New Anterior Controllable Antedisplacement And Fusion Surgery For Cervical Ossification Of The Posterior Longitudinal Ligament:A Biomechanical And Clinical Applied Anatomical Study

BackgroundCervical myelopathy resulted from ossification of the posterior longitudinal ligament(OPLL)frequently developed into neurologic dysfunction,and surgical treatment is usually recommended for those patients.The surgical treatment of OPLL can be divided into anterior and posterior approaches.Furthermore,anterior approach has been reported to outperform posterior approach with respect to the postoperative neural function.However,anterior decompression and fusion has been demonstrated to be risky and technique demanding.In addition,it is difficult to reconstruct the stability of cervical spine in the surgical treatment of long segment OPLL.Therefore,the optimal operation of OPLL is considerably debatableTo address this issue,a new technique named anterior controllable antedisplacement and fusion(ACAF)surgery was designed for the treatment of OPLL,in which the vertebralOPLL complex(VOC)was isolated and hoisted anteriorly without removing the OPLL.By such procedures,it can not only obtain good clinical outcomes due to anterior direct decompression,but also ensure the safety similar to posterior surgery.And it has been found in clinical research that the incidence of complications related to internal fixation was significantly lower than that of traditional anterior surgery.However,in the ACAF surgery,it needs to remove some structures on both sides of the vertebral body to make it isolated,which will aggravate the damage to the internal stability of the cervical spine.Nevertheless,ACAF adopts the method of multi segment screw placement to disperse stress similar to ACDF to avoid the occurrence of stress concentration.Therefore,the stability and biomechanical characteristics of ACAF should be further studied.In the meanwhile,the anatomical study of ACAF is not perfect.For the spine surgeons,it is particularly important to have a good grasp of this new technology and have a thorough understanding of its anatomy,especially in the identification of the anatomical marks of osteotomy,the risk of vertebral artery injury(VAI)and the anatomical relationship between the slotting and spinal cord,nerve root and cervical intervertebral foramen.In this study,the biomechanical properties of ACAF,ACDF and ACCF were compared and analyzed to verify the stability and effectiveness of ACAF technology and provide reference for its clinical application.By constructing the three-dimensional finite element model of ACAF,the biomechanical characteristics of ACAF were compared with those of ACCF and ACDF.Objective to evaluate the risk of vertebral artery injury in ACAF by anatomic study of fresh cadaver specimens and radiographic study of pre-and post-operative OPLL patients,and point out the anatomic relationship between the slotting and the spinal cord,nerve root and cervical intervertebral foramen,so as to provide important anatomical reference for clinical operationObjectiveTo verify the stability and effectiveness of ACAF technoloty by compared with mainstream anterior approachs in the biomechanical stability.To clarify the biomechnical characteristics of ACAF technology by constructing its three-dimensional finite element model.To confirm the safety of ACAF technology in vertebral artery injury,and calrify the anatomical relationship between the slotting and spinal cord,nerve root and cervical intervertebral foramen.To provide scientific reference and theoretical basis for its application in personalized cervical surgery,clinical improvement of surgical techniques and improvement of clinical outcome.Methods1.Cervical areas C1 through T1 were used from 18 cadaveric spines to test flexionextension,lateral bending,and axial rotation ROM(2Nm).The cyclic fatigue loading test was set up with a cycled load of 3Nm(2Hz,3000 cycles).All specimens were randomly divided into three groups according to the surgical procedures: ACDF,ACAF,and ACCF group.They were tested in the following conditions:(1)Flexibility test of intact state,(2)Flexibility test of each model(ACDF,ACAF,ACCF),(3)cyclic loading,n=3000,(4)Flexibility test of each model.2.A finite element model of a healthy cervical spine(C2-C7)was constructed.Three surgical approaches were developed:(1)2-level ACAF at C4 and C5,(2)2-level ACCF at C4 and C5(3)3-level ACDF at C3/C4,C4/C5 and C5/C6.A 75-N follower load with 2 Nm was applied to simulate flexion,extension,lateral bending,and axial rotation.The range of motions(ROMs),endplate stress,fixation system