Partial aplasia of the posterior arch of the atlas with an isolated posterior arch remnant: findings in three cases.
We report the imaging findings in three symptomatic cases of partial aplasia of the posterior arch of the atlas with an isolated posterior remnant of the arch. These cases are instructive in illustrating the mechanism of cord impingement produced by the posterior arch remnant during extension of the cervical spine. Additionally, focal increased T2 signal was observed within the cord at the level of the anomaly in two of the patients. (+info)
Quantitative anatomy of the lateral masses of the atlas and axis vertebrae.
The study was carried out to determine the safe site of entry and the appropriate trajectory of the screw implantation in the lateral masses of atlas (Cl) and axis (C2) during their fixation using the plate and screw technique. Fifty dried specimens of atlas and axis vertebrae were studied. Various dimensions of the lateral masses were quantitatively measured, laying stress on their relationship with the vertebral artery foramen. As the vertebral artery foramen was present entirely in the transverse process in all specimens, screw implantation in the facet of atlas was relatively safe. Best direction of screw implantation in the facet of atlas was observed to be 15 degrees medial to sagittal plane and 15 degrees superior to axial plane. It should be implanted from the middle of the posterior surface of facet. Vertebral artery foramen formed a deep groove in the undersurface of a majority of superior facets of axis. In 15% facets, vertebral artery foramen occupied the entire undersurface of the superior facet. Safe angle for screw implantation in the facet of axis through its pedicle was seen to be 40 degrees medial to sagittal plane and 20 degrees superior to axial plane. Safe site of screw entry in the axis was superior and medial third of the posterior surface of the pedicle. Quality of cancellous bone in the lateral masses in the proposed trajectory of screw in Cl and C2 was good, providing an excellent purchase of the screw. (+info)
Atlas hypoplasia manifesting as myelopathy in a child--case report.
A 14-year-old Japanese boy presented with myelopathy due to atlas hypoplasia with complete posterior arch. Decompressive laminectomy of the atlas produced good neurological recovery, and follow-up T2-weighted magnetic resonance imaging showed disappearance of spinal cord edema. Congenital atlas stenosis may be symptomatic even in children, with no accompanying cervical spondylotic change. Such cases have previously occurred only in Asian adults. A radiological study of the patient's brother showed median cleft formation of the posterior arch of atlas, indicative of a wide spectrum of atlas anomalies and a possible genetic relationship between these anomalies. (+info)
Finite element model of the Jefferson fracture: comparison with a cadaver model.
This study tries to explain the reason why the Jefferson fracture is a burst fracture, using two different biomechanical models: a finite element model (FEM) and a cadaver model used to determine strain distribution in C1 during axial static compressive loading. For the FEM model, a three-dimensional model of C1 was obtained from a 29-year-old healthy human, using axial CT scans with intervals of 1.0 mm. The mesh model was composed of 8200 four-noded isoparametric tetrahedrons and 37,400 solid elements. The material properties of the cortical bone of the vertebra were assessed according to the previous literature and were assumed to be linear isotropic and homogeneous for all elements. Axial static compressive loads were applied at between 200 and 1200 N. The strain and stress (maximum shear and von Mises) analyses were determined on the clinically relevant fracture lines of anterior and posterior arches. The results of the FEM were compared with a cadaver model. The latter comprised the C1 bone of a cadaver placed in a methylmethacrylate foam. Axial static compressive loads between 200 and 1200 N were applied by an electrohydraulic testing machine. Strain values were measured using strain gauges, which were cemented to the bone where the clinically relevant fracture lines of the anterior and posterior arches were located. As a result, compressive strain was observed on the outer surface of the anterior arch and inferior surface of the posterior arch. In addition, there was tensile strain on the inner surface of the anterior arch and superior surface of the posterior arch. The strain values obtained from the two experimental models showed similar trends. The FEM analysis revealed that maximum strain changes occurred where the maximum shear and von Mises stresses were concentrated. The changes in the C1 strain and stress values during static axial loading biomechanically prove that the Jefferson fracture is a burst fracture. (+info)
Posterolateral tunnels and ponticuli in human atlas vertebrae.
The posterolateral tunnel on the superior surface of the first cervical (atlas) vertebra is of normal occurrence in monkeys and other lower animals, but its presence in the form of a tunnel-like canal, for the passage of the third part of the vertebral artery over the posterior arch of the human atlas vertebra is not reported. The aim of the present study was to detect the presence of such a canal, in addition to other types of ponticuli (little bridges) reported by earlier investigators, in macerated atlas vertebrae and routine cadaveric dissections. The posterolateral tunnel was detected in 1.14%, and the posterior and lateral ponticuli in 6.57 and 2% of vertebrae. Probably the bony roof of the posterolateral tunnel serves the purpose of additional lateral extension for the attachment of the posterior atlanto-occipital membrane in quadrupeds, where the load of the head is supported by the extensor muscles of the neck, ligaments and posterior atlanto-occipital membrane. In man, where the weight of the head is borne by the vertical loading of the superior articular process of the atlas, the roof of the tunnel has disappeared. (+info)
Agenesis of the posterior arch of the atlas.
PURPOSE: To illustrate the radiological findings and review the current literature concerning a rare congenital abnormality of the posterior arch of the atlas. CASE REPORT: An adult female without neurological symptoms presented with an absent posterior arch of the atlas, examined with plain films and helical computerized tomography. Complete agenesis of the posterior arch of the atlas is a rare entity that can be easily identified by means of plain films. Although it is generally asymptomatic, atlantoaxial instability and neurological deficits may occur because of structural instability. Computerized tomography provides a means of assessing the extent of this abnormality and can help evaluate the integrity of neural structures. Although considered to be rare entities, defects of the posterior arch of the atlas may be discovered as incidental asymptomatic findings in routine cervical radiographs. Familiarity with this abnormality may aid medical professionals in the correct management of these cases. (+info)
Cervical myelopathy caused by hypoplasia of the atlas and ossification of the transverse ligament--case report.
A 79-year-old Japanese female presented with symptomatic cervical myelopathy caused by a hypoplastic posterior arch of the atlas and ossification of the transverse ligament. Neuroradiological examination demonstrated a hypoplastic posterior arch of the atlas and ossification of the transverse ligament. The cervical spinal cord was compressed at the level of the atlas by both the hypoplastic posterior arch of the atlas and the ossification of the transverse ligament. The patient underwent C-1 laminectomy, which arrested the progressive myelopathy and resulted in a good recovery. Atlas hypoplasia with ossification of the transverse ligament may be associated with Asian ethnicity. (+info)
Vertebrobasilar artery insufficiency in rheumatoid atlantoaxial subluxation.
Cervical myelopathy has become commonly recognized as a complication of rheumatoid atlantoaxial subluxation. A small group of patients with atlantoaxial subluxation may have intermittent symptoms associated with change of head position and which are due to vertebral artery compression. Two such cases are reported, one with necropsy findings of infarction in the area supplied by the vertebrobasilar system. The pathogenesis of the symptomatology and infarction is discussed. (+info)