Fifth Metatarsal Stress Fractures Are Associated With Increased Bone Density and Altered Alignment on Weightbearing CT.

BACKGROUND Stress fractures of the fifth metatarsal (M5) are common among individuals engaging in repetitive impact activities or patients with preexisting deformities. Compared with patients who have traumatic fractures, those with stress fractures often develop delayed union, nonunions, or recurrence. Risk factors such as hindfoot varus and foot adduction have been implicated. The recent advent of weightbearing CT enables the study of specific bone density and orientation characteristics that have not, to our knowledge, previously been explored. Such tools could detect higher risk patients and help trigger potential preventive measures. QUESTIONS/PURPOSES Do patients with an M5 stress fracture present altered three-dimensional orientation and alignment parameters compared with an age- and sex-matched control group? (2) Do the feet and M5s of patients with an M5 stress fracture present different foot ankle offset (FAO) parameters compared with the control group? (3) Do the M5s of patients with an M5 stress fracture present with altered bone density patterns compared with the control group, and is a clinically relevant threshold identifiable? METHODS This institutional review board-approved retrospective case-control study analyzed 15 feet of patients with M5 stress fractures and 15 feet of a control group using weightbearing CT. Between February 2022 and May 2024, a total of 74 patients with available weightbearing CT scans were treated for an M5 fracture. Among those patients, we considered 77% (57) of proximal fractures as potentially eligible. Of those patients, 39% (22 of 57) were included; a further 32% (7 of 22) were later excluded because of metal artifact conflicting with M5 bone density assessment, leaving 68% (15 of 22) for analysis here. Controls were selected from our weightbearing CT archive, matched for age and sex and excluded if any foot disorder or prior intervention was identified. Accordingly, there were seven males and eight females in each group, and five and nine left sides, respectively, in the stress fractures and control groups. The mean ± SD age was 53 ± 13 years for the stress fractures group versus 51 ± 12 years for controls. Mean ± SD BMI was 34.4 ± 10.2 kg/m2 for the stress fractures group and 36.8 ± 8.2 kg/m2 for controls. For the first study question, M5 orientations and baseline foot alignment parameters were evaluated based on Digital Imaging and Communications in Medicine (DICOM) data sets using weightbearing CT software. For the second study question, weightbearing CT software was used to measure the FAO and assess the spatial relationship of the M5 with the foot tripod. For the third study question, segmentation and bone density measurements, using Hounsfield units (HUs), were performed with commercially available and open-source software. Receiver operating characteristic analysis with the Youden index was performed to determine the sensitivity and specificity of the HU M5/HU talus density ratio for identifying stress fractures. RESULTS The stress fractures group exhibited a lower M5 base height at mean ± SD 9 ± 3 mm versus 12 ± 3 mm (p = 0.045), greater ground contact frequency (11 of 15 versus 0 of 15 for the control group; p < 0.001), and an increased median (range) M5/M4 length ratio of 1.06 (0.95 to 1.14) versus 1.01 (0.97 to 1.10) for controls (p = 0.04). Hindfoot varus and foot adduction were associated with stress fractures, as indicated by altered hindfoot alignment and tarsometatarsal angles. The stress fractures group demonstrated a mean 50% increase in the HU M5/HU talus density ratio, at a median (range) of 1.52 (0.9 to 2.3) versus 1.02 (0.97 to 1.1) (p < 0.001). A relative increase by a factor of 1.2 in the HU M5/HU talus density ratio was associated with the stress fractures group with 80% sensitivity and 94% specificity. CONCLUSION Stress fractures of the M5 are known to be associated with hindfoot varus and forefoot adductus. The present study adds that these injuries may also be associated with reduced base height, increased plantarflexion, a longer M5, and higher bone density. Future prospective studies could investigate whether using a threshold of 1.2 for the HU M5/HU talus density ratio to trigger early preventive measures could help decrease the occurrence of stress fractures. LEVEL OF EVIDENCE Level III, prognostic study.