Well a few considerations...
You need something very sharp for what you do, so putting extension tubes on a zoom lens isn't going to give you your best results there. Zoom lenses, especially with a fairly wide range like 18-105mm are quite a bit softer than a macro or even a normal prime lens. You want either a normal sharp prime lens and extension tubes or a dedicated macro lens for sharpness sake.
A dedicated macro lens is going to look more professional and be less hassle, which are both important when working in a professional capacity like a surgeon.
You're going to be indoors, under artificial light (which is almost always 'dim' compared to outdoors, even when its considered bright inside) - so you'll want:
- A good working distance so that you don't block your light source with your lens and body.
- A large aperture so that you can focus in low light and stop down a little to get it ultra sharp but still not be 'slow'.
There's two lenses that generally meet this criteria, depending on your budget.
- Sigma 105mm f/2.8 around $500 US.
- Nikon 105mm f/2.8G ED-IF AF-S VR Micro-Nikkor Lens around $1000 US.
The Sigma is the budget friendly option, but the Nikon is definitely the sharper and better lens. The Nikon brings faster, quieter autofocus and a stabilization (VR) to help you handhold in lower light.
(The Tamron 90mm Macro is another popular telephoto macro lens, but its price about equivalent to the Sigma and the Sigma has slightly more reach).