Implications of humeral short-stem diametral sizing on implant stability.

Background: Shoulder arthroplasty humeral stem design has evolved to include various shapes, coatings, lengths, sizes, and fixation methods. While necessary to accommodate patient anatomy characteristics, this creates a surgical paradox of choice. The relationship between the surgeon's selection of short-stem implant size and construct stiffness, resistance to subsidence and micromotion has not been assessed. Methods: Eight paired cadaveric humeri were reconstructed with surgeon-selected (SS) and 2-mm diametrically larger (SS+2) short-stemmed press-fit implants. Each reconstruction was subjected to 2000 cycles of 90° forward flexion loading, and stem subsidence and micromotion were measured using optical tracking. Compressive stiffness of the stem-bone reconstruction was then assessed by applying a load in-line with the stem axis that resulted in 5 mm of stem subsidence. Results: Increasing stem size by 2 mm resulted in the construct stiffness more than doubling compared to SS stems (-741 ± 243 N/mm vs. -334 ± 120 N/mm; P = .003; power = 0.971). These larger stems also subsided significantly less than their SS counterparts (SS: 1.2 ± 0.6 mm; SS+2: 0.5 ± 0.5 mm; P = .029; power = 0.66), though there were no significant changes in micromotion (SS: 169 ± 59 µm; SS+2: 187 ± 52 µm; P = .506; power = 0.094). Conclusions: The results of this study highlight the importance of proper short-stem sizing, as a relatively small 2 mm increase in diametral size was observed to significantly impact construct stiffness, which could increase the risk of stress shielding and implant loosening. Future work should focus on developing tools that objectively quantify bone quality and aid surgeons in selecting the appropriate size short-stem humeral implants for a particular patient.

The effect of humeral head positioning and incomplete backside contact on bone stresses following total shoulder arthroplasty with a short humeral stem.

BACKGROUND: The use of uncemented humeral stems in total shoulder arthroplasty (TSA) is known to be associated with stress shielding. This may be decreased with smaller stems that are well-aligned and do not fill the intramedullary canal; however, the effect of humeral head positioning and incomplete head backside contact has not yet been investigated. The purpose of this study was to quantify the effect of changes in humeral head position and incomplete head backside contact on bone stresses and expected bone response following reconstruction. METHODS: Three-dimensional finite element models of 8 cadaveric humeri were generated, which were then virtually reconstructed with a short-stem implant. An optimally sized humeral head was then positioned in both a superolateral and inferomedial position for each specimen that was in full contact with the humeral resection plane. Additionally, for the inferomedial position, 2 incomplete humeral head backside contact conditions were simulated whereby contact was defined between only the superior or inferior half of the backside of the humeral head and the resection plane. Trabecular properties were assigned based on computed tomography attenuation and cortical bone was applied uniform properties. Loads representing 45° and 75° of abduction were then applied, and the resulting differentials in bone stress versus the corresponding intact state and the expected time-zero bone response were determined and compared. RESULTS: The superolateral position reduced resorbing potential in the lateral cortex and increased resorbing potential in the lateral trabecular bone, while the inferomedial position produced the same effects but in the medial quadrant. For the inferomedial position, full backside contact with the resection plane was best in terms of changes in bone stress and expected bone response, although a small region of the medial cortex did experience no load transfer. The implant-bone load transfer of the inferior contact condition was concentrated at the midline of the backside of the humeral head, leaving the medial aspect largely unloaded as a result of the lack of lateral backside support. DISCUSSION: This study shows that inferomedial humeral head positioning loads the medial cortex at the cost of unloading the medial trabecular bone, with the same occurring for the superolateral position except that the lateral cortex is loaded at the cost of unloading the lateral trabecular bone. Inferomedial positioned heads also were predisposed to humeral head lift-off from the medial cortex, which may increase the risk of calcar stress shielding. For the inferomedial head position, full contact between the implant and resection plane was preferable.

The sizing and suitability of nonspherical ellipsoid humeral heads for total shoulder arthroplasty.

