Wear rates of highly cross-linked polyethylene humeral liners subjected to alternating cycles of glenohumeral flexion and abduction

Effect of vitamin E-enhanced highly cross-linked polyethylene on wear rate and particle debris in anatomic total shoulder arthroplasty: a biomechanical comparison to ultrahigh-molecular-weight polyethylene

BACKGROUND

Particle-induced osteolysis resulting from polyethylene wear remains a source of implant failure in anatomic total shoulder designs. Modern polyethylene components are irradiated in an oxygen-free environment to induce cross-linking, but reducing the resulting free radicals with melting or heat annealing can compromise the component's mechanical properties. Vitamin E has been introduced as an adjuvant to thermal treatments. Anatomic shoulder arthroplasty models with a ceramic head component have demonstrated that vitamin E-enhanced polyethylene show improved wear compared with highly cross-linked polyethylene (HXLPE). This study aimed to assess the biomechanical wear properties and particle size characteristics of a novel vitamin E-enhanced highly cross-linked polyethylene (VEXPE) glenoid compared to a conventional ultrahigh-molecular-weight polyethylene (UHMWPE) glenoid against a cobalt chromium molybdenum (CoCrMo) head component.

METHODS

Biomechanical wear testing was performed to compare the VEXPE glenoid to UHMWPE glenoid with regard to pristine polyethylene wear and abrasive endurance against a polished CoCrMo alloy humeral head in an anatomic shoulder wear-simulation model. Cumulative mass loss (milligrams) was recorded, and wear rate calculated (milligrams per megacycle [Mc]). Under pristine wear conditions, particle analysis was performed, and functional biologic activity (FBA) was calculated to estimate particle debris osteolytic potential. In addition, 95% confidence intervals for all testing conditions were calculated.

RESULTS

The average pristine wear rate was statistically significantly lower for the VEXPE glenoid compared with the HXLPE glenoid (0.81 ± 0.64 mg/Mc vs. 7.00 ± 0.45 mg/Mc) (P < .05). Under abrasive wear conditions, the VEXPE glenoid had a statistically significant lower average wear rate compared with the UHMWPE glenoid comparator device (18.93 ± 5.80 mg/Mc vs. 40.47 ± 2.63 mg/Mc) (P < .05). The VEXPE glenoid demonstrated a statistically significant improvement in FBA compared with the HXLPE glenoid (0.21 ± 0.21 vs. 1.54 ± 0.49 (P < .05).

CONCLUSIONS

A new anatomic glenoid component with VEXPE demonstrated significantly improved pristine and abrasive wear properties with lower osteolytic particle debris potential compared with a conventional UHMWPE glenoid component. Vitamin E-enhanced polyethylene shows early promise in shoulder arthroplasty components. Long-term clinical and radiographic investigation needs to be performed to verify if these biomechanical wear properties translate to diminished long-term wear, osteolysis, and loosening.

Survivorship of shoulder arthroplasty in young patients with osteoarthritis: an analysis of the Australian Orthopaedic Association National Joint Replacement Registry

BACKGROUND

The treatment of shoulder osteoarthritis in the young patient remains challenging. The higher functional demands and higher expectations of the young patient cohort are often coupled with increased failure and revision rates. Consequently, shoulder surgeons are faced with a unique challenge with implant selection. The aim of this study was to compare the survivorship and reasons for revision of 5 classes of shoulder arthroplasty in patients aged <55 years with a primary diagnosis of osteoarthritis by use of data from a large national arthroplasty registry.

METHODS

The study population included all primary shoulder arthroplasty procedures undertaken for osteoarthritis in patients aged <55 years and reported to the registry between September 1999 and December 2021. Procedures were grouped into the following classes: total shoulder arthroplasty (TSA), hemiarthroplasty resurfacing (HRA), hemiarthroplasty stemmed metallic head (HSMH), hemiarthroplasty stemmed pyrocarbon head (HSPH), and reverse total shoulder arthroplasty (RTSA). The outcome measure was the cumulative percent revision, which was defined using Kaplan-Meier estimates of survivorship to describe the time to the first revision. Hazard ratios (HRs) were calculated from Cox proportional hazards models, adjusting for age and sex, to compare revision rates among groups.

