You can use scissors with a singular verb anytime you want. However, to prevent getting into arguments, you may wish to limit this usage to medical scissors. Or you can say 'if a scissors was good enough for Emily Brontë,
then it's good enough for me'.
The Oxford English Dictionary (OED) notes that using scissors with plural concord is "the usual form." However the same dictionary notes that scissors is also used with singular concord and does not describe this as dialectal or anything that might be considered non-standard. And examples range from 1565 to 2001, including:
1565 T. Cooper Thesaurus at Forfex A sisers, or sheares.
1847 E. Brontë Wuthering Heights I. ix. 164 Now, don't you think the lad would be handsomer cropped?.. Get me a scissors.
1909 Ophthalmol. 5 43 The scissors is inserted into the duct and the cut made as low down as possible.
You might notice this last usage is from the field of ophthalmology.
Google Books provides plenty of other examples of scissors being used with singular concord, including several instances in the medical field. For example:
With a scissors the surgical fascia is incised for 2-3 cm above the suprasternal notch, ...
With a scissors, divide the slip of the diaphragm that attaches to the posterior surface of the xiphoid, ...
From Manual of Pulmonary Surgery (Comprehensive Manuals of Surgical Specialties) by by E.W. Humphrey and D.L. McKeown (2012).
As in your example from the Oxford Dictionary online (borrowed from American Family Physician (link)) there are many uses of singular concord with regard to "medical scissors," but contemporary usage is not limited to this. But singular concord remains the "unusual form" (my quotes).