The bidirectional lung brain-axis of amyloid-ß pathology: ozone dysregulates the peri-plaque microenvironment.

The mechanisms underlying how urban air pollution affects Alzheimer's disease (AD) are largely unknown. Ozone (O3) is a reactive gas component of air pollution linked to increased AD risk, but is confined to the respiratory tract after inhalation, implicating the peripheral immune response to air pollution in AD neuropathology. Here, we demonstrate that O3 exposure impaired the ability of microglia, the brain's parenchymal immune cells, to associate with and form a protective barrier around Aß plaques, leading to augmented dystrophic neurites and increased Aß plaque load. Spatial proteomic profiling analysis of peri-plaque proteins revealed a microenvironment-specific signature of dysregulated disease-associated microglia protein expression and increased pathogenic molecule levels with O3 exposure. Unexpectedly, 5xFAD mice exhibited an augmented pulmonary cell and humoral immune response to O3, supporting that ongoing neuropathology may regulate the peripheral O3 response. Circulating HMGB1 was one factor upregulated in only 5xFAD mice, and peripheral HMGB1 was separately shown to regulate brain Trem2 mRNA expression. These findings demonstrate a bidirectional lung-brain axis regulating the central and peripheral AD immune response and highlight this interaction as a potential novel therapeutic target in AD.

An in vitro comparison of mixed-reality navigation to traditional freehand and patient-specific instrumentation techniques for glenoid guide pin insertion during shoulder arthroplasty.

BACKGROUND: Accurate insertion of the glenoid guide pin in shoulder arthroplasty (RSA) is important for obtaining optimized glenoid component position and orientation. The objective of this study was to evaluate and compare the accuracy of 3 glenoid guide pin insertion techniques: (1) traditional software planning using freehand guide pin insertion (freehand), (2) guide pin insertion utilizing patient-specific instrumentation (PSI), and (3) using a mixed-reality navigation (MR-NAV) system. METHODS: Twenty (20) computed tomography (CT) scans were obtained from patients exhibiting glenoid erosion patterns according to the Walch and Favard classifications. Cases were planned using validated 3-dimensional (3D) preoperative planning software. The CT data were then used to 3D print triplicate plastic models of each glenoid to evaluate the 3 guide pin insertion techniques. The first technique employed traditional software planning with freehand guide pin insertion. The second method used preoperatively planned PSI guides, whereas the third used an MR-NAV system, which provided real-time holographic guidance during guide pin insertion. Once all guide pins had been inserted into the models, an independent optical tracking system and custom digitization device was used to quantify the position and orientation of each guide pin relative to the glenoid. The outcomes for this study included the absolute mean error in guide pin inclination, version, and entry point relative to the preoperative plan. The absolute Total Global Error was also assessed, which was defined as the sum of the absolute guide pin orientation and position error relative to the preoperative plan. RESULTS: No statistically significant differences between MR-NAV and PSI were found for the inclination error (2° ± 1° vs. 2° ± 1°; P = .056), version error (1° ± 1° vs. 1° ± 1°; P > .99), and Total Global Error (5 ± 1 [mm + deg] vs. 5 ± 1 [mm + deg], P > .99), respectively. The freehand technique produced significantly greater error than MR-NAV and PSI for inclination (5° ± 3°, P ≤ .017), version (4° ± 3°, P ≤ .032), and Total Global Error (8 ± 3 [mm + deg], P < .001). No statistically significant differences in the entry point error were observed between all guide pin insertion methods (P ≥ .058). DISCUSSION: These results demonstrate that the precision and accuracy of MR-NAV is comparable to PSI and superior to a freehand technique for glenoid guide pin insertion in vitro. Further study is needed to compare the accuracy of these techniques intraoperatively, in addition to assessing a potential learning curve between surgeons of varying experience with the MR-NAV system.

Superior Capsular Reconstruction using the Long Head of Biceps Tendon: A Biomechanical Assessment of Tenodesis Location and Angle of Fixation.