BACKGROUND: Total shoulder arthroplasty (TSA) implants have evolved to include more anatomically shaped components that better replicate the native state. The geometry of the humeral head is nonspherical, with the frontal diameter of the base of the head being up to 6% larger than the sagittal diameter. Despite this, most TSA humeral head implants are spherical, meaning that the diameter must be oversized to achieve complete coverage, resulting in articular overhang, or portions of the resection plane will remain uncovered. It is suggested that implant-bone load transfer between the backside of the humeral head and the cortex on the resection plane may yield better load-transfer characteristics if resection coverage were properly matched without overhang, thereby mitigating proximal stress shielding. METHODS: Eight paired cadaveric humeri were prepared for TSA by an orthopedic surgeon who selected and prepared the anatomic humeral resection plane using a cutting guide and a reciprocating sagittal saw. The humeral head was resected, and the resulting cortical boundary of the resection plane was digitized using a stylus and an optical tracking system. To simulate optimal sizing of both circular and elliptical humeral heads, both circles and ellipses were fit to the traces. Two extreme scenarios were also investigated: upsizing until 100% total coverage and downsizing until 0% overhang. RESULTS: By switching from a spherical (circular) to an ellipsoid (elliptical) humeral head, a small, 2.3% ± 0.3% increase in total coverage occurred (P < .001), which led to a large, 19.5% ± 1.3% increase in cortical coverage (P < .001). Using a circular head resulted in 2.0% ± 0.1% greater overhang (P < .001), defined as a percentage of the total enclosed area that exceeded the bounds of the humeral resection. As a result of increasing the head size until 100% resection coverage occurred, the ellipse produced 5.4% ± 3.5% less overhang than the circle (P < .001). When the head size was decreased until 0% overhang occurred, total coverage was 7.5% ± 2.8% greater for the ellipse (P < .001) and cortical coverage was 7.9% ± 8.2% greater for the ellipse (P = .01). Cortical coverage was greater for circular heads when the head size was shrunk below -2.25% of the optimal fitted size. DISCUSSION: Reconstruction with ellipsoid humeral heads can provide greater total resection and cortical coverage than spherical humeral heads while avoiding excessive overhang; however, cortical coverage can be inferior when undersized. These initial findings suggest that resection-matched humeral heads may yield benefits worth pursuing in the next generation of TSA implant design.

Examination of Diurnal Variation and Sex Differences in Hippocampal Neurophysiology and Spatial Memory.

Circadian rhythms are biological processes that cycle across 24 h and regulate many facets of neurophysiology, including learning and memory. Circadian variation in spatial memory task performance is well documented; however, the effect of sex across circadian time (CT) remains unclear. Additionally, little is known regarding the impact of time-of-day on hippocampal neuronal physiology. Here, we investigated the influence of both sex and time-of-day on hippocampal neurophysiology and memory in mice. Performance on the object location memory (OLM) task depended on both circadian time and sex, with memory enhanced at night in males but during the day in females. Long-term synaptic potentiation (LTP) magnitude at CA3-CA1 synapses was greater at night compared with day in both sexes. Next, we measured spontaneous synaptic excitation and inhibition onto CA1 pyramidal neurons. Frequency and amplitude of inhibition was greater during the day compared with night, regardless of sex. Frequency and amplitude of excitation was larger in females, compared with males, independent of time-of-day, although both time-of-day and sex influenced presynaptic release probability. At night, CA1 pyramidal neurons showed enhanced excitability (action potential firing and/or baseline potential) that was dependent on synaptic excitation and inhibition, regardless of sex. This study emphasizes the importance of sex and time-of-day in hippocampal physiology, especially given that many neurologic disorders impacting the hippocampus are linked to circadian disruption and present differently in men and women. Knowledge about how sex and circadian rhythms affect hippocampal physiology can improve the translational relevancy of therapeutics and inform the appropriate timing of existing treatments.

Humeral short stem varus-valgus alignment affects bone stress.

The use of uncemented humeral stems in total shoulder arthroplasty (TSA) is associated with stress shielding. Shorter length stems have shown to decrease stress shielding; however, the effect of stem varus-valgus alignment is currently not known. The purpose of this study was to quantify the effect of short stem distal humeral endosteal contact due to varus-valgus angulation on bone stresses after TSA. Three-dimensional models of eight male cadaveric humeri were constructed from computed tomography data. Bone models were reconstructed with a short stem humeral component implant in three positions (standard, varus, and valgus). Modeling was performed at 45° and 75° of abduction and the resulting differentials in bone stress compared to the intact state and the expected time-zero bone response were determined. In cortical and trabecular bone, the standard position (STD) altered bone stress less than the valgus (VAL) and varus (VAR) positions relative to the intact state. For both cortical (p = 0.033) and trabecular (p = 0.012) bone, the VAL position produced a larger volume of bone with resorbing potential compared to the STD position.

The effect of humeral implant thickness and canal fill on interface contact and bone stresses in the proximal humerus.

