Penetration rate
There was a significantly higher penetration rate in bearings with evidence of impingement, possibly because bone and polyethylene debris in the joint acted as third bodies between the articular surfaces. Impingement can be avoided by appropriate surgical technique, and a properly implanted bearing should not impinge against bone or cement. The latest instrumentation sets (Chapter 7) include an anterior mill specifically designed to remove potentially impinging bone from the front of the femoral component.
The mean rate of penetration of the 22 medial bearings with no evidence of impingement, the rate measured in vivo at 10 years and 20 years in clinically successful patients, and the rate in the simulator studies were all very small (0.01–0.02 mm/year). They are an order of magnitude lower than the mean rate of 0.15 mm/year reported for 19 retrievals of the St Georg fixed bearing prosthesis (Ashraf et al., 2004).
They are also much lower than the mean rate of 0.49 mm/year reported for 81 retrievals of various round-on-flat fixed bearing designs by Collier et al. (2007) who attributed these high wear rates to the use of sterilisation by gamma irradiation of the polyethylene in air. This ignores the low penetration rates which we have reported for fully conforming meniscal bearings (Argenson and O’Connor, 1992; Psychoyios et al., 1998), all of which were made of polyethylene which had been irradiated in air. We think that the low contact stresses associated with the large contact areas of the Oxford prosthesis can protect even a vulnerable material from excessive wear (if impingement is avoided). The high contact stresses associated with a round femoral component in contact with a flat or nearly flat tibial component common in fixed bearing designs may yet prove to be beyond the capacity of modern highly cross-linked polyethylene, particularly in view of the known reduction in fracture toughness which has been associated with cross-linking (Oral et al., 2006). However, doping cross-linked UHMWPE with Vitamin E offers the exciting prospect of a material with low wear rate and high fatigue resistance (Oral et al., 2014).
It is not wise to extrapolate long-term wear rates from short-term measurements because the polymer may degrade and oxidise, but, at the rates reported, the OUKA bearings would lose 1 mm of thickness in 50–100 years. The high survival rates of the OUKA up to 20 years after implantation demonstrate (Chapter 10) that, in congruous articulation, polyethylene can survive as long as the patient even when used in thin components (Price & Svard, 2011).
The rate of penetration of the Oxford bearings was also much lower than that reported by Wroblewski (1985) for the acetabular component of the fully congruous Charnley hip (0.19 mm/year). This is not surprising as the projected area of contact is larger in the OUKA than in the Charnley hip, and the contact stresses are correspondingly lower.
A similarly low rate of penetration (0.026 mm/year) was found for a fixed-bearing total knee prosthesis with fully congruent cylindrical articular surfaces (Plante-Bordeneuve & Freeman, 1993), suggesting that it is congruity, rather than the use of mobile bearings, that allows the transmission of high loads with little wear.
Similar data have not been published for mobile bearings articulating with polyradial femoral condyles, but they may not enjoy such low wear rates as they cannot be congruent throughout the range of movement. In one such device, the most frequent cause for revision was bearing failure, and bearing exchange for wear is a well-recognised procedure (Hamelynck et al., 2002; Keblish and Briard, 2004).
Bearing thickness
In incongruent articulations, the wear rate of polyethylene is greater when it is used in a thin layer (Bartel et al., 1986). Perhaps the most important observation made during our retrieval studies, with particular significance for unicompartmental arthroplasty, is that polyethylene used in congruent articulation has a wear rate that is independent of its initial thickness, at least down to 3.5 mm. This fact is less important in TKA where more extensive bone removal allows the use of thick tibial implants, but it is of consequence in unicompartmental prostheses when preservation of bone stock and minimal invasion are required. In fixed-bearing UKA designs, it is thought unsafe to use a polyethylene layer thinner than 6 mm (Marmor, 1976). However, a congruous meniscal bearing that is only 3.5 mm thick at its thinnest point wears no more rapidly than a thicker one.
