Rare Case Of Bilateral Stress Fracture Of Femoral Neck And Osteonecrosis Of Femoral Head

Case Report: Bilateral Femoral Neck Stress Fractures

Femoral neck stress fractures are rare and often recognized as overuse injuries that occur in young athletes or military personnel. A case following osteonecrosis of the femoral head is quite rare; even more uncommon is its occurrence in the bilateral hips. Nishino et al. encountered a case of bilateral femoral neck stress fracture that occurred in a patient with early stage osteonecrosis of the femoral head, and were able to observe progression of stress fracture since before fracture occurred. This is considered to be the first report to capture imaging changes before and after the onset.

Case Presentation

A 41-year-old Japanese woman presented to the clinic with a 4 month history of bilateral groin pain and right buttock pain. Her right hip was more painful than her left hip. There was no history of trauma, alcohol abuse, or steroid use. Her medical history included iron-deficiency anemia diagnosed 2 years earlier, after which she had been on iron supplements.

She was able to walk for approximately 10 minutes without a stick, albeit at a slow speed. Limitations in the passive motion of her bilateral hip joint were observed thus: flexion, 100°, internal rotation 5°, external rotation 15°, and abduction 20°, on both sides. She was able to perform a straight-leg raise of the right limb with substantial pain. The neurovascular status of both lower extremities was intact. The Japanese Orthopaedic Association scoring system for the evaluation of hip-joint function (JOA hip score) was 46 points for her right hip and 56 points for her left hip.

Investigations

Standard radiographs of both hips demonstrated no characteristic findings in either femoral head. MRI of both hips presented a low signal line in the subchondral region of the femoral head in the T1 weighted image and high signal region in almost all of the femoral head in the short tau inversion recovery (STIR). The oblique axial views of the proton density-weighted image showed a low-signal sinuous line in the anteromedial region of the femoral head.

Dual-energy X-ray absorptiometry (DEXA) values were low in both femoral necks. Bone mineral density was 0.909 g/cm² (T-score –0.9, Z-score −0.8) in the lumbar spine, 0.594 g/cm² (T-score −1.8, Z-score −1.4) in the right femoral neck, and 0.529 g/cm² (T-score −2.4, Z-score −2.0) in the left femoral neck.

Laboratory Findings

C-reactive protein (CRP): 0.03 mg/dl (normal range: 0–0.5 mg/dl)
Alkaline phosphatase: 608 IU/l (40–150 IU/l)
Calcium: 9.0 mg/dl (8.4–10.2 mg/dl)
Albumin: 4.4 g/d (3.9–4.9 g/d)
Hemoglobin: 9.7 g/dl (12–16 g/dl)

Bone Turnover Markers

Tartrate-resistant acid phosphatase 5b (TRACP-5b): 463 mU/dl (premenopausal normal range: 120–420 mU/dl)
Total procollagen type 1 N-terminal propeptide (Total P1NP): 72.7 ng/ml (26.4–98.2 ng/ml)

Diagnosis and Management

They diagnosed it as bilateral osteonecrosis of the femoral head and classified it as stage 1 in both femoral heads, according to the Association Research Circulation Osseous (ARCO) classification.

The patient was instructed not to take as much weight as possible using sticks on the right side. Her symptoms improved immediately. However, 6 months later, her symptoms increased slightly without any traumatic episodes. Radiographic findings indicated almost no change. However, a slight signal change in the medial subcapital region was observed in the MRI scan. Her pain gradually increased thereafter, and 10 months after her first visit, walking became difficult.

The JOA hip score decreased to 34 points in both hips; still, no changes could be observed in the radiograph. MRI of both hips showed a nondisplaced subcapital fracture on the medial side of both femoral necks, with bone marrow edema around the fracture. Because the cause of the fractures was not identified, the DEXA and bone turnover markers were measured again, and 25-hydroxy (OH) vitamin D was measured for the first time. All DEXA values decreased.

Updated Bone Mineral Density

Lumbar spine: 0.849 g/cm² (T-score −1.5, Z-score −1.2)
Right femoral neck: 0.527 g/cm² (T-score −2.4, Z-score −2.1)
Left femoral neck: 0.490 g/cm² (T-score −2.7, Z-score −2.4)

Updated Bone Turnover Markers

TRACP-5b: 607 mU/dl
Total P1NP: 52.7 ng/ml

Her 25(OH) vitamin D level was 11.1 ng/dL and she was diagnosed with vitamin D deficiency.

Based on the above results, her diagnosis was bilateral stress fracture of the femoral neck secondary to osteonecrosis of the femoral head.

Treatment and Outcome

The patient underwent internal fixation of both hips with sliding hip screws to stabilize the stress fractures. In addition, the reaming performed before inserting of the sliding hip screw served as core decompression for the femoral heads. The specimens obtained from the reaming were examined histologically. Definitive findings of osteonecrosis such as bone marrow necrosis and loss of osteocyte nuclei in the femoral heads were observed. Postoperative radiographs showed no evidence of displacement of the fractures.

They administered eldecalcitol 0.75 μg per day orally for vitamin D deficiency, and daily subcutaneous injections of teriparatide acetate.

In the immediate postoperative period, the patient began to bear weight as tolerated with the use of an assistive device bilaterally. One month postoperatively, she was able to walk without pain and used a cane part time. She eventually regained full walking ability without a cane 3 months after surgery. Furthermore, her JOA hip score improved to 90 points in both hip joints at 5 months after surgery. Radiographs showed no evidence of recurrent stress fracture in the femoral neck or progression of osteonecrosis. Sequential oblique axial MRI showed that the necrotic region of the femoral head had decreased 5 months after surgery.

Conclusion

The authors concluded that bilateral femoral neck stress fractures are a very rare condition and are often missed. It is important to listen to the patient's complaints and perform an appropriate examination & investigations to diagnose stress fractures. Timely intervention can avoid devastating complications.

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