Yes, aneurysms can be tricky things and naturally you are concerned and confused. Where is the aneurysm located—in the brain or neck? I am assuming you had a brain MRI and an aneurysm was found in the left internal carotid artery. An 11mm aneurysm is considered mildly large (large aneurysms vary in size from 1 to 2.5 cm). One can adopt a wait and watch policy, but saccular aneurysms are known to rupture or bleed as they are often present at bifurcations of blood vessels (area of blood turbulence). If the aneurysm was less than 1 cm, then a wait and watch policy could have been adopted and if the risk of bleeding was low, you could have gone ahead with knee surgery.
Please discuss the risk of complications like rupture and bleed in your case with both your neurosurgeon and your knee surgeon. Take their combined opinion to assess how long you can delay both surgeries. Even after coiling, an aneurysm can recur, rupture or bleed, but chances are less.
So, in my opinion, you must discuss the risks and then first go ahead with aneurysm surgery and then with the knee surgery. Depending on the location, risk of hemorrhage, risk of rupture, family history, etc, your surgeon will estimate the risk.
Also ask your ortho specialist whether there are other options for you except knee replacement surgery. Then if your neuro-surgeon feels you can wait for a year or more and have MRI in between to monitor progress, then you can wait this time.
Once you have undergone the aneurysm surgery with coiling or clipping, you will not be able to undergo MRI again as the prosthesis will alter the magnetic field. So, discuss this aspect also: how you will be monitored for both knee and aneurysm post surgery.
I sincerely hope you will find this information useful.
Hope you get well soon! Good Luck and take care!
Thanks for your most useful comments. The aneurysm is located in the left internal carotid artery at the upper part of the neck (it's not in the brain). I will contact my neuro-surgeon about the issues you raised.