Hironobu Oonishi

Low-Wear Effect of High-Dose Gamma-Irradiated Crosslinked Polyethylene in Total Hip Prostheses

The wear rate of crosslinked sockets irradiated in air with gamma-irradiation of 100 Mrad, which were used in total hip arthroplasty clinically between 1971 and 1978, was 18% of that of non-crosslinked sockets. As the sockets were irradiated in air, the surface was oxidized. The weight-bearing portion of retrieved crosslinked sockets presented a clearly outlined pattern, irregularly lined with smooth ripples of 0.1 um. Delamination, flaking, burnishing, and scratch were not observed. In our wear tests, using cylinder-on-flat, sphero-flat reciprocating, and hip simulator test, crosslinked ultrahigh-molecular-weight polyethylene (UHMWPE) irradiated with 50 to 1000 Mrad showed extremely low wear, even when the UHMWPE was irradiated in the air and the surface was oxidized. In our hip simulator test with 12 channels using 30% bovine serum as lubricant, crosslinked sockets irradiated with 50 to 150 Mrad, in which the oxidized surface was eliminated, showed no wear. After radiation, the mechanical properties of elongation and tensile strength of polyethylene decreased. The hardness and the ultimate tough strength increased. The chemical properties of gel contents, oxidation (C = O), and double bond (C = C) increased.

Clinical Applications of Hydroxyapatite in Orthopedics

This chapter describes since 1982 the use of porous synthetic hydroxyapatite (HA) granules (0.1 to approximately 1.5 mm) interposed at the cement-bone interface to enhance bone bonding, a surgical procedure labeled interface bioactive bone cement (IBBC). HA granules were smeared on the bone surface just before cementing. Because the HA granules used in IBBC were pure polycrystalline HA, they were scarcely absorbed and their osteoconductive activity can continue indefinitely even after the onset of osteoporosis due to aging and even in conditions of extremely low pathological activity of bone. The appearance rate of radiolucent lines and osteolysis was extremely low even over 30 years when IBBC was used. Since 1986, in an attempt to fill the massive bony defect in the acetabulum at revision surgery of total hip arthroplasty, a mixture of HA granules with a size between 0.9∼1.2 mm and 3.0∼5.0 mm was placed densely and firmly into the bone defects. Bone ingrowth was measured to be over 2.5 cm in full depth and the new bone was very stable. Long-term clinical results over 26 years were excellent. On the weight-bearing area, bone ingrowth over 2.5 cm in full depth can be expected. However, on non-weight-bearing area, bone ingrowth is only 0.5 cm in depth. In large cavities after resection of bone tumors, or after removal of pathological fatty bone marrow at joint replacements, excellent stability to provide long-term strong bony support was obtained by filling HA granules firmly into the defects.

Optimal Wear Performance of Ceramic-on-Ceramic Bearings for Total Hip Replacement

Rigid-on-rigid ceramic bearings have been in continuous use since 1970. With the increased concerns over polyethylene (PE) wear debris promoting osteolysis, there has been a renewed interest in such Al2O3/Al2O3 combinations. In this in vitro study, the wear performance of Al2O3/Al2O3 combinations was compared to Al2O3/PE, using 28-mm Bioceram femoral heads and acetabular inlays with three combinations of diametral tolerances. Wear evaluation was performed in a multichannel hip simulator using physiological load-profiles and 90% bovine serum as the lubricant. At the short-term duration of 3.6 million cycles, approximately 3 years use in the patient, the Al2O3/Al2O3 wear rates were exceedingly low, at less than 0.03 mm3 per million cycles. In contrast, the Al2O3/PE combination produced 20.8 mm3 of polyethylene debris per million cycles. This represented over 600-times more wear debris than the Al2O3/Al2O3 combinations at this time interval. Thus, the relative wear performance of the Al2O3/Al2O3 was far superior to Al2O3/PE with serum lubrication. The simulator wear studies are continuing to 10 million cycles (10 years use in patient), to discriminate between the three diametral-tolerance groups for Al2O3/Al2O3 combinations and to confirm the relative volumetric wear performance relative to the standard Al2O3/PE combination.