Ultra-High-Molecular-Weight Polyethylene (UHMWPE) in Total Joint Arthroplasty.

Total joint arthroplasty of the hip and knee are successful orthopedic procedures that reduce pain and improve mobility in patients. As the implanted materials used in these procedures have improved, the lifetime of the implants has now reached more than 20 years. Younger patients are undergoing total joint arthroplasty at increasing rates, which has increased the need for improvements in materials for extended implant longevity. In this review, we aim to provide historical perspective on the evolution of ultra-high-molecular-weight polyethylene in total joint arthroplasty. We review this material with respect to its structure, mechanical properties, wear rates, sterilization effects, crosslinking, and other new polymer innovations. Ultra-high-molecular-weight polyethylene (UHMWPE) has been a cornerstone for total joint arthroplasty and with continued design improvements will continue to be vital to this successful orthopedic operation.

Form and functional repair of long bone using 3D-printed bioactive scaffolds.

Injuries to the extremities often require resection of necrotic hard tissue. For large-bone defects, autogenous bone grafting is ideal but, similar to all grafting procedures, is subject to limitations. Synthetic biomaterial-driven engineered healing offers an alternative approach. This work focuses on three-dimensional (3D) printing technology of solid-free form fabrication, more specifically robocasting/direct write. The research hypothesizes that a bioactive calcium-phosphate scaffold may successfully regenerate extensive bony defects in vivo and that newly regenerated bone will demonstrate mechanical properties similar to native bone as healing time elapses. Robocasting technology was used in designing and printing customizable scaffolds, composed of 100% beta tri-calcium phosphate (ß-TCP), which were used to repair critical sized long-bone defects. Following full thickness segmental defects (~11 mm × full thickness) in the radial diaphysis in New Zealand white rabbits, a custom 3D-printed, 100% ß-TCP, scaffold was implanted or left empty (negative control) and allowed to heal over 8, 12, and 24 weeks. Scaffolds and bone, en bloc, were subjected to micro-CT and histological analysis for quantification of bone, scaffold and soft tissue expressed as a function of volume percentage. Additionally, biomechanical testing at two different regions, (a) bone in the scaffold and (b) in native radial bone (control), was conducted to assess the newly regenerated bone for reduced elastic modulus (Er ) and hardness (H) using nanoindentation. Histological analysis showed no signs of any adverse immune response while revealing progressive remodelling of bone within the scaffold along with gradual decrease in 3D-scaffold volume over time. Micro-CT images indicated directional bone ingrowth, with an increase in bone formation over time. Reduced elastic modulus (Er ) data for the newly regenerated bone presented statistically homogenous values analogous to native bone at the three time points, whereas hardness (H) values were equivalent to the native radial bone only at 24 weeks. The negative control samples showed limited healing at 8 weeks. Custom engineered ß-TCP scaffolds are biocompatible, resorbable, and can directionally regenerate and remodel bone in a segmental long-bone defect in a rabbit model. Custom designs and fabrication of ß-TCP scaffolds for use in other bone defect models warrant further investigation.

Prophylactic Tibial Stem Fixation in the Obese: Comparative Early Results in Primary Total Knee Arthroplasty.

PURPOSE: Obesity is a risk factor for aseptic loosening after total knee arthroplasty (TKA). Prophylactic use of tibial stems may enhance tibial fixation in obese patients. The aim of this study was to determine whether a tibial stem extension decreases rates of early failure in obese patients. MATERIALS AND METHODS: This study included 178 consecutive primary TKAs (143 patients) with a body mass index ≥35 kg/m2. Fifty TKAs were performed with the use of a 30 mm tibial stem extension, and 128 TKAs were performed with a standard tibial component. Patients with two-year clinical follow-up were included. The primary outcome was revision for aseptic loosening. Secondary outcomes were all-cause revision and radiolucent lines (RLLs) on radiographs. RESULTS: Average follow-up was 34 months (range, 24 to 46 months). No failures for aseptic loosening occurred. The occurrence of secondary procedures was not significantly different between groups. Quantification of RLLs revealed no difference between groups. CONCLUSIONS: At early follow-up, no difference was measured in revision rates, need for subsequent procedures, or RLLs between groups.

Staphylococcus lugdunensis Septic Arthritis of a Native Knee A Case Report.

A 67-year-old man presented to orthopaedic care with a painful knee. Workup was consistent with septic arthritis of a native knee, and the patient underwent operative treatment. Cultures from the operating room were speciated to Staphylococcus lugdunensis. To the investigators' knowledge, this is the first reported S. lugdunensis infection in a peripheral joint in the absence of an orthopaedic prosthesis. Although traditionally associated with infectious endocarditis, S. lugdunensis has been identified as a causative agent in many organ systems, including orthopaedic infections. This case report emphasizes the importance of familiarity with this emerging pathogen in the treatment of a septic joint.

