Improvement in the assessment of wear of total knee replacements using coordinate-measuring machine techniques

The influence of implant design and limb alignment on in vivo wear rates of fixed-bearing and rotating-platform knee implant retrievals

Analysis of polyethylene (PE) wear in knee implants is crucial for understanding the factors leading to revision in total knee arthroplasty. Importantly, current experimental and computational methods for predicting insert wear can only be validated against true in vivo measurements from retrievals. This study quantitatively investigated in vivo PE wear rates in fixed-bearing (FB) (n = 21) and rotating-platform (n = 53) implant retrievals. 3D surface geometry of the retrievals was measured using a structured light scanner. Then, a reference surface that included the deformation, but not the wear that the retrievals had experienced in vivo, was constructed using a fully automatic surface reconstruction algorithm. Finally, wear volume was calculated from the deviation between the worn and reconstructed surfaces. The measurement and analysis techniques were validated and the algorithm was found to produce errors of only 0.2% relative to the component volumes. In addition to quantifying cohort-level wear rates, the effect of mechanical axis limb alignment on mediolateral wear distribution was examined for a subset of the retrievals (n = 14 + 26). Our results show that FB implants produce significantly (p = 0.04) higher topside wear rates (24.6 ± 10.1 mm3 /year) than rotating-platform implants (15.3 ± 8.0 mm3 /year). This effect was larger than that of limb alignment, which had a smaller and nonsignificant influence on overall wear rates (+4.5 ± 11.6 mm3 /year, p = 0.43). However, increased varus alignment was associated significantly with greater medial compartment wear in both the FB and rotating-platform designs (+1.7 ± 1.3%/° and +1.8 ± 1.6%/°). Our findings emphasize the importance of implant design and limb alignment on wear outcomes, providing reference data for improving implant performance and longevity.

Knee Wear Assessment: 3D Scanners Used as a Consolidated Procedure

It is well known that wear occurring in polyethylene menisci is a significant clinical problem. At this regard, wear tests on biomaterials medical devices are performed in order to assess their pre-clinical performance in terms of wear, durability, resistance to fatigue, etc. The objective of this study was to assess the wear of mobile total knee polyethylene inserts after an in vitro wear test. In particular, the wear behavior of mobile bearing polyethylene knee configurations was investigated using a knee joint wear simulator. After the completion of the wear test, the polyethylene mobile menisci were analyzed through a consolidated procedure by using 3D optical scanners, in order to evaluate the 3D wear distribution on the prosthesis surface, wear depths, wear rates, amount of material loss and contact areas. The results in terms of wear rates and wear volumes were compared with results of gravimetric tests, finding equivalent achievements.

Comparison of CMM and Micro-CT Volumetric Analysis of Polyethylene Tibial Knee Inserts in Total Knee Replacement

Ultrahigh molecular weight polyethylene (UHMWPE) bearings are used widely in orthopaedic joint replacement as a prominent material for improving the longevity, which is dramatically influenced by wear of polyethylene. Polyethylene tibial knee components from knee simulators under two different input conditions were analyzed using CMM and Micro-CT measurement techniques for volumetric loss with gravimetric measurement as reference. Based on the coordinates and image slices obtained, the surface curve fitting and image digitization methodology were used for the creation of nominal “original” surface in the case of no prewear data provided. The comparison results indicate that gravimetric remains the gold standard and the CMM measurement took less time and had better precision, accuracy, and repeatability compared to Micro-CT measurement technique.

CMM-Based Volumetric Assessment Methodology for Polyethylene Tibial Knee Inserts in Total Knee Replacement

Total knee replacement is a common surgical procedure in orthopaedics. Accurate volumetric wear assessment of the polyethylene knee inserts has been an essential subject for improving the longevity. A new CMM-based methodology was presented to determine volumetric material loss based on curve surface fitting without prewear data, CAD model, or original design of drawings. Both computational and experimental simulated volume removal tests were run to validate the methodology by comparing with the gravimetric measurements. The volume and linear wear of the tibial inserts were calculated using the presented method based on the coordinates acquired by the CMM. The results indicate that the methodology is adequate for clinically retrieved tibial inserts where no prewear data are provided. This technique can also be used for biotribological study of other polyethylene components, since wear and damage can be assessed visually and volumetrically.

