The effects of different articulate curvature of artificial disc on loading distribution.
OBJECTIVE
Deeper insights into the mechanical behavior of lumbar disc prostheses are required. Prior studies on the biomechanical performance of artificial discs were mostly performed with finite element analyses, but this has never been analyzed with altering articulate curvature. This study aimed to ascertain the influence of the geometry of a ball-and-socket disc prosthesis for the lumbar spine.


METHODS
Three-dimensional finite element model of human L4-L5 was reconstructed. Convex, concave, and elliptic artificial disc models were also established with Computer-Aided-Design software. Simulations included: (1) three articulate types of polyethylene (PE) insert were implanted inferiorly and (2) concave and convex PE inserts were implanted on the superior or inferior sides in flexion/extension, lateral bending, and axial rotation in the lumbar spine. Shear stresses and von Mises stresses on PE insert were assessed for their loading distributions.


RESULTS
High shear stresses of all articulate types occurred in flexion, and convex PE insert performed the maximum stress of 23.81 MPa. Under all conditions, stresses on concave PE inserts were distributed more evenly and lower than those on the convex type. Elliptic geometry enabled confining the rotation of the motion unit. The shear force on the convex PE insert on the inferior side could induce transverse crack because the shear stress exceeded yielding shear stress.


CONCLUSIONS
The concave PE insert on the inferior side not only decreased loading concentration but had relatively low stress. Such a design may be applicable for artificial discs.
Department of Physical Therapy and Assistive Technology, National Yang Ming University, Taipei, Taiwan.
Hung-Wen
HW
Wei
Yueh-Feng
YF
Chiang
Yu-Wen
YW
Chen
Cheng-Kung
CK
Cheng
Yang-Hwei
YH
Tsuang
D001696:Biomechanical Phenomena; D017076:Computer-Aided Design; D020342:Finite Element Analysis; D006801:Humans; D007403:Intervertebral Disc / Q000033:anatomy & histology*; D008159:Lumbar Vertebrae / Q000033:anatomy & histology; D008954:Models, Biological; D020959:Polyethylene / Q000737:chemistry; D016059:Range of Motion, Articular; D033081:Shear Strength; D059145:Total Disc Replacement
D020959:Polyethylene