BACKGROUND: Stem size is an important element for successful time zero primary fixation of a press-fit humeral stem in shoulder arthroplasty. Little basic science research, however, has been conducted on the effects of implant thickness and canal fill on load transfer, contact, and stress shielding. The purpose of this finite element study was to determine the effects of varying stem thickness on bone contact, bone stresses, and bone resorption owing to stress shielding. METHODS: Three generic short-stem implant models were developed and varied based on cross-sectional thickness (thinner - 8 mm, medium - 12 mm, thicker - 16 mm). Using a finite element model, three outcome measures were determined (1) the amount of bone-to-implant contact, (2) changes in cortical and trabecular bone stresses from the intact state, and (3) changes in cortical and trabecular strain energy densities which can predict bone remodeling or stress shielding. RESULTS: Increasing the size of the humeral stem had no significant effects on bone-to-implant contact during loading (P > .07). The thinner implant with the lowest canal fill ratio produced significantly lower changes in stress from the intact state in both cortical and trabecular bone (P < .002). In addition, the thinner implant resulted in a substantially lower volume of bone predicted to stress shield and resorb when compared with the medium and thicker stems. DISCUSSION: The results demonstrate that thinner implants and lower canal fill may be beneficial over thicker sizes, provided equal initial fixation can be achieved. The thinner implant has a greater degree of load sharing and increases the mechanical load placed on surrounding bone, reducing the risk of stress shielding and bone resorption.

Regional apparent density correlations within the proximal humerus.

BACKGROUND: Bone quality influences humeral implant selection for shoulder arthroplasty. However, little is known about how well bone near the humeral resection represents more distal cancellous bone. This investigation aimed to quantify the correlations between the apparent density of sites near the humeral head resection plane and cancellous sites throughout the metaphysis. METHODS: Using computed tomography data from 98 subjects, apparent bone density was quantified in 65 regions throughout the proximal humerus. Pearson's correlation coefficient was determined comparing the density between samples from the humeral resection and all supporting regions beneath the resection. Mean correlation coefficients were compared for (i) each sample region with all support regions, (ii) pooling all sample regions within a slice, and (iii) considering sample regions correlated with only the support regions in the same anatomic section. RESULTS: Stronger correlations existed for bone sampled beneath the resection (0.33 ± 0.10≤ r ≤ 0.88 ± 0.10), instead of from the resected humeral head (0.22 ± 0.10≤ r ≤ 0.66 ± 0.14). None of sample region correlated strongly with all support regions; however, strong correlations existed when sample and support regions both came from the same anatomic section. DISCUSSION: Assessments of cancellous bone quality in the proximal humerus should be made beneath the humeral resection not in the resected humeral head; and each anatomic quadrant should be assessed independently.

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.

Double-screw and quadruple-button fixation for the glenoid: Latarjet versus bone block applications.

BACKGROUND: The Latarjet and bone block procedures can be secured with screws or cortical buttons. The purpose of this biomechanical study was to compare quadruple buttons vs. double screws for fixation of anterior glenoid bone grafts. METHODS: Twelve cadaveric scapulae (6 pairs) were denuded, resected, and potted. Pairs were randomized to quadruple-button or double-screw fixation after creation of a 15% anterior glenoid defect. The specimens underwent cyclic uniaxial compressive glenoid loading between 50 and 200 N for 1000 cycles at 1 Hz. Testing was repeated for conjoint tendon loads of 0 N (simulating a bone block procedure), 10 N, and 20 N (simulating a Latarjet procedure). Peak resultant relative coracoid graft displacement was optically tracked at 3 points (superior, central, and inferior) on the edge of the coracoid. RESULTS: No significant differences were found between buttons and screws for bone block applications or with 10 N of conjoint tendon loading (P ≥ .095). At 20 N of conjoint tendon loading, however, the screws were significantly more stable than the buttons (P ≤ .023). During the initial 20-N conjoint load application, all 3 points displaced significantly more with the button reconstruction than with the screws (P ≤ .01). Overall, mean displacements did not exceed 1 mm at any position on the coracoid, regardless of testing condition. CONCLUSIONS: The quadruple-button technique is comparable to screws when the coracoid is used as a bone block or when conjoint tendon loading is minimized. However, at higher conjoint tendon loads, the screws produced a more stable coracoid graft than the buttons.

An in-vitro biomechanical assessment of humeral head migration following irreparable rotator cuff tear and subacromial balloon reconstruction.

