There are two main things that differ between prostheses for THR (and LOTS of smaller differences I won't go into!): cemented vs. uncemented stems and type of articulation surface.
1) cemented vs. uncemented stems
a) CEMENTED: This refers to how the stem in affixed to the femur (as far as I know, nearly all cups are uncemented these days). In cemented stems, the stem is thinner and the space intentionally left between the stem and the cortex of the femur is filled with a cement. Advantages are instant fixation, and works even for femurs with very large canals or when bone quality is poor. Disadvantages include cement tends to degrade and break down with time, especially if the patient is very active, and removing all the little pieces of cement from the canal can be very difficult during revision. Also, nasty chemicals can leach out of the cement (some patients react to this) and the chance of embolism is higher during implantation due to the large bolus of "sludge" that they can SEE (with ultrasound or something) pass through the veins and heart just after cemented THR. Also, quality of the fixation is HIGHLY physician-dependent, and varies with such things as just how the cement is mixed up and applied.
b) UNCEMENTED: Stems are much thicker and are designed to be "press-fit" into the femoral canal which is reamed out to exactly fit the stem; over time, the bone grows onto the stem, locking it in place. Most uncemented stems have a porous coating near the head end of the shaft to give a rough substrate for the new bone growth to attach to. Some have a hydroxyapatite (HA) coating -- HA is a natural component of bone, and stimulates earlier on-growth and therefore early fixation (personally, I'm a big fan of HA coatings). There are also various other differences in stem design to help give rotational stability, prevent the stem impinging on the cup liner, be more flexible, etc. For example, Fetto’s lateral flare stem purports provide better distribution of stresses and therefore less thigh pain and also longer lifetime of stem. Advantages: IF good bony ongrowth occurs, uncemented stems are believed to be able to last longer than cemented stems as there is no cement to break down and bone is self-repairing, believed to be better for active patients. Disadvantages: higher risk of femoral fracture during implantation, requires patient have good bone quality, less stable initial fixation, if well grown in it can make removal very difficult during revision, early designs of uncemented stems proved dissappointing in people and although the manufacturers have addressed the known weaknesses there is simpy no long-term data proving that the new stems will last longer than the cemented stems.
My own research showed that the Osteonics hydroxyapatite-coated uncemented stems had the BEST 10-yr outcomes of any other uncemented or cemented stem, especially in the difficult population of young (under 50) active patients (0% mechanical failure rate!) -- the data simply aren't there for longer periods cause the implants haven't been around that long.
2) type of articulation surface
a) metal or ceramic ball, polyethylene cup liner -- this is your typical prosthesis. Advantage: a known commodity. Poly is belived to provide some shock absorption. Disadvantage: poly wears at a high rate (especially in active patients) which can cause a sloppy fit between ball and socket and increase chances for dislocation or, more commonly, the body mounts an immune response to the poly debris which has a secondary effect of causing bone to be resorbed (osteolysis) which subsequently causes 'aseptic loosening' of the prosthesis -- this latter is the single most common reason for revision and why there is so much research into alternative articulation surfaces.
b) metal or ceramic ball, cross-linked polyethylene cup liner. The poly is subjected to some sort of radiation which causes cross-linking at the molecular level resulting in a substance that degrades much more slowly than standard poly. Advantages: FDA approved in 1998 and therefore readily available, still provides some shock absorption, may last a lot longer than regular poly. Disadvantages: early attempts at cross-linked polys looked great in the lab but didn't hold up in people (degraded rapidly after a few years) -- supposedly the new ones won't do this, but there is no long term data to know for sure -- it'll be interesting to see how this one plays out. Still a poly, therefore the debris can still cause osteolysis. Wear rates lower than regular poly but higher than metal-metal or ceramic-ceramic.
c) metal ball and metal cup (cobalt-chromium alloy) -- the Metasul hip (FDA approved about a year ago) has a metal cup liner inserted into a poly liner in the metal cup, but other m-m hips (still in trial) are solid metal. Biomet recently got FDA approval for a m-m hip, but I don't know anything about it. The first hip replacements were all metal, but due to poorer manufacturing techniques back then they weren't as perfectly spherical and therefore the wear rates were unacceptably high -- now, better machining has re-interested people in this alternative. Advantages: m-m wear rate is much lower than poly, and debris particles are smaller. Disadvantages: metal ions are biochemically active and how these will affect our biochemistry long-term is unknown -- some reports of increased cancer have not been substantiated at this time. Some rumored concerns about denting during major trauma, but this does not seem to be backed up by hard data. No long-term data to show how well these will last.
There is also the experimental metal-metal hip resurfacing technique developed by Amstutz, where much more of the patient's own bone is preserved (the femoral head is carved down a little and capped with metal, but is not removed as in THR). Advantages: less invasive, can do a primary THR if it fails, no thigh pain since there is not stem in the femoral canal, allows patient to be more active (touted for young, active patients) due to lower risk of dislocation because of larger ball. Disadvantages: early designs tried by Amstutz had high failure rates -- hopefully these have been corrected, but again, no long term data to know for sure. Some hypothetical concerns about resurfacing leading to avascular necrosis of femoral ball, but these do not seem to be backed up by any hard data. My own opinion is that this approach makes a lot of sense and when they work out the bugs it will probably be preferable as a first choice (relative to THR) for bad hips, especially in young, active patients -- hopefully, Amtutz's new design (about 5 years in testing so far with good results, I believe) will prove to have the bugs worked out. Currently, the only place you can get this is from Amtutz in L.A. but clinical trials are supposed to start nationwide in the upcoming year (is also available in Europe). Keith had bilateral surface replacements and has an excellent web site discussing resurfacing at http://www.activejoints.com/.
d) ceramic ball and ceramic cup liner -- These have been used in Europe for over 30 years, but the earlier ones had high fracture rates due to poor manufacturing of the ceramic -- improvements in manufacturing have led to much better consistency and low fracture rates (0.03% now). There are actually 2 kinds of ceramics; alumina and zirconia -- the latter is more experimental and when you hear just "ceramic" it means alumina ceramic. Have been in trial in US for several years and are expected to be FDA approved in 1-1.5 yrs. Advantages: lowest wear rate of anything else being tested. Debris particles small and bioinert. Disadvantages: no shock absorbancy therefore chance of fracture with major trauma. Some stem designs may impinge on ceramic cup liner at extremes of range of motion which may cause cracking. No long term data to show how well these will really last.
In summary, the current poly liners are known to be a problem area due to high wear and osteolysis. Three major alternatives being tested are cross-linked poly cup liners, metal cup liners (with metal balls), and ceramic cup liners (with ceramic balls). There is NO long term data to tell which of these will "win out". My own research (and opinions of several prominant orthopedic surgeons I spoke with) leans in favor of the ceramic-ceramic prostheses, but for every opinion there is another one out there to the contrary! Only time will tell...