Simulation of Micro-Fracture Injury in Female Gymnasts-An In Vivo Study

Abstract

Introduction: Injuries due to jumping or falling are common among the participants in sports, e.g., basketball, skiing. It has been reported that 20% of all sports related injuries are due to jumping in games (Garrick and Requa, 1978;). Injury other than falls results in approximately 18,000 deaths in the United States (National Safety Council, 1972). Female gymnasts often place unphysiological extreme repetitive loading on their bodies during the common maneuvers characteristic of this sport. Thus, the necessity to provide quantitative and qualitative analysis of impact behavior of ankles, knees and hips due to jumping is strongly urged. The response of fracture or injury due to jumping is also important in designing safety equipment related to sports. A linear elastic fracture mechanics approach has been used for the analysis. Methods: Five collegiate female gymnasts ranging in age 17–21 years performed a handspring vault landing on a force plate under standardized conditions. Their mean weight and height was 565 N and 168 cm, respectively. The average vertical ground reaction force was 10.8 times the body weight of the gymnasts. Number of cycles to cause the failure of bone due to the repetitive number of landings were obtained by using the following equations: The crack growth model is taken as a0 + Σi Δ ai (1) Where a0 is the initial plan size and ai is the crack growth increment associate with the ith applied load and the process continues until a terminal flaw size is obtained. The fatigue crack propagation rate (FCPR) is defined as the length (or area) propagated during one stress/strain cycle, da/dN. Numerical FCPR model has been proposed by Paris and Erdogan (1963) and provides a functional relationship between the crack growth rate da/dN and the stress intensity factor range Δ, i.e., da/dN = C(ΔK)m (2) Where a is the crack length, N is the number of cycles and C and m are material constants that characterize the crack propagation rate. From equation 2 we can obtain the number of cycles of repetitious jumps to cause failure (Nf) of bone as follows: Nf = ∫afda ┅(3) where, ai and af are initial and final crack length respectively. Using a1 = 10.0 × 10−8, C ΔKm m = ai 1.25, and ΔK = 1.12 (F/A), equation (3) is integrated numerically with an increment of 10 × 10−6 m. to the final value of the crack length (af) 10 × 10−6 m. Then the rate of change of crack length with the number of cycles (da/dN). Results: Typical behavior of crack growth rate characteristics for a force of F = 6.1 kN and m being 1.25, reveals that 6 jumps are sufficient to cause micro-fracture of bone due to landing after vault in gymnasts. For m being 1.5, 2 landings are enough to initiate the micro-fracture in the bone of a gymnasts. This study also indicated that the number of landings for micro-fracture of human bone decreases as the value of m increases. Discussion: These data provide valuable information in the development of an in vivo stress fractures in female gymnasts. The data further reveals that micro-fracture due to landing after vault is sensitive to number of landings. This model is of great significance in computational aspects of bone remodeling, fracture fixation of bone and prosthetic design etc. It shows that shoes, and other supports can also reduce the injury or propagation of bone crack. The model is being extended to evaluate accidents in other areas of sports where falls are involved, e.g., parachuting, ski jumping.