In the original study based on data from Svard, there was no difference in survival between UKA with different bearing thicknesses. More recent data based on 1000 Phase 3 OUKA found that the results were substantially better with 3 mm or 4 mm bearings (94% 15 year survival) compared with 5 mm or more (75% 15 year survival) (Pandit et al., 2015). Although it is not yet clear why the difference is so marked, it confirms that wear is not an issue with the thin mobile bearings.
Volumetric wear
Penetration (linear wear plus creep) is not the only measure of wear. Another measure is the volume of debris generated (volumetric wear). Volumetric wear increases in proportion to the area of contact but, since it reduces with reduction in penetration, the beneficial effect of the low contact pressure at congruous surfaces may more than balance the adverse effect of their large contact areas. Calculations of the mean volume of wear debris produced at the articular surfaces of the Oxford bearing give a figure of about 6 mm3/year (for bearings without impingement). The St Georg fixed-bearing implant had a measured volumetric wear rate of 17.3 mm3/year (Ashraf et al., 2004). No comparable data are available for other knee replacements, but the volumetric acetabular wear rates (assessed in vivo) for various designs of hip prosthesis, which also have congruous surfaces, vary from 26 to 89 mm3/year (Kabo et al., 1993). The tissues around the hip are believed to be able to tolerate a mean of 600 mm3 of polyethylene debris before bone resorption necessitates revision (Hall et al., 1998).
Therefore, it is unlikely that the volume of wear particles from a properly functioning meniscal bearing will cause problems. However, with the accelerated wear rate associated with impingement, it is possible that particle debris accumulated in the long term might become great enough to cause osteolysis.
There is a suspicion that it is the very small particles generated by wear at congruous surfaces that cause osteolysis and aseptic loosening. However, the study by Kendrick et al. (2010) of 47 retrieved Oxford bearings (Fig. 2.13) showed that even those most damaged by impingement came from knees with no evidence of osteolysis. Sathasivam et al. (2001) stated that ‘there is no disadvantage with regard to particle size or type associated with large contact areas’. The survival rates better than 90% at 20 years also imply an absence of significant osteolysis (Price and Svard, 2011).
The debris generated by extra-articular impingement probably consists of larger particles. These may act as third bodies and hasten wear, as suggested by the observed correlation of impingement and increased penetration. Furthermore impingement may cause failure. The surgeon therefore needs to take all necessary precautions to ensure the bearing tracks freely and does not impinge.
Bearing fracture
Figure 2.17 shows a set of eight bearings which had fractured in vivo. We have reported on ten such fractured bearings on which we had data on implantation time (Pegg et al., 2011). Four others were sent to us without such information. Over 500,000 of these bearings have been implanted and these are the only instances of fractures which we have seen, seven from one practice of over 1000 cases. From the latter practice, it is estimated that the fracture rate of Phase 1 bearings was 3.2%, that of Phase 2 bearings was 0.74% and that of Phase 3 was 0.35%. The mean duration of the prosthesis in situ was 16.8 years (range 6.6 to 23.9). Five of the bearings had a minimum thickness of 3.5 mm, three had 4.5 mm and two had 5.5 mm at implantation. The mean age of the patients at implantation was 60.3 years (range 50 to 69) and the mean weight was 80.1 kg (range 70 to 100).
Figure 2.17 Photographs of the femoral and tibial surfaces of the fractured bearings.
All of the fractured bearings showed evidence of impingement and excessive wear (>0.05 mm per year). In most cases, the fractures appeared to be systematic, occurring on the coronal plane through or near the point of minimum thickness of the bearing. Finite element stress analysis confirmed the presence of tensile stresses on this plane needed to resist the forwards facing components of the pressure on the anterior half of the spherical upper surface of the bearing and the backwards facing components of the pressure on the posterior part of the upper surface (Fig. 2.18) (Pegg et al., 2013a). Tensile stress is necessary both to initiate and propagate fatigue cracks.
