Eugene Bahniuk

Some viscoplastic characteristics of bovine and human cortical bone

Multiple cycle tensile creep tests were performed on human and bovine cortical bone specimens. The tests enabled total strain to be decomposed into elastic, linear viscoelastic, creep and permanent plastic components. The results indicate that a stress threshold exists; above which time dependent effects dominate material response and below which the behavior is primarily linear viscoelastic, with time effects playing only a secondary role. A constant stress above the threshold produces a constant steady state creep rate, with the magnitude of the creep rate being an exponential function of the stress magnitude. Additionally, it was found that a major portion of the inelastic strain is always recovered on unloading and that the accumulation of creep strain increases the material compliance on subsequent loadings below the threshold. These two factors suggest that a damage mechanism is responsible for the nonlinear behavior.

Viscoelastic relaxation and regional blood flow response to spinal cord compression and decompression

Study Design. To better understand the relationships between primary mechanical factors of spinal cord trauma and secondary mechanisms of injury, this study evaluated regional blood flow and somatosensory evoked potential function in an in vivo canine model with controlled velocity spinal cord displacement and real-time piston-spinal cord interface pressure feedback. Objectives. To determine the effect of regional spinal cord blood flow and viscoelastic cord relaxation on recovery of neural conduction, with and without spinal cord decompression. Summary of Background Data. The relative contribution of mechanical and vascular factors on spinal cord injury remains undefined. Methods. Twelve beagles were anesthetized and underwent T13 laminectomy. A constant velocity spinal cord compression was applied using a hydraulic loading piston with a subminiature pressure transducer rigidly attached to the spinal column. Spinal cord displacement was stopped when somatosensory evoked potential amplitudes decreased by 50% (maximum compression). Six animals were decompressed 5 minutes after maximum compression and were compared with six animals who had spinal cord displacement maintained for 3 hours and were not decompressed. Regional spinal cord blood flow was measured with a fluorescent microsphere technique. Results. At maximum compression, regional spinal cord blood flow at the injury site fell from 19.0 ± 1.3 mL/100 g/min to 12.6 ± 1.0 mL/100 g/min, whereas piston-spinal cord interface pressure was 30.5 ± 1.8 kPa, and cord displacement measured 2.1 ± 0.1 mm (mean ± SE). Five minutes after the piston translation was stopped, the spinal cord interface pressure had dissipated 51%, whereas the somatosensory evoked potential amplitudes continued to decrease to 16% of baseline. In the sustained compression group, cord interface pressure relaxed to 13% of maximum within 90 minutes; however, no recovery of somatosensory evoked potential function occurred, and regional spinal cord blood flow remained significantly lower than baseline at 30 and 180 minutes after maximum compression. In the six animals that underwent spinal cord decompression, somatosensory evoked potential function and regional spinal cord blood flow recovered to baseline 30 minutes after maximum compression. Conclusions. Despite rapid cord relaxation of more than 50% within 5 minutes after maximum compression, somatosensory evoked potential conduction recovered only with early decompression. Spinal cord decompression was associated with an early recovery of regional spinal cord blood flow and somatosensory evoked potential recovery. By 3 hours, spinal cord blood flow was similar in both the compressed and decompressed groups, despite that somatosensory evoked potential recovery occurred only in the decompressed group.

A damage model for nonlinear tensile behavior of cortical bone.

To describe the time-dependent nonlinear tensile behavior observed in experimental studies of cortical bone, a damage model was developed using two internal state variables (ISVs). One ISV is a damage parameter that represents the loss of stiffness. A rule for the evolution of this ISV was defined based on previously observed creep behavior. The second ISV represents the inelastic strain due to viscosity and internal friction. The model was tested by simulating experiments in tensile and bending loading. Using average values from previous creep studies for parameters in the damage evolution rule, the model tended to underestimate the maximum nonlinear strains and to overestimate the nonlinear strain accumulated after load reversal in the tensile test simulations. Varying the parameters for the individual tests produced excellent fits to the experimental data. Similarly, the model simulations of the bending tests could produce excellent fits to the experimental data. The results demonstrate that the 2-ISV model combining damage (stiffness loss) with slip and viscous behavior could capture the nonlinear tensile behavior of cortical bone in axial and bending loading.

The effects of tibial-femoral angle on the failure mechanics of the canine anterior cruciate ligament

A series of canine femur-ACL-tibia complexes were subjected to tensile tests with axial tibial orientation and 0 degree, 45 degrees or 90 degrees femoral orientation with respect to load direction. A deflection rate of 51.0 cm min-1 was used in all tests. Marked differences occurred in ultimate loads, deflection and energy absorbed as a consequence of differences in femoral-tibial orientation. The mode of structural failure, as determined by post-test examination, also varied markedly as a function of femoral-tibial orientation. It is concluded that differences both in measured mechanical properties and observed failure details are a consequence of varying the loading pattern of the fiber bundles across the finite breadth of the ligament.

Inelastic Strain Accumulation in Cortical Bone During Rapid Transient Tensile Loading

An experimental study examined the tensile stress-strain behavior of cortical bone during rapid load cycles to high strain amplitudes. Machined bovine and human cortical bone samples were subjected to loading cycles at a nominal load/unload rate of +/- 420 MPa/s. Loads were reversed at pre selected strain levels such that load cycles were typically completed in 0.5-0.7 seconds. Axial strain behavior demonstrated considerable nonlinearity in the first load cycle, while transverse strain behavior was essentially linear. For the human bone 29.1 percent (S.D. = 4.7 percent), and for the bovine bone 35.1 percent (S.D. = 10.8 percent) of the maximum nonlinear strain accumulated after load reversal, where nonlinear strain was defined as the difference between total strain and strain corresponding to linear elastic behavior. Average residual axial strain on unloading was 35.4 percent (S.D. = 1.2 percent) for human bone and 35.1 percent (S.D. = 2.9 percent) of maximum nonlinear strain. Corresponding significant volumetric strains and residual volumetric strains were found. The results support the conclusions that the nonlinear stress-strain behavior observed during creep loading also occurs during transient loading at physiological rates. The volume increases suggest that damage accumulation, i.e., new internal surfaces and voids, plays a major role in this behavior. The residual volume increases and associated disruptions in the internal structure of bone provide a potential stimulus for a biological repair response.

Spinal cord monitoring of experimental incomplete cervical spinal cord injury: a preliminary report.

Incomplete spinal cord injuries occur as a result of contusion and mechanical compression of neural tissue. Anterior spinal cord compression may physiologically prevent optimal recovery of spinal cord function for varying periods of time. The aim of this research was to study an animal model of incomplete cervical cord injury with a spinal cord monitoring system utilizing computer-averaged cortical evoked potentials. Two animal models were utilized: a contusion injury by the weight drop method and an anterior cord compression injury. Results indicate that incomplete cord injuries of both types will recover depending upon the amount of initial force or energy applied and the length of time compression is applied. Thirteen compression and 14 contusion injuries were studied. Cortical evoked potentials measured in seventeen dogs paralleled the degree of cord injury as well as recovery.