Factors Associated With Early Discharge in Pediatric Trauma Patients Transported by Rotor: A Retrospective Analysis.

OBJECTIVE: Helicopter emergency medical services (HEMS) play a crucial role in providing timely transport for pediatric trauma patients. This service carries the highest risk of any mode of medical transport and a high financial burden, and patient outcomes are seldom investigated. This study evaluated the characteristics of pediatric trauma patients discharged within 24 hours after transport by HEMS. METHODS: This was a single-center, retrospective analysis on pediatric trauma patients transported by HEMS from 2019 to 2022. Analyses were performed to identify factors associated with discharge within 24 hours. Factors analyzed included vital signs, Shock Index, Pediatric Age-Adjusted scores, management details, and clinical outcomes. RESULTS: A total of 466 pediatric trauma patients were transported by HEMS, including 171 patients (36.7%) who were discharged within 24 hours. There were no differences in the rates of blunt and penetrating injury (P = .583). Patients discharged within 24 hours were more likely to have a higher Glasgow Coma Scale score (14 vs. 11, P < .001) and a lower Injury Severity Score (4.9 vs. 14.7, P < .001), required less prehospital fluid resuscitation (5.5 vs. 11.7 mL/kg, P = .039), and had higher levels of serum calcium (9.3 vs. 8.9 mg/dL, P < .001). They were also less likely to meet criteria for level 1 trauma activation (13.0% vs. 40%, P < .001) or to require prehospital respiratory support of any kind (4.1% vs. 31.1%, P < .001). After arrival at the hospital, they were less likely to require blood transfusions (2.9% vs. 29.8%, P < .001) or tranexamic acid (2.9% vs. 11.5%, P = .001). CONCLUSION: Trauma patients with a high Glasgow Coma Scale score and a low Injury Severity Score who do not require critical care or meet the criteria for high-level trauma activation may be suitable for transportation with lower acuity. Further studies aimed at improving triage and implementing improved criteria for the use of HEMS are paramount.

An overview of the tribological and mechanical properties of PEEK and CFR-PEEK for use in total joint replacements.

Poly-ether-ether-ketone (PEEK) and PEEK composites are outstanding candidates for biomedical applications, such as orthopedic devices, where biocompatibility and modulus match with surrounding tissue are requisite for long-term success. The mechanical properties can be optimized by incorporating fillers such as continuous and chopped carbon fibers. While much is known about the mechanical and tribological behavior of PEEK composites, there are few articles that summarize the viability of using PEEK reinforced with carbon fibers in orthopedic implants. This paper reviews biocompatibility, tribological, and mechanical studies on PEEK and their composites with carbon fibers, notably PEEK reinforced with polyacrylonitrile (PAN)-based carbon fibers and PEEK reinforced with pitch-based carbon fibers, for application in orthopedics and total joint replacements (TJRs). The main objectives of this review are two-fold. Firstly, this paper aims to assist designers in making informed decisions on the suitability of using PEEK and PEEK composites in orthopedic applications; as it is not well understood how these materials perform on the whole in orthopedics and TJRs. Secondly, this paper aims to serve as a centralized paper in which researchers can gain information on the tribological and mechanical advancements of PEEK and PEEK composites.

A methodological framework for nanomechanical characterization of soft biomaterials and polymers.

Nanoindentation utilizes a hard indenter probe to deform the sample surface in order to measure local properties, such as indentation modulus and hardness. Initially intended for characterization of elastic and elastic-plastic materials, nanoindentation has more recently been utilized for viscoelastic solids as well as hydrated and soft biological materials. An advantage to nanoindentation is the ability to determine the nano- and microscale properties of materials with complex microstructures as well as those of limited sample dimension. Nanoindentation finds utility in the characterization of structural tissues, hydrogels, polymers and composites. Nevertheless, testing complexities such as adhesion and surface detection exist in nanoindentation of compliant viscoelastic solids and hydrated materials. These challenges require appropriate modifications in methodology and use of appropriate contact models to analyze nanoindentation data. A full discussion of protocol adjustments has yet to be assembled into a robust nanoindentation testing framework of soft biomaterials and polymers. We utilize existing nanoindentation literature and testing expertise in our laboratories to (1) address challenges and potential errors when performing indentations on soft or hydrated materials, (2) explore best practices for mitigating experimental error, and (3) develop a nanoindentation framework that serves researchers as a primer for nanoindentation testing of soft/hydrated biomaterials and polymers.

