Stitches, formally known as sutures, are medical devices used to hold body tissues together after an injury or a surgical procedure. Their fundamental role is to act as temporary mechanical scaffolding, providing the necessary strength to keep wound edges approximated. This mechanical support is maintained until the body’s natural healing processes generate enough inherent strength to keep the wound closed independently. Sutures are chosen based on the specific tissue being repaired and the expected healing time.
Understanding Suture Material Strength
The strength of a stitch begins with the inherent properties of the material, measured as tensile strength. Tensile strength is the maximum force a suture strand can withstand before it breaks when pulled in a straight line. This measurement is conducted in a laboratory setting. Materials are engineered to possess varying degrees of breaking strength to match the needs of the tissue they are supporting.
Suture materials are divided into two categories: absorbable and non-absorbable. Absorbable sutures (e.g., synthetic polymers like polyglactin) lose their tensile strength predictably and are then broken down by the body. They are used for internal tissues that heal quickly. Non-absorbable materials (e.g., nylon or polypropylene) maintain their strength over a long period and are used for prolonged support or skin closures where they can be removed later.
Material strength is linked to its diameter, classified using the United States Pharmacopeia (USP) gauge system. The system uses numbers with zeros to denote the size; the more zeros, the finer the suture (e.g., 6-0 is thinner and weaker than 2-0). Conversely, sizes without a zero (like size 1 or 2) are the thickest and strongest, reserved for high-tension areas like abdominal fascia. The goal is to use the smallest diameter suture that provides sufficient strength, as larger materials can cause more tissue damage and inflammation.
Factors That Compromise Suture Performance
While the material’s initial tensile strength is measurable, the functional strength of a surgical repair is often limited by other factors. One significant compromise comes from the knot used to secure the stitch. Tying a knot can reduce the suture’s straight-pull tensile strength by 20% to 40%, making the knot the weakest point in the construct.
The choice of knot and the technique used to tie it are crucial for maintaining security and strength. A poorly formed knot can slip and loosen, even if the material is strong, a failure mode known as knot slippage. Knots like the surgeon’s knot or a square knot require a minimum number of throws (often four or five) to achieve reliable security and prevent slippage. The number of throws needed varies significantly between suture types; silk requires more throws than materials like polyglactin.
The location and tension of the wound dictate the performance of the suture. Wounds in high-stress areas (e.g., joints or the abdomen) place greater mechanical load on the stitches, demanding materials with higher intrinsic strength. Conversely, closures in low-tension areas like the face can use finer sutures to minimize scarring. If the tension across the wound is too high, it can lead to premature failure or “cutting through” the tissue, regardless of the suture’s material strength.
Biological interference within the wound environment can undermine the performance of the stitch. Infection or excessive swelling introduces enzymes and inflammatory cells that accelerate the degradation of absorbable sutures, causing them to lose strength faster than intended. Even non-absorbable materials can be affected, as inflammation increases friction and stress on the thread. This biological breakdown can result in the wound edges separating, a complication known as dehiscence, before the body has adequately healed.
The Healing Timeline and Tissue Strength Maturation
The strength provided by the stitches is only a temporary measure, needed until the healing tissue can support itself. Initially, the suture provides virtually 100% of the mechanical strength holding the wound together. The biological process of healing, involving collagen deposition and remodeling, gradually replaces this temporary mechanical support.
The gain in tissue strength follows a characteristic curve, beginning with a latency period where the wound has minimal strength, held only by the clot and the sutures. The rapid increase in tensile strength begins around five to seven days post-injury, corresponding to the proliferative phase where fibroblasts start producing collagen. This new collagen is initially disorganized and weak.
Skin wounds, for example, achieve only about 10% of their final strength after the first week. Strength continues to increase rapidly for about six weeks, but maturation—where initial Type III collagen is replaced by stronger Type I collagen—can continue for a year or more. Even after this prolonged remodeling phase, the healed tissue (scar tissue) rarely regains the full strength of the original, uninjured tissue, often plateauing at 70% to 80% of its native tensile strength.
The timing for removing non-absorbable stitches from the skin is determined by this early stage of tissue strength gain. Stitches are removed when the wound has achieved sufficient early strength to prevent immediate separation of the edges, which is not the same as being fully healed. If sutures are removed too early, the wound edges can pull apart; if left in too long, they increase the risk of track marks and infection. The strength of the final repair is a combination of the suture’s initial support and the body’s successful biological repair process.