Dual mobility (DM) total hip replacements (THRs) were introduced to reduce dislocation risk, which is the most common cause of early revision. Although DM THRs have shown good overall survivorship and low dislocation rates, the mechanisms which describe how these bearings function in-vivo are not fully understood. Therefore, the study aim was to comprehensively assess retrieved DM polyethylene liners for signs of damage using visual inspection and semi-quantitative geometric assessment methods. Retrieved DM liners (n=18) were visually inspected for the presence of surface damage, whereby the internal and external surfaces were independently assigned a score of one (present) or zero (not present) for seven damage modes. The severity of damage was not assessed. The material composition of embedded debris was characterised using energy-dispersive x-ray analysis (EDX). Additionally, each liner was geometrically assessed for signs of wear/deformation [1]. Scratching and pitting were the most common damage modes on either surface. Additionally, burnishing was observed on 50% of the internal surfaces and embedded debris was identified on 67% of the external surfaces. EDX analysis of the debris identified several materials including titanium, cobalt-chrome, iron, and tantalum. Geometric analysis demonstrated highly variable damage patterns across the liners. The incidence of burnishing was three times greater for the internal surfaces, suggesting that this acts as the primary articulation site. The external surfaces sustained more observable damage as evidenced by a higher incidence of embedded debris, abrasion, delamination, and deformation. In conjunction with the highly variable damage patterns observed, these results suggest that DM kinematics are complex and may be influenced by several factors (e.g., soft tissue fibrosis, patient activities) and thus further investigation is warranted.
Total knee arthroplasty (TKA) is highly successful due to pain reduction, patient satisfaction, and increased range of motion (ROM) during activities of daily living (ADL). ROM recovery is critical for successful outcomes, however ROM values are typically captured during routine physical therapy (PT) appointments via simplified measures (e.g. goniometric maximum passive ROM). These measures are imprecise, potentially neglecting patients’ home experiences. Accordingly, improved measurement methods are necessary to realistically represent ROM recovery. A validated inertial measurement unit (IMU) method continuously capturing knee ROM was deployed assessing knee ROM recovery during PT appointments and during patients’ routine daily experiences. Our objectives were to 1) continuously capture knee ROM pre-/post-TKA via IMUs and 2) divide each day's data to PT/non-PT segments comparing ‘gold standard’ ROM measurements (PT periods) with non-invasive home measurements (non-PT periods). Given patients are verbally/physically encouraged during PT, we hypothesized PT and non-PT metrics would be significantly different including 1) greater kinematics, 2) shorter times, and 3) greater activity level during PT compared to non-PT. Following IRB approval, IMUs captured long duration, continuous (8–12 hours/day, ∼50 days) knee ROM pre-/post-TKA. Post-TKA metrics were subdivided to PT/non-PT time periods including maximum ROM, gait phase ROMs (stance/swing), gait times (stride/stance/swing), and activity level. Clinical ROM and patient reported outcome measures (PROMs) were also captured before/after TKA. Statistical comparisons were completed between pre-TKA, post-TKA PT, and post-TKA non-PT metrics. Correlation analyses were completed between IMU, clinical ROM, and PROMs.Introduction
Methods
Large-scale retrieval studies have shown backside wear in tibial inserts is dependent on the surface roughness of the tibial tray. Manufacturers acknowledge this design factor and have responded with the marketing of mirror-finished trays, which are clinically proven to have lower wear rates in comparison to historically “rough” (e.g. grit blasted) trays. While the relationship between wear and surface roughness has been explored in other polymer applications, the quantitative dependence of backside wear rate on quantitative surface finish has not yet been established for modern devices. The present study evaluates small-excursion polyethylene wear on pucks of a variety of surface roughnesses. The objective of this study is to determine where inflection points exist in the relationship between surface roughness and wear rate. An AMTI Orthopod, 6-station pin on disk tribotest was designed to mimic worst-case in vivo backside wear conditions based on published retrieval analyses. Titanium (Ti6Al4V) pucks with six different surface roughness preparations (Sa ranges from 0.06 um to 1.06 um) were characterized with white light profilometry. Never implanted polyethylene tibial inserts (never irradiated, EtO sterilized) were machined into 6 mm diameter cylindrical pins. Fretting-type motion was conducted in a 2mm square pattern at 2Hz under 100 N constant force in 25% bovine serum lubricant for 1.35 million cycles in triplicate. Mass measurements were taken every 225 thousand cycles.Introduction