Abiraterone acetate plus prednisone in non-metastatic biochemically recurrent castration-naïve prostate cancer.

BACKGROUND: Intermittent androgen deprivation therapy (ADT) in biochemically recurrent castration-naïve prostate cancer is non-inferior to continuous therapy. We hypothesised that finite-duration abiraterone acetate plus prednisone (Abi +P) added to ADT will further reduce the duration of treatment exposure by prolonging time to prostate-specific antigen (PSA) recurrence without impacting eugonad state recovery. METHODS: This phase II, randomised, open-label trial enrolled patients with rising PSA ≥ 0.2 ng/ml after radical prostatectomy and/or a PSA ≥ 1 following radiotherapy. Patients were randomised 1:1 to receive Abi (1 g PO daily) + P (5 mg PO daily) + ADT or ADT alone for 8 months. The primary end-point was PSA-free survival difference at 1 year following completion of therapy. RESULTS: Between February 2013 and July 2016, 200 patients were enrolled. Of 100 patients randomised to each arm, 99 in the Abi +P arm and 98 in the ADT arm were evaluable. Median follow-up was 64.4 months. Median PSA-free survival was 27.0 months for the Abi +P-treated group versus 19.9 months for the ADT-treated group (hazard ratio [HR] 0.64, 95% confidence interval [CI] 0.47-0.87). The PSA-free survival at 1 year post-treatment completion was 98% for the Abi +P group and 88% for the ADT group. Median time to eugonad state was 13.1 months for the abiraterone-treated group and 12.8 months for the ADT-treated group. Median eugonad PSA-free survival was 12.5 months for the abiraterone-treated group versus 9.0 for the ADT-treated group (HR 0.72, 95% CI 0.53-0.98). There were no significant between-group differences in androgen deprivation-related adverse events. CONCLUSIONS: In men with biochemically recurrent prostate cancer following definitive treatment of the primary, finite duration treatment with ADT and Abi +P results in a significantly longer PSA relapse-free interval than treatment with ADT alone.

Comparison of macro-, micro- and nanomechanical properties of clinically-relevant UHMWPE formulations.

We characterized a set of eleven clinically relevant formulations of UHMWPE for total joint replacements. Although their molecular and supermolecular structure were quite similar as evidenced by IR, DSC and SAXS measurements, there were slight differences in their crystallinity (DSC crystallinity ranging from 52 to 61%), which were connected with processing conditions, such as the total radiation dose, thermal treatment and/or addition of biocompatible stabilizers. Mechanical properties were assessed at all length scales, using macroscale compression testing, non-instrumented and instrumented microindentation hardness testing (at loading forces ~500 mN), and nanoindentation hardness testing measured at both higher and lower loading (~4 mN and ~0.6 mN, respectively). In agreement with theoretical predictions, we found linear correlations between UHMWPE crystallinity and its stiffness-related properties (elastic moduli, yield stress, and hardness) at all length scales (macro-, micro- and nanoscale). Detailed statistical evaluation of our dataset showed that the accuracy and precision of the applied methods decreased in the following order: non-instrumented microindentation ≥ instrumented microindentation ≥ macromechanical properties ≥ nanoindentation measured at higher loading forces â‰« nanoindentation measured at lower loading forces. The results confirm that microindentation and nanoindentation at sufficiently high loading forces are reliable methods, suitable for UHMWPE characterization.

A 3D-transient elastohydrodynamic lubrication hip implant model to compare ultra high molecular weight polyethylene with more compliant polycarbonate polyurethane acetabular cups.

