Kings of industrial design, Apple utilizes every last bit of available space in their machines. Often overlooked is a very functional piece of the notebook computer, in that it allows it to fold. But the hinge also handles a lot more you may not have known about.
In modern unibody MacBook Pros, that black plastic covers many important pieces other than just hinge hardware.
On the left side is the cable that handles all the video sent to the display. It twists through the hinge assembly before running into the LCD.
On the right, it’s more varied. You have the internal USB cable for the camera and LED. The antenna cables run through here for the Wi-Fi card. The Wi-Fi card resides inside the hinge on many models — it’s quite small. The cable that powers the display backlight (which causes the LCD to illuminate) also runs through the right side.
These cables are protected against abrasion with special covers. They get twisted back and forth all the time as the display is moved.
This assembly stands up to a lot of abuse, day in and day out. But the hinges do loosen, and cables can pinch. Liquid spilled on the hinge area can cause issues, as with most elsewhere in the machine.
The most avoidable hinge problem is loose hinges caused by lifting the machine by the display, or knocking the display from side-to-side. Too much weight on the display, like some snap-on cases, may also cause the hinges to loosen. Overall, the hinge is robust and engineered well. Even on old and heavily used machines, they remain very smooth and reliable.