Psoas Muscle Size Predicts Trauma Outcomes

After obtaining institutional review board approval, a query of our institutional trauma registry across a 5-year time period (2005–2010) was performed. Data for our trauma registry are collected prospectively and reported to the Pennsylvania Trauma Outcomes Study (PTOS) database by trained nurse abstractors. Study inclusion criteria were age ≥55 years, severe injury (Injury Severity Score (ISS) >15), and ICU length of stay >48 h. Patients were excluded if they suffered a critical head injury (defined as a Head/Neck Abbreviated Injury Scale (AIS) score ≥5), did not receive admission cross-sectional imaging of the abdomen, had fractures or preexisting hardware of the 4th lumbar vertebral body, or had a retroperitoneal hematoma that distorted the cross-sectional area of the psoas at the level of the L4 vertebral body. Patient demographics (age, sex, and race), physiologic variables on presentation, mechanism of injury, Injury Severity Score, hospital length of stay (HLOS), ICU length of stay (ILOS), ventilator days, and comorbidities were abstracted from the institutional registry. Morbidity was measured by PTOS-defined occurrences (see “Appendix 1”). Patients meeting all inclusion criteria with no exclusion criteria were then uploaded into a RED Cap database [12 (link)]. Computed tomography (CT) studies of the abdomen were obtained from the medical record and evaluated for each patient by one of three trained reviewers (DG, LE, DH). Prior to abstracting the study CTs, a sample of studies were independently abstracted by the three reviewers and results were compared in order to assess for inter-rater reliability. For each study, the right and left psoas muscle cross-sectional areas (PCSA) were measured at the level of the L4 vertebral body immediately inferior to the origin of the posterior elements. To normalize for body habitus, the cross-sectional area of the L4 vertebral body was also recorded at this level (Fig. 1). Mean PSCA was calculated for each patient, and the ratio between mean PSCA and L4 vertebral body area was calculated using the following formula:Fig. 1

Psoas:lumbar vertebral index was calculated as the ratio between the mean psoas cross-sectional area and the vertebral cross-sectional area at the level of the L4 vertebral body just inferior to the insertion of the posterior elements

$${\text{Psoas:L4 vertebral index = ([right PCSA (mm}}^2)\text{+ left PCSA}\left({\text{mm}}^2\right)]\text{/ 2)/ L4 vertebral CSA}\left({\text{mm}}^2\right).$$
The 50th percentile of psoas:L4 vertebral index (PLVI) value was determined and patients were grouped into high (>0.84) and low (≤0.83) categories based on their relation to the cohort median. Univariate analyses of patient demographic variables, admission vital signs, comorbidities, and outcome measures between the two groups were performed.