Are Rib Versus Spine Anchors Protective Against Breakage of Growing Rods?

Study DesignRetrospective multicenter, case-control study.ObjectiveTo compare the risks of rod breakage and anchor complications between distraction-based growing rods with proximal spine versus rib anchors.Summary of Background DataRod breakage is a known complication of distraction-based growing rod instrumentation.MethodsA total of 176 patients met inclusion criteria: minimum 2-year follow-up, younger than age 9 years at index surgery, non–Vertical Expandable Prosthetic Titanium Rib distraction-based growing rods, and known anchor locations. Mean follow-up was 56 months (range, 24–152 months). Survival analyses using Cox proportional hazards model (accounting for varying lengths of follow-up) of rod breakage, anchor complications, preoperative Cobb angle, number of growing rods, age, and number of levels instrumented were performed using a significance level of p < .05.ResultsThirty-four patients had rib-anchored growing rods and 142 had spine-anchored growing rods. This analysis found that proximal rib-anchored growing rods have a 23% risk of lifetime rod breakage compared with spine-anchored growing rods (6% vs. 29%) (p = .041) without a significant increase in risk of anchor complications (38% vs. 33%) (p = .117). The number of implanted rods (p = .839), age (p = .649), and number of instrumented levels (p = .447) were not statistically significant regarding rod breakage risk, although higher preoperative Cobb angles were significant (p = .014).ConclusionsPreoperative Cobb angle appears to be the most influential factor in determining whether growing rods break (p = .014). Univariate analysis found that rib anchors were associated with less than one-fourth the risk of rod breakage than spine anchors (p = .04) but multivariate analysis found no significant association between anchors and rod breakage (p = .07). This trend suggests that rib-anchored growing rod systems may be associated with less rod breakage because the system is less rigid as a result of some “slop” at the hook–rib interface, as well as the normal motion of the costovertebral joint.