In reply to JimR:
I have some direct, albeit somewhat dated, experience in this industry including providing guidance to surgeons on specific implants and I have attended 100s of hip replacement surgeries.
There are a number of major manufacturers such as Zimmer, Stryker, Johnson and Johnson, B.Braun, Biomet, Smith&Newphew and many more and each manufacturer has a number of 'models' within their portfolio which are used for different circumstances.
The most basic hip implant is the one used in replacing the femoral component if the old person has a slip. The most advanced are staggeringly expensive augmented systems which are used when a previous hip prosthesis has to be replaced because of failure due to breakage or simply time - called a revision hip procecure.
Considerations over what choice is used are often around age, sex, weight, quality of bone, activity levels, complexity, but also often what is 'on the shelf' at the hospital. There are a range of materials of femoral stem metal and a range of fixing techniques, including press-fit and cementing. The same is for the acetabular (socket) which can be screwed, cemented or press fit. The articular surface is created by a femoral head fitted to the stem which can be ceramic or metal and the socket lining can be UHMWP (plastic), ceramic or metal. Ceramic on ceramic is often used for younger active patients and has minimal longer term issues because the material is so tough, together with press-fit stems. Metal on plastic is used but can have longer term problems with plastic particulates damaging the surounding sinovial tissues. There's also been some comment above around the BHR (metal femoral head and acetabular replacement). This is used for the youngest patients with the best underlying bone onto which the prosthesis can be replaced although there are some studies which suggest metal on metal can cause some issues because of particulates, but this might have changed now.
There are a number of units and centres of excellence and within each there will be hip specialist surgeons (who do little else and also conducts the most difficult revisions), general orthopeadic surgeons (who do a range of standard hip/knee replacements) and then smaller surgical units where the surgeon will 'do' hips but will be less frequent and therefore they will only do very easy stuff.
Surgeons will have a preference of what manufacturer they use because of what's on the shelf, they prefer the manufacturer's instruments and they 'work well in their hands' and then some will want the newest tech or model. There are some small longevity differences with different brands but my main selection factor would be to have a good surgeon who has good results regardless of the brand. A poorly implanted top brand would be less prefereable to a good implanted average one.
There is also a relatively new innovation which involves surgical navigation which uses clever IT and sensors to help with measuring biomechanics rather than simply 'feel' and experience, which was the historic method.
Having seen the quality of the work from the 'inside' I would be very careful who I choose and would ask for their history, failure rates, how often they do the procedures and over what period. Are they specialist or jobber, are they using advanced tech etc etc.
Not all hip impants are equal....