Improving the Safety of Pain Pumps After Knee Surgery

Pain pumps are often used after joint surgery to deliver a controlled amount of anesthetic and narcotic. This device provides good pain relief. Patients use less narcotic medication. But previous animal studies have shown that some drugs (lidocaine, bupivacaine) actually cause chondrocytes (cartilage cells) to die.

In this study, the effect of bupivacaine on chondrocytes in human articular cartilage samples is tested. The chondrocytes came from the femoral condyle of patients who had a total knee replacement. The femoral condyle is the round end of the femur (thigh bone).

The cells were scraped off and taken to a lab where they were prepared in a special culture. A system of pain pump to culture was set up. Medication was pumped into the culture plates in such a way as to mimic the way a normal, human joint would be treated.

Medications used included one-per cent lidocaine, one-per cent lidocaine with epinephrine, 0.25 per cent bupivacaine, 0.25 bupivacaine with epinephrine, 0.5 per cent bupivacaine, or 0.5 per cent bupivacaine with epinephrine. The amount of each drug used was similar to the dose given after surgery for pain control. Epinephrine is a stress hormone also known as adrenaline. It has many functions such as increasing the amount of oxygen supplied to the cells.

Samples were treated for 24-, 48-, and 72-hour trials. Control groups were used for comparison. The controls had chondrocytes in a culture media (solution) but without the drugs. The number of live cells in each sample (medication groups and the control groups) were counted before and after the drug trials. Accurate cell counts were done using digitally captured images and special computer software.

After 24-hours there was a significant decrease in the number of live cartilage cells in the medications that contained epinephrine. More than half the chondrocytes in the solution with lidocaine and epinephrine were gone.

The cell death rate was similar after 48-hours for the cells infused with any drug that contained epinephrine. At this point in time, samples containing one-per cent lidocaine also showed a decreased number of live chondrocytes. After 72-hours, cultures with 0.5 per cent bupivacaine showed similar losses of chondrocytes.

Overall, it looked like the death rate of chondrocytes increased with time. The combination of lidocaine and epinephrine had the worst results. These results may help explain reports of severe chondrolysis (breakdown of cartilage cells) after using intra-articular pain pumps.

The authors advise caution when using intra-articular pain pumps for more than 48 hours. Medications that have epinephrine in them should not be used at all. Bupivacaine (0.25 per cent) and lidocaine (one-per cent) are safe and effective during the first 48-hours post-op.

There are still many things that remain unknown about this experiment. Does the pH (acidity) of the medication make a difference in cell death? Would the epinephrine affect chondrocytes of young, healthy patients the same way? The patients in this study getting a total knee replacement were older adults with arthritis. Perhaps these cells are just more susceptible to damage.

Do knee chondrocytes respond differently to these drugs than chondrocytes from the hip or shoulder? Future studies are needed to answer these questions and find the optimal way to use intra-articular pain pumps after surgery.

Jason L. Dragoo, MD, et al. The Effect of Local Anesthetics Administered Via Pain Pump on Chondrocyte Viability. In The American Journal of Sports Medicine. August 2008. Vol. 36. No. 8. Pp. 1484-1488.

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