Incisions do not simply sum

Abstract

Critics of minimally invasive methods sometimes argue that the summed lengths of all trocar sites have a morbidity similar to that for an open incision of equal length. This argument assumes correctly that pain and scarring are proportional to the total tension normal to a linear incision. But the argument also assumes that total tension sums linearly with incision length. This report demonstrates why that premise is not valid. Wounds of various sizes are compared using a simple mathematical model. The closing tension perpendicular to any linear incision is a function of the incision's length, varying symmetrically together with a maximum at the midpoint of length. If tension rises linearly across an incision, integration of the tension relationship demonstrates that the total wound tension actually is proportional to the square of the length. In this report, incisions of various lengths are modeled, and plausible alternative incision scenarios for various procedures (e.g., Nissen, appendectomy) are compared. Total tension rises nonlinearly with increasing wound length. Thus, total tension across multiple incisions is always less than the total tension for an incision of the same total length. For example, an open appendectomy creates 2.7-fold more wound tension than a laparoscopic appendectomy. Similarly, two 3-mm trocars create less total tension than a single 5-mm trocar. Conventional incisions are subject to more total tension than any combination of trocar incisions of equal total length. This inequality yields three clinically relevant corollaries. First, it supports the practice of using the smallest effective trocars (or even no-trocar methods) to minimize pain and scar. Second, addition of a trocar in difficult cases adds relatively little morbidity. Finally, using two small trocars is better than using a single larger trocar.