An arthroscope is the main instrument used for looking into the knee during keyhole surgery - ie arthroscopy.

Surgeon with arthroscope in one hand and fluid irrigation inlet in the other. These are inserted via small 'portals'. Instruments for performing surgical procedures are also inserted via portals.
Components of the arthroscope
Light is shone into the knee through the arthroscope, which has a small lens at the far end to angle the light beam. Fibre-optics relay the view to a monitor which is usually mounted at the surgeon's eye level behind the operating table.
Synonyms:
scope
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CLINICAL PUBLICATIONS
Giants of Orthopaedic Surgery: Masaki Watanabe MD. DeMaio M. Clin Orthop Relat Res. 2013 Aug; 471(8): 2443–2448.