Replace whole strut or just part?
 

2001 E320 4matic wagon. Problem with my front left strut: it pushes up through the top of the shaft when weight is put on the front left side. It started with the front getting a bit slushy and making a bit of noise after having been driven a while. Now, it is very obviously problematic and scrapes against the shaft, making a constant squeaking. (Excuse the lack of proper terminology as I'm only an amateur.)

As you can see from the pictures, part of the strut is shot. I'm guessing I just need to replace the whole shock absorber, but can reuse the spring. Is that right? Can someone explain to me what may have happened here so I can increase my knowledge? Let me know if more info is needed. Other recommendations? Thanks!!

https://cimg2.ibsrv.net/gimg/www.mbw...34cc62c242.jpg
Problem strut: Front left bottom

https://cimg4.ibsrv.net/gimg/www.mbw...89330d7fc0.jpg
Problem strut: Front left top

https://cimg9.ibsrv.net/gimg/www.mbw...660b1d4dbb.jpg
Healthy strut: Front right bottom

https://cimg0.ibsrv.net/gimg/www.mbw...b27798b24a.jpg
Healthy strut: Front right top