A patient with pain over the superior aspect of the left iliac bone region, worsened by prolonged sitting, and localized tenderness, with slightly limited external rotation of the left hip, and normal Erythrocyte Sedimentation Rate (ESR), C-Reactive Protein (CRP), Rheumatoid Factor (RF), and urine analysis, and unremarkable pelvic X-ray, with partial response to Celebrex (celecoxib) and pregabalin, what are the differential diagnoses and next steps?

Differential Diagnoses and Next Investigation

The most likely diagnoses are iliac crest pain syndrome, sacroiliac joint dysfunction, or iliopsoas myofascial dysfunction, and the next investigation should be MRI of the pelvis/hip without contrast to exclude occult stress fracture, subperiosteal hematoma, or early inflammatory pathology. 1, 2, 3

Primary Differential Diagnoses

Iliac Crest Pain Syndrome (ICPS)

• This is the most probable diagnosis given the localized tenderness over the superior iliac region and pain worsened by prolonged sitting 4
• ICPS occurs in 33-58% of patients with low back pain depending on the clinical setting, with localized pain present in 73% of affected patients 4
• Typical pain reproduction occurs with lumbar spine movements (64%) or hip movements (53%), which aligns with the limited external rotation noted in this patient 4

Sacroiliac Joint Dysfunction

• Pain at or around the posterior superior iliac spine is characteristic of SIJ-related pain, and this patient's presentation with localized tenderness and sitting pain is highly consistent 5
• The slightly limited hip external rotation may reflect compensatory movement patterns or referred pain from the SIJ 5
• Sitting pain is a specific feature that strongly suggests SIJ involvement 5

Iliopsoas Myofascial Dysfunction

• Deep iliopsoas pathology can present with pain over the iliac region and limited hip range of motion, particularly in extension and rotation 6
• This diagnosis is supported by the partial response to pregabalin, which may address neuropathic components of myofascial pain 6

Occult Stress (Insufficiency) Fracture

• Despite normal radiographs, stress fractures of the pelvis can be radiographically occult initially and require MRI for definitive diagnosis 1
• The ACR recommends MRI without contrast as the imaging study of choice when initial radiographs are negative but clinical suspicion remains 1

Subperiosteal Hematoma of the Iliac Bone

• Although typically associated with acute trauma, chronic or subacute presentations can occur with persistent localized pain and tenderness 3
• This rare entity mimics other hip pathology and requires cross-sectional imaging for diagnosis 3

Recommended Next Investigation

MRI of the pelvis and hip without IV contrast is the appropriate next imaging study 1, 2

Rationale for MRI

• MRI is the most sensitive test for detecting occult fractures, bone marrow edema, soft tissue pathology, and early inflammatory changes that are not visible on radiographs 1, 2
• The ACR specifically recommends MRI without contrast when initial radiographs are negative or equivocal in patients with persistent bone or joint pain 1, 2
• MRI can identify stress fractures, subperiosteal collections, iliopsoas pathology, sacroiliac joint inflammation, and muscle/fascial abnormalities in a single examination 1, 2, 3

Alternative Diagnostic Approach

• Ultrasound of the iliac region may be useful for detecting subperiosteal hematoma or fluid collections and can guide therapeutic injections 3
• However, ultrasound has limited ability to evaluate bone marrow and deep pelvic structures compared to MRI 3

Diagnostic Injection Strategy

If MRI findings are equivocal or show only mild degenerative changes:

For Suspected SIJ Dysfunction

• Perform a peri-articular sacroiliac joint injection first, as it is effective in 81% of patients with SIJ-related pain and is technically easier than intra-articular injection 5
• Only proceed to intra-articular SIJ injection if the peri-articular injection is ineffective 5
• Complete pain relief with diagnostic injection confirms the SIJ as the pain source 5

For Suspected Iliopsoas Dysfunction

• Deep abdominal palpation of the psoas muscle should reveal tenderness if iliopsoas myofascial dysfunction is present 6
• Trigger point treatment using dry needling followed by postisometric relaxation exercises can be both diagnostic and therapeutic 6

Critical Clinical Examination Points

Perform these specific maneuvers to narrow the differential:

• Sacroiliac joint shear test to assess for SIJ instability or inflammation 5
• Hip flexor contracture testing and pain with stretch maneuvers of the spine and hip to evaluate for iliopsoas involvement 6
• Palpation for tenderness over the medial iliac crest (ICPS) versus posterior superior iliac spine (SIJ dysfunction) 5, 4
• Assess for groin pain, which when present suggests intra-articular hip pathology rather than SIJ or ICPS 5

Common Pitfalls to Avoid

• Do not assume benign mechanical pain without advanced imaging when symptoms persist despite appropriate conservative treatment 2
• The ACR warns that chronic worsening pain in the absence of metabolic derangement warrants structural evaluation, as occult fractures and other serious pathology commonly present with normal laboratory values 2
• Do not rely on normal inflammatory markers (ESR, CRP) to exclude significant pathology—stress fractures, myofascial dysfunction, and early SIJ disease typically have normal inflammatory markers 1, 5, 4
• Pregabalin has limited evidence for non-radicular musculoskeletal pain, and its partial effectiveness does not confirm a neuropathic etiology 7
• Gabapentin may be more effective than pregabalin for non-radicular low back and pelvic pain, though evidence remains limited 7

Treatment Considerations Pending Imaging

• Continue NSAIDs (celecoxib) at the minimum effective dose given normal renal function and absence of cardiovascular risk factors 1
• Consider discontinuing pregabalin if MRI excludes nerve compression or radiculopathy, as evidence does not support its use for non-neuropathic musculoskeletal pain 7
• Physical therapy focusing on hip and lumbar spine mobility, core strengthening, and postural modification for prolonged sitting may provide symptomatic relief while awaiting definitive diagnosis 1, 6