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The healing of 52 diaphyseal osteotomies in rabbit tibiae was followed up histologically from 3 to 24 weeks after rigid intramedullary nailing. The histological evaluation was made from longitudinal sections through the osteotomy area. Particular attention was paid to the fracture healing pattern. A bulky periosteal response was visible in every specimen. At 24 weeks, the external callus was always well remodeled. The osteotomy line rapidly filled with bone from 6 weeks onwards. At 24 weeks, the site of osteotomy was detectable only on the basis of slight irregularity in the cortex. The secondary gap healing seen in 19 specimens was the most common type of bone union from 6 weeks onwards. In 13 specimens, the exact type of osteonal healing was not definable, since a solid union with good cortical reconstruction was always the final outcome. Altogether, four nonunions were detected throughout the study, none of these, however, in the specimens at 24 weeks. Considerable endosteal resorption was detected at 24 weeks, at which time at least one third of the original cortex had disappeared in all specimens. The rigid nail seems to ensure a relatively uneventful healing of the experimental osteotomies. Vast endosteal resorption and some nonunions make the use of medullary reaming in this connection doubtful.

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Transverse osteotomies of the midshaft of the tibia in 75 rabbits were stabilized with intramedullary nails which prevented rotational instability. In a further 36 animals intramedullary nailing was performed without preceding osteotomy. The animals were able to move freely immediately after the operation without any supporting case or splint. The structural changes in the bones were analysed radiographically 3 to 24 weeks after the operation. Fracture repair was accompanied by moderate periosteal callus formation and remodelling of the callus was seen from 9 weeks on. A sleeve of new endosteal bone surrounding the nail was found after 12 weeks. Porotic transformation of the diaphyseal cortical bone was seen in both osteotomized and non-osteotomized bones from 12 weeks onwards. It is concluded that intramedullary nailing provides favourable conditions for fracture repair and enables accurate radiographic assessment of the healing. Attention is focused on the causes and features of secondary porotic transformation of the cortical bone after intramedullary nailing.

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