Ashish L. Oza

Thermal Injury with Contemporary Cast-Application Techniques and Methods to Circumvent Morbidity

BACKGROUND Thermal injuries caused by application of casts continue to occur despite the development of newer cast materials. We studied the risk of these injuries with contemporary methods of immobilization. METHODS Using cylindrical and L-shaped limb models, we recorded the internal and external temperature changes that occurred during cast application. Variables that we assessed included the thickness of the cast or splint, dip-water temperature, limb diameter and shape, cast type (plaster, fiberglass, or composite), padding type, and placement of the curing cast on a pillow. These data were then plotted on known time-versus-temperature graphs to assess the potential for thermal injury. RESULTS The external temperature of the plaster casts was an average (and standard deviation) of 2.7 degrees +/- 1.9 degrees C cooler than the internal temperature. The external temperature of twenty-four-ply casts peaked at an average of 84 +/- 42 seconds prior to the peak in the internal temperature. The average difference between the internal and external temperatures of the thicker (twenty-four-ply) casts (4.9 degrees +/- 1.3 degrees C) was significantly larger than that of the thinner (six and twelve-ply) casts (1.5 degrees +/- 1 degrees C) (p < 0.05). Use of dip water with a temperature of <24 degrees C avoided cast temperatures that can cause thermal injury regardless of the thickness of the plaster cast. A dip-water temperature of 50 degrees C combined with a twenty-four-ply cast thickness consistently yielded temperatures high enough to cause burns. Use of splinting material that was folded back on itself was associated with a significant risk of thermal injury. Likewise, placing a cast on a pillow during curing resulted in temperatures in the area of pillow contact that were high enough to cause thermal damage, as did overwrapping of a curing plaster cast with fiberglass. Attempts to decrease internal temperatures with the application of isopropyl alcohol to the exterior of the cast did not decrease the risk of thermal injury. CONCLUSIONS Excessively thick plaster and a dip-water temperature of >24 degrees C should be avoided. Splints should be cut to a proper length and not folded over. Placing the limb on a pillow during the curing process puts the limb at risk. Overwrapping of plaster in fiberglass should be delayed until the plaster is fully cured and cooled.

Biomechanics of occipitocervical fixation.

Study Design. A human cadaveric biomechanical study comparing occipital fixation techniques. Objectives. To compare ranges of motion between midline and lateral occipital fixation and between rigid and nonrigid occipital fixation of an unstable craniocervical spine. Summary of Background Data. New fixation techniques using rods and screws increase surgical choice on where fixation is placed onto the occiput. Lateral fixation theoretically gives improved resistance to deformation because of its increased effective moment arm and bilateral purchase. Midline fixation allows significantly longer screw purchase. This study compares these two fixation location. Methods. Cadaveric occipital cervical spine specimens were tested biomechanically intact and under six different fixation techniques. Range of motion between the skull and C2 at 1.5 N-m and 2 N-m bending moments was measured in flexion-extension, lateral bending, and axial rotation. Mechanical testing of different rod diameters and a reconstruction plate was performed and compared with biomechanical testing. Results were compared between the intact condition and all fixations, between the medial and lateral fixations, and between the rigid and nonrigid fixations by analysis of variance. Results. The range of motion of all constructs was significantly reduced compared with intact. Significant differences between groups were only seen in lateral bending in fixation placed laterally. Mechanical testing demonstrated that construct stiffness was predicted by area moment of inertia of the rod and plate to a greater degree than variation in placement of occipital screws or locking of the implant. Conclusion. The choice of location of occipital fixation should be based more on the ease of use and instability pattern. The decreased stiffness of the newer small rod systems should be considered.

A method to measure cervical spine motion over extended periods of time.

Study Design. System validation study. Objective. To develop and validate a motion sensor system for measuring cervical spine motion over extended time periods. Summary of Background Data. Many studies using different methodologies have tried to estimate cervical spine motion. These have mostly been carried out in a laboratory setting performing active/passive range of motion or activities of daily living. However, cervical spine performance over extended periods of time in natural environments remains unknown. Methods. A novel motion sensor system, Wisconsin Analysis of Spine Motion Performance (WASP), was validated using 2 benchmarks: a materials testing machine (MTS) and optical motion tracking laboratory. Parameters tested included drift, frequency response, accuracy, effect of sensor orientation, and coupled motions. Applied motions from the MTS and measured motions in subject volunteers under various conditions were compared with WASP using correlation coefficients. Intersubject and intrasubject variability analyses for WASP were also performed. Results. The average WASP slopes for accuracy (compared with MTS) in flexion-extension, lateral bending, and axial rotation were 0.89, 0.93, and 0.38, respectively. The correlation coefficient was 0.99 in all cases. Compared with optical motion tracking, the WASP regression slopes were 1.1, 1.02, and 0.4 and the correlation coefficients were 0.98, 0.92, and 0.93 in the 3 axes of motion. Coupled motion was noted during all subject motions. WASP peak detection algorithm had a 0% error discounting boundary conditions. Conclusion. WASP was accurate in flexion-extension and lateral bending. In axial rotation, WASP was less accurate. However, the system was highly reliable with low intersubject and intrasubject variability. WASP can be used in estimating cervical spine motion with high reliability while keeping in mind the decreased accuracy in measuring axial rotation.

Stiffness of Occipital-Cervical Constructs: Beam Theory

OBJECTIVE The purpose of this study is to show that stiffness of an occipital-cervical construct can be predicted based on rod geometry and material. MATERIALS AND METHODS Various rod-plate implants were tested as previously reported biomechanical studies of occipital-cervical fixation with the exception that no spine was used. A testing frame that holds paired contoured rods and plates to the same position as in the biomechanical testing protocol for occipital-cervical fixation was tested in the flexion-extension direction on a servo-hydraulic testing machine. Stiffness was determined from the plots of applied moment versus angular displacement. The occipital-cervical constructs were then modeled as a curved beam in pure bending in the sagittal plane to calculate the moment of inertia and theoretical stiffness. The Pearson correlation coefficient was used to assess the correlation of the experimental to the theoretical calculated stiffness. Product of inertia and material stiffness were determined for implants from previously published studies and the predicted rank order of this product was compared with the rank order of the observed biomechanical results in each study. RESULTS A strong correlation was observed between the experimental and theoretical stiffness (R2 = 0.85). A strong influence of the inertia was also found on the experimental construct stiffness (R2 = 0.77). In five of six previously published studies, the best experimental performance was predicted using simple mechanical calculations. CONCLUSION This study shows that both the theoretical stiffness and the calculated area moment of inertia are strongly correlated with the experimental stiffness of tested occipital-cervical fixation constructs.

Logarithmic pulse generator for long-term creep and relaxation testing

An existing logarithmic time base pulse generator design has been modified to collect data for creep and relaxation tests over at least 14decades of logarithmic time increments. Creep and relaxation studies of materials, which do not obey time temperature superposition, are done over many factors of ten (decades) of time. To acquire and store data efficiently one employs logarithmic sampling intervals. This device can trigger a data acquisition system to gather the data logarithmically. It generates trigger pulses accurately over 14decades of time.