 patients but to varying degrees. Intraoperative loose reduction and fixation of C1-2 were achieved in one stage. The 4 patients with preoperative neurological deficits were significantly improved after surgery, and their latest follow-ups indicated that their ASIA Impairment Scale grades had improved to E. Postoperative pneumonia occurred in 1 patient but was under complete control after anti-infective therapy and fiber optic-guided sputum suction. CONCLUSIONS: One-stage transoral anterior release, reduction, and fixation is an effective, reliable, and safe means of treating pediatric IAAD. The midterm clinical results are satisfactory, with the technique eliminating the need for interval traction and/or second-stage posterior instrumentation and fusion.

Radiological Evaluation of Craniocervical Region in Patients with Basilar Invagination.

STUDY DESIGN: A retrospective analysis of collected data. OBJECTIVE: Our study aims to present the morphology of cranial-cervical spinal canal in basilar invagination (BI) patients. SUMMARY OF BACKGROUND DATA: BI is characterized by protrusion of the odontoid process into the foramen magnum (C0), leading to compression of the cervicomedullary junction. However, no study has ever clarified the anatomical diameters of spinal canal in patients with BI. METHODS: The study retrospectively examined computed tomography (CT)-based anatomical characteristics in a cohort of 84 patients with and without BI. We measured the anteroposterior diameter (APD) and transversal diameter (TVD) of spinal canal from C0 to C4, together with the area of vertebral canal (Area). Independent samples t test was used for statistical analysis. RESULTS: The APD in the BI group was shorter than the control group from C0 to C2 (C0: 27.98 vs. 35.11 mm, P < 0.001; C1: 11.87 vs. 16.91 mm, P < 0.001; C2: 12.91 vs. 14.84 mm, P < 0.001), but it became longer from C3 to C4. The TVD of the BI group was significantly wider from C0 to C3 (C0: 30.59 vs. 28.54 mm, P < 0.001; C1: 31.31 vs. 25.98 mm, P < 0.001; C2: 21.56 vs. 20.40 mm, P = 0.01; C3: 22.45 vs. 21.23 mm, P = 0.013), and it had no significance at C4. The Area showed no difference between the two groups from C1 to C2, but it turned larger at C3 and C4 in BI patients. CONCLUSION: BI patients may have shorter APD from C0 to C2, which could be the leading cause of neurological compression, necessitating decompression on sagittal plane. Below the pathological levels, BI patients have larger spinal canal than general population. LEVEL OF EVIDENCE: 3.

Atlantoaxial Joint Distraction for the Treatment of Basilar Invagination: Clinical Outcomes and Radiographic Evaluation.

OBJECTIVE: To investigate the causes of partial remission in patients with basilar invagination (BI) and irreducible atlantoaxial dislocation (IAAD) treated with transoral atlantoaxial reduction plate (TARP) without odontoidectomy and quantify the distance of odontoid descent. METHODS: Between August 2010 and July 2012, 22 consecutive patients with BI with IAAD who underwent TARP surgery were reviewed. The preoperative and postoperative radiographic parameters were evaluated. Follow-up data and the symptom treatment interval (STI), defined as the interval between the onset of symptoms and surgical treatment, were assessed. Neurological function was evaluated as neurologic improvement, defined as ([Postoperative Japanese Orthopedic Association (JOA) score] - [Preoperative JOA score])/(17 - [Preoperative JOA score]). The patients were assigned to group A (<50%) or group B (≥50%) based on their level of neurologic improvement. RESULTS: All 22 patients improved clinically to varying degrees. The mean preoperative STI was 105.6 ± 67.6 months for group A and 45.3 ± 46.7 months for group B (P < 0.05). There were no significant between-group differences in follow-up (P > 0.05) or with respect to radiographic parameters (P > 0.05). Persistent brainstem compression was observed in 1 patient, whose symptoms were not adequately relieved after revision surgery (transoral odontoidectomy and posterior decompression and fusion). No fixation failure was observed. CONCLUSIONS: Descent of the odontoid process is useful for treating basilar invagination. TARP surgery without odontoidectomy may pull the dens caudally and ventrally to achieve sufficient decompression of the spinal cord. Neurologic improvement may be associated with STI.

The anatomical study of transoral atlantoaxial reduction plate internal fixation.

