**3.4.5 Wear debris and tissue response**

It has been shown, in vitro and in vivo, that alumina wear debris is biologically inert and well tolerated. Alumina particles induce very little cellular response and formation of granulomatous tissue. The small nature of most alumina on alumina wear particles and the low volume of particles generated leads to a low level of bioactivity. Giant cells have not been observed in contact with alumina wear debris. Lerouge and colleagues compared 12 periprosthetic membranes obtained during revision for aseptic component loosening with an alumina on alumina couple. These were compared with a series of membranes obtained from revisions of a metal on polyethylene bearing. In the alumina on alumina group, the cellular reaction, which was generally mild, was determined to be in response to the zirconia ceramic particles used in the cement as an opacifying agent. No cellular reaction to the alumina particles was noted.(Lerouge 1997) This contrasted with the significant cellular activity noted in the metal on polyethylene group with reaction to the polyethylene debris. Osteolysis associated with alumina on alumina total hip arthroplasty has been infrequently reported. In one study, when an implant made with large-grain-size ceramics, low density, and high porosity was used, large production of debris resulted and osteolysis occurred. Tissue obtained from failed hips with an alumina on alumina couple was shown to have significantly lower prostaglandin E2 (PGE2) levels compared with tissue obtained from hips with metal on polyethylene articulation. Both alumina and polyethylene debris stimulate cellular release of tumor necrosis factor (TNF)-α. However, polyethylene particles cause more release of TNF-α, and in fact the stimulation may be 8 to 10 times greater. Of importance, alumina particles induce macrophage apoptosis, which leads to decreased macrophage activity. This induced apoptosis explains the decreased levels of TNF-α associated with alumina and may also account for the paucity of ceramic-related osteolysis. Ceramic debris may not be bio-inert as initially assumed, because osteolysis has been described in some patients with a COC bearing.(Nam 2007) Some studies describe inflammatory and cytotoxic reactions on the cellular level, but the relationship to material, size, and particle number remains uncertain. It seems that there is less inflammatory reaction compared to MOM or MOP bearings in well-functioning prostheses. In contrast with polyethylene or metallic particles, foreign body reactions are routinely observed. Ion toxicity is not an issue with ceramics.(Mehmood 2008)

### **3.4.6 Ceramic-on-metal articulation**

196 Recent Advances in Arthroplasty

has been reported with products produced before 1990. In recent years, wear rates below 15 µm/year have been consistently reported. Many investigators believe that severe wear is related to clinically exceptional circumstances and that with properly implanted bearing

Ceramic materials may have better biocompatibility than metal alloys, but the relative size, shape, number, reactivity, and local versus systemic distribution of the wear particles have not been fully determined. Hatton et al. investigated the tissues from ten noncemented COC THAs undergoing revision surgery. The tissues from the femoral and acetabular regions demonstrated the presence of intracellular particle agglomerates and mixed pathology, with areas that had no obvious pathology and areas that were relatively rich in macrophages, and over half of the tissues had 60% of necrosis. A bimodal size range of ceramic wear debris was observed, with particles as small as 5 to 90 nm (mean 24 nm) and as large as 0.05 to 3.2 µm (mean 430 nm).(Hatton 2002a) These two types of ceramic wear debris are generated by two different wear mechanisms in vivo, with very small wear particles being generated under normal articulating conditions and larger particles being generated under microseparation conditions. After total hip arthroplasty, the femoral head and the acetabular insert can separate up to 2 mm during the swing phase of a normal gait cycle. When a load is applied at heel strike in the stance phase, the femoral head moves vertically to relocate in the cup. With the geometry of a typical COC prosthesis, separations of only 2 mm will allow the femoral head to contact the rim acetabular liner, resulting in changes in

surfaces, catastrophic wear is essentially nonexistent.(Capello 2008)

the wear performance of the bearing couple (stripe wear). (Stewart 2001, 2003)

Fig. 11. A ceramic head and acetabular liner from a patient with "squeaking" hip with

It is hoped that improvements in the manufacturing of ceramics and ceramic components will minimize or eliminate mechanical problems such as fracture and accelerated wear.

It has been shown, in vitro and in vivo, that alumina wear debris is biologically inert and well tolerated. Alumina particles induce very little cellular response and formation of granulomatous tissue. The small nature of most alumina on alumina wear particles and the low volume of particles generated leads to a low level of bioactivity. Giant cells have not been observed in contact with alumina wear debris. Lerouge and colleagues compared 12

**3.4.4 Microseparation and wear particles** 

evidence of impingement and "stripe wear.

**3.4.5 Wear debris and tissue response** 

In a simulator study, Firkins et al demonstrated superior wear for alumina ceramic-on-metal articulations. Femoral heads 28 mm in diameter made of medical-grade alumina were articulated against acetabular cups manufactured from medical-grade, high-carbon-wrought cobalt-chromium alloy. The implants were tested for 5 million cycles. Wear and surface analyses were performed every 1 million cycles. The results were compared with metal-onmetal articulations that used medical-grade, low-carbon, cobalt-chromium alloy femoral heads against medical-grade, high-carbon, cobalt-chromium alloy sockets. The metal-onmetal bearings initially showed high bedding-in and subsequently developed a steady-state volumetric wear rate of 1.23 mm3 per 1 million cycles. No bedding-in period was observed with the ceramic-on-metal, and a volumetric wear rate of 0.01 mm3 per 1 million cycles was reported. This represents more than a 100-fold decrease in wear with the ceramic-on-metal articulation. Surface analysis of the ceramic head showed no signs of wear or change in surface roughness. Metal particles from both articulations were of nanometer size (6 to 30 nm). In addition, the ceramic-on-metal articulations produced slightly smaller particles although they were far fewer in number. (Firkins 2001) Clinical studies are currently under way to assess the performance of these bearings. The potential advantage of this novel ceramic-on-metal bearing is lower wear and the generation of significantly fewer metal particles compared with currently available metal-on-metal bearing surfaces. This bearing combination allows for the use of large femoral heads, similar to metal-on-metal bearings. With larger ceramic heads, the fracture risk is reduced, and an increased number of femoral

The Bearing Surfaces in Total Hip Arthroplasty – Options, Material Characteristics and Selection 199

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

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

Separation of the ball from the socket in patients with a hip arthroplasty may occur during the swing phase of gait or with impingement of the trunnion on the acetabular rim levering the ball from the socket. When this separation occurs, the contact area of the femoral head on the acetabular liner becomes small and stripe wear can result.(Fig.15) Stripe wear is concerning because volumetric wear associated with this phenomenon is high. In one study, stripe wear produced volumetric wear that averaged 1.24 mm3 per million cycles. Equally concerning, a bimodal array of nanometer- and micrometer-sized particles was created with an enhanced profile of bioreactivity. Separation of the ceramic on ceramic hip articulation is

head.(Ha 2007; D'Antonio 2009)

femoral head (B) **3.4.8.2 Stripe wear** 

head options are available to the surgeon. Potential disadvantages of this new bearing articulation include the risk of a ceramic femoral head fracture and its as-yet unknown clinical performance.(D'Antonio 2009)
