We developed a rat model of limb lengthening to study the basic mechanism of distraction osteogenesis, using a small monolateral external fixator. In 11-week-old male rats we performed a subperiosteal osteotomy in the midshaft of the femur with distraction at 0.25 mm every 12 hours from seven days after operation.Radiological and histological examinations showed a growth zone of constant thickness in the middle of the lengthened segment, with formation of new bone at its proximal and distal ends.Osteogenic cells were arranged longitudinally along the tension vector showing the origin and the fate of individual cells in a single section. Typical endochondral bone formation was prominent in the early stage of distraction, but intramembraneous bone formation became the predominant mechanism of ossification at later stages. We also showed a third mechanism of ossification, 'transchondroid bone formation'. Chondroid bone, a tissue intermediate between bone and cartilage, was formed directly by chondrocyte-like cells, with transition from fibrous tissue to bone occurring gradually and consecutively without capillary invasion.In situ hybridisation using digoxigenin-11-UTPlabelled complementary RNAs showed that the chondroid bone cells temporarily expressed type-II collagen mRNA. They did not show the classical morphological characteristics of chondrocytes, but were assumed to be young chondrocytes undergoing further differentiation into bone-forming cells.We found at least three different modes of ossification during bone lengthening by distraction osteogenesis. We believe that this is the first report of such a rat model, and have shown the validity of in situ hybridisation techniques for the study of the cellular and molecular mechanisms involved in distraction osteogenesis. J Bone Joint Surg [Br] 1997;79-B:824-30. Received 18 November 1996; Accepted after revision 21 March 1997The common principles of current techniques of limb lengthening are osteotomy and slow progressive distraction by an external fixation device.1,2 The type of osteotomy, the timing and rate of distraction, and the apparatus have varied considerably. 3-8 It has been established that slow distraction does not break the bony callus, but actually stimulates osteogenesis and the growth of surrounding soft tissues. The principle has been termed the 'law of tension-stress', 4,5 but the exact mechanism is not understood. We have previously reported the histological findings in lengthened segments in rabbits and concluded that new bone was formed predominantly by endochondral (indirect) ossification.9 Other reports of canine or ovine experiments have established that intramembranous (direct) bone formation is the main mechanism of ossification during distraction osteogenesis. 4,5,10,11 Factors such as the stability of fixation, the timing and rate of distraction, and speciesrelated differences may determine the relative share of endochondral and direct bone formation. Kossmann, Giebel and Glombitza 12 recently described a rat model of tibial lengthening...

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The addition of gadolinium-enhanced MRI to T2-weighted imaging provides better accuracy for detecting cancerous transition zone lesions than the use of T2-weighted imaging alone.

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Doses of sulpiride typically used in human cognitive studies produced low levels of DA D2 receptor occupancy compared to that considered efficacious in the treatment of schizophrenia. The levels of occupancy were sufficient to replicate impairments on a spatial working memory task and impair spatial learning. The relationship between occupancy and working memory was suggestive of presynaptic effects, although the precise mechanism underlying the impairment will require studies of wider ranges of occupancy within and outside of the striatum.