DIFFERENTIAL ROD BENDING FOR RESTORATION OF OPTIMAL THORACIC KYPHOSIS AT POSTERIOR SPINAL FUSION FOR AIS: HOW MUCH SHOULD WE BEND EACH ROD?

In a typical (right-thoracic-left-lumbar) AIS curve, a rigid left rod is often over-bent while a flexible right rod left fairly straight to de-rotate the thoracic spine and simultaneously restore thoracic kyphosis and correct scoliosis. This study aims to quantify the bend for the rigid left rod and flexible right rod to achieve optimal thoracic kyphosis.

Retrospective analysis of 16 AIS patients who achieved postoperative thoracic kyphosis of 20°–30° post-PSF. Patient demographics, surgical parameters, and radiographic measurements were analysed. Intraoperative rod bend angles were measured for both rods. Thoracic kyphosis (T5-T12) and major Cobb angles were assessed preoperatively, at 1.5 and 10.5 months.

In this cohort of 16 patients (mean age 13.67 ± 1.59 years; 94% female), 6 (38%) had LIV L2 or cephalad, 10 patients (62%) had LIV L3 or L4. Overall, the rigid left (5.5mm cobolt-chrome) rod required a significantly greater bend (46.25°±4.23°) than the flexible right (5.5mm titanium) rod (8.38° ± 4.51°). Mean preoperative thoracic kyphosis was 23.84° ± 9.73°; this remained stable post-op (25.06°±3.87° at 1.5 months, 24.94°± 3.21° at 10.5 months; p=0.834). Major Cobb angle improved significantly from 61.63°± 9.62° preoperatively to 17.50°±5.66° at 1.5 months and 19.38° ± 5.64° at 10.5 months (p < 0.001).

To achieve post-op thoracic kyphosis, the rigid left cobolt-chrome rod should be bent approximately 45 degrees and the flexible right titanium rod bent 10 degrees.