Biomechanical analysis of diamond-like carbon coated growth-guidance spinal system under cyclic axial compression

Growth-guidance spinal systems are used in children with progressive spinal deformity to reduce repeated surgical lengthening while allowing continued spinal growth. Their long-term biomechanical behaviour under cyclic loading remains insufficiently characterised. This study assessed whether diamond-like carbon (DLC) coating of rods and sliding screws affects axial stiffness and local kinematics in a growth-guidance spinal system.

Twelve porcine Th11-L7 spine specimens were assigned to three groups: non-instrumented control, non-coated titanium alloy instrumentation, and instrumentation with DLC-coated components. Specimens underwent 100,000 cycles of axial compression. Axial stiffness was evaluated after 100 and 100,000 cycles. Vertebral range of motion from Th12 to L6 was measured using rigid body markers, and local rod-screw interface displacements were assessed using three-dimensional digital image correlation.

Stiffness increased similarly in all groups up to approximately 80,000 cycles. After 100,000 cycles, construct stiffness was approximately 6% lower in DLC coated instrumentation than in non-coated titanium alloy instrumentation. In the coated construct, segmental mobility at Th12-L1 and L5-L6 remained closer to the non-instrumented condition, whereas non-coated titanium alloy instrumentation showed reduced motion at these levels. Digital image correlation demonstrated time-dependent differences in local rod-screw interface kinematics between coated and non-coated constructs.

DLC coating did not increase overall construct stiffness during cyclic axial compression, but it modified local mechanical behaviour and was associated with preservation of segmental mobility at selected levels. Further studies should determine whether these effects are maintained under multi-axis loading and in long-term tribological conditions.