**5.1.1 Advances in polyethylene**

The wear of ultra high molecular weight polyethylene (UHMWPE) is a major contributor to premature failure of THAs. For this reason, a number of years ago, it was questioned

Aseptic Loosening of Total Hip Arthroplasty as a Result of Local Failure of Tissue Homeostasis 351

surfaces made of cobalt, chromium, molybdenum alloy should be discussed with the patient

THA has to withstand repetitive mechanical loading and long term exposure in the human body without compromising the integrity of either implant or bone-implant interface (Tarala et al. 2011). Engineers can help to prevent, at least partially, the development of periprosthetic osteolysis and aseptic loosening by creating a design and surface characteristics that increase the resistance of implant-bone interface to osteolytic granulomas and preventing access of joint fluid to the bone bed. The current evidence is in favour of bone remodeling being responsible for long-term bone-implant fixation. In this line, more biologically active prosthetic surfaces have been introduced into clinical practice (e.g. porous metals, hydroxyapatite coating etc.). Currently, multifunctional surfaces are tested with the aim of improving osseointegration and preventing the formation of biofilm. Recently, attempts have been made to coat implants with mesenchymal stromal (stem) cells (MSCs). The hypothesis is that such cells could enhance bone formation and subsequent implant fixation via intramembranous or endochondral ossification. However, the eventual role and fate of endogenously mobilized or exogenously delivered MSCs in implant fixation

The final outcome of THA depends on a large number of factors including the design of the implant, surgical technique, and patient selection. Currently there are no scientifically proven criteria for matching individuals to particular implant. At the time of surgery, the decision should involve the *age of the patient***,** *his/her local bone status and overall health condition* enabling rough estimation of time in use for the THA. Based on current best evidence, younger more active patients have several options to minimize wear, loosening and osteolysis, and thereby optimize implant longevity. These alternatives include ceramic-onceramic bearings or ceramic head or highly polished cobalt-chrome femoral heads

Another concern relates to *primary diagnosis* and the expecting of worse prognosis especially in patients with osteoarthritis after developmental dysplasia in comparison with those with idiopathic osteoarthritis. This may be associated at least partially with severity of dysplasia (Chougle et al. 2005). In fact, experienced surgeon and choice of implant could also play a role. Recent analysis from the Danish Hip Arthroplasty Registry failed to reveal substantial differences in revision risk between the above groups with follow-up of from 0.5 to 12 years postoperatively (Thillemann et al. 2008). Low revision rate has been observed in both cemented and cementless THA (Sochart and Porter 1997; Rozkydal et al. 2005). Rheumatoid arthritis does not require a special approach even in young patients. Regarding the choice of implant in these patients, a higher risk of revision was found for THAs with uncemented cups and cemented stems according in the Finnish Arthroplasty Registry (Eskelinen et al. 2006). However, the difference in survivorship between cemented and uncemented THAs

Surgically-related variables are those related to wear rate and those that influence the strength of implant-bone fixation. Wear rate can be influenced primarily by *appropriate choice of implant and experienced surgical technique* especially in terms of positioning of the cup and

prior to his/her surgical procedure.

**5.2 Design solutions** 

is still poorly known.

**5.3 Patient selection** 

**5.4 Operative procedures** 

articulating with highly-cross linked polyethylene.

disappeared in patients older than 55 years old (Makela et al. 2011).

whether this material should be retained or withdrawn (Amis 1996). Fortunately, over the recent decades, several processing techniques have been developed to improve the mechanical and tribological performance of UHMWPE. More recent technologies involving high degree cross-linkage of the linear chains of UHMWPE (highly-cross linked polyethylene, HXLPE) have been incorporated into clinical practice for several years. These also differ from company to company according to method used for the elimination of residual free radicals that could lead to oxidative degradation of bearing surface (heating, infiltration of vitamin E etc.). In comparison to ceramic-on-ceramic (CoC) THA, the advantage of HXLPE is greater variety in terms of head sizes and offset. Also the cost of HXLPE is substantially lower than that for hard bearing surfaces. On the other hand, there remains a concern about the risk of fatigue fracture of acetabular liners even though the cases reported were made predominantly of first generation HXLPE (Furmanski et al. 2009). Little is known on the biological activity of HXLPE particles. It was demonstrated moderately increased specific biological activity in comparison to particles of conventional polyethylene (Illgen et al. 2009). Despite that, there is growing evidence that the use of modern HXLPE bearings decrease significantly wear rate and by this way may reduce the risk of premature osteolysis development although the majority of studies report outcomes with less than 10 years follow-up (Kurtz et al. 2011).

### **5.1.2 Advances in ceramic bearings**

The amount of prosthetic particles can be significantly reduced using CoC bearings in THA. Ceramics, in general are very hard materials with excellent biocompatibility. In addition, use of third generation ceramic bearings, reduces the predicted risk for fracture of ceramic bearings to approaching nil. Currently there is enough evidence supporting the further use of CoC bearings in the case of THA (Hannouche et al. 2011). Concerns remain about the "squeaking" of ceramic implants, impingement of the femoral neck on the edge of ceramic liner and microseparations of ceramic surfaces.

Oxinium is a Zr-2.5 Nb alloy that has been oxidized at elevated temperature to grow a zirconium oxide ceramic layer on the surface of the implant. Such surface treatment increases the wear resistance of the implant. However, there is only limited clinical experience with OxiniumTM femoral heads in THA.

### **5.1.3 Advances in metal**

Bearing surfaces made of cobalt, chromium and molybdenum alloys are currently a subject of controversy because of their biological consideration. The main reason is that metallic wear particles can induce implant hypersensitivity, ALVAL (*Part 3.2.2*), may contribute to tissue necrosis and also to induction and perpetuation of periprosthetic particle disease (Watters et al. 2010; Basko-Plluska et al. 2011). In addition, the levels of metal ions in serum and urine also can be elevated for a long time in patients with stable and functional implants in comparison to controls without metallic implants. Although long-term data are not available, the International Agency for Research on Cancer (IARC) has classified implanted foreign bodies consisting of metallic cobalt, nickel and chromium in Group 2B defined as possibly carcinogenic in humans (McGregor et al. 2000). The main concern is that exposure of human cells to cobalt and chromium may induce chromosomal aberrations and damage DNA, potentially leading to carcinoma (Beyersmann and Hartwig 2008). From a purely epidemiological view, the current evidence for clinical carcinogenetic effects of metal-onmetal THAs is limited and rigorous long-term studies are needed to answer the above question. Taken together, above potential consequences from the clinical usage of bearing surfaces made of cobalt, chromium, molybdenum alloy should be discussed with the patient prior to his/her surgical procedure.
