Search

Background: Archived serial radiographs of knee replacements provide indirect evidence of the clinical performance of ultra-high molecular weight polyethylene tibial bearings. Our purpose was to determine the loss of thickness in polyethylene tibial inserts that were of the same design but had been sterilized differently., Methods: Four hundred and sixteen knees with an Anatomic Modular Knee primary total knee replacement had five to eighteen years of follow-up and a posterior cruciate ligament-retaining polyethylene tibial insert that had been sterilized with either gamma radiation in air (from 1987 to 1993) or gamma radiation in an inert gas (from 1993 to 1995), or by means of a nonirradiation gas-plasma method (from 1995 to 2001). Readings were taken from 1975 weight-bearing anteroposterior radiographs of the knee on which the top surface of the metal tibial tray had nearly projected as a line. In each tibiofemoral compartment, loss of polyethylene thickness was calculated as the standard thickness of the insert minus the measured magnification-corrected thickness. For each sterilization method, thickness loss was plotted versus the corresponding follow-up time, and a mixed linear regression model was used to quantify the change in thickness over time. Multiple linear regression analysis was used to determine whether loss of thickness was associated with implant factors, patient variables, and early postoperative limb alignment., Results: According to the mixed model regression coefficients, loss of medial compartment thickness was greatest for inserts that had been sterilized with gamma radiation in air (0.15 mm/yr), least for those sterilized with gamma radiation in an inert gas (0.02 mm/yr), and of an intermediate magnitude for those sterilized with a nonirradiation method (0.06 mm/year); loss of lateral compartment thickness ranged from 0.03 to 0.06 mm/yr. Notably, no insert irradiated in an inert gas showed a loss of thickness of >1 mm, and no nonirradiated insert showed a loss of >2 mm. Loss of medial compartment thickness in inserts sterilized with radiation in air increased significantly with a greater polyethylene shelf age, lesser patient age, and more varus alignment of the limb in the early postoperative period (p < 0.01)., Conclusions: In this total knee design, tibial polyethylene performance improved markedly after discontinuation of the gamma radiation in air sterilization technique. Future concerns are that bearings sterilized with radiation in an inert gas may oxidize in vivo and develop fatigue wear because of free radicals generated during sterilization with radiation and that nonirradiated bearings may undergo greater losses in thickness from routine burnishing since they lack the cross-linking that accompanies sterilization with radiation.

The investigators reviewed 245 fixed-bearing unicondylar arthroplasties that one surgeon performed as treatment of medial compartment osteoarthritis between 1988 and 1997 using a variety of cemented metal-backed tibial components and gamma-irradiated-in-air polyethylene bearings. Multivariate statistical analysis was used to evaluate how the event of revision was influenced by 3 patient factors, 3 implant factors, and 7 factors assessed from preoperative and early postoperative radiographs. Five factors were statistically associated with revision: (younger) patient age, (thinner) tibial component initial thickness, (longer) polyethylene shelf age, (lesser) angular reduction of medial tibial plateau varus, and (more varus) postoperative hip-knee-ankle angle. Besides illustrating deleterious consequences of using gamma-irradiated-in-air polyethylene in medial unicompartmental arthroplasty, our results support reducing varus angulation of the medial tibial plateau and knee at surgery.

Shelf aging of gamma-irradiated-in-air polyethylene tibial components has been associated with increased articular surface wear and an elevated risk for revision. Nine hundred fifty cruciate-retaining inserts of one design were implanted between 1987 and 1996 (shelf age, 1.0 +/- 1.2 years). Less frequently used inserts (smallest/largest sizes, thicker thicknesses, supplemental articular constraint) had longer shelf ages (means ranged from 1.2 to 2.6 years). Survival analysis showed that shelf age (P < .01) and gamma-sterilization in air (P = .01) elevated the risk for revision. Surgeons must remain attentive to identify the shelf-aged gamma-irradiated-in-air polyethylene tibial component while following designs from the era when this sterilization method was used. Recognition is expedited by understanding how shelf life is related to product demand and can be of aid when diagnosing the painful knee.

