CORR Insights®: Cam FAI and Smaller Neck Angles Increase Subchondral Bone Stresses During Squatting: A Finite Element Analysis

Where Are We Now?

Finite element analysis (FEA) is a fascinating method that helps to simplify the visualization of complex interactions. Originating from the engineering sciences, FEA is increasingly used in orthopaedic biomechanics research, not only for implant-related questions but also for issues concerning native human tissues.

Put simply, FEA creates a virtual geometrical model of a structure, which is then further broken up into a limited (finite) number of small meshes. Each of these pieces is then given specific material properties, which are then virtually exposed to external loads or constraints. A variety of output parameters can be calculated with FEA. In biomechanics, typically stresses and strains are computed, color-coded, and displayed in a visual output.

It seems obvious that the more detailed and comprehensive the assumptions underlying any particular FEA might be, the more realistic the simulation will be. Before the era of femoroacetabular impingement (FAI), typical FEA analysis for the native hip joint historically comprised of the static stress distribution in a dysplastic joint [6]. Using simple biomechanical assumptions in standing position (without additional motion of the hip), the joint degeneration in dysplastic hips could be attributed to the increased load of the articular cartilage.

Today, we increasingly use FEA for FAI analysis [4]. In the current study, Ng and colleagues [5] show that a cam deformity leads to increased acetabular cartilage and bone stresses, which coincides with a higher bone density of the acetabulum in patients with symptomatic and asymptomatic hips. Subchondral sclerosis, therefore, might be a natural reaction to a subtle pathomorphology, rather than evidence of degeneration. The study also indicates that valgus configuration can be protective in the setting of an anterolateral cam deformity.

Where Do We Need To Go?

Even with these important findings, several gaps in our knowledge remain. First, a comprehensive FEA simulation of a native hip should include the local and individual variations of cartilage quality, individual hip motions at risk, the simulation of joint instability, and the labral suction seal [2]. The suction seal is created by the labrum, which seals a small amount of joint fluid between the acetabular and the femoral cartilage thereby decreasing wear of the articulating joint surfaces and increasing the joint stability by creating a vacuum phenomenon.

Second, we need to determine whether this process can be applied to clinical practice. Currently, the software is too time consuming, and the resources needed to perform an FEA by far exceed what generally is available in practice. A faster analysis could even open the door for a library of comparative data that could be shared online.

Finally, we need to determine the morphology of older individuals (80 to 90 years of age) who do not experience hip pain and do not have osteoarthritis. The FEA analysis of these hips could define the joint configuration of healthy hips and offer morphological and biomechanical reference values.

How Do We Get There?

A realistic FEA model should incorporate the simulation of a finite element model based on patient-specific morphologies followed by a verification with a biomechanical in-vivo investigation of the acetabular labral seal. Since variations of the cartilage properties in the native hip joint are known, this should be incorporated in the FEA based on modern biochemical MRI methods.

Motion capture analysis should record the most painful motion of an individual patient. The evaluation of joint instability should be based on hip joint stresses, simulated suction seal, and hip motion using a sophisticated motion algorithm. The validity of the FEA in the context of FAI would be emphasized by a direct correlation of the predicted stresses with the actual intraoperative degeneration of the cartilage in a larger patient population or by using animal models [3]. The actual degeneration could be determined by intraoperative cartilage stiffness mapping. Providing part of the motion data online (like Bergmann’s data [1]) could increase the library of pathologies with a complete dataset.

Automating the FEA simulation could cut down on the time-consuming segmentation process for osseous structures, as well as for the cartilage and the labrum. A standardization of high-resolution radiographic protocols (particularly MRI protocols) would make data exchange more practicable and comparisons more valid.

Finally, a complete dataset of painless nonarthritic hips in older individuals in their 80s or 90s should be feasible to acquire from an ethical standpoint and would define the range of successful hip morphologies. By using a combination of three-dimensional CT and MRI analysis, we can perform a patient-specific FEA and compare joint reaction forces of young painful hips to those older individuals with painless nonarthritic hips.