BACKGROUND: A resorbable subacromial balloon has been developed to address humeral head migration following posterosuperior rotator cuff tears. The purpose of this experimental assessment was to quantify the effect of balloon augmentation on humeral head position. METHODS: Eight cadaveric shoulders were subjected to 0°, 30°, 60° and 90° of abduction via a shoulder simulator. The deltoid was activated at 40N, then 80N. The subscapularis and infraspinatus with teres minor were then applied independently and together to create four muscle activation states for each deltoid load. The shoulder was tested intact, torn, then with the balloon. The centre of the humeral head was tracked using active optical markers. RESULTS: When the rotator cuff was torn, the humeral head translated superior by 1.4 ± 1 mm overall (P = 0.009). Following balloon augment, the humeral head translated inferiorly by 2 ± 2 mm relative to the intact state (P = 0.042), and significantly more anterior than the intact (3 ± 2 mm; P = 0.005) state. Rotator cuff variation was only significantly different when the balloon was used, with the subscapularis translating the humeral head posteriorly (P = 0.006). DISCUSSION: The subacromial balloon inferiorized the humeral head compared to the torn state. Unexpected anterior humeral head translation was attributed to the posterosuperior balloon placement relative to the humeral head.

The Effect of Inhomogeneous Trabecular Stiffness Relationship Selection on Finite Element Outcomes for Shoulder Arthroplasty.

An important feature of humeral orthopedic finite element (FE) models is the trabecular stiffness relationship. These relationships depend on the anatomic site from which they are derived; but have not been developed for the humerus. As a consequence, humeral FE modeling relies on relationships for other anatomic sites. The variation in humeral FE outcomes due to the trabecular stiffness relationship is assessed. Stemless arthroplasty FE models were constructed from CT scans of eight humeri. Models were loaded corresponding to 45 deg and 75 deg abduction. Each bone was modeled five times with the only variable being the trabecular stiffness relationship: four derived from different anatomic-sites and one pooled across sites. The FE outcome measures assessed were implant-bone contact percentage, von Mises of the change in stress, and bone response potential. The variance attributed to the selection of the trabecular stiffness relationship was quantified as the standard deviation existing between models of different trabecular stiffness. Overall, variability due to changing the trabecular stiffness relationship was low for all humeral FE outcome measures assessed. The variability was highest within the stress and bone formation potential outcome measures of the trabecular region. Variability only exceeded 10% in the trabecular stress change within two of the eight slices evaluated. In conclusion, the low variations attributable to the selection of a trabecular stiffness relationship based on anatomic-site suggest that FE models constructed for shoulder arthroplasty can utilize an inhomogeneous site-pooled trabecular relationship without inducing marked variability in the assessed outcome measures.

The effect of stemless humeral component fixation feature design on bone stress and strain response: a finite element analysis.

BACKGROUND: Since the advent of stemless implants, several different fixation feature designs have been used to improve primary implant stability. These stemless designs are diverse, and the rationale for their selection and design has not been thoroughly studied. Accordingly, this investigation assessed the effect of stemless implant geometry on the simulated stress and strain response of the proximal humerus. METHODS: Five humeral finite element models were used to examine 10 generic stemless implants with variable fixation features (2 central, 4 peripheral, and 4 boundary crossing). Loads representing 45° and 75° of shoulder abduction were simulated. Implants were compared based on the percentage of implant-bone surface area that remained in contact, the change in bone stress relative to the intact state, and the simulated potential for bone to resorb, remodel, or remain unchanged after reconstruction. RESULTS: The implant-bone contact area was greatest for peripheral, followed by central and boundary-crossing designs. All implants elicited similar bone stress variations, which were greatest 0 to 5 mm beneath the resection and laterally. The simulated potential cortical response was also similar for all implants, with the greatest simulated resorbing potential 0 to 15 mm beneath the resection, and very little expected remodeling. Differences between implants were most prominent within the simulated potential trabecular response, with the central implants having the least bone volume percentage expected to resorb. CONCLUSIONS: Simulated humeral bone response after stemless anatomic shoulder replacement depends on fixation feature geometry. Trade-offs exist between implant types. Centrally pegged implants produced the lowest simulated resorbing potential, whereas peripheral implants had the greatest percentages of implant-bone contact area.

Methods for Post Hoc Quantitative Computed Tomography Bone Density Calibration: Phantom-Only and Regression.

