Separation of Parts Reduces Stiffness and Increases Compliance of the Abdominal Wall in a Rat Model of Chronic Ventral Hernia

Objective: Division of the tendon of insertion of the external oblique (EO) muscle, also called "separation of parts" (SOP), is often performed as an adjunct to ventral hernia repair (VHR).  SOP has been most commonly conceptualized as a "relaxing" maneuver to allow tissue movement and approximation of the abdominal myofascia in the midline.  We hypothesize that SOP increases abdominal wall compliance, counteracting the mechanical changes resulting from a chronic midline ventral hernia.

Methods:  Midline incisional hernias were created in rats and allowed to mature for 7 days; un-operated animals served as controls (N=7 for each group).  For each rat, the abdominal wall was harvested, divided in half at the linea alba, and one side was subjected to SOP while the EO tendon on the other side was left intact.   Each hemi-abdomen was then divided into two samples and the passive mechanical properties were determined using an Instron tensiometer (model 5542; Instron Corporation, Canton, MA) equipped with a 50-N static load set at a crosshead speed of 10mm per minute.

Results:  After 7 days, significant changes in the mechanical properties of the abdominal wall were observed in animals with hernias including decreased elongation at yield and decreased yield strength (data shown in the table).  Release of the EO tendon significantly decreased abdominal wall stiffness in both control and hernia animals.

Conclusion: These data support our hypothesis that even after only 7 days, ventral herniation results in significant myopathy with changes in the mechanical properties of the abdominal wall. ¹ SOP decreases stiffness and increases compliance of the abdominal wall and this effect is preserved despite hernia-induced abdominal wall myopathy.  SOP therefore aids in VHR by specifically counteracting the unfavorable mechanical changes that occur in the abdominal wall after ventral herniation.

Mean (SEM); N=14 measurements for each reported value; * p < 0.05 for hernia vs. control; + p < 0.05 for EO released vs. EO intact

		Un-operated controls	Midline abdominal hernia at 7 days
Elongation (mm)	EO Intact	16.2 (1.0)	12.7 (1.7) *
	EO Released	16.7 (1.3)	11.6 (0.6) *
Yield Strength	EO Intact	19.1 (2.1)	15.8 (1.2) *
	EO Released	13.6 (1.0) +	10.8 (1.9) +
Stiffness (N/mm)	EO Intact	2.6 (0.3)	2.1 (0.2)
	EO Released	1.4 (0.1) +	1.6 (0.1) +