The lateral mass is an important component of a vertebra that maintain stability. The morphology of a subaxial lateral mass is different from that of the lumbar spine: larger articular processes and isthmus, together with the gradual facet surface, constitute the unique columnar lateral mass structure of the cervical spine[11–13]; thus, it bears more longitudinal load.
The cervical nerve roots originate from the cervical cord and exit the spinal canal through the intervertebral foramina in front of the lateral masses. Therefore, to expose a dumbbell tumor originating from the nerve root during surgery, it is necessary to resect the whole or part of the lateral mass. In addition, primary spinal tumors often directly involve the lateral mass, and their treatment even requires total en bloc spondylectomy (including the vertebral body, lateral mass, lamina, transverse process, and other structures). In addition, infection and trauma may lead to bone destruction of lateral masses. After lateral mass destruction of > 50%, cervical stability decreases significantly.[1, 5, 6]
The main internal fixation method used to maintain posterior cervical stability is the fixation of screws, such as lateral mass screw and pedicle screw. Although screw and rod fixation can stabilize the cervical spine and reduce the ROM promptly, stress was not low in the screw-rod fixation group compared to those in other groups, indicating that the screw and rod play an important role in maintaining stability, but are likely to be subjected to high stress-induced instrument failure insidiously. Although pedicle screw implantation with the strongest biomechanical properties can and bear more load and have four times the pull force of the lateral mass screw [16, 17], it cannot reduce the stress of the instrument and carries a high risk of damaging surrounding artery and nerve tissue. In addition, although screw and rod fixation can provide prompt cervical stability, it does not compensate for the bone defect and it is difficult to attain the aim of lateral joint fusion; thus, the risk of long-term instrument failure due to fatigue is insidious. Even if supplemented with common posterolateral bone grafting, it is difficult to achieve bone fusion in time due to the lack of stress stimulation that caused bone graft resorption.
Therefore, our strategy uses a safer lateral mass screw fixation technique combined with lateral mass reconstruction and interfacet bone grafting. Considering the importance and safety of the lateral mass reconstruction, we designed the subaxial lateral mass prosthesis for interbody bone grafting, and its biomechanical superiority was verified using finite element analysis. First, after the lateral mass prosthesis implantation, the ROM was significantly reduced and the model was stable. Second, the weight above the prosthesis can be transmitted along the prosthesis, which is more consistent with the normal three-column transmission of the subaxial cervical spine, thus promoting bone fusion in the prosthesis. Third, it can significantly reduce the stress of the screw and rod.
Minimum ROM, optimal stability
Zdeblick et al., Raynor et al., and Cusick et al., examined the graded resection of facet joints and found that when more than 50% of a facet joint is removed, the displacement during flexion, extension, and rotation significantly increases, and torsional stiffness, shear strength, and the capacity to withstand compression-flexion loads is significantly reduced.[1, 3, 5] Similar results were found in our study, although we did not perform graded excision. The ROM significantly increased after lateral mass resection, especially in the right bending and rotation states, indicating that the lateral mass plays a significant role in limiting excessive movement in these directions.
Ji et al. conducted biomechanical experiments on seven cervical spine specimens and found that, based on unilateral pedicle screw and contralateral lamina screw fixation, the application of a bone graft to reconstruct the lateral mass can effectively reduce the ROM in all directions and could increase stability. However, the reconstruction type used was less stable than bilateral pedicle fixation without lateral mass reconstruction during lateral bending and axial rotation. What was different in our study was the addition of a prosthesis based on bilateral fixation. Compared with the screw-rod fixation group, the ROM after prosthesis implantation was reduced in all movements, especially − 8.3% and − 25.6% during the left and right bending movements, respectively. This may be due to the improvement in the overall structural stiffness of the adjacent segments through the support of the bone graft column and the fixation of the posterior fixation plate.
Hence, the use of lateral mass prostheses can achieve optimal cervical stability based on screw fixation. Simultaneously, this early spinal stability provides a good mechanical environment for subsequent bone fusion.
Prosthetic load sharing promotes interbody fusion
In various movement states, the lateral mass prosthesis shared the weight above the prosthesis, especially in flexion, right bending, and rotation. This further indicates that the lateral mass structure can limit excessive movement and maintain stability in these movements. Similar to interbody fusion during discectomy, we implanted autologous granular bone into the prosthesis. The body’s weight applied to the prosthesis fully stimulates the bone graft in the prosthesis and promotes intervertebral fusion of the interfacet space, which has a higher fusion rate than that of a posterolateral bone graft.
After fusion was achieved, the stress of the prosthesis decreased in the right bending state compared to that before fusion. This may have occurred because the fused bone was tightly attached to the adjacent facet, and the weight over the prosthesis was shared by the fused bone in the prosthesis during right bending. This suggests that the reduced stress of the prosthesis after bone fusion helped to reduce the risk of subsidence.
Reduce the stress of the screw and rod
Compared to the fixation group, the stress on both screws and rods was reduced after prosthesis implantation. This is mainly due to the load-sharing effect of the prosthesis, which transferred the weight from above the prosthesis to along the prosthesis, resulting in stress reduction on the bilateral screw and rod in various motion states (except the left screw and rod during extension).
The stress reduction on the right screw and rod was significantly greater than that on the left screw and rod, especially during the right bending and left rotation, which indirectly indicated that the prostheses could significantly limit these movements and reduce the load on the right screw and rod.
Before and after the fusion of the lateral mass prosthesis, no significant difference was found in the force of the screw and rod mainly because the sliding trend of the prosthesis was limited by the relatively stable and reliable connection between the screw and rod set by the finite element model.
Traditional specimen experiments have only compared the ROM, which cannot reflect the advantages of load sharing and stress reduction of the screw and rod system. Our finite element analysis study properly reflects the above advantages and is a practical analysis of the reconstruction of the lateral mass structure. However, it cannot reflect the fatigue and maximum failure values; thus, the investigation of the overall stability of the model is not comprehensive and can only be used as a supplement to the in vivo experiment.