Spinal manipulative therapy (SMT) is a widely used manual treatment, but it has not been currently possible to provide overall conclusions about the safety of SMT . Stress and strain on the neck during SMT have been found to vary greatly among clinicians and the locations of treatment on the spine . With the continuous intensification of multidisciplinary and interdisciplinary research, the combination of biomechanics on the human musculoskeletal system and manual therapy has gradually become a focus of attention in the field of biomechanics in sports and rehabilitation medicine in which the methodology and technology of traditional Chinese medicine (TCM) is being investigated [27, 28]. In the present study, the biomechanical effects of three conditions were simulated and analyzed during CRM both with and without position-loading in an SSM and PM.
(1) Effect on angular displacement
The results indicated that there was no apparent distinction between the SSM and PM during CRM both with and without position-loading for relative angular displacement. However, the C5-C6 level in the PM experienced large angular displacement during CRM with position-loading due to the thumb thrust force on the left side of the C5 spinous process, corresponding to the experience of clinical therapy. In addition, angular displacement of segments above the C5-C6 level was relatively lower than during CRM with position-loading. However, angular displacement of the segments below the target level was relatively higher. The C5-C6 segment is located in the middle and lower part of the cervical spine, close to the C7-T1 segment. Because of the limited degrees of freedom of the lower surface of T1 in each direction, larger angular displacements could be generated below the target level during loading of the thumb thrust force on the C5 spinous process. Conversely, the C0-C2 segment of the cervical spine experienced a large horizontal rotation, which to a certain degree compensated for the coupling force of the target segment.
(2) Effect on intervertebral discs
During CRM with position-loading, the IDP at C2-C3, C3-C4 and C4-C5 was lower than during CRM without position-loading, consistent with changes in relative angular displacement at the same levels. Furthermore, IDP of the target C5-C6 segment during CRM with position-loading on the PM was lower than that during CRM without position-loading, the former being close to the magnitude of IDP in the C5-C6 segment during CRM without position-loading in the SSM. It is, however, worth noting that the IDP of the C7-T1 segment significantly increased during CRM with position-loading. This is probably due to the limited degrees of freedom of the lower surface of T1 in each direction. Furthermore, these results indicate that stress concentrated at the lateral margins of the fibrous annulus of the intervertebral discs on the rotatory side of the cervical spine during CRM. Previous studies confirm that stress on the disc and the tissues between the cervical nerves, spinal cord and other structures increase following degeneration of the intervertebral discs.  Therefore, we believe that it is safer for patients with intervertebral disc degeneration to undergo CRM with position-loading in order that the increase in IDP is limited.
(3) Effect on articular surface contact pressure
For comparison, the articular surface contact pressure of the C2-T1 levels during CRM without position-loading in the PM was established as 100% of the standard threshold. Results demonstrated that the left articular surface contact pressure of the target C5-C6 segment during rightward CRM without position-loading in the PM was slightly lower than right articular surface contact pressure during leftward manipulation and lower than in the SSM. However, contact pressure at the C5-T1 during leftward CRM with position-loading in the PM and at C6-T1 during rightward CRM with position-loading on the PM were significantly higher than the standard threshold, especially that of the articular surface contact pressure at C5-C6 during leftward CRM with position-loading in the PM. In comparison, contact pressure at C5-C6 during the other three experimental conditions was lower than the standard threshold. The results agree with the characteristics of the pathological mechanism, indicating that the C5-C6 left capsular ligament exhibited linear stiffness, while the stiffness on the right side was nonlinear. Therefore, it is more appropriate for the target C5-C6 pathological segment and segments below during CRM with position-loading to use the same static loading. However, future studies should focus on the rapid increase in articular surface contact pressure and whether this would result in iatrogenic damage or other adverse effects.
There are several limitations to the present study which should be noted. Firstly, the simulations were performed under idealized conditions that did not consider muscle forces in the neck. Because of the significant non-linear material properties of muscles, characteristics of the muscular self-reflex system and control by the brain of the muscles, simulating the function of the muscles was difficult using FE analysis at the present stage. The present study assumed that the volunteer had undergone CRM with complete muscle relaxation with no resistance. Secondly, FE static analysis was used to ensure higher calculation accuracy. Nevertheless, time was not taken into account in this study, even though viscoelasticity of the ligaments is time-dependent. Therefore, FE dynamic analysis of viscoelasticity of the ligaments could be further analyzed to study the biomechanical state of the cervical spine when restored to neutrality following the impulse phase. Future investigations might require consideration of these factors and provide a deeper understanding of the biomechanics of CRM in a sitting position with and without position-loading via finite element analysis and represent a more useful reference for clinical guidance.