BACKGROUND: Massive irreparable rotator cuff tears (MIRCT) treated with superior capsular reconstruction (SCR) using the long head of the biceps tendon have shown satisfactory early results. Different techniques and positions for biceps tenodesis have been described. This study aimed to evaluate the effect of tenodesis location and glenohumeral fixation angle for graft tensioning on the biomechanics of a SCR using a single strand biceps technique. METHODS: Eight cadaveric shoulders were mounted to a custom biomechanical simulator which employed static tone loads to the deltoid and rotator cuff muscles. All cadavers were first tested in the intact condition, and then in the simulated MIRCT condition by sectioning the tendinous insertions of the supraspinatus and upper border of the infraspinatus. SCR using the long head of the biceps tendon was then evaluated. Three biceps tenodesis locations relative to the greater tuberosity (anterior, middle, and posterior) and two glenohumeral fixation angles (0° and 30°) for graft tensioning were tested. An optical tracking system was used to quantify superior-inferior (SI) and anterior-posterior (AP) humeral head translation relative to the glenoid, while the functional abduction force was quantified using a load sensor. All tests were conducted at 0°, 30° and 60° of glenohumeral abduction in a randomized fashion. RESULTS: When assessing isolated superior humeral head migration, all biceps tenodesis locations were effective at decreasing superior migration, with no tenodesis location significantly better than the other (P=0.213). However, biceps grafts tensioned at 30° of glenohumeral abduction were significantly better at reducing proximal humeral migration as compared to graft tensioning at 0° abduction (P=0.008). Posterior humeral head translation observed in the MIRCT condition was significantly reduced when tensioning the biceps tendon at 30° of glenohumeral abduction compared to 0° for all tenodesis locations (P≤0.043). Tenodesis location also significantly influenced posterior humeral head translation (P=0.001), with middle and posterior positions restoring near normal humeral head position when fixed at 30° glenohumeral abduction. All SCR techniques using the biceps tendon improved the functional abduction force relative to the MIRCT condition, although no statistically significant differences were observed relative to the intact condition (P≥0.448). DISCUSSION: SCR using the long head biceps tendon is biomechanically effective in reducing posterosuperior translation of the humeral head in the setting of a MIRCT. Graft tensioning and fixation at 30° of glenohumeral abduction combined with either a middle or posterior tenodesis location on the greater tuberosity most effectively restores near normal time-zero humeral head kinematics.

The Kouvalchouk procedure vs. distal tibial allograft for treatment of posterior shoulder instability: the deltoid "hammock" effect exists.

BACKGROUND: In 1993, Kouvalchouk described an acromial bone block with a pedicled deltoid flap for the treatment of posterior shoulder instability. This procedure provides a "double blocking" effect in that the acromial autograft restores posterior glenoid bone loss and the deltoid flap functions as a muscular "hammock" resembling the sling effect of the conjoint in the Latarjet procedure. The primary aim of this study was to compare the Kouvalchouk procedure to distal tibial allograft (DTA) reconstruction for the management of posterior shoulder instability with associated bone loss, while the secondary aim was to evaluate the deltoid hammock effect. METHODS: Ten upper extremity cadavers were evaluated using a validated shoulder testing apparatus in 0° and 60° of glenohumeral abduction in the scapular plane. Testing was first performed on the normal shoulder state and was followed by the creation of a 20% posterior glenoid defect. Subsequently, the Kouvalchouk and DTA procedures were conducted. Forces of 0N, 5N, 10N, and 15N were applied to the posterior deltoid tendinous insertion on the Kouvalchouk graft along the physiological muscle line-of-action to evaluate the 'hammock" effect of this procedure. Testing was additionally performed on the Kouvalchouk bone graft with the deltoid muscle sectioned from its bony attachment. For all test states, a posteriorly directed force was applied to the humeral head perpendicular to the direction of the glenoid bone defect, with the associated translation quantified using an optical tracking system. The outcome variable was posterior translation of the humeral head at an applied force magnitude of 30N. RESULTS: The Kouvalchouk procedure with the loaded deltoid flap (10N: P = .039 and 15N: P < .001) was significantly better at reducing posterior humeral head translation than the DTA. Overall, increased glenohumeral stability was observed with increased force applied to the posterior deltoid flap in the Kouvalchouk procedure. The 15 N Kouvalchouk was most effective at preventing posterior humeral translation, and the difference was statistically significant compared with the 20% glenoid defect (P = .003), detached Kouvalchouk (P < .001), and 0N Kouvalchouk (P < .001). The 15 N Kouvalchouk procedure restored posterior shoulder joint stability to near normal levels, such that it was not significantly different from the intact state (P = .203). CONCLUSIONS: The Kouvalchouk procedure with load applied to the deltoid was found to be biomechanically superior to the DTA for the management of posterior shoulder instability with associated bone loss. Additionally, the results confirmed the presence and effectiveness of the deltoid "hammock" effect.