OBJECTIVE: To study relevant anatomical features of the structures involved in transoral atlanto-axial reduction plate (TARP) internal fixation through transoral approach for treating irreducible atlanto-axial dislocation and providing anatomical basis for the clinical application of TARP. METHODS: Ten fresh craniocervical specimens were microsurgically dissected layer by layer through transoral approach. The stratification of the posterior pharyngeal wall, the course of the vertebral artery, anatomical relationships of the adjacent structures of the atlas and axis, and the closely relevant anatomical parameters for TARP internal fixation were measured. RESULTS: The posterior pharyngeal wall consisted of two layers and two interspaces: the mucosa, prevertebral fascia, retropharyngeal space, and prevertebral space. The range from the anterior edge of the foramen magnum to C(3) could be exposed by this approach. The thickness of the posterior pharyngeal wall was (3.6+/-0.3) mm (ranging 2.9-4.3 mm) at the anterior tubercle of C1, (6.1+/-0.4) mm (ranging 5.2-7.1 mm) at the lateral mass of C(1) and (5.5+/-0.4) mm (ranging 4.3-6.5 mm) at the central part of C(2), respectively. The distance from the incisor tooth to the anterior tubercle of C(1), C(1) screw entry point, and C(2)screw entry point was (82.5+/-7.8) mm (ranging 71.4-96.2 mm), (90.1+/-3.8) mm (ranging 82.2-96.3 mm), and (89.0+/-4.1) mm (ranging 81.3-95.3 mm), respectively. The distance between the vertebral artery at the atlas and the midline was (25.2+/- 2.3) mm (ranging 20.4-29.7 mm) and that between the vertebral artery at the axis and the midline was (18.4+/- 2.6) mm (ranging 13.1-23.0 mm). The allowed width of the atlas and axis for exposure was (39.4+/-2.2) mm (ranging 36.2-42.7 mm) and (39.0+/-2.1) mm (ranging 35.8-42.3 mm), respectively. The distance (a) between the two atlas screw insertion points (center of anterior aspect of C(1) lateral mass) was (31.4+/-3.3) mm (ranging 25.4-36.6 mm). The vertical distance (b) between the line connecting the two C(1) screw entry points and that connecting the two C(2) screw entry points (at the central part of the vertebrae, namely 3-4 mm lateral to the midline of C(2) vertebrae) was (21.3+/-2.7) mm (ranging 19.4-24.3 mm), with an a/b ratio of 1.3-1.5. The screws of TARP had a lateral tilt of 12.2 degrees+/-0.4 degrees(ranging 10.2 degrees-14.6 degrees) at C(1) and a medial tilt of 7.3 degrees+/-0.3 degrees (ranging 5.1 degrees-9.4 degrees) at C(2) relative to the coronal plane. CONCLUSIONS: An atlanto-axial surgery through transoral approach is safe and feasible. This approach is suitable for an anterior TARP internal fixation, and the design of the internal fixation system should be based on the above anatomical data.

Surgical Outcome and Prognostic Analysis of Transoral Atlantoaxial Reduction Plate System for Basilar Invagination: A Voxel-Based Morphometry Study.

BACKGROUND: The use of a transoral atlantoaxial reduction plate (TARP) system is an effective surgical approach for the treatment of basilar invagination. With the aim of improving the therapeutic efficacy of the TARP operation, we conducted a voxel-based morphometric study to quantitatively investigate the descent of the odontoid process and craniocervical volume changes. METHODS: We enrolled 20 patients with basilar invagination who underwent a TARP procedure. Craniocervical computed tomography (CT) scanning and a 3-dimensional (3-D) reconstruction of the craniocervical junction were performed. Craniocervical volumes and odontoid process descent distances were measured preoperatively and postoperatively. Individual neurological function was evaluated according to the Japanese Orthopaedic Association (JOA) scoring system for cervical disorders. Pearson correlation analysis was applied for statistical testing. RESULTS: Surgical efficacy (the JOA-score improvement rate) was significantly associated with the craniocervical volume improvement rate, the odontoid descent distance, and the absolute craniocervical volume changes (p < 0.01 for all), with correlation coefficients (r) of 0.83, 0.80, and 0.61, respectively. No significant correlation was noted between surgical efficacy and age, symptom duration, preoperative neurological function, odontoid process displacement, or change in clivus-odontoid angle (p > 0.05). The craniocervical volume improvement rate was significantly associated with the odontoid descent distance (r = 0.8; p < 0.01), but it was not associated with the odontoid displacement or the change in the clivus-odontoid angle (p > 0.05). CONCLUSIONS: We found that the odontoid descent distance predicted the craniocervical volume improvement rate following TARP procedures in patients with basilar invagination, and we believe that both can serve as predictors of surgical efficacy. We believe that planning the odontoid descent distance preoperatively may help to improve the efficacy of TARP operations. LEVEL OF EVIDENCE: Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.

