Why not ? The problem here is the secondary displacement inside the cast applied. There are also some authors who immobilize the fracture site using fixateur externe.
of course, undisplaced fracture tibia can be very well treated by cast treatment. however, this treatment requires to be regularly monitored by check x rays and once the fracture margins are sticky partial weight bearing can be allowed.
Conservative treatment is still a method of choise in some non-displaced tibial fractures, Use of ring fixator is however very useful not only in improvement of bone healing, but in correction of bone alignment and deformities, as well.