The open use of nitrogen requires exposition and generous fenestration of bone with a limited penetration of freezing [26]. Ideally, liqiud nitrogen should be applied while avoiding trauma to the surrounding healthy soft tissue to ensure the least additional destabilization of the bone when accessing the tumour. In contrast, earlier cryoprobes were less suitable for this purpose due to poor design and performance [4, 8]. Modern miniature cryoprobes, such as those used in this study, seem to be more suitable. Precise knowledge of the thermal properties of the probe is an absolute prerequisite, as is familiarity with the expansion of the cold zone over time because both the tumour and the tumour-infiltrated bone tissue bordering the tumour are left in place.
In order to determine the cooling performance of the cryoprobe, and in order to be able to follow the freezing process visually, freezing was first done as described by Saliken et al. (see below) and Rewcastle et al. [19, 20] in a homogeneous, transparent reference medium (gelatin). The cold zone expanded in a circle around the cryoprobe and cooled the gelatin to less than -60°C at a distance of 1 cm from the centre of the probe. This freezing differs from those described by Saliken et al. [22] and Rewcastle et al. [19], where different cryoprobes with a comparable cooling capacity and calibre were tested under nearly identical conditions. While the cooling performance of the cryoprobe used by Rewcastle et al. is lower, the edge of the cold zone Saliken observed showed not only somewhat lesser cooling at the corresponding sensors, but also a pear-shaped growth pattern. The use of this probe in the treatment of prostate carcinomas [1] is anatomically advantageous. The prolate field of therapy of the miniature cryoprobe (maximum expansion at the centre of the freezing zone) tested here would be better suited to ablation of bone tumours because even expanded tumours can be treated this way, the probe being centrally inserted and gradually pulled back following one or more freeze-and-thaw cycles [7]. The freezing trials with two probes showed that simultaneous use of more miniature probes creates a synergistic freezing effect as decribed by Saliken [22], Berger [2] and Rewcastle [20] with overlap of both therapy fields. There is a maximum effect after 15 minutes and at a distance of 2 cm between probes; e.g., the Erbokryo CS-6 allows simultaneous ablation with 6 probes to create a tubular field of therapy of at least 14 cm in length, thus allowing optimum treatment of diaphyseal tumours.
As expected, when employing this methodology to bone at body temperature and with circulating blood gave more moderate temperature curves during freezing, both when one or two probes.
The lowest tissue temperature, the prime factor in cell death, should be -50°C [7] or less [3] in neoplastic tissue. Hence, only lesions or tumour-infiltrated bone tissue of about 1.50–2.00 cm or less can be successfully treated with a single 3.2 mm miniature cryoprobe. When applying two adjacent cryoprobes simultaneously, our diaphyseal measurements showed a synergistic freezing effect resuting in a -50°C isotherm of up to 3.00–3.50 cm in diameter. Parting from the recommendations by Baust et al., that the cryosurgeon should aim for a temperature of -30°C in the periphery of the abnormal tissue [3], then the areas potentially treatable with one or two probes would have to be planned accordingly larger.
This study was done on non-malignant bone lesions and not on malignant bone tissue because our main goal was to test the fundamental applicability of this new probe under in vivo conditions in relatively human-like organisms. No tumour models have been bescribed in large animals. Nevertheless, our data can be applied to cryosurgery of malignant bone tissue because freezing at the edge of the dysplastic area, which consists mainly of non-dysplastic bone tissue, is very important in preventing local recurrences.
In principle, the thickness of the 1.1 mm thermal feelers goes with a certain imprecision in measuring the temperature [18], although we made the experience that even if the access hole was drilled precisely, thinner feelers bend very quickly and so cause a much greater measruement error. Moreover, with the thermal feelers used here, temperature is not measured across the entire diameter, but only at the tip of an area some 0.2 mm thick, thus reducing artefacts to an acceptable range.
Neither local nor systemic complications were observed in the animals, which is consistent with the minimally invasive nature of cryosurgery, as was seen when the method was applied to other organs [17, 28]. Although the quantity of individual trials was too low for statistical analysis, it is still evident that adequately low temperatures can be reached in vivo using the miniature cryoprobes that were used here.
Further trials with a larger number of animals have already begun. They need to document the effect the expansion of the zone of necrosis has on bone at body temperature and with a circulating blood. In addition, it is necessary to know the middle- and long-term effects that are to be expected following cryosurgery of bone. In the case of favourable results, cryosurgery with modern miniature probes could be a valuable adjunct to the resection of bone or, in certain cases, provide a viable alternative.