Quantifying in vitro load-sharing in spinal fusion surgical constructs using strain sensor-equipped fixation rods and force sensor-equipped intervertebral cages.

Background context: Posterior and transforaminal lumbar interbody fusion (PLIF, TLIF) are widely recognized techniques for achieving spinal fusion. However, the biomechanical load-sharing interactions between the intervertebral cage, posterior instrumentation, and anatomical structures in the instrumented lumbar spine-particularly before and after cage subsidence and decompression procedures-remain insufficiently understood.

Purpose: This study aimed to quantify the load-sharing proportions of cages, rods, and anatomical structures in various spinal fusion configurations by comparing the load-sharing behavior of fusion components with intact IVD, after microdiscectomy, midline decompression, and cage instrumentation (uni- and bilateral PLIF, and TLIF) with and without endplate disruption.

Study design: Biomechanical cadaveric study.

Methods: Sixteen lumbar spinal segments were mechanically tested using strain sensor-equipped rods and force sensor-equipped cages under uniaxial compression ranging from 0 to 1,000 N. The specimens were randomized into 4 groups based on cage configuration: unilateral PLIF (uPLIF), bilateral PLIF (bPLIF), TLIF, and a group with no cage. Each group included specimens from the T12-L1, L2-L3, and L4-L5 spinal segments. Instrumented fixation rods were mounted after applying a 55 N compression on each side to maintain spinal segment lordosis.

Results: Independent of the applied force, anatomical structures, and cages consistently carried the highest force, with median load proportions at 1,000 N being 44.55% for anatomical structures, 36.3% for cages, and 14.44% for rods (n=12). In the absence of a cage, with an intact IVD, fixation rods contributed less to load carriage with a median of 9.0% (n=4). Endplate disruption, serving as a surrogate for cage subsidence, resulted in a 2.5% increase in the absolute load proportion on rods and a 4.4% increase on anatomical structures at 1000 N, while the proportion on the cages decreased by 14.6%. Removing the cage further increased the proportions on rods by 8.3% and on anatomical structures by 32.9%. Among the decompression steps, only small and full nucleotomy increased the load on the rods, with the median load proportions increasing by 1.6% and 4.8% at 1,000 N, respectively.

Conclusions: This study reveals that regardless of the applied force, anatomical structures and cages consistently carried higher loads compared to the rods. The load taken by the fixation rods was smaller when the IVD was intact and no cage was used, compared to the samples in which cages were inserted. After endplate disruption, cages carried less force whereas rods and anatomical structures took more. Among decompression steps, only small and full nucleotomy increased the load on the rods, consequently decreasing the load on anatomical structures. Anatomical structures contributed more to load carriage than expected, highlighting the importance of their preservation.

Clinical significance: The findings demonstrate that anatomical structures play a critical role in load-sharing during spinal fusion, consistently carrying more than 40% of the load, while cages contribute synergistically by maintaining intervertebral height and supporting fusion. This highlights the importance of preserving anatomical integrity during surgery.