Wear remains a significant challenge in the design of orthopedic implants such as total hip replacements. Early elastohydrodynamic lubrication modeling has predicted thicker lubrication films in hip replacement designs with compliant polycarbonate polyurethane (PCU) bearing materials compared to stiffer materials like ultra-high molecular weight polyethylene (UHMWPE). The predicted thicker lubrication films suggest improved friction and wear performance. However, when compared to the model predictions, experimental wear studies showed mixed results. The mismatch between the model and experimental results may lie in the simplifying assumptions of the early models such as: steady state conditions, one dimensional rotation and loading, and high viscosities. This study applies a 3D-transient elastohydrodynamic model based on an ISO standard gait cycle to better understand the interaction between material stiffness and film thickness in total hip arthroplasty material couples. Similar to previous, simplified models, we show that the average and central film thickness of PCU (∼0.4µm) is higher than that of UHMWPE (∼0.2µm). However, in the 3D-transient model, the film thickness distribution was largely asymmetric and the minimum film thickness occurred outside of the central axis. Although the overall film thickness of PCU was higher than UHMWPE, the minimum film thickness of PCU was lower than UHMPWE for the majority of the gait cycle. The minimum film thickness of PCU also had a larger range throughout the gait cycle. Both materials were found to be operating between boundary and mixed lubrication regimes. This 3D-transient model reveals a more nuanced interaction between bearing material stiffness and film thickness that supports the mixed results found in experimental wear studies of PCU hip implant designs.

Oxidized Zirconium Components Maintain a Smooth Articular Surface Except Following Hip Dislocation.

BACKGROUND: Oxidized zirconium (OxZr) offers theoretical advantages in total hip and knee arthroplasty (THA and TKA, respectively) relative to other biomaterials by combining the tribological benefits of ceramics with the fracture toughness of metals. Yet, some studies have found that OxZr does not improve outcomes or wear rates relative to traditional bearing materials such as cobalt-chromium (CoCr). Separately, effacement of the thin ceramic surface layer has been reported for OxZr components, though the prevalence and sequelae are unclear. METHODS: To elucidate the in vivo behavior of OxZr implants, the articular surfaces of 94 retrieved THA and TKA femoral components (43 OxZr TKA, 21 OxZr THA, 30 CoCr THA) were analyzed using optical microscopy, non-contact profilometry, and scanning electron microscopy. RESULTS: We found that OxZr components maintain a smooth articular surface except following hip dislocation. Three of four OxZr femoral heads revised following dislocation exhibited severe damage to the articular surface, including macroscopic regions of ceramic-layer effacement and exposure of the underlying metal substrate; these components were 23-32 times rougher than pristine OxZr controls. When revised for dislocation, OxZr femoral heads were substantially rougher than CoCr femoral heads (median Sa = 0.431 v. 0.020 µm, P = .03). In contrast, CoCr femoral heads exhibited low overall roughness values regardless of whether they dislocated (median Sa = 0.020 v. 0.008 µm, P = .09, CoCr dislocators v. non-dislocators). CONCLUSIONS: Effacement of the ceramic surface layer and substantial articular surface roughening is not atypical following dislocation of OxZr femoral heads, making OxZr much less tolerant than CoCr to hip dislocation.

Nanomechanical analysis of medical grade PEEK and carbon fiber-reinforced PEEK composites.

Polyether ether ketone (PEEK) and PEEK composites are viable candidates for orthopedic implants owing to their ability for modulus match of surrounding bone tissue. The structural properties of these systems for load-bearing application in the body can be tailored by incorporating carbon fibers; to this end, polyacrylonitrile (PAN) and pitch fibers are commonly incorporated in the PEEK matrix. Mechanical property optimization for a given medical application requires consideration of carbon fiber type and volume fraction, as well as processing conditions for the composite systems. While much is known about the bulk mechanical properties of PEEK and PEEK composites, little is known about the nanomechanical properties of these systems. Insight into nanoscale behavior can offer valuable information about fiber-matrix interactions that may influence long-term integrity of these biomaterials when used in load bearing medical device applications. In this study, we utilize nanoindentation as a method to characterize mechanical behavior of clinical grade PEEK and PEEK composites. We examine PEEK formulations with pitch and PAN fibers and evaluate a range of thermal treatments known to influence polymer microstructure. We use a conospherical tip of 1.5 µm in radius and a conospherical tip of 20 µm radius to determine indentation modulus over different length scales. We correlate these findings with previous characterization on these same PEEK systems using microindentation. A novelty of this work is that we combine nanoindentation with k-means clustering to quantitatively discern the influence of heat treatment and carbon fiber type on the mechanical behavior of PEEK composites and their constituents. We demonstrate that nanoindentation is an effective characterization tool for discerning fiber-matrix interactions and measuring the mechanical behavior in response to thermal treatment and carbon fiber type in PEEK composites. Nanoindentation is shown to be a viable tool for characterizing complex biomaterials and can serve as an effective technique to guide optimization of microstructures for long-term structural applications in the body.

