Mechanical properties of the abdominal wall and biomaterials utilized for hernia repair

Abdominal wall hernias are one of the most common and long-standing surgical applications for biomaterials engineering. Yet, despite over 50 years of standard use of hernia repair materials, revision surgery is still required in nearly one third of patients due to hernia recurrence. To date, hernia mesh designs have focused on maximizing tensile strength to prevent structural failure of the implant. However, most recurrences occur at the biomaterial-tissue interface. There is a fundamental gap in understanding the degree to which a mechanical mismatch between hernia repair materials and host tissue contributes to failure at this interface. This review summarizes the current literature related to the anatomy and mechanics of both human and animal abdominal wall tissues, as well as the mechanical properties of many commonly-utilized hernia repair materials. The studies reviewed here reported greater compliance of the linea alba, larger strains for the intact abdominal wall, and greater stiffness for the rectus sheath and umbilical fascia when the tissues were loaded in the longitudinal direction compared to transverse. Additionally, greater stresses were observed in the linea alba when loaded in the transverse direction compared to longitudinal. Given these trends, a few recommendations can be made regarding orientation of mesh. The most compliant axis of the biomaterial should be oriented in the cranio-caudal (longitudinal) direction, and the strongest axis of the biomaterial should be oriented in the medial-lateral (transverse) direction. The human abdominal wall is also anisotropic, with anisotropy ratios as high as 8-9 reported for the human linea alba. Current biomaterial designs exhibit anisotropy ratios in the range of 1-3, and it is unclear whether an ideal ratio exists for optimal match between mesh and tissue. This is likely dependent on implantation location as the linea alba, rectus sheath, and other tissues of the abdominal wall exhibit different characteristics. Given the number of unknowns yet to be addressed by studies of the human abdominal wall, it is unlikely that any single biomaterial design currently encompasses all of the ideal features identified. More data on the mechanical properties of the abdominal wall will be needed to establish a full set of guidelines for ideal mesh mechanics including strength, compliance, anisotropy, nonlinearity and hysteresis.