Stemless reverse humeral component neck-shaft angle has an influence on initial fixation.

BACKGROUND: Stemless anatomic humeral components are commonly used and are an accepted alternative to traditional stemmed implants in patients with good bone quality. Presently, little literature exists on the design and implantation parameters that influence primary time-zero fixation of stemless reverse humeral implants. Accordingly, this finite element analysis study assessed the surgical implantation variable of neck-shaft angle, and its effect on the primary time-zero fixation of reversed stemless humeral implants. METHODS: Eight computed tomography-derived humeral finite element models were used to examine a generic stemless humeral implant at varying neck-shaft angles of 130°, 135°, 140°, 145°, and 150°. Four loading scenarios (30° shoulder abduction with neutral forearm rotation, 30° shoulder abduction with forearm supination, a head-height lifting motion, and a single-handed steering motion) were employed. Implantation inclinations were compared based on the maximum bone-implant interface distraction detected after loading. RESULTS: The implant-bone distraction was greatest in the 130° neck-shaft angle implantation cases. All implant loading scenarios elicited significantly lower micromotion magnitudes when neck-shaft angle was increased (P = .0001). With every 5° increase in neck-shaft angle, there was an average 17% reduction in bone-implant distraction. CONCLUSIONS: The neck-shaft angle of implantation for a stemless reverse humeral component is a modifiable parameter that appears to influence time-zero implant stability. Lower, more varus, neck-shaft angles increase bone-implant distractions with simulated activities of daily living. It is therefore suggested that humeral head osteotomies at a higher neck-shaft angle may be beneficial to maximize stemless humeral component stability.

Peripheral HMGB1 is linked to O3 pathology of disease-associated astrocytes and amyloid.

INTRODUCTION: Ozone (O3) is an air pollutant associated with Alzheimer's disease (AD) risk. The lung-brain axis is implicated in O3-associated glial and amyloid pathobiology; however, the role of disease-associated astrocytes (DAAs) in this process remains unknown. METHODS: The O3-induced astrocyte phenotype was characterized in 5xFAD mice by spatial transcriptomics and proteomics. Hmgb1fl/fl LysM-Cre+ mice were used to assess the role of peripheral myeloid cell high mobility group box 1 (HMGB1). RESULTS: O3 increased astrocyte and plaque numbers, impeded the astrocyte proteomic response to plaque deposition, augmented the DAA transcriptional fingerprint, increased astrocyte-microglia contact, and reduced bronchoalveolar lavage immune cell HMGB1 expression in 5xFAD mice. O3-exposed Hmgb1fl/fl LysM-Cre+ mice exhibited dysregulated DAA mRNA markers. DISCUSSION: Astrocytes and peripheral myeloid cells are critical lung-brain axis interactors. HMGB1 loss in peripheral myeloid cells regulates the O3-induced DAA phenotype. These findings demonstrate a mechanism and potential intervention target for air pollution-induced AD pathobiology. HIGHLIGHTS: Astrocytes are part of the lung-brain axis, regulating how air pollution affects plaque pathology. Ozone (O3) astrocyte effects are associated with increased plaques and modified by plaque localization. O3 uniquely disrupts the astrocyte transcriptomic and proteomic disease-associated astrocyte (DAA) phenotype in plaque associated astrocytes (PAA). O3 changes the PAA cell contact with microglia and cell-cell communication gene expression. Peripheral myeloid cell high mobility group box 1 regulates O3-induced transcriptomic changes in the DAA phenotype.

Global flyway evolution in red knots Calidris canutus and genetic evidence for a Nearctic refugium.

Present-day ecology and population structure are the legacies of past climate and habitat perturbations, and this is particularly true for species that are widely distributed at high latitudes. The red knot, Calidris canutus, is an arctic-breeding, long-distance migratory shorebird with six recognized subspecies defined by differences in morphology, migration behavior, and annual cycle phenology, in a global distribution thought to have arisen just since the last glacial maximum (LGM). We used nextRAD sequencing of 10,881 single-nucleotide polymorphisms (SNPs) to assess the neutral genetic structure and phylogeographic history of 172 red knots representing all known global breeding populations. Using population genetics approaches, including model-based scenario-testing in an approximate Bayesian computation (ABC) framework, we infer that red knots derive from two main lineages that diverged ca. 34,000 years ago, and thus most probably persisted at the LGM in both Palearctic and Nearctic refugia, followed by at least two instances of secondary contact and admixture. Within two Beringian subspecies (C. c. roselaari and rogersi), we detected previously unknown genetic structure among sub-populations sharing a migratory flyway, reflecting additional complexity in the phylogeographic history of the region. Conversely, we found very weak genetic differentiation between two Nearctic populations (rufa and islandica) with clearly divergent migratory phenotypes and little or no apparent contact throughout the annual cycle. Together, these results suggest that relative gene flow among migratory populations reflects a complex interplay of historical, geographical, and ecological factors.

