In Vivo Biomechanics Study Of Cervical Facet Joints Based On Musculoskeletal Multi-body Dynamics Model

With the increasing use of electronic equipment,the incidence of cervical spondylosis is getting higher,and more and more young people have got cervical spondylosis.Facet tropism and its different orientations with respect to the sagittal plane are common physiological structures,which can affect the biomechanical environment of the cervical spine and lead to cervical degeneration.However,there are few studies on biomechanics of the facet joints in cervical spine,and little is known about the mechanical changes of the facet joints after artificial cervical disc replacement.The musculoskeletal multi-body dynamics model can accurately predict the internal joint motion,contact force,muscle force and ligament force of the prosthetic joint(including adjacent joints and facet joints)under various cervical motions,and a success has been made in the total replacement of hip and knee joints.In this study,a cervical musculoskeletal multi-body dynamic model which can take muscle force,ligament force,intervertebral compressive force(ICF),intervertebral shear force(ISF),facet joint force(FJF)and cartilage deformation simultaneously was developed.Then the effect of facet tropism,different facet joint’s orientations with respect to the sagittal plane and total disc replacement on cervical discs and facet joints were investigated under head flexion,lateral bending and axial rotation.The results are aimed to provide a theoretical basis for clinical treatment and prosthesis design.The main contents including:(1)The biomechanical effects of different levels of facet tropism on cervical.It was found that biomechanics was not affected by facet tropism induced by outward deflection of the right facet joint during head flexion and left lateral bending,because the right facet joint was always kept separated during these two movements.During extension and right lateral bending,facet tropism increased ICFs in the asymmetrical and adjacent levels.Facet tropism increased the ISFs in medial-lateral direction at the asymmetrical level,and the ISFs increased with the increase in asymmetrical angles.Facet tropism increased the FJFs under certain movements obviously.From the perspective of degeneration,increased ICFs,ISFs and FJFs may damage the annulus fibrosis and cause disc herniation.Facet tropism with respect to the sagittal plane may be a factor for cervical degeneration.(2)The effect induced by different orientations of facet joints on cervical biomechanics.It was found that during head lateral bending,posteromedially oriented facet joints can restrict the movement,posterolaterally oriented facet joints can facilitate the movement,and ipsilaterally oriented facet joints can restrict lateral bending to one side but facilitate the lateral bending to another side;During head rotation,facet joints oriented the same direction with the axial rotation restrict the movement,and facet joints oriented opposite direction to the axial rotation facilitate the movement;The ipsilaterally oriented facet joint led to a significant increase in ICFs and ISFs,which may accelerate cervical degeneration.The increased ISFs to one side will lead to disc herniation or cervical spondylolisthesis to the same side.Excessive cervical motion and external torque should be avoided for the ipsilaterally oriented facet joints,and much attention should be paid to ipsilaterally oriented facet joints in clinical treatment and disc replacement.(3)The effect induced by replacement of Pretic-I on cervical biomechanics was studied.It was found that Pretic-I had a significant effect on ICFs in the lower adjacent level,with a maximum increase of 23.17%.From the perspective of degeneration,the replacement of Pretic-I may accelerate the degeneration of the lower adjacent level.FJFs at the implanted level increased significantly under different movements after replacement of Pretic-I,and the increased FJFs may be the potential cause of facet joint degeneration.The ICFs increased rapidly under extreme head movement after replacement,suggesting that extreme head movement should be avoided for patients with Pretic-I replacement.