Treatment of Basilar Invagination With Klippel-Feil Syndrome: Atlantoaxial Joint Distraction and Fixation With Transoral Atlantoaxial Reduction Plate.

BACKGROUND: Klippel-Feil syndrome (KFS) is characterized by congenital vertebral fusion of the cervical spine and a wide spectrum of associated anomalies. In patients with KFS with basilar invagination (BI), compression of the brainstem and upper cervical cord results in neurological deficits, and decompression and occipitocervical reconstruction are required. The highly varied anatomy of KFS makes a posterior occipitocervical fixation strategy challenging. For these patients, the transoral atlantoaxial reduction plate (TARP) operation is an optimal option to perform a direct anterior fixation to achieve stabilization. OBJECTIVE: To evaluate the effectiveness of TARP internal fixation for the treatment of BI with KFS. METHODS: Ten consecutive patients with BI and KFS who underwent TARP reduction and fixation from 2010 to 2012 were reviewed. Clinical assessment and image measurements were performed preoperatively and at the most recent follow-up. Nine patients (9/10) were followed for an average of 31.44 months. RESULTS: Symptoms were alleviated in 9 of 9 patients (100.00%). The odontoid process was ideally corrected with the TARP system. The mean clivus canal angle improved from 124° preoperatively to 152° postoperatively. The average preoperative and postoperative Japanese Orthopedic Association scores were 10.56 (n = 9) and 14.67 (n = 9), respectively, indicating 63.82% improvement. There was bony bridge catenation on the computed tomography scans and no evidence of hardware failure at 6 months. CONCLUSION: The TARP operation is effective and safe for treating patients with BI with KFS. The midterm clinical results were satisfactory.

[The reason and prevention of upper cervical reoperations].

OBJECTIVE: To discuss the reasons for the operation performed on 13 patients with upper cervical disease and to explore the management and prevention of upper cervical disease. METHODS: Thirteen patients with upper cervical disease were retrospectively reviewed. The reason for of reoperations on these patients were analyzed. The measures to reduce upper cervical operational complication and bad prognosis were discussed to avoid reoperations. RESULTS: The reasons for reoperations included 9 cases with unstable or re-dislocated atlantoaxial joint, 10 cases with residual spinal cord compression, 1 case with malposition of odontoid screw, 1 case with adjacent cervical spine regression, 1 case with occipital-cervical fusion failure, 1 case with spinal cord injury during operation, 1 case with bone-plant slipped into canales spinalis, and 1 case with demand to take out internal fixation for aggravated symptom. CONCLUSIONS: The common reasons for upper cervical reoperations were due to instability or redislocation of atlantoaxial joint and residual of spinal cord compression. Some measures such as reducing operate miss, using firm internal fixation and decompressing were advisable to decrease the incidence of reoperations.

Transoral approach for revision surgery of os odontoideum with atlantoaxial dislocation.

Revision surgery for os odontoideum with irreducible atlantoaxial dislocation with a transoral approach is not commonly seen. Typically, management of this type of atlantoaxial dislocation is through posterior fixation and fusion or transoral decompression and posterior fusion. This report describes revision surgery in a patient with os odontoideum who was treated with a transoral approach. A 50-year-old man was diagnosed with os odontoideum and atlantoaxial dislocation in 2007 and was treated surgically with posterior occipitocervical internal fixation and fusion. In 2012, he had recurrence of neck pain and numbness of the limbs. Neurologic function was grade D according to the standard neurologic classification of spinal cord injury from the American Spinal Injury Association. Because this was a revision surgery, the internal fixation implant was removed through a posterior approach and a transoral approach was used for release, reduction, internal fixation, and fusion. Two 6-mm cages filled with autogenous bone were introduced into the lateral mass spaces for bony fusion and distraction, and 2 cervical compressive mini-frames were used for fixation. Complete atlantoaxial reduction and decompression of the spinal cord were achieved. The patient reported improvement of symptoms after surgery. Movement of the extremities increased from grade III force to grade V, and neurologic status improved from American Spinal Injury Association grade D to grade E. A transoral approach for release, reduction, bony fusion, and fixation could be an effective procedure for the treatment of os odontoideum with irreducible atlantoaxial dislocation. It provides a new option for bony fusion and internal fixation of the atlantoaxial joint.

