Abstract
This study of retrieved 28 mm Metasul™ (cemented) revealed for the first time adverse wear effects created by impingement-subluxation in MOM. The 10 cases selected (with femoral stems) had annual follow-up 3–11 years. (1) Unequivocal evidence here shows that all heads routinely subluxed from the Metasul liners.
Femoral stems revealed well-demarcated notches (DN) on necks and trunnions (Fig. 1a: n = 6), shallow cosmetic blemishes (Fig. 1b CB: n = 4), and abrasion by cement (Fig. 1b: PMMA). As demonstrated by EOS radiographs, impingement locations varied with implant positioning, pelvic mobility and patient functionality – both anterior and posterior notching (Fig. 1). The first impingement notch occurred with head located (Fig. 2a), whereas the head had subluxed from the cup at 2nd notch (Fig. 2b). The model demonstrated that patients gained 20° motion by such head-subluxation manoeuvres. It was surprising that there was no collateral damage evident on the liners. Even with severe notching of Ti6Al4V and CoCr stems, the cup rims generally appeared well-polished.
Femoral heads revealed macro-stripe damage on articular surfaces (Fig. 3), as did cups. Basal and polar macro-stripes on heads were always located at hip impingement positions. The equatorial stripes were formed at main-wear zone boundaries. Thus equatorial stripes were likely created by some form of rim-impact damage (micro-separation) or by local ingress of 3rd-body wear particles under the cup rim.
Micro-grooving was evident within these macro-size stripes and frequently featured large raised lips (Fig. 3), interpreted as signs of adverse 3rd-body wear mechanisms, and rarely described.(2) It would appear that large metal particulates were released during MOM impingement-subluxation manoeuvres and circulated the hip joint to producing severe 3rd-body abrasion. Gradual decomposition of such large debris to nano-sized particulates under joint loading would then produce the often-referenced ‘self polishing’ effect of CoCr. EDS studies revealed metal smears on the CoCr surfaces containing the elements of titanium alloy (Ti, Al, V). This was further evidence of impingement-subluxation manoeuvres.(1, 3)
In-vivo cup wear patterns also appeared much larger than those produced in MOM simulators. Such differences likely reflected head-subluxation in vivo, whereby heads unconstrained by the subluxation maneuver were free to orbit up and even cross cup rims, i.e. “edge wear”. This appears to be the first study detailing the adverse wear mechanisms in MOM bearings. There are two limitations to our retrieval study, a) these wear results may not be representative for all MOM designs, and b) it is unknown whether such results have relevance to MOM cases continuing successfully.
