Feasibility of an Analgesic Device Enabling Local Anesthetic Delivery and Neuromodulation After Shoulder/Foot Surgery

Specific Aim: To determine the feasibility and optimize the protocol for the RELAY device following foot and shoulder surgery to prepare for the subsequent randomized, controlled pilot study.

For individuals of childbearing potential, a sample of urine will be collected before any study interventions to confirm a non-pregnant state (this is standard procedure for all surgical patients). The RELAY device combines a catheter-over-needle to permit ultrasound-guided percutaneous insertion with the tip adjacent to a peripheral nerve or plexus, followed by needle removal which leaves the catheter in situ to deliver a bolus of local anesthetic (if desired) and subsequent perineural local anesthetic infusion (if desired). The catheter also has 3 integrated electrodes to enable neuromodulation using the integrated pulse generator and battery.

Participants will have standard external monitors placed and oxygen delivered by facemask or nasal cannula. The peripheral nerve block site(s) will be cleansed with chlorhexidine gluconate and isopropyl alcohol. Intravenous sedation/analgesia with midazolam and/or fentanyl will be titrated for patient comfort as is standard for peripheral nerve block administration. The nerves treated will include the sciatic proximal to the popliteal crease for foot surgery and the brachial plexus for shoulder surgery. The target nerve(s) will be visualized with ultrasound using a transverse cross-sectional (short axis) view and a skin wheal of local anesthetic will be raised inferior to the transducer to anesthetize the skin and then the track towards the target. The RELAY with the integrated needle will be inserted adjacent to the target nerve with an in-plane approach. Dextrose 5% in water (1-20 mL) will be injected via the needle to open a space around the target nerve(s), the catheter advanced, and the needle subsequently withdrawn.

Electrical current will be introduced with increasing intensity via each of the anode electrodes to optimize participant's perceived stimulation (control provided by the Gate Keeper app from an investigator's phone or tablet). Accurate lead placement will be confirmed with subject reports of comfortable sensations over the surgical site without eliciting muscle contractions. The minimum threshold and maximum comfortable amplitudes will be determined along with the optimal frequency, pulse duration, and anode/cathode. Starting from the lowest possible current the investigators increase the current until the participant states that they feel a "buzzing" sensation (some describe it as a "comfortable massage"). That is the minimal sensed current. The investigators then continue increasing with the instructions to let us know when it starts to be less comfortable-and to stop us before it hurts. That is the maximum comfortable current. The stimulator will be then set to deliver the minimum threshold amplitude and turned off for surgery. The pulse generator of the RELAY has a frequency between 1-250 Hz, a pulse duration of 1-200 µs, and a current of 0.001-10 mA (1-10,000 µA). The investigators will maximize frequency, minimize pulse duration, and have participants adjust the amplitude following surgery, as needed.

Local anesthetic (10 mL of lidocaine 2% with epinephrine) will be injected with negative aspiration every 3 mL and resulting sensory block confirmed to ensure accurate catheter tip placement. Following block confirmation (sensory deficits in the expected nerve distributions), the RELAY will be affixed with both surgical adhesive (2-Octyl 2-cyanoacrylate) at the entry site and a chlorohexidine-impregnated occlusive dressing [research specific]. Subsequently, 10 mL of bupivacaine 0.5% with epinephrine will be injected negative aspiration every 3 mL along with a supplemental single-injection saphenous nerve block for foot/ankle surgery.

Intraoperatively, surgeons will be permitted to infiltrate the surgical area with long-acting local anesthetic as their common practice dictates.

Postoperatively, the stimulators will be connected to participants' phones and turned on. If a patient's surgeon requests a postoperative continuous peripheral nerve block, a ropivacaine 0.2% infusion will be provided per standard UC San Diego protocol. After 7 days, participants themselves or their caretaker will remove the occlusive dressing and withdraw the catheter (and integrated electrodes) with gentle traction and rotation at home. The devices are single-use and disposable.

Inclusion criteria

• Adult participants of at least 18 years of age
• Undergoing a rotator cuff repair, total shoulder arthroplasty, ankle arthroplasty/arthrodesis, or foot surgery anticipated to result in moderate-to-severe pain for at least one week
• Planned single-injection peripheral nerve block(s)
• An Android or Apple smartphone able to download the Gate Keeper controller app

Exclusion criteria

• Chronic opioid or tramadol use (daily within prior 2 weeks and duration > 4 weeks)
• Neuro-muscular deficit of the surgical limb
• Compromised immune system based on medical history (e.g., immunosuppressive therapies such as chemotherapy, radiation, sepsis, infection), or other conditions that places the subject at increased risk of infection
• Implanted spinal cord stimulator, cardiac pacemaker/defibrillator, deep brain stimulator, or other implantable neurostimulator whose stimulus current pathway may overlap
• History of bleeding disorder
• Antiplatelet or anticoagulation therapies other than aspirin
• Allergy to skin-contact materials (occlusive dressings, bandages, tape etc.)
• Incarceration
• Pregnancy
• Moderate pain (NRS > 3) in an anatomic location other than the surgical site
• Anxiety disorder
• History of substance misuse
• Inability to communicate with the investigators
• Inability to contact the investigators during the treatment period, and vice versa (e.g., lack of telephone access)
• Allergy to amide local anesthetics
• Morbid obesity (body mass index > 40 kg/m2)