Fastening devices for securing the position of a part of a body (for example, a head) during medical procedures are known. Such medical procedures may include operations. The head of a patient should be secured during operations requiring high precision (for example, neurosurgery of the brain wherein a tumor is removed). In such operations, minimal deviations from a planned incision could have extremely negative consequences for the patient, since important portions of the brain could be unintentionally injured and result in impairment of brain function. To minimize this risk, a patient's head is firmly secured during cranial neurosurgery.
In the prior art, head clamps including several fastening devices may be used to secure a patient's head. The prior art head clamps include stirrup-shaped device that further includes protrusions at least three securing points. A fastening device with a tip is situated at each of the three securing points to secure the patient's head to the head clamp. Three-point head clamps and four-point head clamps are common. In the example of a three-point head clamp, one securing point may be situated on one side of the head and the other two securing points may be situated on the other side of the head. In the example of a four-point head clamp, one securing point is situated on each side of the head and two additional securing points support and/or secure the head from below. For head securing, it may not be sufficient to use fastening devices that do not cut through or into the skin and the bone beneath. Ordinarily, such non-invasive securing devices do not have the ability to firmly secure the head. Thus, pins may be used as fastening devices in combination with head clamps. The pins may be small rod-shaped bodies provided with a tip, which are fastened to the head clamp and inserted into the head. The pins may be driven through the scalp into the cranial bone by turning a securing screw.
Conventional pins in accordance with the prior art may be manufactured from a metallic material. Exemplary materials that have been used include high-grade steel and titanium alloys. These materials exhibit a relatively good resistance to mechanical stress.
Resistance to mechanical stress is not, however, the only criterion that should be considered when selecting the material for a fastening device. In general it may be desirable to produce the fastening device from a material that satisfies the following requirements:
1. Resistance to Mechanical Stress
A patient's head should be secured very firmly and very securely. Local mechanical material stresses on the fastening device are extremely high as the mechanical forces for securing the head are focused on a very small region on the tip of the fastening device. The fastening device, therefore, should not deform under large forces. Moreover, the fastening device should not splinter or otherwise divide because fragments could remain lodged in the patient's head after the fastening device has been removed.
2. Ability to be Sterilized
As the fastening device may be used as an invasive medical product, it is desirable for it to be sterilized. Various types of sterilization are possible, including: heat sterilization methods (for example, steam sterilization and hot air sterilization), cold sterilization methods (for example, gas sterilization and sterilization using ionizing radiation), and sterilization using chemical solutions (for example, aldehydes, peracetic acid, halogens, peroxides, etc.). If sterilization by heat is performed, then the material to be sterilized should exhibit a high resistance to heat and have low water absorbability. In the case of chemical sterilization, the material must be chemically inert.
3. Biocompatibility
As the fastening device may pierce both the skin and the bone of a patient, it may be considered an invasive medical device suitable for short-term use (for example, Class IIa in accordance with Appendix IX of council directive 93/42/EEC). Therefore, it is desirable that the material be biocompatible.
4. Absence of Artifacts in Imaging Methods
Operations requiring a very high level of precision often may be accompanied by imaging methods, including intra-operative imaging. Accordingly, a recording may be taken (for example, using a CT scanner or an MRT scanner) during the operation, wherein it is important to achieve high-quality scan recordings. The use of certain fastening device materials may cause distortion or artifacts in the recordings. Metals are generally radio-opaque as well as some ceramics, whereas plastics are generally radiolucent. Accordingly, conventional fastening devices made of high-grade steel and titanium alloys may cause numerous artifacts.
Efforts have been made to manufacture a head clamp pin from sapphire, which is radiolucent. Such a pin, however, may not withstand the mechanical stresses encounter during use and still shows remaining artifacts.
Finally, the shape of the objects also can contribute to artifacts, for example, tips or edges. In practice, it is difficult to predict whether or not a certain fastening device will cause artifacts or predict the artifacts' shape or magnitude.