RESULTS

There were 1564 shoulder arthroplasty procedures in patients aged <55 years, of which 361 (23.1%) were HRA, 70 (4.5%) were HSMH, 159 (10.2%) were HSPH, 714 (45.7%) were TSA, and 260 (16.6%) were RTSA. HRA had a higher rate of revision than RTSA after 1 year (HRA = 2.51 (95% CI 1.30, 4.83), P = .005), with no difference prior to that time. In addition, HSMH had a higher rate of revision than RTSA for the entire period (HR, 2.69 [95% confidence interval, 1.28-5.63], P = .008). There was no significant difference in the rate of revision for HSPH and TSA when they were compared with RTSA. Glenoid erosion was the most common cause of revision for HRA (28.6% of revisions) and HSMH (50%). Instability/dislocation was the leading cause of revision for RTSA (41.7%) and HSPH (28.6%), and for TSA, the majority of revisions were for either instability/dislocation (20.6%) or loosening (18.6%).

CONCLUSION

These results should be interpreted within the context of the lack of availability of long-term data on RTSA and HSPH stems. RTSA outperforms all implants regarding revision rates at mid-term follow-up. The high early dislocation rate associated with RTSA, as well as the lack of revision options available to address this, indicates that careful selection of patients and a greater appreciation of anatomic risk factors are needed in the future.

Reduced Revision Rates in Total Shoulder Arthroplasty With Crosslinked Polyethylene: Results From the Australian Orthopaedic Association National Joint Replacement Registry

BACKGROUND

Loss of glenoid fixation is a key factor affecting the survivorship of primary total shoulder arthroplasty (TSA). It is not known whether the lower revision rates associated with crosslinked polyethylene (XLPE) compared with those of non-XLPE identified in hip and knee arthroplasty apply to shoulder arthroplasty.

QUESTIONS/PURPOSES

We used data from the Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) to compare the revision rates of primary stemmed anatomic TSA using XLPE to procedures using non-XLPE. In patients receiving a primary stemmed anatomic TSA for osteoarthritis, we asked: (1) Does the rate of revision or reason for revision vary between XLPE and non-XLPE all-polyethylene glenoid components? (2) Is there any difference in the revision rate when XLPE is compared with non-XLPE across varying head sizes? (3) Is there any difference in survival among prosthesis combinations with all-polyethylene glenoid components when they are used with XLPE compared with non-XLPE?

METHODS

Data were extracted from the AOANJRR from April 16, 2004, to December 31, 2020. The AOANJRR collects data on more than 97% of joint replacements performed in Australia. The study population included all primary, stemmed, anatomic TSA procedures performed for osteoarthritis using all-polyethylene glenoid components. Procedures were grouped into XLPE and non-XLPE bearing surfaces for comparison. Of the 10,102 primary stemmed anatomic TSAs in the analysis, 39% (3942 of 10,102) used XLPE and 61% (6160 of 10,102) used non-XLPE. There were no differences in age, gender, or follow-up between groups. Revision rates were determined using Kaplan-Meier estimates of survivorship to describe the time to the first revision, with censoring at the time of death or closure of the database at the time of analysis. Revision was defined as removal, replacement, or addition of any component of a joint replacement. The unadjusted cumulative percent revision after the primary arthroplasty (with 95% confidence intervals [CIs]) was calculated and compared using Cox proportional hazard models adjusted for age, gender, fixation, and surgeon volume. Further analyses were performed stratifying according to humeral head size, and a prosthesis-specific analysis adjusted for age and gender was also performed. This analysis was restricted to prosthesis combinations that were used at least 150 times, accounted for at least four revisions, had XLPE and non-XLPE options available, and had a minimum of 3 years of follow-up.

RESULTS

Non - XLPE had a higher risk of revision than XLPE after 1.5 years (HR 2.3 [95% CI 1.6 to 3.1]; p < 0.001). The cumulative percent revision at 12 years was 5% (95% CI 4% to 6%) for XLPE and 9% (95% CI 8% to 10%) for non-XLPE. There was no difference in the rate of revision for head sizes smaller than 44 mm. Non-XLPE had a higher rate of revision than XLPE for head sizes 44 to 50 mm after 2 years (HR 2.3 [95% CI 1.5 to 3.6]; p < 0.001) and for heads larger than 50 mm for the entire period (HR 2.2 [95% CI 1.4 to 3.6]; p < 0.001). Two prosthesis combinations fulfilled the inclusion criteria for the prosthesis-specific analysis. One had a higher risk of revision when used with non-XLPE compared with XLPE after 1.5 years (HR 3.7 [95% CI 2.2 to 6.3]; p < 0.001). For the second prosthesis combination, no difference was found in the rate of revision between the two groups.