Long-term wear of dual mobility total hip replacement cups: explant study

PURPOSE

Dual mobility THA implantations are increasing. THA survival of more than 15 years is considered long-term survival based on the Charnley prosthesis, which has a median wear rate of 39 to 98 mm³/year. What is the median wear rate of dual mobility THA cups and its repartition on the liner volume? What are the factors influencing dual mobility wear?

METHODS

Thirty-five explants were analysed. The liners were entirely mapped with a fully automatic stripe light scanner that uses green LED light.

RESULTS

The liners had a median wear rate of 38 mm³/year. The distribution of the wear between the two bearings varied greatly. No factor was found influencing wear volumes.

CONCLUSIONS

The median annual wear of dual mobility liners is at least as low as the wear of cemented polyethylene liners and lower than equivalent cementless liners. The two articulations of the dual mobility THA do not cause more wear.

LEVEL OF EVIDENCE

Level IV - case series.

Relationship of surface damage appearance and volumetric wear in retrieved TKR polyethylene liners

Recently developed techniques have enabled volume loss measurements on surgically retrieved total knee replacements (TKR). However, it is not well understood how volume loss relates to polyethylene surface damage appearance. Sixty-four fixed bearing cruciate retaining components retrieved from revision and postmortem surgeries were analyzed for penetration and volume loss on the topside articular surface. An autonomous reconstruction method was used to approximate the original unworn surfaces. Surface damage patterns were also mapped using a video microscope, and each pattern's contribution to volume loss was calculated. With consideration for creep, a total wear rate of 12.9 ± 5.97 mm³ /year was found for the population. The penetration rate was 0.035 ± 0.017 mm/year medially and 0.034 ± 0.011 mm/year laterally, of which the location on the plateau varied greatly. Although striated patterns contributed to most to volume loss, damage patterns generally were only moderate predictors of material volume loss. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 2053-2059, 2017.

Gender-specific total knee arthroplasty in Singaporean women

PURPOSE

To compare the outcome of 145 women who underwent conventional total knee arthroplasty (TKA) with 77 women who underwent gender-specific TKA.

METHODS

Records of 222 women who underwent primary TKA using a conventional (n=145) or gender-specific (n=77) size E or F prosthesis for end-stage osteoarthritis were reviewed. The gender-specific prosthesis has a narrower mediolateral dimension. Patients were assessed for flexion, Oxford Knee Score, Knee Society function and knee scores, and Short Form-36 Health Survey preoperatively and postoperatively (at 6 months and 2 years).

RESULTS

The 2 groups were comparable in terms of age (67.8 vs. 68.1 years, p=0.789), body mass index (28.6 vs. 27.8 kg/m(2), p=0.189), and preoperative scores. 12 women with conventional TKA and 4 women with gender-specific TKA were lost to followup. Compared with women with conventional TKA, women with gender-specific TKA had better flexion at 6 months (116° vs. 121.9°, p=0.007) and 2 years (118.7° vs. 124.6°, p=0.006), better bodily pain score at 2 years (65.1 vs. 72.4, p=0.049), and greater improvement in bodily pain score from baseline to 2 years (30 vs. 38.5, p=0.034).

CONCLUSION

Gender-specific TKA enables better knee flexion and less bodily pain in women who have a high propensity to develop mediolateral overhang of the femoral component.