Materials and Methods
Patella implant research is often overlooked despite its importance as the third compartment in a total knee replacement. Wear and fracture of resurfaced patellae can lead to implant failure and revision surgeries. New simulation techniques have been developed to analyze the performance of patella designs as they interact with the trochlear groove in total knee components, and clinical validation is sought to ensure that these simulations are appropriate. The objective of this work was to subject several patellar designs to patient-derived deep knee bend (DKB) inputs on a 6 degree of freedom (DOF) simulator and compare the resultant wear scars to clinical retrievals. Previously reported DKB profiles were developed based on Introduction
Materials and Methods
With many stakeholders, healthcare decisions are complex. However, patient interests should be prioritized. This maximizes healthcare value (quality divided by cost), simultaneously minimizing costs (objective) and maximizing quality (subjective). Unfortunately, even ‘high value’ procedures like total knee arthroplasty (TKA) suffer from recovery assessment subjectivity (i.e. high assessment variability) and increasing costs. High TKA costs and utilization yield high annual expenditures (∼$22B), including postoperative physical therapy (PT) accounting for ∼10% of total costs (∼$2.3B annually). Post-TKA PT is typically homogenous across subjects ensuring most recover, however recent work shows outcomes unimpacted by PT. Accordingly, opportunities exist improving healthcare value by simultaneously reducing unnecessary PT expenditures and improving outcomes. However, discerning recovery completion relies on discrete ROM measures captured clinically and subjective clinician experience (i.e. intuition about recovery). Accordingly, our goal was developing objective post-TKA performance assessment methods utilizing gait knee ROM and statistical analyses to categorize patient recovery (‘accelerated,’ ‘delayed,’ or ‘normal’). We first established statistical reasons for current post-TKA rehabilitation including risk-reward tradeoffs between incorrectly ascribing ‘poor recovery’ to well-recovering patients (T1 error) or ‘good recovery’ to poorly-recovering patients (T2 error) using methods described by Mudge et al. and known TKA volumes/rehabilitation costs. Next, previously captured gait ROM data from well-healed patients was utilized establishing standard recovery curves. These were then utilized to assess newly captured patient recovery. Following IRB approval, we prospectively captured gait ROM from 10 TKA patients (3M, 69±13 years) 1-week pre-TKA and 6-weeks immediately post-TKA. Performance was compared to recovery curves via control charts/Shewhart rules (daily performance) as well as standard deviation thresholds (weekly performance) establishing recovery as ‘accelerated,’ ‘delayed,’ or ‘normal.’ The categorization was extrapolated to US TKA population and savings/expenses quantified. Statistical analyses were performed in Minitab with statistical significance set to α<0.05.Introduction
Methods
In an effort to provide a TKA bearing material that balances resistance to wear, mechanical failure and oxidation, manufacturers introduced antioxidant polyethylene. In many designs, this is accomplished through pre-blending the polymer with the antioxidant before consolidation and radiation crosslinking. This study reports the wear performance (in terms of thickness change) of a hindered phenol (PBHP) UHMWPE from analysis of an early series of knee retrievals and explores these questions: 1) What is early-time performance of this new bearing material? 2) Is there a difference in performance between fixed and mobile bearings in this design? 3) How does quantitative surface analysis help understand performance at the insert-tray modular interface? A series of 100 consecutive Attune™ knee inserts (DePuy Synthes, Warsaw, IN) received at revision by an IRB approved retrieval laboratory between September 2014 and March 2019 were investigated. In vivo duration was 0–52 months. Both the fixed bearing design (n=74) and the rotating platform mobile bearing design (n=26) were included. Dimensional change was determined by measurement of each insert and compared to the as-manufactured dimensions, provided by the manufacturer. The insert-tray interfaces under the loaded bearing zones were analyzed with light interferometry using an optical surface profiler (NewView™ 7300, Zygo, Middlefield, CT). Statistical analyses to explore relationships between measured variables were conducted using SPSS.Introduction
Methods
In vivo, UHMWPE bearing surfaces are subject to wear and oxidation that can lead to bearing fatigue or fracture. A prior study in our laboratory of early antioxidant (AO) polyethylene retrievals, compared to gamma-sterilized and highly cross-linked (HXL) retrievals, showed them to be more effective at preventing in vivo oxidation. The current analysis expands that early study, addressing the effect of:
