Medical suture creep resistance testing evaluates how a surgical suture deforms under sustained tensile load, a critical performance parameter for wound closure security, knot integrity, and long-term patient safety. This article explains the test principles, sample types, measured parameters, laboratory methods, and regulatory applications for suture creep testing performed to ISO 899-1, ASTM D2990, and related plastics and yarn standards. It is intended for medical device manufacturers, R&D engineers, and quality assurance professionals working with accredited third-party laboratories.
Technical Overview
Creep is the time-dependent, permanent deformation of a material under a sustained mechanical load. In surgical practice, a suture remains under constant tension from the moment it is tied until the wound has healed or the suture is absorbed. During this period, physiological forces, tissue swelling, muscle movement, and hydrolytic degradation continuously act on the filament. If the suture material creeps excessively, the loop elongates, the knot slips or loosens, and approximation force is lost, which can lead to wound dehiscence, hematoma, anastomotic leakage, or recurrence of a repaired hernia.
Creep behavior in polymeric sutures progresses through three classical stages: primary creep (decelerating strain as molecular chains realign), secondary creep (a near-linear, steady strain rate governed by chain slippage and viscoelastic relaxation), and tertiary creep (accelerating deformation ending in rupture). Absorbable materials such as polyglycolide (PGA) and poly(lactide-co-glycolide) show amplified creep rates as hydrolysis cleaves the polymer backbone, while non-absorbable materials such as polypropylene, polyester, and ultra-high-molecular-weight polyethylene (UHMWPE) retain their dimensional stability over many years in vivo.
Suture creep resistance testing is typically adapted from established plastics and textile creep standards, including ISO 899-1 (also published as EN ISO 899-1, GB/T 11546.1, and JIS K 7115) and ASTM D2990, which covers tensile, compressive, and flexural creep and creep-rupture. Baseline tensile and knot-pull strength properties are usually determined beforehand in accordance with ASTM D2256, the single-strand yarn tensile method. Manufacturers frequently engage accredited third-party laboratories to perform these studies for design verification and regulatory submissions.
Testing Samples
Absorbable Synthetic Sutures
Samples include braided polyglactin 910 (PLGA), braided or monofilament polyglycolide, monofilament polydioxanone, and monofilament poliglecaprone 25. Because these materials lose strength through hydrolysis, creep specimens are tested both in the dry as-manufactured state and after hydration in physiological saline or phosphate-buffered saline at 37 °C to replicate in vivo conditions.
Non-Absorbable Synthetic Sutures
Samples include monofilament polypropylene, polyamide (nylon), polyvinylidene fluoride (PVDF), braided polyester (PET), and UHMWPE constructions. These materials are intended to retain closure force for the lifetime of the patient, so long-duration creep characterization is particularly important for cardiovascular, orthopedic, and hernia repair applications.
Natural and Metallic Sutures
Natural filaments such as silk and surgical gut (collagen), as well as metallic suture wires such as 316L stainless steel, are also evaluated. Metallic wires exhibit minimal polymeric creep but can still show time-dependent deformation at crimped connections or wrapped ends, which the test can capture.
Knotted and Constructed Specimens
Because the knot is usually the weakest and most creep-prone element of a suture loop, dedicated knotted specimens are prepared with clinically relevant configurations, including the surgeon's knot, square knot, sliding knots, and barbed suture segments. Loop-style specimens that simulate a closed wound configuration are widely used to measure combined filament-plus-knot creep behavior.
Testing Items
A comprehensive creep resistance study for medical sutures typically includes the following measurable items:
- Instantaneous (elastic) elongation immediately upon load application
- Total creep strain at defined time points, expressed as a percentage of gauge length
- Secondary-stage creep rate (percent strain per hour or per logarithmic decade)
- Time-to-rupture under sustained load, also known as creep rupture or static fatigue life
- Apparent creep modulus as a function of time
- Knot creep elongation and knot slippage distance for knotted specimens
- Tension retention in suture loops, distinguishing creep from stress relaxation effects
- Residual permanent set after load removal, quantifying recovery behavior
- Comparative performance across dry versus hydrated states and 23 °C versus 37 °C environments
Testing Methods
Specimen Preparation and Conditioning
Specimens are cut to a defined gauge length from sterile finished product, including packaging and sterilization exposure (ethylene oxide, gamma, or e-beam) representative of the commercial device. Hydrated test groups are soaked in phosphate-buffered saline at 37 °C for a standardized period before loading, and testing is conducted inside a temperature-controlled environmental chamber or fluid bath to maintain physiological conditions throughout the study.