Quantitative computed tomography (qCT) relies on calibrated bone mineral density data. If a calibration phantom is absent from the CT scan, post hoc calibration becomes necessary. Scanning a calibration phantom after-the-fact and applying that calibration to uncalibrated scans has been used previously. Alternatively, the estimated density is known to vary with CT settings, suggesting that it may be possible to predict the calibration terms using CT settings. This study compares a novel CT setting regression method for post hoc calibration to standard and post hoc phantom-only calibrations. Five cadaveric upper limbs were scanned at 11 combinations of peak tube voltage and current (80-140 kV and 100-300 mA) with two calibration phantoms. Density calibrations were performed for the cadaver scans, and scans of the phantoms alone. Stepwise linear regression determined if the calibration equation terms were predictable using peak tube voltage and current. Peak tube voltage, but not current, was significantly correlated with regression calibration terms. Calibration equation slope was significantly related to the type of phantom (p < 0.001), calibration method (p = 0.026), and peak tube voltage (p < 0.001), but not current (p = 1.000). The calibration equation vertical intercept was significantly related to the type of phantom (p < 0.001), and peak tube voltage (p = 0.006), but not calibration method (p = 0.682), or current (p = 0.822). Accordingly, regression can correlate peak tube voltage with density calibration terms. Suggesting that, while standard qCT calibration is preferable, regression calibration may be an acceptable post hoc method when necessary.

An assessment of proximal humerus density with reference to stemless implants.

BACKGROUND: Shoulder arthroplasty humeral components have undergone several generational changes, with systems now offering shorter stems and stemless options. The stemless humeral implants obtain fixation in the trabecular bone of the proximal humerus through elaborate fixation features. To optimize implant design, the regional variations in bone density within the proximal humerus should be determined. As such, the purpose of this computed tomography-based study was to map the regional variations in bone density of the proximal humerus. METHODS: The trabecular-canal of the proximal humerus was extracted from computed tomography scans of 98 subjects and divided into 13 slices and 5 subsections (central, anterior, posterior, medial, and lateral). The average apparent density (ρAVG) was then quantified in each subsection of the trabecular-canal. RESULTS: Slice depth, subsection, and gender were all significant main effects, with additional significant interactions between slice depth, subsection, and osteoarthritic condition. The slices above the resection plane had the greatest ρAVG, with densities decreasing down the canal. The central subsection had significantly lower ρAVG than the peripheral sections, and the medial subsection tended to have the highest ρAVG (P < .001). Furthermore, the ρAVG of male subjects was significantly greater than that of female subjects (P < .001). CONCLUSIONS: The apparent density of the proximal humerus' trabecular-canal is nonuniform. This has implications for the design of stemless implants, indicating that implants seeking purchase in higher density bone should take advantage of the peripheral regions of the trabecular-canal within the first 15-20 mm beneath the humeral head resection plane.

An analysis of proximal humerus morphology with special interest in stemless shoulder arthroplasty.

BACKGROUND: Shoulder arthroplasty evolution has resulted in the shortening of traditional stemmed humeral components. Newer stemless implants rely on structures that maintain fixation in the metaphyseal region of the proximal humerus. Whereas the overall morphology of the proximal humerus is well understood, the advent of stemless implants requires that additional geometric measures be assessed. This study's purpose was to introduce new anatomic measures to assist with the design of stemless implants. METHODS: Using computed tomography data from 98 subjects (nonarthritic [n = 41], B2 osteoarthritic [n = 26], and symmetric osteoarthritic [n = 31]), shifts in proximal canal direction, bounding diameters along the canal, and canal depth beneath the center of the humeral resection plane were quantified. Traditional articular aspect ratio terms (ie, resection diameter, humeral head height) were also quantified. All measures were reported relative to a humeral coordinate system relevant to stemless implants. RESULTS: Humeral depth, gender, and osteoarthritis were found to have effects on the measured parameters. Of these factors, gender was the most prominent, as men presented with significantly larger canal diameters and depths than women did (P < .001). Osteoarthritis had less of a significant impact on results (P < .001), with the attributed differences in canal path direction and articular aspect ratio being small in absolute value. Canal diameter was found to change significantly as a function of depth beneath the resection plane (P < .001). CONCLUSIONS: This work quantified 3 new morphologic terms relevant to proximal humerus stemless arthroplasty. Together, these outcome measures help define the spatial limits for stemless humeral arthroplasty in an implant-relevant coordinate system.

Quantitative Computed Tomography (QCT) derived Bone Mineral Density (BMD) in finite element studies: a review of the literature.

