Measurement of bone lengthening forces; an experimental model in the lamb

OBJECTIVE: To obtain the mechanical behaviour pattern of the lengthening process. DESIGN. IN VIVO: measurement of forces during bone lengthening in lambs. BACKGROUND: A series of parameters of a mechanical and biological nature have a bearing on all lengthening processes, and most of them are not fully understood. METHODS: A strain-gauge-monitored unilateral fixator was designed and used to obtain data about the changes which took place in the forces of elongation at a rate of 1 mm/day in four lambs while a 3 cm progressive lengthening of the left tibia was being performed, analysing how these forces behaved from day to day, and how they changed in the course of a single day. RESULTS: The maximum forces in all the animals each day occur after distraction, and the forces reach their greatest magnitude between days 21 and 25 after surgery, attaining values of slightly over 8 kg (40-50% of the animal's weight). The maximum daily force starts to drop 1 h after distraction, and continues to decrease gradually throughout the day until it reaches a value slightly greater than the initial force on the previous day. CONCLUSION: This pattern is due to the distraction of soft tissues which gradually adapt to their new situation, thereby reducing the level of stress. RELEVANCE: In the daily bone lengthening procedure, the greatest forces are produced in a short period of time immediately after lengthening. they could be reduced to decrease pain in the patient and loads on the device by performing lengthening over a greater number of steps or using dynamic equipment able to absorb these forces.