Occiput-axis crossing translaminar screw fixation technique using offset connectors: An in vitro biomechanical study.

OBJECTIVE: Fixation with the axis vertebra (C2) using pedicle screws is commonly used to treat an unstable occipitocervical junction; however, it is accompanied by a risk of vertebral artery injury. The occiput-C2 (OC2) crossing translaminar screw fixation technique may avoid this risk, but rod implantation is difficult. Offset connectors can help facilitate this construct. This study aimed to evaluate the stability of a technique for OC2 crossing translaminar screw fixation using offset connectors (C2LAM + OF) in comparison with other methods. PATIENTS AND METHODS: Six fresh-frozen human cadaveric occipital-cervical spines were tested intact under flexion, extension, lateral bending, and axial rotation. These were then made into a type II odontoid fracture model, instrumented with an occipital plate, and tested in the following modes: C2 bilateral pedicle screws (C2P), a single C2 pedicle screw and bilateral C3 lateral mass screws (C2P + C3M), C2 crossing translaminar screws (C2LAM), and C2LAM + OF. The OC2 range of motion (ROM) for each construct was obtained and compared using a repeated-measures analysis. RESULTS: The ROM of the C2LAM + OF construct was found not to be significantly different from that of the C2P and C2P + C3M fixations in every direction (p > 0.05). However, the C2LAM + OF construct was superior to the C2LAM construct in axial rotation (p < 0.05). CONCLUSIONS: OC2 crossing translaminar screw fixation using offset connectors offers similar stability to C2 pedicle screw fixation and is an effective alternative method for treating an unstable occipitocervical junction.

Stresses at the cranial base induced by rapid maxillary expansion.

OBJECTIVE: An analysis of the distribution of stresses at the juvenile and adult cranial base after implementation of a rapid palatal suture expansion was the goal of this study. Of particular interest were stresses occurring near the cranial foramina containing vulnerable structures. MATERIALS AND METHODS: The stresses were simulated and analyzed using a finite elements model of the human cranial base. The model consisted of several skull bones (sphenoid, frontal bone, occipital bone, and the two temporal bones) with a total of 41,556 finite elements. To illustrate the differences between reactions in the juvenile and the adult, the differing bone elasticity was depicted as variations in the modulus of elasticity. RESULTS: At the juvenile cranial base only moderate stresses occurred during rapid palatal suture expansion, apparently precluding the likelihood of any serious complications in the area of the foramina. The situation in the adult, however, was different. Because of the reduced elasticity of the bony structures, considerable stress already occurred on light bending of the pterygoid process, especially in the area of the round foramen, the oval foramen, and the superior orbital fissure, all of which might lead to microfractures with injury of nervous and vascular structures. CONCLUSIONS: The lower the bone elasticity on carrying out a rapid palatal suture expansion, the more important safety measures are for protecting the cranial base. For this reason the pterygomaxillary connection should be severed on both sides in adults when carrying out a surgically assisted palatal suture expansion.

Biomechanical implications of extending occipitocervical instrumentation to include the subaxial spine.

BACKGROUND: No clear biomechanical data exist regarding where to place the caudal end of a screw-rod occipitocervical instrumentation construct. OBJECTIVE: This study examines whether range of motion (ROM) from the occiput to C2 is altered by subaxial extension of occipitocervical instrumentation constructs. METHODS: Cadaver specimens underwent intact biomechanical testing followed by destabilization via an odontoid osteotomy. Subsequent biomechanical testing was performed of four occipitocervical constructs: occipital plate + C2 pars screws (construct 1), occipital plate + C2 pars screws + C4 lateral mass screws (construct 2), occipital plate + C1-C2 transarticular screws (construct 3), and occipital plate + C1-C2 transarticular screws + C4 lateral mass screws (construct 4). RESULTS: All constructs significantly reduced occiput-C2 ROM in all loading modes compared with the intact cervical spine, with one exception (construct 1, lateral bending). No significant ROM differences were noted when C4 lateral mass screws (construct 4) were added to construct 3. Addition of C4 lateral mass screws (construct 2) to construct 1 decreased the ROM in the flexion mode only. No significant ROM differences were seen between construct 2 and construct 3 in any loading mode. CONCLUSION: The addition of subaxial instrumentation to occipitocervical instrumentation constructs in this study decreased occiput-C2 ROM only when the construct was anchored by C2 pars screws and only in flexion. Screws that cross the C1 to C2 articulation provide stable fixation when combined with an occipital plate, and the addition of subaxial instrumentation to this construct for stabilizing the occipitocervical junction does not significantly decrease ROM.

The applicability of the occipital reference base in cephalometrics.

An occipital reference system, located outside the anterior half of the skull, has been designed which reveals sagittal as well as vertical relationships among facial components. The anatomic structures chosen had consistent behavior during growth, were located close to the midsagittal plane, and could be identified in a lateral cephalograph. Among the basal structures of the neurocranium, the occipital bone around the foramen magnum is the first to ossify. This appears to be indispensably necessary as the head is supported by the trunk precisely in the area of this individual bone. The ventrocaudal contour of the basal part of the occipital bone, anterior to the foramen magnum, and the internal occipital (sagittal) ridge, posterior to the foramen magnum, form the reference base. The intersection of the ventrocaudal contour and the anterior outlines of the occipital condyles serve as the key reference point (occipital point O') for constructing the occipital coordinate system whose horizontal axis is oriented to the earth's surface. This orientation is accomplished by a special photographic registration of the natural head position. The coordinate system has its center at the O' point and its abscissa thus adjusted to the horizontal. The use of a template permits the transfer of the occipital reference cross from the first to subsequent radiographs in the same relation to the occipital reference line as first registered. By the aid of a digitizer or a transparent graph paper with millimeter and centimeter squares on which the coordinate cross is marked, all measurements can be made on the radiograph directly as the reference points O' and O'' are marked on the radiograph itself, thus defining the position of the horizontal coordinate. In the past 50 years, a considerable amount has been learned about the function-form relationship in skeletal morphogenesis. It has been recognized that a faulty or poor postural behavior must be regarded as a major contributory factor to the maldevelopment in skeletal morphology and is taken into consideration in this reference system.