Abstract
Growth rods are currently used in young children to hold a scoliosis until the spine has reached a mature length. Only partial deformity correction is achieved upon implantation, and secondary surgeries are required at 6-12 month intervals to lengthen the holding rod as the child grows. This process contains, rather than corrects, the deformity and spinal fusion is required at maturity. This treatment has a significant negative impact on the bio-psychosocial development of the child.
Aim
To design a device that would provide a single minimally invasive, non-fusion, surgical solution that permits controlled spinal movement and delivers three dimensional spinal correction.
Method
Physical and CAD implant models were developed to predict curve and rotational correction during growth. This allowed use of static structural finite element analysis to identify magnitudes and areas of maximum stress to direct the design of prototype implants. These were mechanically tested for strength, fatigue and wear to meet current Industrial standards.
Results
A dynamic hinged construct, was produced. This consisted of carbon nitride coated CoCrMo components assembled in a modular fashion. Five implants were tested under static load to simulate spinal flexion establishing a mean average yield point at a bending moment of 20.8 Nm (SD 2.5 Nm). Six samples were tested for fatigue endurance to 10 million cycles. Two implants were loaded with a 10 Nm maximum bending moment without fracture. Two samples were loaded at 14 Nm with one surviving and one fracturing at 569,048 cycles. Samples loaded at 16 Nm and 17 Nm both fractured at 3,460,359 and 237,613 cycles respectively. Two implants were tested for wear, the first fractured after 290,000 cycles. A second modified implant was tested to ten million cycles and a mean wear rate of 2.03 mg per million cycles was determined during this period. Exposure of the CoCrMo implant substrate was first observed at two million cycles.
Conclusion
The device met all mechanical test criteria necessary for CE marking and allowed progression to implant testing in an ovine model.
