When designing a needle, the first consideration is typically the size of the "hole" (internal diameter of ID) of the tubing. As the intent is usually to pass something through the needle, having an ID as large as possible allows for maximum transfer and/or maximum clearance.

     The second consideration is usually the outer diameter (OD) of the tubing, because this factor is related to patient trauma. The smaller the OD, the less trauma caused.

     To find the "best" combination of these factors has led designers of needles to specify various degrees of thinwall tubing. "Thinwall", "extra thinwall", "ultra thinwall", and "micro thinwall" describe tubing that is available in many gauge sizes. However, reducing wall thickness has consequences. The most critical consequence is that a rapid decrease in stiffness results when the wall size is reduced. This occurs even though the degree of hardness may still be sufficient for good grinding. 

     "Stiffness" is the measure of a tube’s ability to resist bending. We determine stiffness by applying a force to the center of the tube while it is suspended across a span, and then measure the deflection. The stiffness standard for any gauge size is the maximum allowable deflection. GG-N-196 (last amended in 1947) sets standard stiffness performance criteria for regular wall tubing for gauges 13-27. ISO 9626 adds standards for 10 gauge to 30 gauge tubing, regular and thinwall, plus four "extra thinwall" sizes. Unfortunately, many of the tubing dimensions in the thinwall categories addressed by the ISO standard are not the same as those used by the U. S. needle tubing industry, so the ISO standards will not be applicable to all needles produced by U. S. manufacturers. In addition, the U. S. industry has expanded to producing thinwall sizes beyond those addressed by ISO 9626.

     Consequently, designers of many new needle products are left to deal with subjective assessments - such as the "feel" of the device. When long needles are required, for applications such as laparoscopic procedures, ISO tests are simply inadequate. The ISO tests use a maximum span length of only one inch, and cannot predict the performance of much longer needles. In fact, it is not possible to establish a "correct" stiffness for a particular size of tubing - each device’s performance requirements are dictated by the specific application or use of the device.

     The intent is not to suggest that we abandon the stiffness test as a measure of predicting the performance of a needle device. Rather, we need to develop stiffness standards that fit the various applications. For example, the test weight or force applied to the center of the tube should not be so great that it bends the tubing beyond its elastic limit. Also, the length of the span should be in relation to the length of the device being tested. However, if the goal is to measure the strength of the tubing before it kinks, suitable tests which exceed the tube’s elastic limit can be developed. Breakage tests can often suffice in addressing such strength considerations.

     Consistent with both the GG-N and ISO formats, K-Tube has developed stiffness standards for more than 100 tube sizes commonly used by the U. S. tubing industry, utilizing U.S. dimensions. K-Tube’s standards apply to twice the number of tube sizes as the ISO standards; seven times more than the GG-N-196 standards.

     These non-required stiffness standards provide tubing manufacturers a quality reference from one manufacturing lot to another and, in some cases, might be of use to the designer of a new device. The designer should first determine if the load and span length are appropriate, an then determine if the standard deflection is sufficient. This information should be available from all tubing manufacturers, and the actual measured stiffness should always be recorded with each tubing shipment for the user’s reference.