BIOMECHANICAL AND HISTOLOGICAL-EVALUATION OF A CALCIUM-PHOSPHATE CEMENT

It is often difficult to achieve stable fixation of a comminuted fract ure associated with a metaphyseal defect. The injection of a resorbabl e cement into an osseous defect may help to stabilize the fracture and to maintain osseous integrity as the cement is resorbed and replaced by bone, The purpose of the present study was to evaluate the repair o f a metaphyseal defect after treatment with an injectable calcium-phos phate cement. The injectable cement undergoes isothermic curing in viv o to form a carbonated apatite (dahllite),vith a compressive strength of twenty-five megapascals, Either the cement or allograft bone was pl aced in proximal tibial metaphyseal and distal femoral metaphyseal def ects in seventy-two dogs and was evaluated from twenty-four hours to s eventy-eight weeks postoperatively, Histological examination showed th at the cement was osteoconductive; nearly the entire surface area was covered,vith bone two weeks after the injection. The resulting bone-ce ment composite underwent gradual remodeling over time in a pattern tha t was qualitatively similar to the remodeling of normal cortical and c ancellous bone. Osteoclasts were found to resorb the cement and were u sually associated with adjacent ne,v-bone formation, With increasing t ime in vivo, the cement was penetrated by small blood vessels that bec ame surrounded by circumferential lamellae of bone and that closely re sembled evolving haversian systems, This process occurred more rapidly in the cortex than in the medulla, Mechanical testing showed that, by eight weeks, the tibiae that had been treated,vith cement had reached nearly 100 per cent of the torsional strength of the contralateral, c ontrol (intact) tibiae; this finding paralleled the histological obser vations of bone apposition to the cement and rapid restoration of the cortex. At no time was fibrous tissue present between the cement and t he bone, and there was no evidence of acute inflammation, Small partic les of cement were present within occasional macrophages during the pr ocess of cement resorption, but the macrophages disappeared over time and were not associated with fibrosis or unexpected resorption of bone . Resorption of the cement was incomplete in the medullary area at sev enty-eight weeks, but the pattern of cement resorption and bone-remode ling suggested gradual restoration of a physiological proportion of bo ne and marrow in both the cortical and the medullary region with maint enance of mechanical function. CLINICAL RELEVANCE: The results of the present study suggest that an injectable calcium-phosphate cement that sets in situ may be an attractive, structurally competent augmentatio n material for the repair of compromised metaphyseal bone. The high co mpressive strength of this material, as well as its gradual replacemen t by bone, supports its continued evaluation for use in complex metaph yseal fractures or osseous defects.