Maximizing mechanical advantage: surgical technique increases stiffness in spinal instrumentation

• Published in: Spine deformity Vol. 10; no. 2; pp. 295 - 299
• Main Authors: Burke, Corey A., Speirs, Joshua N., Nelson, Scott C.
• Format: Journal Article
• Language: English
• Published: Cham Springer International Publishing 01.03.2022
• Subjects: Adolescent; Biomechanics; Humans; Kyphosis - surgery; Medicine; Medicine & Public Health; Orthopedics; Pedicle Screws; Scoliosis - surgery; Spinal Fusion - methods; Thoracic Vertebrae - surgery; Rod stiffness; Pediatric scoliosis; Flat back; Sagittal balance
• ISSN: 2212-134X; 2212-1358; 2212-1358
• DOI: 10.1007/s43390-021-00425-0

Purpose While there has been a great improvement in the treatment of adolescent idiopathic scoliosis, sagittal deformity correction has remained challenging. Increased rod stiffness has been shown to reduce thoracic flattening. We propose that the surgical technique can increase rod stiffness. A mechanical study was created to quantify the effect this has on construct stiffness. Methods The sagittal bending stiffness of a constrained over contoured rod was measured using four different commonly used instrumentation systems. Pedicle screws were secured into custom printed blocks. One block was completely immobilized, while the other block was subject to four levels of constraint. This includes no constraint, mild constraint, moderate constraint, and maximal constraint with both blocks immobilized. The rod apex was loaded until 1 cm of displacement occurred. The stiffness was then calculated and compared between groups. Results All four rod types showed increased bending stiffness as the construct became more constrained. The moderately constrained and the maximally constrained groups had a significantly higher stiffness compared to the unconstrained groups in all rod types ( p  < 0.05). The 6.0 mm titanium circular rods showed the highest increase in stiffness between maximal and no constraint, which became 3.02 × stiffer. Conclusions Rod stiffness is not only determined by size, shape, and metal alloy, but also by surgical technique. Constraining the spinal instrumentation by first locking the rod to the proximal and distal anchors significantly increases the sagittal bending stiffness. In a mechanical model this technique increases rod bending stiffness regardless of the material or shape.