[Value of intravascular ultrasound in angiographically underestimated coronary findings: exemplified by 2 case reports]. / Bedeutung des intravaskulären Ultraschalls bei angiographisch unterschätztem Koronarbefund: Darstellung anhand zweier Fallbeispiele.

Coronary angiography is considered to be the most important component in the diagnosis of coronary artery disease. Only the lumen may be visualized using contrast radiography of the coronary arteries, however. With intravascular ultrasound, on the other hand, pathological changes in the vascular wall can be recorded. Despite typical symptoms, an angiographical image of the coronary arteries is sometimes unable to show unambiguous stenotic lesions of the coronary arteries. In such cases, intravascular ultrasound provides a new complementary diagnostic tool for detecting even early forms of arteriosclerosis or angiographically underestimated coronary findings. Two cases are used to illustrate this. In those, angiographic visualization of the coronary arteries was unable to produce an unambiguous finding despite typical clinical symptoms, yet intravascular ultrasound could detect considerable arteriosclerotic wall changes.

An experimental technique to induce and quantify complex cyclic forces to the lumbar spine.

The human spine is a complex, heterogeneous nonlinear and viscoelastic structure. In addition, in vivo loading is not uniaxial. Although many studies on the mechanical behavior of the spine under "pure" forces and single cycle load applications exist, little research is conducted with complex cyclic loads. In this study, we developed a technique to induce and quantify controlled complex physiological loads to the lumbar spinal column under cyclic (chronic) conditions. The methods described include specimen preparation and mounting to induce controlled complex loading (cyclic compression-flexion vector was chosen as an example), instrumentation, and biomechanical data to achieve the objectives. The results indicated that the specimen sustained the external load in a combined compression-flexion mechanism without considerable off-axis forces (lateral shears) and moments (lateral bending and torsion). By mounting the anchoring bolt in appropriate places (such as an anterolateral placement to induce compression-flexion-lateral bending), this technique can be used to apply and continuously quantify complex physiological acute or cyclic loads to describe the biomechanics of the spine. This procedure of inducing complex loads eliminates the difficulty in applying the principles of superposition, using the response from individual "pure" forces to account for the nonlinearity and viscoelasticity of the human lumbar spinal column.

Rotational stability of a spinal pedicle screw/rod system.

Although the geometry of spinal instrumentation constructs may significantly affect efficacy, the variation in biomechanical data may not assist the clinician in an appropriate selection. The purpose of the present study was to quantify the effects of transverse fixators on rotational strength of a common pedicle-screw-with-rods system. Pedicle screws were mounted in blocks of polymethyl-methacrylate at angles to reproduce the configuration of placement in the human lumbar spine. Twenty cycles of +/- 12 N-m axial rotation moment was applied, and the steady-state response was used in the analysis. Configurations tested included both medial and lateral placement of longitudinal rods as well as the addition of one or two transverse rods. Up to a 20% difference in stiffness was noted between medial and lateral placement of longitudinal rods when no transverse rods were mounted. A maximum difference in flexibility of 6% was noted between the use of one and two transverse rods. For medially placed rods, a single transverse connector will add significant rotational stiffness even for shorter rod lengths; for laterally placed longitudinal rods, only the longer rod lengths need a transverse connector.

Fusion rate and biomechanical stiffness of hydroxylapatite versus autogenous bone grafts for anterior discectomy. An in vivo animal study.

STUDY DESIGN: The fusion rate and biomechanical stiffness were evaluated for 56 goat spinal units from 14 animals that had anterior discectomies and grafting procedures completed using hydroxylapatite and autogenous bone and survived for 6, 12, and 24 week healing times. OBJECTIVES: Harvested spinal units underwent radiographic imaging to assess fusion, biomechanical testing in axial compression, flexion, extension, lateral bending, and axial rotation to assess strength, and histological analysis. The above results were compared for the two procedures and the different healing times. SUMMARY OF BACKGROUND DATA: Because of some of the complications associated with the use of autogenous iliac crest bone graft in spine fusions, there has been considerable interest in the use of calcium phosphate ceramics as a possible substitute for a grafting material. One of the attractive features of calcium phosphate ceramics is the resulting strong bond that is formed with the host bone unlike other inert compounds. METHODS: Surgeries were done at four sites on each animal with two in the cervical spine and two in the lumbar spine. Radiography was done during the survival time and postsacrifice. Biomechanical testing was done on the day of sacrifice under physiological loads. Both hard tissue sections and decalcified sections were histologically evaluated. RESULTS: A 55% fusion rate for bone preparations and a 50% fusion rate for the hydroxylapatite (HA) units was found for the 12 and 24 week preparations. The HA preparations were better at maintaining disc space height. The biomechanical analysis revealed significantly higher stiffness values for fused preparations than for nonfused samples under extension, lateral bending, and axial rotation. Fused units demonstrated no statistical difference in biomechanical stiffness between HA versus autogenous bone units for any mode of loading. CONCLUSIONS: Our results indicate that these dense, nonresorbable hydroxylapatite blocks perform as well as autogenous bone for anterior spinal fusions in this animal model. The use of this hydroxylapatite material in anterior spine fusions may have some clinical validity.

