Christine L. Abraham

Patient-specific Analysis of Cartilage and Labrum Mechanics in Human Hips with Acetabular Dysplasia

BACKGROUND Acetabular dysplasia is a major predisposing factor for development of hip osteoarthritis (OA), and may result from alterations to chondrolabral loading. Subject-specific finite element (FE) modeling can be used to evaluate chondrolabral mechanics in the dysplastic hip, thereby providing insight into mechanics that precede OA. OBJECTIVE To evaluate chondrolabral contact mechanics and congruency in dysplastic hips and normal hips using a validated approach to subject-specific FE modeling. METHODS FE models of ten subjects with normal acetabula and ten subjects with dysplasia were constructed using a previously validated protocol. Labrum load support, and labrum and acetabular cartilage contact stress and contact area were compared between groups. Local congruency was determined at the articular surface for two simulated activities. RESULTS The labrum in dysplastic hips supported 2.8-4.0 times more of the load transferred across the joint than in normal hips. Dysplastic hips did not have significantly different congruency in the primary load-bearing regions than normal hips, but were less congruent in some unloaded regions. Normal hips had larger cartilage contact stress than dysplastic hips in the few regions that had significant differences. CONCLUSIONS The labrum in dysplastic hips has a far more significant role in hip mechanics than it does in normal hips. The dysplastic hip is neither less congruent than the normal hip, nor subjected to elevated cartilage contact stresses. This study supports the concept of an outside-in pathogenesis of OA in dysplastic hips and that the labrum in dysplastic hips should be preserved during surgery.

A new discrete element analysis method for predicting hip joint contact stresses

Quantifying cartilage contact stress is paramount to understanding hip osteoarthritis. Discrete element analysis (DEA) is a computationally efficient method to estimate cartilage contact stresses. Previous applications of DEA have underestimated cartilage stresses and yielded unrealistic contact patterns because they assumed constant cartilage thickness and/or concentric joint geometry. The study objectives were to: (1) develop a DEA model of the hip joint with subject-specific bone and cartilage geometry, (2) validate the DEA model by comparing DEA predictions to those of a validated finite element analysis (FEA) model, and (3) verify both the DEA and FEA models with a linear-elastic boundary value problem. Springs representing cartilage in the DEA model were given lengths equivalent to the sum of acetabular and femoral cartilage thickness and gap distance in the FEA model. Material properties and boundary/loading conditions were equivalent. Walking, descending, and ascending stairs were simulated. Solution times for DEA and FEA models were ~7 s and ~65 min, respectively. Irregular, complex contact patterns predicted by DEA were in excellent agreement with FEA. DEA contact areas were 7.5%, 9.7% and 3.7% less than FEA for walking, descending stairs, and ascending stairs, respectively. DEA models predicted higher peak contact stresses (9.8-13.6 MPa) and average contact stresses (3.0-3.7 MPa) than FEA (6.2-9.8 and 2.0-2.5 MPa, respectively). DEA overestimated stresses due to the absence of the Poissons effect and a direct contact interface between cartilage layers. Nevertheless, DEA predicted realistic contact patterns when subject-specific bone geometry and cartilage thickness were used. This DEA method may have application as an alternative to FEA for pre-operative planning of joint-preserving surgery such as acetabular reorientation during peri-acetabular osteotomy.

Computed tomography arthrography with traction in the human hip for three-dimensional reconstruction of cartilage and the acetabular labrum

AIM To develop and demonstrate the efficacy of a computed tomography arthrography (CTA) protocol for the hip that enables accurate three-dimensional reconstructions of cartilage and excellent visualization of the acetabular labrum. MATERIALS AND METHODS Ninety-three subjects were imaged (104 scans); 68 subjects with abnormal anatomy, 11 patients after periacetabular osteotomy surgery, and 25 subjects with normal anatomy. Fifteen to 25 ml of contrast agent diluted with lidocaine was injected using a lateral oblique approach. A Hare traction splint applied traction during CT. The association between traction force and intra-articular joint space was assessed qualitatively under fluoroscopy. Cartilage geometry was reconstructed from the CTA images for 30 subjects; the maximum joint space under traction was measured. RESULTS Using the Hare traction splint, the intra-articular space and boundaries of cartilage could be clearly delineated throughout the joint; the acetabular labrum was also visible. Dysplastic hips required less traction (∼5 kg) than normal and retroverted hips required (>10 kg) to separate the cartilage. An increase in traction force produced a corresponding widening of the intra-articular joint space. Under traction, the maximum width of the intra-articular joint space during CT ranged from 0.98-6.7 mm (2.46 ± 1.16 mm). CONCLUSIONS When applied to subjects with normal and abnormal hip anatomy, the CTA protocol presented yields clear delineation of the cartilage and the acetabular labrum. Use of a Hare traction splint provides a simple, cost-effective method to widen the intra-articular joint space during CT, and provides flexibility to vary the traction as required.

