Atypical fractures of the ulna: Effect of bowing of the ulna on fracture location and occurrence.

Introduction: Ulnar fractures associated with long-term bisphosphonate (BPs) therapy are rare, and the nature and extent of this potential risk remains unknown. Although ulna is generally considered a "straight bone", it actually features a bow anatomically. For this reason, we speculated that ulnar bow may have a role in the development of atypical ulnar fractures (AUFs). Therefore, we compared the location and depth of ulnar bow between AUF patients and patients with atypical fractures other than the ulna. We aimed to answer: (1) whether a correlation exists between the location of the ulna bow and the location of AUFs, (2) whether the degree of ulnar bow affects the occurrence of AUFs.

Hypothesis: Ulnar bowing could play a critical role in the location and occurrence of AUFs.

Methods: We retrospectively reviewed the radiographs and medical records of 64 patients with atypical fractures admitted to our department between May 2010 to July 2020. The bow of the ulna was measured using anteroposterior (AP) and lateral radiographs. Bone angulation was described as apex of deformity, with apex lateral bow designated as AP plane bowing and apex posterior bow marked as lateral plane bowing.

Results: In all patients with atypical fractures, bow locations were measured at the proximal third level to the index line in 68% of AP plane and 72% of lateral plane. In patients with AUFs, fracture sites occurred in the range of 20% to<40%, except in one patient. Fracture site versus apex lateral bow location and fracture site versus apex posterior bow location showed a statistically significant correlation coefficient of 0.81 (p<0.001) and 0.69 (p=0.003), respectively. In lateral plane, there was a significant difference between AUF patients and patients with atypical fractures other than the ulna in ulnar bow depth (p=0.014). However, no statistically significant differences were found in AP plane (p=0.110).

Discussion: In AUFs, fracture site was highly correlated with ulnar bowing location, and, as the degree of apex posterior bow increased, occurrence of AUFs increased. These findings are helpful in understanding the role of bowing as the ulnar geometry in the development of AUF and early identification of the location of suspicious AUF.

Level Of Evidence: IV.