Background: A recent report linked shelf-aging of unicondylar polyethylene tibial components with accelerated fatigue wear and rapid failure. We retrospectively reviewed our experiences with another unicondylar knee system to investigate the relationship between the shelf age of the polyethylene components and clinical outcome., Methods: One hundred Single Compartment Replacement unicompartmental knee arthroplasties were performed with cement between 1990 and 1996. The median shelf age for the polyethylene inserts was 1.7 years. At the time of the review, four knees had been lost to follow-up, sixteen were in patients who had died, and nineteen had been revised. Sixty-one knee replacements remained in situ at a mean and standard deviation of 8 +/- 2 years postoperatively., Results: With revision as the end point, Kaplan-Meier survivorship analysis demonstrated a six-year rate of survival of 96% when the shelf age of the insert was less than the median shelf age but only 71% when the median shelf age was exceeded. Fatigue wear was identified on all retrieved components except for two that had shelf ages under one year and had been revised within three years. At four to six years of clinical follow-up, the group of knees in which the median shelf age had been exceeded also had poorer Knee Society knee scores and function scores., Conclusions: Aging on the shelf accelerated fatigue failure of polyethylene inserts sterilized with gamma irradiation in air and compromised our intermediate-term clinical outcomes with this unicondylar knee replacement system. Attention to the relationship between the shelf age and the clinical performance of the polyethylene component continues to be warranted with unicondylar knee arthroplasty implants.

Background: Radiographic measurements of the wear of total hip arthroplasty implants are indirect measurements based on radiographic determinations of the location of the femoral head relative to the acetabular component. Using the simplest case of zero wear, we assessed the reproducibility and accuracy of two software applications designed to quantify wear from clinical radiographs., Methods: After a cobalt-chromium head was glued into the polyethylene liner of a titanium shell, one cross-table lateral radiograph and three anteroposterior pelvic radiographs were made for twelve permutations of acetabular component angulation. The three anteroposterior radiographs differed only with regard to the cephalocaudal positioning of the prosthesis relative to the x-ray tube. To assess method reproducibility, each anteroposterior radiograph was assumed to be both the initial and the latest follow-up radiograph of a wear analysis. To assess method accuracy, each anteroposterior radiograph was paired in a wear analysis with each of the two anteroposterior radiographs made when the component was in the same angulation but at a different cephalocaudal position relative to the tube (one radiograph was the initial follow-up radiograph while the second was the latest follow-up radiograph). The analyses of reproducibility and accuracy were performed both with and without inclusion of the lateral radiograph made with the component in the same angulation., Results: Both methods fared well in the reproducibility analyses, with mean linear and volumetric wear values of 0.00 to 0.07 mm and 0 to 24 mm(3), respectively. In the accuracy analyses, the mean linear and volumetric wear values derived with the two methods were 0.26 to 0.40 mm and 78 to 126 mm(3), respectively., Conclusions: Whereas the results of the reproducibility analyses showed that the methods were consistent in determining the relative positions of the head and shell from a given anteroposterior radiograph or pair of anteroposterior and lateral radiographs, the non-zero wear results obtained in the accuracy analyses proved that these positional determinations were often inaccurate. Thus, while contemporary software methods may yield reproducible results, their accuracy is limited by their inability to correctly determine the position of the head relative to the acetabular component.

Sixty-six posterior cruciate-retaining inserts of one fixed-bearing design were revised because of wear or osteolysis. Tibial polyethylene thickness was estimated from standard anteroposterior radiographs and compared with the original or final minimum thicknesses. The most accurate method involved measuring the shortest distance from each femoral condyle to a transverse line through the middle of the baseplate's superior surface. On radiographs acquired 6 weeks after arthroplasty, the mean error was -0.1 mm (underestimate) +/- 0.6 mm (standard deviation), with 72% of the measurements within 0.5 mm of the original minimum thickness and 87% within 1.0 mm. The method was less accurate for radiographs made before revision, for which the mean error was 0.6 mm (overestimate) +/- 1.0 mm. In this study, 41% and 70% of the prerevision measurements were accurate to within 0.5 and 1.0 mm, respectively.