Mismatch-induced growth reductions in a clade of Arctic-breeding shorebirds are rarely mitigated by increasing temperatures.

In seasonal environments subject to climate change, organisms typically show phenological changes. As these changes are usually stronger in organisms at lower trophic levels than those at higher trophic levels, mismatches between consumers and their prey may occur during the consumers' reproduction period. While in some species a trophic mismatch induces reductions in offspring growth, this is not always the case. This variation may be caused by the relative strength of the mismatch, or by mitigating factors like increased temperature-reducing energetic costs. We investigated the response of chick growth rate to arthropod abundance and temperature for six populations of ecologically similar shorebirds breeding in the Arctic and sub-Arctic (four subspecies of Red Knot Calidris canutus, Great Knot C. tenuirostris and Surfbird C. virgata). In general, chicks experienced growth benefits (measured as a condition index) when hatching before the seasonal peak in arthropod abundance, and growth reductions when hatching after the peak. The moment in the season at which growth reductions occurred varied between populations, likely depending on whether food was limiting growth before or after the peak. Higher temperatures led to faster growth on average, but could only compensate for increasing trophic mismatch for the population experiencing the coldest conditions. We did not find changes in the timing of peaks in arthropod availability across the study years, possibly because our series of observations was relatively short; timing of hatching displayed no change over the years either. Our results suggest that a trend in trophic mismatches may not yet be evident; however, we show Arctic-breeding shorebirds to be vulnerable to this phenomenon and vulnerability to depend on seasonal prey dynamics.

Unexpected diversity in socially synchronized rhythms of shorebirds.

The behavioural rhythms of organisms are thought to be under strong selection, influenced by the rhythmicity of the environment. Such behavioural rhythms are well studied in isolated individuals under laboratory conditions, but free-living individuals have to temporally synchronize their activities with those of others, including potential mates, competitors, prey and predators. Individuals can temporally segregate their daily activities (for example, prey avoiding predators, subordinates avoiding dominants) or synchronize their activities (for example, group foraging, communal defence, pairs reproducing or caring for offspring). The behavioural rhythms that emerge from such social synchronization and the underlying evolutionary and ecological drivers that shape them remain poorly understood. Here we investigate these rhythms in the context of biparental care, a particularly sensitive phase of social synchronization where pair members potentially compromise their individual rhythms. Using data from 729 nests of 91 populations of 32 biparentally incubating shorebird species, where parents synchronize to achieve continuous coverage of developing eggs, we report remarkable within- and between-species diversity in incubation rhythms. Between species, the median length of one parent's incubation bout varied from 1-19 h, whereas period length-the time in which a parent's probability to incubate cycles once between its highest and lowest value-varied from 6-43 h. The length of incubation bouts was unrelated to variables reflecting energetic demands, but species relying on crypsis (the ability to avoid detection by other animals) had longer incubation bouts than those that are readily visible or who actively protect their nest against predators. Rhythms entrainable to the 24-h light-dark cycle were less prevalent at high latitudes and absent in 18 species. Our results indicate that even under similar environmental conditions and despite 24-h environmental cues, social synchronization can generate far more diverse behavioural rhythms than expected from studies of individuals in captivity. The risk of predation, not the risk of starvation, may be a key factor underlying the diversity in these rhythms.

Overhead arm positioning in the rehabilitation of elbow dislocations: An in vitro biomechanical study.

