Abstract:

            Purpose:  To demonstrate the impact of inconsistent bite line positioning during pre and post lateral cervical radiographic examinations, and to suggest certain additional imaging studies if the bite line cannot be consistently maintained.  Methods:  Radiographic measurements of relative flexion and extension in the atlantal-occipital (AO) and atlantal-axial (AA) joints were taken from neutral lateral cervical and cervical flexion and extension radiographs of twenty subjects.  Results:  The average relative AO flexion was –0.9 degrees and 12.0 degrees of extension, while the average relative AA flexion and extension values were 8.5 degrees and 2.8 degrees, respectively. In addition, 12 out of the 20 subjects (60%) exhibited paradoxical motion at the AO joint during cervical flexion. Out of these 12 subjects, 10 also displayed excessive relative AO extension (beyond 7.5 degrees).  Conclusions:  If a bite line deviation exists in pre and post lateral cervical radiographic examinations, dynamic cervical flexion and extension radiographs should be taken to calculate the maximum tolerances in the upper cervical spinal joints.  If these tolerances are exceeded, the measurement of the cervical lordosis from the back of the second cervical vertebra (C2) and seventh cervical vertebra (C7) may be altered, thus incorporating the possibility of a 20.3% measurement error on the post lateral cervical radiograph. Key Indexing Terms: [Bite Line; Radiographs; Upper Cervical; Joint Dysfunction]    

J Manipulative Physiol Ther 2003; 26:E000  

Introduction

            In recent literature, it has been shown that the cervical curve is directly influenced by the position of the bite line.  The bite line is a line parallel to the chewing surface of the dentition. In some cases, a flattened object, such as a tongue depressor, is inserted into the patient’s mouth to more clearly demonstrate the bite line during radiographic imaging. In one study, it was concluded that the degree of the cervical curvature, measured from the posterior vertebral bodies of the second and seventh cervical vertebrae (C2, C7) with Ruth Jackson stress lines, can be changed up to 6.9 degrees with up to a 13.9 degree change in the bite line.[i]  The same authors have previously published a mathematical model of a normal cervical spine.2,3  This model demonstrates the average cervical curve to be 34 degrees.  Therefore, according to these two studies, a bite line deflection of 13.9 degrees or less may change the cervical curve measurement by 20.3%.  Furthermore, the aforementioned authors have previously stated that, “the atlanto-occipital joint acts as the pivot for the flexion/extension motion of the cranium. The occiput- to- C2 articulations average about 23 degrees of flexion/extension.”  From this they concluded, “slight head nodding occurs in the upper cervical spine, and does not affect curve measurements from C2-C7.”4  Due to the seemingly contradictory results of these studies, it became necessary to reevaluate their conclusions collectively.

Even though this last study mentioned above was very thorough and well documented, it is inevitably based upon an assumption.  This article assumes that any individual, prior to the onset of chiropractic care, will have normally functioning atlantal-occipital (AO) and atlantal-axial (AA) joints.  When these joints are working correctly (i.e. normal or full ranges of motion5,6), the conclusions of these studies may indeed be accurate.  However, there may be evidence to suggest that an unknown portion of the population may display a functional deficit at one or both of these upper cervical joints7-9 Due to the possibility of these functional deficits, the authors propose a method of evaluating the upper cervical joints radiographically by analyzing lateral cervical flexion and extension x-rays. This method may determine if these joints are in fact working normally in each patient.  The authors propose that upper cervical joint dysfunction may compromise the reliability of pre and post lateral cervical radiographs if consistent patient positioning is not maintained on pre and post lateral cervical radiographs.

            According to the literature, there exists an average of approximately 14.0 degrees of flexion and extension in the AO joint and approximately 12.5 degrees of flexion and extension of the AA joint.5,6   This breaks down into 7.0 degrees in either direction at the AO joint and 6.3 degrees in either direction at the AA joint.  Therefore, these ranges of motion are the maximum amounts that can occur at these joints before the measurement of the cervical lordosis from C2-C7 is affected.  However, these are the values for a normally functioning upper cervical spine.  When the AO and/or AA joints are restricted, these normal ranges are decreased, which may lead to compensatory flexion and extension in the middle to lower cervical joints so that the proper amount of global flexion and extension can still be achieved. 

