Causes / typical injury mechanism: A tarsal navicular stress fracture is the result of repetitive microtraumas of the navicular bone, often from overuse, incorrect technique, or improper equipment when engaging in vigorous weight-bearing activities such as running and jumping.

Classic history and presentation: Navicular stress fractures typically occur in young, active athletes involved in vigorous weight-bearing activities. Patients often present after failed conservative measures with a 6-month or longer history of vague pain on the dorsomedial or medial aspect of the midfoot / hindfoot, radiating distally to the forefoot. Often the pain is aggravated by activity and relieved by rest.

Typically, there is no single inciting event that the patient recalls, such as a pop or snap.

Prevalence: In athletes, it is estimated that up to 35% of all foot and ankle stress fractures are navicular stress fractures. Prevalence has been increasing over time, likely due to earlier symptom recognition and advanced imaging.

One study described female sex as a risk factor for navicular stress fractures (since female sex is a risk for stress fractures in general); however, other studies have found that the majority of navicular stress fractures occur in male athletes in their 20s.

Risk factors:

• Prior history of a stress fracture
• Unfavorable foot mechanics
• Anatomic factors – cavovarus foot, reduced ankle dorsiflexion (such as from a tight gastrocnemius), short first metatarsal, metatarsus adductus, equinus contracture
• Improper footwear or technique during athletics
• Increasing volume and intensity of physical activity
• Menstrual irregularity
• Low body mass index
• Diet poor in calcium and/or vitamin D

Pathophysiology: A combination of mechanical and vascular causes have been proposed as risk factors for injury. The location of the navicular bone between the talar head and cuneiform predisposes it to unique stresses that may contribute to development of a stress fracture. The first and second metatarsocuneiform joints transmit forces through the navicular bone and talar head medially during a foot strike. These forces are not shared by the lateral aspect of the navicular bone. When contracted, the posterior tibial tendon also contributes to tension medially. These net forces lead to shear stress through the middle third of the navicular bone.

The medial and lateral portions of the navicular bone receive blood supply through the medial tarsal branches of the dorsal pedis artery and branches of the lateral tarsal artery, leaving the central area hypovascularized. However, further studies have shown that this central hypovascularized zone is only present in approximately 10% of patients, and nearly 60% of patients do not have any zone of complete avascularity.

Additionally, anatomic variations of cavovarus foot, reduced ankle dorsiflexion, and short first metatarsal amplify the compression loading through the navicular bone.

Grade / classification system: Bone stress injuries of the navicular are classified on CT (other than type 0.5) using the Saxena classification.

• Type 0.5: Stress reaction on MRI; normal CT
• Type I: Fracture in the dorsal cortex
• Type II: Fracture extends from the dorsal cortex into the navicular body
• Type III: Complete fracture through both cortices