The mechanics of corneal deformation and rupture for penetrating injury in the human eye.

Penetrating eye injuries are surgical emergencies with guarded visual prognosis. The purpose of the current study was to determine the force required to rupture the cornea with a penetrating object, and to study how this force is affected by the object geometry. Thirty-six human cadaveric eyes from donors of various ages were characterized for diameter, axial length, and pre-test intraocular pressure. In order to investigate the effects of specimen storage time on the tissue response, half of the specimens were tested within two weeks of donor expiration, and half of the specimens were stored at -4°C for 12-18 months. Indenters of three different diameters (1.0, 1.5, and 2.0mm) were lowered into the apex of the cornea until rupture. Resistance to displacement (stiffness), displacement at failure, and the force at failure were determined. Multi-variable regression analysis was used to determine associations of the input variables (indenter size, test speed, and tissue postmortem time) on the mechanics of the tissue response. Twenty-nine of the 36 specimens failed at the indenter location in the cornea, four failed at the limbus, and three failed in the sclera near sites of muscle attachment. The average force at failure caused by the 1.0mm, 1.5mm, and 2.0mm indenters increased from 30.5±5.5N to 40.5±8.3N to 58.2±14.5N, respectively (p<0.002). The force at failure was associated with the donor age (p<0.001), and globe diameter (p<0.041), but was not associated with pre-test intraocular pressure, tissue postmortem time, axial length, or speed of the indenter. This study has quantified the force-displacement and failure response of a large series of human cadaveric eyes subjected to penetrating indentation loads on the cornea. The results provide useful data for characterizing the relationship between corneal rupture and the geometry of a penetrating object.

Finite element analysis and experimental evaluation of penetrating injury through the cornea.

Penetration injuries of the eye are among the most frequent causes of permanent visual impairment resulting from trauma. The purpose of this study was to determine the peak strain at which rupture occurs in the cornea due to a penetrating object. Probes of varying diameters (1.0, 1.5, and 2.0mm) were pressed into the apex of the cornea of 36 human cadaveric eye specimens until perforation or rupture of the specimen at the cornea, limbus, or sclera occurred. An axisymmetric finite element model of the human globe was created to replicate the experimental set-up. The models were used to map the force-displacement response of the experiments and quantitatively determine a peak strain at which the eye ruptures. For the experiments, the average force at failure increased from the smallest to largest probe (p<0.002). The average forces at failure are as follows: 30.5±5.5N (1.0mm probe); 40.5±8.3N (1.5mm probe); 58.2±14.5N (2.0mm probe). The force-displacement responses of the finite element models of all three probe sizes bounded and tracked the experimental data. In all cases, the peak strain at failure in the cornea was located on the posterior surface of the cornea, directly adjacent to the corneal apex. This strain was in the range of 29% to 33% for all models analyzed. In addition to determining an objective failure strain of corneal tissue, the model developed in this study can provide quantitative information for understanding the risk of penetrating eye injuries.

Surgical management of complete blepharoptosis with no levator function from a compressive third nerve palsy.

A 94-year-old monocular woman with a posterior communicating artery aneurysm developed a compressive third nerve palsy with complete blepharoptosis and abduction of her seeing eye. It was believed that she was not a good neurosurgical candidate for aneurysm repair. Her ptosis was managed successfully with an in-office Whitnall sling procedure combined with a superior tarsectomy. The author describes this safe and effective method for surgical management of complete ptosis with zero levator function.

Oral gabapentin for the treatment of postoperative pain after photorefractive keratectomy.

