CathInfo.com

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There has been much discussion among theologians as to what constitutes the essential matter of this sacrament. Some, e.g. Aureolus and Petavius, held that it consists in the imposition of hands. Others, with St. Thomas, Bellarmine, and Maldonatus, maintain that it is the anointing with chrism. According to a third opinion (Morinus, Tapper) either anointing or imposition of hands suffices. Finally, the most generally accepted view is that the anointing and the imposition of hands conjointly are the matter. The "imposition", however, is not that with which the rite begins but the laying on of hands which takes place in the act of anointing.

It also says this about the Eastern rites:

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The Eastern Church omits the imposition of hands and the prayer at the beginning, and accompanies the anointing with the words: "the sign [or seal] of the gift of the Holy Ghost."

The conclusion is that according to the generally accepted view the imposition of hands is part of the essential matter in the Roman rite, and in the Eastern rites it is not.

Therefore, when a bishop confirms in the Roman rite, that means it is essential for validity that he imposes his hands on the head of the confirmants.

Quote from: PapalSupremacy

Quote

There has been much discussion among theologians as to what constitutes the essential matter of this sacrament. Some, e.g. Aureolus and Petavius, held that it consists in the imposition of hands. Others, with St. Thomas, Bellarmine, and Maldonatus, maintain that it is the anointing with chrism. According to a third opinion (Morinus, Tapper) either anointing or imposition of hands suffices. Finally, the most generally accepted view is that the anointing and the imposition of hands conjointly are the matter. The "imposition", however, is not that with which the rite begins but the laying on of hands which takes place in the act of anointing.

It also says this about the Eastern rites:

Quote

The Eastern Church omits the imposition of hands and the prayer at the beginning, and accompanies the anointing with the words: "the sign [or seal] of the gift of the Holy Ghost."

The conclusion is that according to the generally accepted view the imposition of hands is part of the essential matter in the Roman rite, and in the Eastern rites it is not.

Therefore, when a bishop confirms in the Roman rite, that means it is essential for validity that he imposes his hands on the head of the confirmants.

Negatory.  It can be matter without being essential matter; those are two different things.

If the Sacrament remains valid in the Eastern Rite without imposition then it's not matter essential to the Sacrament.  St. Thomas therefore clearly had it right.