The current generation of hard-on-hard hip replacement bearings was designed to minimize wear. This was achieved not only by using low abrasion cobalt-chrome and ceramic materials, but also by promoting hydrodynamic lubrication, which necessitates smooth surfaces, low clearance between the articulating components, and larger diameters.1 Theory was supported by hip simulator testing.2 However, current clinical results have been less convincing, and national registries show that large diameter metal bearings have the highest revision rate of any bearing.3 Failure is blamed primarily on wear of the bearing surfaces, which leads to adverse tissue reaction.4 In vivo conditions are different to those employed in simulators during pre-clinical testing, and this is related, in part, to suboptimal component placement during surgery.5 A large retrieval study demonstrated a relationship between excessive bearing surface wear and high cup inclination.6 This finding was replicated in simulator studies.7
Additional failure mechanisms for such bearings have also been suggested. These include wear and corrosion of the taper junction between the head and the stem,5, 8 and loosening of press-fit cups.6 For these failure scenarios to occur, the respective interfaces must become overloaded. In this study, we hypothesized that high friction moments in large diameter bearings play an important role in these failures. Friction factors of around 0.15 have been measured in simultors for large diameter metal bearings in lubricated conditions,9 which translates to a joint friction moment of 7.5 Nm for a bearing with 50 mm diameter during gait loading of 2000N for average body weight.10 See below for the definition of friction factor. Although the joint moment and taper axes are not aligned, moments of this magnitude might challenge axial taper capacities and also cup anchorage capacities. Furthermore, local relative interface motion is likely to occur at moments below those necessary for overloading of the complete interface. Moments will increase even further as the lubrication mechanism is compromised, which has been reported for high cup inclination or anteversion.
So far, little clinical experience exists with large diameter ceramic bearings. They have the potential for lower wear and less toxic particles than metal bearings.11 Volumetric wear rates have been found to be an order of magnitude lower for ceramic bearings than metal implants, and to remain low even at high cup angles.12 However, the inclusion of ceramic chips increased the friction factor by 26 times the normal condition in serum, demonstrating that disrupted lubrication can lead to dangerously high moments.
In this study, we proposed that friction moments for large diameter ceramic-on-ceramic bearings are higher than those for large diameter metal-on-metal designs in compromised lubrication conditions. Therefore, moments were measured in a hip simulator for medium and large diameter metal and ceramic bearings at varying cup inclination angles in serum and in dry conditions as a hypothetical worst case.
Bearings were tested in a hip simulator described in detail elsewhere.9 The device is mounted on a standard servohydraulic mechanical testing machine, which provides the joint force and to which the head is mounted and is forced downwards in the cup (Fig. 1). The cup is cemented into a container that oscillates around a horizontal axis running through the joint center to simulate walking kinematics. The cup inclination angle can be varied by rotation around a third orthogonal axis, also running through the joint center. The container can be filled with lubricant so that the bearing is fully submerged.
Loading consisted of a sequence of static and dynamic joint force and motion combinations (Fig. 2). The sequence represents two-legged stance (650N; “2” in Fig. 2), one-legged stance (1700N; “3”), and walking (2000N; “4”). The walking conditions represent 10 gait load cycles, according to those measured in patients,10 which have been studied previously.9 Simplified flexion-extension motion was sinusoidal with an amplitude of ±20°. The initial and final sets of increasing and decreasing force levels were used to check for symmetry of the moment measurement (“1” in Fig. 2). Measurements of the last 10 cycles at a constant high joint force of 1700N (“6” in Fig. 2) representing the one-legged stance phase of gait are presented.
Two clinically used bearing designs were investigated. A resurfacing metal bearing (Adept®, Finsbury Orthopaedics Ltd., Surrey, UK) was tested with 40 and 50 mm bearing diameters. A composite ceramic bearing (Delta Motion®, DePuy Orthopaedics Inc., Warsaw, IN) was tested with a modular head and 32 and 48 mm bearing diameters. Five samples of each type and size were tested, except for the dry metal condition, for which only three samples per type were available.
Twenty-five percent fetal bovine serum (“Gold” Kraeber GmbH & Co., Germany, 3.5–4.5 g/dl protein before dilution: 86% serum albumin/14% globulins) was used at room temperature. Prior to each test, bearings were thoroughly washed in water, followed by 96% ethanol, and then rinsed again in water. For dry conditions, bearings were washed similarly (after testing in serum), followed by drying gently with tissue paper and for 5 min in air at room temperature, mounted in the testing machine.
