Well, the reasons depend upon who you talk to about the issue. One side argues that the term "OMM" doesn't really reflect what services the specialty offers anymore, it's become too dated and narrow. When you go see a OMM specialist, he or she may order xrays, mri's, ct scans, labwork, postural studies, emg's, prescribe medication, and do manipulation. So, the idea is that what you're really being trained to practice is non-surgical orthopedic medicine, but that's even more of a mouthful than Neuromusculoskeletal Medicine.
The new residency requirements for NMS (which can be viewed on the AOA-website) incorporate rotations in rheum, ortho, rehab, neuro, etc.
As for there being no research in manipulative medicine, that isn't true. It's out there, it just hasn't cumulated like other areas of medicine.
--dave
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David Russo, MS3
UNTHSC/TCOM
Research Fellow, Department of Osteopathic Manipulative Medicine