In the case of the fabella a comparison view of the other knee is unlikely to be helpful.

The fabella is a common normal ossicle lying behind the knee. An experienced observer is unlikely to consider it to be an abnormality but, if there is uncertainty in the mind of the referring clinician, a comparison view of the other knee might be requested. This would be a useful decision if the bone in question was present on the other side in close to 100% of people. The incidence of the fabella was studied in the local population and it was found that it was bilateral in only 63% of people. This percentage is lower than that reported elsewhere (81-85%).1-4 Therefore, there was no reason to X-ray the opposite side to confirm that the ossicle in question was a fabella. If the local incidence of fabella had been a little lower, say 50%, it would have meant that tossing a coin would have been just as reliable as a "comparison view". Therefore, in case of doubt, the best advice seems to be to refresh the memory by consulting a reference book of normal variants and to take comparison views sparingly.