Background: Treating angular deformities with temporary hemiepiphysiodesis in skeletally immature patients can avoid the need for osteotomies. Recently, newer implants have been introduced as alternatives to the Blount staple, which was commonly used for the past 60 years. The purpose of this project was to develop a porcine model that can be used to compare various methods of temporary hemiepiphyseal arrest. In this pilot study, we compared the Blount staple and the eight-Plate. We hypothesized that the Blount staple, a fixed angle device, tethers growth immediately, whereas the eight-Plate, having mobile arms, has a certain lead time to take the slack out of the system until it can begin to tether growth.
Methods: A porcine model was selected due to its widespread availability and because its size approximates human pediatric dimensions. Thirty-day-old, 15-kg piglets underwent surgery to insert an eight-Plate in each right proximal medial tibia and a Blount staple in each left proximal medial tibia. The insertion technique was identical to the surgical procedure performed in humans, including the use of image intensification. Anteroposterior and lateral view tibial radiographs were obtained monthly until sacrifice at 3 months after insertion to measure the medial proximal tibial angle.
Results: At 1 month follow-up, a large varus deformity up to 30 degrees had already occurred with both devices. In some piglets, the implants were even dragged, by growth, through the epiphysis during the first two months. With further follow-up, the Blount staples frequently migrated to the proximal metaphysis. We observed rebound in four tibiae after implant migration, with partial restoration of the normal limb alignment.
Conclusions: The porcine model for temporary hemiepiphysiodesis seems to have promise for future studies. However, the unusually rapid rate of growth causes us to recommend more frequent observation periods, such as weekly instead of monthly radiographic observation.