I had this situation on a 2005 joint return. I was told if the spouse cannot sign because of injury or disease and tells their spouse to sign, he or she can sign their spouse's name in the proper space on the return followed by the words "By (your name) Husband (or Wife." They need to also be sure to sign in the space provided for their own signature. They should also attach a dated statement signed by them to the tax return. The statement should include the form number of the return they are filing, the tax year, the reason their spouse cannot sign, and that theirspouse has agreed to them signing for him or her.
We followed these instructions and the return was accepted as filed by the IRS. Hope this helps.