Role of collisions in erosion of regolith during a lunar landing.

The supersonic gas plume of a landing rocket entrains lunar regolith, which is the layer of loose solids covering the lunar surface. This ejection is problematic due to scouring and dust impregnation of surrounding hardware, reduction in visibility for the crew, and spoofing of the landing sensors. To date, model predictions of erosion and ejection dynamics have been based largely on single-trajectory models in which the role of interparticle collisions is ignored. In the present work, the parameters affecting the erosion rate of monodisperse solids are investigated using the discrete element method (DEM). The drag and lift forces exerted by the rocket exhaust are incorporated via one-way coupling. The results demonstrate that interparticle collisions are frequent in the region immediately above the regolith surface; as many as 20% of particles are engaged in a collision at a given time. These collisions play an important role both in the erosion dynamics and in the final trajectories of particles. In addition, a direct assessment of the influence of collisions on the erosion rate is accomplished via a comparison between a "collisionless" DEM model and the original DEM model. This comparison shows that the erosion dynamics change drastically when collisions are considered and that the erosion rate is dependent on the collision parameters (coefficient of restitution and coefficient of friction). Physical explanations for these trends are provided.

Persistent ischiorectal fistula with supralevator origin secondary to a chronic tubo-ovarian abscess: report of a case and review of the literature.

BACKGROUND: Chronic tubo-ovarian abscess is an uncommon finding in postmenopausal women. This abscess may rupture or fistulize to adjacent organs into the ischiorectal space. CASE: A gravida three, para three, postmenopausal woman with extensive sigmoid diverticulosis presented with perianal fistula of 2 years' duration. Magnetic resonance imaging showed the tract to have a supralevator origin adjacent to the sigmoid colon. She had no recent instrumentation other than preoperative colonoscopy. Intraoperatively, the fistula tract origin was noted to be from a right tubo-ovarian abscess. She was treated with right salpingo-oophorectomy and tract excision/sealing. At 4-month follow-up, the fistula tract was healed with no further drainage. CONCLUSIONS: Tubo-ovarian abscess should be considered in the differential diagnosis of supralevator fistula in postmenopausal women when no other source can be localized.

H theorem for contact forces in granular materials.

A maximum entropy theorem is developed and tested for granular contact forces. Although it is idealized, describing two-dimensional packings of round, rigid, frictionless, cohesionless disks with coordination number Z=4, it appears to describe a central part of the physics present in the more general cases. The theorem does not make the strong claims of Edwards' hypothesis, nor does it rely upon Edwards' hypothesis at any point. Instead, it begins solely from the physical assumption that closed loops of grains are unable to impose strong force correlations around the loop. This statement is shown to be a generalization of Boltzmann's assumption of molecular chaos (his stosszahlansatz), allowing for the extra symmetries of granular stress propagation compared to the more limited symmetries of momentum propagation in a thermodynamic system. The theorem that follows from this is similar to Boltzmann's H theorem and is presented as an alternative to Edwards' hypothesis for explaining some granular phenomena. It identifies a very interesting feature of granular packings: if the generalized stosszahlansatz is correct, then the bulk of homogeneous granular packings must satisfy a maximum entropy condition simply by virtue of being stable, without any exploration of phase space required. This leads to an independent derivation of the contact force statistics, and these predictions have been compared to numerical simulation data in the isotropic case. The good agreement implies that the generalized stosszahlansatz is indeed accurate at least for the isotropic state of the idealized case studied here, and that it is the reductionist explanation for contact force statistics in this case.

Adenocarcinoma developing at an ileostomy: report of a case and review of the literature.

Primary adenocarcinoma of a permanent ileostomy is a rare and unusual complication. We report a case of primary adenocarcinoma arising at an ileostomy site 46 years after total proctocolectomy for Crohn's colitis. In addition, we performed a literature search and found 36 such cases reported. Based on the results of this case and literature review, we concur with the previously reported theory that the etiology of this phenomenon is likely the result of colonic metaplasia in the ileal mucosa, which eventually progresses to carcinoma. Common presenting symptoms include a bleeding, friable mass, difficulty fitting the stomal appliance, and bowel obstruction. Once confirmed by biopsy, appropriate surgical en bloc excision and stomal relocation is the mainstay of therapy. Lymph node metastasis occurs in 19 percent of patients and survival is at least 85 percent. Adjuvant therapy may be of additional benefit. Patient education is important for early detection as the lesion typically appears an average of 27 years after the original operation.

Intraoperative gamma probe localization of the ureters: a novel concept.

BACKGROUND: Ureteral stent placement to localize the ureters during operations is an invasive procedure. The aim of this study was to evaluate the feasibility of using the gamma probe to intraoperatively identify the ureters after intravenous injection of a radiopharmaceutical agent. STUDY DESIGN: Ten patients undergoing elective abdominal operations were prospectively enrolled in this study. An average dose of 4.5 mCi (range 2.8 to 5.3 mCi) of technetium Tc 99m-labeled diethylenetriamine pentaacetic acid ((99m)Tc-DTPA) was administered intravenously before localization of the ureters. The gamma probe was used to localize the ureters. Correct identification of the ureters was confirmed when gentle manipulation induced a typical ureteral peristaltic pattern. RESULTS: Gamma counts were significantly elevated in all ureters examined. Compared with background counts, gamma counts were increased over the ureter in all patients, with an average increase of 465%. The technique was modified after use in the first 3 patients and standardized for patients 4 through 10. Data from those seven patients were analyzed. Both ureters were correctly identified using the gamma probe at a mean of 15 minutes (median, 10 minutes) after a single (99m)Tc-DTPA injection (range 4 to 41 minutes). The mean background count was 80 counts per second (cps, range 50 to 130 cps). The mean ureter count was 393 cps (range 128 to 700 cps). The average percent increase of each ureter count compared with its specific background count was 465% (range 256% to 1,077%). The difference was statistically significant for all values (p < 0.001). CONCLUSIONS: This novel technique of gamma probe localization of the ureters may offer a noninvasive approach for ureteral identification.

