In this cross-sectional study, we recruited patients who had undergone an appendicectomy due to appendicitis, and we divided them into two groups based on whether they had developed an IH or not. We explored the relationship between the psoas’ CT measurements and IH. After adjusting for age, gender and whether the participants were smoking or non-smoking, the psoas’ CT attenuation was deemed a protective factor for IH, and FIR was deemed a risk factor. In addition, it was also noted that PMI and sarcopenia hardly had an effect on the occurrence of IH at all.Psoas atrophy is mainly manifested by volume reduction and morphological changes, with an unobvious deposition of fat. Therefore, previous studies on IH have mainly focused on the psoas’ muscle area and skeletal muscle index. However, in addition to volume reduction, muscle atrophy may pathologically exhibit a fatty infiltration and muscle fibre loss [3, 11]. Some authors believe that CT attenuation may accurately reflect the number of muscle fibres and the degree of fat deposition . Muscle CT attenuation is related to various diseases: paraspinal muscle density is 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 standard deviation decrease of just 1 in the thigh muscle HU value conferred a nearly 40% increase in the risk of hip fracture . For critically ill adult patients being treated with mechanical ventilation systems, 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 lengths of stay . Furthermore, cancer patients with cachexia and low muscle CT values had a poor prognosis .
Skeletal muscle density inversely correlated with the length of hospitalisation when following complex abdominal wall hernia surgery . In our study, patients with a lower psoas CT attenuation were more likely to develop IH. It could be that the CT attenuation of the psoas correlates with that of the abdominal wall muscles . A decreased CT attenuation of the muscles is independently associated with muscle weakness . Therefore, a lower CT attenuation of the psoas indirectly reflects a weakness of the abdominal wall muscles, which are susceptible to hernias .
In our study, patients with a higher FIR of the psoas were prone to develop IH. Fatty infiltration in skeletal muscle has been identified as a possible cause for loss of muscle quality . Fatty infiltration induces insulin resistance, which impairs the normal capacities for protein synthesis, and subsequently contributes to muscle atrophy [19, 20]. In previous studies, MRI has often been applied to quantitatively measure the muscle’s fat content [21, 22], and they concluded that paraspinal fatty infiltration, rather than the muscle’s cross-sectional area, 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, a fatty infiltration in the multifidus muscles at L5-S1 could be correlated with the disc bulge at the stenosis segment and the reduction of lumbar lordosis . Since MRI examinations are not routinely performed on hernia patients, the MRI-measured muscle fat content in these patients is not widely taken into consideration. Previous published research on CT imaging mostly used HU value to indirectly represent muscle fat content. However, since muscle CT attenuation can be affected by previous surgeries and the deposition of high-density substances, such as calcium and bleeding, it may not effectively reflect the muscle’s fat content. Wang C et al. reported that the psoas’ CT attenuation in patients with osteoporosis fractures was unexpectedly higher than those without osteoporosis fractures , which may be related to intra-muscular haemorrhages or muscle repair following a fracture. In this case, CT attenuation cannot accurately reflect the muscle’s fibre content or the degree of fat accumulation. Although relatively complex, measuring the intramuscular fat area or muscle FIR by defining a CT threshold can more accurately reflect the degree of muscle fatty infiltration. So far, few studies have used CT to evaluate fatty infiltration in the muscle. One study that did, by Peter et al., showed that as shoulder strength increases following a shoulder arthroplasty, the rotator cuff fatty infiltration, measured by CT, decreases . To our knowledge, this is the first study to explore the potential relationship between the FIR of the psoas and IH. We concluded that when compared with CT attenuation, FIR was more closely related to IH.
Previous studies that have investigated the roles of abdominal muscles in malignancies, mostly measured the muscle area in the L3 or L4 cross-sectional image, including the psoas, erector spinae, quadratus lumborum, transversus abdominus, rectus abdominus, and the internal and external obliques. The corresponding skeletal muscle index might predict the prognosis of various malignancies [9, 27]. CT-determined sarcopenia, that was determined by measuring the level L3 cross-section muscles area, was not a risk factor for the occurrence of IH ; although it could prolong the postoperative hospital stay . It is very labour-intensive to measure all the muscle areas in a cross-sectional image, since it must be performed using a specific post-processing software by defining the range of the CT values to exclude intermuscular fat. To only measure the psoas area at level L3 is simple however, and the corresponding muscle index (PMI) correlates with the whole-body muscle mass . A decrease in PMI indicates a decline in the whole-body muscle mass (including the abdominal wall muscles), which results in a decreased functional capacity. Thus, having a low PMI may constitute a potential risk factor for IH. Although PMI at level L3 was a protective factor for IH in our research’s univariate regression analysis, it was not statistically significant in multivariate regression analysis. Sarcopenia meanwhile, as defined by PMI’s cut-off values in a previous study (based on an Asian population), was not associated with IH in neither the univariate nor multivariate regression analyses.
Our study demonstrated that psoas’ CT attenuation and FIR were associated with IH, but PMI and sarcopenia were not. PMI and sarcopenia reflect muscle mass, while CT attenuation and FIR are associated with psoas’ intramuscular adipose. We therefore speculate that the occurrence of IH is more related to muscle quality than quantity.
Among our study population, smoking was deemed a significant risk factor for IH, and this is consistent with previous publications [30, 31]. One hypothesis is that tobacco leads to atheroma, which reduces the blood supply to the abdominal wall. In addition, a decreased collagen deposition in surgical test wounds has been found among smokers .
In our study, BMI was not deemed an influencing factor of IH. However, many studies have shown that BMI, especially BMI > 30 (obesity), is a risk factor for IH [33, 34]. Obesity is likely to increase intra-abdominal pressure by putting mechanical stress on the abdominal incision, thus predisposing the occurrence of a hernia. However, the BMI in our population (26.1 in the IH group, 25.6 in the non-IH group) is lower than in those studies, and only 10.0% of our patients had a BMI > 30. Therefore, BMI may not have identified as a risk factor because the majority of our patients were non-obese.
Our univariate analysis identified older age and female sex as significant risk factors for IH, while no significant difference was noted in the multivariate analysis. Different studies report inconsistent results on whether age or gender are risk factors for incisional hernias [35,36,37]. These discrepancies in the age-specific or sex-specific influences on the risk for IH may reflect differences in the study population and diseases. We also showed two non-significant trends in the multivariate analysis: hypertension and diabetes, both of which are consistent with previous research [35, 36].
This study has several limitations. First, wound closure techniques and wound infection are recognised risk factors for IH. However, some patients could not recall the relevant information, since more than 10 years or even decades had passed since their appendectomy to this admission. Taking recall-bias into consideration, wound closure techniques and wound infection were not included in the statistical analysis. Second, the area under the curves of CT attenuation and FIR were not very high in our study. The purpose of our study was to screen the influencing factors of IH after appendectomy, and to provide a basis for further research. A perfect IH prediction model will be constructed in our follow-up research. Third, the cross-sectional retrospective study design limits our ability to ascertain causality and may cause the inevitability of selection bias. Thus, our conclusion needs to be verified using a larger sample size.