Hip Replacements, by Matthew Osso
The “pelvic girdle” is made up of two hip bones, which are each made of three main parts: the ilium, the ischium, and the pubis.
As can be seen by the figure, the upper region of the hip bone is the ilium; this section forms the sacroiliac joint. The ilium supports the sacrum on either side, which in turn supports the spine. The ishium is the bottom posterior piece of the bony hip; this is the part that humans sit on. The pubis is the bottom anterior part and the two pubic bones are connected by fibrous cartilage. This is also where the bladder rests within the human body. The large socket seen in the hip structure toward the bottom is known as the acetabulum. This socket forms the “ball and socket” joint with the femur and is a major weight-bearing joint.  Hip Replacement Surgery is usually either partial or total; the difference being how much of the hip is actually replaced during the surgery. For most partial surgeries, it is only the femoral head (the “ball”) that is replaced, with the acetabulum (the “socket”) remaining the same. The reasoning for partial instead of total would be more of an injury whereas total is typically needed for those with severe arthritis.  When looking at Total Hip Replacement Surgery there are two main types, cemented and cementless. Cemented replacements most commonly use polymethylmethacrylate as the cement and patients with such surgeries can usually move well soon after the surgery. The only issue with cemented replacements is if the cement fails and the prosthesis loosens or the bond between the cement and the bone breaks. Even so, this is generally a very reliable method. Cementless replacements are those which are directly attached to the bone without the need of cement. The surface of such implants are said to be “conductive to attracting new bone growth”. The main issue with this type of replacement is the need for very high precision in order for the bone to actually grow onto the implant. Due to this, motion directly following the procedure is usually assisted in order to allow the bone time to grow a connection with the implant. This type of implant is still being developed, but for most cases it is used in younger patients who tend to be more active and have better overall bone quality. 
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Images and Video
[E] Stefan Kreuzer, MD., http://www.youtube.com/watch?v=HRki3UDYa08
[F] Barry Waldman, MD., http://www.youtube.com/watch?v=-hm2ogoSc1Y