Comparison of morphological parameters and interference patterns of the hip between normal and femoral acetabular impingement patients
Authors:   , Chandrasekaran Charanya , Siegler Sorin 
Published: July 2024

Femoral Acetabular Impingement is a clinical condition of the hip joint that is characterized by an abnormal morphology causing an early impingement between the femur and the acetabulum. This interface between the bones can be due to the aspherical head of the femur (cam impingement) or due to the acetabular over coverage of the femoral head that is characteristic of a protrusion at the rim of the acetabulum (pincer impingement). Femoral acetabular impingement (FAI) serves as one of the mechanisms for the development of the osteoarthritis of the hip. Treatment of FAI prevents the onset of osteoarthritis by removing the abutment between the femur and the acetabulum to restore normal conditions and range of motion. Timely treatment reduces the degeneration of the joint and prevents labral tears and lesions. Current treatment for FAI includes Osteoplasty which involves removal of excess bone that causes early interference and Chondroplasty to remove and repair the damaged cartilage. Several other techniques of open and minimally invasive arthroscopic surgeries have also been employed in the treatment of FAI. However, the post-surgical results of the treatment are less favorable in many cases and it leads to revision surgeries or Total Hip Replacements. The purpose of this study is to test the assumption that early interference at the hip joint and FAI are due to abnormal morphology of the bone. It is performed by comparing the morphology and the patterns of bone interference during motion of the hip between healthy subjects and patients diagnosed with FAI. This study proposes a method to identify the occurrence, onset and progression of impingement in FAI patients at various angles of motion simulation. This method also identifies the location of the interference based on the geographic zones of the femur. The analysis included 10 FAI subjects and 7 age-matched healthy subjects and used ANALYZE[trademark] software to develop patient specific, image based, three dimensional, dynamic models of the hip joint. These 3D bone models were used for the measurement of twelve morphological parameters that represent common clinical parameters for the acetabulum, femur and the acetabulum-femur relations. These models were also used to carry out simulations using ADAMS[trademark] software at various angles of flexion, adduction and internal rotation to quantify and detect the zones of impingement. An Interference Detection Algorithm (RAPID[trademark]) was used to detect the contact between the femur and the acetabulum during simulation. In order to compare the patterns of interference, the femoral head was divided into 18 zones, 6 zones starting from anterior-inferior to anterior-superior and each further divided into medial, superior and lateral regions to produce the 18 zones. The interference patterns were quantified using distance maps and were compared within the subjects and between the normal and FAI subjects at each simulated position. The morphological parameters were compared between the healthy and FAI subjects using statistical analysis based on t-test. Alpha angle was found to be a significantly different between normal and FAI subjects. The FAI subjects had interference at the anterior-inferior region of femoral head-neck junction starting at 100° flexion with 20° adduction and going through 100° flexion combined with 20° adduction and 40° internal rotation. In contrast, for the normal subjects, 3 subjects did not have interference in any of the simulated positions and 4 subjects have interference only at the extremes of motion such at 100° flexion with 20° adduction combined with 30° at the anterior-inferior region of femoral head-neck junction. A linear model was developed to predict the impingement based on the alpha angle of the subjects. Correlation analysis between impingement and morphological parameters were carried out. The major concentration of the impingement is in the geographic zones of the femur that correspond to the excessive bony bump on the femoral head causing it to be aspherical.