Functional assessment of the ex vivo vocal folds through biomechanical testing: A review.

The human vocal folds are complex structures made up of distinct layers that vary in cellular and extracellular composition. The mechanical properties of vocal fold tissue are fundamental to the study of both the acoustics and biomechanics of voice production. To date, quantitative methods have been applied to characterize the vocal fold tissue in both normal and pathologic conditions. This review describes, summarizes, and discusses the most commonly employed methods for vocal fold biomechanical testing. Force-elongation, torsional parallel plate rheometry, simple-shear parallel plate rheometry, linear skin rheometry, and indentation are the most frequently employed biomechanical tests for vocal fold tissues and each provide material properties data that can be used to compare native tissue to diseased or treated tissue. Force-elongation testing is clinically useful, as it allows for functional unit testing, while rheometry provides physiologically relevant shear data, and nanoindentation permits micrometer scale testing across different areas of the vocal fold as well as whole organ testing. Thoughtful selection of the testing technique during experimental design to evaluate a hypothesis is critical to optimize biomechanical testing of vocal fold tissues.

Large deformation compression induced crystallinity degradation of conventional and highly crosslinked UHMWPEs.

The effect of a large compressive plastic deformation on the melt temperature (Tm), lamellar thickness, crystallinity, and density of four UHMWPEs (two conventional and two highly crosslinked) was examined. The materials were prepared from a single batch of medical grade GUR 1050 resin (Ticona, Bayport, TX, USA). The two conventional UHMWPEs were as-received (virgin) and gamma radiation sterilized at 30 kGy in a nitrogen atmosphere (radiation sterilized). The two highly crosslinked UHMWPEs were each irradiated at 100 kGy and then post-processed with one of either two thermal treatments: annealing, which was done below the melt transition temperature (Tm), at 110 degrees C for 2h (110 degrees C-annealed); and, remelting, which was done above Tm, at 150 degrees C (150 degrees C-remelted). Differences in changes upon compression between the materials were examined using ANCOVA analyses. The 150 degrees C-remelted material showed a significant change in Tm and lamellar thickness upon compressive plastic deformation whereas the other three UHMWPE materials did not. However, all of the materials showed significantly decreased crystallinity and density upon compressive deformation. The findings of this study support that microstructural evolution during compressive deformation is a function of UHMWPE formulation, as affected by irradiation and post-irradiation heat treatment.

Periprosthetic Joint Infection in Patients Receiving TNFα Antagonists.

Tumor necrosis factor-α antagonists (anti-TNFα) have become increasingly more common as a treatment for rheumatoid arthritis (RA). However, there has been an increased incidence of severe infections in patients taking anti-TNFα therapy. We present a case series of RA patients treated with anti-TNFα therapy that had previously underwent TJA and subsequently developed periprosthetic infections. All patients had a well-functioning implant for a period of 1 to 14 years prior to the development of infection. Each patient underwent two to five different joint replacements, and four patients developed infection in multiple sites. The infections proved difficult to eradicate with four patients requiring multiple procedures, and one patient ultimately requiring a hemipelvectomy. This study suggests that periprosthetic infections acquired by patients on anti-TNFα therapy are challenging to eradicate and treat; highlighting the need for the establishment of guidelines for perioperative and long-term management of anti-TNFα therapy, and infection monitoring in joint replacement patients.

Preliminary investigation of a novel technique for the quantification of the ex vivo biomechanical properties of the vocal folds.

The human vocal fold is a complex structure made up of distinct layers that vary in cellular and extracellular matrix composition. Elucidating the mechanical properties of vocal fold tissues is critical for the study of both acoustics and biomechanics of voice production, and essential in the context of vocal fold injury and repair. Both quasistatic and dynamic behavior in the 10-300 Hz range was explored in this preliminary investigation. The resultant properties of the lamina propria were compared to that of the nearby thyroarytenoid muscle. Er, quantified via quasistatic testing of the lamina propria, was 609±138 MPa and 758±142 MPa in the muscle (p=0.001). E' of the lamina propria as determined by dynamic testing was 790±526 MPa compared to 1061±928 MPa in the muscle. Differences in E' did not achieve statistical significance via linear mixed effect modeling between the tissue types (p=0.95). In addition, frequency dependence was not significant (p=0.18).

Monotonic and fatigue behavior of five clinically relevant conventional and highly crosslinked UHMWPEs in the presence of stress concentrations.

