Useful scientific theories are useful: A reply to Rouder, Pratte, and Morey (2010)

In a recognition memory experiment, Mickes, Wixted, and Wais (2007) asked a simple question: Would the same result-namely, a higher mean and variance of the memory strengths for the targets as compared with the lures-be evident if one used a 20-point confidence scale and then simply computed the relevant distributional statistics from the ratings themselves instead of estimating them by fitting a Gaussian model to ROC data? And if an unequal-variance model were suggested by the ratings data, would the magnitude of the estimated ratio of the standard deviations based on the ratings (s Lure / s Target ) be similar to the magnitude of the estimated ratio obtained by fitting a Gaussian model to ROC data ( Lure / Target )? A priori, agreement between the two ratio estimates seems unlikely, because there are many reasons why they might disagree. For example, if the Gaussian assumption is not valid, then disagreement between the two estimates seems more likely than agreement. In addition, if the rating scale does not approximate an interval scale, or if it covers only a limited range of the memory strength dimension, then, again, disagreement seems more likely than agreement. Somewhat surprisingly, From these results, 1. The two experiments reported here support a conclusion that is commonly drawn from ROC analysis-namely, that the memory strengths of the targets are more variable than the memory strengths of the lures. (p. 864) 2. The close agreement between the model-based ROC analysis and the model-free ratings method supports not only an unequal-variance model, but also the idea that the memory strengths are distributed in such a way that fitting a specifically Gaussian model to the data yields accurate conclusions (even if the true underlying distributions are not strictly Gaussian). (p. 864

Novel Methodology for Using Radiostereometric Analysis to Monitor Fracture Healing

Background: Radiostereometric Analysis (RSA) is a method for performing highly accurate three-dimensional measurements in-vivo using sequential radiographs. RSA has been used extensively for monitoring prosthesis fixation in hip and knee replacements. Recently, there has been increasing interest in applying RSA towards the monitoring of fracture healing. Objective: The objective of this study was to evaluate the feasibility of using RSA to measure strain, stress, and plate migration in a distal femur fracture model. Methods: Femoral sawbones with a distal femur fracture were used as models. A distal femur condylar locking compression plate (LCP) was used to reduce the fracture model. Stainless steel screws were used to fasten the plate to the sawbone. In addition, translucent polyester screws were composed, embedded with 1mm steel beads, and fastened to the most proximal and distal portions of the plate. This allowed for recognition by the RSA imaging modality. The femoral sawbones were then placed in a mechanical testing complex and RSA X-rays taken at different forces of compression. The radiographs were analyzed for plate migration using the 1mm steel beads as points of reference. Results: Preliminary data indicate that it is possible to use a plate model that incorporates a micro-bead system to measure migration. Further analysis will quantify the amount of migration to determine whether significant changes occur at different stages of compression. Conclusion: The ability to measure plate migration in a Radiostereometric Analysis X-ray model is an important step towards improving the ability of orthopedic surgeons to monitor fracture healing and prevent non-union. The next stage of this research will involve using this model in clinical trials of distal femur fractures and building a database to correlate levels of plate migration with surgical outcome

Biomechanical investigation of a novel ratcheting arthrodesis nail

<p>Abstract</p> <p>Background</p> <p>Knee or tibiotalocalcaneal arthrodesis is a salvage procedure, often with unacceptable rates of nonunion. Basic science of fracture healing suggests that compression across a fusion site may decrease nonunion. A novel ratcheting arthrodesis nail designed to improve dynamic compression is mechanically tested in comparison to existing nails.</p> <p>Methods</p> <p>A novel ratcheting nail was designed and mechanically tested in comparison to a solid nail and a threaded nail using sawbones models (Pacific Research Laboratories, Inc.). Intramedullary nails (IM) were implanted with a load cell (Futek LTH 500) between fusion surfaces. Constructs were then placed into a servo-hydraulic test frame (Model 858 Mini-bionix, MTS Systems) for application of 3 mm and 6 mm dynamic axial displacement (n = 3/group). Load to failure was also measured.</p> <p>Results</p> <p>Mean percent of initial load after 3-mm and 6-mm displacement was 190.4% and 186.0% for the solid nail, 80.7% and 63.0% for the threaded nail, and 286.4% and 829.0% for the ratcheting nail, respectively. Stress-shielding (as percentage of maximum load per test) after 3-mm and 6-mm displacement averaged 34.8% and 28.7% (solid nail), 40.3% and 40.9% (threaded nail), and 18.5% and 11.5% (ratcheting nail), respectively. In the 6-mm trials, statistically significant increase in initial load and decrease in stress-shielding for the ratcheting vs. solid nail (<it>p </it>= 0.029, <it>p </it>= 0.001) and vs. threaded nail (<it>p </it>= 0.012, <it>p </it>= 0.002) was observed. Load to failure for the ratcheting nail; 599.0 lbs, threaded nail; 508.8 lbs, and solid nail; 688.1 lbs.</p> <p>Conclusion</p> <p>With significantly increase of compressive load while decreasing stress-shielding at 6-mm of dynamic displacement, the ratcheting mechanism in IM nails may clinically improve rates of fusion.</p

Displacement Across a Fracture Gap with Axial Loading of Far Cortical Locking Constructs

