**3.4.7 Ceramic advantages**

The potential advantages of using a ceramic on ceramic articulation for total hip arthroplasty can be quickly summarized as decreased wear and elimination of osteolysis. Osteolysis from wear debris is commonly viewed as the major obstacle blocking the development of a "lifetime" hip replacement. The need to eliminate wear and osteolysis has been magnified by the extension of indications for total hip arthroplasty to younger, more active, healthier patients with long life expectancies. The potential for decreased wear is derived from the tribologic properties inherent to alumina. Alumina can be highly polished. Alumina bearings are also very hard, and this characteristic increases their resistance to scratching. The hardness minimizes third-body wear from entrapped bone, polymethylmethacrylate, or metal debris derived from surgical instruments or component fretting. Alumina has ionic properties and therefore, in combination with body fluids, has better wettability than chrome cobalt. The fluid film that develops on ceramic surfaces decreases frictional drag and adhesive wear. Wear rates for modern ceramic on ceramic articulations have been shown to be as low as 4 µm/year. This low wear rate coupled with less alumina bioreactivity minimizes the likelihood of osteolysis. With currently used implant designs, osteolysis has not been reported with follow-up as long as 18.5 years.

### **3.4.8 Ceramic concerns**

One significant drawback of ceramic materials is their inherently lower strength and toughness under tension and bending, which are the loading modes that favor the initiation and propagation of cracks. One such adverse loading arises from the mismatch at the taper junction between the metal stem and ceramic head. To prevent this from leading to early fracture, the tolerances at the taper are matched and may be manufacturer-specific. The surgical placement of the components may also predispose the ceramic components to high stresses and initiate fracture. For instance, a third body (bone cement or bone fragment) left in the taper interface or impingement of the femoral component on the rim of the ceramic acetabular liner secondary to malpositioning of the components could also initiate ceramic fracture.(Barrack 2004)

### **3.4.8.1 Fracture (Fig.12)**

Improved material processing, smaller grain sizes, fewer impurities, laser etching, and proof testing have greatly diminished the risk of catastrophic in vivo fracture. The risk of ceramic fracture is estimated to have decreased nearly 100-fold in the last two decades. In 1990 the incidence of fracture was approximately 0.8%, and today is likely between 0.004% and 0.010%. Nonetheless, this complication is devastating and still occurs.(Min 2007) Even with proof testing, it is unlikely that failure by fracture will be eliminated. Although theoretically proof testing eliminates weaker components, flawed products that are likely to fail are not always eliminated. Proof testing theoretically is designed to be stringent enough to remove components with manufacturing flaws that are likely to clinically fail. However, the test must be nondestructive and not cause damage to the tested part. No proof test currently available is 100% effective. So, it must be remembered that production errors can and still do occur. In 1998 a manufacturing change resulted in a high fracture rate of ceramic balls. About one in three components clinically failed, and this was despite negative proof testing of all fractured devices. The production of ceramics is far more demanding than the manufacture of metal and polyethylene components, and the incidence of catastrophic failure for ceramics will always be higher than with other materials. Ceramic component fracture may occur secondary to poor surgical technique. Improper component insertion predisposes the implant to fracture. Impaction of the femoral head on the trunnion should be performed only after ensuring it is concentrically placed. Placing the head nonconcentrically on the trunnion or not cleaning and drying it properly leads to stress concentrations in the femoral head.(Poggie 2007) In addition, placing a ceramic head on a damaged trunnion also leads to stress concentration and a significantly reduced burst strength with the potential for fracture.(Anwar 2009) It is also possible to nonconcentrically place the ceramic liner in the metal acetabular shell. However, the adverse effects and longterm consequences of this error have not been reported. Ceramic component fracture is a double-edged sword. After fracture the patient is confronted with immediate debilitating pain and the need for emergency revision surgery. However, secondarily, revision of a fractured ceramic component carries the risk of a less than optimal outcome. Because of trunnion damage, revision with a ceramic head is usually not possible. Ceramic fracture debris embedded in the soft tissue can cause third-body wear and premature failure owing to accelerated wear if a metal and polyethylene articulation is used for the revision. During revision of fractured ceramic components, it is now recommended to perform meticulous synovectomy and debridement to remove as much fracture debris as possible and revise the femoral head with a metallic alloy, such as CoCr, to avoid further catastrophic failure. Another option on the femoral side is to revise the femoral stem and use a ceramic femoral head.(Ha 2007; D'Antonio 2009)

Fig. 12. Retrieved Ceramic-on-Ceramic implants showing fractured acetabular liner (A) and femoral head (B)