CONCLUSION

These AOANJRR data demonstrate that noncrosslinked, all-polyethylene glenoid components have a higher revision rate compared with crosslinked, all-polyethylene glenoid components when used in stemmed anatomic TSA for osteoarthritis. As polyethylene type is likely an important determinant of revision risk, crosslinked polyethylene should be used when available, particularly for head sizes larger than 44 mm. Further studies will need to be undertaken after larger numbers of shoulder arthroplasties have been performed to determine whether this reduction in revision risk associated with XLPE bears true for all TSA designs.

LEVEL OF EVIDENCE

Level III, therapeutic study.

Wear and damage in retrieved humeral inlays of reverse total shoulder arthroplasty-where, how much, and why?

BACKGROUND

Polyethylene (PE) wear and material degradation have been reported as complications in reverse total shoulder replacements (rTSAs). In this regard, scapular notching is associated with more clinical complications. Therefore, the purposes of the study were to quantify the linear and volumetric wear, as a measure for the amount of removed material, and to qualitatively assess the PE damage modes to describe the material degradation in retrieved rTSA humeral PE inlays that contribute to failure of shoulder replacements. Furthermore, this study aimed to evaluate the effect of scapular notching on PE wear and rim damage of the humeral components.

METHODS

The total study population of 39 humeral inlays contains 2 cohorts that were used for the damage mode analysis and for the wear analysis, respectively. The extent and presence of wear damage modes in 5 defined zones were assessed by a grading system for all PE joint replacements. For quantitative wear analysis the most frequent design (n = 17) was chosen. Using a coordinate-measuring machine and postprocessing software, volumetric wear measurements for the retrieved humeral PE inlays were undertaken. Furthermore, prerevision radiographs were analyzed for scapular notching. Finally, retrieval findings were correlated with clinical and radiographic data to consider the effect of notching and to identify risk of failures for these prostheses.

RESULTS

Damage on the rim of the humeral PE inlays was more frequent and severe than on the intended articulation surface. Irrespective of the damage mode, the inferior rim zone sustained the greatest amount of wear damage followed by the posterior zone. Burnishing, scratching, pitting, and embedded particles are most likely to occur in the articular surface area, whereas surface deformation, abrasion, delamination and gross material degradation are predominantly present in the inferior and posterior rim zones. The retrieved inlays exhibited a mean volumetric wear rate of 296.9 mm³/yr ± 87.0 mm³/yr. However, if the notched and non-notched components were compared, a significant higher volumetric wear rate (296.5 ± 106.1 mm³/yr) was found for the notched components compared to the non-notched group (65.7 ± 7.4 mm³/yr). Generally, there was a significantly greater incidence of damage and greater amount of wear if scapular notching occurred.

CONCLUSION

The notched components showed a 5-fold increase in PE wear rate. Therefore, scapular notching has a strong effect on PE damage and wear. If scapular notching can be clinically avoided, the PE wear performance is in a similar magnitude as found for hip and knee replacements.

Prosthetic Bearing Surfaces in Anatomic and Reverse Total Shoulder Arthroplasty

Total shoulder arthroplasty (TSA) and reverse TSA have provided an effective treatment for glenohumeral osteoarthritis; however, longevity of the procedure may be limited by osteolysis and polyethylene wear. In TSA, glenoid component failure occurs through several mechanisms, the most common being aseptic loosening and polyethylene wear. Newer bearing surfaces such as highly cross-linked ultra-high-molecular-weight polyethylene, vitamin E processing, ceramic heads, and pyrolytic carbon surfaces have shown improved wear characteristics in biomechanical and some early clinical studies. The purpose of this review is to provide a historical perspective and current state of the art of bearing surface technology in anatomic and reverse TSA.

In vivo volumetric and linear wear measurement of reverse shoulder arthroplasty at minimum 5-year follow-up

BACKGROUND

Reverse shoulder arthroplasty is quickly becoming the most frequently performed glenohumeral joint replacement. The purpose of this study was to evaluate the volumetric and linear wear rates of ultrahigh-molecular-weight polyethylene humeral liners in vivo at a minimum 5-year follow-up.

