ON THE RISK OF ZYGOMA FRACTURE FROM BASEBALL LOADING Joel D. Stitzel1, Paul F. Vinger2, and Stefan M. Duma1 1 Impact Biomechanics Laboratory, Mechanical Engineering, Virginia Tech 2 Tufts University School of Medicine, New England Medical Center Email: email@example.com, Web: www.ibl.vt.edu adult population was based on 1985 Humanscale data for the US population. The mass used for the 9-11, 12-14, and adult age groups was 31.5, 50.4, and 73 kg, respectively. Mass scaling was performed using the technique developed by Eppinger et al (1984). This technique utilizes a scaling relationship that takes into account occupant mass and an assumed increase in bone size and strength that accompanies this mass. Known force values are scaled using the scaling relationship to obtain equivalent force values for the mass of interest. The scaled data was used to obtain risk functions for the two age groups. Risk functions were established with the failure data using a logit formulation, where the probability of injury is related to the average of failure data and a function of the standard deviation of the data (Duma, 2000). The logit formulation was selected versus the normal distribution in order to produce a closed form solution. Given an impact force, these risk functions can be used to predict injury due to commonly used baseball types (Equation 1). INTRODUCTION It is estimated that 16 million children play some form of organized baseball in the US. Little league baseball is the world’s largest sporting organization, with 190,000 teams in over 80 countries. From 1994-1998, the mean estimated yearly baseball related injury rate in children from 5-14 years of age was 103,731 (Yen, 2000). The most commonly injured body part was the face, followed by fingers, head, ankles, wrists, mouth, knees, and hands. Softer baseballs, with weight, liveliness, and surfaces similar to standard hard balls, have been designed to reduce these injuries. However, there is no data on the risk of fracture due to baseball loading in the literature with respect to younger children. The purpose of this study is to predict the risk of zygoma fracture from impact with a range of baseballs of varying hardness. In particular, this paper develops injury risk functions to predict the probability of fracture of the zygoma for children in the range from 9-11 and 12-14 years of age, and compares these to adult values. METHODS Existing research on the danger of fracture due to the type of impact loading that may be seen in a facial impact with a baseball is limited to three studies. Nahum et al. (1968), Schneider and Nahum (1972), and Hodgson (1967) quantified the peak force necessary to fracture the zygoma with a cylindrical impactor, for automobile safety. For this study, the peak force at fracture from these three studies was selected (28 tests resulting in failure from the three studies). Data was then mass scaled to a standard mass that would be expected for the two age groups of children. Average mass for the two age groups was based on actuarial data from the Metropolitan Life Insurance Company. Average mass for the Risk = 1+ e 1 x −m − b , b= σ 3 π (1) Where m is the mean of the failure data, b is a function of the standard deviation of the data, σ, and x is the peak force in Newtons. The peak force that might be seen on the zygoma due to a baseball impact was obtained from work performed by Vinger et al. (1999). This work correlated impact velocity of increasingly harder baseballs to peak orbital force. This orbital force was analyzed relative to the failure data for the zygoma and then again for the mass scaled failure data. Regression equations were used to determine the impact force on the orbit for two age groups, using pitch velocities of 35 Risk and 55 mph for the 9-11 and 12-14 year age groups, respectively. These velocities were chosen because they are the average pitch speeds seen at these age levels (Vinger et al., 1999). RESULTS AND DISCUSSION 1.0 0.8 0.6 0.4 0.2 0.0 0 1000 2000 3000 4000 9-11 12-14 Adult The risk functions established for the adult, the 9-11 year age group, and the 12-14 year age group demonstrate a notably increased risk of fracture due to baseball impact to the younger population (Figure 1, Equations 24). The 50th percentile of the risk function for 9-11 year olds is approximately half the force that is required for the adult age group, with 12-14 year olds falling about halfway between this (Figure 1). Using pitch velocities and average masses of children seen in the two age groups, large differences in risk are seen (Table 1). As ball hardness increases, risk of fracture of the zygoma also increases. In both categories, ball CD’s above 35 result in scaled risks above 96%. It is only for adults subjected to lower velocity pitches (9-11 year category) that there is any risk below 50%, and this is only for the softest two types of balls tested. Baseball velocities likely to be thrown by little league pitchers posess a substantial risk of fracturing the zygoma should a direct impact occur. Mass scaling injury criteria using known cadaver failure data to a younger population demonstrates this increase. Even with the softest ball types now available, the maximum force imparted in a direct hit to the zygoma is sufficient for fracture. This risk of injury underscores the need for facial protection in little league baseball. Force (N) Figure 1. Risk of zygoma fracture versus peak baseball impact force for the adult, 911, and 12-14 year old populations Table 1. Risk of zygoma fracture for the adult and two selected age groups Adult 9-11 Years mass = mass = 31.5 kg 73.0 kg velocity = 35 mph 12-14 Years mass = 50.4 kg velocity = 55 mph Ball Force, Risk, Scaled Force, Risk, Scaled CD, lb adult, N adult Risk adult, N adult Risk 25 35 50 266 291 250 1104 1674 1915 2034 2163 2288 0.13 0.42 0.59 0.66 0.74 0.80 0.60 0.96 0.99 0.99 1.00 1.00 2295 3126 3335 3643 3672 3745 0.80 0.98 0.99 0.99 0.99 1.00 0.96 1.00 1.00 1.00 1.00 1.00 REFERENCES Risk 9 −11 years = 1+ e 1 Peak Force −1020.83 − 207.62 (2) Risk 12−14 years = 1+ e Risk Adult = 1+ e 1 Peak Force −1394.8 − 283.68 (3) 1 Peak Force −1786.77 − 363.40 (4) Duma SM. (2000). Injury Criteria for the Small Female Upper Extremity; University of Virginia Eppinger RH, Marcus JH, Morgan RM. (1984). SAE Technical Publication no. 840885 Hodgson VR. (1967). American Journal of Anatomy, 120, 113. Nahum AM, Gatts JD, Gadd CW, Danforth J. (1968). Proceedings of the 12th Annual Stapp Car Crash Conference, 302. Schneider DC, Nahum AM. (1972). Proceedings of the 16th Annual Stapp Car Crash Conference, 186. Vinger PF, Duma SM, Crandall J. (1999). 117, 354. Yen KL, Metzl JD. (2000). Pediatr Emerg Care, 16, 215.