Pedicle morphology of the immature thoracolumbar spine.

STUDY DESIGN: Human vertebral morphologic data were compiled from anatomic skeletal collections from three museums. OBJECTIVES: To quantify the morphometric characteristics of the pedicles of the immature thoracolumbar spine. SUMMARY OF BACKGROUND DATA: Little is known of pedicle morphology of the immature spine as related to pedicle screw fixation. METHODS: A total of 75 anatomic skeletal specimens were acquired from C1 to L5 in the age range of 3 to 19 years. The data were collected and analyzed using a computerized video analysis system. Each vertebral pedicle was measured in the axial and sagittal planes. The measurements included the minimum pedicle width, the pedicle angle, the distance to anterior cortex, and anteroposterior and interpedicular spinal canal diameters. RESULTS: Wide variation in pedicle morphology between specimens at each vertebral level was found in the young population. In general, compared with the average adult data, a younger spine demonstrated a near uniform reduction in the linear pedicle dimensions at each vertebral level. Pedicles from the lower lumbar vertebrae attained dimensions acceptable for standard screw sizes at an earlier age than in the thoracic vertebrae. CONCLUSIONS: The data in this study indicates that pedicle screws may be used in the adolescent spine. However, care should taken to accurately ascertain pedicle size before surgery so that improper use of screws can be avoided. Growth of the pedicles in relation to the spinal canal indicates that the increase in pedicle size is lateral to the spinal canal.

Estimation of critical power with nonlinear and linear models.

Sixteen young, healthy males each performed five to seven randomly assigned, exhaustive exercise bouts on a cycle ergometer, with each bout on a separate day and at a different power, to compare estimates of critical power (PC) and anaerobic work capacity (W') among five different models: t = W'/(Pmax-PC) (two-parameter nonlinear); t = (W'/P-PC))-(W'/(Pmax-PC)) (three-parameter nonlinear); P.t = W' + (PC.t) (linear (P.t)); P = (W'/t) + PC (linear (P)); P = PC + (Pmax-PC)exp(-t/tau) (exponential). The data fit each of the models well (mean R2 = 0.96 through 1.00 for each model). However, significant differences among models were observed for both PC (mean +/- standard deviation (SD) for each model was 195 +/- 29 W through 242 +/- 21 W) and W' (18 +/- 5 kJ through 58 +/- 19 kJ). PC estimates among models were significantly correlated (r = 0.78 through 0.99). For W', between-model correlations ranged from 0.25 to 0.95. For a group of six subjects, the ventilatory threshold for long-term exercise (LTE Tvent; 189 +/- 34 W) was significantly lower than PC for all models except the three-parameter nonlinear (PC = 197 +/- 30 W); PC for each model was, however, positively correlated with LTE Tvent (r = 0.69 through 0.91).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of pedaling speed on the power-duration relationship for high-intensity exercise.

Seven males (age = 20.4 +/- 0.3 yr) each performed a total of eight exhaustive exercise bouts (four at 60 rpm and four at 100 rpm) in order to determine the influence of pedaling frequency on the parameters of the power-duration relationship for high-intensity cycle ergometry. The power-endurance time data for each subject at each rpm were fit by nonlinear regression to extract parameters of the hyperbolic: (P - theta PA). t = W', where P = power output, t = time to exhaustion, and theta PA and W' are constants. theta PA (the power asymptote, in watts (W] reflects an inherent characteristic of aerobic energy production during exercise, above which only a finite amount of work (W', in joules) can be performed, regardless of the rate at which the work is performed. theta PA at 60 rpm (235 +/- 8 W) was significantly (15.9 +/- 4.5%, P less than 0.05) greater than theta PA at 100 rpm (204 +/- 11 W), thus confirming our hypothesis that endurance would be compromised while cycling at the higher pedaling frequency. In contrast, W' was not significantly (P greater than 0.05) affected by cadence (16.8 +/- 1.7 kJ at 60 rpm vs 18.9 +/- 2.2 kJ at 100 rpm). Our data are consistent with the implications of previous investigations which demonstrated a greater cardiorespiratory and blood/muscle lactate response during constant-power exercise while cycling at high vs low rpm and indicate that the theoretical maximum sustainable power (i.e., theta PA) during cycle ergometry in untrained males is greater at 60 rpm than at 100 rpm.