Breaking force of the rabbit growth plate and its application to epiphyseal distraction

The in vitro breaking forces of the distal femoral growth plates of young rabbits were measured as a background to the design of a bone lengthening method, using epiphyseal distraction. The mean breaking force in 16 femora was 12.98 +/- 3.48 kg and the mean strain was 0.91 +/- 0.33 mm. The mean stress in 10 femora was 14.51 +/- 3.88 kg/cm2. The procedure was repeated, after applying a 1.0 kg dead weight to 6 femora for 24 hours and the breaking force was then 15.01 +/- 4.70 kg, with a mean strain of 0.85 +/- 0.62 mm. A further 8 rabbits then underwent epiphyseal distraction for 2 days in vivo, with 1 or 2 kg forces delivered to two parallel K wires by a pair of spring devices, whereupon the femora were removed and tested as before. The breaking force on the distracted side was now only 8.91 +/- 3.71 kg, compared with 13.99 +/- 3.40 kg on the control side. Although not fractured, these plates had obviously been weakened. The clinical implication of this is discussed.