2.1

Sample

Ten cervical spines and heads from cryopreserved cadavers (9 males and 1 female; mean age: 74 years, range: 63–85 years) were examined in this study. To determine suitability for inclusion, all specimens were visually checked for evidence of prior surgery, trauma, or any anatomical abnormalities that would influence ROM. In addition, all specimens were required to be free of any disease or contamination that would influence connective tissue integrity. All specimens were from a body donor program. The study was approved by a local ethical committee.

2.2

Anatomical and biomechanical procedure

All specimens were stored in a freezer at −14 °C and thawed to room temperature 24 h prior to data collection. To prepare each specimen, first, all spinal segments caudal to C2 were removed by disarticulating C2 from C3 by cutting through the C2-C3 intervertebral disc and zygapophysial facet joint capsules. Second, all muscle tissue in the specimens was removed with care taken not to disrupt any ligamentous tissues. Third, the cranial posterior third of the skull was removed using a wide posterior wedge cut as described by to allow for the brain to be removed and the visualization of the foramen magnum. Integrity of the posterior arch of atlas was maintained during this procedure. Forth, the brainstem, spinal cord, dura and part of the tectorial membrane were carefully removed to expose the alar ligament. Finally, for the purpose of attaching the measurement sensors the mandible and upper maxilla were removed. Once the specimen was ready for testing, a metallic handlebar was attached to the skull by three points, one through each auditory canal and one through the top of the head. The handlebar is pictured on Fig. 1 , and it was designed to move the head without contacting any sensors attached to the specimens.

Test setup: C0-C2 set up before mobilization in side bending once C2 has been fixated with screws, the handlebar has been fixated to the cranium and sensors have been attached.

After the specimen was prepared, it was fixed to the load cell (MC3A Force and Torque Sensor, AMTI, MA, USA) which measured the force required to generate side bending of the head. More specifically, the C2 vertebra was screwed to a metallic support which was secured to the load cell. C2 was attached in the anatomical mid-position, aligned with the axes of the load cell. The head was aligned with the C2 position before each side bending motion was performed. To find the neutral head position, an anatomical Frankfurt horizontal plane was laterally marked on the head (through the external auditory meati and the infraorbital foraminae), a vertical line was also marked on the center of the face. These two lines were aligned with the horizontal and vertical references given by two red light lasers (previously calibrated to be horizontal and vertical). Head neutral position was checked before C2 was fixed to ensure that both were aligned in a neutral position after C2 fixation.

An optical motion capture system (Vicon, TS series, Oxford, UK) consisting of four cameras was used to examine the 3D motion of the head over C2 during side bending. Retroreflective spherical markers placed on the head and C2 ( Fig. 1 ) were used to define head and C2 coordinate systems. Each specimen was side bent four times. The first two side bending motions were used as a warm-up to reduce the influence of soft tissue viscoelasticity ( ), with all measurements being recorded on the third motion (alar ligament intact). After transecting the right alar ligament, side bending was once more registered. To simulate the passive side bending stress test, all movements were induced manually in the frontal plane between 0.5 and 5.0°/s as recommended by . The movement ended when the researcher perceived a marked resistance. All side bending movements for all specimens were performed by the same researcher with more than 15 years of experience in manual therapy. To prevent dehydration and ensure that specimens remained physiologically viable, the room temperature was maintained between 17.0° and 17.8° Celsius and the humidity was maintained between the 47–52%.

SPSS statistical software (version 20.0) for Windows was used for all statistical analyses. A descriptive analysis of side bending angles and forces applied during the experiment was performed. The differences between normal and transected specimens was compared using the Wilcoxon rank test. Shapiro-Wilk test was used to identify the normal distribution of the sample. The level of significance was set at alpha = 0.05. The average and standard deviation corridors of the results from the 10 specimens were obtained following the method described by . This method avoided discontinuities in the averaged curve due to the different ranges of results between the specimens.