Delayed surgery does not reduce the interface strength between the surfaceof a bone fracture and a self-curing injectable hydroxyapatite (Norian SRS)

The shear strength of the interface between bone and an injectable calcium phosphate bone substitute that cures to form a carbonated apatite (Norian S RS) was measured and related to the time that elapsed between injury and su rgery. Eleven rabbits had a 3 mm drill hole made in the retropatellar aspec t of the distal femur. After one week, the rabbits were reoperated on and t he hole filled with Norian SRS after it had been cleaned gently with a spon ge. During the same procedure a similar hole was: burred in the opposite fe mur and filled with Norian SRS after a few minutes ("immediate injection"). Four other rabbits had only one side operated on with immediate injection of Norian SRS to compare with the opposite untreated side. The rabbits were killed four days after the injection of Norian SRS. The femurs were prepar ed and sawed perpendicularly to the burr channels to produce discs 3.5 mm t hick. A push out test of the Norian SRS plug within the bone disc was done to measure the force at failure. In the immediately injected specimens the failure occurred at a mean of 28 N (range 5-57) compared with 42 N (range 2 5-65) in the specimens injected after a one week delay. There was no signif icant difference between delayed and immediate treatment (95% confidence in terval -5 to 133). The histological examination showed that 9 of the 12 spe cimens had Norian SRS still adherent to half or more of the circumference o f the hole. This indicates that the failure occurred in the Norian SRS rath er than in the bone or at the interface.