CERAMIC ON CERAMIC: WHY TO INCREASE THE DIAMETER

Abstract

Volumetric wear study in metal-polyethylene bearing couple, has demonstrated that the wear rate is reduced by the decrease of the diameter of the ball-heads. On the other hand, small ball-heads introduce some limitations: the stability, the subluxation and the dislocation of the prostheses are directly correlated with the diameter and are often cause of failure.

The crosslinked polyethylene, promising lower wear rates, seems to have higher Function Biological Activity (FBA) because of its smaller but more aggressive particles[1]. In ‘70s, the alumina ceramics has been introduced in the hip prostheses due to its high wear resistance and its bulk material and debris biocompatibility. Laboratory test and long terms clinical experience confirmed that the BIOLOX®forte/BIOLOX®forte bearing couple offers a reduction of two orders of magnitude of the linear wear rate (in vivo results 0.005 mm/year) if compared with metal-polyethylene (0,2 mm/year), it does not produce reaction[2] and it has a high reliability (fracture rate = 0,01% )[3]. Moreover, it has been noticed that for each mm of linear wear of the liner, the correspondent penetration of the head reduces the Range Of Motion (ROM) of approximately 7°.

The low friction coefficient of the alumina, reduces the friction torque and the stresses between the bone and the cup[4]. Greater diameters (32 mm and 36 mm) offer different advantages. The ceramic ball-heads of greater dimensions have better mechanical characteristics. The minimal fracture load of BIOLOX®forte ball-heads (L neck) increases from 55 KN of the 28mm ball-heads to 65 KN of the 32 mm until to 90 KN of the 36 mm ones[5]. This superior characteristics allows to increase reliability (fracture rate of 32 mm ball-heads is 0,004%)[3]. The luxation risk is one of the most important parameter for the reliability of prostheses. Bigger diameter increases the luxation distance and consequently decreases the risk of subluxation and dislocation of the prostheses. The ROM is directly correlated to the diameter of the head. With ball-heads of 36 mm it can be caught up a ROM of approximately 136°. This is an advantage for the reliability of the system because it reduces the risk of impingement that is often cause of failure[6,7]. The lengths of the neck (+/− 3,5 mm for 28 mm ball-heads) can be increased. With 32 mm ball-heads there is an excursion of +/− 4 and the increase of the diameter reduces the necessity of a XL neck.

Due to its biocopatibility, low wear characterisitcs and mechanical aspect, ceramic on ceramic bearing couple of bigger diameter seems to be the right solution for long terms results for active and young patients.