Nomogram Predicts Need for Sciatic Nerve Block Post-TKA

Although the use of femoral nerve catheters is commonplace after total knee arthroplasty (TKA), many patients will still require a postoperative sciatic nerve block for pain rescue. That cumbersome reality may be avoided in the future, however, thanks to the development of a nomogram that predicts which patients will need supplemental analgesia after TKA.

“The femoral nerve catheter covers the pain very well after total knee replacement,” said Wael Saasouh, MD, clinical research fellow in the Department of Outcomes Research at the Cleveland Clinic Foundation. “Nevertheless, 10% to 20% of those patients will have some pain afterwards and will need another intervention, usually the sciatic nerve block. But it’s not cost-effective to just put sciatic nerve catheters in everybody, so we’re trying to predict who will need them and who won’t, so we can preemptively perform a sciatic nerve block before the pain sets in.”

A Large Data Set

To that end, Dr. Saasouh and his colleagues used data from 2,329 patients, 276 (12%) of whom received a postoperative sciatic nerve block and 2,053 (88%) of whom did not. Patients who received a preoperative sciatic nerve block, underwent multiple TKA surgeries during the queried time window or were missing outcome data were excluded. The researchers performed univariate and multivariate logistic regression analyses for 18 possible predictors of receiving a supplemental sciatic nerve block after the procedure. The model showed statistical significance for age, body mass index, presence of depression, intraoperative anesthetic management, joint infiltration and duration of surgery.

The nomogram was developed from these data (Figure). “A nomogram is a number-based model that predicts the risk for a certain intervention or outcome,” Dr. Saasouh said in an interview with Anesthesiology News. “Based on these predictors, you come up with a single cumulative number that reflects likelihood.”

Figure. A nomogram for predicting the need for sciatic nerve block after total knee arthroplasty.
Use the nomogram by locating the patient’s position for each predictor variable, which has corresponding prognostic points (top axis). Point values are summed to arrive at a total point axis. Directly below is the predicted probability for receiving a sciatic nerve block postoperatively. For example, a total score of 335 corresponds to a probability of 0.1 (10%).

The nomogram is used by first locating the patient position on each variable’s individual scale, which has corresponding prognostic points. Point values for all the predictor variables are determined consecutively, and then summed to arrive at a total point axis. Directly below the total point axis is the predicted probability for receiving a postoperative sciatic nerve block. For example, a total score of 335 corresponds to a predicted probability of 0.1 (10%) for receiving a sciatic nerve block after TKA.

“If you look at age,” he continued, “an age of 80 will give you 12 points, while an age of 20 will give you 52 points. The more points you get, the greater the likelihood that you will need a rescue sciatic nerve block.”

As promising as the tool may be, Dr. Saasouh recognized that it needs to be validated. “This is based on a retrospective study that was done at our institution,” he commented at the 2015 annual meeting of the American Society of Anesthesiologists (abstract A1297). “So while we expect it to hold true with its own data set, we have no way of confirming that unless we apply it to a data set outside the institution. But the concept is there.” Large-scale studies will not only help validate the nomogram but also identify new potential variables to improve its accuracy.

As it stands, clinicians use their clinical judgment to predict which TKA patients will require a supplemental sciatic nerve block. “Too few people will actually perform a sciatic nerve block prior to surgery, because less than one in five patients will need it,” Dr. Saasouh said. “So why go through the trouble? It would be nice to have some way to predict who will need it and who won’t.” Ultimately, postoperative recovery should be improved in these individuals.

Patients also may benefit from alternative analgesic approaches that are gaining traction among anesthesiologists. “For example, we have the adductor canal block, which may be another valid option for analgesia,” he said. “Local anesthetic approaches are changing, too. So maybe a different technique or combination of approaches will replace current regimens. It all depends on what kind of practice you have at your institution.”