Knee osteoarthritis (OA) is a common condition causing pain, stiffness, and difficulty walking. Many patients with advanced (Grade III) knee OA also experience neuropathic pain - burning, tingling, or electric shock sensations - which is harder to treat with standard physiotherapy. Oral pregabalin works for neuropathic pain but causes side effects like dizziness and drowsiness. This study tests whether delivering pregabalin through the skin using ultrasound (phonophoresis), combined with calf muscle stretching (Muscle Energy Technique), can reduce nerve pain and improve walking better than ultrasound alone.

Knee osteoarthritis is the most prevalent degenerative joint disease globally, characterized by progressive loss of articular cartilage, subchondral bone remodeling, synovial inflammation, and capsular thickening. While traditionally considered a nociceptive pain condition, accumulating evidence demonstrates that a significant subset of patients - particularly those with advanced radiographic severity - exhibit neuropathic pain features. This neuropathic component arises from peripheral nerve sensitization, central sensitization, and structural changes in both peripheral and central nervous systems.

Grade III knee osteoarthritis (Kellgren-Lawrence classification) represents severe cartilage loss with joint space narrowing, osteophyte formation, and subchondral sclerosis. In this population, neuropathic pain prevalence is substantially elevated compared to earlier grades. Clinical descriptors include burning sensations, electric shock-like pains, tingling, and allodynia.

Pregabalin, a gabapentinoid anticonvulsant, exerts its analgesic effect by binding to the alpha-2-delta subunit of voltage-gated calcium channels, reducing calcium influx and subsequent release of excitatory neurotransmitters including glutamate, norepinephrine, and substance P. While effective for neuropathic pain, oral administration produces dose-dependent central nervous system side effects that frequently limit tolerability and compliance.

Phonophoresis represents a transdermal drug delivery method utilizing therapeutic ultrasound to enhance skin permeability and drive medication into underlying tissues. The proposed mechanism involves cavitation, acoustic streaming, thermal effects, and alterations in stratum corneum lipid bilayer structure. Previous studies have demonstrated successful phonophoretic delivery of gabapentin (a pregabalin analog) for post-mastectomy neuralgia, with superior analgesia compared to topical gel alone.

Concurrently, lower extremity muscle dysfunction is well-documented in knee osteoarthritis. The gastrocnemius muscle, spanning both knee and ankle joints, exhibits increased stiffness measured by shear wave elastography. This stiffness alters plantar pressure distribution, increases knee joint compressive forces during gait, and contributes to reduced walking speed, stair-climbing difficulty, and overall mobility limitation.

Muscle energy technique is a manual therapy method classified under osteopathic and physiotherapeutic approaches. Post-isometric relaxation, a specific MET variant, involves voluntary isometric contraction of the target muscle against a precisely applied counterforce, followed by passive stretching during the subsequent relaxation phase. This technique is theorized to reset muscle spindle sensitivity, reduce Golgi tendon organ inhibition, and lengthen fibrotic or shortened muscle fibers.

Study Objectives

Primary Objective: To evaluate the effect of pregabalin-encapsulated hydrogel phonophoresis combined with gastrocnemius MET on neuropathic pain (measured by DN-4) in patients with Grade III knee osteoarthritis, compared to aqua gel phonophoresis with MET.

Secondary Objectives:

To compare the effect of the experimental intervention versus control on pain intensity (NPRS).

To compare the effect on functional mobility and stiffness (WOMAC total and subscale scores).

To document any adverse events associated with pregabalin phonophoresis.

Hypotheses

Null Hypothesis (H₀): There is no significant difference between pregabalin-encapsulated hydrogel phonophoresis plus MET versus aqua gel phonophoresis plus MET in reducing neuropathic pain, pain intensity, or improving mobility in Grade III knee osteoarthritis.

Alternative Hypothesis (H₁): Pregabalin-encapsulated hydrogel phonophoresis plus MET produces significantly greater improvement in neuropathic pain, pain intensity, and mobility compared to aqua gel phonophoresis plus MET.

Detailed Methodology

Screening and Recruitment

Potential participants will be identified from outpatient departments of Madinah Teaching Hospital, Mujahid Hospital, Inmotion Rehabilitation Clinic, and Jaffar Physiotherapy Clinic in Faisalabad, Pakistan. Flyers and physician referrals will be used. All interested individuals undergo a two-stage screening process:

Initial telephone screening: Age range, diagnosis of knee osteoarthritis, absence of obvious exclusion criteria.

In-person clinical screening: Confirmation of Grade III Kellgren-Lawrence radiograph (available within prior 12 months or newly obtained), DN-4 administration, physical examination, and review of medical history.

Randomization and Allocation Concealment

After providing informed consent and completing baseline assessments, eligible participants will be randomly assigned to either Group A or Group B in a 1:1 ratio. Randomization sequence will be generated using an online random number generator (Randomizer.org) with variable block sizes of 4 and 6. Allocation concealment will be maintained using sequentially numbered, opaque, sealed envelopes prepared by an investigator not involved in participant recruitment or outcome assessment.

