The side-bending stress test is a pre-manipulative screening test for assessing upper cervical instability. To our knowledge, there is no study that simulates the clinical application of side bending stress test before and after alar ligament transection with fixation of C2.
To simulate the effect of alar ligament transection in the side bending stress test for an in vitro validation.
In vitro study.
After the dissection of the superficial structures to the alar ligament and the fixation of C2, ten cryopreserved upper cervical spines were manually mobilized in right and left lateral flexion with and without right alar ligament transection. Upper cervical lateral flexion range of motion and mobilization force were measured with the Vicon motion capture system and a load cell respectively.
The right alar ligament transection increased the upper cervical spine (UCS) range of motion (ROM) in both side bendings (1.30°±1.54° and 1.88°±1.51° increase for right and left side bending respectively). As an average, with standardized forces of 2N, 4N and 6N, right alar ligament transection increased both right and left lateral flexion UCS ROM.
This in vitro study simulates the clinical application of the side bending stress test with intact and right transected alar ligament. Unilateral transection of the alar ligament revealed a predominantly bilateral increase in upper cervical side bending and variability in the mobilization force applied during the test.
This in vitro study simulates the clinical application of the side bending stress test.
C2 fixation in our study provided a reference for measuring upper cervical side bending.
Transection of right alar ligament increased upper cervical side bending bilaterally.
Manual therapists use cervical spine manipulation and mobilization to treat many different types of musculoskeletal dysfunctions ( ; ). For the safe and effective practice of these techniques in the upper cervical spine, the identification of instability is fundamental. The stability of the craniocervical junction and the physiological mobility of the upper cervical spine in the frontal plane depend on the integrity of the alar ligament. This ligament is formed by two portions that connect the odontoid process to the lateral part of the foramen magnum of the skull.
According to , pure side bending of the cervical spine is accompanied by immediate ipsilateral rotation of the lower cervical spine (C2 and below) and contralateral rotation of the upper cervical spine (C0-C1, C1-C2). suggested that coupled movements associated with cervical side bending are a direct consequence of alar ligament tension. Disruption of this bone-ligament-bone system ( ) may potentially increase the risk of neurovascular compromise in the upper cervical region during upper cervical rotation and side bending motions ( ).
recommend the application of pre-manipulative screening before the use of spinal manipulation to identify any such disruption of the bone-ligament-bone system. They suggested that pre-manipulative testing include stabilization of the spinous process and lamina of C2 (to prevent both side bending and rotation of the segment) followed by a movement of the occiput into pure side bending. They theorized that if stabilization of C2 is effective, no upper cervical side bending will occur if the bone-ligament-bone system is intact. However, if motion occurs, then laxity of the alar ligament is suspected ( ).
measured the origin-insertion length of the alar ligament in vivo using MRI on 16 participants between the ages of 18 and 35. During their study, they compared the ligaments resting length with its length during pure side bending. They concluded that side bending causes an increase in length (a median between-side difference of 1.15 mm) of the contralateral alar ligament during side bending. investigated the effect of the transection of unilateral and bilateral alar ligaments on upper cervical side bending in vitro. They identified a 16.5% increase in the contralateral side bending ROM following alar ligament transection in the first three cervical segments (2.3°) compared to nontransected specimens (13.9°±4.6°). Unlike the clinical side bending test where C2 is stabilized, Panjabi et al. fixated C3. The purpose of this study is to more accurately simulate the clinical side bending stress test by examining the effects of an intact and transected alar ligament on the side bending test in vitro with C2 fixated.
Materials and methods
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.
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.
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.
Fig. 2 illustrates the amount of force applied and the resultant side bending movement for all ten specimens with both alar ligaments intact (illustrated in black) and with the right alar ligament cut (illustrated in grey). When reading the graphs, positive flexion values indicate right side bending, and negative values indicate left side bending. Table 1 contains the side bending angles recorded for each specimen (normal and transected) when the applied forces were 2N, 4N, and 6N, as well as the force applied to achieve maximum ROM. Table 2 shows the comparison of the side bending angles and associated forces for each specimen (normal and transected).
|Right side bending||Left side bending|
|2 N||4 N||6 N||F. Max||ROM Max||2 N||4 N||6 N||F. Max||ROM Max|