The development and evaluation of individualized templates to assist transoral C2 articular mass or transpedicular screw placement in TARP-IV procedures: adult cadaver specimen study.

OBJECTIVES: The transoral atlantoaxial reduction plate system treats irreducible atlantoaxial dislocation from transoral atlantoaxial reduction plate-I to transoral atlantoaxial reduction plate-III. However, this system has demonstrated problems associated with screw loosening, atlantoaxial fixation and concealed or manifest neurovascular injuries. This study sought to design a set of individualized templates to improve the accuracy of anterior C2 screw placement in the transoral atlantoaxial reduction plate-IV procedure. METHODS: A set of individualized templates was designed according to thin-slice computed tomography data obtained from 10 human cadavers. The templates contained cubic modules and drill guides to facilitate transoral atlantoaxial reduction plate positioning and anterior C2 screw placement. We performed 2 stages of cadaveric experiments with 2 cadavers in stage one and 8 in stage two. Finally, guided C2 screw placement was evaluated by reading postoperative computed tomography images and comparing the planned and inserted screw trajectories. RESULTS: There were two cortical breaching screws in stage one and three in stage two, but only the cortical breaching screws in stage one were ranked critical. In stage two, the planned entry points and the transverse angles of the anterior C2 screws could be simulated, whereas the declination angles could not be simulated due to intraoperative blockage of the drill bit and screwdriver by the upper teeth. CONCLUSIONS: It was feasible to use individualized templates to guide transoral C2 screw placement. Thus, these drill templates combined with transoral atlantoaxial reduction plate-IV, may improve the accuracy of transoral C2 screw placement and reduce related neurovascular complications.

The development and evaluation of individualized templates to assist transoral C2 articular mass or transpedicular screw placement in TARP-IV procedures: adult cadaver specimen study

OBJECTIVES: The transoral atlantoaxial reduction plate system treats irreducible atlantoaxial dislocation from transoral atlantoaxial reduction plate-I to transoral atlantoaxial reduction plate-III. However, this system has demonstrated problems associated with screw loosening, atlantoaxial fixation and concealed or manifest neurovascular injuries. This study sought to design a set of individualized templates to improve the accuracy of anterior C2 screw placement in the transoral atlantoaxial reduction plate-IV procedure. METHODS: A set of individualized templates was designed according to thin-slice computed tomography data obtained from 10 human cadavers. The templates contained cubic modules and drill guides to facilitate transoral atlantoaxial reduction plate positioning and anterior C2 screw placement. We performed 2 stages of cadaveric experiments with 2 cadavers in stage one and 8 in stage two. Finally, guided C2 screw placement was evaluated by reading postoperative computed tomography images and comparing the planned and inserted screw trajectories. RESULTS: There were two cortical breaching screws in stage one and three in stage two, but only the cortical breaching screws in stage one were ranked critical. In stage two, the planned entry points and the transverse angles of the anterior C2 screws could be simulated, whereas the declination angles could not be simulated due to intraoperative blockage of the drill bit and screwdriver by the upper teeth. CONCLUSIONS: It was feasible to use individualized templates to guide transoral C2 screw placement. Thus, these drill templates combined with transoral atlantoaxial reduction plate-IV, may improve the accuracy of transoral C2 screw placement and reduce related neurovascular complications. .

[A biomechanical study of anterior screw fixation for type II odontoid fracture with anteroinferior-posterosuperior fracture line].

