Clinical application of ultrasound combined with X-ray in assessing callus formation for pediatric limb lengthening: a prospective cohort study

Background: Early monitoring of callus formation is crucial for adjusting the lengthening strategy. While routinely used X-ray has limitations in early callus observation during distraction, ultrasound offers distinct advantages owing to its high soft-tissue resolution. Therefore, this study investigates the application of ultrasound combined with X-ray in monitoring callus formation in pediatric limb lengthening, aiming to provide a more comprehensive assessment method and guidance strategy for limb lengthening. Materials and Methods: From January 2022 to October 2023, 71 limbs undergoing limb lengthening surgery were prospectively included. Ultrasound and X-ray examinations were conducted at the start of lengthening, 1 week later, and biweekly following discharge to monitor new bone formation. Ultrasound visualized the shape and number of callus, while X-ray observed the degree of mineralization and medullary canal reformation in the distraction gaps. Results: A callus staging system was established, with ultrasound as the primary reference in the early stage and X-ray in the mineralization stage. Based on this system, we proposed a novel limb lengthening guidance strategy: in the third week, patients in stage IIa should reduce the rate to 2/3 mm/day. In the fifth week, patients remaining in stage IIa should consider stopping the lengthening process or receiving a bone morphogenetic protein injection; those in stage IIb should reduce the rate to 2/3 mm/day. At the same assessment time point, patients with higher staging had a faster lengthening rate and a lower healing index. Compared with X-ray alone, ultrasound combined with X-ray more accurately determined callus stages, optimized treatment plans, and significantly reduced external fixator wearing time (225.21 ± 61.47 vs. 260.43 ± 93.56 days, P = 0.006), healing index (1.38 ± 0.54 vs. 1.74 ± 0.77 months/cm, P = 0.001), and incidence of lengthening-related adverse events (8.45 vs. 20.99%, P = 0.041). Conclusions: Ultrasound combined with X-ray provides earlier and more comprehensive callus assessment for pediatric limb lengthening, aiding in treatment plan adjustment, and improving therapeutic effects.