R. Nassutt

The acting wear mechanisms on metal-on-metal hip joint bearings: in vitro results

Metal-on-metal (MOM) hip joint bearings are currently under discussion as alternatives to metal-on-polymer (MOP) bearings. Some criteria under scrutiny are the wear resistance, the influence of wear particles on the surrounding tissue, as well as the frictional torque. In order to understand and control the wear behavior of such a bearing a close correlation between the microstructures of the alloys used and the acting wear mechanisms has to be found. Thus, commercially available CoCrMo-balls were tested against self mating concave pins in a physiological fluid at 37°C under reciprocating sliding wear (1 Hz). The compressive load was 750 N (body weight). For 2×106 cycles tests were carried out continuously and with periodically occurring resting periods. On the basis of the observed wear appearances the acting wear mechanisms are defined and evaluated as to their contribution to the wear behavior. Due to the high local contact stresses surface fatigue prevails initially. Cr– and Mo–carbides are fractured and torn off the surfaces bringing about additional surface fatigue by indentations and initiating abrasion. The weight loss can be predominately attributed to these mechanically dominated wear mechanisms. In a parallel occurring tribochemical reaction layers are generated from denatured proteins. These adhere rigidly to the surfaces and cover parts of the contacting surfaces avoiding adhesion. Thus, the wear behavior is mainly influenced by the alternating balance between surface fatigue and abrasion on the one side and by tribochemical reactions on the other side.

Long-term in vivo alterations of polyester vascular grafts in humans.

OBJECTIVES To examine the influence of in vivo hydrolysis on the physical properties of polyester grafts and their correlation to the period of implantation in the human body. MATERIALS AND METHODS Sixty-five explanted vascular grafts were obtained after 0-23 years of implantation due to suture aneurysms (18), occlusion (12), graft infection (12), failure of graft material (7) and post-mortem (16). The surface was examined by scanning electron microscopy, the molecular integrity by infra-red spectroscopy and physical strength by probe puncture. RESULTS Scission of macromolecular chains and loss of strength were shown. It was demonstrated that hydrolytic degradation of polyester takes place with increasing time of implantation in humans. Analysis by linear regression showed that polyester grafts lose 31.4% of their bursting strength in 10 years and 100% in 25-39 years after implantation. CONCLUSIONS Regular follow-ups of patients with aged vascular grafts and the precise documentation of implanted materials are necessary to estimate graft degradation.

The influence of resting periods on friction in the artificial hip

Insufficient tribologic performance of total joint components is a major cause of prostheses failure. Wear has been studied intensively using testing machines that apply continuous motions. Human locomotion, however, is not well represented by continuous motions alone. Singular events and resting periods are a substantial part of daily activities. Resting does influence adhesion in the artificial joint with possible effects on friction, wear, and loosening. The current study evaluated the effects of resting on the frictional properties of hip prosthesis components. The activity measurements of 32 patients with artificial hip replacements were analyzed for resting durations of the hip. A pin-on-ball screening device was used to determine friction after characteristic resting periods and during continuous oscillating motion. All common articulation pairings were investigated. Prolonged and frequent resting periods of the hip were found for the patients. Initial friction increased with increasing resting duration for all tested materials (between 41% and 191%). The metal-on-metal articulations showed the highest friction level (0.098 for sliding) and the highest increase (191%) in friction with resting duration (0.285 after resting periods of 60 seconds). A high static frictional moment after resting periods might present a risk for aseptic implant loosening. Therefore, large head diameters of metal-on-metal joints should be used with caution, especially when additional unfavorable risk factors such as obesity, weak bone-implant interface, or high activity level are present.

Investigation on stick phenomena in metal-on-metal hip joints after resting periods

Abstract Insufficient understanding of tribological behaviour in total joint arthroplasty is considered as one of the reasons for prosthesis failure. Contrary to the continuous motion input profiles of hip simulators, human locomotion contains motion interruptions. These occurring resting periods can cause stick phenomena in metal-on-metal hip joints. The aim of the present study was to investigate the tribological sensitivity of all-metal bearings to motion interruptions on in vitro test specimens and retrieved implants. Friction and wear with and without resting periods were quantified. Unlike the metal-on-polyethylene joints, the static friction of metal-on-metal joints increased up to μs = 0.3 with rest, while wear appeared to be unaffected. This effect is caused by the interlocking of firmly adhered carbon layers, which were generated from the protein-containing lubricant through tribochemical reactions. Since more than 80 per cent of the retrieved implants exhibited macroscopically visible carbon layers, the increase in friction presumably also occurs under physiological conditions, which is then transferred to the bone-implant interface. These recurrent tangential stress peaks should be considered for the design features of the cup-bone interface, in particular when larger-sized implant heads are used.

