Biomechanical And Clinical Research Of Subaxial Malalignment After Occipitocervical Fusion

Occipitocervical fusion techniques have evolved extensively since their initial description in the 1930 s.It has made satisfactory effect for restoring the stability of the occipitocervical junction(CVJ),increasing the fusion rate and maintaining the reduction.However,in recent years,some patients have developed subaxial malalignment after occipitocervical fusion,even kyphosis or subluxation,and part of them require secondary surgery,which has attracted the attention of doctors.Previous studies about this mainly focused on the radiographic follow-up of the O-C2 angle and C2-C7 angle,but its biomechanal mechanism has not been clarified yet.On the other hand,measurement of the O-C2 angle is difficult to achieve due to the split of specimens in vitro biomechanical studies,however,the posterior occipitocervical angle(POCA)is available.But a previous study pointed out the reproducibility of POCA especially in cases with a high degree of deformity still needs further verification.Objective1.To investigate the intra-and inter-observer reliability of POCA through 3 investigators repeated measurements and statistical analyzes.2.To reveal the influence of different POCA on the range of motion(ROM)and the intradiscal pressure(IDP)of subaxial cervical spine through in vitro biomechanical study on human cadaveric cervical vertebrae specimens.To reveal the biomechanical mechanism of the development of subaxial malalignment after occipitocervical fusion.To provide theoretical basis for choice of ideal POCA in the occipitocervical fusion.3.Through clinical follow-up,to evaluated the relationship between O-C2 angle,POCA and C2-C7 angle before and after occipitocervical fusion surgery as well as the last follow-up,to reveal the clinical significance of POCA.Methods1.O-C2 angle and POCA were measured the on the standard lateral cervical radiographs of 30 healthy people and 30 patients with occipitocervical lesions requiring occipitocervical fusion.The measurement was conducted by three experienced orthopedists for twice,each time separated by at least 2 weeks.Data of the measurement was recorded in separated document avoiding interaction.Intra-and inter-observer intraclass correlation coefficients(ICC)of the two methods were evaluated.Also,O-C2 angle and POCA of the two groups were analyzed.2.Eight recently dead cervical cadaveric specimens were used in vitro biomechanical study.The DePuy Synthes occipitocervical fusion system,biomechanical testing machine,OptiTrack? motion capture system,060 miniature pressure sensor and its matching P3 strain gauge and recorder and other experimental tools were also used.The hybrid test model was applied to simulate the three-dimensional motion of the cervical spine.The test was divided into 6 working conditions.The first was the normal group without internal fixation,and the other 5 were experimental groups with the POCA fixed at different angles in the occipitocervical fixation.They were: 1.Neutral position group(POCA=111°),2.Neutral-SD(POCA=101°),3,Neutral+SD(POCA=121°),4,Neutral-2SD(POCA=91°),5,Neutral position + 2SD group(POCA = 131°).In the experiment,range of motion(ROM)and intervertebral Pressure(IDP)of each intervertebral motion segment(C3/4,C4/5,C5/6,C6/7)in the four directions of flexion,extension,left lateral bending and left axial rotation were monitored and recorded.Data of the ROM and IDP were analysed.3.A retrospective study of 30 patients underwent occipitocervical fusion surgery performed by the same surgeon in the orthopaedic department of the Second Affiliated Changzheng Hospital,Naval Military Medical University from January 2005 to December 2016 was performed.The O-C2 angle,POCA,and C2-C7 angle of the neutral position cervical lateral X-rays before and after operation also the last follow-up were measured.The correlation between O-C2 angle,POCA,and C2-C7 angle was analyzed and evaluated.Results1.The O-C2 angle of the healthy group in this study was: 19.72°±6.80°,POCA: 111.13°±9.11°;in patient group O-C2 angle: 7.12°±11.70°,POCA: 121.23°±14.64 °;there were statistical difference between the two groups(P < 0.001,P = 0.003).In the healthy group,mean intraobserver ICC of O-C2 angle for 3 surgeon were 0.975,0.948,and 0.974 respectively,and interobserver ICC was 0.983;mean intraobserver ICC of POCA were 0.984,0.856,0.945 respectively,and interobserver ICC was 0.948.In the patienet group,mean intraobserver ICC of O-C2 angle were 0.990,0.988,and 0.986 respectively,and the interobserver ICC was 0.995;mean intraobserver ICC of POCA were 0.998,0.997,and 0.997 respectively,and interobserver ICC was 0.999.2.After the occipitocervical fixation,the ROM in all directions from C0 to C3 decreased significantly compared with the normal group,while the ROM in each direction of C3-C7 segments had a significant increase compared with the normal.POCA has significant influence on the flexion and extension test,and ROM of each segment showed similar trend.In flexion,with the increase of POCA,ROM of each segment decreases;while in extension,the trend is opposite.While POCA fixed at the neutral position,although ROM of each segment both in flexion and extension was still significantly larger than that of the normal,there was no extreme increase like that of flexion test when POCA was too small and that of extension test when POCA was too large.Although ROM of each experimental group in the left rotation and bending increased significantly after internal fixation,the change of POCA had no significant influence on ROM in these two test.In IDP test,with the increase of POCA,IDP tends to decrease both in flexion and extension.3.The symptoms of patients after occipitocervical fusion surgery were significantly improved.The preoperative O-C2 angle was negatively correlated with POCA and C2-C7 angle.POCA was positively correlated with C2-C7 angle.O-C2 angle,POCA and C2-C7 angle of the postoperative and at the final follow-up were all improved comparing with the preoperative.O-C2 angle,POCA and C2-C7 angle at the final follow-up were not significantly different from those after surgery.O-C2 angle of postoperative and of the final follow-up were negatively correlated with POCA and C2-C7 angle,but there was no significant correlation between POCA and C2-C7 angle.The change of O-C2 angle at the last follow-up negatively correlated with the change of POCA and the change of C2-C7 angle,and there was a positive correlation between the change of POCA and that of C2-C7 angle.Conclutions1.There was a significant difference in O-C2 angle and POCA between the healthy and patient group.These two methods of measurement have high intra-and inter-observer reliability both for the healthy and people with occipitocervical lesions.POCA angle is a clinically practical and highly reliable observation index,which can be used clinically and experimentally.2.After occipitocervical fixation,ROM of the fixed segment was significantly reduced,while ROM of the rest of motor segments had increased.When POCA fixed in neutral position,compared with overflexion and hyperextension,it can avoid extreme increased motion during flexion and extension,and the intervertebral pressure in each motion segment is approximately normal.Therefore,in occipitocervical fixation,it's best to fix POCA at the neutral position of about 111.13 ± 9.71 ° to obtain the best biomechanical environment.3.The occipitocervical fusion surgery can effectively improve the occipitocervical alignment and maintain the reduction.POCA is an important parameter of the whole cervical alignment,its change is negatively correlated with the change of O-C2 angle and positively correlated with the change of C2-C7 angle.In order to reduce the occurrence of subaxial malalignment after occipitocervical fusion,the operation should not be simply over-retroverted,rather caution should be taken about the choice of POCA.The overall alignment of the cervical spine should be considered comprehensively.For patients with small C2-C7 angle,POCA should not be reduced too much comparing with that of pre-operation.