Methods: 

            Twenty sets of cervical radiographs, provided by various chiropractors in private practice, were analyzed for atlantal-occipital and atlantal-axial joint function. These films included a neutral lateral cervical and cervical flexion and extension views. The radiographs were taken according to the specific patient positioning methods outlined by Jackson et al.10 The subjects represented in the sample radiographs were between the ages of 18-40. The histories of each subject are not reported because the purpose of this study is to investigate the existence and effects of upper cervical joint dysfunction, not the cause of the dysfunction. 

Each AO and AA joint was evaluated on the neutral lateral radiograph to determine their positions in relation to each other via a vertical gravity line.  A skull base line, an atlas plane line, and a C2 disc plane line were all constructed on each of the 20 sets of radiographs to evaluate relative atlantal-occipital and atlantal-axial flexion/extension.  The skull base line was constructed by connecting two points just posterior and just anterior to the occipital condyle-cranial base junction.  The atlas plane line was created using a point in the center of the anterior tubercle of atlas, and a point halfway between the posterior tubercle and lateral mass on the posterior arch of atlas.  Marking both the anterior-inferior and the posterior-inferior corners of the C2 vertebral body provided the reference points for the C2 disc plane line. The line is constructed parallel to the C2 disc.  The position of the atlantal-occipital joint was evaluated by measuring the angle created by the intersection of the skull base line and the atlas plane line on each radiograph.  The atlantal-axial joint was evaluated by measuring the angle formed by the intersection of the atlas plane line and the C2 disc plane line.  These lines and measurements were performed on all 60 radiographs.  The relative amounts of flexion and extension at both joints were calculated by taking the end value of the dynamic view (flexion or extension) and subtracting the value of that joint in the neutral position (neutral lateral cervical).  For example, when an occiput, initially positioned in 5 degrees of extension on the atlas in the neutral lateral cervical radiograph, repositions to 2 degrees of extension in the flexion radiograph, the net result is a relative flexion of 3 degrees in the AO joint complex. 

Results:

Utilizing the methods mentioned above, measuring all the angles on each film, only two of the sample subjects demonstrated a normal active range of motion at the AO joint. Most of these radiographs also revealed a deficit at the AA joint.  The average relative amounts of flexion were –0.9 degrees (the assigned negative value indicates paradoxical motion that will be defined later in this article) in the AO joint and 8.5 degrees of flexion in the AA joint.  As previously mentioned, these values should be 6.5 degrees and 5.0 degrees respectively.  Therefore, on average, a 7.4 - degree functional deficit existed at the AO joint and 2.2 degrees of excessive motion occurring at the AA joint.  In extension, the average relative amounts in the AO and AA joints were found to be 12.0 degrees and 2.8 degrees, respectively.  These values are normally expected to be 7.0 degrees (AO) and 6.3 degrees (AA). Therefore, our findings suggest an average of 5.0 degrees of excessive AO joint extension and an average AA joint extension deficit of 3.5 degrees. 

Paradoxical motion, which can be defined as a motion that occurs opposite of the expected motion, is an indicator of kinematic instability, according to White and Panjabi.11 In our subjects, AO and AA joint extension occurred during full cervical flexion in a significant portion of the sample size.  In fact 12 out of the 20 patients (60%) exhibited paradoxical motion at the AO joint during cervical flexion, compared to 1 out of 20 patients (5%) exhibiting paradoxical motion in the AO joint during cervical extension. Paradoxical motion occurred with much less frequency at the AA joint. Flexion and extension at this joint displayed roughly the same amount of paradoxical motion (10% in flexion and 15% in extension). 

Another factor of kinematic instability, according to White and Panjabi, is the presence of excessive motion in a given joint complex.11  Our sample subjects demonstrated excessive AO joint extension in 15 out of 20 cases (75%), as compared to only 3 out of 20 (15%) in AA joint extension.  The opposite was true in AO and AA joint flexion with excessive motion occurring in 2 out of 20 (10%) and 13 out of 20 (65%), respectively.  Thus, our sample group displayed evidence of kinematic instability of the upper cervical spine in both quantity and quality of motion.  It is also interesting to note that 10 out of the 12 subjects (83%) who displayed paradoxical motion at the AO joint during cervical flexion also demonstrated excessive AO joint extension during cervical extension.  Further studies need to be conducted to investigate any possible links or relationships between these two indicators of kinematic instability.

