This study is an in vivo comparative analysis of the total cervical axial range of motion and atlantoaxial rotation.

Twenty healthy volunteers, all medical staff, including 10 males and 10 females, were recruited for this study. The inclusion criteria were age above 18 years, no previous cervical spine surgery or diagnosed pathology, and no history of chronic neck pain or shoulder-related disorder. The age of the participants was 34 ± 6 years, and the body-mass-index was 27 ± 4. The technique was explained to the volunteers, and informed consent was obtained from each participant.

Active ROM was assessed using a common orthopedic handheld goniometer with 1° increments. ROM was measured in a sitting position, and subjects were sitting comfortably, with their feet flat on the floor and hands relaxed by their sides. They were asked to sit up straight and position their head in a neutral position, as described elsewhere [15]. Clinical measurements were performed for each direction of rotation, and the sum is reported as total clinical neck rotation (cROTneck). Measurements of atlantoaxial rotation, namely, C1-2 rotation, during left and right head rotation were performed in terms of the flexion-rotation test (FRT). The sum of the left and right rotation is reported as cROTC1−2.

The FRT is a commonly used test to analyze the ability of atlantoaxial joints in axial rotation [16]. It was shown to be reliable to analyze C1-2 ROM by Hall et al. [17]. With the subjects sitting in the upright position, the head was actively flexed, followed by active left and right rotation to take the measurements.

Landmarks for clinical measurements included the vertex and a line imagined from the top view connecting the nose to the opisthion (Fig. 1). For the statistical analysis, ROM in the left and right directions were summed and reported as total ROM values.

Drawings illustrating clinical measurement of C1-2 axial rotation during the flexion-rotation test

Clinical measurement of total cervical rotation and C1-2 axial rotation in the head-flexion position was performed by an experienced orthopedic surgeon. Measurements were performed three times for each direction of rotation, and the average was taken for statistical analysis.

To benchmark the clinical measurements with a standard of reference, cROTneck and cROTC1−2 were studied in detail using functional MRI as described below.

ROM was studied using MRI with the subjects in a comfortable supine position. For the study, a 0.5 Tesla Philips Gyroscan NT 5 Magnetic Resonance scanner (Philips Medical Systems, Reigate, U.K.) was used. Axial T1-weighted images from the base of the skull to T1 were obtained. The image slices were 4 mm thick with a 0.4 mm interslice gap. At first, a set of images was acquired with the subject supine and the head in a neutral position. Next, the head was turned to the individual maximum left and right positions, and the imaging was repeated. The subject was asked to remain in the rotated position without movement for the duration of the investigation. Because axial rotation of the head is accompanied by some tilting of the vertebrae relative to the axial plane, the initial‚ scout or planning MRI was repeated after each change in head position, as recommended by Roche et al. [2].

Analysis of the MRI slices was performed by three observers, and the means were used for statistical analysis. Digital measurements with 0.1-mm increments were performed on the axial MRI slices using commercially available software (Escape Medical Viewer V3, Escape Thessaloniki, Greece). The measurements were performed as shown in Fig. 2, similar to a technique applied for C1-2 ROM measurements in three previous studies using functional CT scanning [12, 13, 18]. The C1 angle and the C2 angle were measured in the left and right head-rotation positions.

The technique for measuring the C1-2 angle on MRI slices. Green-line: midsagittal plane. α = right mROTC1; α′ = right mROTC2; β = left mROTC1; β′ = left mROTC2; Separation angle C1-2 right side = α—α′. Vice versa for the left side. Total axial rotation of C1-2 is defined by the net sum of both separation angles

Maximum rotation of C1 to either side (mROTC1) was defined by the angle formed between the midsagittal line and the C1 bisector line, which connects the anterior and posterior C1 tubercle, crossing the tangent line of the posterior surface of the C1 lateral masses.

The rotation of the axis vertebra was assessed by drawing a bisector line for C2 that crossed the posterior cortical wall tangent at its widest level. The midsagittal line of C2 was calculated to the vertical axis delineating the maximum rotation of C2 to either side (mROMC2). Axial rotation of the subaxial spine is expressed by mROTC2. Left and right mROTC1 and mROTC2 were calculated, and the separation angle between C1 and C2 was defined. This is the algebraic difference between the angle subtended by C1 and the angle subtended by C2.

Because the contribution of occipitoatlantal rotation to total neck rotation is negligible [1,2,3], C0 angles were not created, and maximum C1 rotation was defined as maximum head and neck rotation in this MRI study (mROTC1). Maximum C1 rotation is the ability of the total cervical spine to rotate to the left (mROTneck−left) and the right (mROTneck−right). For the statistical analysis, total left and right neck rotation (mROTC1) was summed and reported as total neck rotation (mROTneck). The sum of the separation angle C1-2 in left and right neck rotation was reported as the ability of C1-2 to perform axial rotation (mROTC1−2).

Statistical analysis included a direct comparison of cROTneck and mROTneck as well as of cROTC1−2 and mROTC1−2. To study the amount of C1-2 axial rotation contributing to total cervical rotation using clinical and MRI-based measurements, C1-2 axial rotation was expressed as the percentage of C1-2 rotation for each modality studied (%cROTC1−2 and %mROTC1−2).

Data were checked for consistency and normality. ANOVAs with and without the assumption of homogeneity of variance were used. Levene's test was used to assess variance homogeneity. Paired and unpaired Student’s t tests were used for pairwise comparisons. Pearson and Spearman correlation coefficients were computed to analyze correlations. Intraclass correlation coefficients (absolute agreement, ICCs) were computed. All reported tests were two-sided, and p-values < 0.05 were considered statistically significant. All statistical analyses in this report were performed using NCSS (NCSS 10, NCSS, LLC. Kaysville, UT) and STATISTICA 13 (StatSoft, Tulsa, OK).