Incidental appendectomy: 18-year pathologic survey and cost effectiveness in the nonmanaged-care setting.

BACKGROUND: Incidental appendectomy (IA) remains a controversial issue. The role of IA in the nonmanaged-care setting has not been evaluated recently. This study evaluates the cost-benefit of IA based on current third-party reimbursements and reports the incidence of pathology from routine IA during an 18-year period. STUDY DESIGN: A retrospective review was performed for all patients who underwent open intestinal operations for nonappendiceal pathology by a single colon and rectal surgeon between 1988 and 2006. Patient records were reviewed for surgical indication, procedure, pathology reports, and complications. A cost-benefit analysis for IA versus laparoscopic appendectomy was performed using previously published epidemiologic data for risk of appendectomy. RESULTS: During this period, 341 patients (mean age 62.9 years) underwent IA during open intestinal operation. Malignancy was the indication for operation in 61.6%. Pathologic findings of clinical significance in the appendix were present in 2.6% of specimens. Combined reimbursements for laparoscopic appendectomy for nonruptured and ruptured appendicitis are 8,500.95 dollars and 15,870.37 dollars, respectively. For patients with a benign surgical indication, there was cost-benefit for IA during open operation for men younger than 55 years and women younger than 50 years of age. For patients with malignant disease, there was cost-benefit for men and women younger than 45 years of age. CONCLUSIONS: IA can be performed safely during open operation for other bowel pathology. Rate of clinically significant appendiceal pathology is low but not negligible. For patients with third-party payor status, IA can be more broadly performed during open gastrointestinal operation when no additional surgeon reimbursement is obtained.

Colorectal lymphangioma.

Lymphangiomas of the colon are historically rare benign tumors. Only 331 cases have been reported in the world medical literature between 1931 and 2004. With widespread use of colonoscopy, however, they are being found more frequently. We report the case of a 74-year-old woman in whom a colonoscopy revealed a 3 x 4-cm submucosal lesion in the cecum that was eventually diagnosed as a lymphangioma. A CT of the abdomen showed a soft-tissue mass in the cecum and a low-density hepatic lesion. An endoscopic ultrasound of the colon showed a 3 x 4-cm hypoechoic lesion with internal septa arising from the submucosal layer of the cecum. This lesion resembled a vascular malformation; therefore a biopsy specimen was not taken. Pathologic findings of a specimen taken after a subsequent right hemicolectomy identified a submucosal lymphangioma. Published reports indicate that colonoscopy cures most lesions smaller than 2.5 cm in diameter. Resection should be reserved for larger lesions or those in patients exhibiting protein-losing enteropathy.

Unusual bacterial infections and colorectal carcinoma--Streptococcus bovis and Clostridium septicum: report of three cases.

Several types of unusual bacterial infections have been reported in association with colorectal malignancy. In the three cases described, uncommon infections resulted in further workup and subsequent resection for occult colonic malignancy. Awareness of this association is critical for early diagnosis and management.

Elegance of disordered granular packings: a validation of Edward's hypothesis.

We have found a way to analyze Edwards' density of states for static granular packings in the special case of round, rigid, frictionless grains assuming a constant coordination number. It obtains the most entropic density of single grain states, which predicts several observables including the distribution of contact forces. We compare these results against empirical data obtained in dynamic simulations of granular packings. The agreement is quite good, helping validate the use of statistical mechanics methods in granular physics. The differences between theory and empirics are mainly related to the coordination number, and when the empirical data are sorted by that number we obtain several insights that suggest an underlying elegance in the density of states.

Granular contact force density of states and entropy in a modified Edwards ensemble.

A method has been found to analyze Edwards' granular contact force probability functional for a special case. As a result, the granular contact force probability density functions are obtained from first principles for this case. The results are in excellent agreement with the experimental and simulation data. The derivation assumes Edwards' flat measure--a density of states (DOS) that is uniform within the metastable regions of phase space. The enabling assumption, supported by physical arguments and empirical evidence, is that correlating information is not significantly recursive through loops in the packing. Maximizing a state-counting entropy results in a transport equation that can be solved numerically. For the present this has been done using the "mean-structure approximation," projecting the DOS across all angular coordinates to more clearly identify its predominant nonuniformities. These features are (1) the grain factor Psi related to grain stability and strong correlation between the contact forces on the same grain and (2) the structure factor Upsilon related to Newton's third law and strong correlation between neighboring grains.

Comment on "Mechanical analog of temperature for the description of force distribution in static granular packings".

It has been proposed by Ngan [Phys. Rev. E 68, 011301 (2003)] that the granular contact force distribution may be analytically derived by minimizing the analog of a thermodynamic free energy, in this case consisting of the total potential energy stored in the compressed contacts minus a particular form of entropy weighted by a parameter. The parameter is identified as a mechanical temperature. I argue that the particular form of entropy cannot be correct and as a result the proposed method produces increasingly errant results for increasing grain rigidity. This trend is evidenced in Ngan's published results and in other numerical simulations and experiments.