Tissue distortion and clinical outcomes in field block versus infiltrative anaesthesia for small cutaneous excisions: a randomized controlled trial

Local anaesthesia is integral to dermatologic surgery, providing patient comfort, safety, and optimal operative conditions in outpatient settings. The two most commonly employed techniques for small cutaneous excisions are infiltrative anaesthesia and field block anaesthesia. Infiltrative anaesthesia, delivered directly along the incision line, is technically simple and rapid but often causes tissue oedema and geometric distortion at the operative site. This may obscure margin clarity, alter orientation, and affect the accuracy of excision and wound closure. In contrast, field block anaesthesia involves depositing the anaesthetic circumferentially around the lesion rather than within it. By anaesthetising the peripheral sensory supply while sparing the central operative field, it preserves tissue planes and native topography. These differences may influence precision, particularly in trunk and extremity excisions where accurate margin visualisation is crucial.

Rationale

Although both techniques are widely practised, their relative impact on early post-injection tissue distortion has not been objectively characterised in dermatology. Distortion can alter surgical planning, biopsy interpretation, closure alignment, and ultimately cosmetic outcome. This study minimises operator variability by standardising the dose to lesion ratio  to ensure that any observed difference arises from technique rather than anaesthetic volume.

Novelty

This trial introduces a Composite Distortion Index (CDI) a quantitative measure of percentage change in ellipse geometry between predefined fiducials to objectively assess distortion. No prior dermatologic study has validated or applied such a metric under uniform dosing conditions.

Expected Outcomes

Field block anaesthesia is expected to cause less tissue distortion and provide better intraoperative visualization while maintaining comparable analgesia, safety, efficiency, and cosmetic outcomes. The findings may enable evidence based selection of anesthetic technique in routine small cutaneous excisions.

MethodologyThis prospective, randomized controlled trial will be conducted at the Dermatology Outpatient Department, JIPMER, Puducherry following approval from Postgraduate Research Monitoring Committee (PGRMC) and Institutional Ethics Committee (IEC). The study will be registered with CTRI before participant recruitment.

ScreeningEligibility will be confirmed against inclusion and exclusion criteria. Relevant comorbidities and current medications including anticoagulants and antiplatelets will be reviewed. Drug allergies particularly to local anaesthetics or adrenaline will be documented. Written informed consent will be obtained.

Baseline assessmentDemographic data Fitzpatrick phototype anatomical site and clinical description of the lesion will be recorded. The planned excision ellipse and four fiducials at twelve oclock three oclock six oclock and nine oclock positions will be inked before anaesthesia.

Baseline ellipse measurements including apex to apex length L0 and mid width W0 dimensions will be measured with a sterile calibrated Vernier calliper and the planned long axis length in centimetres will be documented.

Laxity will be measured one centimetre from the lesion edge at the site of intended injection.

Glide in millimetres will be measured perpendicular to the relaxed skin tension line to firm resistance with two readings and a third reading will be taken if the difference is greater than two millimetres.

Pinch in millimetres will be measured as a gentle vertical skinfold with the same replicate rule.

The mean of the two readings will be recorded for both glide and pinch.

The Laxity Index will be calculated as zero point six multiplied by glide in millimetres plus zero point four multiplied by pinch in millimetres and laxity class will be assigned. Class one will be less than twelve millimetres class two twelve to less than eighteen millimetres and class three eighteen millimetres or more. Borderline values within plus or minus one millimetre of a boundary will be flagged for review.

The lesion size category will be determined from the longest pre injection diameter and classified as one centimetre or less one point one to two centimetres or two point one to two point five centimetres.

A standardised pre injection photograph of the inked ellipse will be taken and will be cropped appropriately to preserve assessor blinding.

Dose planningThe total planned anaesthetic volume will be standardised as one point two multiplied by the planned long axis length in millilitres and rounded to the nearest zero point one millilitre. Rescue allowances in increments of zero point five millilitres will be specified if required at the time of incision.

Equipment including sterile calliper timer one millilitre syringes and twenty seven to thirty gauge needles will be prepared.

Calibration of the calliper including zero verification will be confirmed before use.

RandomizationBased on the Laxity Index and lesion size category each participant lesion will be assigned to one of nine strata comprising three laxity classes and three size categories.

Randomization stratified by laxity class and lesion size category will be performed immediately prior to anaesthesia. Participants will be randomized in a one to one ratio to field block or infiltrative anaesthesia. Randomization will occur at the patient level with one lesion per participant using centralized computer based stratified block randomization to ensure allocation concealment.

InterventionsGroup A Field blockThe solution used will be two percent lignocaine with adrenaline in a concentration of one is to two hundred thousand using a twenty six gauge needle in the subdermal plane with aliquots of approximately zero point zero five to zero point one millilitre.

Anaesthetic will be deposited circumferentially ten millimetres outside the planned ellipse with a tolerance of plus or minus two millimetres using short circumferential runs at three to five millimetre spacing while avoiding radial drift toward the incision line.

The total planned anaesthetic volume will be one point two multiplied by the planned long axis length in centimetres.

If rescue anaesthesia is required it will be administered in increments of zero point five millilitres and the timing and total volume will be recorded.

