CORR Insights®: Saphenous Nerve Block From Within the Knee Is Feasible for TKA: MRI and Cadaveric Study

Where Are We Now?

Although we still have much to learn about pain after TKA, advancements in anesthesiology techniques and collaborative efforts with orthopaedic surgeons have improved postoperative TKA treatment [3,7]. Indeed, today’s improved fast-track protocols allow for outpatient joint replacement [15], but making postoperative treatment a comfortable experience for the patient depends largely on pain-control techniques that reduce morphine consumption such as multimodal-analgesia approaches and locoregional anesthesia [3].

The added value in postoperative pain control of locoregional anesthesia has been well demonstrated [5]. The evolution from femoral nerve block (FNB) to adductor canal block (ACB) was inspired by the wish to avoid quadriceps muscle weakness, which can slow down a patient’s early mobilization and cause occasional falls [2]. The addition of an ACB improves the analgesic effects of any type of local infiltration analgesia (LIA), a technique to control pain following hip or knee surgery that has become a part of most multimodal pain protocols. Researchers developed LIA as a response to an important limitation of FNB that generally covers the anterior surface of the knee, with remaining pain in the posterior region [6, 12]. The use of LIA would likely be more efficient, if surgeons knew exactly where to infiltrate it, instead of injecting many areas of the knee with analgesics [5, 8].

In the current study, Kavolus and colleagues underscore two important topics: (1) Anatomically identifiable neurological structures can be infiltrated individually around the knee. This finding may contribute to the success of surgeon-performed LIA combined with selective sensory nerve blockade as part of a multimodal pain protocol [8]. The study findings may also help us understand why LIA works efficiently in TKA, but less in THA to my knowledge. The specific anatomy of the hip being potentially less convenient or less accessible for effective LIA than the knee [11]. (2) Surgeons can access the saphenous nerve intraoperatively, allowing them to perform a distal saphenous nerve block with a minimal amount of resources. Compared to ultrasound-guided infiltration performed by an anesthesiologist, performing the block intraoperatively can reduce costs and time for TKA surgery, especially in an academic environment or centers where no preanesthesia rooms are available [14].

Where Do We Need To Go?

For patients who undergo TKA, a pain protocol that includes a full sensory knee blockade with long-acting agents [13] that eliminate pain after surgery and reduce the local inflammatory response of the surgical trauma is ideal. These long-acting agents could also help specific patient subgroups such as those at risk for severe acute or chronic pain [3], patients who developed a pathological pain trajectory in the first days after a previous procedure [7], or those who developed or have neuropathic pain [4]. Additionally, long-lasting sensory knee blockades could be useful for patients with abnormal temporal summation [9] or those predisposed to longer-term pain after surgery [1, 16]. Depending on the cost of such a protocol, it might be proposed on a patient-specific basis for patients presenting with preoperative identifiable risk factors [3], or as a standard treatment for all TKA patients [1].

The use of local knee anesthesia or peripheral nerve blockade may reduce the centralization of pain and potentially the peripheral sensitization effect of the inflammatory factors released by the surgery [10]. LIA blocking the local receptors, combined with a distal saphenous block limiting the ascendant pain pathway, should reduce morphine consumption [12]. One of the advantages of a pure sensory nerve block is the possibility of obtaining these results without impairing early mobility [2]. If all of this can be obtained with long-acting local anesthetics, then we might have the ideal multimodal pain protocol [13].

The current study shows that performing a saphenous block from within the knee joint can be performed by the surgeon. But it remains to be seen whether this surgeon-performed block will be as good as or better than the ultrasound-guided block by an anesthesiologist. And if it proves to be reliable, it should be analyzed for its reduction of the total costs of the surgical procedure.

Another issue remains: Can the posterior nerve structures in the same way be infiltrated from within the joint using reliable anatomical reference points like this study? This might be important, given that posterior knee pain remains after many locoregional anesthetic approaches used after TKA. But given the proximity of the major posterior neurovascular structures, and the distance of these structures from the anteriorly based TKA approach, surgeons may be reluctant to inject posteriorly. Even so, specific anatomical landmarks might be identified at the posterolateral corner or around the posterior aspect of the knee that surgeons can use to guide LIA towards or away of the posterior nervous structures.

It is also important to identify the most-effective dosages for LIA approaches around the knee, and whether these vary based on the nerve(s) being injected. Do we need more local anesthetic in the anterior or the posterior part of the knee? Can we obtain a more effective LIA technique by infiltrating specific nerves and therefore use lower dose and volume? The advantage of adding other agents to the local anesthetic should be evaluated depending on the specific pain receptors known to be present in specific areas of the knee joint.

Furthermore, I believe that surgeons should move away from the end-of-procedure, all-at-once injection of LIA. Performing these infiltrations progressively may create a better pre-emptive and preventive blockade of the joint. Additionally, the surgeon can use a greater volume of local anesthetics since potential absorption would be spaced over time.

Finally, I believe we need to investigate whether long-acting anesthetics are suitable for all these purposes; if the approaches successfully block only sensory nerves, longer-acting drugs should help decrease pain for a longer period without causing sustained motor blockades that can delay rehabilitation. This might become an issue if we use them also on the posterior nerves, from within the knee joint, resulting in motor blockades for longer periods while being in doubt about more permanent neurovascular damage.

How Do We Get There?

Researchers should conduct randomized controlled trials to determine the ideal LIA cocktail, examining toxicity levels depending on the volume and amount of absorption of each anesthetic either injected all at once at the end of the procedure or progressively during the surgery as a step-by-step infiltration. These studies should help us determine whether a different dose or volume makes the LIA technique more efficient in controlling pain as well if it prolongs the analgesia period.

Combinations of adjuvant drugs working on different receptors present in the joint should be analyzed for their effectiveness for different patient subgroups. Furthermore, the efficacy of these adjuvant drugs should be tested for their effect on the specific sensory nerve blockades, such as the ACB. Ideally, the patients included would be stratified for being at risk for severe acute or chronic pain and evaluated for their outcome, both in the short- and long-term, depending on the adjuvants used in the LIA cocktail.

Future randomized controlled trials should also evaluate the influence of morphine-free anesthesia on the immediate outcome after TKA. Furthermore, it would be interesting to determine the risk of medical complications under this specific protocol. Finally, TKA with a minimal (or no) use of morphine should be studied prospectively in terms of possible improvements to the long-term outcome. A lower rate of persistent postsurgical pain at 1 year or more after surgery, or a lower risk of revision TKA for unexplained pain would emphasize the importance of this analgesic strategy and the potential advantages of well-executed local infiltration and saphenous nerve blocks.