Advances in FAI Imaging: a Focused Review

Abstract

Purpose of review: Femoroacetabular impingement (FAI) is one of the main causes of hip pain in young adults and poses clinical challenges which have placed it at the forefront of imaging and orthopedics. Diagnostic hip imaging has dramatically changed in the past years, with the arrival of new imaging techniques and the development of magnetic resonance imaging (MRI). This article reviews the current state-of-the-art clinical routine of individuals with suspected FAI, limitations, and future directions that show promise in the field of musculoskeletal research and are likely to reshape hip imaging in the coming years.

Recent findings: The largely unknown natural disease course, especially in hips with FAI syndrome and those with asymptomatic abnormal morphologies, continues to be a problem as far as diagnosis, treatment, and prognosis are concerned. There has been a paradigm shift in recent years from bone and soft tissue morphological analysis towards the tentative development of quantitative approaches, biochemical cartilage evaluation, dynamic assessment techniques and, finally, integration of artificial intelligence (AI)/deep learning systems. Imaging, AI, and hip preserving care will continue to evolve with new problems and greater challenges. The increasing number of analytic parameters describing the hip joint, as well as new sophisticated MRI and imaging analysis, have carried practitioners beyond simplistic classifications. Reliable evidence-based guidelines, beyond differentiation into pure instability or impingement, are paramount to refine the diagnostic algorithm and define treatment indications and prognosis. Nevertheless, the boundaries of morphological, functional, and AI-aided hip assessment are gradually being pushed to new frontiers as the role of musculoskeletal imaging is rapidly evolving.

Keywords: Advanced imaging; Artificial intelligence; Computed tomography; Femoroacetabular impingement; Hip; Magnetic resonance imaging.

Fig. 1

Natural history of femoroacetabular impingement as known in 2020. dGEMRIC, delayed gadolinium-enhanced MRI of cartilage; MRI, magnetic resonance imaging; OA, osteoarthritis

Fig. 2

Overview of current standard of care and future perspectives. MRI, magnetic resonance imaging; CT, computed tomography; MRA, magnetic resonance arthrography; AP, anteroposterior; Dx, diagnosis; Tx, treatment; PGx, prognosis

Fig. 3

Cam morphology. Radiographs (a, b), direct arthro-magnetic resonance examination (c, d), and corresponding surgical hip dislocation procedure (e, f) in a former athlete. a AP hip–centered radiograph and b cross-table view show a clear femoral head-neck convexity indicative of a cam morphology (white arrow heads). c Coronal proton density sequence and d corresponding radial proton density–weighted image at 1:00 o’clock, showing cam morphology (red arrow heads and red curved line), later confirmed by direct observation (curved yellow line in (e)). Also focal cartilage defect, labrum degeneration (c), and chondral delamination (d) are depicted. e Surgical hip dislocation caption showing a cam morphology extending from 10:00 to 3:00 o’clock. f Corresponding acetabular examination confirms extensive acetabular delamination under probing (black arrow heads)

Fig. 4

Schematic representation of the hypothesized mechanism of pincer FAI. a Normal spherical femoral head and acetabulum, which is congruent with the femoral head, provide the hip a wide range-of-motion. A pincer deformity (b) can cause pincer impingement against the femoral neck, especially during terminal flexion of the hip (c) leading to a typical pattern of circumferential acetabular cartilage damage (d). e Acetabular rim ossification and labral ossification associated with acetabular overcoverage, findings usually seen in pincer FAI. (f) MRA of the right hip of a female 26-year-old field hockey player, and arthroscopy (g), same athlete as in (e), revealing (i) a small-sized globular labrum with (ii) peripheral cartilage thinning and (iii) overcoverage of the acetabulum

Fig. 5

Automated segmentation and quantification of femoral parameters based on a 3D MRI dataset of a 30-year-old elite soccer player. a Volumetric 3D MRI alpha-angle (α°) automated measurements made at different points around the femoral head/neck junction. α° measured at 9 o’clock (posterior); 10, 11, and 12 o’clock (superior); and 1, 2, and 3 o’clock (anterior). b 3D generated model representing the radial extension of the cam deformity (orange and red line representing increased alpha angles). c 3D generated model of the corresponding acetabulum with important landmark clock-face references. d Polar plot (2D) of the automated 360° α° measurements around the FHN, representing the Ω° angle (gray straight lines) and corresponding perimeter (red line) for a given α° threshold (55°). Red lines represent increased α°’s for a given threshold. The Ω° is formed by two lines intersecting the center of the femoral neck at the level of the head-neck junction. The most posterior line posteriorly intersects the point at which the α° angle begins to be abnormal beyond a best-fitting circle and the anterior line at the point where the α° angle returns to normal

Fig. 6

a MRI volumetric imaging with reformats in all planes allow measurements between examinations and for research purposes, namely to pin-point anatomical landmarks to measure spinopelvic parameters (b). b Spinopelvic parameters schematics. Several steps are methodologically advised such as (1) Correction of tilt on the coronal plane: aligning the superior edges of the femoral heads or the inferior margins of the ischial tuberosities. (2) Correction of rotation in the axial plane: aligning both posterior acetabular wall margins and the antero-superior iliac spines (ASIS). (3) Defining the anterior pelvic plane (APP) (correction for tilt in the sagittal plane): aligning the ASIS and the anterior edge of the pubic symphysis. The APP is thus defined by three bony landmarks, the ASIS on both sides, and the pubic symphysis. The angle between the APP and the horizontal is defined as the APP angle

Fig. 7

Labrum tear patterns (a–d) and cartilage lesion patterns (e–g). a Intrasubstance labrum degeneration; sagittal plane. b Intrasubstance labrum degeneration and hypertrophy; radial plane at 1.5 o’clock. c Labral-chondral separation (= labral detachment); sagittal plane. d Intrasubstance labrum tear; sagittal plane. e Grade 2: partial thickness cartilage damage (peripheral acetabular chondral delamination involving the chondro-labral junction; outside-in pattern). f Complete cartilage loss (focal full-thickness acetabular cartilage defect). g Complete cartilage loss (diffuse full-thickness acetabular cartilage loss)

Fig. 8

Pathway for the imaging management and assessment of femoroacetabular impingement syndrome (FAIS). W, with; Wo, without; AP, anteroposterior

Fig. 9

Synthetic CT derived from MRI-based information. T1-weighted MRI (left column), CT (middle column), and synthetic CT (right column) images of the pelvic area. The sCT images, reconstructed from the T1wMRI images using a pre-trained deep learning algorithm strongly, resemble the CT images, the current standard for 3D imaging of osseous morphology. The 3D nature of the synthetic CT reconstruction facilitates multiplanar reconstructions, as demonstrated by the coronal (upper row) and axial (middle row) images as well as 3D renderings (lower row). (Courtesy of P.R. Seevinck, University Medical Center Utrecht, The Netherlands; boneMRI v1.1, MRIguidance BV, Utrecht, The Netherlands)

Fig. 10

MRI cartilage quantitative imaging. Pre-operative imaging of a 25-year-old male with cam-type FAI. a radial T2* mapping color–coded measurements at 3.0 T show decreased T2* relaxation times in the central compartment (antero-superior quadrant). b Proton density fat–saturated corresponding radial morphological sequence. c Corresponding MRI-derived 3D model with superimposed cartilage quantitative mapping

Fig. 11

MRI-based 3D modeling and dynamic virtual assessment. Virtual ROM detects impingement areas at specific degrees of motion represented on the right. In this 35-year-old male with a cam deformity, there was impingement noted at 17° of internal rotation with 90° of flexion and 20° adduction