Biomechanical Comparison Analysis of Cervical Plate Systems for Anterior Cervical Discectomy and Fusion: Constrained vs. Semi-constrained Systems

Kang, A Ram; Kim, Jin Hwan; Lee, Sung Jae

Abstract

The purpose of this study were to constructed multi-level cervical spine finite element (FE) model and to investigate changes in load distribution and range of motion (ROM) at index level and adjacent levels at immediately after anterior cervical discectomy and fusion (ACDF) and after full bony union using constrained and semi-constrained cervical plate systems(dynamic plate, variable screw). A FE model of intact cervical spine (C3-6) was created from computer tomography (CT) images of the healthy adult (male, 26 years, no pathologies). The post-op FE models (C5-6 with ACDF, cage with bone graft) were constructed by modifying a intact cervical FE model. Four different configurations of the model were considered: Type 1-Rigid plate+Fixed screw, Type II-Rigid plate + Variable screw, Type III-Dynamic plate + Fixed screw, Type IV-Dynamic plate + Variable screw. The bone-cage and bone screw interface behavior were accomplished via 'tie' contact condition and friction coefficient of 0.2 to assume fusion and non-fusion, respectively. The inferior endplate of C6 vertebral body was constrained in all directions. Loading condition used hybrid protocol with follower load of 73.6N at superior endplate of C3 vertebral body. In non-fusion cases, load at the bone graft increased from Type I to IV (2.3 --> 6.8%, flexion; 2.9 --> 7.6%, extension) while plate load was reduced (91.4 --> 75.7%, 89.4 --> 71.8%, respectively) and the rest went through the facets. Similar trend was observed in fusion cases at bone graft/fusion (6.4 --> 12.4%, 7.4 --> 10.4%) and plate (83.3 --> 71.3%, 80.6 --> 67.2%). As compared to the intact spine, ROM of Type IV was most similar to those of the intact spine; least reduction at the index (-36%) and least increase (+7%) at the adjacent level. This was followed by Type III, II, and I and this trend was more evident in non-fusion cases than after fusion. With more degree of freedom (Type I --> Type IV), the load sharing became more favorable for bony union across the graft/cage area. Simultaneously, excessive changes in motion at the index and adjacent levels were reduced. These biomechanical changes could contribute to promoting fusion process. Our study predicted more biomechanically sound characteristics of semi-constrained cervical plate system as compared to the conventional constrained system in terms of load sharing and motion kinematics.