METHODS

Radiostereometric analysis was used to image 15 patients at terminal range of motion in forward flexion, abduction, external rotation, and internal rotation and with the arm at the side. The relative position and orientation of the glenosphere and polyethylene were identified for each arm position. The apparent intersection of the glenosphere into the polyethylene was recorded as wear. Mean volumetric and linear wear rates were recorded, and Pearson correlation coefficients were applied to the 36-mm liners to assess the relationship between the wear rate and term of service.

RESULTS

The mean reverse shoulder arthroplasty term of service at the time of imaging was 8 ± 1 years (range, 6-11 years). The mean volumetric and linear wear rates for the 36-mm liners (n = 13) were 42 ± 22 mm³/yr (r = 0.688, P = .009) and 0.11 ± 0.03 mm/yr (r = 0.767, P = .002), respectively. The mean volumetric and linear wear rates for the 42-mm liners (n = 2) were 114 ± 44 mm³/yr and 0.17 ± 0.01 mm/yr, respectively. No single arm position was able to capture all recorded wear individually.

CONCLUSION

This study showed volumetric and linear wear rates of approximately 40 mm³/yr and 0.1 mm/yr, respectively, for the 36-mm polyethylene liners. The 42-mm liners showed higher wear rates, although a greater number of subjects is required for conclusive results. In vivo wear of reverse total shoulder arthroplasty is multidirectional and perceptible.

Reverse Total Shoulder Arthroplasty: Technique, Decision-Making and Exposure Tips

PURPOSE OF REVIEW

The goal of this review is to introduce surgical decision-making pearls for reverse shoulder arthroplasty and describe optimization of surgical exposure for reverse shoulder arthroplasty.

RECENT FINDINGS

While the technology of reverse shoulder replacement and the associated prosthetic options have expanded, the principles involved in successfully exposing the humerus and glenoid in arthroplasty remain the same. Reverse shoulder replacement should be considered in arthroplasty situations with rotator cuff disease, deformity, bone loss, and instability as part of the diagnosis. Optimal exposure in reverse shoulder arthroplasty can be obtained by (1) releasing deltoid adhesions, (2) removal of humeral osteophytes, (3) generous humeral head cuts, (4) thorough humeral and glenoid capsular release and (5) optimal glenoid retractor placement. Neuromuscular paralysis can also aid glenoid exposure.

The effect of vitamin E-enhanced cross-linked polyethylene on wear in shoulder arthroplasty-a wear simulator study

BACKGROUND

Wear of the polyethylene glenoid component and subsequent particle-induced osteolysis remains one of the most important modes of failure of total shoulder arthroplasty. Vitamin E is added to polyethylene to act as an antioxidant to stabilize free radicals that exist as a byproduct of irradiation used to induce cross-linking. This study was performed to assess the in vitro performance of vitamin E-enhanced polyethylene compared with conventional polyethylene in a shoulder simulator model.

METHODS

Vitamin E-enhanced, highly cross-linked glenoid components were compared with conventional ultrahigh-molecular-weight polyethylene glenoids, both articulating with a ceramic humeral head component using a shoulder joint simulator over 500,000 cycles. Unaged and artificially aged comparisons were performed. Volumetric wear was assessed by gravimetric measurement, and wear particle analysis was also subsequently performed.

RESULTS

Vitamin E-enhanced polyethylene glenoid components were found to have significantly reduced wear rates compared with conventional polyethylene in both unaged (36% reduction) and artificially aged (49% reduction) comparisons. There were no differences detected in wear particle analysis between the 2 groups.

CONCLUSION

Vitamin E-enhanced polyethylene demonstrates improved wear compared with conventional polyethylene in both unaged and artificially aged comparisons and may have clinically relevant benefits.

