RESUMEN
Objective To investigate the stress characteristics of atlanto-axial bony structure under conditions of anteflexion,posterior extension,lateral flexion,and rotation after artificial atlanto-odontoid joint arthroplasty using three-dimensional finite element method and to improve the orientation of artificial atlantoodontoid joint from perspective of stress.Methods A three-dimensional finite element model of prosthetic atlanto-odontoid joint arthroplasty was created from CT images of the artificial atlantoodontoid joint and cervical vertebrae using software Mimics,Freeform,and Ansys.Stress characteristics of the model dealt with proneness,posterior extension,lateral flexion,or rotation loads were observed.Biomechanical performance of the bony structure of the model was analyzed and the orientation in improving the prosthesis was discussed.Results Anteflexion loading produced a maximum stress of 0.138 ×l08 N/m2 at the junction of lateral mass and posterior arch of the atlas,and 0.201 × 108 N/m2 at axial nail hole,contact point of plates with the axis,and posterior arch of the axis.Posterior extension loading produced a maximum stress of 0.666 × 107 N/m2 at junction of lateral mass and posterior arch of the atlas and 0.254 × 108 N/m2 at arch of the axis.Besides,stress concentration occurred at atlantoaxis nail hole.Right bending produced a maximum stress of 0.124 × 108 N/m2 at nail hole of right mass of atlas and 0.178 × 108 N/m2 at right contact point of the axis with plates.Right rotation produced a maximum stress of 0.847 × 107 N/m2 at junction of lateral mass and posterior arch of the atlas and 0.170 × 109 N/m2 at contact point of the axis with plates.The finite element model comprised 28 620 nodes and 107 441 units and provided good defining of the structural properties of artificial atlanto-odontoid joint arthroplasty.Under different loading conditions,the stress was mainly distributed in contact point of the vertebral body with plates,nail holes,junction of lateral mass and posterior arch of the atlas,and axial pedicle.Conclusions Prosthetic atlanto-odontoid joint scatters a part of the stress and alters the stress distribution of the atlas and axis from the intact condition.Finite element method can obtain complete analysis of the stress distribution of the artificial atlanto-odontoid joint arthroplasty.
RESUMEN
Objective To evaluate the clinical effect and safety of the occiput-cervicle or C1-C2 internal fixation and bone graft fusion in treatment of the unstable atlas fracture.Methods A retrospective study was performed in 38 patients with unstable atlas fractures treated by the occiput-cervicle or C1-C2 internal fixation and bone graft fusion from October 2004 to March 2009.Six patients with comminuted atlas fracture combined with instability of the occipito-atlantoid articulations were treated with occiput-C2 fusion(five patients)and with occiput-C3 fusion(one patient).There were seven patients with typical Jefferson fractures,three with semiring fractures,eight with atlas fractures combined with Anderson type Ⅱ odontoid process fractures,three with atlas fractures combined with Hangman's fractures (two patients with Levine and Edwards type Ⅲ Hangman's fractures were treated with occiput-C3 fusion and one patient Levine and Edwards type Ⅱ Hangman's fracture was treated with C1-C2 fusion),three with atlas fracture combined with lower cervicle injury,six with rupture of transverse ligament combined with instability of atlanto-axial joint(Dickman transverse ligament type Ⅰ injury)and two with comminuted fracture of the lateral mass associated with bony avulsion of the medial tubercle and transverse ligament(Dickman transverse ligament type Ⅱ injury).Of all,five patients were treated with occiput-C2 fusion,three treated with occiput-C3 fusion and 30 treated with C1-C2 fusion.Results All the patients were followed up for a range of 12-46 months(average 28 months),which showed improvement of clinical symptoms in some extent postoperatively.The operation time ranged from 80 to 190 min ates(average 135 minates),with intraoperative blood loss for 200-3 300 ml(average 460 ml)and average fluoroscopic time for 60 seconds.There were no neurological deficits,vertebral artery related complications or other complications in all the patients during the surgical operation.No neurological deficit was aggravated after the patient's mobilization with brace three days after operation.The enous plexus of blood vessel at C1-C2 rupture induced by the use of electrocautery was found in three patients who showed no cerebral hemodynamic deficit after hemostasis with hemostatic sponge and cotton piece.The follow-up X-ray and CT manifested osseous fusion in all the patients,with no looseness or breakage of the screws.The late follow-up showed pain associated with movement and limited range of motion in four patients(11%)and occipital neuralgia in one.Conclusions An occiput-cervicle fixation fusion or a C1-C2 fixation fusion combined with short external fixation can reestablish the upper cervical stability and prevent further injury of the spinal cord and nerve function and hence is an ideal option for C1 burst fracture with or without rupture of the transverse ligament.
RESUMEN
Objective To investigate feasibility and safety of anterior pedicle screw fixation tunnel in the axis so as to provide theoretic evidence for further clinical application.Methods Thirty-two dry axis specimens were used for anterior pedicle screw placement,and the length of pedicle crews tunnel was measured by a digital caliper.The extraversion angle and the downslope angle were determined by CT cross-sectional scanning and reconstruction.Results All the anterior pedicle screws were positioned well,with no perforation of the pedicle.The anchoring point was at the vertex between the vertebral body and the superior articular process,with axis pedicle length of(28.4 ± 2.5)mm,the extraversion angle of (28.5 ±2.3)°and the downslope angle of(15.5 ±2.0)°.Conclusions Anterior pedicle screw fixation tunnel is feasible in the axis and can keep away from some important anatomy structures including transverse foramen,vertebral artery groove and vertebral canal to provide large safety space.
RESUMEN
Objective To identify the feasibility and the anatomical parameters of posterior transarticular pedicle screw fixation in the thoracic spine, provide a reference for clinical applications. Methods Twenty human cadaveric thoracic spine segments were dissected posteriorly and anteriorly, with care taken to expose the laminas and pedicles. The entrance point of transarticular pedicle screws was located in the 7 mm away from the above at the inferior margin of the lamina and the inside at the exterior margin, respectively. Posterior transarticular pedicle screws implantation was performed under direct visualization into T1,2,T5,6 and T9,10 Under direct abservation, the feasibility of posterior transarticular pedicle screw fixation was assessed. Then a CT was done. On the morphologic CT scan, the angle and length of the transarticular pedicle crew trajectory were measured. Results The thoracic transarticular pedicle screw trajectory were caudal tilting in the sagittal plane and lateral tilting in the axial plane with successful placement. Screws were placed across the facet joint, and from the inferior articular process of upper thoracic vertebra into the pedicle of lower thoracic vertebra. There was little difference between different number of thoracic vertebrae of the angle, but without significance. The average angles of the screws were 41.4°+3.2°caudal tilting in the sagittal plane and 2.1°±0.7° lateral tilting in the axial plane. The average trajectory lengths were (40.6±4.9)mm, and the lengths increased gradually from upper thoracic vertebra to middle and lower. There were significant differences statistically among T1,2, T5,6 and T9.10 (F=74.09, P＜0.01 ). Conclusion Posterior transarticular pedicle screw fixation is feasible, and there are some directions for implantating the screws. Transarticular pedicle fixation in the thoracic spine affords an alternative to standard pedicle screw placement for thoracic stabilization.