抄録
In recent years cementless hip joint protheses have been gaining increasing popularity for various reasons, especially in cases of revision, in young persons with hip joint problems. The main problem with this prosthesis lies in the prevention of loosening and the inhibition of femoral bone stress shielding. A radiological study was subsequently undertaken to investigate the relationship between stem elasticity and femoral bone stress shielding.
The stems used were the Isoelastic Stem (Robert Mathys Co.) and the Osteonics titanic femoral prostheses, whose coefficients of elasticity are very different, as is well known. Isoelastic Stems were inserted in 7 males and 8 females, 8 patients with coxarthrosis and 7 patients with aseptic necrosis of the femoral head (ANF), while Osteonics titanic stems were inserted in 9 males and 9 females, 11 patients with coxarthrosis and 7 patients with ANF. Those in the Isoelastic group averaged 54.3 years in age (43 to 66), and those in the titanic 51.6 years (19 to 66) . Prior to surgery, patients were examined to see whether they had bone cortex of sufficient thickness to bear non-cement type stems, and those with it were randomized into Isoelastic and titanic prosthesis groups. The patients were observed for 5.5 years on an average.
According to the zone classification of Gruen et al., in postoperative patients in the Isoelastic group, bone reaction such as thinning or thickening of the bone cortex was absent in all but one, and stem loosening was observed also in only one. In the titanic group, on the other hand, bone cortex thinning apparently due to stress shielding in zones 6 and 7 was observed in A-P projections in 16 of 17. Furthermore, stress shielding in zone 1 and bone cortex thickening in zones 3 to 5 was observed in lateral projections with high frequency in all patients.
Stress shielding was thus often observed in the inner aspect of the calcar area of the femur in cases where titanic stems of high rigidity were used, while stress shielding did not occur with Isoelastic stems of low elasticity. These results suggest that hip joint protheses of the lowest possible elasticity will provide stems which are effective in the prevention of stress shielding, both theoretically and clinically.
