Abstract
Introduction: Vertebral end plate is both inervated and has been shown to be a source of pain. Clinical experience shows some degree of end plate subsidence, usually posteriorly, in cases of total disc arthroplasty (TDA). It follows therefore that this may be a cause of pain. It is theorized that in quiet standing, a prosthesis with a posteriorly placed centre of rotation (COR) will have maximal end plate stresses posteriorly unless the centroid of the prosthesis end plate lies over the COR of the prosthesis. The rationale is that a posteriorrly weighted non uniform static end plate stress distribution will be produced in order to satisfy the static requirement that the sum of all moments be zero. It is further theorized that removal of portions of anterior end plate to move the centroid closer to the pivot point will make the stress distribution more normal.
Methods: A ball and socket prosthesis with a posterior articulation was subjected to static compression with an Instron testing machine against a foam block (Sawbones 1522-11) with the lower part of the specimen on rollers to allow lateral translation. Load was applied normal to the lower end plate in displacement control at a constant rate of 3 mm/min. Load displacement curves, lateral translation of the lower endplate and subsidence angle into the foam was measured. The experiment was repeated with various shaped end plates with the same outside footprints, though with various cutouts of the endplate footprint, such that the footprint area was reduced though with the centroid now lying over the prosthesis COR.
Results: With the standard prosthesis subsidence was noted to be associated with a translatatory movement of the inferior assembly and tilting, with the posterior portion subsiding more than the anterior portion. The prosthesis continued to subside with an increase in the tilt angle and liftoff of the anterior portion of the end plate until the calculated centroid of the subsidence footprint in the foam was over the prosthesis COR. With an end plate cutout the inferior assembly did not translate and the prosthesis subsided in a parallel fashion. This behavior was unchanged by varying the geometry of the cutout. The area of the modified cutout prosthesis was 69% (564 sq mm cf 817 sq mm) of the area of the original end plate, while the yield load was 89% (2.34 kN cf 2.62 kN) of the original. The Yield stress was higher in the modified prosthesis (4.13 MPa cf 3.21 MPa)
Discussion: A potential reason for end plate subsidence in TDA’s may be mismatch between the position of the end plate centroid and the COR. In static loading this causes a tendency to posterior subsidence and tilting. Removal of part of the end plate footprint such that the centroid is moved closer to the COR results in less of reduction in yield load than would be anticipated by loss of surface area alone. There is also abolition of subsidence by tilting.
Correspondence should be addressed to Dr Owen Williamson, Editorial Secretary, Spine Society of Australia, 25 Erin Street, Richmond, Victoria 3121, Australia.
