Hi Sorry to hear about your problem.
Going by what you have described, you have not noticed a swelling behind your knee, before your surgery. If you have noticed it only after surgery then the possibilities are, especially with you plaster cast on, is a possible infection and an abcess. However there are other possibilities which could
1) Infected lymph nodes behind your knee
2) Baker's cyst
3)
Aneurysm of the artery behind the knee
4)
Varicose vein of the small sephanous vein
The correct diagnosis is best clinched with
complete blood count and ESR looking for infection and an
ultrasound scan or an MRI scan.
Remember let not a knife be put on this swelling without diagnosis, as you can have a dangerous consequence of cutting a
popliteal artery aneurysm, with disastrous consequences.
Hope this helps.
Dr Purushottam Reddy Padala MS(Ortho), FRCS
Consultant orthopaedic and foot and ankle surgeon
HAPPY FEET FOOT CLINICS
Hyderabad