Cutaneous Anesthesia of the Lateral Femoral Cutaneous Nerve

Background:

Total hip arthroplasties may be associated with moderate to intense postoperative pain. It is essential that pain treatment, besides being analgesic, has a minimum of side effects, especially those opioid-related, in order to achieve low morbidity, high patient satisfaction and to encourage the functional rehabilitation.

Currently, there is no "gold standard" for pain treatment after THA, and there exists a large variety of treatment options. Some patients who have undergone THA, can be difficult to sufficiently treat with conventional analgesics without reaching additional morphine doses where side effects are unacceptable. It is therefore important to explore other treatment options, including nerve blockade.

Nerve blocks have previously been used after THA operations, but have been hampered by the fact, that the nerve blocks have an effect on the motorial nerve, inhibiting early mobilization and possibly increasing tendencies to fall when mobilized. The Lateral Femoral Cutaneous Nerve (LFCN) is purely sensorial and can therefore relieve the patient of pain without influencing the early mobilization.

In two previous trials, the investigators have studied the effect of LFCN blocks in pain treatment after THA. In the recent study, the LFCN block was applied in patients after THA, who still had visual analogue scale (VAS) mobilization pain > 40 mm when lifting their leg stretched. The LFCN block alleviated significantly the average pain at group level. Meanwhile 42% of patients in the study had no or limited effect of the block. The reason for this may be due to the fact, that LFCN block does not cover all types of pain after THA, but also the fact, that the nerve innervation area has individual differences in the anatomical distribution. We are currently investigating this matter in another study.

LFCN arise from the lumbar plexus, L1-L3. It progresses profound to m. psoas major and over m. iliacus towards the anterior superior iliac spine (ASIS). LFCN perforates the inguinal ligament in varying distance from the ASIS. Afterwards it makes a roughly 90 degrees lateral turn, divides into 3-5 branches and innervates skin lying on the lateral, proximal part of the leg. LFCN is found to have multiple anatomical variations therefore it is described to innervate both the lateral, as well as the anterior, parts of the thigh.

A single study has described LFCN as being placed in it's own canal created by a splitting of the Iliac Fascia creating a lumen in which the nerve lies. Therefore, it is hypothesized that a larger injection volume of local anaesthetic could migrate both proximal and distal within the above-described canal, spreading to additional local cutaneous nerves arising from LFCN, or even other nerves (iliohypogastric, ilioinguinal or subcostal), thereby giving rise to a greater blocked distribution area. An unwanted result of a larger injection volume is the possibility of involvement of the femoral nerve, which would result in motoric blockage of the Quadriceps muscle. It has previously been described that Iliac fascia block (injection profound to the iliaca fascia) has a great diversity in area distribution as well as blockage of the femoral nerve. By injecting the local anaesthetic directly into LFCN own canal we seek to minimize the risk of femoral nerve involvement.

Methods:

Prior to applying the nerve block, the measurement methods are tested and baseline values are registered for each leg of each participant.

The effect of a nerve block in healthy individuals can be studied by a number of validated measurement techniques. These techniques consist of mapping the affected anatomical area with temperature discrimination test (cold sensation) by using alcohol soaked gauze and pin-prick test by using a Von Frey filament, and of pain during tonic heat-stimulation of the skin (VAS tonic heat stimulation). The distribution of the sensory block is thus evaluated by high precision.

It is furthermore important to investigate whether the nerve blockade affects the femoral nerve's motorial innervation by testing m.Quadriceps' motoric function.

Course of study:

1. Baseline values are measured.
2. An orthopedic surgeon draws two hypothetical incisional lines corresponding to the posterior and to the anterior-lateral access to the hip joint on each leg of each participant. The incisional lines will be drawn with UV paint only visible under UV light. No other than the person drawing the lines and the participant will see where the lines are drawn.
3. An ultrasound guided LFCN block is then applied by a doctor with extensive experience in applying ultrasound guided blocks. Each trial participant will get two injection of Ropivacaine 0.75%. 8 ml in one leg and 16 ml in the other leg. Neither trial participants, doctors,investigator or outcome assessors will know which volume the participant receives in which leg, that is the trial is blinded.
4. After the LFCN block is applied a line called "Line A" is drawn on each leg. Line A goes from trochanter major to the femoral lateral epicondyle and is extended cranial. A line 0A is drawn perpendicular on line A at the trochanter major point. 5 cm above line 0A, is line -1A drawn up to -3A, and 5 cm below line 0A is line 1A drawn, and so forth to line 15A is reached.
5. One hour after the nerve block is applied to the right leg, the motorial and sensorial tests will be carried out on each leg, and the outcome will be registered and documented with photography. The test will be carried out on the right leg and then on the left leg in following order:

• Maximum isometric voluntary contraction
• Temperature discrimination test (alcohol soaked gauze)
• Pin-prick (Von Frey filament)
• Photo documentation
• Drawing of the incisional lines under UV-light
• Photo documentation
• Measurement of the incisional lines
• Pain stimulation test. The subjects will be monitored with blood pressure, pulse oximetry and ECG during the trial. The study extends for each participant from the introductory conversation to all testing is complete.