Determination of Baseline Tensile Properties
The ultimate tensile strength and elongation of each suture size and material are first measured in accordance with ASTM D2256 or equivalent single-strand yarn methods. These baseline values establish the load levels for the creep phase and allow creep stress to be expressed as a percentage of ultimate strength.
Constant-Load Creep Procedure
Dead-weight or lever-arm creep frames, or closed-loop servo-controlled universal testing machines operating in load-hold mode, apply a constant force to the specimen. Typical load levels range from 10% to 75% of ultimate tensile strength, selected to bracket the physiological tension range expected in the target surgical application. Elongation is recorded continuously using contact extensometers, laser sensors, or video extensometry, and tests run from short screening durations such as 1 hour to long-duration studies of 100, 500, or 1000 hours. Load levels may also be escalated stepwise on a single specimen to generate an isochronous stress-strain curve.
Data Analysis and Reporting
Strain-versus-time curves are generated for each specimen, and the primary, secondary, and, where applicable, tertiary creep stages are identified. The test report documents the material, size, gauge length, knot configuration, load level, environmental conditions, creep strain at each time point, creep rate, failure mode and time-to-rupture if applicable, and statistical summary across replicates, typically a minimum of five specimens per condition.
Applications
Creep resistance data for medical sutures support a wide range of engineering, quality, and regulatory objectives:
- Material selection and formulation comparison during new suture development, including evaluations of coatings, braiding patterns, and monofilament extrusion parameters
- Design verification for wound closure devices, hernia mesh fixation systems, tendon and ligament repair constructs, and cardiovascular suturing applications such as valve annuloplasty and vessel ligation
- Regulatory submissions, including premarket notifications and conformity assessments, where accredited third-party laboratory reports provide objective performance evidence
- Batch release testing and supplier qualification for incoming suture materials
- Assessment of sterilization method, shelf aging, and real-time or accelerated degradation effects on long-term dimensional stability
- Comparative benchmarking of absorbable versus non-absorbable constructions for surgical specialties such as ophthalmic, plastic, gastrointestinal, and orthopedic surgery
FAQ
What is creep resistance in a medical suture?
Creep resistance is the ability of a suture to resist gradual, time-dependent elongation while holding a constant tensile load. High creep resistance means the suture loop maintains its circumference and approximation force over the healing period, reducing the risk of knot loosening and wound separation.
Why is suture creep testing performed at 37 °C in hydrated conditions?
Polymer properties are strongly temperature- and moisture-dependent. Body temperature and physiological fluid soften most suture polymers and accelerate hydrolytic degradation of absorbable materials, so testing in phosphate-buffered saline at 37 °C provides a far more realistic prediction of in vivo performance than dry room-temperature testing alone.
What load levels and durations are typically used?
Loads are usually set at 10% to 75% of the suture's ultimate tensile strength, determined by preliminary tensile testing to ASTM D2256. Screening tests may last 1 to 24 hours, while verification studies commonly extend to 100, 500, or 1000 hours to capture the steady-state creep rate and, where relevant, creep rupture.
What is the difference between creep and stress relaxation in sutures?
Creep occurs under a constant applied load, with elongation increasing over time. Stress relaxation occurs under a constant imposed strain, with internal tension decaying over time. Both mechanisms reduce the effective closure force on a wound, and modern studies often measure loop tension decay to capture both effects together.
Which suture materials offer the best creep resistance?
Stainless steel wire, UHMWPE, PVDF, and polypropylene generally exhibit the lowest creep rates, which is why they are preferred for long-term load-bearing applications. Braided polyesters provide good stability, nylons are moderate, and absorbable polymers such as polyglactin and poliglecaprone show the highest creep rates, particularly as degradation progresses.
Is creep resistance testing mandatory for regulatory approval?
Requirements depend on the intended use and the risk profile claimed by the manufacturer. For sutures used in sustained-load applications such as tendon repair, hernia fixation, or cardiovascular procedures, creep data derived from ISO 899-1 or ASTM D2990 methodologies is commonly expected as part of design verification, typically generated by accredited third-party laboratories.