Sensory Distribution of Lateral Femoral Cutaneous Nerve Block

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. LFCN is purely sensorial and can therefore relieve the patient of pain without influencing the early mobilization. In addition, the application of nerve blocks is very easy and virtually without complications.

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. It is therefore relevant to describe the anatomical distribution of a LFCN block in order to determine whether the variations in anatomy is a contributing factor of the high number of patients with no effect of the blockade.

An earlier study investigated the proportion of the incisional line associated with total hip arthroplasty that was covered by the LFCN block. However the study was not blinded or randomized, and the description of the anatomical distribution of the block was inadequate.

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 a pain stimulation test with determination of the heat pain threshold (Heat Pain Detection Threshold, HPDT) and pain during tonic heat-stimulation of the skin (VAS tonic heat stimulation). The duration and distribution of the sensory block is thus evaluated by high precision.

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

Course of study:

• Baseline values are measured.
• 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.
• An ultrasound guided LFCN block is applied by a specialist in anesthesia. Each trial participant will get an injection of Ropivacaine (active ingredient) in one leg and an injection of isotonic NaCl (placebo) in the other leg. Neither trial participant, doctors or investigator will know which treatment the participant receives in which leg, that is the trial is blinded.
• 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.
• 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
• Heat pain detection threshold test
• 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.

Inclusion criteria

• Participants who have given their written consent after understanding the content and limitations og the study.
• ASA 1-2.
• BMI ≥18 and ≤ 30

Exclusion criteria

• Participants who can not cooperate in the study.
• Participant who can not speak or understand danish.
• Allergies to the drugs used in the study.
• Alcohol consumption that exceeds 21 units per week.
• Drug abuse by the investigators judgement.
• Daily consumption of prescription required analgesic drugs within the last four weeks.
• Consumption of over the counter analgesic drugs within the last 48 hours.
• Neuromuscular defects, former surgery or trauma to the lower extremities.
• Diabetes mellitus.
• Pregnant women (women in the fertile age must have used safe contraception and test negative with a urine-HCG to be able to participate in the study).