A cannula includes an instrument valve housing and a valve assembly disposed within the instrument valve housing. The valve assembly includes a seal assembly (270) having at least one seal member (272,274,276) including a ring portion (272a,274a,276a) and a septum seal (272b,274b,276b) extending across the ring portion. The valve assembly further includes a guard assembly (130) and a centering mechanism. The guard assembly (130) includes a plurality of guard members. Each guard member includes a ring portion, and a flap portion having first and second flap sections and a groove disposed between the first and second flap sections. The centering mechanism includes an annular ring and a plurality of spokes extending outwardly from the annular ring. The assemblies are supported within the annular ring which is disposed within the instrument valve housing. The flap portions of the guard members are configured to engage and stretch the septum seal (270) during reception of a surgical instrument through the valve assembly.