Each bearing was tested at five discrete cup angles, first in lubricant and then dry, in the following sequence: 0°, 15°, 33°, 45°, 60°, and repeat 33° (Fig. 3). The cup angle represents the relationship between the polar axis of the cup and the joint force vector. The 33° condition represents a standard physiological relationship between the joint force vector acting at 12° to the vertical, measured during gait,10 and a 45° clinical cup angle (45°–12° = 33°), similar to the generally intended inclination angle of the acetabular plane.13 The orthogonal relationship between the applied joint load and rotation axis is a necessary simplification for the equipment used. Zero degrees represents a cup with an extremely shallow cup angle (12° inclination clinically) that would protrude laterally from the superior aspect of the acetabulum. Sixty degrees represents a steep cup inclination (72° inclincation clinically), even steeper than the mean angle of 59° for retrieved metal bearings demonstrating “edge wear”.6
Measurements for the 33°cup angle were repeated once for each bearing (termed 33° and 33° [repeat]), both lubricated and dry, to check for repeated measures effects. The metal bearings were tested dry only at 33° followed by 60°.
Output Parameters and Data Analysis
Peak moments were sampled for 10 consecutive cycles, and the mean moment M was calculated. The friction factor µ, based on joint force F and radius R, was calculated from M according to µ = M/(R·F). To confirm that there was no effect of repeated testing of each bearing, a paired t-test was used to compare moments at 33° and 33° (repeat) for each bearing material/diameter/lubrication condition separately. The influence of cup inclination on friction factor was investigated by ANOVA, with material/diameter/lubrication conditions tested for homogeneous subgroups. The effect of material, diameter, and lubrication condition on friction factor was tested by ANOVA for the 33°cup angle only. The probability of type I error was set to 5%. PASW Statistics 18.0 was used for the calculations.
No significant difference was observed between friction factors measured at 33° or 33° (repeat) for any bearing (0.10 < p < 0.45, Fig. 4). The mean ratios between the first 33° measurement and 33° (repeat) ranged from 0.81 (for 48 mm ceramic in serum) to 1.20 (for 48 mm ceramic dry) for all bearings. The overall mean ratio for all measurements was 1.01 ± 0.12.
The larger diameter (48 mm) ceramic bearing in serum was the only combination of material and lubrication condition with a significant effect of cup angle (Fig. 4). An increase in friction factor was observed at the highest inclination of 60°. For this condition, the friction factor increased by a ratio of 2.0 ± 0.7 to µ = 0.11 ± 0.04 (M = 4.6 Nm) from a mean of µ = 0.06 ± 0.02 (M = 2.4 Nm) for all other cup angles (p = 0.01). The smaller diameter ceramic bearing (32 mm) showed a similar, weak tendency (ratio: 1.4 ± 0.5, p = 0.47). Other bearings did not show an influence of cup orientation (mean ratio: 1.0 ± 0.1).
The overall highest friction factors were observed for ceramic bearings in dry conditions (Fig. 4). At a physiological cup angle of 33°, the moments reached Mmax = 26.5 Nm for the larger diameter and Mmax = 16.0 Nm for the smaller diameter (mean friction factors: µmean = 0.58 ± 0.08 and µmean = 0.49 ± 0.14, respectively; Table 1, Fig. 4). Mean friction factors for the metal bearings in dry conditions were 1.7–2.1 times lower than for ceramic bearings. The opposite was observed in the lubricated condition, where mean friction factors for ceramic bearings were found to be 1.5 (smaller diameter) to 3.2 times (larger diameter) lower than those for metal bearings (Table 1).
Table 1. Moments and Friction Factors for Each Combination of Bearing Material, Lubrication Condition and Diameter (Big/Small: for Metal 50/40 mm; for Ceramic: 48/32 mm), at a Physiological Cup Angle of 33°
Friction Factor [−]
Larger Diameter (Ø)
p values are given for large-small diameter comparison (n = 5, except n = 3 for metal–metal dry).
At the physiological cup orientation (33°), the friction moments for metal bearings increased by a ratio of 1.9 between serum and dry conditions for the smaller diameter bearing (Table 1), and by 1.7 for the larger bearing (Table 1). For ceramic bearings, the consequence of loss of lubrication was much greater, with an increase by a ratio of 10.1 between serum and dry conditions for the larger diameter bearing and 4.9 for the smaller diameter bearing (Table 1).