OBJECTIVE: To investigate the biomechanical effect of anterior screw fixation on the type II fractured odontoid process. METHODS: Twenty fresh human C1-C2 vertebrae specimens were harvested and randomly divided into three groups. The angle of type II fracture line was 0 degree in group I (n=6), 17 degrees in group II (n=8) and 25 degrees in group III (n=6). The fractures were treated by anterior screw fixation. Insertion torque,maximal axial pullout force and stiffness of the bone-screw were tested. RESULTS: There was no significant difference of screw insertion torque and the pull-out strength between each group. The displacement of the odontoid fragment had an association to the angle of the fracture line,the displacement of the small angle was significantly higher than that of the large one (P < 0.5). No significant difference of structure stiffness of the bone-screw was found between each group. CONCLUSION: Anterior screw fixation is feasible for type II odontoid fracture with certain fracture line extends from anteroinferior to posterosuperior.

Applied anatomy of transoral atlantoaxial reduction plate internal fixation.

STUDY DESIGN: A C1-C2 operation by the transoral approach was simulated to study the anatomic stratification, various structures, and adjacent relationships. The anatomic parameters in relation to transoral atlantoaxial reduction plate (TARP) internal fixation were measured. OBJECTIVES: To study relevant anatomic features of the structures involved in TARP internal fixation through transoral approach for treating irreducible atlantoaxial dislocation, so as to provide anatomic basis for the clinical application of TARP. SUMMARY AND BACKGROUND DATA: Irreducible anterior atlantoaxial dislocation (IAAD) with ventral spinal cord compression is difficult for surgical correction. Despite previous description of direct plate internal fixations through the transoral approach, the problem has not been fully resolved: the Harms' plate lacked a locking mechanism while the other plates unable to achieve immediate reduction of the atlantoaxial joint. We therefore designed the TARP system with which the decompression, reduction, internal fixation, and fusion procedures could be completed in the same transoral approach. The anatomic structures and stratification involved in the transoral approach, which were seldom addressed in previous anatomic studies, need to be clarified for internal fixation with TARP system. METHODS: Twenty fresh craniocervical specimens were microsurgically dissected layer by layer according to a transoral approach. Stratification of the posterior pharyngeal wall, course of the vertebral artery, anatomic relationships of the adjacent structures of the atlas and axis, and closely relevant anatomic parameters for TARP internal fixation were measured. RESULTS: The posterior pharyngeal wall consisted of two layers and two interspaces: the mucosa, prevertebral fascia, retropharyngeal space, and prevertebral space. The range from the anterior edge of the foramen magnum to C3 could be exposed by this approach. The thickness of the posterior pharyngeal wall was 3.6 +/- 0.3 mm (range, 2.9-4.3 mm) at the anterior tubercle of C1, 6.1 +/- 0.4 mm (range, 5.2-7.1 mm) at lateral mass of C1 and 5.5 +/- 0.4 (range, 4.3-6.5 mm) at the central part of C2, respectively. The distance from the incisor tooth to the anterior tubercle of C1, C1 screw entry point, and C2screw entry point was 82.5 +/- 7.8 mm (range, 71.4-96.2 mm), 90.1 +/- 3.8 mm (range, 82.2-96.3 mm), and 89.0 +/- 4.1 mm (range, 81.3-95.3 mm), respectively. The distance between the vertebral artery at atlas and the midline was 25.2 +/- 2.3 mm (range, 20.4-29.7 mm) and that between the vertebral artery at the axis and the midline was 18.4 +/- 2.6 mm (range, 13.1-23.0 mm). The allowed width of the atlas and axis for exposure was 39.4 +/- 2.2 mm (range, 36.2-42.7 mm) and 39.0 +/- 2.1 mm (range, 35.8-42.3 mm), respectively. The distance (a) between the two atlas screw insertion points (center of anterior aspect of C1 lateral mass) was 31.4 +/- 3.3 mm (range, 25.4-36.6 mm). The vertical distance (b) between the line connecting the two C1 screw entry points and that connecting the two C2 screw entry points (at the central part of the vertebrae, namely, 3 to 4 mm lateral to the midline of C2 vertebrae) was 21.3 +/- 2.7 mm (range, 19.4-24.3 mm), with an a/b ratio of 1.3 to 1.5. The screws of TARP had a lateral tilt of 12.2 degrees +/- 0.4 degrees (range, 10.2 degrees -14.6 degrees ) at C1 and a medial tilt of 7.3 degrees +/- 0.3 degrees (range, 5.1 degrees -9.4 degrees ) at C2 relative to the coronal plane. CONCLUSION: An atlantoaxial surgery through transoral approach is safe and feasible. This approach is suitable for an anterior TARP internal fixation, and the design of the internal fixation system should be based on the above anatomic data.