Study Design.
An
in vitro biomechanical study using porcine
lumbar segments as specimens.
Objective.
To evaluate the effects of interbody
cage support and endplate strength on the stability of instrumented segments.
Summary of Background Data.
The anterior
lumbar interbody fusion (
ALIF)
cage is widely used to restore disc height and support the anterior column. Transpedicle or posterior spinal fusion or
facet screw fixation (FSF) can improve the stability of the vertebra-instrumented segments. The
cage position can affect the anterior support and
initial stability of the
ALIF region, but there is no consistent data on its biomechanical effects on
ALIF and
ALIF/FSF segments.
Methods.
Nine variations of 3 instrumentation modes (intact,
ALIF,
ALIF/FSF) and 3
cage positions (type I, anterolateral; type II, mediolateral; and type III, posteromedial) are tested under 5
lumbar motions. The range of motion and axial displacement are used as comparison indices for the different variations.
Results.
The
cage placement serves as support for the intervertebral loads while the posterior fixation behaves as lever to further enhance the anterior support. At the endplate-
cage interfaces, the endplate strength directly affects the
cage subsidence. Type III exhibits higher stability for standing due to the greater strength of the endplate in the posterior region. Otherwise, type I consistently has higher stability for all other types of motion.
Conclusion.
The
initial stability of the
ALIF region is affected by the moment arm and the mechanical strength of the engaged endplates. Type I has greater moment arm and provides more efficient support to the instrumented segments. Endplate strength provides an ability to withstand
lumbar loads and suppress the
cage subsidence. Bone quality at the endplate-
cage interfaces must therefore be cautiously evaluated preoperatively.
Level of Evidence: N/A
