149 - Does Sacral Geometry Affect Biomechanical Behavior and Facet Forces af...

#149 Does Sacral Geometry Affect Biomechanical Behavior and Facet Forces after Total Disc Replacement at the Lumbosacral Junction?

Oral Posters: Deformity

Presented by: S. Robinson

Author(s):

S.T. Robinson (1)
R.B. Delamarter (1)
M.T. Svet (1),M.F. Metzger (1)

(1) Cedars Sinai Medical Center, Spine Center, Surgery, Los Angeles, CA, USA

Abstract

Aim: The lumbosacral junction presents unique biomechanical challenges to motion preservation due to its orientation, caudal location, and consequential shear loading. Reports of inferior total disc replacement (TDR) outcomes at L5/S1 compared to levels above have been reported including increased facet joint pain, decreased post-operative ROM, and greater vertebral translation1,2. The degree of sacral slope has yet to be investigated in relation to surgical treatment of degenerative disc disease with motion preservation devices. The purpose of this study is to determine whether there is a relationship between the obliquity of the sacral slope and increased biomechanical risk factors for TDR leading to reduced clinical outcomes at L5/S1.

Methods: Seven non-degenerative cadaveric L5/S1 motion segments were cleaned of non-structural soft tissue and potted with their sacral endplate parallel to the potting surface, allowing experimental control of the sacral slope from 20 o to 70o, in 10 oincrements, using a novel testing apparatus. Specimens were subjected to 800 N of force distributed between an axial and an anterior shear component corresponding to the degree of sacral slope set by the testing apparatus. Specimens were first tested intact at each sacral angle in random sequence. Six specimens were re-tested after implantation of a Prodisc-L, preserving one intact specimen for load-to-failure comparison. Motion-tracking data was recorded using Optotrak cameras and facet loads were captured via customized pressure sensor arrays. Afterward all specimens were loaded to failure to determine ultimate load, stiffness, and maximum anterior displacement at 40o sacral slope.

Results: The percent change in anterior displacement after device insertion was inversely related to the sacral angle, approaching near intact values at 50o (20o>40o,50o,60o,70o;30 and 40 o>50 o,60o,70o p< 0.05), Figure 1A. Both the intact disc and TDR demonstrated an increase in the amount of anterior displacement as the sacral slope increased. Percent change in the facet pressure was not significantly different when comparing the various sacral slopes (p< 0.05), although a general decreasing trend was observed as the sacral angle increased, Figure 1B. The intact spine had higher axial and shear stiffness and less anterior dislocation when compared to all of the Prodisc-instrumented specimens during destructive testing.

Discussion: The results of this study provide support for clinical reports indicating suboptimal interactions between the implant and facets at the lumbosacral joint after instrumentation. While further investigation is needed, we hypothesize that the amount of distraction (increased disc height) and shallow ball-and-socket design of the TDR facilitates greater anterior translation and facet load as observed at lower sacral angles (20 -50o). As the sacral angle increases, the device is forced into an extended posture and shear load is transferred from the facets to the polyethylene inlay reducing facet pressures and anterior dislocation to near-intact values.

References:

1. Siepe, C.J., et al., Spine 2010;35:1191-2003.

2. Park, C.K., et al., Spine, 2008;33:1755-61.