Eli R. Groppo

Intramuscular and blood pressures in legs positioned in the hemilithotomy position : clarification of risk factors for well-leg acute compartment syndrome.

Background: Acute compartment syndrome has been widely reported in legs positioned in the lithotomy position for prolonged general surgical, urologic, and gynecologic procedures. The orthopaedic literature also contains reports of this complication in legs positioned on a fracture table in the hemilithotomy position. The purpose of this study was to identify the risk factors for development of acute compartment syndrome resulting from this type of leg positioning. Methods: Eight healthy volunteers were positioned on a fracture table. Intramuscular pressures were continuously measured with a slit catheter in all four compartments of the left leg with the subject supine, in the hemilithotomy position with the calf supported, and in the hemilithotomy position with the heel supported but the calf free. Blood pressure was measured intermittently with use of automated pressure cuffs. Results: Changing from the supine to the calf-supported position significantly increased the intramuscular pressure in the anterior compartment (from 11.6 to 19.4 mm Hg) and in the lateral compartment (from 13.0 to 25.8 mm Hg). Changing from the calf-supported to the heel-supported position significantly decreased intramuscular pressure in the anterior, lateral, and posterior compartments (to 2.8, 3.4, and 1.9 mm Hg, respectively). The mean diastolic blood pressure in the ankle averaged 63.9 mm Hg in the supine position, which significantly decreased to 34.6 mm Hg in the calf-supported position. Changing to the heel-supported position had no significant effect on the diastolic blood pressure in the ankle (mean, 32.8 mm Hg). The mean difference between intramuscular pressure and diastolic blood pressure in the supine position was approximately 50 mm Hg in each of the four compartments. This mean difference significantly decreased to <20 mm Hg in the calf-supported position and then, when the leg was moved into the heel-supported position, significantly increased to approximately 30 mm Hg in all compartments. Conclusions: The combination of increased intramuscular pressure due to external compression from the calf support and decreased perfusion pressure due to the elevated position causes a significant decrease in the difference between the diastolic blood pressure and the intramuscular pressure when the leg is placed in the hemilithotomy position in a well-leg holder on a fracture table. Combined with a prolonged surgical time, this position may cause an acute compartment syndrome of the well leg. Leaving the calf free, instead of using a standard well-leg holder, increases the difference between the diastolic blood pressure and the intramuscular pressure and may decrease the risk of acute compartment syndrome.

Lower body positive-pressure exercise after knee surgery

Lower body positive pressure allows unloading of the lower extremities during exercise in a pressurized treadmill chamber. This study assessed the preliminary feasibility of lower body positive pressure exercise as a rehabilitation technique by examining its effects on gait mechanics and pain, postoperatively. Fifteen patients who had arthroscopic meniscectomy or anterior cruciate ligament reconstruction participated in this study. Patients exercised for 5 minutes at 2.0 mph under three body weight conditions (normal body weight, 60% body weight, and 20% body weight) in random order. Bilateral ground reaction force, electromyographs, and dynamic knee range of motion were collected, and pain was assessed using a visual analog scale. Ground reaction forces for surgically treated and contralateral extremities were reduced 42% and 79% from normal body weight conditions when ambulating at 60% and 20% body weight, respectively. After meniscectomy, ambulatory knee range of motion decreased only at 20% body weight (37°), compared with normal body weight conditions (49°). Peak electromyographic activity of the biceps was maintained at all body weight conditions, whereas that of the vastus medialis was reduced at 20% body weight. Pain relief was significant with lower body positive pressure ambulation after anterior cruciate ligament reconstruction. This study showed that lower body positive pressure exercise is effective at reducing ground reaction forces, while safely facilitating gait postoperatively. Level of Evidence: Therapeutic study, Level II-1 (study of untreated controls from a previous randomized controlled trial)