The posterior metal wire used as an x-ray marker appeared to be implicated in some of the specimens and, since 1999, this has been replaced by two tantalum balls in short holes on the medial and lateral edges at the back of the bearing. However, Lim et al. (2014) have reported an instance of a fractured Phase 3 bearing with a tantalum ball posterior marker. They remarked on evidence of impingement on the retrieved specimen.
Figure 2.18. The anteriorly directed pressure at the front of the upper surface of the bearing and the posteriorly directed pressure at the back can generate the type of tensile stresses necessary to propagate fatigue cracks but they have been observed only in severely worn bearings. (This material has been reproduced from the Journal of Engineering in Medicine: Proceedings of the Institution of Mechanical Engineers Part H. 2013 Vol 227 pp 1213-23. Fracture of mobile unicompartmental knee bearings, Pegg E, Murray DW, Pandit HG, O’Connor JJ, Gill HS. Permission is granted by the Council of the Institution of Mechanical Engineers.)
We conclude that fracture occurs only after significant reduction of bearing thickness due to impingement-induced wear and that fracture can be avoided if impingement is avoided. As bearing thickness is reduced, the anteroposterior tensile stresses are increased. The most recent instrumentation, Microplasty (Chapter 7), introduces an anterior mill specifically designed to help avoid anterior impingement.
Comparison between fixed and mobile
With fixed bearing UKA, a divot appears within the surface of the tibial components almost as soon as they are used, partly as a result of creep and partly by wear, (Fig. 2.19). This tends to constrain the movement of the femoral component on the tibia. In contrast with the mobile bearing Oxford implant, Figure 2.20(a), except for the effects of friction, there is free movement of the mobile bearing both in the short and long term.
The differences between mobile and fixed have significant implications for function in the longer term. Figure 2.20(a) and (b) shows schematic diagrams of the response to an oblique load of both mobile bearing (a) and fixed bearing (b) UKA with the cruciate ligaments. With the mobile bearing implant, the femur can translate freely along the plateau, allowing one of the cruciate ligaments to stretch and to develop tensile forces to balance the component of the oblique force parallel to the tibial plateau. The interface between the implant and the tibia is not involved in this balance and the bone/implant stresses remain compressive, ideal for fixation. (More detailed descriptions of the interactions between the mobile bearing implant and the ligaments are given in Chapter 3 and the Appendix, with animations on the website www.oxfordpartialknee.com also relevant.)
After a period of usage, the divot worn in the surface of the fixed bearing implant resists anteroposterior translation of the femur on the tibia by developing shear forces parallel to the bone/implant interface (Fig. 2.20(b)), while allowing both cruciate ligaments to slacken. These shear forces tend to tilt the implant, with the development of tensile forces. The shear and tensile forces will be transmitted to the implant/bone interface and may cause loosening.
Figure 2.19 Divot in a fixed bearing UKA, unpublished photograph by Professor W Plitz, Munich.
Figure 2.20 Interface reactions to an oblique load (shown in brown), mobile (left) v fixed (right). Compressive stresses at the bone/implant interface are shown in blue, tensile stresses are shown in red, shear stresses in green.
If the cruciate ligaments are not loaded normally, they may, with time, fail. Evidence that this happens comes from a study by Argenson et al. (2002) which showed, at 10 years after Miller-Galante fixed bearing arthroplasty, many knees were functioning like ACL deficient knees which increases the loading in the PFJ. Studies of the OUKA show that not only at one but also 10 years post operation, the kinematics following OUKA are virtually normal, with normal bearing movement (Price et al., 2004). This difference may explain in part why, after the mobile bearing arthroplasty, there do not seem to be problems with the patellofemoral joint in the long term whereas, after the fixed bearing, progression of the OA into the patellofemoral joint is one of the commonest causes for failure (Argenson et al., 2013).
