Biomechanics of Stress Fractures: The Fifth Metatarsal Case

Research Type: Level 3 - Retrospective cohort study, Case-control study, Meta-analysis of Level 3 studies Introduction/Purpose: Stress fractures of the 5th metatarsal are common among individuals engaging in repetitive impact activities, or patients with preexisting deformities. Compared to traumatic fractures, outcomes are often worsened by delayed union, non-unions, or recurrence. Risk factors such as hindfoot varus and foot adduction have been implicated while specific anatomical and bone density characteristics of the 5th metatarsal remain underexplored. We hypothesized that stress fractures of the 5th metatarsal are associated with distinct alignment, and bone density patterns compared to healthy controls. Methods: This IRB-approved retrospective case-control study analyzed 15 feet with stress fractures and 15 control feet, using weight-bearing computed tomography (WBCT). Fifth metatarsal offsets, orientations, and baseline foot alignment were evaluated from Digital Imaging and Communications in Medicine (DICOM) datasets using WBCT software. Segmentation and bone density measurements (normalized Hounsfield Units [HU]) were performed with commercially available and open-source software. Results: No significant differences were observed in age, BMI, sex, or laterality between groups (p > 0.05). Fracture cases exhibited lower 5th metatarsal base height (p = 0.045), greater ground contact frequency (p < 0.001), and longer 5th metatarsal length (p = 0.038). Hindfoot varus and foot adduction were significantly associated with fractures, indicated by altered hindfoot alignment and tarsometatarsal angles. While absolute HU values showed no difference, fracture cases demonstrated a mean 50% increase in normalized HU values (p < 0.001). A ratio of 1.2 in 5th metatarsal over talus HU was associated to stress fracture cases with a 80% sensitivity and 94 % specificity. Conclusion: Stress fractures of the 5th metatarsal were known to be associated with hindfoot varus and forefoot adductus. The present study adds that they are associated with reduced base height, increased plantarflexion, a longer 5th metatarsal and higher bone density. A threshold of 1.2 for the Metatarsal over Talus HU ratio could possibly be used as an indicator for early preventive measures. Biomechanics of Stress Fractures: The Fifth Metatarsal Case. The figure illustrates 2D and 3D measurements including 5th metatarsal height, forefoot arch angle, foot ankle offset. We also illustrate the segmentation process to obtain 3D measurements and density values. The chart demonstrates the ROC curve for 5th metatarsal density as a predictor of stress fractures.