Sixty-nine male Sprague Dawley rats were divided into three groups of 23 animals each and osteotomies were performed in group 1 with a power saw, in group 2 with the Erb:Yag laser, and in group 3 with the Hol:YAG laser. Two animals of each group were sacrificed 1 week, 4, 8, and 12 weeks after operation for histologic investigation, and five animals of each group at 4, 8, and 12 weeks after osteotomy for torque testing. Anterior-posterior (AP) radiographs were taken at the same time points and investigated for callus formation and development of pseudoarthrosis. All tibiae osteotomied with the Hol:YAG laser (group 3) developed pseudoarthrosis within 12 weeks and, therefore, torque testing could not be performed for this group. Biomechanical measurements of bone treated by power saw or Erb:YAG laser osteotomies, respectively, showed no significant statistical difference in the stability of bone between the two groups. Histologic examination after 1 week exhibited fibrous tissue at the site of osteotomy in rats of all three groups and additionally carbonization in rats of group 3. Saw osteotomies resulted in more callus formation than Erb:YAG osteotomies, but both techniques provoked a certain reunion within 8 weeks. Hol:YAG laser-treated osteotomies, however, exhibited formation of dense fibrous tissue, carbonization and no callus formation within 12 weeks. Radiographic pictures showed more callus formation for saw osteotomies as compared to those performed with the Erb:YAG laser. For Hol:YAG laser osteotomies pseudoarthrosis was identified also radiologically.

One hundred twenty-two patients with tuberculous or pyogenic spondylitis were investigated retrospectively. Patient histories, laboratory tests, and radiographic findings were compared statistically between the two groups. Significant differences were calculated for the interval between onset of symptoms and diagnosis, erythrocyte sedimentation rate, mean vertebral loss at discharge, and sclerosis of the vertebral bodies involved. Open or closed biopsy was performed in 91 patients. The result provided a clear distinction between tuberculous and pyogenic spondylitis in 62.2%, either by means of histology or by culture growth. In pyogenic spondylitis, staphylococci were the most predominant bacteria isolated. Neurologic deficits were demonstrated in 17.8% of patients with tuberculous spondylitis and 22.7% with pyogenic spondylitis. At follow-up examinations, only two patients still had a motor deficit. Additionally, pain, gibbus formation, and bony fusion were evaluated, but no significant differences were found. The combination of several unspecific findings such as patient history, erythrocyte sedimentation rate, and radiographic assessment can lead to the correct diagnosis. A definitive diagnosis is established by means of biopsy, histologic evidence, and bacterial culture.Spinal infections are relatively rare, so larger series of patients for comparative stud-

Erb:YAG and Hol:YAG laser ablation rates of fibrocartilage and nucleus pulposus were measured in vitro simulating clinical conditions. After ablation macroscopic and microscopic appearance of the ablation site was investigated. Hol:YAG and Erb:YAG laser mean ablation rates increased almost linearly with rising energies, showing higher total ablation rates for the Hol:YAG laser due to its higher achievable energy density. At comparable energy densities the Erb:YAG laser appears to be more effective with respect to the corresponding ablation rates. Consequently, the ablational threshold proved to be lower for the Erb:YAG laser. Whereas during Hol:YAG laser ablation, some smoke formation and considerable tissue shrinking occurred, these effects could not be observed during Erb:YAG laser ablation. Consequently macroscopic and microscopic inspection showed some thermal damage after Hol:YAG and only minimal alterations after Erb:YAG laser ablation. Adjacent thermal damage was determined and proved to be lower for the Erb:YAG laser. In our opinion the characteristics of each laser system provide certain advantages for special clinical indications.

We develop a quantum theory of the nonlinear interaction between intense surface acoustic waves and electrons in a quantum well in the regime of moving quantum wires and dots. In the quantum nonlinear regime, the sound attenuation exhibits quantum oscillations and dramatically decreases with increasing quantization. In the case of dynamically created electron dots formed by two acoustic waves, the waves can propagate without any dissipation in the limit of high sound intensity and, hence, the electron quantum film acts as an acoustically quasitransparent material.

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