Uncommon Sites of Calcium Hydroxyapatite Deposition: The Forgotten Cause of Pain Syndromes Around the Hip

Presentation Format: Educational poster presentation.

Purpose or Learning Objective: (1) To demonstrate atypical manifestations of hydroxyapatite crystal deposition disease (HADD) around the hip as various pain syndromes, and (2) to review imaging findings to enable confident diagnosis and management, preventing unnecessary investigations.

Methods or Background: HADD is characterized by intra-articular or periarticular deposition of hydroxyapatite crystals of uncertain etiology. Stages have been described with distinct imaging features: Deposits appear as round-to-ovoid calcification in the formative and resting phases, and ill-defined calcification during the resorptive phase. The resorptive phase correlates with clinical symptoms of localized pain, tenderness, and reduced mobility. The hip is the second most frequently involved site but almost always described around the gluteus medius insertion.

Results or Findings: We illustrate cases of greater trochanteric pain syndrome, ischiofemoral impingement, and piriformis syndrome. Magnetic resonance imaging (MRI) demonstrated low signal foci at the greater trochanter (gluteus medius insertion), quadratus femoris (just lateral to the ischial tuberosity), and proximal piriformis (adjacent to the sciatic nerve at the greater sciatic notch), respectively. Associated tendinopathy, bursitis, and/or muscle edema were evident, indicating inflammation. Hip radiographs confirmed calcific deposits in keeping with HADD.

In the patient with piriformis syndrome, the calcific deposit was visible as a hyperechoic focus on ultrasonography and direct pressure-reproduced sciatica, likely due to irritation from inflammatory change. The patient's symptoms markedly improved following steroid injection and low-energy shock wave to this area.

Because symptoms can be acute and severe, and imaging appearances can be aggressive with bony erosion and extensive soft tissue or marrow edema, HADD can mimic trauma, infection, or malignancy. Hydroxyapatite appears as focal areas of low signal on all MR sequences; hence correlative radiographs are useful to identify small deposits. Radiographs can be used to assess the extent and morphology of calcifications. Computed tomography (CT) best evaluates osseous involvement and can predict consistency of deposits, which is important if planning intervention.

Conclusion: Ischiofemoral impingement and piriformis syndrome related to calcium hydroxyapatite have not been previously described. Although uncommon, knowledge of tendon anatomy is crucial in recognizing atypical presentations of calcium hydroxyapatite deposition. Correlation with radiographic or CT imaging enables a prompt diagnosis, facilitates management, and prevents mistaking HADD for more aggressive pathology.

No conflict of interest has been declared by the author(s).

Publication History

Article published online:
03 June 2021