Evaluation of the Direction of External Force Input to the Skull and Its Influence on the Severity of Traumatic Brain Injury.

BACKGROUND: Recent demographic changes in Japan have led to an increasing incidence of traumatic brain injury (TBI) resulting from low-energy mechanisms such as falls among the elderly. However, the influence of the direction of external force input on the severity of head injury has not been well characterized in clinical cohorts. This study aimed to evaluate the relationship between the direction of cranial impact and the severity of TBI, with particular attention to the anatomical fracture site and the need for emergency neurosurgical intervention. METHODS: This retrospective cohort study included 231 patients with blunt head trauma admitted to Gunma University Hospital between April 2018 and March 2023. The impact direction was classified as longitudinal or transverse according to injury patterns and prehospital documentation. Skull fractures were categorized into five anatomical regions: frontal, parietal, occipital, temporal, and basilar skull. The primary outcomes were the occurrence of severe TBI, defined as head AIS ≥ 5, and the performance of emergency craniotomy. Multivariate logistic regression analyses were performed to estimate odds ratios (ORs) and 95% confidence intervals (CIs), controlling for age and antithrombotic use. Model calibration was verified using the Hosmer-Lemeshow test, and sensitivity analysis was conducted by comparing minimally adjusted and primary models. RESULTS: Among the included patients, 70% sustained injuries due to falls, and more than half were aged ≥ 65 years. The proportion of severe TBI was higher in the transverse impact group (23%) than in the longitudinal group (13%). Temporal and basilar skull fractures were independently associated with severe TBI (temporal: OR 3.84, 95% CI 1.63-9.05; P = 0.002; basilar: OR 7.61, 95% CI 2.78-20.9; P < 0.001) and emergency craniotomy (temporal: OR 3.13, 95% CI 1.29-7.16; P = 0.011; basilar: OR 2.90, 95% CI 1.00-8.36; P = 0.049). No significant group differences were observed in mortality at 14, 30, or 180 days. The overall in-hospital mortality rate was 18.2% (42/231), comparable to previously reported rates in moderate-to-severe TBI cohorts. The median Glasgow Outcome Scale-Extended (GOSE0 at 180 days did not differ significantly between groups (P = 0.289), although survivors in the transverse group had better functional outcomes (P = 0.042). CONCLUSIONS: Temporal and basilar skull fractures, as well as transverse impact mechanisms, are associated with higher injury severity and a greater likelihood of emergency craniotomy but are not independent predictors of mortality. These findings suggest that the direction of cranial impact provides important biomechanical insight into injury severity and may aid in prehospital triage and preventive strategies for head trauma in aging populations.