LOAD-SHARING ANALYSIS IN SPINAL FUSION USING SENSOR-EQUIPPED RODS AND CAGES

Spinal fusion surgery is a common treatment for many spinal diseases, providing stabilization and symptom relief. However, complications like pseudoarthrosis and implant failure can necessitate revision surgery. Understanding the biomechanics of fusion components is crucial for predicting and preventing such issues. This study quantifies the load-sharing proportions of fusion rods and cages equipped with strain gauges and force sensors under various fusion configurations. Load ratios are compared in human cadaveric samples under various conditions, including intact intervertebral discs, after microdiscectomy, midline decompression, and following cage instrumentation (PLIF and TLIF) with and without artificially induced cage subsidence. Spinal samples from fresh-frozen human cadavers (T12–L5), without a history of spinal disorders, are used. Dissections are carefully performed to preserve intervertebral discs and bony processes. Customized 3D-printed clamps secure the spinal segments in the testing machine. Instrumentation includes four pedicle screws and two fusion rods with strain gauges to measure strain in multiple directions. Cages with piezo-resistive load sensors are used in cage-included samples to measure compressive forces. Axial compressive forces up to 1000 N are applied. One group utilizes only rod sensors to measure load ratios before and after microdiscectomy. Other groups include rod sensors and one of the three cage configurations, comparing load ratios before and after artificially induced cage subsidence. A novel approach converts rod strain into force by removing anatomical structures between two vertebrae, measuring strain while applying specific loads. This method quantifies load-sharing proportions across fusion components under varying conditions. At 1000 N, the median load proportion for cages was 36.3%, with subsidence reducing this absolute proportion by 14.6% (p < .001; N=12; 4 samples per cage configuration). Cage subsidence increased the load proportion on the rods from 14.4% to 16.9% (p = .003) and on the anatomical structures from 44.6% to 49.0% (p < .001). Without a cage and with an intact IVD, fusion rods carried a median load proportion of 9.0% (n=4). Partial and full nucleotomy increased the load on rods by 1.6% (p = .096) and 4.8% (p = .155), respectively. The study reveals that cage subsidence reduces the load carried by the cages while increasing the load on the rods and anatomical structures. Among the decompression steps, partial and full nucleotomy resulted in an increased load on the rods, accompanied by a corresponding decrease on the anatomical structures.