Genetic Heritability of Urinary Stone Risk in Identical Twins

PURPOSE Quantitative measurements of urinary parameters are valuable clinical tools for predicting the risk of nephrolithiasis. To our knowledge no previous studies have evaluated the heritability of urinary stone risk in identical twins. Because these individuals share identical genetics, the R defined by their phenotypic data are theoretically equal to the entire population H2. MATERIALS AND METHODS A total of 12 sets of healthy homozygous twins, including 4 males and 8 females for a total of 24 individuals, with a mean age of 25.9 years (range 21 to 36) volunteered for this study. All subjects provided informed written consent before assessment. Urinary stone risk profiles were done elsewhere on 2 consecutive days after 5 days of a standardized diet (170 mEq Na and 2500 kcal). Linear regression was performed on the data to determine R. Because identical twins were used, R was theoretically an estimate of H2. RESULTS Certain urinary stone risk markers were highly heritable, including urinary calcium (94%), oxalate (94%), citrate (95%), uric acid (96%) and brushite supersaturation (90%), as determined by genotype (H2 90% or greater). Uric acid supersaturation (58%) and urinary sodium (64%) had low degrees of heritability. CONCLUSIONS H2 is a measure of how much of the total variance in phenotype results from differences in genotype, as opposed to environmental differences. For example, an H2 of 95% for citrate suggests that genetic differences account for 95% of the variation in urinary citrate and environmental differences account for the remaining 5%. Therefore urinary calcium, oxalate and citrate are primarily determined by genotype, while environmental factors, particularly those that impact urine pH and urinary volume, may be increasingly important for determining uric acid supersaturation.

Renal Stone Risk in a Simulated Microgravity Environment: Impact of Treadmill Exercise With Lower Body Negative Pressure

PURPOSE Prolonged exposure to microgravity during spaceflight causes metabolic changes that increase the risk of renal stone formation. Studies during the Gemini, Apollo, Skylab and Shuttle missions demonstrated alterations in renal function, fluid homeostasis and bone resorption that result in increased urinary supersaturation of calcium oxalate, brushite, sodium urate and uric acid. Developing countermeasures to increased urinary supersaturation is an important priority as the duration of space missions increases. MATERIALS AND METHODS A total of 11 sets of identical twins remained on 6-degree head down, tilt bed rest for 30 days to simulate prolonged microgravity. One twin per pair was randomly selected to exercise while supine in a lower body negative pressure chamber 6 days weekly for 40 minutes, followed by 5 minutes of resting lower body negative pressure at 50 mm Hg. The other twin served as a nonexercise control. Pressure in the exercise lower body negative pressure chamber (52 to 63 mm Hg) was adjusted to produce footward forces equivalent to those for upright running on Earth at 1.0 to 1.2 x body weight. Pre-bed rest urinary stone risk profiles were done elsewhere after 5 days of a standardized diet, consisting of 170 mEq sodium, 1,000 mg calcium, 0.8 gm/kg animal protein and 2,500 kcal, and then throughout the bed rest and recovery phases of the protocol. RESULTS A significant increase in urinary calcium after just 1 week of bed rest was noted in the nonexercise control group (p = 0.001). However, no such increase was noted in the exercise group. Brushite supersaturation increased significantly from bed rest in each group, although the increase was significantly higher in the nonexercise control group than in the exercise group (p = 0.006). Calcium oxalate supersaturation increased during bed rest in the exercise group (p = 0.004). It trended toward a higher level in the nonexercise control group, although this did not achieve significance (p = 0.055) Mean urine volume +/- SD was significantly higher in the nonexercise control group than in the exercise group at bed rest week 2 and at week 3 (2.01 +/- 0.21 vs 1.63 0.18 l and 2.03 +/- 0.22 vs 1.81 +/- 0.20, respectively). Urinary pH was significantly higher in the nonexercise control group than in the exercise group at week 1 and week 3 (6.62 +/- 0.7 vs 6.49 +/- 0.5 and 6.58 +/- 0.6 vs 6.49 +/- 0.8, respectively, p = 0.01). CONCLUSIONS Bed rest significantly alters the urinary environment to favor calculous formation. Lower body negative pressure chamber treadmill exercise offers some protection against increases in stone risk during simulated microgravity, particularly with regard to the risks of hypercalciuria and brushite stone formation. The use of lower body negative pressure to augment aerobic exercise in space may decrease the risk of stone formation in astronauts. Adjunct measures, including aggressive hydration and alkalinization therapy, should be considered.