In Vivo Ranges of Motion of Cervical Segments in Patients with Cervical Spondylosis During Dynamic Neck Motions

In Vivo Ranges of Motion of Cervical Segments in Patients with Cervical Spondylosis During Dynamic Neck Motions

Cervical spondylosis is a degenerative condition affecting the sub-axial cervical spine, often leading to symptoms such as axial neck pain, cervical radiculopathy, and cervical myelopathy. One of the most common surgical treatments for this condition is anterior cervical discectomy and fusion (ACDF). However, ACDF has been associated with the development of progressive adjacent segment degeneration (ASD) in over 90% of patients, with up to 25.6% requiring re-operations due to symptomatic ASD. Recent advancements in total disc replacement (TDR) prostheses aim to restore motion capability at the diseased levels, but clinical outcomes and post-operative re-operation rates have not been superior to ACDF. The etiology of ASD remains unclear, but one possible explanation is that the spinal motion restored after ACDF or TDR does not match the in vivo physiological motion of the patient-specific cervical spine. Given the lack of in vivo data on cervical motions in symptomatic patients, this study evaluates the in vivo range of motion (ROM) of each sub-axial cervical segment in spondylotic patients with symptoms at C5–6 using a dynamic imaging technique and compares these data with those of an asymptomatic control group matched for age and sex.

The study involved eight spondylotic patients (four females, four males; age range: 26–51 years; Pfirrmann grades: III to V at C5–6; all patients had ossification of the posterior longitudinal ligament at C5–6, producing myelopathy or radiculopathy) and ten asymptomatic subjects (four females, six males; age range: 30–59 years; Pfirrmann grades: I to III at C5–6) without prior spinal disorders. The cervical spine of each subject was imaged using a 3 Tesla MRI scanner with a spine surface coil and a proton density-weighted sequence. The MRI images were imported into 3D solid modeling software to construct 3D anatomical vertebral models of the cervical spine. A dual fluoroscopic imaging system (DFIS) was used to capture dynamic images of the cervical spine during neck motions. The subjects performed full ranges of flexion-extension and left-right axial twisting neck motions, and their cervical spines were imaged for less than 3 seconds for each activity to ensure the collection of a full dynamic cycle. Three full cycles were imaged for each subject. The fluoroscopic images and 3D vertebrae models were imported into Rhinoceros solid modeling software to create a virtual DFIS, which mimics the actual DFIS. Using a previously validated 3D-2D registration technique, the 3D models of the vertebrae were independently translated and rotated in six degrees of freedom until their projections matched the osseous outlines on the fluoroscopic images. The positions and alignments of the C3-C7 vertebrae captured on the paired fluoroscopic images were reproduced using the 3D vertebral models.

Intervertebral kinematics were calculated as the relative motion of the superior vertebrae with respect to the inferior vertebrae at the C3–4, C4–5, C5–6, and C6–7 levels using Cartesian coordinate systems. The ROMs in principal rotational directions of the neck for the vertebral segments during the two functional activities (flexion-extension and left-right twisting) were reported. The ROM was represented by the difference between the maximal and minimal values of each motion throughout a cycle. The averages of the three trials were used to represent the ROMs of the subject. Student’s t-tests were used to analyze the differences in ROMs between the healthy and spondylotic groups at C3–4, C4–5, C5–6, and C6–7 levels, with statistical significance set at P < 0.05.

During the flexion-extension motion of the neck, the ROM of C3–4 was 14.5° ± 5.7° in the healthy group and 13.8° ± 3.6° in the spondylotic patients (P > 0.05). At the adjacent level C4–5, the ROM was 16.7° ± 2.7° in the healthy group and 16.9° ± 2.5° in the spondylotic patients (P > 0.05). At the index level C5–6, the mean ROM was 15.1° ± 3.6° in the healthy group and 18.4° ± 5.9° in the spondylotic patients. The ROM of the healthy group was significantly lower than the spondylosis group (P 0.05).

During the left-right twisting motion of the neck, the ROM of C3–4 was 12.3° ± 2.9° in the healthy group and 11.8° ± 4.4° in the spondylotic patients (P > 0.05). At the adjacent level C4–5, the ROM was 10.9° ± 2.9° in the healthy group and 7.5° ± 2.4° in the spondylotic patients. The ROM of the healthy group was significantly higher than the spondylosis group (P < 0.05). At the index level C5–6, the mean ROM was 11.6° ± 3.8° in the healthy group and 6.3° ± 2.4° in the spondylotic group. The ROM of the healthy group was significantly higher than the spondylosis group (P 0.05).

