Saddles. Saddles are quite effective at holding a rider on a horse. Thus Lance based charges can still be effectively used. If you were to compare a pre-stirrup Kataphractoi charge to a medieval post-stirrup Knight charge you would probably give the advantage to the Knights. But its not a major difference that you would see based purely on the use of stirrups.

Stirrups probably give a rider more control of a horse, but weight in a charge is largely pushed through the lance, into the human torso, and then into the saddle.

Here's a picture of a recreated Roman saddle mentioned by /u/Thecna2.

The other thing to remember is that a cavalry charge is primarily a morale attack - to frighten the opponent into breaking ranks and fleeing. A line of fully armoured men on armoured horses moving forward at a gallop (though not sure if cataphracts would break into a gallop) is incredibly frightening regardless. It wasn't about killing the opponent in the charge, most casualties in battle resulted from the pursuit when one side fled.