EFFECT OF THE BONE/HA RATIO ON THE INITIAL STABILITY OF ACETABULAR CUP FIXED WITH IMPACTION BONE-GRAFTING -DYNAMIC MIGRATION TRACED BY A HIGH-SPEED PHOTOGRAPHY CAMERA-

Abstract

In revision total hip arthroplasty (THA), it is essential to cope with the bone stock loss. The acetabular bone loss is reconstructed by bulk bone grafts, bone chips, bone cement or jumbo cup. The impaction bone-grafting (IBG) technique is a technique that can restore acetabular bone loss, while enough bone allografts are not easy to obtain and the quality is not always sufficient. Thus we mixed hydroxyapatite (HA) granules into bone chips to supplement the volume and the mechanical strength of allografts. To investigate the dynamic migration of cemented cup fixed with IBG, we made acetabular bone defect models and the migration of the cup was traced by a high-speed photography camera.

Composite test blocks were used as synthetic acetabulum models. A hemisphere defect of 60mm in diameter was made. We tested 4 different bone/HA ratio; 100%/0%, 75%/25%, 50%/50% and 0%/100%. Each group consisted of 6 specimens. The grafted materials were impacted using impactors. Then, a 46 mm polyethylene cup was fixed with bone cement. The specimens were clamped to the MTS mechanical tester at an angle of 20 degrees. A dynamic load of 150 N to 1500 N with a frequency of 1 Hz was applied for 15 minutes, followed by a dynamic load of 300 N to 3000 N for the same time period. Then the load was released for 15 minutes. The cup migration was traced by the camera during loading and releasing. This camera captures 15 images per second thus it enables us to trace the migration of the cup during cyclic loading. The cup migration at the end of 3000N loading was measured. Elastic recoil was defined as the difference between the migration at the end of 3000N loading and that when the load reached to 0N. Visco-elastic recoil was defined as the difference between the migration at the release of loading and that after 15 minutes. Data were investigated by Pearson’s correlation coefficient test.

A strong negative correlation (r = −0.71) was observed significantly between the amount of the migration and bone/HA ratio. In elastic recoil, statistically significant correlation was (r = −0.55) observed. In visco-elastic recoil, there is no correlation between the amounts of the visco-elastic recoil and bone/HA ratio.

In the reconstruction of bone defects, initial stability of the cup is a first step to expect the long term survival. The initial stability depends on the mechanical properties of the grafted materials. To supplement the volume and mechanical strength of bone allografts, we added HA granules to the bone chips. In the current study, the cup migration was smaller by adding HA granules. Elastic recoil was affected, while visco-elastic recoil was not affected. These results indicated that the mixture of HA granules to bone chips stabilized the cup during loading period and load releasing period.