Solar system constraints on multi-field theories of modified dynamics
  Any viable theory of modified Newtonian dynamcs (MOND) as modified gravity is likely to require fields in addition to the usual tensor field of General Relativity. For such theories the MOND phenomenology emerges from an effective fifth force probably associated with a scalar field. Here I consider the constraints imposed upon such theories by solar system phenomenology, primarily by the absence of significant deviations from inverse square attraction in the inner solar system as well as detectable local preferred frame effects. The current examples of multi-field theories can be constructed to satisfy these constraints and such theories lead inevitably to an anomalous non inverse-square force in the outer solar system. 
R.H. Sanders