Sara, there has been a Cochrane abalysis covering this subject, which identiifed only 6 studies that met the crietria of randomised controlled trial of thromboprophylaxis in lower limb immobilisation. If I recall correctly, most, if not all of the trials were some kind of fractionated heparin versus placebo. They found that rates of VTE in lower limb imobilisation in the placebo arms was between 4 and 40%, and that on balance, thromboprohylaxis was probably justified....Here is the link...

http://summaries.cochrane.org/CD006681/PVD_low-molecular-weight-heparin-for-prevention-of-venous-thromboembolism-in-adults-with-lower-leg-immobilization-in-an-outpatient-setting

Having said that the AACP still recommend no thromboprophylaxis, but comment that further research is required in this area:

http://www.ncbi.nlm.nih.gov/pubmed/22315265 (sorry, pubmed version only as document is very large)

I guess the issue is one of assessing and balancing risk in a predominantly outpatient population... The novel oral anticoagulants (as opposed to S.c. LMWH) may have some role to play here....

There are no firm guidelines but I would definitely advocate if there is a previous history of VTE. If other risk factors for VTE are present then you could also make a case for thromboprophylaxis e.g. strong family history of VTE, very obese, malignancy.