Study design: In vitro study of the effect of hemiepiphyseal implant on biomechanical properties of porcine thoracic motion segments.
Objective: Determine whether implantation of a titanium clip-screw construct alters spine biomechanical properties.
Summary of background data: Growth modification is under investigation as a treatment of early adolescent idiopathic scoliosis. Biomechanical property changes due to device implantation are essential to characterize immediate postoperative treatment effects.
Methods: In vitro biomechanical tests were conducted on 18 thoracic functional spinal units. Specimens were tested before and after implantation of a clip-screw construct in lateral bending, flexion-extension, or axial rotation (n = 6 per loading direction). Pure moments were applied, and range of motion, stiffness, and neutral zone were measured. Axial translations were determined bilaterally.
Results: Implantation of the clip-screw construct decreased range of motion in lateral bending by 19% (P < 0.0003), flexion-extension by 11% (P < 0.04), and axial rotation by 8%. Mean stiffness in lateral bending toward and away from the treated side increased by 20% (P < 0.007) and 33%, respectively. In flexion and extension, mean stiffness increased by 10% and 16%, respectively. Treatment decreased the neutral zone in lateral bending toward and away from the instrumented side by 30% (P < 0.0003) and 47% (P < 0.02), respectively. In flexion and extension, neutral zone decreased by 20% (P < 0.04) and 26% (P < 0.007), respectively. In axial rotation toward and away from the treated side, mean neutral zone decreased by 22% (P < 0.04) and 7%, respectively. Range of axial translation decreased on the ipsilateral side by 49% (P < 0.001) and increased on the contralateral side by 17%.
Conclusion: Implantation of a titanium clip-screw construct decreased range of motion by less than one-fifth, increased stiffness by one-third or less, and decreased the neutral zone by less than one-half. Range of axial translation decreased on the instrumented side and increased contralaterally. This study suggests that most of the flexibility of the spine is preserved in the immediate postoperative period after implantation of the spinal hemiepiphyseal construct.