manufacturing-variables on as-manufactured UHMWPE; in vivo time on these initial properties; identifying important factors in selecting UHMWPE for the hip or knee. After our prior report, our IRB-approved retrieval laboratory received an additional 96 consecutive AO-retrievals (19 hips, 77 knees: in vivo time 0–6.7 years) of three currently-marketed AO-polyethylenes. These retrievals represented two different antioxidants (Vitamin E and Covernox) and two different delivery methods: blending-prior-to and diffusing-after irradiation cross-linking. Consecutive HXL acetabular and tibial inserts, received at retrieval, with in vivo time of 0–6.7 years (260 remelted, 170 annealed) were used for comparison with AO-retrievals. All retrievals were analyzed for oxidation and trans-vinylene index (TVI) using a Thermo-Scientific iN10 FTIR microscope. Mechanical properties were evaluated for 35 tibial inserts by uniaxial tensile testing using an INSTRON load frame. Cross-link density (n=289) was measured using a previously published gravimetric gel swell technique. Oxidation was reported as maximum ketone oxidation index (KOI) measured for each bearing. TVI was reported as the average of all scans for each material. Cross-link density and mechanical properties were evaluated as a function of both TVI and oxidation.Introduction
Methods
Wear of polyethylene tibial inserts has been cited as being responsible for up to 25% of revision surgeries, imposing a very significant cost burden on the health care system and increasing patient risk. Accurate measurement of material loss from retrieved knee bearings presents difficult challenges because gravimetric methods are not useful with retrievals and unworn reference dimensions are often unavailable. Geometry and the local anatomy restrict in vivo radiographic wear analysis, and no large-scale analyses have illuminated long-term comparative wear rates and their dependence on design and patient factors. Our study of a large retrieval archive of knee inserts indicates that abrasive/adhesive wear of polyethylene inserts, both on the articular surface and on the backside of modular knees is an important contributor to wear, generation of debris and integrity of locking geometry. The objective of the current study is to quantify wear performance of tibial inserts in a large archive of retrieved knees of different designs. By assessing wear in a large and diverse series, the goal is to discern the effect on wear performance of a number of different factors: patient factors that might help guide treatment, knee design factors and bearing material factors that may inform a surgeon's choice from among the array of arthroplasty device options. An IRB approved retrieval database was queried for TKA designs implanted between 1997 and 2017. 1385 devices from 5 TKA designs were evaluated. Damage was ranked according to Hood's method, oxidation was determined through FTIR, and wear was determined through direct measurement of retrieved inserts using a previously established protocol. Design features (e.g. materials, conformity, locking mechanisms, stabilization, etc.) and patient demographics (e.g. age, weight, BMI, etc.) were cataloged. Multivariate analysis was performed to isolate factors contributing to wear, oxidation, and damage.Introduction
Methods
While advances in joint-replacement technology have made total ankle arthroplasty a viable treatment for end-stage arthritis, revision rates for ankle replacements are higher than in hip or knee replacements [1]. The questions asked in this study were (1) what retrieved ankle devices demonstrate about ankle arthroplasty failures, and (2) how do these failures compare to those seen in the hip and the knee? An IRB-approved retrieval laboratory received retrieved polyethylene inserts and surgeon-supplied reason for revision from 70 total-ankles (7 designs, including five currently-marketed designs) from 2002 to the present. All retrievals were rated for clinical damage. Polyethylene inserts received six months or less after retrieval (n=45) were analyzed for oxidation using Fourier Transform Infrared (FTIR) spectroscopy, reported as maximum ketone oxidation index [2]. Insert sterilization method was verified using trans-vinylene index [3]. Oxidation measured in the 45 ankle inserts versus their time in vivo was compared to oxidation rates previously published for gamma-sterilized hip and knee polyethylene retrievals [6]. Statistical analysis was performed using IBM SPSS v.22.Introduction
Materials and Methods
