In vivo distraction force and length measurements of growing rods: which factors influence the ability to lengthen?

STUDY
DESIGN: Prospective, intraoperative force measurement in consecutive lengthening procedures in a series of growing-rod patients undergoing lengthening.
OBJECTIVE: The purpose of this study was to measure the forces and amount of distraction over time in early onset scoliosis patients treated with growing rods. SUMMARY OF BACKGROUND DATA: Growing rods are one of the current techniques used in the treatment of early onset scoliosis, and the goal of the growing-rod technique is to achieve deformity correction, maintaining spinal growth at the same time. Gradual stiffening or spontaneous fusion of the spine can interfere with the ability to lengthen. In addition, diminished acquired length with serial distraction are common observations and need to be evaluated and quantified.
METHODS: Distraction forces were measured prospectively during 60 consecutive lengthening procedures in 26 patients. All patients had single submuscular rod constructs with side-to-side connectors. For each measurement, output from a transducer on a dedicated pair of distraction calipers was recorded at zero load status and the force was then recorded at every 1 mm lengthening; length was obtained at each event and was recorded in millimeters.
RESULTS: The force required to distract the spine doubled at the 5th lengthening procedure (mean 368 N ± 54 N), and the distraction force was significantly higher at the fifth lengthening compared with the previous lengthening (P <0.01). Mean length achieved at each distraction decreased over time such that by the fifth lengthening, consistently 8 mm or less was achieved.
CONCLUSION: Distraction forces increase significantly after repeated lengthening of growing-rod constructs, and the length obtained at each procedure exhibits a decreasing trend.