Validation of In Vivo Linear and Volumetric Wear Measurement for Reverse Total Shoulder Arthroplasty Using Model-Based Radiostereometric Analysis

Presently, polyethylene wear measurement of reverse total shoulder arthroplasty (rTSA) is restricted to in vitro, in silico, and retrieval analysis, with no method for the quantification of in vivo wear of well-functioning implants. The purpose of this study was to validate the use of model-based radiostereometric analysis (MBRSA) as a measurement tool for in vivo rTSA wear using a phantom setup. Six additively manufactured polyethylene inserts were fabricated, one unworn control and five to represent known wear patterns, and individually fit within the rTSA components. Each insert was imaged using standard radiostereometric techniques and analyzed using MBRSA. From the position and orientation estimation provided by MBRSA, a micro-computed tomography model of the control insert was virtually placed within the metaphyseal tray. The apparent intersection of the glenosphere into the insert was recorded as wear. This method enables wear measurements with a linear precision of 0.21 mm and a bias of 0.36 ± 0.13 mm, and a volumetric precision of 49.3 mm³ , with a bias of 48.9 ± 24.3 mm³ . This technique allows for the in vivo measurement of polyethylene wear without the requirement of marker beads or baseline radiographs, expanding the potential for in vivo wear measurements to larger populations and retrospective analysis. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1620-1627, 2019.

Quantitative evaluation of retrieved reverse total shoulder arthroplasty liner surface deviation and volumetric wear

Polyethylene wear is a known complication in total joint arthroplasty, however, in vivo wear rates in reverse total shoulder arthroplasty (RTSA) remain largely unknown. This study aimed to quantify volumetric and surface deviation changes in retrieved RTSA humeral liners using a novel micro-computed tomography (μCT)-based technique. After IRB-approval, 32 humeral liners (single manufacturer and model) with term-of-service greater than 90 days were analyzed. Clinical demographics and surgical data were collected via chart review. Unworn liners were used as geometric controls. Retrieved and unworn liners underwent μCT scanning. Retrieved liner volumes were isolated, co-registered to controls of matching geometry, and surface deviations of the articulation surface and rim were computed. Differences in total volume loss (TVL), volumetric wear rate (VWR), and surface deviation were reported. Semi-quantitative grading evaluated rim damage presence and severity. Mean term-of-service for all liners was 2.07 ± 1.33 years (range: 0.30-4.73). Mean TVL and VWR were 181.3 ± 208.2 mm³ and 114.5 ± 160.3 mm³ /year, respectively. Mean articulation and rim surface deviations were 0.084 ± 0.065 and 0.177 ± 0.159 mm, respectively. Articulation surface deviation was positively correlated to term-of-service. Rim damage was present on 63% of liners and correlated significantly to rim surface deviation. This study reports in vivo wear rates of retrieved RTSA implants. Our results demonstrate volumetric and articulation surface wear in select RTSA liners that is correlated to term-of-service. Calculation of in vivo wear rates can help bridge the gap between clinical outcomes and experimental models such as wear simulations and computational models. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2007-2014, 2018.

What can be learned from an analysis of 215 glenoid component failures?

BACKGROUND

Glenoid component failure is a prevalent mechanical complication of anatomic total shoulder arthroplasty. The objective of this study was to identify surgeon-controlled factors that may be addressed to reduce the rate of glenoid component failure that is sufficiently symptomatic to merit surgical revision.

METHODS

We reviewed the clinical and radiographic features of 215 total shoulder arthroplasties that we revised for symptomatic glenoid component failure.

RESULTS

Glenoid component failure was associated with poor patient self-assessed shoulder function (mean Simple Shoulder Test score, 3.0 ± 2.7). These shoulders often showed multiple failure modes; 72% had glenoid component loosening, 69% had polyethylene wear, 51% had glenohumeral decentering, and 25% had humeral component loosening. Metal-backed/hybrid and keeled glenoid designs had higher rates of loosening (P = .010), malposition (P = .007), dislocation (P < .001), and early failure (P = .044) in comparison to pegged designs. Glenoid components with cement on the backside were more prevalent among those revised sooner than 5 years after the index surgery (P < .001).

CONCLUSIONS

Glenoid component failure remains a major cause of poor patient outcomes after total shoulder arthroplasty. The occurrence of severe glenoid component failure might be reduced by paying attention to glenoid component design and insertion technique, restoring the normal balance of the humeral head in the center of the glenoid, and considering a reverse total shoulder when the shoulder is unstable because of soft tissue deficiency.

Neer Award 2017: wear rates of 32-mm and 40-mm glenospheres in a reverse total shoulder arthroplasty wear simulation model

BACKGROUND

Larger glenosphere diameters have been used recently to increase prosthesis stability and impingement-free range of motion in reverse total shoulder arthroplasty. The goal of this study was to evaluate the rate of polyethylene wear for 32-mm and 40-mm glenospheres.

