Medicare Reimbursement_63685

Hello Everyone,

Neurosurgeon performed Implantation of Neurostimulator electrodes and Neurostimulator pulse generator.

We coded the following CPT® 63655,63685. But Medicare appended the Modifier 51 as default to CPT® 63685 and reduced the payment.

Can any one explain me why medicare appended 51 modifier to CPT® 63685.