Nondestructive microimaging during preclinical pin-on-plate testing of novel materials for arthroplasty

The purpose of this study was to determine the ability of micro-computed tomography to quantify wear in preclinical pin-on-plate testing of materials for use in joint arthroplasty. Wear testing of CoCr pins articulating against six polyetheretherketone plates was performed using a pin-on-plate apparatus over 2 million cycles. Change in volume due to wear was quantified with gravimetric analysis and with micro-computed tomography, and the volumes were compared. Separately, the volume of polyetheretherketone pin-on-plate specimens that had been soaking in fluid for 52 weeks was quantified with both gravimetric analysis and micro-computed tomography, and repeated after drying. The volume change with micro-computed tomography was compared to the mass change with gravimetric analysis. The mean wear volume measured was 8.02 ± 6.38 mm(3) with gravimetric analysis and 6.76 ± 5.38 mm(3) with micro-computed tomography (p = 0.06). Micro-computed tomography volume measurements did not show a statistically significant change with drying for either the plates (p = 0.60) or the pins (p = 0.09), yet drying had a significant effect on the gravimetric mass measurements for both the plates (p = 0.03) and the pins (p = 0.04). Micro-computed tomography provided accurate measurements of wear in polyetheretherketone pin-on-plate test specimens, and no statistically significant change was caused by fluid uptake. Micro-computed tomography quantifies wear depth and wear volume, mapped to the specific location of damage on the specimen, and is also capable of examining subsurface density as well as cracking. Its noncontact, nondestructive nature makes it ideal for preclinical testing of materials, in which further additional analysis techniques may be utilized.

Manufacturing lot affects polyethylene tibial insert volume, thickness, and surface geometry

To perform wear measurements on retrieved joint replacement implants, a reference geometry of the implant's original state is required. Since implants are rarely individually scanned before implantation, a different, new implant of the same kind and size is frequently used. However, due to manufacturing variability, errors may be introduced into these measurements, as the dimensions between the retrieved and reference components may not be exactly the same. The hypothesis of this study was that new polyethylene tibial inserts from different manufacturing lots would demonstrate greater variability than those from the same lot. In total, 12 new tibial inserts of the same model and size were obtained, 5 from the same lot and the remainder from different lots. The geometry of each tibial insert was obtained using microcomputed tomography. Measurements of tibial insert volume, thickness, and three-dimensional surface deviations were obtained and compared between tibial inserts from the same and different manufacturing lots. Greater variability was found for the tibial inserts from different manufacturing lots for all types of measurements, including a fourfold difference in volume variability (p < 0.001) and a maximum of 0.21 mm difference in thickness (p < 0.001). Investigators should be aware of this potential confounding error and take steps to minimize it, such as by averaging together the geometries of multiple new components from different manufacturing lots for use as the reference geometry.

The radiological assessment of total and unicompartmental knee replacements

Radiological assessment of total and unicompartmental knee replacement remains an essential part of routine care and follow-up. Appreciation of the various measurements that can be identified radiologically is important. It is likely that routine plain radiographs will continue to be used, although there has been a trend towards using newer technologies such as CT, especially in a failing knee, where it provides more detailed information, albeit with a higher radiation exposure. The purpose of this paper is to outline the radiological parameters used to evaluate knee replacements, describe how these are measured or classified, and review the current literature to determine their efficacy where possible.

An autonomous mathematical reconstruction to effectively measure volume loss on retrieved polyethylene tibial inserts

Wear of the polyethylene tibial component is a major factor in the success of total knee replacements. However, sampling resolution and the challenges of estimating original surfaces with relatively complex articulating geometries have limited the accuracy of volumetric measurements of wear on surgically retrieved inserts. A mathematical model analyzed volume error due to sampling resolution and found that 100 × 100 μm(2) point spacing reduced error below 1 mm(3). Small volumes of material were progressively removed from the topside of three unworn tibial inserts, after which each component was weighed and digitized with a laser coordinate measuring machine. Six inserts worn in knee simulator tests and nine surgically retrieved inserts visually scored for damage were also digitized. For these tests, the original surface of an insert was mathematically reconstructed from unworn regions of the same component, and volume loss and its spatial distribution were calculated. Volume loss estimated by autonomous reconstruction correlated strongly to mass removed manually (R(2) = 0.954, slope = 1.02 ± 0.04), mass lost during simulator testing (R(2) = 0.935, slope = 1.01 ± 0.07) and visual damage scores separated by size (R(2)large = 0.9824, R(2)small = 0.9728). These results suggest that an autonomous mathematical reconstruction can be used to effectively measure volume loss in retrieved tibial inserts.

