What is your differential and most likely diagnosis?

 The differential includes stress fracture, medial tibial stress syndrome (shin splints), compartment syndrome (chronic exertional vs acute), and hematoma. The diagnosis in this case is a stress fracture. 

• Pearl: Repetitive stress injuries to the tibia exist on a spectrum ranging from benign (shin splints) to more severe (stress fractures). Image 1 shows a stress fracture line but initial plain films are often negative or with only slight periosteal changes, as in Image 2.

Image 2. Case courtesy of Dr Matt Skalski, Radiopaedia.org, rID: 21318 

What are the risk factors for this injury?

Tibial stress fractures are fatigue injury patterns. Any abrupt increase in training (duration, intensity, etc.) can lead to injury. Individuals with poor bone health are also at risk, especially females.

What exam findings can help differentiate?

Patients with tibial stress fractures are more likely to have pinpoint pain that worsens with ambulation, whereas those with shin splints will exhibit a more diffuse injury pattern and ambulate without difficulty. 

• Pearl: The Hop Test – Have the patient hop on the affected leg 10 times if tolerated. Patients with shin splints can likely do this without difficulty, whereas those with stress fractures are more likely to report pain in the first few hops.

What is the high risk x-ray finding for this injury? What is the significance of this?

The "Dreaded Black Line" can be seen on lateral x-rays of the tibia and would indicate an anterior stress fracture. These are at a higher risk for malunion and may need early surgical fixation. See an example on Orthobullets. 

How is this injury managed in the emergency department?

Posterior and mid-tibial stress fractures are typically managed nonoperatively and can be discharged from the ED with crutches and instructions for activity restrictions and protected weightbearing. They should follow-up with sports medicine or orthopedics in 1-2 weeks. Consider earlier surgical consultation for anterior tibial stress fractures.