M. Kandasamia et al.

The Knee 2009, 16, 98

Femoral nerve block (FNB) is a well documented option for post-operative analgesia following major knee surgery. However, motor blockade may be prolonged preventing early mobilisation thereby increasing the length of stay. In addition, as a consequence of persistent quadriceps weakness, patients have an increased risk of falling. We present a series of five patients who underwent total knee replacement with spinal anaesthesia and FNB who fell, sustaining complete wound disruption — including a patient with peri-prosthetic fracture requiring further surgery and prolonged hospital stay.

◊ Reviewed by P. Narchi, MD
Anesthesia Department, Centre Clinical, Soyaux, France

In this descriptive study, the authors question the safety of femoral nerve catheters after TKR. They described 5 cases of fall on day 1 and 2 post-surgery in a series of 250 patients undergoing TKR representing an incidence of 2%. All these falls led to wound disruption (with one case of periprosthetic fracture) leading to re-intervention and significant longer hospital stay. In the past, patients in this institution had apparently been managed with lumbar epidural analgesia not resulting in any report of fall. The authors conclude that while the quality of analgesia has been highly promoted in the literature, experts have underestimated the risks related to early mobilization of these patients.

Early physiotherapy and mobilization is among the first challenges after TKR and many studies have shown that early physiotherapy will decrease both the duration of hospital stay and the requirement for physiotherapy after discharge. Many institutions start passive physiotherapy after surgery still in the PACU in order to accelerate knee rehabilitation. Today, there is no doubt that regional analgesia using local anesthetics (whether epidural or peripheral nerve catheters) facilitate this early mobilization of the operated knee. As compared to epidural analgesia, the femoral catheter has the following advantages: Significantly less urinary retention and hypotension and no risk of epidural hematoma and abscess at comparable analgesia and “dynamic pain” control.

The presence of motor weakness and the risk of fall which has been raised by the authors is a serious issue which may lead to surgical complications as mentioned in this article. Besides the inter-individual variability of nerve sensitivity to local anesthetics, many factors contribute to unpredicted nerve responses to local anesthetics. First, quadriceps weakness is mainly influenced by the amount of local anesthetics surrounding the femoral nerve which is generally correlated to the volume and concentration administered. Second, the proximity of the tip of the catheter to the femoral nerve in some cases causes motor weakness despite the use of diluted low-volume local anesthetics. Indeed, many studies have shown that local anesthetic requirements are significantly lower when using stimulating versus non-stimulating catheters. In addition, the recent development of ultrasound-guided blocks and very precise placement of perineural catheters close to nerves may lead to a decrease in the postoperative use of local anesthetics for pain control. Finally, in some cases, “unrecognized” intraneural placement of these catheters could also account for some prolonged postoperative motor weakness.

All these factors leading to quadriceps weakness postoperatively can be prevented by the following recommendations: first, patient-controlled regional analgesia (PCRA) with or without low basal infusion rate should be preferred to continuous infusion or to regular bolus injection through the catheter. PCRA has been shown to decrease requirements for local anesthetics by 30% as compared to continuous infusion. Interestingly, in many institutions, this PCRA concept is controlled rather by the physiotherapist than by the patient. Indeed, the physiotherapist can adapt the infusion and the bolus according to his own mobilisation criteria which helps to avoid motor weakness at an optimal analgesia during mobilization. Second, the key factor in avoiding quadriceps weakness remains regular assessment of pain and motor weakness by nurses. Any unexplained motor weakness should be communicated to the Acute Pain Team and should lead to a decrease in infusion rate of local anesthetics.

I fully agree with the authors who stopped using clonidine as an adjuvant to local anesthetics postoperatively and recommended to use lower concentrations of local anesthetics. However, I question the rationale of their decision to avoid continuous infusion of local anesthetics to prevent motor weakness: indeed, continuous basal infusion will need to be replaced by the regular injection of boluses. This means to expose the femoral nerve regularly and acutely to significant amounts of local anesthetics inducing not only analgesia but also some degree of motor weakness during the first 2-3 hours after each injection.

It should be mentioned that falls in the postoperative period can be related not only to quadriceps weakness but also to the use of sciatic block (in addition to the femoral nerve catheter) which is part of analgesic regimens in many institutions. Sciatic block with long-acting local anesthetics is frequently used as a single shot to control posterior pain after TKR during the first night post-surgery. Residual sensory blockade can persist on the first day and may lead (in case of early patient mobilization) to loss of control of the foot and hence to falls.

Finally, many recent articles have demonstrated that the combination of wound infiltration and intra-articular catheter (over 24 hours) using a combination of ropivacaine, epinephrine, and ketorolac provides better pain relief, better quadriceps strength, and shorter duration of hospital stay after TKR as compared to femoral catheters.