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Quantifying in vitro load-sharing in spinal fusion surgical constructs using strain sensor-equipped fixation rods and
Mohsen Khodaee1, Philipp Sager2, Tobias Götschi2
1Department of Orthopedic Surgery, Spine Biomechanics, Balgrist University Hospital, University of Zurich, Zurich, Switzerland; Department of Information Technology and Electrical Engineering, Energy-Efficient Circuits and Intelligent Systems, ETH Zurich, Zurich, Switzerland.
Anatomical structures and interbody cages carry the majority of the load in spinal fusion surgeries like posterior and transforaminal lumbar interbody fusion (PLIF, TLIF). Preserving these structures is crucial for successful outcomes.
Area of Science:
- Spine biomechanics
- Spinal fusion surgery
- Orthopedic research
Background:
- Posterior and transforaminal lumbar interbody fusion (PLIF, TLIF) are established spinal fusion techniques.
- Load-sharing biomechanics between cages, instrumentation, and anatomical structures in instrumented lumbar spines are not fully understood, especially concerning cage subsidence and decompression.
Purpose of the Study:
- To quantify load-sharing proportions of cages, rods, and anatomical structures in various spinal fusion configurations.
- To compare load-sharing behavior before and after microdiscectomy, midline decompression, and cage instrumentation (PLIF, TLIF) with and without endplate disruption.
Main Methods:
- A biomechanical cadaveric study involving sixteen lumbar spinal segments.
- Mechanical testing under uniaxial compression (0-1,000 N) using strain sensor-equipped rods and force sensor-equipped cages.
- Specimens were randomized into groups for unilateral PLIF (uPLIF), bilateral PLIF (bPLIF), TLIF, and no cage, with and without endplate disruption.
Main Results:
- Anatomical structures (44.55%) and cages (36.3%) consistently carried the highest loads at 1,000 N, with rods carrying 14.44%.
- Endplate disruption (cage subsidence) increased load on rods and anatomical structures while decreasing it on cages.
- Cage removal significantly increased load on rods and anatomical structures.
Conclusions:
- Anatomical structures and cages are primary load-bearers in spinal fusion, with rods carrying less load, especially with intact IVDs and no cage.
- Endplate disruption alters load distribution, increasing reliance on rods and anatomical structures.
- Preserving anatomical integrity is vital, as these structures significantly contribute to load carriage.