Discussion:

Due to the small, nonrandom sample size of this study, any conclusions made from the given results are premature. However, the occurrence of upper cervical spine dysfunction in this study is consistent with the previous findings of Kraemer and Patris14, and Skippings and Taylor.15 

            Given that reproducing patient positioning on pre and post imaging studies can be quite difficult, many line analysis systems may contain a large degree of mensuration error. However, in the present study, the patient positioning methods utilized have previously shown to produce a mensuration error of about 3-6%10.

Many chiropractic radiologists agree that the bite must be consistently maintained in the same position on pre and post radiographs in order to more accurately quantify cervical lordosis restoration.  Any chiropractic technique focused on obtaining consistent, quantifiable results must take the necessary precautions to avoid exaggerated or embellished measurements.  In addition to reproducible patient positioning, the inter- and intra-reliability of marking the radiographs is of key importance, as outlined by Jackson et al10-12and Rochester.13

            Radiographic analysis of upper cervical spine dysfunction may also have other implications. Previous studies have correlated upper cervical dysfunction with headaches of cervicogenic origin.16-30 Several authors have also documented the related anatomy,17-23 symptomatology24-26, neurophysiology17,27-30, and proposed mechanisms18,27 involved in cervicogenic headache, as well as treatment options in various disciplines.31-36  Conservative manual therapy trials have shown promise in treating cervicogenic headache through the manipulation of the upper cervical spine.34-36  Additional studies should attempt to correlate the presence of both upper cervical spine dysfunction and cervicogenic headache symptoms.

If it is previously known that maintaining the same bite line position from pre to post imaging studies will not be achieved, cervical flexion and extension views may be taken to properly calculate the ranges of motion in the AO and AA joints.  This may then be used as a tolerance for the greatest amount of bite line deviation that can exist for each patient from pre to post evaluation.  If the bite line deviation exceeds the calculated tolerance of the upper cervical joints, any quantitative evaluation of the cervical curvature may be rendered inaccurate, and the post radiographic evaluation may need to be re-performed.  This provides an inherent check and balance system to ensure quantitative accuracy for a reliable comparison of pre treatment and post treatment radiographs.  From this the chiropractic physician may more accurately chart a patient’s progress throughout a given treatment protocol.

            For accurate evidence-based outcome measures to be achieved, universal standards should exist so that all chiropractic care can be consistently evaluated regardless of the chiropractic techniques employed.  Currently, the vast number of chiropractic technique systems makes it difficult to objectively compare the outcome measures of each chiropractic method.  Due to the questionable reliability and validity of certain chiropractic diagnostic procedures, such as palpation and inclinometry37-42, radiographic biomechanical evaluation standards should be employed.  In this regard, the authors agree with the previous conclusions by Harrison et al that, “the use of radiography for identification of any abnormal lateral cervical configuration is absolutely mandatory.”43 However, their conclusion, based upon the evidence presented in this study, could be expanded to include the position and function of the upper cervical spine. 

Conclusion:

The process of evaluating the relative flexion and extension of the upper cervical spine from its neutral position may determine the maximum allowable tolerances of the upper cervical joints for each patient. These tolerances serve as the limits for which the upper cervical joints (and therefore the bite line) can vary before the curvature of the cervical spine is altered.  These tolerances may decrease the amount of error inherent in measuring cervical curve measurements on pre and post lateral cervical radiographs.  Omission of this process in radiographic analysis of lateral cervical x-rays may result in a measurement error of up to 20.3%, in an unknown portion of the population.  These tolerances, as well as upper cervical spine dysfunction, cannot be evaluated by visualization or surface curve measurements.44,45  These findings may provide an indication for conducting biomechanical radiographic evaluations in instances of cervicogenic headache and other symptoms18,25-27.  Future research should include a reproduction of this study on a larger sample size before these findings are applied to the population.

Acknowledgement:

            We would like to thank Tim Pugh, DC; Gary Lawrence, DC; and William Watt, DC for their time and effort in providing the materials necessary for this article.

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