Interfascial Hydrodissection Added to Ultrasound-Guided Piriformis Injection for Piriformis Syndrome

Piriformis syndrome is a clinical pain condition characterized by buttock pain that may radiate to the posterior thigh or leg. Symptoms may be associated with dysfunction of the piriformis muscle, irritation of the sciatic nerve, and abnormalities within the surrounding soft-tissue planes. Because its clinical presentation may resemble lumbar radiculopathy or primary hip disorders, diagnosis is generally based on clinical examination, exclusion of other important causes of symptoms, and response to an ultrasound-guided piriformis injection.

The gluteus maximus overlies the piriformis muscle and forms a broad fascial interface with it. In some participants, restricted gliding, increased tension, or adhesions within this plane may contribute to persistent gluteal pain in addition to abnormalities involving the piriformis muscle itself. Despite the potential clinical importance of this interface, most interventional treatments for piriformis syndrome are directed primarily toward the piriformis muscle.

Ultrasound-guided piriformis injection is used in clinical practice for both diagnostic and therapeutic purposes. Interfascial hydrodissection is an emerging ultrasound-guided technique in which fluid is introduced between adjacent tissue layers. The procedure may separate restricted fascial planes, improve gliding between the gluteus maximus and piriformis muscles, reduce local mechanical irritation, and modulate nociceptive input from pain-sensitive free nerve endings and small sensory nerve branches within or adjacent to the fascial plane. However, the additional clinical benefit of hydrodissection over piriformis injection alone remains uncertain.

This is a prospective, randomized, assessor-blinded, parallel-group, active-controlled, single-center clinical trial. Adults with clinical findings consistent with piriformis syndrome will undergo a detailed clinical assessment and review of available routine imaging to exclude lumbar radiculopathy, major hip pathology, and other important causes of buttock or leg pain. The clinical examination will include palpation of the piriformis muscle and the FAIR, Freiberg, Pace, and Beatty maneuvers.

Participants who meet the initial eligibility criteria will receive a single ultrasound-guided local anesthetic injection into the piriformis muscle. This injection will have both diagnostic and therapeutic intent. The early pain response will be used to support the diagnosis and determine eligibility for randomization, while the same injection will serve as the common active treatment received by both randomized groups. Approximately 110 participants are expected to undergo the pre-randomization diagnostic-therapeutic piriformis injection, of whom approximately 96 are expected to demonstrate at least a 50 percent reduction in overall pain and proceed to randomization.

Participants who demonstrate at least a 50 percent early reduction in overall pain compared with their pre-injection pain level will be randomly assigned in a 1:1 ratio to one of two parallel groups. The active-control group will receive no additional invasive procedure. The intervention group will receive ultrasound-guided interfascial hydrodissection of the tissue plane between the gluteus maximus and piriformis muscles after the early response assessment. All invasive procedures will be completed during the same study visit under standardized sterile conditions.

The early pain assessment used to determine eligibility for randomization will be recorded separately from the post-randomization outcome assessments. The first comparative follow-up assessment will be performed 1 hour after hydrodissection in the intervention group and at the corresponding post-randomization time point in the active-control group. Subsequent assessments will be performed at 1 week, 1 month, and 3 months.

The piriformis injection and, when assigned, the hydrodissection procedure will be performed by a physical medicine and rehabilitation physician experienced in ultrasound-guided interventions. Baseline and follow-up outcome assessments will be conducted by a different physical medicine and rehabilitation physician working in the same clinic. The outcome assessor will not participate in the procedures, will not have access to treatment allocation records, and will remain unaware of treatment allocation throughout follow-up. Participants will be instructed not to disclose their group assignment to the outcome assessor.

After completion of the 1-week assessment, participants in both groups will begin the same standardized home-based piriformis stretching program and will continue the program for 6 weeks. The exercise will be demonstrated by a member of the rehabilitation team, written instructions will be provided, and adherence will be recorded during follow-up.

The primary objective is to determine whether adding interfascial hydrodissection produces a greater reduction in overall pain intensity from the pre-injection baseline to 3 months than the piriformis injection alone.

Secondary evaluations will include changes in overall pain at 1 hour, 1 week, and 1 month; pain at rest, during activity, at night, while sitting, while standing, and while lying down; hip-related function measured using the Non-Arthritic Hip Score; participant-reported global improvement; analgesic medication use; the need for additional or repeat treatment; and adverse events.

By comparing a diagnostic-therapeutic piriformis injection and standardized home-based stretching program with the same care supplemented by interfascial hydrodissection, this study aims to determine whether addressing the gluteus maximus-piriformis fascial interface provides additional and sustained clinical benefit in participants with piriformis syndrome.

