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Biomedical subjects

J C Dick

Publications and source records attributed to J C Dick.

8 recordsLinked to original sources
PubMed2001

Notch sensitivity of titanium alloy, commercially pure titanium, and stainless steel spinal implants.

STUDY DESIGN: This is a biomechanical study comparing the fatigue life of stainless steel (SS), titanium alloy (TI), and commercially pure titanium (CP) rods that were notched using techniques essential to spine surgery. OBJECTIVES: To show the difference in fatigue life of the three rods and the effect on fatigue life of marks caused by techniques unavoidable in spine surgery. SUMMARY OF BACKGROUND DATA: Past studies have shown a decrease in the fatigue life of titanium when there is a defect or notch in the surface. This has been demonstrated with spinal and fracture implants, prosthetic joints, and in the materials science literature. Previous authors have recommended avoidance of techniques that mark the surface of titanium implants. METHODS: Four Texas Scottish Rite Hospital rods of each metal were prepared in four different ways: 1) no notch, 2) a French bender notch, 3) a top-loading connector--left on, 4) a top-loading connector--removed. The rods were fatigued in four point bending up to 1 million cycles. Before fatigue testing, two samples of each preparation were examined with electron microscopy. RESULTS: No SS rods failed. No TI rods without a notch or connector removed failed. One TI rod with the connector left in place failed (106,635 cycles). Three of the TI rods in the French-bender group failed (average, 87,663 cycles). One CP rod without a notch failed (358,957 cycles). No CP rods with the connector left in place failed. All CP rods with the connector removed failed (average, 230,094 cycles). All CP rods in the French bender group failed (average 120,817 cycles). The following differences were significant (P < 0.05): SS-TI and SS-CP in the French bender group; SS-CP and TI-CP in the connector removed group. Electron micrographs performed before fatigue testing showed cracks in the TI and CP rods that were marked with the French bender. CONCLUSION: Caution is recommended when bending TI or CP rods, or placing a connector and removing it when using CP rods in situations in which fatigue failure is a concern.

Alloys↗
PubMed1997

Anterior instrumentation of the thoracolumbar spine. A biomechanical comparison.

STUDY DESIGN: To evaluate the fatigue strength and stiffness of four anterior thoracolumbar fixation devices using a corpectomy model without load-sharing bone graft to test the devices under the worst case scenario of instability. OBJECTIVES: To gain a more thorough understanding of the biomechanical qualities of anterior fixation devices to improve clinical application and design. SUMMARY OF BACKGROUND DATA: For many surgeons, the anterior approach has become the treatment of choice for patients with compression of the spinal cord, whether it is caused by trauma, tumor, or infection. When stabilization is needed, anterior fixation devices have been advocated for many years to avoid the additional approach required for posterior fixation. Many of these devices, however, have an unacceptably high rate of hardware failure. Recently, several new devices for anterior fixation have been marketed with purported advantages in fatigue life and ease of use. METHODS: Four implants, the Synthes Anterior Thoracolumbar Locking Plate, the Kaneda device, a Texas Scottish Rite Hospital anterior construct, and the Z-Plate were attached to vertebral models and tested for stiffness in multiple planes on a modified Materials Testing System machine. They then were fatigued to failure on an Instron testing machine. RESULTS: The Anterior Thoracolumbar Locking Plate was the stiffest in axial compression, lateral flexion, and torsion. The Texas Scottish Rite Hospital anterior construct was the least stiff in flexion-extension, with no significant differences in the stiffness of the anterior thoracolumbar locking plate, that of the Kaneda device, and that of the Z-Plate. Fatigue life exceeded 80,000 cycles for the anterior thoracolumbar locking plate and averaged 26,472 cycles for the Z-Plate, 6915 cycles for the Teas Scottish Rite Hospital construct, and 4419 cycles for the Kaneda device. CONCLUSIONS: The significantly greater fatigue life of the Anterior Thoracolumbar Locking Plate and the Z-Plate may predict a lower incidence of hardware failure than with previous anterior devices. This has been confirmed in preliminary clinical studies with the Z-Plate. Further clinical studies are needed to show if these lower failure rates will continue over a long-term period.

Compressive Strength↗
PubMed1997

Mechanical evaluation of cross-link designs in rigid pedicle screw systems.

