Working Diagnosis:
Stress fracture of left tibia

Unable to rule out possibility of malignant or infectious process

Treatment:
An open surgical biopsy with curettage and bone grafting of left medial tibial lesion was performed.

Outcome:
Intraoperative cultures were negative for infectious process. Pathology indicated a periosteal aneurysmal bone cyst. Break-apart FISH (fluorescence in-situ hybridization) probe was positive for USP6 gene rearrangement, indicating the lesion was a primary ABC and not the result of an underlying malignancy. Case Photo #9 Case Photo #10
Radiographs at one month revealed incorporation of the synthetic graft material without evidence of focal lucencies. Local recurrence was not evident. The patient returned 6 weeks later for follow-up radiographs which showed continued consolidation of the graft material. At the time of this presentation, no local recurrence was evident. At 10 weeks, the patient was allowed weight bearing as tolerated.

Author's Comments:
An aneurysmal bone cyst (ABC) is a benign, blood-filled, fibrous cyst most commonly presenting in the second decade of life. ABCs deform the surrounding bone and can therefore become very destructive. This lesion typically presents with swelling and pain in the area of the cyst and may be associated with activity-related pain. The cause is unknown but is hypothesized to be related to a disruption of blood vessels or, in some cases, a pre-existing tumor.

An ABC and a tibial stress fracture have similar presentations. A tibial stress fracture classically presents as focal activity-related leg pain that gradually progresses over several weeks. Definitive diagnosis is made with imaging by the presence of a radiolucent line on radiographs or bone marrow edema on MRI.

Proximal tibial pain in a runner may have multiple etiologies. The case discussed here represents an unusual presentation of an ABC with initial clinical findings and imaging characteristic of a tibial stress fracture. This case highlights the results of close observation and repeated evaluations for a frequently encountered clinical scenario. An ABC is a benign bone lesion that may have an atypical presentation and requires a high index suspicion to obtain a definitive diagnosis and timely treatment to minimize long-term negative outcomes.

Editor's Comments:
This is an interesting case which highlights the importance of follow-up with patients. With a relatively common presentation of a runner with a "tibial stress fracture", conservative measures such as rest, physical therapy (to address biomechanical forces and muscle strength/ pliability), and relative activity modifications should address the issue and lead to complete healing. This particular patient underwent a very thorough initial assessment with plain radiographs, an MRI, AND a CT. Realization that there was no resolution of the patient's symptoms was the key to this case. As providers in sports medicine, it is equally important to schedule follow-up appointments as it is to get initial complaints evaluated. More importantly, it is imperative to keep searching and exploring if an injury is not behaving the way one would expect.

References:
1. Biermann JS, Stanitski CL, Common benign lesions of bone in children and adolescents. J Pediatr Orthop B. 2002;22:268-273
2. Harris JD, Varner KE. Stress fractures of the tibia. In: Miller L, Kaeding CC, eds. Stress Fractures in Athletes. Switzerland: Springer International Publishing; 2015:137-147
3. Tis JE. (2019). Nonmalignant bone lesions in children and adolescents. In Torcia, MM(Ed.), UpToDate. Retrieved February 1, 2019, from https://www.uptodate.com/contents/nonmalignantbonelesionsinchildrenandadolescents
4. Wyers MR. Evaluation of pediatric bone lesions. Pediatr Radiol. 2010;40:468-473

Acknowledgments:
The authors would like to acknowledge Dr. David Greenberg and Dr. Nancy Phillips for their contributions to this case study.

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