1994
 PERFORMANCE IMPROVEMENT OF SURGICAL INSTRUMENTATION THROUGH THE USE OF NlTINOL MA TERlALS

Design considerations and material selection play important roles in the development of surgical instruments. The substitution of shape memory or superelastic Nitinol for conventional materials used in instrumentation for endoscopic and interventional procedures can lead to a significant improvement in overall perfonnance of the instruments. Simplicity of design, reduced number of parts and ease of assembly and disassembly result in cost reductions. The paper describes examples of Nitinol instruments and accessories already on the market or in clinical evaluation INTRODUCTION Diagnosis and therapy in medicine have advanced considerably during recent years. Therapeutic procedures, in panicular, show impressive results mainly in the area of minimally invasive, or key-hole surgery. The application of endoscopic procedures. catheterization and pereutaneous interventions minimizes the trauma of access and pain with the same or, in many cases even better, therapeutic success rates. Design of and material selection for devices and instruments for interventional procedures is dictated by considerations like reliability. safety. hygiene. functionality. biocompatibility. dimensions and, last not least, cost The operating physician is asking for instruments which combine multi-functionality with ease of use and minimum size. The healthcare provider adds lowest cost to the list of requirements. Conventional instruments arc: either reusable, made from stainless steel precision crafted for highest performaoce and best feel. or disposable. made from plastics or combinations of plastic aod stainless steel. The use of Nitinol. in particular superelastic Nitinol. allows the improvement of many instruments and devices by reducing the number of parts. simplifying assembly aod improving the overall performance through the characteristic propenies of the material. INSTRUMENTS FOR ENDOSCOPIC PROCEDURES Minimally invasive endoscopic surgery is intended to minimize the trauma of access and pain without compromising exposure of the operating field. In addition to avoiding large painful access wounds, the instruments used for dissection are small and fine, and thus the tissue trauma inherent to surgical dissection is reduced further. Olber benefits include diminished cost of therapy due to a reduced hospital stay and accelerated recovery with early return to full activity. Over the past several years the number of endoscopic surgery procedures has grown explosively [ I]. Endoscopic surgery involves several procedure types: ,. Laparoscopic • Thoracoscopic • Endoluminal • Perivisceral endoscopic • Inn-articular Among these approaches, the laparoscopic approach has been the most significant advance in general surgery in recent years [2]. Endoscopic surgery is characterized by the small size of a number of access ports, and requires specialized instruments capable of perfonning various tasks inside the body while making a gas-tight scal to Ibe entry port into Ibe body. Instruments available today are miniaturized versions of instruments used in conventional open surgery or are modified gynaecological instruments. Thus, compromises in performance have to be accepted. Forceps and scissors have hinged jaws. which limits miniaturisation. The opening gap of the jaws as well as their length is limited, making precise dissection difficult . .. ~g .. -, -I• Figure 1 (left): Conventional design of hinged instrument tips [3] Figure 2 (right): Dimensions of freedom of a conventional instrument [4] Another problem with endoscopic surgery vs. open surgery is the reduced number of degrees of freedom for manipulation. Most laparoscopic instruments have two to three degrees of freedom [4]: translation (lbe movement of Ibe instruments in Ibe direction of Ibeir longitudinal axis), axial rotation (rotation of the instrument around its longitudinal axis) and relative rotation around the entry point To overcome this problem, the instruments should be steerable or artiCUlating. The requirement of perfect hygiene made disposable instruments very popular. However, single use instruments using plastic components for many functional parts, do not provide the exactness and repeatability of precision crafted steel instruments. Moreover, cost per procedure recently became a major issue. Although less expensive than a reusable inslI'Ulllent, cost per procedure can be much higher in Ibe case of single use instruments. Reusable instruments, on Ibe olber hand, have to be cleaned and sterilized after each use. The more complex Ibey are, Ibe more difficult is Ibe disassembly and reassembly for cleaning purposes. New approaches have to be chosen in designing and producing instruments which overcome the problems associated with cost-effective, safe and reliable endoscopic procedures. By substituting steel parts in instruments with parts made from Nitinol, significant perfonnance improvements can be achieved. Nitinol alloys provide a unique combination of properties, not found in any other material. which makes these alloys particularly interesting for medical applications: shape recovery (thermal or mechanical) cxtrem elasticity low deformation force/stress constant force over wide strain range high strength good ductility corrosion resistance biocompatibility excellent MRI visibility sufficient radioopacity Hingeless Instruments Conventional instrument development efforts have taken the fonn of miniaturization of mechartical linkages in hinged-type designs. resulting in highly complicated systems with many individual parts, which are difficult to assemble. Worldwide coSt containement efforts favor the use of reusable or hybrid instruments, which have to be cleaned and sterilized after each use. As mentioned above, ease of assembly and disassembly, therefore, becomes an issue.