• Ultrasound-Guided Piriformis Injection (Procedure): Before randomization, participants who meet the initial clinical eligibility criteria will receive one ultrasound-guided injection of 5 mL of 2% lidocaine into the piriformis muscle. The injection will have both diagnostic and therapeutic intent. The early pain response will be used to support the diagnosis and determine eligibility for randomization. In participants who demonstrate at least a 50 percent reduction in overall pain and proceed to randomization, the same injection will constitute the common active treatment received by both arms.
• Ultrasound-Guided Interfascial Hydrodissection (Procedure), also known as Gluteus Maximus-Piriformis Interfascial Hydrodissection: After assessment of the early pain response and randomization, participants assigned to the experimental arm will receive one ultrasound-guided interfascial hydrodissection of the tissue plane between the gluteus maximus and piriformis muscles. Hydrodissection will be performed using a total volume of 20 mL consisting of 18 mL of 0.9% sodium chloride and 2 mL of 1% lidocaine.
• Standardized Home-Based Piriformis Stretching Program (Behavioral): After completion of the 1-week assessment, participants in both arms will begin the same standardized home-based piriformis stretching program and will continue the program for 6 weeks. The exercise will be demonstrated by a member of the rehabilitation team, written instructions will be provided, and adherence will be recorded during follow-up.

Primary outcomes

• Change From Pre-Injection Baseline in Overall Pain Intensity on the 11-Point Numeric Rating Scale at 3 Months [Pre-injection baseline and 3 months after randomization]: Overall pain intensity will be measured using an 11-point Numeric Rating Scale ranging from 0 (no pain) to 10 (worst imaginable pain). The change score will be calculated as the 3-month score minus the pre-injection baseline score. More negative change scores indicate greater improvement in pain.

Secondary outcomes

• Change From Pre-Injection Baseline in Overall Pain Intensity at Earlier Follow-Up Time Points [Pre-injection baseline and 1 hour, 1 week, and 1 month after randomization]: Overall pain intensity will be measured using an 11-point Numeric Rating Scale ranging from 0 (no pain) to 10 (worst imaginable pain). Change scores will be calculated separately at 1 hour, 1 week, and 1 month by subtracting the pre-injection baseline score from the score at each follow-up time point. More negative change scores indicate greater improvement in pain.
• Change From Pre-Injection Baseline in the Situation-Specific Pain Profile [Pre-injection baseline and 1 week, 1 month, and 3 months after randomization]: The situation-specific pain profile consists of 6 prespecified component ratings: pain at rest, during activity, at night, while sitting, while standing, and while lying down. Each component will be measured separately using an 11-point Numeric Rating Scale ranging from 0 (no pain) to 10 (worst imaginable pain). Change scores for each component will be calculated separately by subtracting the pre-injection baseline score from the score at each follow-up time point. More negative change scores indicate greater improvement. The component scores will be reported separately, and no total or combined pain score will be calculated.
• Change From Baseline in Hip-Related Function Measured by the Non-Arthritic Hip Score [Baseline and 1 week, 1 month, and 3 months after randomization]: Function will be assessed using the Non-Arthritic Hip Score, a 20-item participant-reported questionnaire scored from 0 to 100. Higher scores indicate better hip-related function. Change scores will be calculated separately at each follow-up time point by subtracting the baseline score from the follow-up score. More positive change scores indicate greater functional improvement.

Sex: All, ages: 18 Years to 70 Years

Inclusion Criteria:

* Age 18 to 70 years.
* Buttock or gluteal pain that may radiate to the posterior thigh and/or leg.
* Medical history and physical examination findings consistent with piriformis syndrome.
* Tenderness on palpation over the piriformis muscle.
* A positive result on at least one of the following piriformis provocation maneuvers: FAIR, Freiberg, Pace, or Beatty.
* Exclusion of lumbar radiculopathy, clinically significant hip pathology, and other major causes of buttock or leg pain by clinical assessment and, when clinically indicated, review of available routine imaging.
* Ability to understand and sign the written informed consent form.
* Ability and willingness to attend the scheduled follow-up visits.

Additional criterion for randomization:

\- At least a 50 percent early reduction in overall pain after the single ultrasound-guided local anesthetic piriformis injection.

Exclusion Criteria:

* Active lumbar radiculopathy, clinically significant lumbar disc herniation, spinal stenosis, or another spinal disorder that could explain the participant's symptoms.
* Clinically significant neurological deficit.
* Marked limitation of lumbar spine and/or hip range of motion.
* Previous surgery involving the lumbar spine or hip region.
* Pregnancy or breastfeeding.
* Known allergy or hypersensitivity to lidocaine or another local anesthetic used in the study.
* Current anticoagulant treatment or a clinically significant coagulation disorder.
* Active infection at the injection site.
* Body mass index greater than 35 kg/m².
* Active inflammatory or infectious disease that may affect participant safety or study assessments.
* Active psychiatric condition that may interfere with informed consent, adherence to study procedures, or outcome assessment.
* Uncontrolled hypertension.
* Uncontrolled diabetes mellitus.
* Decompensated cardiac, hepatic, or renal failure.
* Vascular or neoplastic disease that could explain the symptoms or increase the risk of the study procedures.
* Injection, dry needling, or a similar intervention in the gluteal region within the previous 3 months.
* Inability or unwillingness to complete the scheduled follow-up visits.

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• Kubra Neslihan Kurt Oktay, MD, Principal investigator, University of Health Sciences, Haydarpasa Numune Training and Research Hospital, Department of Physical Medicine and Rehabilitation