METHODS

Glenospheres (32 mm and 40 mm, n = 6/group) and conventional polyethylene humeral liners underwent a 5-million cycle (MC) wear simulation protocol. Abduction-adduction and flexion-extension motion profiles were alternated every 250,000 cycles. At each interval, mass loss was determined and converted to volume loss and wear rate. At 0, 2.5 MC, and 5 MC, liners were imaged using micro-computed tomography to determine surface deviation. White light interferometry was performed on liners and glenospheres at 0 and 5 MC to quantify surface roughness. Wear particle morphology was characterized by environmental scanning electron microscopy.

RESULTS

Total volume loss was significantly higher in 40-mm liners from 1.5 MC onward (P < .05). Overall, volumetric wear rate was significantly higher in 40-mm liners compared with 32-mm glenospheres (81.7 ± 23.9 mm³/MC vs. 68.0 ± 18.9 mm³/MC; P < .001). However, micro-computed tomography surface deviation results demonstrated increased linear penetration on 32-mm glenospheres compared with 40-mm glenospheres (0.36 ± 0.03 µm vs. 0.28 ± 0.01 µm; P = .002). Surface roughness measurements showed no difference for liners; however, increased roughness was noted for 40-mm glenospheres at 5 MC compared with 32 mm (P < .05).

CONCLUSION

Larger glenospheres underwent significantly greater polyethylene volume loss and volumetric wear rates, whereas smaller glenospheres underwent greater polyethylene surface deviations. The enhanced stability provided by larger glenospheres must be weighed against the potential for increased polyethylene wear.

Validation of a protocol based on Raman and infrared spectroscopies to nondestructively estimate the oxidative degradation of UHMWPE used in total joint arthroplasty

As a matter of fact, the in vivo oxidative degradation of highly cross-linked polyethylene (HXLPE) still remains one of the limiting factors that affect the long term survivorship of joint replacements. Recent studies clearly pointed out that also the new generation of highly cross-linked and remelted polyethylene components in total hip and knee replacement underwent unexpected oxidation after 5-10years of implantation. The standard methodology to investigate the oxidation of polyethylene (PE) relies on the use of infrared spectroscopy, which, if from one hand is a reliable technique for the detection of oxidized species containing carbonyl group, on the other hand it is not capable of discriminating the fraction of carboxyl acids that is responsible for chain scission and subsequent deterioration of the mechanical properties of the polymer. In the present study we validate a new protocol based on Raman spectroscopy, which is suitable on assessing the structural degradation of polyethylene induced by oxidation. Following in vitro accelerated aging experiments, the oxidation index (OI) of different commercially available HXLPEs, as calculated by infrared spectroscopy according to ASTM standard, has been univocally correlated to the most severe variation of crystalline phase (αc), as calculated by Raman spectroscopy. In each material, locations with equal values of OI showed different degree of recrystallization induced by chain scission, confirming that infrared spectroscopy might overestimate the effective mechanical degradation of the polymer. In addition, as compared to the standards based on infrared spectroscopy, this new method of assessing oxidation enables to investigate the degradation occurring on the original surface of HXLPE components, due to the nondestructive nature of Raman spectroscopy and its high spatial resolution.

STATEMENT OF SIGNIFICANCE

In the present study we validate a new protocol based on Raman spectroscopy, which is suitable on assessing the structural degradation of polyethylene induced by oxidation. In fact, the standard methodology to investigate the oxidation in polyethylene relies on the use of infrared spectroscopy, which is capable of detecting the presence of oxidized species containing carbonyl group, the main products of oxidation in polyolefins. If from one hand this technique enables quantitative analysis of oxidation, on the other hand it is not capable of discriminating the fraction of species with carbonyl groups responsible for the chain scission. In fact, esters, ketones and carboxyl acids are products of oxidation with carbonyl groups commonly formed on polyethylene at the end of the oxidative cascade initiated by the presence of free radicals, but only the latter are responsible for the chain scission and the subsequent deterioration of the mechanical properties. The oxidation index as obtained according to the ASTM standards is not univocally correlated to a certain degree of mechanical deterioration, but, in simple words, two retrievals with the same amount of carbonyl groups might have had different degradation of the mechanical properties. Recrystallization is a direct consequence of the reduction of molecular weight that occurs after chain scission. Raman spectroscopy (RS) is a viable non-destructive method to assess the fraction of crystalline phase in polyethylene and, due to its high spatial resolution, is perfectly suitable to analyze the microstructural modification at the mesoscopic scale, where the effects of oxidation manifest themselves. The aim of the present paper is twofold: i) to compare the microstructural modifications caused by in vitro oxidation on 5 different types of polyethylene currently available on the market of joint replacements; ii) to establish a protocol based on the comparative analysis of IR and RS results to obtain a phenomenological correlation capable to judge the mechanical deterioration of the material induced by the oxidative degradation.

