Abstract
Objectives:
Femoroacetabular impingement (FAI) is a common hip condition that affects an estimated 54.4 persons per 100,000 with a higher prevalence in females. FAI is defined as abnormal contact between the femur and acetabulum leading to impingement, often as a result of bone deformities of the acetabulum or femoral neck. Diagnosis of FAI is often suspected based on history and physical exam and is typically confirmed with advanced imaging. Computed tomography (CT) has long been considered the gold standard for three-dimensional (3D) osseous morphology evaluation, while MRI has been used for soft tissue evaluation. Diagnosis often involves both modalities. While effective, this method is logistically burdensome, expensive, and may expose patients to ionizing CT scan radiation. Our institution incorporated a novel axial MRI sequence using in- and out-of-phase (ioMRI) sequences into an FAI-specific MRI to improve evaluation of osseous morphology.
The aim of this study was to compare the ioMRI protocol to CT imaging with regard to FAI osseous evaluation. We performed a retrospective case-control series that compared musculoskeletal (MSK) radiologist dictated FAI measurements between ipsilateral hip CT and ioMRI studies. We hypothesize that the FAI measurements taken from the ioMRI will be similar to those obtained from CT.
Methods:
We conducted a retrospective electronic medical record (EMR) review of two attending sports fellowship-trained orthopedic surgeons to identify FAI patients who underwent both hip CT scan and ipsilateral ioMRI between May 2014 and May 2024. Each included patient must have had both imaging studies and had formal imaging interpretations from our institution’s fellowship-trained musculoskeletal radiologists. All reports contained each of the following hip morphometrics: midcoronal angle, midsagittal angle, acetabular version at 1, 2, and 3 o’clock, maximum femoral α-angle, femoral neck angle, and femoral neck version. Patient demographic data were also recorded.
A two-way mixed model with absolute agreement was used to calculate interclass correlation coefficients (ICC’s) to compare ioMRI and CT scan measurement agreement. Results were interpreted as follows: minimal correlation < 0.2, poor correlation 0.2 to < 0.4, moderate correlation 0.4 to < 0.6, strong correlation 0.6 to ≤ 0.8, and almost perfect correlation > 0.8 (Koo 2016). Paired t-tests were conducted to compare means between CT and MRI hip measurements. All statistical testing was performed in RStudio (Version 2024.04.2).
Results:
Our initial chart review yielded 178 patients. After inclusion and exclusion criteria were applied, our final population studied included 95 patients (68 female). Flow diagram of patient data is depicted in Figure 1). Average patient age was 28.6 years and average BMI was 25.3 (Table 1).
Average results for each hip morphometric measurement are reported in Table 2. ICC’s comparing modalities are reported in Table 3. An almost perfect correlation was found for the acetabular version measurement at 3 o’clock. Strong correlation between CT and MR imaging was found for femoral neck version measurement and acetabular version at 1 and 2 o’clock. Moderate correlation was found for the midcoronal angle, midsagittal angle, maximum α-angle, and femoral neck angle measurements.
Our inter-method ICC’s were notable for moderate to almost perfect correlation across the board. Acetabular version at 3 o’clock had an ICC of 0.801, which is nearly indistinguishable. Likewise, acetabular version at 1 and 2 o’clock as well as femoral neck version all had strong ICC values of 0.617, 0.665, and 0.731 respectively.
Conclusions:
Our study found moderate to strong inter-method agreement between CT and ioMRI for all FAI evaluation hip measurements. This represents statistically significant correlation. This finding also demonstrates that ioMRI is able to provide osseous imaging that is fairly comparable to CT. Surgeons may anticipate that ioMRI measurements are reasonably reliable at estimating these measurements. However, several ioMRI measurements only had moderate ICC values, and CT imaging is likely still necessary for a thorough evaluation. The best performing measurements, acetabular version at 1, 2, and 3 o’clock, were all derived from the novel axial sequence of the ioMRI. Future work may consider applying this imaging technique to the other measurements. Our study is limited by its retrospective nature, and we did not assess inter-rater agreement between the many (>10) reading radiologists. However, the large number of reading radiologists extends real world applicability of our findings. Presently, ioMRI protocol studies likely cannot be used in place of CT scans for FAI osseous evaluation, but can provide a similar evaluation of hip osseous morphology. Surgeons may consider using isolated ioMRI in circumstances where CT may be contraindicated (cost, pregnancy, young age).
