Load distribution on intervertebral cages with and without posterior instrumentation.

ABSTRACT

BACKGROUND CONTEXT Posterior and transforaminal lumbar interbody fusion (PLIF, TLIF) are well-established procedures for spinal fusion. However, little is known about load sharing between cage, dorsal construct, and biological tissue within the instrumented lumbar spine. PURPOSE: The aim of this study was to quantify the forces acting on cages under axial compression force with and without posterior instrumentation. STUDY DESIGN: Biomechanical cadaveric study. METHODS: Ten lumbar spinal segments were tested under uniaxial compression using load cell instrumented intervertebral cages. The force was increased in 100N increments to 1000N or a force greater than 500N on one load cell. Each specimen was tested after unilateral PLIF (uPLIF), bilateral PLIF (bPLIF) and TLIF each with/without posterior instrumentation. Dorsal instrumentation was performed with 55N of compression per side. RESULTS: Cage insertion resulted in median cage preloads of 16N, 29N and 35N for uPLIF, bPLIF, and TLIF. The addition of compressed dorsal instrumentation increased the median preload to 224N, 328N, and 317N, respectively. With posterior instrumentation, the percentage of the external load acting on the intervertebral cage was less than 25% at 100N (uPLIF 14.2%; bPLIF 16%; TLIF 11%), less than 45% at 500N (uPLIF 31.8%; bPLIF 41.1%; TLIF 37.9%) and less than 50% at 1000N (uPLIF 40.3%; bPLIF 49.7%; TLIF 43.4%). Without posterior instrumentation, the percentage of external load on the cages was significantly higher with values above 50% at 100N (uPLIF 55.6%; bPLIF 75.5%; TLIF 66.8%), 500N (uPLIF 71.7%; bPLIF 79.2%; TLIF 65.4%), and 1000N external load (uPLIF 73%; bPLIF 80.5%; TLIF 66.1%). For absolute loads, preloads and external loads must be added together. CONCLUSIONS: Without posterior instrumentation, the intervertebral cages absorb more than 50% of the axial load and the load distribution is largely independent of the loading amplitude. With posterior instrumentation, the external load acting on the cages is significantly lower and the load distribution becomes load amplitude dependent, with a higher proportion of the load transferred by the cages at high loads. The bPLIF cages tend to absorb more force than the other two cage configurations. CLINICAL SIGNIFICANCE: Cage instrumentation allows some of the compression force to be transmitted through the cage to the screws below, better distributing and reducing the overall force on the pedicle screws at the end of the construct and on the rods.