How do CAD models compare with reverse engineered manufactured components for use in wear analysis?

BACKGROUND

To accurately quantify polyethylene wear in retrieved arthroplasty components, the original geometry of the component must be estimated accurately using a reference geometry such as a computer-aided design (CAD) model or a never-implanted insert. However, differences may exist between the CAD model and manufactured inserts resulting from manufacturing tolerances.

QUESTIONS/PURPOSES

We quantified the deviations between CAD models and newly manufactured inserts and determined how these deviations compared with using a never-implanted insert as a reference geometry.

METHODS

We obtained five cruciate-retaining (CR) and five posterior-stabilizing (PS) tibial inserts and their CAD models. The inserts were scanned and reconstructed using microcomputed tomography (micro-CT). Differences in volume and surface geometry were measured among (1) the individual inserts; (2) between the inserts and a CAD model; and (3) between the inserts and a reference geometry constructed from multiple scanned inserts averaged together.

RESULTS

The micro-CT volumes were, on average, 0.4% smaller (34-178 mm(3)) than the CAD model volumes. The mean deviation between the CAD model and insert surface geometry was 25.7 μm smaller for CR and 36.8 μm smaller for PS. The mean deviation between the inserts and an averaged reference geometry was 1.4 μm larger for CR and 0.4 μm smaller for PS.

CONCLUSIONS

Deviations exist between manufactured tibial inserts and CAD models that could cause errors in wear measurements. Scanned inserts may better represent the preimplantation geometry of worn inserts than CAD models, depending on the manufacturing variability between lots.

CLINICAL RELEVANCE

The magnitude of the error in estimation of the preimplantation geometry of a retrieved component could add or subtract the equivalent of 1 year of wear.

Assessment of Knee Implant Alignment Using Coordinate Measurement on Three-Dimensional Computed Tomography Reconstructions

The rapid development of navigation systems designed to improve implant alignment calls for precise methods to verify positioning. One new approach to assess knee implant alignment is coordinate measurement (CM) using reference objects on 3-dimensional reconstructions of computed tomographic (CT) image sequences. The objectives of this study were to determine whether implant alignment can be accurately and precisely examined with CT-based CM, and whether correct positioning of a transversal support implant can be attained using a specially developed aiming instrument. Average deviation of CT measurements from those made with a digital caliper remained within the caliper’s range of error. This level of accuracy was observer independent. CT-based CM can be used for accurate and precise alignment verification. As well, precise alignment of transversal support can be achieved during implantation with a conventional aiming device, assuming the bony landmarks are appropriately recognized.

The in vivo linear and volumetric wear of hip resurfacing implants revised for pseudotumor

BACKGROUND

Metal-on-metal arthroplasty-related pseudotumors can cause severe local destruction of bone and soft tissues. The cause of pseudotumors is unknown, although some authors have implicated metal wear debris. The aim of this study was to measure the location and magnitude of wear on resurfacing devices that were retrieved during revision procedures for pseudotumor (the pseudotumor group) and for other reasons (the control group).

METHODS

We examined thirty-six hip-resurfacing implants, which were divided into two groups: eighteen implants from patients with a diagnosis of pseudotumor and eighteen control implants. Implant orientation and patient demographics were recorded. Three-dimensional, contactless metrology was used to scan the surface of the femoral and acetabular components to a resolution of 20 nm. Linear and volumetric wear were measured, and the components were examined for evidence of edge wear.

RESULTS

There was three times more total linear wear and over six times more total volumetric wear of the femoral and acetabular components in the pseudotumor group as compared with that in the control group. The mean linear wear rate and standard deviation of the femoral components in the pseudotumor group (8.4 ± 8.7 μm/yr) were significantly greater than those in the control group (2.9 ± 3.9 μm/yr; p = 0.01). The mean volumetric wear rate of the femoral components was also significantly greater in the pseudotumor group (3.3 ± 5.7 mm3/yr) than it was in the control group (0.8 ± 1.2 mm3/yr; p = 0.009). Seventeen of eighteen subjects in the pseudotumor group had edge wear, compared with six of eighteen in the control group (p < 0.001).

