This paper discusses the effects of small interactions like hyperfine structure or Zeeman effect on ultra-cold collisions. These interactions are usually neglected during collisions, but it is shown here that they may have large dynamical consequences in the particular case of collisions of ultra-cold atoms interacting through the resonance dipole-dipole terms (proportional to r -3 , where r is the internuclear distance). These collisions involving an excited atomic pair are responsible for most of the escape processes in laser traps and a very interesting prospect is to reduce the escape rate through the use of external fields.