Temsirolimus versus Pazopanib (TemPa) in Patients with Advanced Clear-cell Renal Cell Carcinoma and Poor-risk Features: A Randomized Phase II Trial.

BACKGROUND: Temsirolimus has level 1 evidence for initial treatment of poor-risk patients with advanced renal cell carcinoma (mRCC), but its efficacy has not been directly compared with an antiangiogenic tyrosine kinase inhibitor (vascular endothelial growth factor receptor tyrosine kinase inhibitor [VEGFR TKi]) in this setting. OBJECTIVE: To evaluate temsirolimus versus pazopanib as first-line therapy in patients with mRCC, predominant clear-cell features, and clinical characteristics of a poor prognosis. DESIGN, SETTING, AND PARTICIPANTS: A randomized (1:1) phase II trial in 69 treatment-naïve mRCC patients and with three or more predictors of short survival for temsirolimus was conducted during 2012-2017 in a single academic cancer center. Crossover to the alternative treatment upon discontinuation of the first-line agent was permitted. INTERVENTION: Mechanistic target of rapamycin inhibitor temsirolimus and VEGFR TKi pazopanib. OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS: The primary endpoint was progression-free survival (PFS), and the secondary endpoints were overall survival (OS), objective response rate (ORR), safety, and patient-reported outcomes (PROs). Radiographic response was assessed by blinded radiologists. Efficacy outcomes were adjusted by prior nephrectomy status, prior interleukin-2 treatment, and the International Metastatic Renal Cell Carcinoma Database Consortium (IMDC) score. RESULTS AND LIMITATIONS: Thirty-five patients received temsirolimus and 34 received pazopanib upfront; 72% overall had poor risk by IMDC. Median PFS in the first line was 2.7mo with temsirolimus and 5.2mo with pazopanib (adjusted hazard ratio [HR] 1.36, 95% confidence interval [CI] 0.84-2.22; p=0.210). Median OS was 7.1mo with temsirolimus and 11.9mo with pazopanib (adjusted HR 1.16, 95% CI 0.70-1.93; p=0.558), and ORRs were 5.9% and 21.2%, respectively (adjusted odds ratio 5.2, 95% CI 0.9-29.3; p=0.062). PRO measures favored pazopanib. Five patients discontinued first-line therapy due to adverse events. CONCLUSIONS: Temsirolimus and pazopanib had modest activity in patients with poor-risk clear-cell mRCC, and therefore their use should be discouraged in this setting. PATIENT SUMMARY: We evaluated outcomes of advanced renal cell carcinoma patients presenting with aggressive features when treated with temsirolimus or pazopanib as first-line therapy. Survival was <1yr for most, suggesting that more efficacious alternative treatments should be favored for these patients.

Effect of carbon fiber type on monotonic and fatigue properties of orthopedic grade PEEK.

Carbon-fiber reinforced (CFR) PEEK implants are used in orthopedic applications ranging from fracture fixation plates to spinal fusion cages. Documented implant failures and increasing volume and variety of CFR PEEK implants warrant a clearer understanding of material behavior under monotonic and cyclic loading. To address this issue, we conducted monotonic and fatigue crack propagation (FCP) experiments on orthopedic grade unfilled PEEK and two formulations of CFR PEEK (PAN- and pitch-based carbon fibers). The effect of annealing on FCP behavior was also studied. Under monotonic loading, fiber type had a statistically significant effect on elastic modulus (12.5 ±â€¯1.3 versus 18.5 ±â€¯2.3 GPa, pitch versus PAN CFR PEEK, AVG ±â€¯SD) and on ultimate tensile strength (145 ±â€¯9 versus 192 ±â€¯17 MPa, pitch versus PAN CFR PEEK, AVG ±â€¯SD). Fiber type did not have a significant effect on failure strain. Under cyclic loading, PAN CFR PEEK demonstrated an increased resistance to FCP compared with unfilled and pitch CFR PEEK, and this improvement was enhanced following annealing. Pitch CFR PEEK exhibited FCP behavior similar to unfilled PEEK, and neither material was appreciably affected by annealing. The improvements in monotonic and FCP behavior of PAN CFR PEEK is attributed to a compound effect of inherent fiber properties, increased fiber number for an equivalent wt% reinforcement, and fiber aspect ratio. FCP was shown to proceed via cyclic modes during stable crack growth, which transitioned to static modes (more akin to monotonic fracture) at longer crack lengths. The mechanisms of fatigue crack propagation appear similar between carbon-fiber types.