Background: Wear of the polyethylene tibial bearing is a leading cause of failure of knee replacements done prior to the current decade. The objective of this study was to determine how patient-related factors, implant-related factors, and limb or tibial component alignment influenced the amount of thickness loss in polyethylene tibial bearings that were retrieved at the time of revision surgery or after the death of the patient. Methods: We retrieved polyethylene tibial bearings from eighty-one unicondylar and eighty-nine total knee replacements that had been performed because of osteoarthritis with varus deformity from 1984 to 1998. All of the polyethylene bearings had been sterilized with gamma radiation in air. Polyethylene loss was quantified as the change in the minimum bearing thickness per years in vivo (the mean time in vivo [and standard deviation] was 8 ± 4 years). Multiple linear regression was used to assess whether polyethylene loss was associated with age, weight, gender, varus angle of the tibial component, postoperative hip-knee-ankle angle, initial thickness of the polyethylene, shelf age of the polyethylene, and either the type of polyethylene (for total knee replacements, which were of one posterior cruciate ligament-retaining design) or the manufacturer (for unicondylar knee replacements), and to determine the magnitude by which polyethylene loss would change if any of the significant risk factors were changed. Results: The mean loss (and standard deviation) of polyethylene thickness in the medial compartment of total knee replacements (0.33 ± 0.28 mm/yr) and that in medial unicompartmental knee replacements (0.49 ± 0.40 mm/yr) were significantly (p < 0.05) associated with the same three variables: patient age, postoperative hip-knee-ankle angle, and shelf age of the polyethylene. A total knee bearing with a one-year increase in shelf age, a unicondylar knee bearing with a six-month increase in shelf age, a patient who was ten years younger at the time of operation, or a limb that was aligned in 5° more varus (less valgus) had similar effects on the loss of polyethylene thickness in the medial compartment; the coefficients of the linear regression equations indicated that any one of these changes would increase polyethylene loss by 0.11 to 0.14 mm/yr. Conclusions: The wear-related loss of thickness in gamma-irradiated-in-air polyethylene bearings from unicondylar and total knee replacements implanted in osteoarthritic knees with varus deformity is influenced mainly by the shelf age of the polyethylene, the age of the patient, and the postoperative angulation of the knee in the coronal plane. Clinical Relevance: Although polyethylene bearings have not been sterilized with gamma radiation in air since the end of the last decade, many arthroplasty patients have polyethylene bearings that were sterilized with use of this method. An understanding of the findings of this study may be of value as these patients return for follow-up care. Whether the study findings have relevance to bearings sterilized with other methods is unclear and will remain so for many years. Level of Evidence: Prognostic Level II. See Instructions to Authors for a complete description of levels of evidence. [ABSTRACT FROM AUTHOR]

Conclusions: Backside wear was correlated with the relative motion between the polyethylene insert and the metal baseplate. New locking mechanism designs directed toward better methods of securing the polyethylene insert to the tibial tray are needed to minimize the generation of particulate wear debris at the modular interface. [ABSTRACT FROM AUTHOR]

Background: The results of recent studies documenting the backside wear of polyethylene inserts retrieved from total knee implants call into question the stability of the locking mechanisms of modular tibial components. Wear of the metal tibial baseplate suggests that the capture mechanisms of some modular fixed-bearing tibial components do not adequately restrict in vivo motion of the insert. The purposes of this study were (1) to present a method for evaluating locking-mechanism stability and (2) to investigate the stability of modular tibial components after an interval in vivo.Methods: We measured the anteroposterior and mediolateral motion between the polyethylene insert and the tibial tray in a variety of modular total knee tibial components. A uniaxial mechanical testing machine was used to evaluate the stability of ten unimplanted components (control group), fifteen implants obtained from patients who were undergoing revision total knee arthroplasty (revision group), and fifteen devices retrieved post mortem (autopsy group). We applied loads along the anteroposterior and mediolateral axes of the tibial component and recorded the maximum insert displacement that occurred. From this value, we calculated an insert-motion index, the magnitude of a two-dimensional vector that represented the total motion in the transverse plane.Results: For the control group, the mean insert-motion index was 64 +/- 13 microm (range, 6 to 157 microm); for the revision group, it was 341 +/- 51 microm (range, 104 to 718 microm); and for the autopsy group, it was 380 +/- 45 microm (range, 122 to 657 microm). The insert-motion index for the control group was significantly lower than that for the revision group (p = 0.001) or autopsy group (p < 0.001).Conclusions: Motion between the polyethylene insert and the metal baseplate in contemporary modular tibial designs increases after a period of in vivo loading.Clinical Relevance: Although there are several advantages to the use of modular tibial components, these advantages must be weighed against the disadvantage of backside wear debris secondary to motion of the modular insert. Debris from backside wear combined with wear from the articular side might account for the increasing prevalence of osteolysis since modular components have become widely used. [ABSTRACT FROM AUTHOR]

The article reports that according to a study Polyethylene Damage Increases With Varus Implant Alignment in Posterior-stabilized and Constrained Condylar Knee Arthroplasty. It mentions that the kinematic and anatomical differences that exist between mechanically varus knees with osteoarthritis (OA) and mechanically valgus knees with OA suggest that the role of limb alignment should be examined independently for each preoperative diagnosis.