Blinding

This is a double-blind trial:

Participant blinding: Participants are unaware of whether they receive pregabalin-encapsulated hydrogel or aqua gel. Both hydrogels are identical in appearance, viscosity, odor, and packaging.

Outcome assessor blinding: The assessor performing baseline and post-intervention measurements (NPRS, DN-4, WOMAC) will have no knowledge of group assignment and will not be present during treatment sessions.

Therapist blinding: Not possible, as the therapist prepares the gel and knows which is being applied. To minimize bias, therapists will follow a standardized script and avoid any verbal cues regarding expected outcomes.

Intervention Protocol Details

Common Procedures for Both Groups:

Each session lasts approximately 25-30 minutes and follows this sequence:

Hot pack application (10 minutes): Standard hydrocollator hot pack (temperature approximately 70°C, insulated with layers of toweling) applied to the posterior knee and upper calf to increase tissue temperature, blood flow, and muscle extensibility prior to MET.

Phonophoresis (5 minutes): The participant lies supine with the affected knee slightly flexed on a rolled towel for comfort. The therapist applies the assigned gel (pregabalin-encapsulated or aqua gel) to the medial, lateral, and posterior aspects of the knee joint. Ultrasound unit (Sonopuls 490, Enraf-Nonius) parameters: continuous mode, frequency 1 MHz, intensity 1 W/cm². The sound head (5 cm² effective radiating area) is moved in a slow circular motion (approximately 2-3 cm/sec) over an area twice the size of the sound head. Total treatment time: 5 minutes, including all three knee regions.

Gastrocnemius Muscle Energy Technique - Post-Isometric Relaxation (10-12 minutes):

Position: Supine, legs extended, ankle in neutral.

Therapist position: Standing at the foot of the treatment table on the side of the affected leg.

Grip: One hand stabilizes the distal femur just proximal to the knee joint; the other hand holds the plantar aspect of the foot with palmar contact.

Step 1 - Isometric contraction: Therapist instructs: "Push your foot down into my hand, like pointing your toe. Do not move - just push against me. Use about 25% of your maximum strength." Contraction held for 7-10 seconds. Therapist provides sufficient resistance to prevent any joint movement (true isometric).

Step 2 - Relaxation: Therapist instructs: "Now relax completely. Let me do the work."

Step 3 - Stretch phase: Immediately following relaxation, therapist passively dorsiflexes the ankle to stretch the gastrocnemius, moving to the first point of tissue resistance or until the participant reports mild stretch (never pain). The stretch is held for 10 seconds while maintaining knee extension.

Step 4 - Repeat: The cycle (contract-relax-stretch) is repeated 4-5 times per session.

Between repetitions, the muscle is returned to neutral for 2-3 seconds of rest.

Group-Specific Procedures:

Group A (Experimental) - Pregabalin-Encapsulated Hydrogel

Preparation: Pregabalin powder (pharmaceutical grade, purity ≥99%) is incorporated into a water-soluble, ultrasound-conductive gel base at a concentration of 5% w/w. The gel is prepared under sterile conditions in the university pharmacy. Each 5-minute phonophoresis session delivers approximately 0.5 mL of gel per cm² of treatment area.

Stability and Storage: Prepared gel is stored at 4°C and used within 14 days of preparation. Each participant receives freshly prepared gel weekly.

Group B (Active Comparator) - Aqua Gel

Composition: Standard ultrasound conductive gel containing water, carbomer, propylene glycol, methylparaben, and propylparaben. No active pharmaceutical ingredient.

Appearance: Identical to pregabalin gel in color (transparent), viscosity, and packaging.

Adherence Monitoring

Attendance recorded at each session.

Participants missing more than 3 consecutive sessions or 5 total sessions are considered protocol violators and will be excluded from per-protocol analysis (but included in intention-to-treat analysis).

Concomitant Medications and Therapies

Participants are instructed to maintain their usual medications (excluding new prescriptions for pain) and to avoid any other new treatments (acupuncture, other manual therapy, new exercise programs) during the 4-week study period. Any changes in concomitant therapy will be recorded.

Adverse Event Monitoring

At each session (16 total), participants are asked: "Have you experienced any unusual symptoms or problems since your last visit?" Any reported adverse events are documented, including onset, duration, severity, relationship to intervention (definite, probable, possible, unlikely, unrelated), and action taken (none, treatment modification, treatment discontinued, medical referral).

Predicted potential adverse events:

Local skin irritation (redness, itching) from gel or ultrasound

Muscle soreness following MET (expected, typically mild, resolves within 24-48 hours)

Temporary increase in knee pain (uncommon)

Any serious adverse event (defined as hospitalization, disability, or life-threatening) will result in immediate unblinding, participant withdrawal, and reporting to the ethics committee within 24 hours.