STUDY DESIGN: In vitro biomechanical study. INTRODUCTION: Elbow stiffness is a common complication following elbow dislocation. Overhead exercises have been proposed to initiate early motion to reduce stiffness through employing gravity to stabilize the elbow. The implications of this position with regard to elbow kinematics after dislocation have not been reported. PURPOSE OF THE STUDY: To determine the influence of the overhead position on elbow stability following combined medial and lateral collateral ligament (MCL and LCL) injuries. METHODS: Passive and simulated active extension were performed on 11 cadaveric elbows with the arm in the overhead, dependent, and horizontal positions and with the forearm in pronation, neutral, and supination. Internal-external rotation (IER) and varus-valgus angulation (VVA) of the ulnohumeral joint were assessed for the intact elbow and after simulated MCL-LCL injury. Repeated-measures analyses of variance were conducted to analyze the effects of elbow state, arm position, forearm rotation, and extension angle. RESULTS: During passive extension with the arm overhead, the pronated position resulted in more internal rotation than supination (-2.6 ± 0.7°, P = .03). There was no effect of forearm rotation on VVA. The overhead position increased internal rotation relative to the dependent position when the forearm was neutral (-8.5 ± 2.5°, P = .04) and relative to the horizontal position when the forearm was supinated (-12.7 ± 2.2°, P= .02). During active extension, pronation increased valgus angle compared to the neutral (+1.2 ± 0.3°, P= .04) and supinated (+1.5 ± 0.4°, P= .03) positions, but did not affect IER. There was no difference between active and passive motion with the arm overhead (P > .05). DISCUSSION: Movement of the injured elbow in the overhead position most closely replicated kinematics of the intact elbow compared to the other arm positions. CONCLUSIONS: Overhead elbow extension results in similar kinematics between an intact elbow and an elbow with MCL and LCL tears. As such, therapists might consider early motion in this position to reduce the risk of elbow stiffness after dislocation.

The Effect of Forearm Position on Wrist Joint Biomechanics.

PURPOSE: All active motion wrist joint simulators have been designed to simulate physiologic wrist motion; however, a main difference among them is the orientation of the forearm (horizontal or vertical with respect to gravity). Moreover, the effect of forearm orientation on experimental results has yet to be quantified, but it may be an important variable. Thus, the purpose of this study was to determine the effect of forearm orientation on wrist kinematics and contact mechanics. METHODS: Eight cadaveric upper limbs were cycled through a flexion-extension motion using an active motion wrist simulator. Motion trials were performed in 3 forearm orientations (gravity-neutral, gravity-flexion, and gravity-extension). A computed tomography-based joint congruency technique was used to examine radiocarpal joint contact and joint contact centroid translation in the 3 tested orientations. RESULTS: At full wrist extension and wrist flexion, radioscaphoid contact area was greatest in the gravity-extension orientation. Radiolunate contact area was similar among all 3 forearm orientations. The radioscaphoid contact centroid was consistent among the 3 tested positions with the wrist in neutral wrist position. In contrast, the radioscaphoid contact centroid translated radially in the gravity-neutral position relative to the gravity-flexion position in extreme extension. There were no differences in radiolunate centroid contact position in the 3 forearm orientations. CONCLUSIONS: This study demonstrates that forearm orientation affects contact mechanics and end-range carpal kinematics. Future biomechanical studies should report forearm orientation and discuss the implication of the forearm orientation used on the experimental results. CLINICAL RELEVANCE: This study provides evidence that the wrist joint is sensitive to forearm positions consistent with activities of daily living and rehabilitation protocols.

A Biomechanical Evaluation of the ECRL Tenodesis for Reconstruction of the Scapholunate Ligament.

PURPOSE: Reconstruction of the scapholunate ligament (SLL) in the setting of dynamic instability remains a surgical challenge, with lack of consensus on the best reconstructive procedure. Reconstruction of only the dorsal component may lead to volar gapping and abnormal wrist kinematics. This cadaveric active motion simulation study determined whether scapholunate (SL) motion, angulation, and contact are restored following open reconstruction using the extensor carpi radialis longus (ECRL) tenodesis, which reconstructs both the volar and the dorsal SLL components. METHODS: Seven fresh-frozen cadaveric upper limbs (mean age, 68 ± 10.1 years) underwent a 4-stage protocol of cyclic dart-throw motion and flexion-extension motion (utilizing an active wrist motion simulator that used tendon load/motion-controlled actuation. Scaphoid and lunate motion, relative scaphoid translation, SL angle, and dorsal-volar SL diastasis were measured with (1) wrist ligaments intact, (2) following complete sectioning of the SLL, and (3) following SL reconstruction using the ECRL tenodesis technique. RESULTS: Complete SLL sectioning resulted in a typical pattern of SL instability. Following the ECRL tenodesis, lunate extension was not corrected. Scaphoid flexion, however, was not significantly different from the native state in FEM but remained significantly flexed during dart-throw motion. Differential dorsal and volar gapping did not significantly improve following ECRL tenodesis (dorsal, 1.2-2.3 mm; volar, 1.1-1.7 mm). CONCLUSIONS: This biomechanical study demonstrates that the ECRL tenodesis did not fully restore native carpal kinematics, despite dorsal and volar SLL, and scaphotrapeziotrapezoid reconstruction. TYPE OF STUDY/LEVEL OF EVIDENCE: Therapeutic IV.

