To obtain the mechanical behaviour pattern of the lengthening process. DESIGN. IN VIVO: measurement of forces during bone engthening 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