For nearly 58% of total knee arthroplasty (TKA) revisions, the reason for revision is exacerbated by component malalignment. Proper TKA component alignment is critical to functional outcomes/device longevity. Several methods exist for orthopedic surgeons to validate their cuts, however, each has its limitations. This study developed/validated an accurate, low-cost, easy to implement first-principles method for calculating 2D (sagittal/frontal plane) tibial tray orientation using a triaxial gyroscope rigidly affixed to the tibial plateau of a simulated leg jig and validated 2D tibial tray orientation in a human cadaveric model. An initial simulation assessed error in the sagittal/frontal planes associated with all geometric assumptions over a range of positions (±10°, ±10°, and −3°/0°/+3° in the sagittal, frontal, and transverse planes, respectively). Benchtop experiments (total positions - TP, clinically relevant repeated measures - RM, novice user - NU) were completed using a triaxial gyroscope rigidly affixed to and aligned with the tibial tray of the fully adjustable leg-simulation jig. Finally, two human cadaveric experiments were completed. A similar triaxial gyroscope was mounted to the tibial tray of a fresh frozen human cadaver to validate sagittal and frontal plane tibial tray orientation. In cadaveric experiment one, three unique frontal plane shims were utilized to measure changes in frontal plane angle. In cadaveric experiment two, measurements using the proprosed gyroscopic method were compared with computer navigation at a series of positions. For all experiments, one rotation of the leg was completed and gyroscopic data was processed through a custom analysis algorithm.Introduction
Methods
Subluxation and dislocation are frequently cited reasons for THA revision. For patients who cannot accommodate a larger femoral head, an offset liner may enhance stability. However, this change in biomechanics may impact the mechanical performance of the bearing surface. To our knowledge, no studies have compared wear rates of offset and neutral liners. Herein we radiographically compare the in-vivo wear performance of 0mm and 4mm offset acetabular liners. Two cohorts of 40 individuals (0mm, 4mm offset highly crosslinked acetabular liners, respectively) were selected from a single surgeon's consecutive caseload. All patients received the same THA system via the posterior approach. AP radiographs were taken at 6-week (‘pre’) and 5-year (‘post’) postoperative appointments. Patients with poor radiograph quality were excluded (n0mm=5, n4mm=4). Linear and volumetric wear were quantified according to Patent US5610966A. Briefly, images were processed in computer aided design (CAD) software. Differences in vector length between the center of the femoral head and the acetabular cup (pre- and post-vector, Figure 1) allow for calculation of linear wear and wear rate. The angle (β) between the linear wear vector and the cup inclination line was quantified (Figure 1). Patients with negative β were excluded from volumetric analyses (n0mm=11, n4mm=7). Volumetric wear was accordingly calculated accounting for wear vector direction. The results from three randomly selected patients were compared to results achieved using the “Hip Analysis Suite” software package (UChicagoTech).Introduction
Methods
The optimum UHMWPE orthopaedic implant bearing surface must balance wear, oxidation and fatigue resistance. Antioxidant polyethylene addresses free radicals, resulting from irradiation used in cross-linking, that could oxidize and potentially lead to fatigue damage under cycles of in vivo use. Assessing the effectiveness of antioxidant (AO) polyethylene compared to conventional gamma-sterilized or remelted highly cross-linked (HXL) polyethylene is necessary to set realistic expectations of the service lifetime of AO polyethylene in the knee. This study evaluates what short-term antioxidant UHMWPE retrievals can reveal about: (1) oxidation-resistance, and (2) fatigue-resistance of these new materials. An IRB-approved retrieval laboratory received 25 AO polyethylene tibial insert retrievals from three manufacturers with in vivo time of 0–3 years. These were compared with 20 conventional gamma-inert sterilized and 30 HXL (65-kGray, remelted) tibial inserts of the same in vivo duration range. The retrievals were (1) analyzed for oxidation and trans-vinylene index (TVI) using an FTIR microscope, and (2) inserts of sufficient size and thickness were evaluated for mechanical properties by uniaxial tensile testing using an INSTRON load frame. Oxidation was reported as maximum oxidation measured in the scan from the articular surface to the backside of each bearing. TVI was reported as the average of all scans for each material. Average ultimate tensile strength (UTS), ultimate elongation (UE), and toughness were the reported mechanical properties for each material.Introduction
Methods