PURPOSE: To evaluate oral gabapentin for postoperative pain after photorefractive keratectomy (PRK). DESIGN: Prospective, nonrandomized clinical trial. METHODS: In additional to a standard regimen of topical antibiotics, topical steroids, and topical tetracaine as required, all PRK patients at our laser vision center were treated after surgery for pain for a two-month period with Percocet (oxycodone/acetaminophen) [Endo Pharmaceuticals; Chadds Ford, Pennsylvania, USA] 5 mg/325 mg as required for three days (control group). Patients completed a pain assessment survey using a faces pain scale (from zero through 6) on the evening of surgery and each subsequent morning and evening until postoperative day 3. A successive cohort of patients received Neurontin (gabapentin) [Pfizer, New York, New York, USA] 300 mg thrice daily (first dose administered two hours or more before the procedure) as an oral pain medication for three days, and the same survey data were collected. RESULTS: Data were collected on 141 patients in each cohort. Successful pain management score (defined as faces zero through 2 on the scale) differences did not reach statistical significance between the two cohorts except on the morning of the second postoperative day, when gabapentin was superior. On all postoperative days, patients in the oxycodone/acetaminophen cohort used significantly less tetracaine eye drops as required. The percent of patients rating overall pain experience as better than expected was 35% and 36%, those rating pain experience as about what was expected was 50% and 49%, and those rating pain experience as worse than expected was 15% and 15% in the oxycodone/acetaminophen and gabapentin cohorts, respectively. CONCLUSIONS: We found no difference in overall subjective pain management ratings between gabapentin and oxycodone/acetaminophen for postoperative PRK pain, although gabapentin was associated with significantly more frequent use of anesthetic eye drops as required.

Accuracy, repeatability, and clinical application of spherocylindrical automated refraction using time-based wavefront aberrometry measurements.

PURPOSE: To evaluate the clinical utility of spherocylindrical automated refraction (AR) based on time-based wavefront technology compared with subjective manifest refraction (MR) for the purpose of prescribing eyeglasses. DESIGN: Prospective observational case series. PARTICIPANTS: A convenience sample of 105 eyes of 53 patients (ages 19 to 87) with corrected acuity no worse than 20/40(-2). METHODS: Manifest refraction was performed on all subjects followed by three consecutive wavefront measurements at both a 4-mm and 6-mm optical zone using the 3D Wave OPD Scan ARK 10,000 aberrometer. MAIN OUTCOME MEASURES: Repeatability of multiple consecutive ARs; difference in the sphere, cylinder, and axis of AR compared with MR; difference between the corrected visual acuity based on AR- and MR-derived acuity. RESULTS: Repeatability analysis showed that the initial wavefront refraction was not significantly different from the mean of the three refractions. At the 4-mm zone, the absolute value (+/- standard deviation) of the difference between the initial AR compared with the MR was 0.34+/-0.28 diopters (D) for sphere, 0.22+/-0.21 D for cylinder, and 12.5+/-15.8 degrees for axis. At the 6-mm zone, these values were 0.32+/-0.29 D for sphere, 0.25+/-0.24 D for cylinder, and 11.8+/-14.6 degrees for axis. When 4-mm AR visual acuity was superior to MR acuity (19.0% of eyes) it was 0.28+/-0.24 Snellen lines better, and when inferior (53.3% of eyes) it was 0.47+/-0.37 lines worse. When 6-mm AR acuity was superior to MR acuity (16.2% of eyes) it was 0.32+/-0.22 Snellen lines better, and when inferior (60.0% of eyes) it was 0.48+/-0.48 lines worse. The absolute value of the difference in spherical equivalent of the first AR measurement at the 4-mm zone compared with the MR value was 0.33+/-0.23 D, and at the 6-mm zone it was 0.30+/-0.27 D. CONCLUSIONS: The wavefront-derived AR values reflect the physical optical system, but not necessarily subjectively derived values, and the impact of this disagreement on patient satisfaction is unknown. Future studies evaluating this technology and its role in clinical ophthalmology are warranted.

Delayed-onset expulsive choroidal hemorrhage attributed to an acute elevation in systemic blood pressure following traumatic globe rupture.