These results indicate that the two neck motions lead to differences in angular ROMs between the spondylotic and healthy control groups. The spondylotic patients had significantly increased ROM at the diseased (C5–6) level during the flexion-extension motion of the neck, resulting in laxity at the index level. The ROMs were reduced at the diseased (C5–6) level and the proximately adjacent level (C4–5) of the spondylotic patients during the left-right twisting motion of the neck, resulting in stiffening of both the index and proximal adjacent levels. These data indicate that abnormal motion patterns of necks suffering from spondylosis are associated with physiological conditions in neck motions and specific cervical levels.

The study found that spondylosis was associated with a reduced ROM of the adjacent segment during the left-right twisting, which is a common daily functional activity of the neck. This highlights that data measured in a single neck motion such as flexion-extension may not comprehensively represent the effect of spondylosis on cervical behavior in other neck motions. Correspondingly, motion-preserving implants that were developed by considering only a single loading condition may not restore cervical motions under other loading conditions. Therefore, the effects of cervical spondylosis on cervical motion presented in this study could have important clinical implications.

Contemporary motion-preserving surgical treatments mostly aim to restore segment motion to “normal” levels, but post-operative complications such as ASD are still often reported. Spinal degenerative changes may result in significantly different motion patterns at the index level before surgery, indicating the long-term adaption of surrounding spinal structures to the disease status. Therefore, decompression surgery which causes minimal iatrogenic changes may be an alternative to ACDF. In addition, re-definition of design objectives for the motion-preserving implants (by further considering spinal tissue load sharing instead of uniquely restoring motion to “normal” levels) is necessary to improve clinical outcomes. Current TDRs commonly using artificial disks with metal-on-polyethylene articulations provide minimal resistance to intervertebral axial rotations. To match the normal intervertebral ROMs (15.1° ± 3.6° in neck flexion-extension vs. 11.6° ± 3.8° in neck left-right twisting) at the index (C5–6) level, TDR could be more suitable for neck flexion-extension motion than for neck left-right twisting. Recently, hybrid application of fusion and TDR has been reported in clinical studies to treat cervical spondylosis. It is shown that the hybrid surgery may have the biomechanical advantages to synergize the over-constraint of fusion and minimal resistance of TDRs. An in vivo study is warranted to compare the biomechanical functions of various surgical techniques including segmental fusion, TDR, and hybrid surgeries.

There are several limitations that should be noted when interpreting our data. A post hoc power analysis showed that the powers were 72% and 90% for the left-right twisting ROMs at the adjacent (C4–5) and index (C5–6) segments, respectively, but there was only a power of 25% for the flexion-extension ROM at the index segment. Due to the small sample size, whether spondylosis alters segment motion at the diseased level during neck flexion-extension rotation should be further validated. Furthermore, the lack of evaluation of soft tissue status also presents a limitation. Because soft tissue maladies may be associated with abnormal cervical ROMs, it is necessary to further quantify soft tissue changes using MRI in future studies (e.g., T2 values of the discs). In addition, we only included patients with spondylosis at C5–6. Future studies should also include other cervical degenerative pathologies such as single-level and multi-level cervical degenerations. Investigations of these patients pre- and post-operatively in a prospective, longitudinal fashion should be conducted in order to investigate the kinematic changes of the adjacent segments after surgery and to explore the biomechanical factors related to ASD development.

In conclusion, this study investigated the ROMs of a patient cohort with spondylosis at C5–6 during dynamic flexion-extension and left-right twisting neck motions using a dynamic imaging technique. Compared to those of asymptomatic subjects, it was revealed that spondylosis caused higher ROMs at the diseased level during neck flexion-extension, but lower ROMs at both the diseased and proximally adjacent levels during the left-right neck twisting. It indicates that spondylosis affects the ROMs of both the diseased and adjacent levels, depending on the neck motion scenarios. These data could provide valuable insights into the improvement of cervical surgery. We suggest that motion-preserving treatments should further consider pre-operative spinal disease status to restore physiological segmental ROM.

doi.org/10.1097/CM9.0000000000001209

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