Hard Tissue Replacements - Biomaterials - page 240

shell allows worn polyethylene liners to be exchanged. In cases of repetitive dislocation of the hip after
surgery, the metallic shell allows for replacement of the old liner with a more constrained one, to pro-
vide additional stability. Great effort has been placed on developing an effective retaining system for the
insert, as well as on maximizing the congruity between the insert and the metallic shell. Dislodgment of
the insert results in dislocation of the hip and damage of the femoral head, since it contacts the metallic
shell directly. Micromotion between the insert and the shell produces additional polyethylene debris
which can eventually contribute to bone loss (Friedman et al. 1994).
The hip joint is a ball-and-socket joint, which derives its stability from congruity of the implants,
pelvic muscles, and capsules. The prosthetic hip components are optimized to provide a wide range of
motion without impingement of the neck of the prosthesis on the rim of the acetabular cup, to prevent
dislocation. The design characteristics must enable implants to support loads that may reach more than
eight times body weight (Paul 1976). Proper femoral neck length and correct restoration of the center
of motion and femoral offset decrease the bending stress on the prosthesis-bone interface. High stress
concentration or stress shielding may result in bone resorption around the implant. For example, if
the femoral stem is designed with sharp corners (diamond-shaped in a cross-section), then the bone
in contact with the corners of the implant may necrose and resorb (Glyn-Jones et al. 2005). In order to
preserve femoral bone, hip resurfacing is also available. In the hip resurfacing, only part of the femoral
head is removed and replaced with a metallic shell, thus preserving the neck, intertrochanteric region,
and proximal femur intact for future revisions (Amstutz and Le Duff 2009) (Figure 10.8c and d). While
preserving bone is an attractive concept, achieving consistent long-term fixation without utilizing the
proximal femur for fixation remains a challenge (Malviya et al. 2010).
Load bearing and motion of the prosthesis produce wear debris from the articulating surface,
and from the interfaces where there is micromotion, for example, stem-cement interface. Bone chip,
cement chip, or broken porous coating are often entrapped in the articulating space and cause severe
polyethylene wear (third-body wear). The principal source of wear under normal conditions is the
UHMWPE bearing surface in the cup. Approximately 150,000 particles are generated with each
step and a large proportion of these particles are smaller than 1 micron (McKellop et al. 1995). Cells
from the immune system of the host, for example, macrophages , are able to identify the polyethylene
particles as foreign and initiate a complex inflammatory response. This response may lead to rapid
focal bone loss ( osteolysis ), bone resorption, loosening, and/or fracture of the bone. Recently, low-
wear UHMWPE has been developed using a crosslinking of polyethylene molecular chains. There
are several effective methods of crosslinking polyethylene, including irradiation of crosslinking,
peroxide crosslinking, and silane crosslinking (Shen et al. 1998; McKellop et al. 1999). Gamma-
radiated crosslinked polyethylene has been clinically tested and shows very low wear rate (Garcia-
Rey et al. 2008; Manley and Sutton 2008; Rajadhyaksha et al. 2009). Numerous efforts are underway
to modify the material properties of articulating materials to harden and improve the surface fin-
ish of the femoral head (Friedman et al. 1994). Currently, metal-metal and ceramic-ceramic hip
prostheses are used widely due to their low wear rate (Lang et al. 2008; Tateiwa et al. 2008; Durr
2009). However, nano-sized particles from the metal-metal prosthesis may cause complex biological
responses (Papageorgiou et al. 2007; Callaghan et al. 2008; Endres et al. 2008; Manley and Sutton
2008; Mehmood et al. 2008; Hallab and Jacobs 2009; Kanaji et al. 2009). This biological reaction
can take place around the joint, in the form of a large sterile effusion, microscopic tissue necrosis,
or metallosis, and are collectively referred to as ARMD (adverse reaction to metal debris) (Langton
et al. 2010). More specifically, Willert et al. (2005) described a local tissue reaction with an aseptic
lymphocyte-dominated vasculitis-associated lesion, having histologic features that include diffuse
perivascular infiltrates of T and B lymphocytes and plasma cells, high endothelial venules, mas-
sive fibrin exudation, accumulation of macrophages with drop-like inclusions, and infiltrates of
eosinophilic granulocytes and necrosis. There are a few reports of squeaking noises from a ceramic-
ceramic hip system during patient walking. However, the cause of the noise is unknown (Ranawat
and Ranawat 2007).