Predicting Orbital Fractures from Baseball Impact

Zygoma fracture as a result of baseball impact presents a particular concern due to its prevalence and importance as a structure of the orbit. The purpose of this study was to develop risk functions for zygoma fracture based on ball type and velocity. A normal distribution was assumed for previous data on the fracture strength of the zygoma. The fracture strength data was mapped with the force exerted by a baseball as a function of velocity to develop the zygoma fracture risk functions. The risk functions showed that a major league ball has a 50% chance of causing fracture at a speed of 31 mph. Experimental evaluation of the risk functions was performed using six cadaver tests and two baseballs of different stiffness values. Tests with a softer baseball did result in injury, however they occurred at velocities 50% higher than the major league ball with fewer resulting bone fragments. The experimental results validated the risk functions at the lower and upper level. Post test analysis of the skull was performed using CT imaging including 3D reconstruction as well as autopsy. The injuries observed in the post test analysis included fractures of the zygomatic arch, frontal process and the maxilla, zygoma suture, with combinations of these creating comminuted, tripod fractures of the zygoma. INTRODUCTION ittle league baseball is the most popular sport among youth athletics. Approximately 16 million children participate in some form of organized baseball in the United States. The average annual injury rate between 1994 and 1998 among children 5 to 14 years of age was 103,731 (Yen et a, 2000) per year. Facial fractures resulting from baseball impact can have detrimental effects on the skeletal structure of the face. In particular, a zygoma fracture can present injury to the facial structure as well as the orbit (Fan et al 2002, Green et al 1990, Hampson 1995, Krsarsai et al 1999). Preventing these injuries in organized sports like baseball is accomplished through protective equipment and the use of balls with a lower stiffness. The benefit of these devices can be improved if the forces necessary to cause injury are known as a function of ball characteristics. L Injury Biomechanics Research 220 The zygoma has been the focus of previous research due to the prevalence of facial injuries in sporting and automotive events (Arbogast et al 2002, Gassner et al 1999, Gillespie et al 1987, Hodgson 1967, Nahum et al 1968, Nyquist et al 1986, Schneider et al 1972, Vinger et al 1999). The purpose of this study was to develop risk functions to predict the probability of zygoma fracture from impact with a baseball. The data used to develop the risk functions was based on previous experimental work performed on cadavers using rigid impactors (Hodgson 1967, Nahum et al 1968, Schneider et al 1972). Determining the forces exerted by a baseball has also been done previously and therefore this information will be utilized as well. The result of combining the two types of research is a risk function that is based on the type of ball and the speed of the ball at impact. This is a more intuitive relationship that can be utilized to aid in the mitigation of facial injuries. METHODOLOGY The methods for the current study are divided into two parts. The objective of part one was to develop risk functions for zygoma fracture from baseball impact. The objective of part two was to evaluate the validity of the risk functions by performing experimental tests on human cadavers. After the experimental evaluation, computed tomography (CT) images as well as autopsy provided injury information. Part 1: Zygoma Fracture Risk Functions In order to develop functions for predicting the risk of zygoma fracture, previous research on the breaking strength of the zygoma was utilized. Three studies that performed impact tests to the zygoma were selected. Hodgson (1967), Nahum et al. (1968) and Schneider and Nahum (1972) used rigid impactors to determine the breaking strength of the zygoma to dynamic impacts (Table 1). A cylindrical rod with an area of 1 in was used in the three tests to impact the zygoma of the cadaver subjects. The force during impact was recorded using load cells attached to the cylindrical impactors. The peak forces were used in the current study to develop risk functions for zygoma fracture due to baseball impacts, assuming similar loading areas between the rigid impactors and baseballs. Table 1. Peak Forces (N) Resulting in Zygomatic Fracture Nahum 1968 Schneider 1972 Hodgson 1967 1828.2 1970.6 1396.7 158