Biomechanical Investigation of Head Injuries Caused by Baseball Bat Strikes with Different Bat Sizes and Velocities: A Finite Element Simulation Study

Objective: Traumatic brain injury (TBI) represents a significant clinical problem, with the biomechanical mechanisms of striking from different blunt instruments remaining unclear. This study aims to quantitatively evaluate TBI severity under blunt strikes and to assess the effects of strike velocity and blunt instrument size on biomechanical responses to provide a finite element approach for investigating injury mechanisms and informing clinical diagnosis. Methods: A head finite element model incorporating an outer cortical-cancellous-inner cortical bone structure was developed and verified against a previous cadaveric impact study. Strike velocities and blunt instrument parameters, obtained from experiments in which a long bat (LB) and a short bat (SB) were used to strike a dummy head, were applied as the loading conditions in the finite element simulation. Kinetic energy (KE), internal energy (IE), impact force, von Mises stress on skull, intracranial pressure (ICP), and Head3ms acceleration were analyzed as indicators of injury severity. Results: Simulated force and ICP responses agreed with cadaveric experimental data within a 9.8% error. With increasing strike velocity (10–30 m/s), KE, IE, impact force, ICP, and Head3ms all rose, while von Mises stress evolved from localized to dispersed distribution. Head3ms reached an injury threshold of 80 g at a strike velocity of 10 m/s, and ICP peaks for LB and SB exceeded the brain injury threshold (235 kPa, ≈1760 mmHg) at 12 m/s and 14 m/s, respectively. At the same velocity, LB generated higher KE, IE, impact force, ICP and Head3ms than SB. At 30 m/s, LB generated 390 J KE and 29.0 kN peak force, which were 50.0% and 11.1% higher than those of SB (260 J, 26.1 kN). Conclusion: This study reveals that increasing strike velocity and employing a larger blunt instrument elevate biomechanical responses, resulting in von Mises stress transitioning from localized concentration to multipolar dispersion. Specifically, when striking the head with the LB at velocities exceeding 12 m/s or with the SB exceeding 14 m/s, the impacts indicate a severely life-threatening level. These findings deepen our understanding of the mechanisms of blunt TBI. The constructed and validated finite element model can be repeatedly used for computer simulations of TBI under various blunt striking conditions, providing a scientific basis for clinical diagnosis and surgical planning.