Abstract
Introduction
The change of position of the distal pedicle screws with growing rods in relation to vertebral bodies was described as pedicle screws migration. Pedicle screws are subjected to serial distractive forces pushing them down with every distraction; additionally there is continuous growth of the vertebral bodies during the treatment period. These two factors can affect the change of position of the pedicle screws in relation to the vertebrae during the use of growing rods. To our knowledge, this finding has never been studied, confirmed, or quantified.
Methods
This is a retrospective review of the radiographs and operative notes of 23 consecutive cases of early-onset scoliosis treated with single growing rods. Age at index surgery ranged from 4 years 2 months to 8 years 9 months, and the number of distractions was four to 11 per patient. Measurements were done on post-index and latest follow-up true lateral radiographs. With optimum initial position of the screws in the pedicle, we calculated the distance between the upper end plate and the pedicle screw (distance superior to the screw [SS]) and the distance between the screw and lower-end plate (distance inferior to the screw [IS]). We expressed this ratio as a percentage: SS/IS x 100%. Any increase in this percentage with time denoted a more caudal position; however, a change in the percentage of less than 10% was regarded as insignificant.
Results
Seven cases were excluded because of inadequate radiographs for measurements. Within the distal construct, measurements of the upper pedicle screw showed that six cases had a change of less than 10% and were regarded as insignificant. Ten cases had changes of more than 10%, five had changes between 10% and 50%, three between 50% and 100%, and two more than 100%. We identified two types of migration: the first within the pedicle with pedicle elongation, and the second through and distal to the pedicle. None of the patients had adverse clinical consequences related to this change.
Conclusions
Change of screw position with time is a frequent occurrence in the distal pedicle screws with single growing rods. Two different types were identified. With growing rods some degree of distal pedicle screw migration in this paediatric age group may not be regarded as a complication of treatment.
