Introduction: With a four-rod construct in pedicle subtraction osteotomy, the secondary rods are installed to add strength by sharing loads across the osteotomy. Because of this, the bend radius of the secondary rods has a direct effect on the strain of the primary rods.
Methods: As installed during clinical trials, sensors were attached to each primary rod to measure strain at their apex. Testing included secondary rods in three configurations: under-bent, matching, and over-bent. The strain of the primary rods was measured before and after the installation of the secondary rods to determine the difference in strain values.
Results: The direction and amplitude of this strain is dictated by the difference in bend radius of the secondary rods compared to the installed primary rods. Testing with under-bent secondary rods increased the strain on the primary rods by as much as 2,600µstrain. Testing with over-bent secondary rods showed a 1,700µstrain change on the primary rods, this change resulted in the primary rods being forced in the opposite direction of the desired correction. In this case, the strain measured at the apex of the rods were fighting each other in opposing directions.
Conclusions: Installing a secondary rod with an un-matched bend radius negatively impacts the amount of strain at the apex of the primary rod. This was demonstrated in simulated testing and clinical trials. Data collected during surgery showed strain increases as high as 61% on the primary rod.
With a lower strain present at the apex of the primary rod, there is a reduced risk of exceeding the rod yield point.
A wireless sensor will aid the surgeon with reducing and balancing rod loads. In-situ bending of secondary rods can reduce or neutralize the strain(load) induced in the primary rods, thereby promoting longer construct life.
Patient Care: As a result of having this quantitative data, the surgeon can make adjustments to the curvature of the secondary rods for reductions in rod strain and to balance the construct loads. In doing so, the surgeon can optimize the amount of correction while remaining within the limits of the rod yield strength. The improved balance and reductions in rod strain can promote longer construct life.
Learning Objectives: From this provided information, participants will understand of the importance of knowing the strain on primary rods used in a pedicle subtraction osteotomy. Through the use of wireless strain sensors, surgeons will have real-time, quantitative data of the forces within the construct. With this information, surgeons can use surgical techniques, such as in situ bending, to reduce the strain that secondary rods exert on the primary rods. This information allows the surgeon to determine if additional rod shaping is required to maintain a strain balance or avoid loading conditions that may cause the rods to be loaded beyond their yield point.