Chronic anterior tibial stress fractures in athletes: No crack but intense remodeling.

Purpose: Delayed healing of anterior tibial stress fractures in athletes is related to high tensional forces acting across a putative fracture gap. These forces lead to crack propagation and create strains that exceed tissue differentiation thresholds for new bone to form in the gap. The "dreaded black line" is a radiographic hallmark sign of stress fractures considered to represent a transverse fracture gap. However, whether a fracture gap truly exists at the microscopic level remains unclear. The aim of this study was to describe the area of the "dreaded black line" microscopically and to identify signs of delayed healing.

Methods: Between 2011 and 2016, we included seven athletes with chronic anterior mid-shaft tibial stress fractures. The fracture site was excised as a cylindrical biopsy. The biopsy was evaluated with micro-CT and histology. The formation of new bone in the defect was evaluated radiographically.

Results: The "dreaded black line" seen on pre-operative radiographs in all patients could not be seen on the microscopic level. Instead, the area of the putative crack showed widened resorption cavities, lined with active osteoblasts, and surrounded by immature bone. This area of intense remodeling seemed to create a false impression of a fracture line on radiographs. Complete cortical continuity was restored at the biopsy site at median 8 months (range 6-13 months).

Conclusion: Tibial stress fractures in athletes normally show no fracture defect, but a region of increased remodeling. The healing process is already ongoing but seems mechanically insufficient.