Glenoid baseplate screw fixation in reverse shoulder arthroplasty: does locking screw position and orientation matter?

BACKGROUND: The longevity of a reverse total shoulder arthroplasty prosthesis can be compromised by glenoid baseplate loosening. Circular baseplate designs can be secured with superoinferior (SI) locking screws and anteroposterior (AP) compression screws or vice versa (AP-locking and SI-compression screws). This biomechanical cadaveric study investigated screw position (locking screws SI vs. AP and compression screws AP and SI) and screw orientation (parallel vs. divergent) to determine quantitative differences in baseplate micromotion. METHODS: Ten paired fresh-frozen cadaveric scapulae (n = 20) were implanted with a standard circular baseplate (∅ = 29 mm). The specimens were randomized into SI-locking or AP-locking screw configurations with the screw orientation directed either parallel or angled divergently at 15°. This yielded a total of 4 groups for statistical comparison: SI-lockingparallel, SI-lockingdivergent, AP-lockingparallel, and AP-lockingdivergent, which were subjected to axial eccentric loading on the implanted baseplates, similar to the American Standard of Testing of Materials standard for shoulder joint arthroplasty. RESULTS: In both static and cyclic testing, there were no statistically significant differences (P = .6) in micromotion between SI-locking (2.9 ± 0.8 µm) and AP-locking (3.5 ± 1.5 µm) configurations. In addition, there were no statistically significant differences (P = .2) in the divergent screw orientation group (2.0 ± 0.7 µm) vs. the parallel group (4.0 ± 1.5 µm). CONCLUSION: All configurations of screw position and screw orientation tested in a circular reverse baseplate have similar time-zero fixation in an intact glenoid bone model. In addition, the resultant micromotions for all configurations tested fell far below the 150 µm threshold for bone ongrowth.

The effect of hemiarthroplasty implant modulus on contact mechanics: an experimental investigation.

BACKGROUND: Hemiarthroplasties cause damage to the cartilage that they articulate against, which is a major limitation to their use. This study investigated the use of lower-stiffness materials to determine whether they improve hemiarthroplasty contact mechanics and thus reduce the risk of cartilage damage. METHODS: Eleven fresh-frozen cadaveric upper extremities were disarticulated and fixed in a custom-built jig that applied a static load of 50 N to the radiocapitellar joint. Flexion angles of 0°, 45°, 90°, and 135° were tested with radial head implants made of cobalt-chrome (CoCr) and ultrahigh-molecular-weight polyethylene (UHMWPE) compared with the native radial head. A Tekscan thin-film sensor was used to measure the contact area and contact pressure between the radius and capitellum. RESULTS: UHMWPE and CoCr were too stiff in the application of hemiarthroplasty, resulting in lower contact areas and higher contact pressures relative to the native joint. The native contact area was, on average, 42 ± 20 mm2 larger than that of UHMWPE (P < .001) and 55 ± 24 mm2 larger than that of CoCr (P < .001). UHMWPE had a contact area 13 ± 10 mm2 greater than that of CoCr (P = .014). DISCUSSION AND CONCLUSION: This study shows that even though UHMWPE has a stiffness several times lower than CoCr, the use of this material in hemiarthroplasty led to only a minor improvement in contact mechanics. Neither implant restored contact similar to the native articulation. Investigations into new materials to improve the contact mechanics of hemiarthroplasty should focus on materials with a lower stiffness than UHMWPE.

The influence of reverse arthroplasty humeral component design features on scapular spine strain.

