Abrasive water jet cutting as a new procedure for cutting cancellous bone--in vitro testing in comparison with the oscillating saw.

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.

The water jet as a new tool for endoprosthesis revision surgery--an in vitro study on human bone and bone cement.

In revision surgeries of endoprostheses, the interface between implant and bone cement or bone must be loosened. Conventional tools have many disadvantages because of their size and limited range. Taking advantage of the selective and athermic cutting process, a plain water jet is already used in order to cut soft tissues. This study investigates the possibilities of both a plain and an abrasive water jet as cutting tools for revision surgery. Samples of the mid-diaphysis of human femora and bone cement (CMW3) were cut with a plain water jet (PWJ) and an abrasive water jet (AWJ) at two different jet-to-surface angles (30 degrees,90 degrees ) and at five different pressure levels (30, 40, 50, 60, 70 MPa). For a PWJ a selective pressure range was identified, where only bone cement was cut. Injecting a bio-compatible abrasive (lactose) to the jet stream resulted in significantly higher cut depths in both materials. Material removal in bone was significantly less at the smaller jet-to-surface angle for both techniques. No clear selectivity between bone and bone cement was observed for application of the AWJ. However, the material removal rate was significantly higher for bone cement than for bone at all pressure levels. The results indicate that an AWJ might be an alternative tool for cement removal. The possibility for localised cutting at interfaces could be an advantage for revision of a non-cemented prosthesis.

[The pulsed water jet for selective removal of bone cement during revision arthroplasty]. / Der gepulste Wasserstrahl zur selektiven Knochenzemententfernung in der Revisionsendoprothetik.

Conventional tools used in prosthetic revision surgery have a limited range of action within the narrow cement mantle. Water jet cutting technology permits tiny and precisely controlled cuts, and may therefore be an alternative method of bone cement removal. Our study compares the cutting performance on bone cement (PMMA) and bone of a pulsed water jet and a continuous water jet. The aim of the study was to establish whether selective removal of PMMA is possible. 55 bone specimens (bovine femora) and 32 specimens of PMMA were cut with a continuous and a pulsed water jet at different pressures (40 MPa, 60 MPa) and pulse frequencies (0Hz, 50Hz, 250Hz). To ensure comparability of the results, the depths of cut were related to the hydraulic power of that part of the jet actually impinging on the material. While for PMMA the power-related depth of cut increased significantly with the pulse frequency, this did not apply to bone. The cuts produced in bone were sharp-edged. Since PMMA is more brittle than bone, the water jet caused cracks that enlarged further until particles of bone broke away. Although selective removal of PMMA without doing damage to the bone was not possible at the investigated settings of the jet parameters, the results do show that a pulsed water jet can cut bone cement much more effectively than bone. This is an important advantage over conventional non-selective tools for the removal of bone cement.