Five formulations of clinically relevant UHMWPE (conventional, moderately crosslinked annealed and remelted, and highly crosslinked annealed and remelted) were investigated in a physiologically relevant environment. Their monotonic stress-strain behavior in the presence of notches of two different severities and at two different displacement rates was examined using a custom developed video based system. It was found that both an elevation of yield stress and a truncation of orientation hardening took place under monotonic loading and that these changes were found to be material and elastic stress concentration factor dependent. The fatigue behavior of these materials was examined using the same geometries via a stress-life approach with failure defined as fracture of the specimen in the 1000 to 100,000 cycle lifetime range. The results were modeled using the Basquin relationship (σ=AN(b), where σ=stress and N=lifetime, and A and b are experimentally derived constants) via maximum likelihood estimation methods to account for specimen runout (no failure at 250,000cycles). The conventional material was found to have a greater slope, b, and intercept, A, than the crosslinked materials as well as appearing to have less variance in its failure distributions.

The effects of loading conditions and specimen environment on the nanomechanical response of canine cortical bone.

Bone is a viscoelastic connective tissue composed primarily of mineral and type I collagen, which interacts with water, affecting its mechanical properties. Therefore, both the level of hydration and the loading rate are expected to influence the measured nanomechanical response of bone. In this study, we investigated the influence of three distinct hydration conditions, peak loads and loading/unloading rates on the elastic modulus and hardness of canine femoral cortical bone via nanoindentation. Sections from three canine femurs from multiple regions of the diaphysis were tested for a total of 670 indentations. All three hydration conditions (dry, moist and fully hydrated tissue) were tested at three different loading profiles (a triangular loading profile with peak loads of 600, 800 and 1000 µN at loading/unloading rate of 60, 80 and 100 µN/s, respectively; each test was 20s in duration). Significant differences were found for both the elastic modulus and hardness between the dry, moist and fully hydrated conditions (p≤0.02). For dry bone, elastic modulus and hardness values were not found to be significantly different between the different loading profiles (p>0.05). However, in both the moist and fully hydrated conditions, the elastic modulus and hardness were significantly different under all loading profiles (with the exception of the moist condition at the 600- and 800-µN peak load). Given these findings, it is critical to perform nanoindentation of bone under fully hydrated conditions to ensure physiologically relevant results. Furthermore, this work found that a 20-s triangular loading/unloading profile was sufficient to capture the viscoelastic behavior of bone in the 600- to 1000-µN peak load range. Lastly, specific peak load values and loading rates need to be selected based on the structural region for which the mechanical properties are to be measured.

Notch sensitivity of PEEK in monotonic tension.

Poly(ether-ether-ketone) (PEEK) has been used as a load bearing orthopaedic implant material with clinical success. All of the orthopaedic applications contain stress concentrations (notches) in their design; however, little work has been done to examine the stress-strain behavior of PEEK in the presence of a notch. This work examines both the stress-strain behavior and the fracture behavior of neat PEEK in a uniaxial loaded condition, and in circumferentially grooved round bar specimens with different elastic stress concentration factors. It was found that the material shows ductile necking in the smooth condition and that this is almost completely suppressed in the notched conditions. Additionally, the deformation and fracture micromechanisms changed drastically, from one of plastic deformation and void coalescence to one dominated by crazing and brittle fast fracture. This change in mechanism was explained via Neuber's theory of stresses at a notch.

Notched stress-strain behavior of a conventional and a sequentially annealed highly crosslinked UHMWPE.

Contemporary total joint replacement designs contain stress-risers such as fillets, grooves, and undercuts; therefore, it is of interest to analyze the behavior of UHMWPEs in the presence of such design-related stress-risers. This study examined the engineering and true axial stress-strain behavior of smooth cylindrical and notched cylindrical test specimens, under applied axial tensile loading (2 displacement rates, 37 degrees C) for a conventional and a highly crosslinked second generation UHMWPE. Both materials were prepared from ram extruded GUR 1050. The conventional material (30 kGy) was gamma sterilized at 30 kGy in an inert N(2) environment. The sequentially annealed material (SA) was gamma irradiated at 30 kGy and annealed for 8h at 130 degrees C. The irradiation-annealing process was repeated two more times for an overall irradiation dose of 90 kGy. Differential scanning calorimetry (DSC) was utilized to investigate changes in crystallinity and lamellar thickness distributions upon loading. Fractographic analysis of scanning electron microscope (SEM) images of fracture surfaces was performed to investigate changes in fracture micromechanism with notching. Both the 30 kGy and SA materials, in the smooth condition, demonstrated substantial ductility and orientation hardening. With the introduction of a notch, both materials demonstrated an elevation in the yield stress (notch strengthening) and a reduction in the ultimate stress and ultimate strain at both displacement rates. Additionally, it was found that the uniaxial stress-state (smooth condition) allowed for greater changes in crystallinity and the lamellar thickness distributions, when compared to the untested materials, than the triaxial stress-state induced by the notched geometry.