A number of operating room issues are of importance when making choices about knee replacement.
Tourniquet use in knee replacement
Total knee replacement without a tourniquet tends to be better for the patient with respect to was superior to thromboembolic events such as deep vein thrombosis (DVT) and other related complications. The actual amount of blood loss does not seem to be a major factor, as surgeons are able to seal vessels as they work even if there is no tourniquet being used.
Underlying tissue damage when a tourniquet is used may slow down early rehabilitation.
If a tourniquet is used, releasing it before wound closure decreases post-operative complications, although the procedure may take longer as the surgeon gets control of any bleeders.
The effects of a tourniquet used in total knee arthroplasty: a meta-analysis Zhang W, Li N, Chen S, Tan Y, Al-Aidaros M and Chen L. J Orthop Surg Res. 2014;9(1):13. Published 2014 Mar 6. doi:10.1186/1749-799X-9-13
Timing of tourniquet release in total knee arthroplasty: A meta-analysis. Zhang P, Liang Y, He J, Fang Y, Chen P and Wang J. Medicine (Baltimore). 2017 Apr;96(17):e6786. doi: 10.1097/MD.0000000000006786..
Cement use in knee replacement
Knee replacements are designed to be cemented or cementless. The material used to fix the former is correctly called polymethyl methacrylate, and it is an acrylic polymer of two separate powdered substances that are mixed together in the operating room shortly before application. This reaction causes heat to be released. Addition of another chemical allows the cement to show up on X-ray. Antibiotics may also be incorporated into the powder before mixing.
Because the cement preparation is a chemical process, the timing and temperature have to be monitored carefully in the operating room, and the surgical team have to wait until the mix is deemed to be optimal for implantation of the prosthesis. Vacuum centrifugation is used to reduce the porosity of the mix. The cement and prosthesis are firmly pushed into place and held there, while any excess cement that squeezes around the edge is removed and discarded.
At the time the cement is applied, it is common for the patient's blood pressure to fall, and this is carefully monitored by the anaesthetist.
Bone cement. Vaishya R, Chauhan M and Vaish A. J Clin Orthop Trauma. 2013 Dec; 4(4): 157–163.
Anaesthesia for knee replacement
There are a number of options for anaesthesia, and choice is usually dictated by obesity or frailty of the patient and, of course, their age.
General anaesthetic
With general anaesthesia the patient is fully asleep. Usually an agent is injected to relax the patient so fully that it is necessary to have the patient on a ventilator during the procedure, but this is reversed before the patient is woken up. The anaesthetist may also have used techniques to drop the blood pressure during the time of the incision and bone cuts if a tourniquet is not being used, so that bleeding is minimal, but the blood pressure will be returned to normal if there has been any drop from the use of the cement.
Sometimes general anaesthesia is combined with regional anaesthesia (epidural, spinal or nerve blocks) for relief of pain after surgery, but this is done only once the patient is asleep so they will not be aware of the procedures themselves. This can be a bit of a problem because the regional anaesthesia can make the lower limbs a bit weak until the anaesthesia wears off, so the patient needs to be made aware of this.
Epidural anaesthetic
With epidural anaesthesia, when used on its own, the patient is awake. A fine plastic catheter is introduced by the anaesthetist via the back to the area around the spine, which is called the epidural space. It is taped there and left in place during the procedure and usually for some time afterwards for pain relief. An anaesthetic solution is injected into the catheter and titrated carefully, so the any part of the body below the catheter is numb and not aware of the incisions of the surgeon.
Once the patient is out of the recovery room, the catheter can remain in place and the staff can be instructed about topping up the anaesthetic agent just enough so the limbs can move but pain does not kick in.
Spinal anaesthetic
Spinal anaesthesia is simple but effective. The anaesthetist inserts a needle into the spinal canal and injects and quantity of long-acting anaesthetic agent in a single dose, calculated from the patient's weight. By tipping the patient's couch up or down and checking the limb for numbness, the agent can be distributed to ensure that the knee and lower limb feel nothing. The doctor needs to take care that the agent does not track too high, as it will affect blood pressure and breathing. An intravenous drip is always inserted, and the fluid flow can be used to keep the blood pressure stead, and there are also medications that can be injected to do the same.
Once the patient leaves the recovery room the spinal anaesthesia can start to wear off, and pain management is not so efficient as it is with an epidural.
Local nerve blocks
A local nerve block is when an individual nerve is 'paralysed' after having local anaesthetic agent injected around it. It is possible to do a knee replacement with local nerve blocks alone, but that is challenging. Usually nerve blocks are used to offer pain relief after the procedure, when they may be injected while the patient is still under the influence of general, epidural or spinal anaesthesia.
Anaesthesia and analgesia for knee joint arthroplasty. O'Donnell R and Dolan J. BJA Education January 2018, Volume 18, Issue 1, Pages 8–15.