A common phenomenon occurring as a result of reverse total shoulder arthroplasties (RSA) is scapular notching. While bone loss of the scapula may be quantified using radiographic techniques,[1] the material loss on the humeral bearing has not been quantified. Depending on their functional biological activity, a high volume of polyethylene wear particles has been shown to be related to osteolysis, bone loss and ultimately, loosening of implants in other joints.[2] In order to understand the threshold for osteolysis in the shoulder, it is important to have a method that can accurately quantify the amount of material loss. The aim of this research was to (I) create and validate a method for quantifying material loss from a single humeral implant design which can then (II) be used to measure retrieved devices. Measurement of the surface topography of the implant was completed using coordinate measurement machine (CMM). The resulting point cloud was then imported into MATLAB and run through a custom algorithm to determine the volumetric wear of the humeral liner. Two never implanted humeral liners with an artificially damaged material loss were used for validation purposes. Each component was scanned three times, analyzed using the custom MATLAB program, and compared to gravimetric analysis (Figure 1). Following validation, an IRB-approved database was queried to identify 10 retrieved components of the same design which were then analyzed using the validated method.Introduction
Methods
Fretting corrosion at the junction of the modular head neck interface in total hip arthroplasty is an area of substantial clinical interest. This fretting corrosion has been associated with adverse patient outcomes, including soft tissue damage around the hip joint. A number of implant characteristics have been identified as risk factors. However, much of the literature has been based on metal on metal total hip arthroplasty or subjective scoring of retrieved implants. The purpose of this study was to isolate specific implant variables and assess for material loss in retrieved implants with a metal on polyethylene bearing surface. All 28mm and 32 mm femoral heads from a 12/14 mm taper for a single implant design implanted for greater than 2 years were obtained from our institutional implant retrieval laboratory. This included n = 56 of the 28 mm heads (−3: n = 10, +0: n = 24, +4: n = 13, and +8: n = 9), and n = 23 of the 32 mm heads (−3: n = 2, +0: n = 8, +4: n = 1, and +8: n = 6). There were no differences between groups for age, gender, BMI, or implantation time. A coordinate measuring machine was used to acquire axial scans within each head, and the resulting point clouds were analyzed with a custom Matlab program. Maximum linear wear depth (MLWD) was calculated as the maximum difference between the material loss and as-machined surface. Differences in MLWD for head length, head diameter, stem material, and stem offset were determined.Background
Methods
Highly cross-linked (HXL) polyethylene has demonstrated clinical advantages as a wear resistant acetabular bearing material in total hip arthroplasty (THA) [1]. This study focuses on locating and quantifying the maximum linear wear of retrieved acetabular poly liners using a coordinate measuring machine (CMM). Specifically, HXL liners are compared to a baseline of conventional, non-HXL bearings. An IRB-approved retrieval laboratory received 63 HXL acetabular bearing retrievals from 5 manufacturers with in vivo durations of 1.01–14.85 years. These were compared with 32 conventional, non-HXL controls (including gas plasma, gamma-barrier and EtO) from 3 manufacturers with in vivo durations of 1.03–20.89 years. Liners were mounted in a tripod of axial contacts with the liner face positioned in a vertical plane. Each bearing was scanned with a CMM dual-probe head, with one horizontal probe scanning the articular surface and the other scanning the non-articular, sequentially. Surface-normal wall thickness values along each latitude were calculated using a custom developed algorithm (Figure 1). Because the liners are axially symmetric as manufactured, deviation in wall thickness at a given latitude represents linear wear [4].Introduction
Methods
Irradiated, thermally stabilized, highly cross-linked UHMWPE bearings have demonstrated superior wear performance and improved in vitro oxidation resistance compared with terminally gamma-sterilized bearings, yet retrieval analysis reveals unanticipated in vivo oxidation in these materials. There has been little evidence to date that oxidation in these materials is leading to degradation of mechanical properties, but since oxidation has previously been shown to cause chain scission in other materials, there is the potential for oxidation to cause decreased molecular weight and crosslink density. The aim of this study was to determine whether measured in vivo oxidation in highly cross-linked tibial bearings corresponds with a decreasing crosslink density. Retrieval analysis for three tibial bearing materials reveals that crosslink density is decreasing following in vivo duration, and that the change in crosslink density is strongly correlated with oxidation. The results suggest that oxidation in highly cross-linked materials is causing chain scissions that may impact the material properties. If the correlation between oxidation and duration continues, then as longer duration, more oxidized devices are retrieved there is a potential for measurable mechanical property changes.