CONCLUSIONS

Implants that were retrieved because of pseudotumor had a significantly higher wear rate and prevalence of edge wear than the control implants did. There was a strong association between pseudotumor and the high levels of wear debris that are generated during edge-loading. However, not all patients with high wear developed pseudotumors, and not all pseudotumors had high wear; therefore, other factors are most likely involved in the cause of pseudotumors.

A pictographic atlas for classifying damage modes on polyethylene bearings

Evaluation of medical devices retrieved after in vivo service provides unique evidence related to the physiological environment in which the biomaterials performed. This study implements a training procedure for evaluating polyethylene bearings of joint prostheses obtained after pre-clinical tests or explanted after in vivo function. A total of 161 damage regions on 45 bearings were evaluated by four observers. An illustrated Damage Mode Atlas was developed as a reference guide, inclusive of both photographs and concise written descriptions of 16 specific damage modes that are typical for polyethylene bearings. Utilizing the Damage Mode Atlas to train new researchers improved the damage pattern analysis, including more accurate identification of damage modes and improved inter-rater reliability. This Damage Mode Atlas is a useful supplementary tool for conducting Stage II non-destructive analysis of explanted polyethylene bearings used for joint replacement, in accordance with international guidelines for evaluating explanted medical devices.

Determination of reference geometry for polyethylene tibial insert wear analysis

Geometric wear analysis techniques require unworn geometries to serve as a reference in wear measurement. A method to create a reference geometrical model is described for retrieval studies when the actual unworn geometry is unavailable. Never-implanted tibial inserts were scanned with micro-computed tomography. Two, 3, or 6 insert surfaces were coaligned and averaged to create reference geometries. Individual inserts were compared with each other (manufacturing variability) and with the reference geometries (reference variability). The 3-dimensional deviations between the surfaces were recorded. The reference variability was reduced to 8.3 ± 39 μm, vs manufacturing variability of 15 ± 59 μm. Deviations were smallest on the articular surfaces where most wear occurs and were significantly less than the reported insert wear rate of 20 μm/y.

In vitro quantification of wear in tibial inserts using microcomputed tomography

BACKGROUND

Wear of polyethylene tibial inserts can decrease the longevity of total knee arthroplasty. Wear is currently assessed using laboratory methods that may not permit backside wear measurements or do not quantify surface deviation.

QUESTIONS/PURPOSES

We developed and validated a technique to quantify polyethylene wear in tibial inserts using microcomputed tomography (micro-CT), a nondestructive high-resolution imaging technique that provides detailed images of surface geometry in addition to volumetric measurements.

METHODS

Six unworn and six wear-simulated polyethylene tibial inserts were evaluated. Each insert was scanned three times using micro-CT at a resolution of 50 μm. The insert surface was reconstructed for each scan and the insert volume was calculated. Gravimetric analysis was performed for all inserts, and the micro-CT and gravimetric volumes were compared to determine accuracy. We created three-dimensional surface deviation maps.

RESULTS

Micro-CT generated high-quality three-dimensional renderings of the insert surface geometry. Between-scan precision was 0.07%; we observed no difference between micro-CT and gravimetric volume measurements.

CONCLUSIONS

Micro-CT can provide precise and accurate volumetric measurements in addition to quantifiable three-dimensional surface deviation maps for the entire insert surface. The technique has the potential to evaluate wear in wear simulator trials and retrieval studies.

CLINICAL RELEVANCE

This micro-CT technique combines the benefits of volumetric and surface scanning methods to quantify wear across all surfaces of polyethylene components with a single tool. When applied in wear simulator and retrieval studies, these measurements can be used to evaluate and predict the wear properties of the components.