Influence of interface condition and implant design on bone remodelling and failure risk for the resurfaced femoral head

Resurfacing of the femur has experienced a revival, particularly in younger and more active patients. The implant is generally cemented onto the reamed trabecular bone and theoretical remodelling for this configuration, as well as uncemented variations, has been studied with relation to component positioning for the most common designs. The purpose of this study was to investigate the influence of different interface conditions, for alternative interior implant geometries, on bone strains in comparison to the native femur, and its consequent remodelling. A cylindrical interior geometry, two conical geometries and a spherical cortex-preserving design were compared with a standard implant (ASR, DePuy International, Ltd., UK), which has a 3° cone. Cemented as well as uncemented line to line and press-fit conditions were modelled for each geometry. A patient-specific finite element model of the proximal femur was used with simulated walking loads. Strain energy density was compared between the reference and resurfaced femur, and input into a remodelling algorithm to predict density changes post-operatively. The common cemented designs (cylindrical, slightly conical) had strain shielding in the superior femoral head (>35% reduction) as well as strain concentrations (strain>5%) in the neck regions near the implant rim. The cortex-preserving (spherical) and strongly conical designs showed less strain shielding. In contrast to the cemented implants, line to line implants showed a density decrease at the centre of the femoral head, while all press-fit versions showed a density increase (>100%) relative to the native femur, which suggests that uncemented press-fit implants could limit bone resorption.

Friction and Wear Properties of Metal/Metal Hip Joints

Insufficient understanding of tribological behaviour of total joint components is considered as one of the major reasons for premature prosthesis failure. In the previous years, wear modes and mechanisms of artificial hips have been intensively studied using testing machines applying continuous motion and load profiles. Human locomotion, however, is of erratic shape containing motion interruptions among other irregularities. These occurring resting phases could influence adhesive friction at the artificial articulation with possible effects on wear and loosening. Initial frictional behaviour after resting periods and subsequent wear of metal-on-metal hip components have been investigated. While the initial frictional moment of artificial hip components was found to increase significantly with increasing resting duration, wear was hardly affected. These results therefore rather point to an interlocking phenomenon than adhesion. Still, the results are of concern because the increase in friction is likely to occur under physiological conditions too and may cause tangential stress peaks at the bone/ implant interface accelerating fatigue failure. In particular, big head sizes should be used with caution because the frictional moment is a function of coefficient of friction and ball diameter. Application of a Novel Testing and Analysis Method Durch Verschleis in der Artikulation erzeugte Abriebpartikel werden als Hauptursache des fruhzeitigen Versagens kunstlicher Huftgelenke angesehen. Verschleis und dessen Mechanismen wurden in den vergangenen Jahren intensiv auf Huftsimulatoren untersucht. Allerdings fanden diese Untersuchungen zeitgerafft, mit ununterbrochenen Bewegungs- und Lastkollektiven statt. Betrachtet man hingegen menschliche Bewegungsablaufe, beinhalten jene Unterbrechungen und andere Unregelmasigkeiten. Diese Ruhephasen beeinflussen unter Umstanden die Adhasionsneigung des Kunstgelenkes, was wiederum Auswirkungen hinsichtlich Verschleis und Lockerung der Prothese haben konnte. In dieser Studie wurde deshalb die tribologische Sensitivitat der Metall/Metall-Artikulation hinsichtlich Bewegungsunterbrechungen untersucht. Obwohl das Losbrechmoment nach Ruhephasen betrachtlich ausfiel, bewirkte dies keinerlei zusatzlichen Verschleissanstieg der Metall/Metall-Artikulation und ist deshalb eher als mechanisches Verzahnen, denn als Folge von Mikroverschweissungen zu interpretieren. Da dieser initiale Reibwertanstieg jedoch im Bereich physiologisch relevanter Ruhephasen zum Tragen kommt, gebuhrt ihm entsprechende Aufmerksamkeit. Die wahrend des Bewegungsstarts entstehenden Reibmomentspitzen konnen die Knochen/ Implantatverbindung von Becken und Pfanne gefahrden, indem sie Ermudung bewirken. Da das Reibmoment eine Funktion von Reibwert und Prothesenkopfdurchmesser des Kugelgelenks darstellt, sind grosse Kopfe mit Vorsicht zu verwenden.

Influence of cooling on curing temperature distribution during cementing of modular cobalt-chromium and monoblock polyethylene acetabular cups.

Total hip replacements for older patients are usually cemented to ensure high postoperative primary stability. Curing temperatures vary with implant material and cement thickness (30°C to 70°C), whereas limits for the initiation of thermal bone damage are reported at 45°C to 55°C. Thus, optimizing surgical treatment and the implant material are possible approaches to lower the temperature. The aim of this study was to investigate the influence of water cooling on the temperature magnitude at the acetabulum cement interface during curing of a modular cobalt-chromium cup and a monoblock polyethylene acetabular cup. The curing temperature was measured for SAWBONE and human acetabuli at the cement–bone interface using thermocouples. Peak temperature for the uncooled condition reached 70°C for both cup materials but was reduced to below 50°C in the cooled condition for the cobalt-chromium cup (P = .027). Cooling is an effective method to reduce curing temperature with metal implants, thereby avoiding the risk of thermal bone damage.