BACKGROUND: Finite element modeling of human bone provides a powerful tool to evaluate a wide variety of outcomes in a highly repeatable and parametric manner. These models are most often derived from computed tomography data, with mechanical properties related to bone mineral density (BMD) from the x-ray energy attenuation provided from this data. To increase accuracy, many researchers report the use of quantitative computed tomography (QCT), in which a calibration phantom is used during image acquisition to improve the estimation of BMD. Since model accuracy is dependent on the methods used in the calculation of BMD and density-mechanical property relationships, it is important to use relationships developed for the same anatomical location and using the same scanner settings, as these may impact model accuracy. The purpose of this literature review is to report the relationships used in the conversion of QCT equivalent density measures to ash, apparent, and/or tissue densities in recent finite element (FE) studies used in common density-modulus relationships. For studies reporting experimental validation, the validation metrics and results are presented. RESULTS: Of the studies reviewed, 29% reported the use of a dipotassium phosphate (K2HPO4) phantom, 47% a hydroxyapatite (HA) phantom, 13% did not report phantom type, 7% reported use of both K2HPO4 and HA phantoms, and 4% alternate phantom types. Scanner type and/or settings were omitted or partially reported in 31% of studies. The majority of studies used densitometric and/or density-modulus relationships derived from different anatomical locations scanned in different scanners with different scanner settings. The methods used to derive various densitometric relationships are reported and recommendations are provided toward the standardization of reporting metrics. CONCLUSIONS: This review assessed the current state of QCT-based FE modeling with use of clinical scanners. It was found that previously developed densitometric relationships vary by anatomical location, scanner type and settings. Reporting of all parameters used when referring to previously developed relationships, or in the development of new relationships, may increase the accuracy and repeatability of future FE models.

Comparison of proximal humeral bone stresses between stemless, short stem, and standard stem length: a finite element analysis.

BACKGROUND: The stem lengths of humeral components used in shoulder arthroplasty vary; however, the literature on these devices is limited. This finite element study investigates the effect of humeral component length on stresses in the proximal humerus. METHODS: Intact and 3 reconstructed (standard length, short, and stemless implants) finite element models were created from shoulder computed tomography scan data (N = 5). Loading was simulated at varying abduction angles (15°, 45°, and 75°). The average bone stress (represented as a percentage of intact values) was reported at 8 transverse slices. In addition, the overall average change in cortical and trabecular bone stresses was quantified. RESULTS: Cortical bone stresses in the most proximal slice for the standard (58% ± 12%) and short (78% ± 10%) stem models were significantly reduced compared with the intact (100%) and stemless (101% ± 6%) models (P = .005). These reductions persisted in the second cortical slice for the standard stem compared with the intact, stemless, and short models (P = .025). Interestingly, stresses in the trabecular bone within these proximal slices were significantly elevated when stemless implants were used compared with all other implants (P < .001), regardless of abduction angle. CONCLUSION: Reducing stem length produced humeral stresses that more closely matched the intact stress distribution in proximal cortical bone. Opposing trends presented in the proximal trabecular bone, probably because of differences in load transfer when shorter stems are used. Accordingly, the results suggest that implant stem length is 1 variable that can be modified in an attempt to better mimic intact bone stresses during humeral component insertion, provided stem fixation is adequate.

The effect of static muscle forces on the fracture strength of the intact distal radius in vitro in response to simulated forward fall impacts.

The distal radius fracture (DRF) is a particularly dominant injury of the wrist, commonly resulting from a forward fall on an outstretched hand. In an attempt to reduce the prevalence, costs, and potential long-term pain/deformities associated with this injury, in vivo and in vitro investigations have sought to classify the kinematics and kinetics of DRFs. In vivo forward fall work has identified a preparatory muscle contraction that occurs in the upper extremity prior to peak impact force. The present investigation constitutes the first attempt to systematically determine the effect of static muscle forces on the fracture threshold of the distal radius in vitro. Paired human cadaveric forearm specimens were divided into two groups, one that had no muscle forces applied (i.e., right arms) and the other that had muscle forces applied to ECU, ECRL, FCU and FCR (i.e., left arms), with magnitudes based on peak muscle forces and in vivo lower bound forward fall activation patterns. The specimens were secured in a custom-built pneumatic impact loading device and subjected to incremental impacts at pre-fracture (25 J) and fracture (150 J) levels. Similar fracture forces (6565 (866)N and 8665 (5133)N), impulses (47 (6)Ns and 57 (30)Ns), and energies (152 (38)J and 144 (45)J) were observed for both groups of specimens (p>0.05). Accordingly, it is suggested that, at the magnitudes presently simulated, muscle forces have little effect on the way the distal radius responds to forward fall initiated impact loading.