914 THE JOURNAL OF BONE AND JOINT SURGERY BONE DENSITY AFTER RIGID PLATE FIXATION OF TIBIAL FRACTURES A DUAL-ENERGY X-RAY ABSORPTIOMETRY STUDY G. C. JANES, D. M. COLLOPY, R. PRICE, J. M. SIKORSKI From the University of Western Australia and the Sir Charles Gairdner Hospital, Perth, Western Australia We used dual-energy X-ray absorptiometry (DEXA) to measure the bone mineral content (BMC) of both tibiae in 13 patients who had been treated for a tibial fracture by rigid plate fixation. Within two weeks of plate removal the BMC was significantly greater in the bone that had been under the plate than at the same site in the control tibia. An unplated area of bone near the ankle showed a significant decrease in BMC at the time of plate removal with subsequent return to the level of the control tibia during the ensuing 18 months. We conclude that osteoinductive influences outweigh the potential causes of osteopenia, such as stress shielding and disuse, and that, contrary to expectation, demineralis- ation is not a factor in the diminished strength of the tibia after plating for fracture. J Bone Joint Surg [Br] 1993 ; 75-B :914-7. Received 26 June 1992; Accepted after revision 5 May 1993 When diaphyseal fractures of the tibia are treated by open reduction and application ofa metal plate, utilising the principle ofdynamic compression, the implant diverts load away from the bone (stress shielding) and it is widely believed that the bone then becomes osteoporotic (Akeson et al 1976). In animal experiments bone has been shown to have reduced strength characteristics after the removal of rigid internal fixation devices (Tonino et al 1976; L#{226}ftmann, Sigurdsson and Stromberg 1980; L#{225}ftmann et al 1989). The causes of this weakness are thought to be due to two G. C. Janes, MB BS, Lecturer J. M. Sikorski, MD, FRACS, Professor University Department of Surgery (Orthopaedics), 2nd Floor, ‘M’ Block, Queen Elizabeth II Medical Centre, Nedlands 6009, Western Australia. D. M. Collopy, MB BS, Orthopaedic Registrar R. Price, PhD, Scientist in Charge, Bone Structural Unit, Department of Endocrinology Sir Charles Gairdner Hospital, Verdun Street, Nedlands 6009, Western Australia. Correspondence should be sent to Dr G. C. Janes. ©l993 British Editorial Society of Bone and Joint Surgery 0301 -620X/93/6672 $2.00 different mechanisms, cortical atrophy and the effect of the residual screw holes. Uhthoff and Dubuc (1971) plated osteotomised canine femora for periods ranging from 2 to 30 weeks. They found that this resulted in cortical thinning, trabeculation of the cortex and persistence of disorgan- ised woven bone at the osteotomy site. Most studies have shown that the previously plated segment of bone has thin cortices and an enlarged medullary cavity from endosteal resorption (Akeson et al 1976; Moyen et al 1978 ; Paavolainen et al 1978 ; Sl#{228}tis et al 1978 ; Stromberg and Dalen 1978; Terjesen and Benum 1983; L#{226}ftmann et al 1989). Carter, Vasu and Harris (1981), however, in animal studies reported thickening of the cortical bone adjacent to the plate due to new bone formation. Two hypotheses, which are not mutually exclusive, are proposed to explain the phenomenon of cortical atrophy observed in animal models. The first, and most widely held view, is that the rigid plate diverts compres- sive bending and torsional stresses away from the bone, which reacts by reducing its cortical thickness and bone mass in accordance with Wolff’s law (Wolff 1892 ; Carter et al 1984; Perren 1991). The second hypothesis is that the rigid plate compromises the periosteal blood supply causing ischaemia and resorption ofthe underlying bone. This latter concept has led to the development of the ‘limited contact - dynamic compression plate’ (Perren 1991). The increased porosity (trabeculation) of cortical bone has been investigated by microscopy, microradiog- raphy(Paavolainen et al 1978 ; Sl#{228}tis et al 1978 ; Stromberg and Dalen 1978; Terjesen, Nordby and Arnulf 1985) cross-sectional point counting (Stromberg and Dalen 1978), single and dual photon absorptiometry (Terjesen and Benum 1983; Rosson, Petley and Shearer 1991), and by gamma ray absorption (Tonino et al 1976). All studies found an increase in the porosity of the cortical bone under the plate. Most of them, however, were performed on intact animal long bones. L#{225}ftmann et al (1989) combined an osteotomy with plating and reported an initial increase in bone mineralisation, as measured by ash weight, only to find that it soon returned to normal and subsequently became atrophic. Some studies have