Impact of transforaminal lumbar interbody fusion on rod load: a comparative biomechanical analysis between a cadaveric instrumentation and simulated bone fusion

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BACKGROUND: Recent research has demonstrated the potential of implant load monitoring to assess posterolateral spinal fusion in a sheep model. This study investigated whether such a system could monitor bone fusion after interbody fusion surgery by biomechanically testing of human cadaveric lumbar spines in two states: following a transforaminal lumbar interbody fusion (TLIF) procedure and after simulating bone fusion. METHODS: Eight human cadaveric spines underwent a TLIF procedure at L4-L5. An implantable sensor system was attached to one rod, while two strain gauges were attached to the contralateral rod (dorsally and ventrally) to derive implant load changes during unconstrained flexion-extension (FE), lateral bending (LB) and axial rotation (AR) motion. The specimens were retested after simulating bone fusion at L4-L5. Range of motion (ROM) of L4-L5 was measured during each loading mode. RESULTS: ROM decreased in the simulated bone fusion state in all loading directions (p ≤ 0.002). Compared to the TLIF motion, the remnant motion after simulated fusion was 53 ± 21 % in FE, 40 ± 12 % in LB, and 49 ± 16 % in AR. In both states, measured strain on the posterior instrumentation was highest during LB motion. All sensors detected a significant decrease in load-induced rod strain after simulated bone fusion in LB (p ≤ 0.002). The strain measured by the implantable strain sensor, the dorsal strain gauge, and the ventral strain gauge decreased to 49 ± 12 %, 49 ± 17 %, and 54 ± 17 %, respectively. CONCLUSION: Rod load measured via strain sensors can monitor fusion progression after a TLIF procedure when measured during isolated LB of the lumbar spine. This study provides the basis for further development and understanding of in vivo implant load data.