This is the first study in a UK elderly trauma population that examines both masseter and psoas muscle groups as indices for sarcopenia and their association with outcome. The findings of the study do not support our initial hypothesis; M-CSA is not proportionately comparable to P-CSA in predicting health outcomes in older trauma patients.
The odds of inpatient mortality are three times higher for older trauma patients with PLVI sarcopenia compared to those without. We also found that PLVI sarcopenia is an independent risk factor for reduced survival two years following injury and is associated with reduced likelihood of being discharged home. Our findings are consistent with several previous studies examining psoas sarcopenia in trauma populations [4, 7].
There was no statistical association between sarcopenia defined by either muscle group and inpatient complications or length of stay. Our study, like most others, uses muscle size as a proxy for sarcopenia. The operational definition of sarcopenia encompasses muscle quality and more importantly, physical strength. Overlooking these dimensions may explain why we did not note any prognostic associations with these important clinical outcomes .
Correlation between M-CSA and PLVI was weakly positive but M-CSA was not a predictor of overall mortality or any other measured health outcomes in our population. Our results contradict findings from other studies that have reported a positive association between M-CSA and mortality at different time points in the trauma population [11,12,13]. There may be several reasons for this. For example, in our study, the average CSA in patients with masseter sarcopenia was much higher— 438.5 ± 49.1 mm2 in females and 420.7 ± 70.4 mm2 in males. Two other studies quoted average sarcopenic values as 224 mm2  and 343 mm2  in females, and 281 mm2  and 418 mm2  in males. This heterogeneity highlights the importance of establishing standardised cut-offs, ideally referenced by healthy, non-hospitalised populations to prevent variations and over-diagnosis.
There were also differences in stratification of sarcopenia; we defined sarcopenia as the lowest quartiles in masseter and psoas populations regardless of sex. Sarcopenia was more prevalent amongst females in both groups including the PLVI group, in whom we adjusted for body stature. Other studies have defined sarcopenia with sex-based cut-offs below the median  or one standard deviation below mean .
Furthermore, our results may be impacted by exclusion bias; 34.2% of patients were excluded from statistical analysis due to inadequate visualisation of bilateral masseters, compared with only 6.6% in the PLVI group (Table 1). We may have failed to capture patients with reduced muscle quality. M-CSA was measured along the longitudinal axis, which requires reconstructing the imaging plane to align with the proximal and distal attachments. If accurate M-CSA measurement relies on higher-quality imaging or is technically more challenging, its viability as a metric for sarcopenia may be limited.
On post hoc analysis, our study is not sufficiently powered to show the observed 12% 2-year survival difference between the two masseter sub-groups. An estimated sample size of 605 patients would be required to reduce the probability of a type II error.
This study, nevertheless, suggests that M-CSA is not as robust a prognostic indicator as PLVI in these patients. Furthermore, localized masseter muscle atrophy can occur in patients with tooth loss, dental prosthesis or other causes for reduced masticatory function [9, 18, 19]. Caution needs to be exercised in using it as a surrogate for global muscle deconditioning. Masseter composition is also affected by body surface area  and craniofacial structure [20, 21]. Adjusting for body habitus and stature has relevance in achieving reliable sarcopenia measurements. Height and weight can be used to achieve this, but in the acute clinical setting, such as major trauma or emergency surgery, where the clinical application of sarcopenia measurement lies in augmenting emergent decision-making and prognostication, accurate height and weight measurements may not be readily available. Thus, sarcopenia measurement should ideally rely upon independent predictors of stature that can be measured on the same opportunistic imaging modality. We adjusted for this in our study using the L4 vertebral body CSA as part of the PLVI, but we are not aware of an available target for adjustment for stature in masseter sarcopenia quantification. Alternatively, combining CSA measurement with other metrics of masseter composition— be it, masseter volume, thickness or density— may increase sensitivity .
Our study is limited by virtue of its retrospective, single-centre design. We adjusted for injury severity, but confounders, such as comorbidity index, ethnicity and any operative interventions, were not examined. We did not measure the inter-rater reliability for the muscle CSA measurements. While psoas is the most commonly used muscle group in radiological evaluation of central sarcopenia in the trauma population , there is only one other study that looks specifically at PLVI . Conversely, this showed an association of PLVI with morbidity but not in-hospital mortality. Differences in PLVI cut-offs and determinants of inpatient morbidity could explain this disparity.
The prospect of muscle segmentation on volumetric CT imaging using deep learning algorithms provides exciting opportunity for further work in this area and may overcome many of the challenges in sarcopenia measurement, improving precision and validity [23, 24]. The trauma population is unique in the challenges it imposes given the heterogeneity of injuries inflicted— in terms of severity, quantity and distribution of affected body areas. This makes prognostication and clinical decision-making more difficult. Given that many patients in a trauma or neurosurgical setting only undergo CT imaging of the head or neck, it is crucial that future studies focus upon cranial as well as abdominal imaging modalities to develop pragmatic clinical applications for opportunistic sarcopenia assessment. Some studies have indicated that morphometric analysis of temporal muscle thickness [25,26,27] or zygomatic thickness  may be suitable craniofacial surrogates of central sarcopenia. Composite analysis of all facial muscles may serve to enhance diagnostic accuracy. Combining sarcopenia as an objective metric with clinical frailty scoring may allow multi-dimensional frailty assessment that can augment prognostication and clinical decision-making. It may serve to identify patients that will benefit most from multi-disciplinary interventions and navigate decision-making around procedural interventions, discharge planning and palliation.