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Commentary & Perspective | ||||||||
Commentary & Perspective on We commend Komistek et al. for their pioneering investigations in this important line of inquiry. In this in vivo study, they analyzed twenty subjects with use of video fluoroscopy to determine whether the femoral head and the acetabular component of total hip implants "separate" (or more precisely "slide," since a portion of the femoral head remains in contact with the cup) during level gait. They observed sliding that exceeded 0.75 mm, their threshold value, in all ten patients who had a metal-on-polyethylene implant, while patients with metal-on-metal components had no detectable sliding. The authors attributed this difference to the effect of fluid-film cohesion, in which the film connects the metal femoral head to the metal acetabular liner during gait. Komistek et al. suggested that the cohesive forces generated were sufficient to support the lower limb's weight and the inertial forces during the swing phase of gait, so that sliding was inhibited. On the basis of their observations, they also suggested that femoral-head sliding increases shear force during impulse-loading cycles with potentially detrimental effects on the wear rates of polyethylene. But while the role of fluid-film cohesion and the occurrence of impulse loading in wear patterns are appealing speculations, it should be recognized that they are not substantiated by either experimental data or published studies. Consistent with the notion that sliding rather than complete separation occurs, Bergmann et al.1 demonstrated in telemetric studies of metal-on-polyethylene hip prostheses in five patients that the resultant force in the hip-joint was always—i.e., even in swing phase—20% of the patient's body-weight. This indicates that there is contact between the femoral head and the acetabular component throughout the gait cycle. Furthermore, in contrast to the hypothesis of Komistek et al., Bergmann et al. have shown that peak loads never occurred during heel strike, which calls into question the suggestion that ‘impulse loading’ is associated with metal-on-polyethylene articulations. While Komistek et al. attributed their observations to differences in the composition of the articulating couples between the two groups, it may well be that other subtle factors related to the implant, the patient, and/or the surgery contributed to their findings. Given the small number of patients evaluated, however, it is unlikely that the contribution of patient and surgery-related factors could be ascertained. Furthermore, since the authors evaluated only one implant system, it is unclear if the findings of this study can be generalized to other implant designs. The authors implied that the observed sliding may be a cause of the multidirectional wear vectors that have been found on retrieved polyethylene articulating surfaces. Another possible explanation is that the observed ball/cup kinematics are an effect of the multidirectional wear vectors. In this scenario, the ball always stays firmly in contact with the cup while rolling and sliding along the path from one wear vector to the other. On the basis of data from our post-mortem2 and recent hip-simulation3 studies of well-functioning acetabular cups, multi-directional wear vectors were observed frequently. In our hip-simulation study3, we oberved multidirectional wear vectors in the absence of preceding "separation." Also in the aforementioned studies, we observed two penetration maxima (wear vectors) that were connected by a toroidal wear track. The first maximum occurred close to the dome of the polyethylene cup, and the second occurred peripherally within the superolateral quadrant. On the basis of the location of these two maxima, a penetration depth of 0.4 mm is sufficient to exceed the apparent separation threshold of 0.75 mm when the ball travels from the central to the peripheral maximum (see below†). Since initial "bedding-in" of the polyethelene articulation causes rapid penetration during the first year of service (>0.2 mm/year in our post-mortem study; R2 = 0.98, p < 0.006), a penetration depth of >0.4 mm is conceivable for the patients in Komistek's study. Furthermore, it must be noted that the error analysis and the model-fitting process presented in this paper are based on unworn polyethylene liners, which may or may not contribute to an increase of apparent sliding. Since metal components are not susceptible to such deep penetration but rather stay constant in geometry, this would explain why no sliding was observed with metal-on-metal bearings. Although the separation (sliding) theory looks very attractive, it has not been proven with use of a methodology other than fluoroscopic imaging. Clearly, additional research is required to definitively establish whether the articulating surfaces of the hip joint separate or slide during the normal gait cycle and, if so, whether this actually damages the polyethylene. While Nevelos et al.4 have shown that micro-separation between the head and cup can increase the wear of alumina-on-alumina components, there is no evidence that bearings coupled with polyethylene behave in the same fashion. In fact, Clarke and colleagues5 have shown that a constant 0.87 kN load can be regarded as the equivalent of a physiological gait load-profile with 2 kN peak magnitudes in polymer articulations. Thus, polymers seem to be relatively insensitive to impulse loading, at least within the physiologic range studied by Clarke et al. These considerations point to a pressing need for a deeper understanding of the relationship between joint kinematics and wear. Continued advancements in this area are necessary to improve the fidelity of joint simulations and their value as accurate predictors of long-term wear behavior. *The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. One or more of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity (Zimmer, Inc.) In addition, a commercial entity (Zimmer, Inc.) paid or directed, or agreed to pay or direct, benefits to a research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated. †p = 0.75 / ((sin α1 – sin α2)2 + (cos α1 – cos α2)2)1/2 – c; with p = penetration depth (assuming equal depth of the two maxima), c = clearance between head and cup, α1 (α2) = 0 … 180°, vector angle of the central peripheral penetration maximum in the sagittal plane. References 1. Bergmann G, Graichen F, Rohlmann A. Investigation of potential separation between head and cup of total hip implants. In: Transactions of the 48th Annual Meeting of the Orthopaedic Research Society, 2002 Feb 10-13; Dallas, TX. Chicago: Orthopaedic Research Society; 2002. p 982. | ||||||||
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