Suture tensile testing provides researchers some of the key parameters when determining where the suture can be used and for how long it is intended to remain. Once in production, this type of mechanical testing is necessary to ensure the sutures meet the required design and performance specifications.
Surgical sutures are made of a variety of materials such as PDS, poly(ethylene terephthalate), polypropylene, polyamide, Stainless Steel and ePTFE in addition to suitable coatings and additives. One of the risks identified by the US FDA with surgical sutures is suture breakage, thus strength data of sutures is critical to the physical and performance characteristics.
Suture Physical Characteristics
The diameter of the suture, the needle attachment, and the tensile strength data provide information on the physical characteristics of absorbable or non-absorbable sutures. FDA recommends running tensile tests on at least the largest and the smallest sizes of the suture and presenting the information with a graph that illustrates the residual tensile strength for a length of time that aligns with the time specified in its intended use.
Suture Performance Characteristics
The length of time a certain suture is used for depends on the suture’s intended use. Tensile strength testing data for absorbable sutures should include information on how the tensile strength changes over time. For non-absorbable sutures, information on whether there has been a significant loss of tensile strength over time needs to be included and labelled accordingly.
Types of Suture Tests
Typical suture tensile tests and recommended equipment are described below.
- Straight-Pull Test
- A tensile test method where the suture specimen is clamped between two tensile grips and pulled apart
- Knot-Pull Test
- A tensile test method where the suture specimen is tied into a knot in the center of the grips then pulled apart
- Suture Creep Test
- A creep test method where the suture specimen is pulled to a desired load and held at that load for a certain amount of time
- ASTM Test Method
- ASTM D2256 is a testing standard designed to measure the tensile properties of threads and yarns. This test method is by far the most common specification that is followed for determining the breaking strength, elongation, and energy of threads, yarns, or other single strand textiles. This is a relatively simple tensile test whereby the material is pulled at a constant rate of speed until it breaks. The specimen can be tested in three different physical configurations: straight, knotted, or looped.
Universal testing machines equipped with a load cell with the right grips and a closed loop controller that can run tests at a specified rate and generate the required data is recommended for use to test sutures. Below is a featured product configuration for tensile suture testing.
Testing Machine: eXpert 7600 Universal Tensile Testing Machine
eXpert 7600 series Universal Testing Machines are single column testing systems with capacities up to 5kN. The eXpert 7601 system was selected as the featured suture test configuration as it is designed with the industries largest vertical test space at the appropriate force ranges for suture strengths.
Sutures do not exhibit high forces and the eXpert 7601 system can be used up to 1kN. In addition, gauge length or the distance between the two grips is typically 10 inches (254 mm) and the eXpert 7600 systems are designed with a vertical test space up to 61inches (1,549 mm) that allows testing longer length or high elongation materials.
Controller: MTESTQuattro Controller & Software
A controller capable of pulling the sample at a specified rate, identify peak load and the breaking strength, and record test data is needed to complete the setup. Both MTESTQuattro PC-based controller and eP2 Digital controller are closed loop controllers that offer this capability.
In addition, reports for suture tensile testing can be customized using MTESTQuattro software from ADMET to include additional properties such as knot pull strength, creep, and absorbable vs. non-absorbable suture identification. Cyclic testing can also be performed with one of ADMET’s fatigue testing systems.
Grips: Pneumatic Rope and Thread Grips
We recommend pneumatic rope and thread grips to clamp suture specimens. These grips are designed to avoid specimen slippage with the samples being wrapped around the guided horn and then secured. This clamping mechanism also prevents premature specimen break at the grip jaw faces.
As mentioned earlier, the testing machine must be able to accommodate specimens with large lengths if GT-T thread grips are used. At least 5.9 inches (15 cm), 9.84 inches (25 cm), and 11.8 inches (30 cm) of specimen length is required for the 200N capacity GT-0.2T, 2kN capacity GT-2T, and 5kN GT-5T grips, respectively. GT-T thread grips are used with sample diameters up to 3mm. For larger specimen diameters or smaller lengths, GR-T rope grips can be used.
Prior to starting suture testing, make sure the specimen is lined up properly in between the grips to avoid recording skewed data.
Testing the tensile strength (measure of the average tensile load required to bring the material to a complete break) of sutures can help determine how likely the stitch is to break and the amount of stitch separation needed to hold tissue. Testing the elongation (average increase in length) of suture materials helps determine how likely the suture is to hold when stretched and the amount of pain the stitch can cause the patient when used in areas of high mobility.
Product developers and researchers need equipment to accurately measure, record, and graph the tensile strength and performance data of sutures to comply with the FDA guidelines. Contact an ADMET Sales Engineer for further information on suture testing equipment.