Wear simulation strategies for reverse shoulder arthroplasty implants

Reverse total shoulder arthroplasty is a clinically accepted surgical procedure; however, its long-term wear performance is not known. The purpose of this work is to review wear simulator testing of reverse total shoulder arthroplasty, to develop a wear simulator protocol for reverse total shoulder arthroplasty, and to test it by performing a pilot study. The review of wear simulator testing in the literature revealed considerable variation in protocols. A combination of our own cadaveric testing and those of other research groups helped in determining the magnitude and direction of joint loading for the development of the present protocol. A MATCO orbital-bearing simulator was adapted using custom fixtures to simulate a circumduction motion of the shoulder under mildly adverse conditions, and a pilot study gave wear rates within the wide range found in the literature. Arguments were presented in support of the currently developed protocol, but it was also suggested that, rather than rely on one protocol, a series of simulator wear protocols should be developed to fully test the implant wear performance in reverse total shoulder arthroplasty.

Effect of size and dimensional tolerance of reverse total shoulder arthroplasty on wear: An in-silico study

Although huge research efforts have been devoted to wear analysis of ultra-high molecular weight polyethylene (UHMWPE) in hip and knee implants, shoulder prostheses have been studied only marginally. Recently, the authors presented a numerical wear model of reverse total shoulder arthroplasties (RTSAs), and its application for estimating the wear coefficient k from experimental data according to different wear laws. In this study, such model and k expressions are exploited to investigate the sensitivity of UHMWPE wear to implant size and dimensional tolerance. A set of 10 different geometries was analysed, considering nominal diameters in the range 36-42mm, available on the market, and a cup dimensional tolerance of +0.2, -0.0mm (resulting in a diametrical clearance ranging between 0.04-0.24mm), estimated from measurements on RTSAs. Since the most reliable wear law and wear coefficient k for UHMWPE are still controversial in the literature, both the Archard law (AR) and the wear law of UHMWPE (PE), as well as four different k expressions were considered, carrying out a total of 40 simulations. Results showed that the wear volume increases with the implant size and decreases with the dimensional tolerance for both the wear laws. Interestingly, different trends were obtained for the maximum wear depth vs. clearance: the best performing implants should have a high conformity according to the AR law but low conformity for the PE law. However, according to both laws, wear is highly affected by both implant size and dimensional tolerance, although it is much more sensitive to the latter, with up to a twofold variation of wear predicted. Indeed, dimensional tolerance directly alters the clearance, and therefore the lubrication and contact pressure distribution in the implant. Rather surprisingly the role of dimensional tolerance has been completely disregarded in the literature, as well as in the standards. Furthermore, this study notes some important issues for future work, such as the validation of wear laws and predictive wear models and the sensitivity of k to implant geometry.

Contact mechanics of reverse total shoulder arthroplasty during abduction: the effect of neck-shaft angle, humeral cup depth, and glenosphere diameter

BACKGROUND

Implant design parameters can be changed during reverse shoulder arthroplasty (RSA) to improve range of motion and stability; however, little is known regarding their impact on articular contact mechanics. The purpose of this finite element study was to investigate RSA contact mechanics during abduction for different neck-shaft angles, glenosphere sizes, and polyethylene cup depths.

METHODS

Finite element RSA models with varying neck-shaft angles (155°, 145°, 135°), sizes (38 mm, 42 mm), and cup depths (deep, normal, shallow) were loaded with 400 N at physiological abduction angles. The contact area and maximum contact stress were computed.

RESULTS

The contact patch and the location of maximum contact stress were typically located inferomedially in the polyethylene cup. On average for all abduction angles investigated, reducing the neck-shaft angle reduced the contact area by 29% for 155° to 145° and by 59% for 155° to 135° and increased maximum contact stress by 71% for 155° to 145° and by 286% for 155° to 135°. Increasing the glenosphere size increased the contact area by 12% but only decreased maximum contact stress by 2%. Decreasing the cup depth reduced the contact area by 40% and increased maximum contact stress by 81%, whereas increasing the depth produced the opposite effect (+52% and -36%, respectively).

