In agreement with Webb et al. [60
] the ventral synovial folds were larger than the dorsal synovial folds and the right ventral and dorsal synovial folds were larger than the left ventral and dorsal synovial folds; however, in the present study these did not reach significance. All subjects were right-hand dominant with one being ambidextrous; therefore, side-to-side differences might be the result of a dominance effect. Further studies including left-hand dominant subjects are needed to clarify this issue. The degree of an individual’s sagittal and coronal head tilt [48
] might also influence the size of the ventral and dorsal synovial folds and right and left synovial folds, respectively. Based upon observations from cadaver studies, it is suggested that the morphology of the synovial folds changes in association with increasing age and/or articular degeneration [15
]. In agreement with Friedrich et al. [17
] and Webb et al. [60
], neither of these age-related observations was verified by the results of the present study.
The size of the synovial folds was related to subject anthropometry. The dimensions of the subject’s body as a whole, rather than the regional dimensions of the head and neck, were strongly related to synovial fold volume. The larger synovial fold volume was associated with taller subjects and with those who weighed less and had a lower BMI. This is in contrast to the dimensions of the articular facets, spinal cord and some cervical muscles, which typically increase in size in association with greater body size [10
]. Although, in the main, synovial fold volume was not as strongly related to the dimensions of the head and neck, there was a general trend for synovial fold volume to increase with increasing neck length and to increase with decreasing head and neck circumference, i.e. larger synovial folds were generally associated with long slender necks and smaller heads.
At low-speed motor vehicle collisions, occupants with smaller neck circumference and lower BMI experience greater head acceleration as compared to occupants with a larger neck circumference and greater BMI, respectively [23
]. Freeman et al. [16
] found an increased risk of association between BMI and chronic neck pain following a motor vehicle collision but did not find an increased risk in the female population. Thus Freeman et al. [16
] have suggested that body mass and neck circumference rather than female gender [24
] are most likely to result in the development of neck pain following a whiplash trauma. Females generally have smaller necks and less body mass as compared to males, which may explain why this relationship is thought to have been attributed to gender [16
]. The finding in the present study that taller individuals with less body mass and lower BMI, and longer more slender necks, had larger synovial folds is of potential relevance to the understanding of the biomechanics and pathoanatomy of a whiplash injury. Larger synovial folds may be more vulnerable to being pinched and bruised between the articular surfaces following the application of a traumatic force. Contusions of the synovial folds and occult fractures of the articular processes are two of the most common injuries that affect the cervical articulations at post-mortem following motor vehicle trauma [29
]. Taller individuals with less body mass, lower BMI and long slender necks that have larger synovial folds may be more prone to bruising of the synovial folds following whiplash trauma but less vulnerable to damage affecting the hyaline articular cartilage and articular facets. In contrast, shorter individuals with greater body mass and BMI and shorter thicker necks that have smaller synovial folds may be more prone to articular cartilage and subchondral bone damage following motor vehicle trauma.
The function of the synovial folds is not known. In the present study, larger lateral atlanto-axial synovial folds were typically associated with a greater range of flexion, extension and rotation but not lateral flexion. This is in agreement with the atlanto-axial joints having the greatest range of rotation in the cervical spine [8
] and the observation that the synovial folds of the lateral atlanto-axial joints are larger than the synovial folds of the cervical zygapophysial joints [62
]. Furthermore, the lateral atlanto-axial joints have a moderate range of flexion and extension and a small range of segmental lateral flexion [45
]. The measurement of cervical ROM is interpreted as an indication of the state of the anatomic structures within or around the joints [58
] and the function of the cervical spine is evaluated by assessing its ROM. The presence of a relationship between cervical ROM and the size of the synovial folds would suggest that the synovial folds are involved in the facilitation of mobility rather than stability.
Cervical range of motion forms an integral component of spinal evaluation and is the principal criterion in the quantification of musculoskeletal impairment [40
]. In patients with whiplash-associated disorder, all planes of motion are reduced with the sagittal plane movements the most affected [5
]. Patients with headache also demonstrate decreased motion [13
]. The cause of the reduced range of motion associated with neck pain and headache is not known but suggested reasons include mechanical changes in the tissues or pain inhibition [5
]. Because the measurement of cervical range of motion is interpreted as an indication of the state of the anatomic structures within or around the joint complex, abnormal results may indicate abnormalities affecting the cervical articular structures [58
]. It is currently not known as to what these abnormalities might be; however, injuries to the synovial folds in patients with neck pain and/or headache might be related to the reduction in cervical range of motion observed in these patient groups.
In principle, multiple regression analysis could have been used to investigate the relationship between synovial fold volume (outcome variable) and age, anthropometrics and cervical range of motion (predictor variables) and in addition assess the potential for interactions between the predictor variables. However, the present study was a pilot study with a small number of subjects. Undertaking multiple regression analysis with a small sample size is not reliable and a sample size of at least 10–15 participants per predictor variable is recommended [14
]. Although it would be possible to tease out potential relationships, to do so in a reliable way would require considerably more participants. Despite this, it was prudent to explore potential interactions between composite measures of age, body dimensions, neck dimensions and cervical range of motion as a first step in looking at such relationships. The very strong correlation between BMI and neck morphometry is of particular interest as a high level of collinearity threatens the validity of multiple regression analyses making it difficult to assess the individual importance of a predictor, increasing the probability of a type II error and producing predictor equations that are not reliable. A larger study is planned for the future as these relationships require further careful consideration. The results of the present study will be used to select suitable predictor variables and the method in which these variables are entered into the regression model.
To date, it has not been possible to visualise specific morphological changes using standard cervical MR imaging in patients with neck pain [32
]. Recent MR studies have described the possible presence of visible changes to muscles and ligaments in neck pain patients after whiplash [9
]. The reliability of quantitative methods [9
] appears to be superior to that of qualitative methods [36
]. The reliability of the quantitative MR measure of synovial fold volume used in the present study has been previously reported [60
]. The measurement method proved to have high levels of intra-observer and inter-observer reliability, ICC 0.99 and ICC 0.82, respectively [60
]. In the present study, the reliability of the method for measuring synovial fold volume was tested further by examining the consistency of synovial fold dimensions that were imaged and measured on different occasions (test–retest reliability). The test–retest reliability of the method was excellent (ICC 0.99) and consistent with that reported in previous studies [9