Three materials were used for this study: a short-duration retrieved mobile-bearing conforming tibial insert with minimal oxidation (non-oxidized); a shelf-aged, oxidized, non-conforming fixed bearing tibial insert (oxidized); and standard NIST 1050 bar stock (NIST). Utilizing both conforming and non-conforming devices tests the technique over a range of articular curvatures, while testing a highly oxidized material tests the feasibility of maintaining the native surface when machining wear pins with compromised mechanical properties. FTIR analysis was performed at the articular surface of the devices near where the pins were taken, using ketone peak height as an indicator of oxidation. Wear rates were determined using a six station AMTI OrthoPod with an applied load of 100 N in multidirectional motion for a total of 2 million cycles. The oxidized material had a surface ketone level of 0.26, the non-oxidized device had a ketone level of 0.05, and the NIST sample had a ketone level less than 0.01. Two pins of each material were machined to ¼″ diameter with a length of the through thickness of the tibial inserts; soak controls were also produced. Figure 1 shows mass loss data for all six pins tested. Wear rates between the two pins of each group were fairly repeatable, and the wear rates of the different groups could be easily differentiated. The pins machined from NIST bar stock showed the best match-up, but pins machined from retrieved devices also showed good repeatability, with the non-conforming device showing better results than the conforming device. The ability to produce repeatable wear results with pins machined from
A common feature of retrieved ceramic-on-ceramic (CoC) hips is the presence of metal transfer on the femoral head. This metal transfer represents an important change in the articulating surface and can have consequences in terms of lubrication, friction, wear, and squeaking. Given the potential impact of metal transfer on the performance of CoC bearing couples, a good understanding of the factors surrounding its occurrence is warranted. This study documents the metal transfer onto a ceramic femoral head with two subluxations onto the rim of the cup which occurred during surgery. This metal transfer is compared to that on other ceramic heads retrieved for various reported reasons, including squeaking, pain and loosening. The first ten retrieved alumina heads of current ceramic technology (Ceramtec, Plochingen, Germany) submitted to our retrieval laboratory were assessed to document the phenomenon of metal transfer. Nine devices underwent in vivo service (mean duration 32 mo., range 13 to 84) and the tenth device was removed intra-operatively and serves as an instructive control case. It was impacted onto a trunnion and during final testing for stability subluxed anteriorly over the titanium lip of the cup. The metal transfer was immediately noted by the surgeon and the head was removed. All ceramic heads were examined under light microscopy (Nikon Dissecting Microscope, Tokyo, Japan) and white light optical profilometry (NewView 7300, Zygo, Middlefield, CT). The control ceramic head showed two distinct metal transfer streaks from two discrete subluxation events that were documented by the surgeon (IMT). Those streaks are aligned in a direction approximately 24o to the right (clockwise) of a line through the polar apex of the head and parallel to the axis of the femoral neck. Microscopy and profilometry indicate that they were laid down in a direction from equator-toward-pole. Seven of the retrieved ceramic heads showed streaks of metal transfer that are very similar to those on the control ceramic head in terms of: alignment (equator-toward-pole, 20 to 45o off-axis) width (tapered point growing to approximately 1.0 to 1.5 mm), depth of metal deposition (0.25 to 0.40 μm), and depositional texture. It is notable that the metal transfer streaks commonly observed on retrievals bear a close resemblance to that caused by a single intra-operative event wherein a hip abduction force pulled the head into contact with the titanium cup/liner rim. An important implication is that this demonstrates that metal transfer can occur with a single instance of rim contact, wherein the femoral head is forced against the metal cup rim. If metal transfer onto the head were to occur during final reduction of the hip, its presence may well be undetected and any deleterious in vivo impact of the metal transfer would be in effect from the day of surgery.
