Differential Diagnosis
- Single most likely diagnosis
- Sacroiliac (SI) joint dysfunction: The patient's pain is exacerbated by sitting on a hard surface or dangling their left leg, which can put stress on the SI joint. The presence of a benign-appearing high T2 signal focus within the right sacral ala, adjacent to the right SI joint, suggests possible SI joint pathology. The lack of relief from corticosteroid injections in the hip joint and buttocks, as well as the worsening of pain after physical therapy, also points towards SI joint dysfunction as the primary cause of pain.
- Other Likely diagnoses
- Femoral head lesion: The complex lesion within the posterior femoral head could be causing pain, especially if it is related to the same process causing the lesion in L4. However, the lack of perilesional edema and the intact cortex make this less likely.
- L4 vertebral body lesion: The heterogeneous lesion within the right side of the L4 vertebral body could be causing referred pain to the posterior hip. The presence of areas with near-fluid signal intensity within the lesion suggests possible cystic or necrotic components.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Infection (e.g., osteomyelitis or septic arthritis): Although the MRI findings do not strongly suggest infection, it is essential to consider this possibility, especially if the patient has a history of fever, chills, or recent travel.
- Malignancy (e.g., metastatic disease or primary bone tumor): The presence of multiple lesions (femoral head and L4 vertebral body) raises the concern for malignancy, although the MRI appearance is nonspecific.
- Rare diagnoses
- Avascular necrosis of the femoral head: The lesion within the posterior femoral head could be related to avascular necrosis, although the lack of perilesional edema and the intact cortex make this less likely.
- Inguinal canal lipoma causing referred pain: The presence of a lipoma within the left inguinal canal could potentially cause referred pain to the posterior hip, although this is a less common cause of pain in this region.
Regarding the treatment options:
- If the femoral lesion is the cause of pain, cyst removal or other surgical interventions may be considered. However, as you mentioned, cyst removal may cause blood circulation loss to the femoral head, and hip replacement might be a safer option in some cases.
- SI corticosteroid injection could potentially lessen pain if the SI joint is the primary cause of pain. However, given the patient's history of worsening pain after corticosteroid injections in the hip joint and buttocks, this should be approached with caution.
- Physical therapy and other conservative management options should be tailored to the patient's specific needs and avoided if they exacerbate the pain.