DISCUSSION

The location of the contact patch and maximum contact stress in this study matches the area of damage seen frequently on clinical retrievals. This finding suggests that damage to the inferior cup due to notching may be potentiated by contact stresses. Increasing the glenosphere diameter improved the joint contact area and did not affect maximum contact stress. However, although reducing the neck-shaft angle and cup depth can improve range of motion, our study shows that this also has some negative effects on RSA contact mechanics, particularly when combined.

Porous metals and alternate bearing surfaces in shoulder arthroplasty

Total shoulder arthroplasty (TSA) provides an effective solution for the treatment of glenohumeral arthritis. However, long-term outcomes have been limited by glenoid component aseptic loosening and polyethylene (PE) wear. Previous attempts to improve glenoid fixation with metal-backed glenoids resulted in inferior results. Newer component designs that contain porous metal allow for biological ingrowth of the prosthesis, potentially improving longevity and overall outcomes. Porous metal can also improve humeral component fixation, obviating the need for cement and simplifying revision surgery. Advances such as highly cross-linked polyethylene (HXLPE), vitamin E-doped HXLPE, and alternate bearing surfaces like ceramics and pyrolytic carbon have proven to provide superior wear characteristics in other joint replacements and may prove beneficial in the shoulder as well.

Numerical and experimental investigations for the evaluation of the wear coefficient of reverse total shoulder prostheses

In the present study, numerical and experimental wear investigations on reverse total shoulder arthroplasties (RTSAs) were combined in order to estimate specific wear coefficients, currently not available in the literature. A wear model previously developed by the authors for metal-on-plastic hip implants was adapted to RTSAs and applied in a double direction: firstly, to evaluate specific wear coefficients for RTSAs from experimental results and secondly, to predict wear distribution. In both cases, the Archard wear law (AR) and the wear law of UHMWPE (PE) were considered, assuming four different k functions. The results indicated that both the wear laws predict higher wear coefficients for RTSA with respect to hip implants, particularly the AR law, with k values higher than twofold the hip ones. Such differences can significantly affect predictive wear model results for RTSA, when non-specific wear coefficients are used. Moreover, the wear maps simulated with the two laws are markedly different, although providing the same wear volume. A higher wear depth (+51%) is obtained with the AR law, located at the dome of the cup, while with the PE law the most worn region is close to the edge. Taking advantage of the linear trend of experimental volume losses, the wear coefficients obtained with the AR law should be valid despite having neglected the geometry update in the model.

Wear rates of retentive versus nonretentive reverse total shoulder arthroplasty liners in an in vitro wear simulation

BACKGROUND

Although short-term outcomes of reverse total shoulder arthroplasty (rTSA) remain promising, the most commonly cited complication remains prosthetic instability. A retentive rTSA liner is commonly used to increase system constraint; however, no studies have evaluated the rate of polyethylene wear. Our hypothesis was that more constrained retentive liners would have higher wear rates than nonretentive liners.

METHODS

Six nonretentive and six retentive rTSA non-cross-linked polyethylene liners were subjected to 4.5 million cycles of alternating cycles of abduction-adduction and flexion-extension motion loading profiles. The rTSA liners were assessed for gravimetric wear loss, 3-dimensional volumetric loss by novel micro-computed tomography analysis, and particulate wear debris analysis.

RESULTS

Volumetric wear rates were significant at 7 specific time points (1.0, 2.0, 2.5, 3.25, 3.75, 4.0, and 4.5 million cycles) throughout testing between nonretentive and retentive liners; however, overall mean volumetric wear rate was not statistically significant (P = .076). Total volume loss between liner test groups was found to be significant starting after 3.5 million cycles of testing. Maximum and mean surface deviations were found to be larger for retentive liners vs. nonretentive liners by micro-computed tomography analysis across the entire articulation surface.

DISCUSSION AND CONCLUSION

Retentive liners undergo significantly greater volume loss and greater surface deviation compared with nonretentive liners, most notably at later time points representing extended implantation times. Additional stability afforded by retentive liners should be balanced against the potential for increased wear and potential for subsequent polyethylene wear-induced aseptic loosening.