No significant effect of bearing diameter on friction factor was found at 33° cup angle for the metal bearings, either in serum or dry (p = 0.372 and p = 0.725, respectively, Table 1). For the ceramic bearings in the lubricated condition, the larger diameter showed a significantly lower friction factor than the smaller diameter bearing (µ = 0.051 ± 0.022 and µ = 0.096 ± 0.028, respectively, p = 0.001, Table 1). Ceramic in dry conditions showed a non-significant tendency in the opposite direction (Table 1, p = 0.12).
Audible squeaking occurred only in dry conditions. It was mild and intermittent for the metal bearings, occurring only at medium joint loads. For the smaller ceramic bearing, squeaking was louder and occurred more continuously at medium joint loads. Squeaking was loudest for the larger ceramic bearing, and was continuous and independent of load.
Fine scratching was observed on the metal components after testing in serum. Scratches on the cup reflected the range of oscillating motion (±20°) for respective cup angles. On the head, similar fine scratches were localized at the pole over an area of about 10 mm in diameter. With dry testing of the metal bearings, the fine scratches became deeper matt striations on both surfaces (Fig. 5). Less damage was observed for the dry ceramic bearings. The surface remained shiny, and rubbing with pencil graphite revealed only single scratches on the cup in some cases.
Large diameter (≥36 mm) ceramic-on-ceramic bearings are currently being introduced clinically as an alternative to large metal bearings, eliminating the danger of adverse reactions to metallic debris, while maintaining the advantage of resistance to dislocation. However, other suggested failure mechanisms of large metal bearings, such as loosening of the acetabular cup interface and wear of the taperlock with the stem, may be related to high moments, which are not exclusive to metal articulations and can occur with any hard-on-hard bearings with diminished lubrication. Our results show that, in the lubricated condition, large ceramic bearings can provide superior friction characteristics to metal bearings, with friction factors two times lower. However, without lubricant, maximum friction moments for ceramic bearings increased to more than double those for large metal bearings (26.0 Nm vs. 12.1 Nm, respectively, at a physiological cup angle of 33° [45° clinically]). Problems related to friction moments in large metal bearings could thereby become even more acute for large diameter ceramic bearings in extreme conditions.
The revival of large diameter metal bearings has become controversial due to their high revision rates.3 Their failure mechanism remains inconclusive, but primarily involves metal wear.4, 5, 14 This has been related to compromised lubrication at steep cup angles. Compromised lubrication may also generate joint moments sufficiently large to turn out cups, which has been reported clinically for large diameter metal bearings6 and for ceramic bearings.15
Another unresolved issue relates to wear of the taper connection of large diameter modular metal heads and the stem.5, 8 This may also be related to high joint moments. Turn-off capacities of the modular taper between head and stem can be less than 10 Nm along the taper axis.16 The friction moments and turn-off capacities cannot be directly compared since the joint load and the taper axes are not aligned. However, the first clinical failure was recently reported due to wear of the ceramic head on the stem interface for a 36 mm ceramic bearing,17 highlighting the relevance of the issue also to ceramics. High friction moments could also contribute to overload of the femoral bone-implant interface, particularly for shorter implant designs.18
Standard Sized Bearings
Maximum moments measured for the ceramic bearings in serum at a physiological cup angle of 33° (45° clinically) were similar for the larger and smaller diameters (3.7 and 3.4 Nm, respectively). These magnitudes are similar to moments for smaller 28 mm diameter metal-on-metal bearings,9, 19 as well as metal-on-polyethylene in similar conditions,20 and as such would not be expected to cause problems clinically.
Effect of Cup Inclination
Runaway wear in large metal bearings has been related to high cup inclination angles6 and has been reproduced in hip simulators.21 Lubrication becomes compromised as the joint force vector approaches the edge of the cup. Although friction is not directly related to wear,1 both factors would be expected to increase in bearings with compromised lubrication. However, friction moments measured for the metal bearings in serum were not affected by cup inclination in our study. This could be due to poor lubrication, even at physiological cup angles during the running-in period represented by this short-term study, with no further disruption possible by inclining the cup. Hydrodynamic lubrication is proposed to be essential for low wear in large metal bearings and becomes increasingly effective with increasing bearing diameter.1, 2 Our finding that friction factor was independent of diameter for metal bearings in serum (µ[mean] ≈ 0.15) suggests that there was no hydrodynamic lubrication. Instead, boundary lubrication for the metal bearings was provided by the serum, since the friction factor almost doubled to µ(mean) ≈ 0.28 in dry conditions.
