A 19-year-old middle-distance runner has dull dorsal midfoot pain worsened by push-off. Plain films taken twice are normal. CT eventually shows a linear fracture through the central third of the tarsal navicular with sclerosis but no displacement. The reason this particular fracture carries the highest rate of nonunion among tarsal stress fractures is:

• A Repetitive shear from the spring ligament prevents fracture apposition
• B The navicular is covered entirely by articular cartilage, preventing callus formation
• C The central third of the navicular is relatively avascular, being a watershed zone between medial and lateral arterial supplies ✓
• D The fracture line always propagates into the talonavicular joint, destroying congruity

Explanation

The tarsal navicular is supplied proximally by branches of the dorsalis pedis and medially by branches of the posterior tibial artery, leaving the central third as a watershed zone with sparse vascularity; fractures here heal poorly and risk avascular necrosis. Because plain radiographs frequently miss the fracture, CT or MRI is needed when suspected. Undisplaced fractures require strict non-weight-bearing cast immobilization for 6 to 8 weeks, and displaced or ununited fractures need screw fixation, sometimes with bone grafting.

Reference: Campbell's Operative Orthopaedics, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.