Some good resources on this - The CMS Medicare Claims Processing Manual, Chapter 12, Section 30.6.6.B. CPT Assistant has some good explantory articles, for example, Sept, 1998 - "Appropriate Use of Modifier -25," and May, 2003, "Use of Modifiers -25 and -27."
The rules have been consistent for years. Essentially, almost every pateint will have an E/M service on their first visit. After that, if the doctor says come back next week and we'll give you a repeat Synvisc injection - the pateitn comes back, doctors says hi, how are you - any better/worse, etc? Nothing new? O.K., lets proceed with the injection and administers it - here's what to watch out for (bleeding, swelling, whatever) - patient goes home.
The pateint came for the purpose of having the injection, nothing new, no suprises - the injection is all you charge (and medication, if supplied by an independent physician office).
If, however, patient comes back for the second injection, and mentions that he has a swollen area on the proximal calf on the other leg - the doctor looks at it, diagnoses a spider bite, gives some cream samples, and goes ahead with the injection on the one knee - AND properly documents this- you have an E/M in addition to your injection. The expectation was the patient was coming in for an injection and just a routine - hi, how's it going, nothing new or unexpected - and something unexpected, unplanned was there that needed looking into.
Have a great day!