stress,bone-screw interfacial stress and graft stress among three surgical approaches were compared.3.Computed tomographic(CT)scan data of 28 patients were retrospectively studied.7 radiographic parameters were evaluated: UP(uncinate process)tips distance,TF(transverse foramen)-UP tips distance,TF-LWL(the ipsilateral limited wedging line)distance,LDLD(the limited distance of lateral decompression),the maximum oblique angle of LWL,TF-LWG(the lateral wall of groove)distance,and width of groove.11 fresh cadaveric spines undergoing ACAF surgery were also studied.2 anatomic parameters were evaluated: width of groove,LWG-TF distance.4.Radiographic data of consecutive 47 patients treated for cervical OPLL and stenosis were retrospectively reviewed.3 postoperative parameters were evaluated: the decompression width,the satisfaction rate of decompression at the entrance zone of intervertebral foramina and the transverse diameter of spinal cord in the decompression levels.3 fresh cadaveric spines undergoing ACAF surgery were also studied.4 anatomic parameters were evaluated: the width of groove,the distance between the bilateral origins of ventral rootlets,the length of ventral rootlet from their origin to the intervertebral foramina,the descending angle of ventral rootlet.Results1.After operation,the segmental and total ROM in all directions showed a significant reduction for all groups.Then they tended to be larger after fatigue test.The crossover effect of evaluating time and operation method showed no significant,which meant that the change trends of segmental and total ROM were parallel among groups.However,the postoperative and fatigue ROM in ACCF group tended larger in all directions,and no differences were found between ACDF and ACAF group.2.ACAF and ACDF decreased more ROMs after surgery than ACCF,especially in flexion and lateral bending.ACCF have higher stress peaks in the fixation system than those of ACAF and ACDF,especially in flexion.The maximum von Mises stresses of the bone-screw interfaces at C3 and C6 of ACCF were higher than those of ACAF and ACDF.The maximum von Mises stresses of the grafts of ACCF and ACAF were much higher than those of ACDF.The maximum von Mises stresses of the endplate of ACCF were much higher than those of ACAF and ACDF.3.The UP tips distance increased from C3 to C6 and tended to be larger in males.The UP tip-TF distance and LWL-TF distance were smallest at C4,but both were larger than 2mm.Maximum oblique angle decreased from C3 to C6.Postoperatively,both radiographic and cadaveric measurements showed the width of groove was larger than UP tips distance,but LWG-TF distance were larger than 2 mm in all levels.4.The groove created in ACAF surgery obviously included the bilateral origins of ventral rootlets.The rootlets tended to be vertical from the rostral to the caudal direction as their takeoff points from the central thecal sac became higher and farther away from their corresponding intervertebral foramina gradually.The decompression width was significantly greater in ACAF group.The satisfactory rate of decompression at the entrance zone of intervertebral foramina tended to be better in the left side and ACAF group.ConclusionThis biomechanical study and finite element analysis confirm that ACAF and ACDF present similar biomechanical stability,which is significantly better than ACCF.The theory of multi-segment screw placement is also applicable to ACAF technology.The risk of internal fixation failure and cage subsidence in ACAF are less than that in ACCF,but the its risk of internal fixation failure and rostal and caudal screw loosening are still higher than that in ACDF,especially in axial rotation.In terms of bone fusion,ACAF is greater than ACDF.Furthermore,it is confirmed by anatomical and radiographic studies that UP can be used as anatomical landmarks to avoid VAI during ACAF surgery.Radiographic and cadaveric measurements verified the safety of ACAF surgery,even for those special cases with wedging and lateral slotting.ACAF can decompress the entrance zone of intervertebral foramina effectively and its decompression width includes the origins and massive running part of bilateral ventral rootlets.This research will provide a scientific reference for the interpretation of the current clinical effects of ACAF technology and the prediction of longterm complications,and it possesses an important guiding significance for improving clinical practice and improving the prognostic rehabilitation program.Combined with the clinical superiorities of ACAF,it is likely to be an alternative method for the treatment of multilevel cervical OPLL.