The hip joint consists of a round ball, the femoral head, and the matching concavity, the acetabulum. In a total joint replacement both are operated upon. Classically, the femoral head is replaced by a ball perched on a stem (Figure 1), and the acetabulum is resurfaced with a plastic or metal/plastic cup (Figure 2). A number of variations on this theme exist.

Figure 1		Figure 1

Stem and Ball

The stem and ball can be made of one solid piece, or, more commonly, the ball and stem are two separate components, in which case the implant is said to be “modular”. The part of the stem that protrudes from the bone consists of a tapered cone (a “Morse taper”), and the hollow ball (“head”) is literally tapped and wedged onto this Morse taper. The head is manufactured with a “neck” extension, and this neck extension varies in length. The surgeon chooses the neck length that gives the best fit. If the neck is too long, the surgeon won’t be able to get the femoral head (ball) back into the cup, and/or the patient’s leg will have been lengthened excessively. If the neck is too short, the construct is loosey-goosey, and the hip can pop out (“dislocate”) unexpectedly.

The femoral head is made of metal or ceramic. The metal can be stainless steel, titanium, or cobalt-chrome. Titanium is rarely used because it is relatively soft and wears down (slowly) when it repeatedly rubs against the cup. Ceramics represent a large class of materials. They are smoother than any of the metals and lead to less friction and torque. A ceramic femoral head articulating against a ceramic cup therefore lasts longer than any other combination of materials used in hip replacement surgery. Certain ceramics, however, have been found to crack and ceramics are expensive.

The diameter of the femoral head varies. Your own femoral head measures somewhere between 45mm and 55mm in diameter (about 2 inches). In a hip replacement the head measures 22mm, 26mm, 28mm, 32mm, or 36mm.

The reasons for this relate to principles in physics you learned in high school (unless you were out that day). And if you didn’t care for physics then, I suggest you skip this paragraph. One of the complications of hip replacement surgery is loosening. In time, the fixation of the implant to the bone can loosen and become painful. On the cup side of the implant, loosening is related to torque. Every time you take a step or, more significantly, get up from a chair the femoral head (ball) applies a twisting moment (torque) to the cup. This torque is proportional to the diameter of the femoral head. The smaller the head diameter, the less the torque, the longer it will take for the cup to (painfully) loosen.

The size of the femoral head also influences the rate at which the plastic in the cup will wear out. The larger the head, the quicker the wear.

For all of these reasons the inventor of hip replacement surgery, Sir John Charnley, chose a smallish 22mm head size (22.25mm to be exact) for his pioneering hip replacement, the Low Friction Arthroplasty as he called it.

But there is another side to this femoral head business: taken as an isolated parameter (which no surgeon should do), a small head size increases the risk of dislocation which occurs when the head “pops out” of the cup. This explains the existence of 26mm, 28mm, and 32mm head sizes. The loosening rate and wear have been shown to be significantly increased with 32mm heads, which explains why you are no longer likely to receive an implant with such a head size.

In short, the surgeon has the choice between a smaller head that will last longer but will be more prone to dislocation and a larger size that will not last as long but will be less prone to dislocate. The day loosening and wear are solved you will see the re-emergence of larger head sizes.

The surface texture of a press-fit stem is usually rough. By presenting a sandpaper surface to the surrounding bone it allows bone to grow into the tiny interstices of the implant surface. Engineers have worked out just how rough the sandpaper finish should be. Since the growth of bone into the implant is a biological and chemical process it is hardly surprising to find that bone grows into certain surfaces better than it does into others. Bone grows particularly well into something called hydroxyapatite (HA), a white substance that coats certain implants. Such a coating is not essential. Bone also grows into titanium alloys, cobalt-chrome, and carbon.

The Cup

Most cups in use today consist of a metal shell that is impacted into the pelvis. It is literally hammered into place with a mallet. A plastic cup called “the liner” is in turn impacted into this shell.

Figure 3

The cup may be entirely made of plastic (“all-poly” in the orthopedic lingo) in which case it is cemented into place (see Figure 3). Indeed, press-fit plastic cups have not worked well to date.

Unusual Cups

In patients at particular risk for dislocation – patients who have already suffered a few dislocations, for example, a so-called constrained cup can be utilized. With this construct, the femoral head snaps into the cup and comes out with difficulty. Why is this not used in every case? The constraint increases the torque on the cup and increases the risk of loosening.

Certain models feature a ceramic liner. These are matched to a ceramic femoral head. The friction of ceramic on ceramic is lower than metal on plastic and lower even than ceramic on plastic. Cost, design considerations, and politics have limited the availability of these constructs.

All-Metal Cups

These have been re-introduced. Although they failed when first utilized in the 1950s, some investigators feel that new metallurgy will lead to success. The friction associated with a metal head rubbing on a metal cup would intuitively appear to be high. This is indeed what happened in the 1950s. But with today’s designs and metals the friction may in fact be lower than that seen between metal and plastic.

I believe that it is best to tailor the implant to the patient.

*Excerpted from What Your Doctor May NOT Tell You About Hip and Knee Replacement Surgery, Warner Books 2004.