Incisions Do Not Linearly Sum

Introduction: Critics of minimally invasive methods sometimes argue that the sum of lengths of all trocar sites are similarly morbid to a “conventional” incision of equal length. This argument assumes correctly that pain and scarring are proportional to total tension normal to a linear incision. But the argument also assumes that total tension sums linearly with incision length. Here, I demonstrate why that premise is not valid.
Methods: Closing tension perpendicular to any linear incision (i.e. elliptical incision with a>>b) is a function f(L) of the length L of the incision, varying symmetrically along L with a maximum at L=(L/2). A constant C incorporates tissue mechanical properties for the passive abdominal wall, assumed to be directionally independent in the plane of the body wall, and not to vary with strain rate). If tension rises linearly with slope m,i.e. f(L) =mL, total tension then becomes

Incisions of various lengths are modeled, and plausible alternative incision scenarios for various procedures (Nissen, appendectomy, etc) are compared.
Results: Total tension rises non-linearly with increasing wound length. As a result, total tension across multiple incisions is less than the total tension for an incision of the same total length. (Approximating f(L) using a nonlinear equation does not alter this result.) For example, an appendectomy performed via one 10mm incision and two 5 mm incisions creates around 37% of the relative tension created by a single 20mm incision. A Nissen fundoplasty performed with five 5 mm incisions produces only 20% of the tension across a 25 mm incision. Even two 3 mm trocars can create less total tension than a single 5 mm trocar.
Conclusion: 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, adding a trocar in difficult cases adds relatively little morbidity. Finally, using two small trocars may be better than using a single larger one.

Session: Poster

Program Number: P078

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