The mean treatment effect of fusion on adjacent segment disc space height was -0.44 SDs ( 95 % CI , -0.77 to -0.11 ; P = 0.01 ; as-treated analysis ) ; there was no group difference for posteroanterior displacement ( 0.18 SDs , 95 % confidence interval , -0.28 to 0.64 , P = 0.45 ) .