Could a patient with pain during active hip flexion but not passive flexion have a muscle tear other than rectus abdominis, possibly involving the hip flexor muscles, such as the iliopsoas muscle?

Differential Diagnosis: Pain with Active but Not Passive Hip Flexion

Yes, iliopsoas muscle pathology is the most likely alternative diagnosis when a patient has pain with active hip flexion but not passive flexion, and this should be your primary consideration beyond rectus abdominis. 1, 2

Understanding the Clinical Pattern

The key distinguishing feature you've identified—pain with active but not passive hip flexion—is pathognomonic for a muscle or tendon injury rather than intra-articular pathology. 1, 3

• Active hip flexion without pain during passive movement specifically implicates the hip flexor muscle-tendon unit, as passive motion does not load these structures. 2
• This pattern effectively rules out intra-articular hip pathology (FAI syndrome, labral tears), which typically causes pain with both active and passive hip flexion, particularly in the FADIR position. 4, 1

Primary Hip Flexor Muscles to Consider

Iliopsoas Complex (Most Likely)

The iliopsoas is the primary hip flexor and should be your first consideration after excluding rectus abdominis. 1, 2

• The iliopsoas consists of the psoas major, psoas minor, and iliacus muscles, functioning as the strongest hip flexor. 2
• Iliopsoas pathology presents with anterior hip or groin pain, weakness with resisted hip flexion, and pain specifically with active hip flexion activities. 1, 3
• Weakness with resisted hip flexion in abduction is particularly suggestive of iliopsoas syndrome. 3
• Iliopsoas injuries can occur from repetitive active hip flexion or acute flexion injuries, and spontaneous tears are rare but possible, particularly in elderly patients or those with risk factors like steroid use. 3, 5

Diagnostic Approach for Iliopsoas Pathology

Start with plain radiographs to exclude osseous pathology, followed by MRI or ultrasound if radiographs are negative. 1

• Plain radiographs are first-line to rule out structural abnormalities, arthritis, or fractures. 1
• MRI is highly sensitive and specific for detecting iliopsoas tendinitis, tears, and bursitis. 1
• Ultrasound is an excellent alternative that allows dynamic evaluation of the iliopsoas tendon and can guide therapeutic injections. 1
• Physical examination should include resisted hip flexion testing, which reproduces pain in iliopsoas pathology. 3, 6

Other Hip Flexor Muscles (Less Common)

Sartorius

• The sartorius assists with hip flexion, abduction, and external rotation. 2
• Isolated sartorius tears are uncommon but possible, particularly at the anterior superior iliac spine origin.

Tensor Fasciae Latae (TFL)

• TFL contributes to hip flexion and abduction.
• Isolated TFL pathology is rare but should be considered if pain is more lateral.

Rectus Femoris

• The rectus femoris crosses both the hip and knee, functioning as a hip flexor and knee extensor.
• Tears typically occur at the proximal musculotendinous junction and present with anterior thigh pain with active hip flexion. 7

Critical Diagnostic Pitfalls to Avoid

Always exclude lumbar spine pathology, as radicular pain can mimic hip flexor muscle pain. 8, 1, 9

• Lumbar spine pathology must be screened in all hip pain evaluations, as referred pain can present as anterior hip or groin discomfort. 8, 9
• Spine pathology typically presents with sharp, lancinating pain radiating down the leg, worsened by sitting or standing. 9

Do not confuse iliopsoas pathology with intra-articular hip conditions:

• FAI syndrome and labral tears cause pain with both active AND passive hip flexion, particularly in the FADIR position. 4, 1
• A negative FADIR test helps rule out intra-articular pathology. 8, 4

Consider that iliopsoas pathology can coexist with intra-articular hip conditions:

• Iliopsoas symptoms are often associated with FAI syndrome and labral pathology. 1
• If both active and passive movements cause pain, consider coexisting pathology requiring MRI evaluation. 1

Treatment Algorithm for Iliopsoas Pathology

Initial conservative management should be attempted for 4-6 weeks before considering injections. 1, 3

• Activity modification and avoidance of repetitive hip flexion activities. 1, 3
• Physical therapy focusing on iliopsoas stretching and strengthening of hip stabilizers. 1, 3
• NSAIDs for pain control and anti-inflammatory effect. 1, 3

If symptoms persist after 4-6 weeks, proceed with ultrasound-guided corticosteroid injection into the iliopsoas bursa or peritendinous region. 1

• This provides both diagnostic confirmation and therapeutic benefit. 1
• All patients in one large dance population series responded to conservative treatment without requiring injections or surgery. 3