Group B Infiltrative anaesthesiaThe solution and equipment will be the same as Group A using two percent lignocaine with adrenaline one is to two hundred thousand with a twenty six gauge needle in the subdermal plane and aliquots of zero point zero five to zero point one millilitre.

Anaesthetic will be deposited along or just outside the inked ellipse one to two millimetres from the incision line in short tangential runs at three to five millimetre spacing avoiding intralesional tracks.

Total anaesthetic volume and rescue protocol will be the same as Group A.

Both groupsStandard antiseptic precautions will be followed and sterile draping will be used. Buffering or warming if used will be applied identically across groups. Ellipse measurements will be taken with a sterile calibrated Vernier calliper through a drape window to maintain assessor blinding. Total milligrams of lignocaine administered will be tracked using a conversion of twenty milligrams per millilitre with milligram per kilogram safety checks as per institutional policy and without a volume cap. Each rescue increment and its timing relative to the sixty one hundred twenty and one hundred eighty second measurements will be documented.

Post procedure careHaemostasis will be ensured and the operative site will be cleansed and covered with a plain petroleum based non adherent dressing. The wound will be kept dry for twenty four to forty eight hours after which gentle cleansing and light daily dressing will be advised. Paracetamol will be prescribed for analgesia and non steroidal anti inflammatory drugs or unnecessary antiplatelet agents will be avoided. Routine antibiotics will not be administered. Patients will be instructed to avoid strenuous activity or soaking until the wound is sealed and sutures are removed. They will be advised to report any increasing pain erythema discharge fever haematoma wound dehiscence or persistent numbness. Suture removal will be performed after ten to twelve days for upper limb trunk or abdomen and after twelve to fourteen days for back or lower limb. Complications and any unplanned interventions will be documented. At the eight week visit scar outcome and satisfaction will be assessed using the Patient and Observer Scar Assessment Scale visual analogue scale and standardised photographs by a blinded assessor.

Outcome assessment methods Primary outcomeComposite Distortion Index at 120 secondsThe ellipse and four fiducials at twelve three six and nine oclock positions will be inked before anaesthesia. Baseline apex to apex length L0 and mid width W0 will be measured by a blinded measurer using a sterile calibrated Vernier calliper through a drape window. Immediately after delivery of the planned dose a timer will be started. At one hundred twenty seconds the measurer will record L120 and W120. Fractional changes in apex to apex length and mid width will be calculated.

Change in length will be calculated as the absolute difference between L120 and L0 divided by L0.

Change in width will be calculated as the absolute difference between W120 and W0 divided by W0.

Composite Distortion Index at 120 seconds will be calculated as one hundred multiplied by the square root of the mean of the squared fractional changes in length and width.

The Composite Distortion Index represents the overall geometric distortion of the planned excision ellipse. It is calculated using the root mean square of fractional changes in the long and short axes expressed as a percentage. The root mean square approach is used to ensure that opposing directional changes in the two axes do not cancel each other out thereby providing a single composite measure of tissue distortion. Duplicate blinded readings will be taken and the mean value will be used.

Secondary parameters1.Composite Distortion Index at 60 seconds

2.Composite Distortion Index at 180 seconds

3.Injection pain

Immediately after completing the planned dose the participant will rate the maximum pain after injection on a numerical rating scale from zero to ten where zero indicates no pain and ten indicates worst imaginable pain. Any pre emptive use of analgesics or anxiolytics will be recorded.

4.Rescue anaesthesia

Any additional anaesthetic administered after the planned dose will be recorded as yes or no along with increments of zero point five millilitres cumulative rescue volume in millilitres injection sites and timing relative to the sixty one hundred twenty and one hundred eighty second measurements. Multiple rescues are permitted and total dose in milligrams will be tracked.

5.Time to onset of anaesthesia

Starting immediately after the planned dose pin prick sensation will be tested every five seconds at the two long side midpoints at three and nine oclock positions using a consistent twenty six gauge stimulus with occasional sham touches. Onset will be defined as two consecutive responses of no sharp sensation at both sites. If onset is not achieved by one hundred eighty seconds the time will be recorded as one hundred eighty seconds.

6.Complications at seven to ten days

At the seven to ten day review haematoma infection and wound dehiscence will be assessed and recorded using predefined criteria. Haematoma will be defined as one requiring evacuation or causing tense swelling. Infection will be defined by purulence or erythema or tenderness requiring prescribed antibiotics. Dehiscence will be defined as wound separation requiring intervention. Interim events will be captured from unscheduled visits.

7.Surgeon ease of technique and workflow

Immediately after wound closure the surgeon will rate overall ease and workflow on a five point Likert scale where one indicates very difficult and five indicates very easy.

8.Cosmesis at eight weeks and patient satisfaction

At the eight week visit the Patient and Observer Scar Assessment Scale will be used where feasible otherwise a visual analogue scale from zero to ten will be used. The observer component will be completed by a blinded assessor using direct examination and standardised coded photographs. Patient satisfaction will be recorded on a five point Likert scale.

Composite Distortion Index at 60 seconds and 180 seconds following injection of anaesthesia will be considered confirmatory secondary outcomes while all other secondary parameters will be considered exploratory.