October AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 8 3 1 cles within the joint articulation. Wear- induced biological failure is most com- monly seen adjacent to hip replace- ment implants when accumulation of debris leads to a cell response cascade activating osteoclasts (bone resorb- ing cells). This type of wear results in chronic inflammation, bone loss, asep- tic loosening, and implant failure [3] . It is one of the most important factors af- fecting the longevity of contemporary joint replacement. Using thermal treatment and high- energy irradiation to achieve highly crosslinked ultrahigh molecular weight polyethylene (a process developed in the 1990s to address wear) greatly re- duces the risk of particulate debris, al- beit at the expense of other critical material properties [4] . HXLPE is less susceptible to oxidative degradation and is more wear resistant compared with non-crosslinked UHMWPE. How- ever, it exhibits lower toughness and ultimate tensile strength, and is more susceptible to fatigue and crack propa- gation, particularly when used in knee arthroplasty. The time-dependent viscoelastic material properties of polymers lead to yielding, creep, and fatigue, result- ing in implant deformity, particularly in cases where the material is used at (or near) its yield stress. In addition, the dynamic mechanical stresses applied to bearing materials are complex and prone to forming heterogeneous stress distributions [5] . Varying load stress- es and kinematics, particularly around the knee joint, taken together with sub- par polymeric bearing materials have in the past led to UHMWPE fracture—with large pieces of polymer breaking away from the implant [6] (Fig. 2). The use of HXLPE in knee arthro- plasty is still a controversial topic and there are few studies that document the clinical performance of HXLPE fol- lowing knee replacement surgery [7] . Ma- terial delamination is a challenge even without the immediate onset of frac- ture and fatigue. Mechanical mismatch and limited conformity between the bearing material and adjacent surface (e.g., metal, ceramic, bone, or cartilage), as well as accumulation of de- bris aggregates, drive delamination. Further, difficulty in mimicking in vivo environments with ex vivo testing is currently a limiting factor in the search for superior polymer- ic bearing materials. Thus, the advancement of ex vivo characteriza- tion methods and test- ing environments must occur alongside development of long-lasting, superior bearing materials. RECENT DEVELOPMENTS AND FUTURE DIRECTIONS Reconstructing hip and knee joints with synthetic components has prov- en to be challenging over the years, and there is significant interest in de- veloping novel technologies to further improve the clinical performance of contemporary polymeric bearing mate- rials. Two prominent clinical challeng- es with orthopedic implants are related to wear-induced aseptic loosening and implant infection. Many recent studies focus on im- proving mechanical properties and wear resistance, enhancing biocom- patibility, and reducing the risk of in- fection and inflammation of polymeric materials. New processing and steril- ization techniques have been devel- oped and continue to be studied to overcome these problems. The studies aim to reduce the fraction of low mo- lecular-weight chains in the polymer, optimize the orientation and compac- tion of polymer chains, and modify and harden the surface of UHMWPE bearing components. Incorporating vitamin E into UHM- WPE reportedly improves wear re- sistance and mechanical strength [8] . Vitamin E is an antioxidant that func- tions as a scavenger, picking up excess free radicals that do not cross-link. Design solutions include identifying (a) (b) (c) (d) Fig. 1 — Total hip and total knee replacement systems: (a) Continuum acetabular system, (b) ADM and MDM acetabular systems, (c) polyethylene components available for the Persona personalized knee system, and (d) tibial trays in their articulating position between the femoral and tibial metal components. Note: Continuum and Persona are trademarks of Zimmer Biomet. ADM and MDM acetabular systems are trademarks of Stryker. (Image adapted from Advanced Polymers in Medicine, 2015; 129-145. Springer, Cham. https://doi.org/10.1007/978-3-319-12478-0_5 ) (a) (b) Fig. 2 — Two cases out of 49 polyethylene-based knee implants with the highest wear scored: (a) at implant revision and (b) at autopsy. (Image adapted from Journal of Orthopaedic Surgery and Research, 2008; 3-8. https://doi.org/10.1186/1749-799X-3-8 )