John Dumbleton

The basis for a second-generation highly cross-linked UHMWPE

Highly cross-linked ultra-high molecular weight polyethylene (UHMWPE) decreases wear at the hip by more than 50% compared with conventional UHMWPE. However, melted highly cross-linked polyethylene may be susceptible to fatigue cracking, and annealed highly cross-linked polyethylene may be susceptible to in vivo oxidation. The second-generation highly cross-linked UHMWPE (X3 HXPE) uses a sequential irradiation and annealing process. It preserves mechanical strength properties and has the highest survivorship in functional fatigue testing. The free radical content is low, and its performance under accelerated aging is the same as virgin UHMWPE. Hip simulator studies with 32-mm acetabular components demonstrated 97% wear reduction compared with conventional UHMWPE, and 62% compared with a clinically successful first-generation annealed highly cross-linked polyethylene. The crystallinity, density, and tensile strength of the X3 HXPE material was unchanged by oxidative challenge. X3 HXPE material articulating on cobalt-chromium alloy yields a volumetric wear rate very similar to that of metal-on-metal articulations, but eliminates the concerns of metal ion release. Wear particles generated from the X3 HXPE were the same size as those produced from conventional UHMWPE. Preliminary results suggest X3 HXPE can be used for cups larger than 36 mm.

Hydroxyapatite-Coated Prostheses in Total Hip and Knee Arthroplasty

Hydroxyapatite-coated implants have demonstrated extensive bone apposition in animal models. The osseous interface develops even in the presence of gaps of 1 mm and relative motion of up to 500 mum. Development of implant-bone interfacial strength is due to the biological effects of released calcium and phosphate ions, although surface roughness leads to increased interface strength in the absence of interface gaps. The clinical results at fifteen years after total hip replacements have demonstrated that hydroxyapatite-coated femoral stems perform as well as, and possibly better than, other types of cementless devices, with the added benefit of providing a seal against wear debris. Hydroxyapatite-coated acetabular components must have a mechanical interlock with bone in order to take advantage of the coating effects. Clinical analyses of these types of designs at seven years have indicated good survivorship. The performance of a hydroxyapatite-coated implant depends on coating properties (thickness, porosity, hydroxyapatite content, and crystallinity), implant roughness, and overall design. The most reliable predictor of the performance of a device is success in long-term clinical studies.

A Highly Crosslinked UHMWPE for CR and PS Total Knee Arthroplasties

X3 is a highly crosslinked ultrahigh molecular weight polyethylene (UHMWPE) produced by a sequential irradiation and annealing process. The sequential process results in a material with a free radical content that is 1% that of conventional UHMWPE gamma sterilized in nitrogen resulting in an oxidation resistance similar to that of virgin UHMWPE. Yield strength and ultimate tensile strength exceed American Society for Testing and Materials minimum rates for UHMWPE. Simulator testing of contemporary cruciate retaining (CR) and posterior-stabilized knee inserts (Triathlon) manufactured by the sequential process demonstrated 68% and 64% less wear, respectively, compared to conventionally processed inserts. The wear and mechanical integrity of sequentially processed posterior-stabilized inserts was unaffected by accelerated aging, whereas conventional UHMWPE exhibited increased wear, cracking, and delamination.

Calcium Phosphates: A Survey of the Orthopaedic Literature

For over two decades, calcium phosphates have been the focus of many laboratory and clinical investigations (1–8). Particular interest has surrounded calcium hydroxyapatite (HA), a naturally occurring calcium phosphate present in tooth enamel and vertebrate bone. In the early 1980’s, the dental community began using HA blocks and coatings to augment bone and encourage fixation in restorative dental procedures ; the chemical stability and excellent biocompatibility of HA made it an attractive material choice (1, 5, 9, 10). Subsequently, the orthopaedic community investigated and began using HA for bone defect obliteration and as an implant coating, with encouraging results (3, 11, 12). More recently, attention has been given to biphasic calcium phosphates (BCP), which combine HA and tricalcium phosphate (TCP) in different ratios. Solid, porous, and granular forms of HA and BCP materials have been employed for filling defects (13–16). Current studies involving the use of block HA as a drug delivery system and the use of HA with either bone cement or growth factors all show promise, as do HA composite materials, such as BCP (17–19).

Chapter 14 – Highly Crosslinked and Annealed UHMWPE

Publisher Summary This chapter explores the sequential irradiation and annealing process that is shown to eliminate free radicals more effectively and form crosslinks than a single cumulative radiation dose of the same magnitude followed by annealing. The sequential and annealing process produces a microstructure that is not significantly different from that of ultra-high molecular weight polyethylene (UHMWPE) gamma sterilized in nitrogen (N2/Vac). The use of sequential irradiation and annealing provides the same level of crosslinking at a lower radiation dose with a consequent improvement in mechanical properties that allows X3 to be used for total knee replacement (TKR). The experience with highly crosslinked UHMWPEs over the last decade shows a progressive development in understanding the relationship between UHMWPE structure (crosslink density and crystallinity) and properties (strength, ductility, and wear). Highly crosslinked UHMWPEs were introduced clinically in the 1970s by Oonishi and coworkers and by Grobbelaar and associates, and reintroduced by Wroblewski and colleagues in the 1980s. The crosslinking methods used were not based directly on knowledge of UHMWPE structure–property relationships, and clinical use was limited. There was no commercial introduction, but these materials are of historical interest.