Abstract
Introduction and Objective
Local cartilage defects in the knee are painful and mostly followed by arthritis. In order to avoid impaired mobility, the osteochondral defect might be bridged by a synthetic compound material: An osteoconductive titanium foam as an anchoring material in the subchondral bone and an infiltrated polymer as gliding material in contact with the surrounding natural cartilage.
Materials and Methods
Titanium foam cylinders (Ø38 mm) with porosities ranging from 57% to 77% were produced by powder metallurgy with two different grain sizes of the space holder (fine: 340 ± 110 μm, coarse: 530 ± 160 μm). The sintered titanium foam cylinders were infiltrated with UHMWPE powder on one end and UHMWPE bulk at the other end, at two different temperatures (160 °C, 200 °C), using a pressure of 20 MPa for 15 minutes. Smaller cylinders (Ø16 mm) were retrieved from the compound material by water jet cutting. The infiltration depths were determined by optical microscopy. The anchoring of the UHMWPE was measured by a shear test and the mechanical properties of the titanium foam were verified by a subsequent compression test. The tribological behaviour was investigated in protein containing liquid using fresh cartilage pins (Ø5 mm) sliding against a UHMWPE disc with or without a notch to simulate the gap between the implant and the surrounding cartilage. Friction coefficients were determined in a rotation tribometer and the cartilage wear in a multidirectional six-station tribometer from AMTI (load 10 – 50 N, sliding speed 20 mm/s, 37 °C).
Results
UHMWPE could be infiltrated into titanium foam by 1.1 – 1.3 mm with fine pores and by 1.5 – 1.8 mm with coarse pores. The infiltration was neither dependent on the type of UHMWPE (powder or bulk) nor on the temperature. The polymer was so well anchored inside the titanium foam pores that the shear forces for the compounds exceeded the shear strength obtained for a UHMWPE-cylinder. This effect was due to the increased stiffness of the compound plug. Uniaxial compression of the titanium foams after the shear-off of the polymer revealed yield strengths ranging from 50 – 88 MPa for porosities of 62 – 73%. The Ø16 mm samples yielded beyond physiological loads in the knee (≥ 10x body weight) and behaved in a strain hardening and fully ductile manner, reaching deformations of at least 50 % of their initial height without the appearance of macroscopically visible cracks. For smaller plug diameters down to Ø8 mm, however, the lower porosity / higher strength foam should be used to limit elastic deformation of the compound to < 0.1 mm. Pore size did not significantly influence the strength and stiffness values. The elevated coefficient of friction between cartilage and UHMWPE of about 1 was not negatively affected by the presence of the gap. The height loss of the cartilage pin after 1 hour (respectively after 3600 reciproque wear cycles) was 0.2 ± 0.1 mm using a flat disc. For discs with a 1 mm wide V-notch, the wear increased to 0.9 ± 0.3 mm.
Conclusions
The tested titanium foams are well suited to act as an anchoring material in the subchondral bone as mechanical properties can be tailored by choosing the adequate porosity and as bone ingrowth has previously been demonstrated for the used pore sizes. UHMWPE is not an ideal gliding partner against cartilage because the friction coefficients of frictions were high. The presence of a V-notched gap was detrimental for cartilage wear. More hydrophilic polymers like PCU should be tested as potential gliding materials.