Phase II Study of Two Weeks on, One Week off Sunitinib Scheduling in Patients With Metastatic Renal Cell Carcinoma.

Purpose Standard frontline treatment of patients with metastatic renal cell carcinoma currently includes sunitinib. A barrier to long-term treatment with sunitinib includes the development of significant adverse effects, including diarrhea, hand-foot syndrome (HFS), and fatigue. This trial assessed the effect of an alternate 2 weeks on, 1 week off (2/1) schedule of sunitinib on toxicity and efficacy in previously untreated patients with metastatic renal cell carcinoma. Methods Patients started with oral administration of 50 mg sunitinib on a 2/1 schedule and underwent schedule and dose alterations if toxicity developed. The primary end point was < 15% grade ≥ 3 fatigue, diarrhea, or HFS. With 60 patients, the upper bound of the CI would fall below the published 4/2 schedule grade ≥ 3 toxicity rate of 25% to 30%. Results Fifty-nine patients were treated between August 2014 and March 2016. Seventy-seven percent were intermediate or poor risk per Memorial Sloan Kettering Cancer Center criteria. With a median follow-up of 17 months, 25% of patients experienced grade 3 fatigue, HFS, or diarrhea; 37% required a dose reduction, and 10% discontinued because of toxicity. The overall response rate was 57%, median progression-free survival was 13.7 months, and median overall survival was not reached. At 12 weeks, Functional Assessment of Cancer Therapy-General scores dropped between 0% and 10% from baseline, with less reduction in patients who continued treatment longer. Conclusion The primary end point of decreased grade 3 toxicity was not met; however, treatment with a 2/1 sunitinib schedule is associated with a lack of grade 4 toxicity, a low patient discontinuation rate, and high efficacy.

Material properties of ultra-high molecular weight polyethylene: Comparison of tension, compression, nanomechanics and microstructure across clinical formulations.

This is the first study to simultaneously measure material properties in tension, compression, nanoindentation as well as microstructure (crystallinity and lamellar level properties) across a wide variety of clinically relevant ultra-high molecular weight polyethylene (UHMWPE) formulations. Methodologies for the measurement of UHMWPE mechanical properties-namely elastic modulus, yield stress, yield strain, ultimate strength, energetic toughness, Poisson's ratio, hardness and constitutive variables-are evaluated. Engineering stress-strain behavior is compared to true stress-strain behavior for UHMWPE across a range of cross-linking and antioxidant chemistry. The tensile mechanical properties and constitutive behavior of UHMWPE are affected by resin type, antioxidant source and degree of cross-linking. Poisson's ratio is shown to be affected by resin type, antioxidant addition, and cross-linking dosage. Relationships between bulk mechanical properties from different measurement methodologies as well as microstructure are analyzed across all material formulations using Spearman rank correlation coefficients. Modulus and yield strength correlate in both tension and compression. Similarly, tensile and compressive properties including modulus and yield strength correlate strongly with crystallinity (Xc) and lamellar thickness (D). This work has broad application and provides a basis for interpreting the mechanical behavior of UHMWPE used in orthopedic implants.

Micromechanisms of fatigue crack growth in polycarbonate polyurethane: Time dependent and hydration effects.

Polycarbonate polyurethane has cartilage-like, hygroscopic, and elastomeric properties that make it an attractive material for orthopedic joint replacement application. However, little data exists on the cyclic loading and fracture behavior of polycarbonate polyurethane. This study investigates the mechanisms of fatigue crack growth in polycarbonate polyurethane with respect to time dependent effects and conditioning. We studied two commercially available polycarbonate polyurethanes, Bionate® 75D and 80A. Tension testing was performed on specimens at variable time points after being removed from hydration and variable strain rates. Fatigue crack propagation characterized three aspects of loading. Study 1 investigated the impact of continuous loading (24h/day) versus intermittent loading (8-10h/day) allowing for relaxation overnight. Study 2 evaluated the effect of frequency and study 3 examined the impact of hydration on the fatigue crack propagation in polycarbonate polyurethane. Samples loaded intermittently failed instantaneously and prematurely upon reloading while samples loaded continuously sustained longer stable cracks. Crack growth for samples tested at 2 and 5Hz was largely planar with little crack deflection. However, samples tested at 10Hz showed high degrees of crack tip deflection and multiple crack fronts. Crack growth in hydrated samples proceeded with much greater ductile crack mouth opening displacement than dry samples. An understanding of the failure mechanisms of this polymer is important to assess the long-term structural integrity of this material for use in load-bearing orthopedic implant applications.

