BACKGROUND: Nonfusion techniques, such as the posterior dynamic distracting device (PDDD), for adolescent idiopathic scoliosis (AIS), aim to correct deformity and preserve motion. PDDD relies on 2 pedicle screws cranially but only a single caudal screw, a design that may predispose to implant complications. This study evaluated the role of caudal screw parameters-depth, trajectory, and diameter-on mechanical failure, revision surgery, and curve correction.
METHODS: A review of AIS patients from a multicenter registry who underwent PDDD with ≥2 years of follow-up was performed. Radiographic measures included caudal screw depth, rod-to-caudal screw angle, and caudal screw-to-upper endplate angle postoperatively and at final follow-up or before revision.
RESULTS: A total of 155 patients (mean age 14.4 y, range 10 to 19 y) were included with a mean f/u of 2.3 years. Initial coronal deformity was 46 degrees, which improved to 21 degrees at final f/u. Construct breakage occurred in 17% and overall mechanical failure in 27%. Revisions for mechanical failure occurred in 14%. Revisions, breakage, and mechanical failure were all correlated with more convergent, negative, initial screw-toupper endplate angle (P=0.025, P=0.003, and P<0.001, respectively). Mechanical failure correlated with lower final caudal screw depth (P=0.025), lower initial actuator-to-caudal screw angle (P=0.010), and initial and final convergent screw-to-upper endplate angulation (P<0.001 and P=0.010, respectively). Multivariable logistic regression identified initial caudal sagittal angle (OR: 0.893; P=0.003) and Lenke curve type (OR: 0.446; P=0.049) as independent predictors of mechanical failure, with the final model demonstrating strong predictive capacity (AUC=0.755).
CONCLUSION: PDDD achieved meaningful curve correction, but with notable mechanical failure rates. Convergent caudal screw angulation, but not screw depth or diameter, was associated with construct breakage, mechanical failure, and revision surgery, emphasizing the importance of sagittal screw trajectory for optimal outcome. Furthermore, caution should be exercised with Lenke type 5 curves.
LEVELS OF EVIDENCE: Level III.
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