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

Effectiveness of Interfascial Hydrodissection Added to Ultrasound-Guided Piriformis Injection in Piriformis Syndrome: A Prospective Randomized Controlled Trial

The gluteus maximus overlies the piriformis muscle and forms a broad fascial interface with it. Restricted movement, increased tension, or adhesions between these tissue layers may contribute to persistent buttock pain in piriformis syndrome. Interfascial hydrodissection is an ultrasound-guided procedure in which fluid is introduced between the gluteus maximus and piriformis muscles to gently separate restricted fascial planes, improve tissue gliding, reduce local mechanical irritation, and potentially modulate nociceptive input from pain-sensitive nerve endings within or adjacent to the fascial interface.

The goal of this clinical trial is to learn whether adding ultrasound-guided interfascial hydrodissection to a piriformis injection provides better pain relief and improves function in adults with piriformis syndrome.

Piriformis syndrome can cause buttock pain that may spread to the back of the thigh or leg. Interfascial hydrodissection is a procedure in which fluid is injected between the gluteus maximus and piriformis muscles. The fluid gently separates the tissue layers and may help them move more freely.

The main questions this trial aims to answer are:

• Does adding interfascial hydrodissection to a piriformis injection lower overall pain more than the piriformis injection alone at 3 months after treatment?
• Does it provide greater improvement in pain at earlier follow-up visits, hip-related function, and overall recovery?

All participants will first receive one ultrasound-guided local anesthetic injection into the piriformis muscle. The early pain response will help support the diagnosis, and the same injection will serve as the common treatment received by both groups. Participants whose pain improves by at least 50 percent will be randomly assigned to either continue with the piriformis injection alone or receive additional interfascial hydrodissection. After the 1-week assessment, participants in both groups will begin the same home-based piriformis stretching program.

Participants will:

• Receive one ultrasound-guided piriformis injection and, depending on group assignment, may also receive interfascial hydrodissection
• Have follow-up assessments later on the treatment day and at 1 week, 1 month, and 3 months
• Follow the same home-based stretching program after the 1-week assessment
• Report their pain, daily function, overall improvement, pain medicine use, additional treatment needs, and any side effects

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.

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.