Squeaking of ceramic-on-ceramic (CoC) hips is a clinical phenomenon that is concerning with regard to the long term performance of these joint devices. Investigations into the cause of the squeaking have focused on patient factors and demographics, surgical placement, and other non-ceramic components in the devices. The current study tests latest-generation CoC devices to measure the vibration modes and frequencies of the components individually as well as assembled in the complete surgical construct. Audio data from clinical cases of squeaking hips were analysed to determine the frequencies present. Retrieved CoC hips (n = 7) and never-implanted CoC bearing couples (n = 3) were tested in the laboratory for squeaking under loaded articulation. Bovine serum was introduced into the CoC articulation and dried to promote stick-slip motion at the articulation. Squeaking sounds from the in vitro tests were recorded for audio analysis. Low mass, high frequency-response ceramic shear piezoelectric accelerometers (PCB Piezotronics) were adhered to the hip components along multiple axes to measure vibrations during testing. Clinical audio shows that squeaking occurs at fundamental frequencies in the range of 1 to 3 kHz, with harmonics above the fundamental frequency. Retrieved CoC bearing couples squeaked at fundamental frequencies from 1.5 kHz to 3.8 kHz. Fourier Transform analysis of the audio closely matched the concurrent output from the accelerometers mounted directly on the ceramic components. This held true even in the absence of metal components in the system. With metal components included in the test construct (acetabular shell, acetabular cup, femoral stem), those components also vibrated at the same frequencies as the ceramic bearing couples, indicating that the CoC articulation is the source of the vibrations, with metal components conducting and emanating the sound. The never-implanted bearing couples were made to squeak and vibrated at fundamental frequencies ranging from 1 kHz to 8 kHz. Squeaking from CoC hips can be reproduced in the lab using components from clinical retrievals. Instrumentation of the explanted hips confirms that the vibration frequencies of the ceramic components themselves match the audible squeaking. The squeaking of ceramic components mounted with soft polymers and with no metal contact at any point indicates that the ceramic components themselves are the source of the clinical squeaking. The measured vibration of ceramic components in the audible range is an observation not predicted by modeling studies reported in the literature to date.
Irradiation cross-linking of UHMWPE has been shown to reduce wear while generating free radicals that oxidise in the presence of oxygen or oxidising species. Various methods have been used to minimise or eliminate the effect of these free radicals including below-melt annealing, remelting, Vitamin E infusion, or the use of other antioxidants. Each method has benefits and drawbacks with respect to wear properties, mechanical properties, and chemical properties. Accelerated aging techniques are used to evaluate the efficacy of new methods in stabilising free radicals in highly cross-linked UHMWPE. Various procedures have been described for aging standard gamma-air sterilised UHMWPE to produce oxidation levels that represent shelf-aged bearings. An important factor in evaluating and comparing these aging techniques is validating that they reproduce the profile of oxidation (depth and magnitude) seen both in gamma-air, shelfaged polyethylene and in clinical retrievals. Moreover, the resulting oxidation level in the aged UHMWPE should predict the fatigue and/or wear damage seen in retrieved gamma-air inserts and liners. The present study compared clinically relevant UHMWPE samples aged with ASTM 2003-00, (Method B: 70°C, 5 atm O2, 14 days) and a published lower temperature, lower oxygen-pressure environment (63° C, 3 atm O2, 28 days). Longer aging times (35 to 42 days) were also tested to examine oxidation rate and time to onset of mechanical degradation. Both published methods result in oxidation of gamma-air and gamma-barrier sterilised polyethylene, but have little effect on remelted or antioxidant stabilised crosslinked polyethylene. These aging protocols, however, did not bring standard polyethylene to the critical oxidation level necessary for the fatigue damage that is seen in retrieved inserts and liners. Oxidation of gamma-air and gamma-barrier sterilised UHMWPE increases exponentially with time on the shelf or in the two aging environments. Of note, longer aging times (35 to 42 days) that bring standard UHMWPE to sufficiently high oxidation levels for fatigue to occur also cause increased oxidation levels in remelted UHMWPE. Oxidation increases were the smallest in antioxidant UHMWPE, though still detectable. While this oxidation is not high enough in remelted material or antioxidant material to cause the fatigue damage seen in gamma-air sterilised UHMWPE, it does raise concerns about the published aging techniques and the long term stability of the new materials in vivo. Relying on artificial aging techniques that do not adequately challenge even gamma-air polyethylene may conceal unforeseen weaknesses of new materials. Using longer aging times for existing techniques or novel aging approaches may be necessary to effectively evaluate the long term stability of new bearing materials.