Fatigue fracture of a cemented Omnifit CoCr femoral stem: implant and failure analysis.

A cemented, cast CoCr alloy, Omnifit Plus femoral stem was retrieved following mid-stem fracture after 24 years in vivo. The patient was an active 55-year-old male with a high body mass index (31.3) and no traumatic incidents before stem fracture. Fractographic and fatigue-based failure analyses were performed to illuminate the etiology of fracture and retrospectively predict the device lifetime. The fracture surfaces show evidence of a coarse grain microstructure, intergranular fracture, and regions of porosity. The failure analysis suggests that stems with similar metallurgical characteristics, biomechanical environments, and in vivo durations may be abutting their functioning lifetimes, raising the possibility of an increased revision burden.

Notch fatigue of ultrahigh molecular weight polyethylene (UHMWPE) used in total joint replacements.

Ultrahigh molecular weight polyethylene (UHMWPE) has remained the primary polymer used in hip, knee and shoulder replacements for over 50 years. Recent case studies have demonstrated that catastrophic fatigue fracture of the polymer can severely limit device lifetime and are often associated with stress concentration (notches) integrated into the design. This study evaluates the influence of notch geometry on the fatigue of three formulations of UHMWPE that are in use today. A linear-elastic fracture mechanics approach is adopted to evaluate crack propagation as a function of notch root radius, heat treatment and Vitamin E additions. Specifically, a modified stress-intensity factor that accounts for notch geometry was utilized to model the crack driving force. The degree of notch plasticity for each material/notch combination was further evaluated using finite element methods. Experimental evaluation of crack speed as a function of stress intensity was conducted under cyclic tensile loading, taking crack length and notch plasticity into consideration. Results demonstrated that crack propagation in UHMWPE emanating from a notch was primarily affected by microstructural influences (cross-linking) rather than differences in notch geometry.

Analysis of severely fractured glenoid components: clinical consequences of biomechanics, design, and materials selection on implant performance.

BACKGROUND: The longevity of total shoulder replacement is primarily limited by the performance of the ultrahigh-molecular-weight polyethylene (UHMWPE) glenoid component in vivo. Variations in glenoid design (conformity, thickness), biomechanics (joint kinematics), and UHMWPE material selection (sterilization, cross-linking) distinguish total shoulder replacements from hip and knee arthroplasty devices. These variables can lead to severe mechanical failures, including gross fracture. METHODS: Sixteen retrieved glenoids with severe fracture were analyzed. The explant cohort included 3 material groups (gamma-sterilized Hylamer; gamma-sterilized UHMWPE; and gas plasma-sterilized, remelted, highly cross-linked UHMWPE [HXL]) and a range of conformities (0- to 10-mm radial mismatch). Analysis included fractography (optical and scanning electron microscopy) and Fourier transform infrared spectroscopy for oxidative analysis. RESULTS: Fracture primarily occurred along the exterior rim for all 16 explants. Fourier transform infrared analysis and fractography revealed significant oxidative embrittlement for all gamma-sterilized glenoids. Fatigue striations and internal flaws were evident on the fracture surface of the HXL glenoid, with little oxidation detected. CONCLUSIONS: Fracture initiated at the external rim of all devices. Elevated oxidation levels and visible material distortion for representative gamma-sterilized conventional and Hylamer devices suggest oxidative embrittlement as a driving force for crack inception and subsequent fracture. Brittle fracture of theHXL glenoid resulted from a combination of elevated contact stress due to a nonconforming surface, an internal flaw, and reduced resistance to fatigue crack growth. This demonstrates that glenoid fracture associated with oxidation has not been eliminated with the advent of modern materials (HXL) in the shoulder domain. LEVEL OF EVIDENCE: Basic Science Study; Implant Retrieval Study.

