Incisional hernia is a frequent and well described complication to abdominal surgery, which causes discomfort, pain, and reduction of quality of life. Nearly 350,000 hernia repairs are performed annually, costing approximately $3 billion dollars in the USA . The main known risk factors for IH development include surgical site infection, obesity, smoking, older age, gender (mostly reported as female), hypertension, diabetes, and use of corticosteroids [10–14].
CT scan has become a routine examination for abdominal IH, especially for huge hernia. Besides demonstrating the location, size and content of hernia, as well as helping to determine the presence of intestinal obstruction and necrosis, CT examination of the abdomen can quantitatively measure body composition, in particular visceral fat area and abdominal wall subcutaneous fat content, abdominal wall muscle and psoas muscle mass. CT measurement of body composition is quite reliable. For example, cross-sectional CT measurements of muscle and subcutaneous fat tissue are consistent with autopsy results . Furthermore, fat and fat-free tissue measurement at L3 level using CT strongly correlate with whole-body fat and fat-free mass . Numerous studies have delineated that visceral fat area and subcutaneous fat area measured on CT image can predict re-herniation and surgical site infection [17, 18]. Moreover, visceral obesity, rather than elevated BMI, is associated with IH after colorectal surgery [19, 20]. To date, there is very limited knowledge on the relationship between psoas CT measurement parameters and susceptibility to IH.
In this study, we recruited patients who underwent appendicectomy due to appendicitis, and divided them into two groups based on whether they had developed IH or not. We explored the relationship between psoas CT measurements and IH. After adjusting for age, gender and smoking, psoas CT attenuation was a protective factor for IH, and FIR a risk factor for IH; while PMI and sarcopenia hardly affect the occurrence of IH.
Muscles are composed of muscle fibers and intramuscular adipose tissue. Unlike erector spinae, psoas atrophy is mainly manifested by volume reduction and morphological changes, with unobvious fat deposition. Therefore, previous studies on IH had mainly focused on psoas muscle area and skeletal muscle index (SMI). However, in addition to volume reduction, muscle atrophy may pathologically exhibit fatty infiltration and muscle fibers loss [5, 6]. Some authors believe that CT attenuation may accurately reflect the number of muscle fibers and the degree of fat deposition . Muscle CT attenuation is related to various diseases: paraspinal muscle density was associated with facet joint osteoarthritis, spondylolisthesis and disc narrowing at the same level ; lower thigh muscle CT attenuation could increase the risk of hip fracture, and a 1 SD decrease in thigh muscle HU value conferred a nearly 40% increase in the risk of hip fracture ; for critically ill adult patients treated with mechanical ventilation, those with a lower skeletal muscle CT value at admission had a higher 6-month mortality rate, and a 10 HU increase in muscle density was associated with a 14% decrease in hospital length of stay ; cancer patients with cachexia and low muscle CT values had a poor prognosis .
Skeletal muscle density inversely correlated with length of hospitalization after complex abdominal wall hernia surgery . In our study, patients with lower psoas attenuation were more likely to develop IH. CT attenuation of abdominal wall muscles may correlate with that of psoas . Decreased CT attenuation of muscle is independently associated with muscle weakness . Therefore, lower CT attenuation of psoas indirectly reflects weakness of the abdominal wall muscle, which is susceptible to hernia .
In our study, patients with higher FIR of psoas were prone to develop IH. Fatty infiltration of skeletal muscle has been identified as a possible cause of loss of muscle quality . Fatty infiltration induces insulin resistance, which impairs normal capacity for protein synthesis, subsequently contributing to muscle atrophy [30, 31]. Previous studies often applied magnetic resonance imaging (MRI) to quantitatively measure muscle fat content [32, 33], and concluded that paraspinal fat infiltration, rather than muscle CSA, was associated with high-intensity pain/disability and structural abnormalities in the lumbar spine . Moreover, in patients with L4-5 single-segment degenerative lumbar spinal stenosis, fatty infiltration in the multifidus muscles at L5-S1 may be correlated with the disc bulge at the stenosis segment and reduction of lumbar lordosis . Since MRI examination is not routinely performed on hernia patients, MRI-measured muscle fat content in these patients is not widely-applied. Prior literatures about CT imaging mostly used HU value to indirectly represent muscle fat content. However, since muscle CT attenuation can be affected by previous surgery and deposition of high-density substances, such as calcium and bleeding, it may not effectively reflect fat content of muscle. WC et al reported that psoas CT attenuation in patients with osteoporosis fracture was unexpectedly higher than those without osteoporosis fracture , which may be related to intra-muscular hemorrhage or muscle repair after fracture. In this case, CT attenuation cannot accurately reflect muscle fiber content as well as the degree of fat accumulation. Although relatively complex, measuring intramuscular fat area or muscle FIR by defining CT threshold can more accurately reflect the degree of muscle fat infiltration. When intramuscular fat area was quantitatively measured by CT, rotator cuff fat infiltration conferred functional recovery after shoulder arthroplasty . To our knowledge, this is the first study to explore potential relationship between FIR and IH. We concluded that compared with CT attenuation, FIR was more closely related to IH.
Previous studies, to investigate the roles of abdominal muscles in malignancies, mostly measured muscle area of L3 or L4 cross-sectional image, including psoas, erector spinae, quadratus lumborum, transversus abdominus, rectus abdominus, as well as internal and external obliques. The corresponding SMI might predict prognosis of various malignancies [7, 38]. Based on SMI (calculated by measuring L3 level cross-section muscles), sarcopenia was not a risk factor for IH ; but it prolonged postoperative hospital-stay .
Since the measurement method mentioned above needs to be performed on a specific post-processing software by defining the range of CT values to exclude intermuscular fat, which is very labor-intensive, its application in clinical is very limited. To measure psoas area at L3 level only is simple, and corresponding PMI correlates with the whole-body muscle mass . A decrease in PMI indicates a decline in whole-body muscle mass (including abdominal wall muscles), which results in decreased functional capacity. Thus, PMI may be a potential risk factor for IH. In our research, PMI at L3 level was a protective factor for IH in univariate regression analysis, but not statistically significant in multivariate regression analysis. Sarcopenia, as defined by cut-off values in previous study (based on Asians), was not associated with IH neither in univariate nor in multivariate regression analysis.
This study has several limitations. First, wound infection after surgery can be prone to IH. However, some patients did not remember clearly whether they had postoperative infection or not, since more than ten years or even decades had passed from appendectomy to this admission. Taking recall-bias into consideration, wound infection was not included in statistical analysis. Second, cross-sectional retrospective study design limits our ability to ascertain causality, and may cause selection bias inevitably. Thus, our conclusion needs to be verified by a larger sample size.