Purpose: Far cortical locking has been proposed for reducing stiffness and promoting greater dynamic stability in locked plating constructs. Prior studies have shown reduced stiffness with axial loading of these constructs, leading to a theoretical increase in inter-fragmentary motion and secondary bone healing. The purpose of this study was to examine strain across a fracture gap using far cortical locking constructs in a biomechanical model of distal femoral fractures. Methods: Fourth generation sawbones were cut transversely along the distal diaphysis and plated with distal femoral buttress plates and cortical locking screws. Far cortical locking (FCL) specimens were predrilled in the lateral cortex and control specimens were plated with a standard locked plating construct. The constructs were loaded sequentially with 100, 200, and 400 lbs of force on a mechanical test frame. Displacement across the fracture gap measured in pixels using an optical system. Results: Strain across the fracture gap increased with progressive loading from zero to 400 lbs in both groups. Strain also decreased in a linear fashion from medial to lateral across the fracture gap in both constructs (Figure 1). Standard locking constructs exhibited an average 28% greater strain than the far cortical locking constructs at all loading forces. Control specimens exhibited greater lateral displacement of the distal segment relative to the plate (Figure 2), consistent with higher shear forces compared to FCL specimens. Conclusions: In all specimens, there was considerable strain seen with loading that increased in characteristic fashion from lateral to medial. Overall, FCL constructs exhibited both lower strain, and importantly, lower shear, than measured in controls. This biomechanical model suggests that FCL changes loading across the femoral diaphysis in complex ways, and that assumptions about strain approaching zero on the lateral side of the distal femur with conventional locking or FCL may be incorrect

Patient Flow in Peri-Operative Services: Analysis of Factors that Impact OR Turn Over Time

Introduction: The University Campus of UMass Memorial Medical Center has 16 operating rooms. The average Turn Over Time (TOT), defined as patient out of OR to next patient in, is 43 minutes. This is a source of dissatisfaction for the surgeons and has an impact on the patient experience. We did an observational study to map and time patient flow from the Surgical Admission Unit (SACU) to the Operating Room and the process for OR turn over and arrival of the next patient. The goal was to assess how the process could be modified to enhance the patient experience and improve surgeon satisfaction. Methods: Observation and hand recording of three rooms in the University OR was performed from 6/13/2011 – 8/5/2011 (Orthopedic and Neurosurgery cases). Time points for all processes directly involving the patient were collected. Data collection was done by three undergraduate students. Support for the students was provided by Orthopedics, Anesthesiology and Peri-Operative Services. Results: See Figure 1 for patient flow and turn over process map. Subsequent figures show time intervals from SACU to OR case completion, call for cleaning, completion of cleaning and arrival of next patient. Cleaning was complete in 18 minutes but TOT was 43 minutes because nondependent tasks were being performed in series (Figure 5). Summary: Nondependent tasks are being performed in series in the SACU and during OR turn over. There is an opportunity to decrease patient length of stay in the SACU. By doing parallel practice OR TOT can be decreased from 43 to 19 minutes if the anesthesia team is allowed to bring the patient in the room once the room is clean. This process can enhance the patient experience and improve surgeon satisfaction

A direct test of the unequal-variance signal detection model of recognition memory

Analyses of the receiver operating characteristic (ROC) almost invariably suggest that, on a recognition memory test, the standard deviation of memory strengths associated with the lures (sigma(lure)) is smaller than that of the targets (sigma(target)). Often, sigma(lure)/ sigma(target) approximately = 0.80. However, that conclusion is based on a model that assumes that the memory strength distributions are Gaussian in form. In two experiments, we investigated this issue in a more direct way by asking subjects to simply rate the memory strengths of targets and lures using a 20-point or a 99-point strength scale. The results showed that the standard deviation of the ratings made to the targets (S(target)) was, indeed, larger than the standard deviation of the ratings made to the lures (S(lure)). Moreover, across subjects, the ratio S(lure)/ S(target) correlated highly with the estimate of sigma(lure)/ sigma(target) obtained from ROC analysis, and both estimates were, on average, approximately equal to 0.80.</p

Useful scientific theories are useful: A reply to Rouder, Pratte, and Morey (2010)

In a recognition memory experiment, Mickes, Wixted, and Wais (2007) reported that distributional statistics computed from ratings made using a 20-point confidence scale (which showed that the standard deviation of the ratings made to lures was approximately 0.80 times that of the targets) essentially matched the distributional statistics estimated indirectly by fitting a Gaussian signal-detection model to the receiver-operating characteristic (ROC). We argued that the parallel results serve to increase confidence in the Gaussian unequal-variance model of recognition memory. Rouder, Pratte, and Morey (2010) argue that the results are instead uninformative. In their view, parametric models of latent memory strength are not empirically distinguishable. As such, they argue, our conclusions are arbitrary, and parametric ROC analysis should be abandoned. In an attempt to demonstrate the inherent untestability of parametric models, they describe a non-Gaussian equal-variance model that purportedly accounts for our findings just as well as the Gaussian unequal-variance model does. However, we show that their new model-despite being contrived after the fact and in full view of the to-be-explained data-does not account for the results as well as the unequal-variance Gaussian model does. This outcome manifestly demonstrates that parametric models are, in fact, testable. Moreover, the results differentially favor the Gaussian account over the probit model and over several other reasonable distributional forms (such as the Weibull and the lognormal).</p