Bruce Field

Section Modulus Is the Optimum Geometric Predictor for Stress Fractures and Medial Tibial Stress Syndrome in Both Male and Female Athletes

Background Various tibial dimensions and geometric parameters have been linked to stress fractures in athletes and military recruits, but many mechanical parameters have still not been investigated. Hypotheses Sedentary people, athletes with medial tibial stress syndrome, and athletes with stress fractures have smaller tibial geometric dimensions and parameters than do uninjured athletes. Study Design Cohort study; Level of evidence, 3. Methods Using a total of 88 subjects, male and female patients with either a tibial stress fracture or medial tibial stress syndrome were compared with both uninjured aerobically active controls and uninjured sedentary controls. Tibial scout radiographs and cross-sectional computed tomography images of all subjects were scanned at the junction of the midthird and distal third of the tibia. Tibial dimensions were measured directly from the films; other parameters were calculated numerically. Results Uninjured exercising men have a greater tibial cortical cross-sectional area than do their sedentary and injured counterparts, resulting in a greater value of some other cross-sectional geometric parameters, particularly the section modulus. However, for women, the cross-sectional areas are either not different or only marginally different, and there are few tibial dimensions or geometric parameters that distinguish the uninjured exercisers from the sedentary and injured subjects. In women, the main difference between the groups was the distribution of cortical bone about the centroid as a result of the different values of section modulus. Last, medial tibial stress syndrome subjects had smaller tibial cross-sectional dimensions than did their uninjured exercising counterparts, suggesting that medial tibial stress syndrome is not just a soft-tissue injury but also a bony injury. Conclusion The results show that in men, the cross-sectional area and the section modulus are the key parameters in the tibia to distinguish exercise and injury status, whereas for women, it is the section modulus only.

Experimental and finite element analysis of tibial stress fractures using a rabbit model

AIM To determine if rabbit models can be used to quantify the mechanical behaviour involved in tibial stress fracture (TSF) development. METHODS Fresh rabbit tibiae were loaded under compression using a specifically-designed test apparatus. Weights were incrementally added up to a load of 30 kg and the mechanical behaviour of the tibia was analysed using tests for buckling, bone strain and hysteresis. Structural mechanics equations were subsequently employed to verify that the results were within the range of values predicted by theory. A finite element (FE) model was developed using cross-sectional computer tomography (CT) images scanned from one of the rabbit bones, and a static load of 6 kg (1.5 times the rabbits body weight) was applied to represent running. The model was validated using the experimental strain gauge data, then geometric and elemental convergence tests were performed in order to find the minimum number of cross-sectional scans and elements respectively required for convergence. The analysis was then performed using both the model and the experimental results to investigate the mechanical behaviour of the rabbit tibia under compressive load and to examine crack initiation. RESULTS The experimental tests showed that under a compressive load of up to 12 kg, the rabbit tibia demonstrates linear behaviour with little hysteresis. Up to 30 kg, the bone does not fail by elastic buckling; however, there are low levels of tensile stress which predominately occur at and adjacent to the anterior border of the tibial midshaft: this suggests that fatigue failure occurs in these regions, since bone under cyclic loading initially fails in tension. The FE model predictions were consistent with both mechanics theory and the strain gauge results. The model was highly sensitive to small changes in the position of the applied load due to the high slenderness ratio of the rabbits tibia. The modelling technique used in the current study could have applications in the development of human FE models of bone, where, unlike rabbit tibia, the model would be relatively insensitive to very small changes in load position. However, the rabbit model itself is less beneficial as a tool to understand the mechanical behaviour of TSFs in humans due to the small size of the rabbit bone and the limitations of human-scale CT scanning equipment. CONCLUSION The current modelling technique could be used to develop human FE models. However, the rabbit model itself has significant limitations in understanding human TSF mechanics.

Optimizing the Mould Die Design and Production Cycle

This paper will report on a collaborative research project that includes participants from local manufacturing industries, universities and government. The major outcome sought from the collaboration is the enhancement of training and education in the polymer-manufacturing sector, with specific emphasis on the “injection mould” design and manufacture. The practical outcome sought from the research includes determining design priorities and minimizing associated design errors. Experimental procedures for evaluating alternative design and manufacture philosophies will be completed in parallel with the development of an open reporting system that will lead to a growing body of knowledge for the optimal design of dies. Future die designers will then have an available and evolving system of recording and informing successful die design and manufacture techniques. Improved training methods will be an associated benefit. Specifically, this paper will: 1. Introduce the collaborative research project and describe the proposed programme of activities; 2. Outline the perceived need for this work within Australian polymer manufacturing companies, with particular reference to a significant local trend towards private sector provision of training and learning. This provides an associated opportunity for innovative teaching; and, 3. Detail the outcomes of the first phase of our collaborative program — a substantial survey of engineering and production managers in the polymer industry sector.Copyright