BACKGROUND: Reverse shoulder arthroplasty (RSA) humeral implant parameters have been previously studied with respect to range of motion, deltoid function, and stability. However, limited literature exists on the influence of humeral design features on scapular spine strain. The purpose of this cadaveric biomechanical simulator study was to evaluate the role of humeral component lateralization and neck-shaft angle (NSA) on scapular spine strain. METHODS: Eight fresh-frozen cadaveric shoulders were tested using an in vitro shoulder simulator. A custom-designed modular RSA system was implanted that allowed for the in situ adjustment of humeral lateralization and NSA. Scapular spine strain was measured by strain gauges placed along the acromion and scapular spine in clinically relevant positions representative of the Levy fracture zones. All testing was conducted in both abduction and forward elevation. RESULTS: In Levy zones 2 and 3, increasing humeral lateralization caused significant incremental decreases in scapular spine strain at 0° and 90° abduction (P < .042). Strain decreases as high as 34% were noted with increases in humeral lateralization from -5 to 15 mm (P = .042). Changing NSA had no statistically significant effect on scapular spine strain (P > .14). CONCLUSIONS: Some humeral implant design features in RSA have effects on scapular spine strain. Humeral component lateralization had significant effects, whereas adjusting NSA resulted in no substantial differences in scapular spine strain. Understanding humeral component variables is important to allow for design optimization of future RSA implants.

Comparing internal fixation constructs for scapular spine insufficiency fractures following reverse shoulder arthroplasty.

INTRODUCTION: There is limited research on the surgical management techniques for scapular spine fractures after reverse shoulder arthroplasty (RTSA). As such, the purpose of this in vitro biomechanical study was to compare 4 fixation constructs to stabilize scapular spine insufficiency fractures. METHODS: Twelve paired fresh-frozen cadaveric scapulae (N = 24) were randomized into 4 fixation groups: subcutaneous border plating (± hook) and supraspinatus fossa plating (± hook). A Levy type II fracture was simulated. Each specimen was cyclically loaded incrementally up to 700 N in 50 N steps or until failure. Between 50 and 200 N construct stiffness was measured, and stability failure was defined as displacement greater than 2.5 mm. RESULTS: Seventy-nine percent (19 of 24) of the specimens failed before the maximum load of 700 N. The average survival force with subcutaneous border plating was 480 ± 80 N compared with 380 ± 30 N for supraspinatus fossa plating (P = .3). Fixation construct failure was significantly more likely with fossa plating over subcutaneous plating (P = .012). The presence of the lateral plate hook was beneficial in preventing failure of the lateral acromion (P = .016). CONCLUSION: When appropriately surgically indicated, a dorsally applied plate to the subcutaneous border of the scapular spine with a lateral inferior supporting hook may be advantageous for internal fixation of type II scapular spine insufficiency fractures after RTSA.

The role of biceps loading and muscle activation on radial head stability in anterior Monteggia injuries: An in vitro biomechanical study.

INTRODUCTION: Little evidence-based information is available to direct the optimal rehabilitation of patients with anterior Monteggia injuries. PURPOSE OF THE STUDY: The aims of this biomechanical investigation were to (1) quantify the effect of biceps loading and (2) to compare the effect of simulated active and passive elbow flexion on radial head stability in anterior Monteggia injuries. STUDY DESIGN: In vitro biomechanical study. METHODS: Six cadaveric arms were mounted in an elbow motion simulator. The effect of biceps loading, simulated active and passive elbow flexion motions was examined with application of 0N, 20N, 40N, 60N, 80N, and 100N of load. Simulated active and passive elbow flexion motions were then performed with the forearm supinated. Radial head translation relative to the capitellum was measured using an optical tracking system. After testing the intact elbows, the proximal ulna was osteotomized and realigned using a custom jig to simulate an anatomical reduction. We then sequentially sectioned the anterior radiocapitellar joint capsule, annular ligament, quadrate ligament, and the proximal and middle interosseous membrane to simulate soft tissue injuries commonly associated with anterior Monteggia fractures. RESULTS: Greater magnitudes of biceps loading significantly increased anterior radial head translation. However, there was no significant difference in radial head translation between simulated active and passive elbow flexion except in the final stage of soft tissue sectioning. There was a significant increase in anterior radial head translation with progressive injury states with both isometric biceps loading and simulated active and passive motion. CONCLUSIONS: Our results demonstrate that anatomic reduction of the ulna may not be sufficient to restore radial head alignment in anterior Monteggia injuries with a greater magnitude of soft tissue injury. In cases with significant soft tissue injury, the elbow should be immobilized in a flexed and supinated position to allow relaxation of the biceps and avoid movement of the elbow in the early postoperative period.

