Is total knee replacement more painful than total hip replacement?

During its use in pain management the patient-controlled analgesia (PCA) devices are capable of registering the course of treatment at patient request, the condition of drug delivery and total amount of drug being given. The patients could determine the need of medication to their own satisfaction while forced treatment by the bias of the health care personnel is avoided and the safety of patients is further warranted. In pain relief with this device, the number of requests for analgesia and the dose of analgesic used can be easily measured. Therefore, it is more objective to compare the pain intensity among different types of operation when PCA device is used. Using PCA morphine consumption as a parameter, we attempted to elucidate the difference of intensity of pain associated with total hip and total knee replacements by comparing their morphine requirement.

In this prospective cohort study, 50 patients who underwent either total hip replacement (THR, n = 24) or total knee replacement (TKR, n = 26) were enrolled. After recovery from general anesthesia when the patients first complained intense pain in the recovery room, morphine was given intravenously in titration with a calculated loading dose in 30 min to achieve an acceptable analgesia (VAS < or = 3) followed by morphine PCA at 1 mg bolus with a lockout interval of 6 min. The patients were then followed for 48 h. During and at the end of the course the data relevant to pain score, total dose, demand, delivery, and adverse effects were recorded for assessment.

With the use of PCA, the pain scores were similar in both surgical groups in the 48 h observation. Total consumption of morphine in THR was 13.2 +/- 8.1 mg as against 19.7 +/- 5.7 mg in TKR in postoperative day 1 and 25.2 +/- 12.7 mg as against 34.1 +/- 13.9 mg in postoperative day 2 (P < 0.05, t-test). Demand/delivery ratio was not statistically significant between the 2 groups at 24 and 48 h (t-test). Minor adverse effects were seen in both groups but the differences were not significant.

Using PCA morphine consumption as parameter, we can distinguish the magnitude of pain intensity between 2 major orthopedic surgeries. The deeper and more extensive operation would in total hip replacement does not mean that it is a more painful procedure than total knee replacement. Several speculations are proposed.