Finite Element Analysis Examining the Effects of Cam FAI on Hip Joint Mechanical Loading Using Subject-Specific Geometries During Standing and Maximum Squat

Abstract

Background: Cam femoroacetabular impingement (FAI) can impose elevated mechanical loading in the hip, potentially leading to an eventual mechanical failure of the joint. Since in vivo data on the pathomechanisms of FAI are limited, it is still unclear how this deformity leads to osteoarthritis. Purpose: The purpose of this study was to examine the effects of cam FAI on hip joint mechanical loading using finite element analysis, by incorporating subject-specific geometries, kinematics, and kinetics. Questions: The research objectives were to address and determine: (1) if hips with cam FAI demonstrate higher maximum shear stresses, in comparison with control hips; (2) the magnitude of the peak maximum shear stresses; and (3) the locations of the peak maximum shear stresses. Methods: Using finite element analysis, two patient models were control-matched and simulated during quasistatic positions from standing to squatting. Intersegmental hip forces, from a previous study, were applied to the subjectspecific hip geometries, segmented from CT data, to evaluate the maximum shear stresses on the acetabular cartilage and underlying bone. Results: Peak maximum shear stresses were found at the anterosuperior region of the underlying bone during squatting. The peaks at the anterosuperior acetabulum were substantially higher for the patients (15.2±1.8 MPa) in comparison with the controls (4.5±0.1 MPa). Conclusions: Peaks were not situated on the cartilage, but instead located on the underlying bone. The results correspond with the locations of initial cartilage degradation observed during surgical treatment and from MRI. Clinical Relevance: These findings support the pathomechanism of cam FAI. Changes may originate from the underlying subchondral bone properties rather than direct shear stresses to the articular cartilage.

Patient-Specific Anatomical and Functional Parameters Provide New Insights into the Pathomechanism of Cam FAI

“…This mechanical impingement is typically observed at the limit of ROM [31,32] when the aspherical femoral head comes into contact with lateral aspects of the anterosuperior labrum and acetabulum. In the presence of a larger cam deformity, it further reduces the clearance between the femoral head-neck junction and the labrum, therefore imposing an obstruction and inducing elevated stresses [35].…”

“…Moreover, other common radiographic measures of the femoral head and neck and acetabulum have been associated with symptomatic FAI [5,10,26,40], but it remains unclear as to which combination of parameters plays a role in identifying patients at risk of developing symptoms. Although patients have demonstrated higher hip stresses [35] and different hip kinematics at higher ROMs such as during maximal squatting [31,32], it is still unclear why many individuals with cam deformity do not exhibit any clinical signs [22,38,40].…”

Increased Hip Stresses Resulting From a Cam Deformity and Decreased Femoral Neck-Shaft Angle During Level Walking

“…3). The femur model was oriented using the kinematics data during the loading condition and was permitted to translate in the loading direction [16,37]. A quasistatic loading scenario, using the highest resultant hip forces during terminal stance, was selected for comparison with hip contact forces applied at the femoral head.…”

“…A quasistatic loading scenario, using the highest resultant hip forces during terminal stance, was selected for comparison with hip contact forces applied at the femoral head. Because cartilage may be most susceptible to shear stresses under quasistatic loading [27], maximum shear stresses were examined on each participant's acetabular cartilage and labrum to examine adverse loading conditions [15,37]. A process flowchart from subject-specific input data to resultant hip stresses is summarized (Fig.…”

“…It has also been demonstrated that subchondral bone density is higher in individuals with a symptomatic or asymptomatic cam morphology [52,53]; however, previous finite element simulations neglected subject-specific bone material properties. Previous finite element studies also attempted to delineate pathomechanics of cam FAI using hip reaction loads from instrumented hip prostheses [8,15,18] or from inverse dynamics [37], which is typically limited to external and inertial forces and neglects muscle activity [26,54]. Because muscles contribute greatly to hip contact forces, it may be more appropriate to implement rigid body dynamics and musculoskeletal modeling (including gravitational, inertial, and muscle forces) to calculate joint contact loads.…”