Intravertebral pressure changes caused by spinal microtrauma.

Clinical studies indicate variations in intravertebral pressures in patients with and without low back pain. It is known that not all patients with back pain have abnormal lumbar radiographs and, furthermore, microfractures of the endplate may be one of the causes in the origin of low back pain. Consequently, this study was conducted to determine the interrelationship between microtrauma and intraosseous pressures in the lumbar spine. Miniature pressure transducers were inserted into the vertebral bodies and spinous processes of human cadaver spinal units. Radio-opaque medium was injected into the nucleus to fluoroscopically monitor the movement of the fluid from the disc as the preparation was loaded up to the initiation of microtrauma (before reaching the ultimate load-carrying capacity). The onset of injury was evidenced by the microfracture of one of the two endplates and impregnation of the contrast medium into the spongiosa. After relaxation, another cycle of loading was applied by limiting the deflections to the maximum compression sustained under the intact configuration. The load, stiffness, and energy-absorbing capacities were lower (P < 0.05) for the injured specimen compared with the intact configuration. The intraosseous pressures were higher (P < 0.05) in the vertebral body and the spinous process of the vertebra where the endplate exhibited microtrauma in the injured cycle compared with the intact cycle. In contrast, the intraosseous pressures in the vertebral body and the spinous process at the level where the endplate remained intact were not significantly different between the two cycles of loading.(ABSTRACT TRUNCATED AT 250 WORDS)

Cyclic compression-flexion loading of the human lumbar spine.

STUDY DESIGN: The present study was designed to investigate the biomechanical behavior of the lumbar spine under controlled complex physiologic situations with chronic input. OBJECTIVE: The objective was to determine the response of the human cadaver lumbar spinal column under repetitive compression-flexion forces. SUMMARY OF BACKGROUND DATA: Studies have been conducted in the past to determine the biomechanical response of the spine under uniaxial or pure forces. There is no methodology that can be used to apply and continuously quantify the fatigue response of the lumbar spinal column under controlled combined complex loading vectors (e.g., compression flexion). METHODS: Intact cadaver lumbar columns (L1-L5) were mounted with the superior end in contact with a ball-transfer mount, inducing a flexion load to the spine while allowing multiple degrees of freedom. The distal portion of the specimen was attached to a six-axis load cell to quantify the force sustained by the specimen during the entire loading cycle. The applied load and piston deformation and the generalized six-axis force histories were gathered as a function of time using a digital data acquisition system. RESULTS: The stiffness versus number of cycles (K-N) response exhibited nonlinear characteristics. The stiffness increased initially and then stabilized after 1,000-2,000 cycles of loading, delineating the viscoelastic characteristics of the spine. The initial stiffness increase before stabilization was found to be significantly different (P < 0.025) compared to the stiffness beyond 2,000 cycles. CONCLUSIONS: The data suggest that the fatigue response can be understood by cyclically loading the ligamentous lumbar spine preparation to approximately 2,000 cycles.

Correlation of microtrauma in the lumbar spine with intraosseous pressures.

This study was conducted to determine the relationship between intraosseous pressure and vertebral microtrauma in the lumbar spine. Functional spinal units were excised from human cadavers. Radio-opaque dye was injected into the nucleus. Miniature transducers were inserted into the vertebrae to record intraosseous pressures. Compressive loading was applied quasistatically (2 mm/sec) until injury occurred. Movement of the contrast medium was monitored under fluoroscopy. The subchondral endplate was the most vulnerable component for initiation of injury to the lumbar spine segment. In the initial stages of loading, the vertebral endplates gradually bulged outward, with the contrast medium staying within the nucleus. However, at higher physiologic load levels, before reaching the limiting load, the deformations increased, resulting in buckling of one of the endplates. This was followed by the contrast medium impregnating the spongiosa. Microlevel trauma was not observed radiographically after load removal, indicating that one cannot always equate a normal radiograph with normal spinal anatomy. Mean forces, deformations, stiffnesses, energies, and strains were 7.8 kN (+/- 1.4), 5.23 mm (+/- 0.78), 1940 N/mm (+/- 226), 18.7 J (+/- 4.4), and 35.5% (+/- 3.7), respectively. Pressure in the vertebral body containing the injured endplate before the onset of microtrauma was different (P < 0.05) from the pressure after injury; the pressures in the body containing the intact endplate, however, were not statistically different. Significant differences (P < 0.05) in the intraosseous pressures occurred between the two spinal levels at low-level physiologic loads before the onset of microtrauma.(ABSTRACT TRUNCATED AT 250 WORDS)