Statistical Analysis Plan

Sample Size Justification:

Based on a previous study of gabapentin phonophoresis for neuropathic pain (Alsharidah 2024), assuming an effect size (Cohen's d) of 0.8 for the primary outcome (DN-4), with α = 0.05 (two-tailed) and power = 80%, the required sample size per group is 26 participants. Accounting for an anticipated 15% dropout rate, the target enrollment is 30 participants per group, total N = 60.

Analysis Populations:

Intention-to-treat (ITT): All randomized participants analyzed according to original group assignment, regardless of protocol adherence. Missing data handled using last observation carried forward (LOCF) for primary analysis, with sensitivity analysis using multiple imputation.

Per-protocol (PP): Participants who completed at least 12 of 16 sessions (75% adherence) with no major protocol violations.

Descriptive Statistics:

Baseline demographic and clinical characteristics will be summarized using means and standard deviations (normally distributed continuous variables), medians and interquartile ranges (non-normally distributed continuous variables), and frequencies and percentages (categorical variables).

Primary Outcome Analysis (DN-4):

Within-group comparison: Paired t-test (if normally distributed) or Wilcoxon signed-rank test (if non-normal).

Between-group comparison: Independent t-test or Mann-Whitney U test, comparing change scores (post-intervention minus baseline).

Secondary Outcome Analyses (NPRS, WOMAC):

Same parametric/non-parametric approach as primary outcome.

For WOMAC, total score and subscale scores (pain, stiffness, physical function) analyzed separately.

Secondary Analyses:

Analysis of covariance (ANCOVA) with baseline score as covariate to adjust for potential baseline differences.

Subgroup analyses (exploratory) by sex, age group (<60 vs ≥60 years), and bilaterality (unilateral vs bilateral involvement).

Significance Criterion:

Two-tailed p < 0.05 considered statistically significant for primary outcome. For secondary outcomes, p < 0.05 considered nominal significance; no adjustment for multiple comparisons due to exploratory nature.

Software:

IBM SPSS Statistics version 25 (IBM Corp., Armonk, NY, USA).

Data Management and Quality Control

Paper case report forms (CRFs) are completed during each visit.

Data entry: Double-entry by two independent research assistants.

Range and logic checks programmed in SPSS.

Source documents (medical records, CRFs) retained for 10 years per institutional policy.

Secure storage: Locked filing cabinet for paper records; password-protected, encrypted electronic database.

Ethical Considerations

Approval obtained from the Research and Ethics/Technical Committee, The University of Faisalabad.

Conducted in accordance with the Declaration of Helsinki (2013 revision) and Good Clinical Practice guidelines (adapted for non-pharmaceutical trials).

Informed consent: Written, dated, and signed by participant or legal guardian. Includes detailed explanation of procedures, potential risks (mild skin irritation, muscle soreness), benefits, alternatives, right to withdraw without penalty, and confidentiality protections.

Participants may withdraw at any time without affecting their medical care.

No compensation provided for participation (no funding for incentives).

Results will be disseminated through thesis publication, peer-reviewed journal, and registered on ClinicalTrials.gov regardless of outcome direction.

Inclusion Criteria:

1. Diagnosed Grade III knee osteoarthritis confirmed by Kellgren-Lawrence grading scale (radiograph within past 12 months or newly obtained).
2. Neuropathic pain component confirmed by DN-4 questionnaire (score ≥4).
3. Age between 40 and 70 years inclusive.
4. Both male and female participants.
5. Unilateral or bilateral knee involvement (if bilateral, the more symptomatic knee selected for assessment and treatment).
6. Willing and able to provide written informed consent.
7. Available to attend 4 sessions per week for 4 consecutive weeks (total 16 sessions).
8. Able to understand and follow verbal instructions (for MET).

Exclusion Criteria:

1. Diabetic neuropathy (diagnosed or DN-4 score attributable to diabetes rather than knee OA).
2. Active skin lesions, wounds, infections, or dermatitis on the knee or calf.
3. Metal implants (including knee replacement), pacemaker, or other electronic implant (contraindications for ultrasound).
4. Received intra-articular knee injection (corticosteroid or hyaluronic acid) within the last 3 months.
5. Grade I, II, or IV knee osteoarthritis (Kellgren-Lawrence).
6. Aseptic arthritis of knee (rheumatoid, psoriatic, gouty).
7. Any lower extremity deformity (genu varum >15°, genu valgum >15°, fixed flexion deformity >15°).
8. Previous knee replacement surgery (partial or total).
9. Known allergy or hypersensitivity to pregabalin, gabapentin, or any gel components.
10. Pregnant or lactating women (ultrasound safety in pregnancy not established for non-obstetric use).
11. Concurrent participation in another interventional study.
12. Diagnosis of fibromyalgia, complex regional pain syndrome, or other centralized pain disorder that might confound neuropathic pain assessment.
13. Active malignancy or history of knee malignancy.
14. Deep vein thrombosis or thrombophlebitis in lower limb.