STUDY DESIGN: This study was designed to evaluate the biomechanical performance of 5 different cross-link brands to determine which design characteristics are biomechanically desirable. METHODS: The Cotrel-Dubousset, Isola, Puno Winter Byrd, Rogozinski, and Texas Scottish Rite Hospital systems were assembled to vertebral models according to the manufacturer's specifications. Three constructs were tested for each brand of instrumentation: without cross-links, with one cross-link, and with two cross-links. Four modes of loading: axial, torsional, flexion-extension, and lateral-flexion were used. Load-displacement curves were plotted. The stiffness was calculated from the slope of these curves. OBJECTIVES: Five different rigid pedicle screw systems were tested to determine: 1) what are the characteristics of cross-link design that are most effective in limiting torsional motion; 2) whether two cross-links are more effective than one; and 3) whether cross-linkage increases the construct stiffness in lateral bending. SUMMARY OF BACKGROUND DATA: Cross-linkage has been shown to increase the torsional stiffness of rod and screw constructs. Increased construct stiffness has been correlated with higher fusion rates. RESULTS: Increases in axial, flexion-extension, or lateral-flexion stiffness, with the addition of one or two cross-links, were not statistically significant. In torsional loading, increases in stiffness within brands were statistically significant in every case. The average increase was 44% with one added cross-link and 26% with two. The magnitude of the increase in torsional stiffness was compared with the cross-sectional area of the respective cross-link. Greater stiffness correlated with larger cross-sectional area (r = 0.81 for one cross-link, and r = 0.60 for two). CONCLUSION: The use of cross-linkage in spinal fusion increases torsional stiffness in pedicle screw and hook constructs. This study 1) confirmed the effectiveness of cross-linkage in limiting torsional motion and showed the superiority of two cross-links to one cross-link in limiting torsional motion, 2) showed that increase of torsional stiffness of a cross-linked construct is proportional to the cross-sectional area of the cross-link, and 3) demonstrated that cross-links do not increases stiffness in the lateral flexion mode.

Biomechanical Phenomena↗
PubMed1997

Posterior lumbar interbody fusion. A biomechanical comparison, including a new threaded cage.

STUDY DESIGN: In vitro biomechanical testing was performed on eight lumbar calf spines. OBJECTIVES: To compare the initial stiffness of a standard method of posterior lumbar interbody fusion using structural autograft with the same procedure using additional posterior instrumentation. These constructs also were compared to a new titanium implant. SUMMARY OF BACKGROUND DATA: Posterior lumbar interbody fusion is gaining wide acceptance for the treatment of segmental spinal instability, spondylolisthesis, and discogenic pain. Many methods have been described, including use of autograft or allograft bone, in either structural or nonstructural form, with or without additional fixation. A new threaded titanium interbody implant has been designed to increase initial stability while allowing bony ingrowth for fusion. METHODS: Eight lumbar calf spines were subjected to axial compression, sagittal moments (flexion-extension), and axial torque while displacement was measured. Stiffness was calculated from the load displacement curves for each construct under each load pattern. RESULTS: The posterior lumbar interbody fusion by bone graft alone was the least stiff construct of all modes tested. In two of eight specimens the bone graft dislodged posteriorly into the canal during torsional testing. The titanium interbody implant was similar in stiffness to the bone graft posterior lumbar interbody fusion with posterior instrumentation group in all three modes. They were both significantly stiffer than the normal spine, the destabilized spine, and the posterior lumbar interbody fusion by bone graft alone (P < 0.05). CONCLUSIONS: In this model, the posterior lumbar interbody fusion with bone graft alone had less initial stiffness than that of the intact spine. The addition of posterior instrumentation or interbody implants can increase initial stiffness significantly.

Animals↗
PubMed1994

A biomechanical comparison evaluating the use of intermediate screws and cross-linkage in lumbar pedicle fixation.

In lumbar fusion, controversy remains regarding the effectiveness of cross-linking and the necessity of placing pedicle screws at the intermediate levels of the segment to be fused. The purpose of this study is to evaluate the stiffness of various rod/screw constructs used to instrument a three-level fusion with specific emphasis on the effect of cross-linking and the intermediate pedicle screws. Nine lumbar calf spines were mounted at L1 and L5. Pedicle screws (TSRH, Danek, Memphis, TN) were then placed bilaterally in the L2, L3, and L4 pedicles. Random sequence testing of the following constructs was then conducted: TSRH rods connected bilaterally to the L2 and L4 pedicles with and without a cross-link, and rods connected bilaterally at the L2, L3, and L4 levels with and without a cross-link. The tests were conducted on a modified MTS testing machine (MTS, Minneapolis, MN) and consisted of cyclic application of axial load, torsion, and flexion and extension. The tests yielded axial, sagittal, and torsional stiffness values. Statistical analysis was performed using log transformation and Fischer's test of least significant difference. In axial testing the use of additional screws in the intermediate pedicles increased stiffness an average of 160% (p = .007). The addition of a cross-link did not increase stiffness with axial loading. In flexion testing the six-screw construct was 84% stiffer when compared with the four-screw construct (p = 0.0001). There was no significant change in flexion stiffness with addition of cross-links. In torsional testing the six-screw construct was 38% stiffer than the four-screw construct (p = 0.042).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