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Computational simulation of growth after temporal proximal tibial hemiepiphysiodesis in idiopathic genu valgum
Introduction Length growth at the epiphyseal plate mainly depends on the cellular senescence of chondrocytes. The growth rate is influenced by the ratio between shear and hydrostatic stress, age, gender, and hormones as well as location and thickness of the growth plate. Uneven growth of the proximal tibia can lead to an increase of the medial proximal tibial angle (MPTA). MPTA is physiologically 85°-90° in children over the age of 7. Higher values are associated with genu valgum and significantly higher contact forces occurring at the lateral tibial plateau raising the chance of pain and degenerative joint disease. Temporary hemiepiphysiodesis is an effective procedure to treat angular deformities. In this computational study the guided growth by medial temporal hemiepiphysiodesis is simulated.
Methods A finite element model of the proximal tibia matching the dimensions of a 9-year-old boy is established in ANSYS 19.2. The model is adjusted to a MPTA of 95°. A plate is virtually implanted on the medial side. Growth is assumed to be equally distributed between a biological baseline of 3mm/year and growth mechanically stimulated by octahedral shear stress and hydrostatic stress. The relative contributions to the elongation of the proximal tibia from the hypertrophic zone are 73 % and from the proliferating zone 27 % according to literature. Growth due to proliferation is modeled by isotropic thermal expansion, growth in the hypertrophic zone as unidirectional expansion in the direction of the highest absolute principal stress. Loads are allocated to the medial and lateral tibial plateau according to literature values depending on the MPTA.
Results A MPTA of 87° is predicted after 14 months corresponding to a mean angular correction of 0.6°/month. The growth measured at the lateral tibial plateau during that time is 7 mm.
Discussion Finite element simulation can predict epiphyseal growth after temporary hemiepiphysiodesis and can be useful to estimate time needed for guided growth. To wait for a possible spontaneous recovery and to avoid relapses surgery is usually performed at the end of growth. However, sometimes early treatment is necessary due to severe deformities restricting normal movement. Computational models could help to assess growth after implant removal and optimize removal time to prevent relapses. The present model contains some limitations: As loads play a key role in influencing bone growth, a significant problem remains the lack of knowledge about loads acting in the pediatric knee joint and dynamic load cases are simplified to one value here. Factors influencing the baseline growth are not included. Patient specific growth rates could be rated by reviewing preceding follow-up X-rays. In further research, simulations could be applied to test hypotheses of the development of joint deformities.
Keywords finite element, hemiepiphysiodesis, pediatric deformities
Korrespondenzadresse Andreas Lipphaus, Lehrstuhl für Produktentwicklung, Arbeitsgruppe Biomechanik, Ruhr-Universität, Universitätsstraße 150, 44801 Bochum, Deutschland
E-Mail [email protected]
05 March 2021 (online)
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