Responses in cortical bone following elevated intramedullary pressure

A model for nontraumatic osteonecrosis was studied to define regional necrotic and reparative changes in sheep femora one month following 12 hours of elevated IMP. Following epidural anesthesia, a drill-needle was placed laterally in the femoral diaphyses of both hind limbs in seven adult ewes (4--6 years of age). IMP was elevated for twelve hours on one randomly chosen side, but not elevated on the contralateral control side.;Fluorochromes were used to assess mineral apposition rate in IMP elevated and control femurs. Oxytetracycline (F1) was injected intravenously 15 days prior to artificial IMP elevation (day 1). One day prior to elevating IMP (day 14) xylenol orange (F2) was injected. On day 28, calcein green (F3) was administered followed by alizarin complexone (F4) on day 42. The control period (one) between F1 and F2 is t1; between F2 and F3 and between F3 and F4 define periods two (t2) and three (t3) respectively. The sheep were killed by lethal injection and femora were harvested on day 44.;Lacunae with and without nuclei were used as an index of bone viability. Sections from IMP elevated sides had a significantly lower percentage of filled lacunae than control sides. Significantly greater linear appositional bone growth rate (mineral apposition), labeled surface (active formation surface) and surface area of new bone existed along the endosteal surface in all treated sites during t3, but not during t2. A significantly greater number of F4 labeled osteons was also observed in the cortex on the treated side relative to earlier periods on the same side, and all periods on the control side. This study demonstrated bone necrosis is greatest near the endosteum and in the medial diaphyseal cortex. Elevated IMP caused osteocyte death while simultaneously stimulating new bone growth along the endosteal surface and reparative events in the cortex. New bone growth resulted from increased linear bone growth rate, and increased endosteal surface actively involved in mineral apposition. These results indicate that a single bout of elevated IMP for 12 hours can initiate marrow necrosis, bone necrosis, remodeling and fracture which are characteristics of many forms of osteonecrosis.