The authors describe a 78-year-old woman who suffered a traumatic partial dehiscence of a penetrating keratoplasty on the day prior to presentation. While awaiting surgical repair, the patient experienced an expulsive choroidal hemorrhage necessitating a primary evisceration of the eye. This case is unique because the hemorrhage can be largely attributed to the acute dramatic rise in systemic blood pressure that immediately preceded it. Management considerations for patients with open-globe injuries who have poorly controlled systemic hypertension should include close monitoring of vital signs in a controlled setting, anxiolysis, aggressive intervention for hypertensive lability, and hastening of surgical repair regardless of nothing by mouth status.

Critically reappraising the literature-driven practice of analgesia administration for acute abdominal pain in the emergency room prior to surgical evaluation.

BACKGROUND: Classic teaching is that narcotic analgesia in the setting of an acute abdomen can alter physical examination findings and should therefore be withheld until after a surgeon's examination. METHODS: A telephone survey of emergency medicine physicians representing 60 US hospitals was conducted to assess the current practices and opinions regarding the early administration of narcotic analgesia in this setting. Relevant literature was also reviewed for methodological errors. RESULTS: Fifty-nine of 60 (98.3%) respondents reported that it is their practice to administer analgesia prior to surgical evaluation. Of these, only 9 of 59 (15.3%) reported always informing the surgeon prior to dosing the patient. The two most common motivations cited were that patient discomfort takes precedence (52 of 59; 88.1%) and that the literature supports the practice to be safe (51 of 59; 86.4%). CONCLUSIONS: It is common for emergency medicine physicians to medicate acute abdomen patients prior to surgical evaluation. Numerous significant study limitations and design flaws were found that question the validity of the four clinical trials supporting this practice. Because many physicians base their clinical decisions on these trials, a careful analysis of their shortcomings, as well as our own personal experiences and practice recommendations, is discussed.

Spontaneous nontraumatic hemoperitoneum due to a bleeding arteriovenous malformation on the serosal surface of the transverse colon: a case report.

Arteriovenous malformations of the gastrointestinal tract are a very common entity and a frequently cited cause of lower gastrointestinal bleeding in adults. They are characteristically found in the mucosa or submucosa of the bowel wall, and the vast majority do not cause any symptoms. We discuss the rare case of an extraluminal arteriovenous malformation of the transverse colon in an elderly woman who presented to the emergency department in hypovolemic shock after collapsing at home. An exploratory laparotomy was performed after a diagnostic peritoneal lavage yielded gross blood. The patient was found to have a spontaneously bleeding lesion confined to the serosal surface of the right transverse colon, which histological examination revealed to be an arteriovenous malformation. The lesion was excised and the patient recovered without sequelae. To date there have been no other documented cases of an arteriovenous malformation on the serosal surface of a visceral abdominal organ.

The development of an interactive game-based tool for learning surgical management algorithms via computer.

BACKGROUND: We have previously demonstrated the potential efficacy of a computer-assisted board game as a tool for medical education. The next logical step was to transfer the entire game on to the computer, thus increasing accessibility to students and allowing for a richer and more accurate simulation of patient scenarios. METHODS: First, a general game model was developed using Microsoft Visual Basic. A breast module was then created using 3-D models, radiographs, and pathology and cytology images. The game was further improved by the addition of an animated facilitator, who directs the players via gestures and speech. Thirty-three students played the breast module in a variety of team configurations. After playing the game, the students completed surveys regarding its value as both an educational tool and as a form of entertainment. 10-question tests were also administered before and after playing the game, as a preliminary investigation into its impact on student learning. RESULTS: After playing the game, mean test scores increased from 6.43 (SEM +/- 0.30) to 7.14 (SEM +/- 0.30; P = 0.006). The results of the five-question survey were extremely positive. Students generally agreed that the game concept has value in increasing general knowledge regarding the subject matter of breast disease and that the idea of following simultaneously the work-up of numerous patients with similar problems is a helpful way to learn a work-up algorithm. CONCLUSIONS: Postgame surveys demonstrate the efficacy of our computer game model as a tool for surgical education. The game is an example of problem based learning because it provides students with an initial set of problems and requires them to collect information and reason on their own in order to solve the problems. Individual game modules can be developed to cover material from different diagnostic areas.