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The quality of bone cuts is assessed by the accuracy and biological potency of the cut surfaces. Conventional tools (such as saws and milling machines) can cause thermal damage to bone tissue. Water jet cutting is nonthermal; that is, it does not generate heat. This study investigates whether the abrasive jet cutting quality in cancellous bone with a biocompatible abrasive is sufficient for the implantation of endoprostheses or for osteotomies. Sixty porcine femoral condyles were cut with an abrasive water jet and with an oscillating saw. alpha-lactose-monohydrate was used as a biocompatible abrasive. Water pressure (pW = 35 and 70 MPa) and abrasive feed rate (m = 0.5, 1, and 2 g/s) were varied. As a measure of the quality of the cut surface the cutting gap angle (delta) and the surface roughness (Ra) were determined. The surface roughness was lowest for an abrasive feed rate of m = 2 g/s (jet direction: 39 +/- 16 microm, advance direction: 54 +/- 22 microm). However, this was still significantly higher than the surface roughness for the saw group (jet direction: 28 +/- 12 microm, advance direction: 36 +/- 19 microm) (p < 0.001 for both directions). At both pressure levels the greatest cutting gap angle was observed for a mass flow rate of m = 1 g/s (pW = 35 MPa: delta = 2.40 +/- 4.67 degrees ; pW = 70 MPa: delta = 4.13 +/- 4.65 degrees), which was greater than for m = 0.5 g/s (pW = 35 MPa: delta = 1.63 +/- 3.89 degrees ; pW = 70 MPa: delta = 0.36 +/- 1.70 degrees) and m = 2 g/s (pW =70 MPa: delta = 0.06 +/- 2.40 degrees). Abrasive water jets are suitable for cutting cancellous bone. The large variation of the cutting gap angle is, however, unfavorable, as the jet direction cannot be adjusted by a predefined value. If it is possible to improve the cutting quality by a further parameter optimization, the abrasive water jet may be the cutting technique of the future for robotic usage.

Results of testing are useful to the surgeon in making a choice of reconstruction technique.

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Schlüsselwörter: Knochen, Knpchenzement, Endoprothese, Revision, Wassersträhl Konventionelle Werkzeuge haben bei Prothesenrevisionen nur eine limitierte Reichweite im schmalen Zementmantel. Das Druckwasserstrahlschneiden erlaubt eine schmale und präzise Schnittführung und kommt daher als alternative Methode für die Knochenzemententfernung in Frage. Diese Studie vergleicht die Schneidleistung eines gepulsten Wasserstrahls mit jener eines kontinuierlichen Wasserstrahls in Knochenzement (PMMA) und Knochen. 55 Knochen-Proben (Rinderfemora.) und 32 Proben aus PMMA wurden mit einem kontinuierlichen und einem gepulsten Wasserstrahl bei unterschiedlichen Druckniveaus (40 MPa, 60 MPa) und Pulsfrequenzen (0 Hz, 50 Hz, 250 Hz) gekerbt. Um einen Vergleich zu ermöglichen, wurden die Kerbtiefen auf die hydraulische Leistung des auf das Material auftreffenden Strahlanteils bezogen. Während die leistungsbezogenen Kerbtiefen

A case of lytic lesion of the pelvis in a 23-year-old woman is presented. A biopsy led to the diagnosis aneurysmal bone cyst (ABC). Due to the histologically very aggressive growth of the tumor, a low malignant osteosarcoma could not be excluded. In an initial operation the tumour, affecting the sacrum, the iliac crest and the lower lumbar spine was resected. Temporary restabilisation of the pelvic ring was achieved by a titanium plate. The histological examination of the entire tumour confirmed the diagnosis ABC. After 6 months, the MRI showed no recurrence. The observed tilt of the spine to the operated side on the sacral base prompted a second surgical procedure: a transpedicular fixation of L5 and L4 was connected via bent titanium stems to the ischium, where the fixation was achieved by two screws. This construction allowed the correction of the base angle and yielded a stable closure of the pelvic ring. The patient has now been followed for 6 years: the bone grafts have been incorporated and, in spite of radiological signs of screw loosening in the ischium, the patient is fully rehabilitated and free of symptoms. Pedicle screws in the lower spine can be recommended for fixation of a pelvic ring discontinuity.Key Words: aneurysmal bone cyst, pelvic reconstruction, spino ischial spondylodesils, pelvic tumor A 23-year-old woman presented with a low back pain (LBP) history of 2 years. Plain film radiographs and an MRI of the lumbar spine had not shown any pathologies. The patient had been treated with drugs and physical therapy. The patient was referred to our hospital since the pain increased, especially during the night with referral to the legs. A new X-ray examination of the spine showed a cystic structure in the sacral bone. Immediately following this discovery, images of the whole pelvis were taken, revealing a lytic lesion of the left pelvis involving the ilium near the acetabular roof, the sacrum near the spinal canal, and also the left L5-lamina (Fig. 1). An open biopsy was performed. The histological examination revealed multiple communicating vascular spaces separated by fibrous septa. Some areas showed suspicious bone lesions. The histological diagnosis was aneurysmal bone cyst (ABC) but, due to the very aggressive growth of the tumour, a low malignant osteosarcoma could not be excluded.Therefore the strategy of operative treatment was an initial wide tumour resection and temporary stabilisation of the pelvic ring. After histological examination of the entire lesion, a subsequent operation was planned to reconstruct the pelvic ring with custom-made implants and graft material.A combined anterior and posterior approach in a lateral decubitus position was used to carry out the wide tumour resection. The tumour had spread into the L5-lamina, the left part of the sacrum, the iliac crest, and the ileum down to the level of the back side of the articular cartilage of the acetabulum. Primary stabilisation of the pelvic ring was achieved by mounting a 12-hole titanium osteosynthetic plate between t...

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