Seasonal survival and reversible state effects in a long-distance migratory shorebird.

Events during one stage of the annual cycle can reversibly affect an individual's condition and performance not only within that stage, but also in subsequent stages (i.e. reversible state effects). Despite strong conceptual links, however, few studies have been able to empirically link individual-level reversible state effects with larger-scale demographic processes. We studied both survival and potential reversible state effects in a long-distance migratory shorebird, the Hudsonian Godwit Limosa haemastica. Specifically, we estimated period-specific survival probabilities across the annual cycle and examined the extent to which an individual's body condition, foraging success and habitat quality during the nonbreeding season affected its subsequent survival and reproductive performance. Godwit survival rates were high throughout the annual cycle, but lowest during the breeding season, only slightly higher during southbound migration and highest during the stationary nonbreeding season. Our results indicate that overwintering godwits foraging in high-quality habitats had comparably better nutritional status and pre-migratory body condition, which in turn improved their return rates and the likelihood that their nests and chicks survived during the subsequent breeding season. Reversible state effects thus appeared to link events between nonbreeding and breeding seasons via an individual's condition, in turn affecting their survival and subsequent reproductive performance. Our study thus provides one of the few empirical demonstrations of theoretical predictions that reversible state effects have the potential to influence population dynamics.

Weak effects of geolocators on small birds: A meta-analysis controlled for phylogeny and publication bias.

Currently, the deployment of tracking devices is one of the most frequently used approaches to study movement ecology of birds. Recent miniaturization of light-level geolocators enabled studying small bird species whose migratory patterns were widely unknown. However, geolocators may reduce vital rates in tagged birds and may bias obtained movement data. There is a need for a thorough assessment of the potential tag effects on small birds, as previous meta-analyses did not evaluate unpublished data and impact of multiple life-history traits, focused mainly on large species and the number of published studies tagging small birds has increased substantially. We quantitatively reviewed 549 records extracted from 74 published and 48 unpublished studies on over 7,800 tagged and 17,800 control individuals to examine the effects of geolocator tagging on small bird species (body mass <100 g). We calculated the effect of tagging on apparent survival, condition, phenology and breeding performance and identified the most important predictors of the magnitude of effect sizes. Even though the effects were not statistically significant in phylogenetically controlled models, we found a weak negative impact of geolocators on apparent survival. The negative effect on apparent survival was stronger with increasing relative load of the device and with geolocators attached using elastic harnesses. Moreover, tagging effects were stronger in smaller species. In conclusion, we found a weak effect on apparent survival of tagged birds and managed to pinpoint key aspects and drivers of tagging effects. We provide recommendations for establishing matched control group for proper effect size assessment in future studies and outline various aspects of tagging that need further investigation. Finally, our results encourage further use of geolocators on small bird species but the ethical aspects and scientific benefits should always be considered.

The effect of short-stem humeral component sizing on humeral bone stress.

BACKGROUND: Several humeral stem design modifications for shoulder arthroplasty, including reduced stem length, changes to metaphyseal geometry, and alterations to implant surface texture, have been introduced to reduce stress shielding. However, the effect of changes in the diametral size of short-stem humeral components remains poorly understood. The purpose of this finite element study was to quantify the effect of varying the size of short-stem humeral components on the changes in bone stress from the intact state to the reconstructed state. METHODS: Three-dimensional models of 8 male cadaveric humeri (mean age, 68 ± 6 years; all left-sided humeri) were constructed from computed tomography data using Mimics software. Each humerus was then reconstructed with 2 short-stem components (Exactech Preserve), one having a larger diametral size (SH+) and one having a smaller diametral size (SH-). Modeling was conducted for loading states consistent with 45° and 75° of abduction, and the resulting changes in bone stress compared with the intact state and the expected bone response were determined. RESULTS: The smaller (SH-) short-stem implant produced humeral cortical and trabecular bone stresses that were closer to the intact state than the larger (SH+) short-stem implant at several locations beneath the humeral head resection (P ≤ .032). A similar trend was observed for expected bone response, where the smaller (SH-) short-stem implant had a smaller proportion of bone that was expected to resorb following reconstruction compared with the larger (SH+) short-stem implant for several slice depths in the medial quadrant (P ≤ .02). DISCUSSION: These findings may indicate that smaller short-stem components are favorable in terms of stress shielding.