Effect of processing, sterilization and crosslinking on UHMWPE fatigue fracture and fatigue wear mechanisms in joint arthroplasty.

Ultra high molecular weight polyethylene (UHMWPE) has been used as a bearing surface in total joint replacements (TJR) for nearly five decades. This semi-crystalline polymer has extraordinary energetic toughness owing to its high molecular weight and entanglement density. However, it is challenged by a need to offer a combined resistance to fatigue, wear and oxidation in vivo. The processing, sterilization treatment, and microstructural tailoring of UHMWPE has evolved considerably in the past 50 years but an optimized microstructure remains elusive. This review seeks to provide an overview of this processing history to address two primary questions: First, how does microstructure affect fatigue fracture and fatigue wear mechanisms in UHMWPE? And second, can microstructure be optimized to provide resistance to fatigue, oxidation and wear in vivo? Previous literature demonstrates that while crosslinking improves resistance to adhesive/abrasive wear, it also reduces resistance to fatigue crack propagation and fatigue wear by restricting molecular mobility and rendering the polymer more brittle. Crystallinity improves fatigue resistance but generally increases elastic modulus and concomitant contact stresses in vivo. The presence of fusion defects or oxidation reduces further fatigue resistance and enhances fatigue wear. Thus, UHMWPE microstructural evolution comes with trade-offs. Currently there is no singular formulation of UHMWPE that is ideal for all TJR applications.

Effects of elastase and collagenase on the nonlinearity and anisotropy of porcine aorta.

We use enzymatic manipulation methods to investigate the individual and combined roles of elastin and collagen on arterial mechanics. Porcine aortic tissues were treated for differing amounts of time using enzymes elastase and collagenase to cause degradation in substrate proteins elastin and collagen and obtain variable tissue architecture. We use equibiaxial mechanical tests to quantify the material properties of control and enzyme treated tissues and histological methods to visualize the underlying tissue microstructure in arterial tissues. Our results show that collagenase treated tissues were more compliant in the longitudinal direction as compared to control tissues. Collagenase treatment also caused a decrease in the tissue nonlinearity as compared to the control samples in the study. A one hour collagenase treatment was sufficient to cause fragmentation and degradation of the adventitial collagen. In contrast, elastase treatment leads to significantly stiffer tissue response associated with fragmented and incomplete elastin networks in the tissue. Thus, elastin in arterial walls distributes tensile stresses whereas collagen serves to reinforce the vessel wall in the circumferential direction and also contributes to tissue anisotropy. A microstructurally motivated strain energy function based on circumferentially oriented medial fibers and helically oriented collagen fibers in the adventitia is useful in describing these experimental results.

Fractography and oxidative analysis of gamma inert sterilized posterior-stabilized tibial insert post fractures: report of two cases.

BACKGROUND: Highly crosslinked ultra-high molecular weight polyethylene (UHMWPE) has shown success in reducing wear in hip arthroplasty but there remains skepticism about its use in Total Knee Replacement (TKR) inserts that are known to experience fatigue loading and higher local cyclic contact stresses. METHODS: Two Legacy Posterior-Stabilized (LPS) Zimmer NexGen tibial implants sterilized by gamma irradiation in an inert environment with posts that fractured in vivo were analyzed. Failure mechanisms were determined using optical and scanning electron microscopy along with oxidative analysis via Fourier Transform Infra-Red (FTIR) spectroscopy. RESULTS: Micrographs of one retrieval revealed fatigue crack initiation on opposite sides of the post and quasi-brittle micromechanisms of crack propagation. FTIR of this retrieval revealed no oxidation. The fracture surface image of the second retrieval indicated a brittle fracture process and FTIR revealed oxidation in the explant. CONCLUSIONS: These two cases suggest that crosslinking of UHMWPE as a manufacturing process or sterilization method in conjunction with designs that incorporate high stress concentrations, such as the tibial post, may reduce material strength. Moreover, free radicals generated from ionizing radiation can render the polymer susceptible to oxidative embrittlement. CLINICAL RELEVANCE: Our findings suggest that tibial post fractures may be the results of in vivo oxidation and low level crosslinking. These and previous reports of fractured crosslinked UHMWPE devices implores caution when used with high stress concentrations, particularly when considering the potential for in vivo oxidation in TKR.