In contrast, hydrodynamic lubrication is suggested for the larger ceramic bearing in serum at physiological angles, since the highest cup angle (60° representing 72° clinically) caused the friction factor to double. This indicates disrupted lubrication, with friction factors approaching those for the metal bearings in serum. However, even at steep cup angles the lubrication was not completely disrupted, since friction factors for the ceramic bearings increased even further, by more than five times, in unlubricated conditions.
The dry condition used to represent an extreme adverse case may in fact represent extreme situations occurring in vivo. Charnley20 performed his original bearing tests under constant load in both lubricated and dry conditions, so as to include the worst-case situation in his friction measurements. This becomes further justified by the squeaking phenomenon occasionally observed clinically for ceramic bearings, which can be reproduced in the laboratory in dry conditions.22 Under lubricated conditions squeaking has been reproduced in the laboratory only with metal transfer23 (increasing friction) or by the interposition of a third body ceramic chip,12 which increased the friction coefficient to values even higher than those presented in our study.12 The dry condition in the simulator is unlikely to match the in vivo conditions exactly, where some form of body fluid is expected in or around the joint. Clinically, the dry condition might occur locally at the contact area between head and cup, rather than over the whole joint area.
For clinically established ceramic bearings with smaller diameter (28 and 32 mm) large variations in squeaking rates have been reported, with no evidence of a relationship between squeaking and failure. Increased wear has been reported for clinically squeaking retrievals,24 but long term revision rates are as low as those for any other conventional bearing.3 Thus, despite potentially high friction coefficients for poorly lubricated (squeaking) ceramic bearings, their relatively small diameter may generate moments safely below the system capacity. Likewise, conventional metal-on-metal bearings with smaller diameters might survive compromised fluid lubrication conditions due to their smaller effective moment arm, while moments in larger bearings might exceed the taper capacity in similar conditions. This may partly explain why large diameter modular metal bearings perform worse than smaller diameters.3 Consequently large diameter ceramic bearings might demonstrate similar problems to large diameter metal bearings in adverse conditions, since unlubricated friction factors were about two times higher than for metal bearings in this study. On the other hand, the better wettability of ceramics may provide greater protection against drying out than for metal bearings.
This study is limited by the simplified testing conditions. Measurements were short term, employed a constant joint force, and kinematics were in a single plane. The friction moments measured might therefore be higher than those in well-functioning hips. Friction factors were somewhat higher than in other publications, which may be due to different test devices or loading conditions. For example, a reduced swing-phase load was used in other studies, which reduces friction moments.12, 19, 25 Since physiological conditions are unknown, and in vivo joint moment measurements unavailable, no direct clinical comparison can be made.
The lubricant used was only a representation of synovial fluid. Although surfaces were thoroughly cleaned after testing in serum, some protein may have remained on the surfaces during dry testing. However, surfaces were not observed microscopically in this study. Directly post-operatively a bearing is likely to be lubricated (perhaps by blood serum), and a layer of protein will probably form on the surfaces26 prior to disruption of the lubrication some time later, for whatever reason. The dry condition has not been explicitly demonstrated to occur clinically. The evidence cited is that clinical squeaking can only be replicated in extreme conditions: dry22 or with dried serum,27 with metal transfer23 or third body ceramic chips.28 Large ceramic bearings are relatively new and reports of their clinical performance are not yet available.
Ceramic bearings demonstrated lower friction factors than metal bearings in lubricated conditions, with the largest sized bearing exhibiting the lowest friction factor, similarly to other studies. Our study focused not on optimum lubrication but on an extreme condition with no lubrication. The possibility of this condition occurring during rim loading of the bearing is speculated to be a major factor for the high clinical failure rate of large metal-on-metal articulations. As anticipated, friction moments increased twofold for metal bearings in dry conditions, but by more than fivefold for 48 mm diameter ceramic bearings, reaching a maximum torque of over 25 Nm. But these high values can only occur if lubrication is heavily compromised, for whatever reason. In such a condition, larger diameter ceramic bearings may also amplify the clinical problem of squeaking. The advantage of large diameter ceramic bearings in lubricated conditions must be carefully weighed against the potentially higher risk in adverse conditions, which might occur in some clinical cases. The rate of incidence of such adverse situations in patients is unknown and cannot be estimated. The better wettability of ceramics might provide greater protection against loss of lubrication than for metal bearings.
This study was supported financially by Depuy International Ltd., UK, producer of the implant brands tested. The first author and senior author receive institutional funding from, and are consultants to, Depuy International Ltd., UK.