Accuracy of 3D dual echo steady state (DESS) MR arthrography to quantify acetabular cartilage thickness

To deploy and quantify the accuracy of 3D dual echo steady state (DESS) MR arthrography with hip traction to image acetabular cartilage. Clinical magnetic resonance imaging (MRI) sequences used to image hip cartilage often have reduced out‐of‐plane resolution and may lack adequate signal‐to‐noise to image cartilage.

Patient-specific chondrolabral contact mechanics in patients with acetabular dysplasia following treatment with peri-acetabular osteotomy.

OBJECTIVE Using a validated, patient-specific finite element (FE) modeling protocol, we evaluated cartilage and labrum (i.e., chondrolabral) mechanics before and after peri-acetabular osteotomy (PAO) to provide insight into the ability of this procedure to improve mechanics in dysplastic hips. DESIGN Five patients with acetabular dysplasia were recruited in this case-controlled, prospective study. Models, which included anatomy for bone, cartilage, and labrum, were generated from computed tomography (CT) arthrography scans acquired before and after PAO. Cartilage and labrum contact stress and contact area were quantified overall and regionally. Load supported by the labrum, expressed as a percentage of the total hip force, was analyzed. RESULTS Percent cartilage contact area increased post-operatively overall, medially, and superiorly. Peak acetabular contact stress decreased overall, laterally, anteriorly, and superiorly. Average contact stress decreased overall, laterally, anteriorly, and posteriorly. Only average contact stress on the superior labrum and peak labrum stress overall decreased. Load supported by the labrum did not change significantly. CONCLUSIONS PAO was efficacious at medializing cartilage contact and reducing cartilage contact stresses, and therefore may minimize deleterious loading to focal cartilage lesions, subchondral cysts, and cartilage delaminations often observed in the lateral acetabulum of dysplastic hips. However, the excessively prominent, hypertrophied labrum of dysplastic hips remains in contact with the femoral head, which continues to load the labrum following PAO. The clinical ramifications of continued labral loading following PAO are not known. However, it is plausible that failure to reduce the load experienced by the labrum could result in end-stage hip OA following PAO.

Anatomic changes in the inguinal region after hip arthroscopy: implications for femoral nerve block

Patients may experience significant pain after hip arthroscopy. Two patients who experienced severe pain after arthroscopic hip surgery, despite receiving multimodal pharmacologic therapy, are presented. In both cases, ultrasound-guided femoral nerve block provided excellent analgesia. However, during ultrasound, significant anatomic changes were noted in the inguinal region. In this report, we describe anatomic changes in the inguinal region that are visible with ultrasound after hip arthroscopy.

Changes in chondrolabral mechanics, coverage, and congruency following peri‐acetabular osteotomy for treatment of acetabular retroversion: A patient‐specific finite element study

Using a validated finite element (FE) protocol, we quantified cartilage and labrum mechanics, congruency, and femoral coverage in five male patients before and after they were treated for acetabular retroversion with peri‐acetabular osteotomy (PAO). Three‐dimensional models of bone, cartilage, and labrum were generated from computed tomography (CT) arthrography images, acquired before and after PAO. Walking, stair‐ascent, stair‐descent, and rising from a chair were simulated. Cartilage and labrum contact stress, contact area, and femoral coverage were calculated overall and regionally. Mean congruency (average of local congruency values for FE nodes in contact) and peak congruency (most incongruent node in contact) were calculated overall and regionally. Load supported by the labrum was represented as a raw change in the ratio of the applied force transferred through the labrum and percent change following surgery (calculated overall only). Considering all activities, following PAO, mean acetabular cartilage contact stress increased medially, superiorly, and posteriorly; peak stress increased medially and posteriorly. Peak labrum stresses decreased overall and superiorly. Acetabular contact area decreased overall and laterally, and increased medially. Labral contact area decreased overall, but not regionally. Load to the labrum decreased. Femoral head coverage increased overall, anterolaterally, and posterolaterally, but decreased anteromedially. Mean congruency indicated the hip became less congruent overall, anteriorly, and posteriorly; peak congruency indicated a less congruent joint posteriorly. Clinical relevance: Medialization of contact and reductions in labral loading following PAO may prevent osteoarthritis, but this procedure increases cartilage stresses, decreases contact area, and makes the hip less congruent, which may overload cartilage.

Cartilage Contact Mechanics Following Treatment of Acetabular Retroversion With Peri-Acetabular Osteotomy: A Case Study

Acetabular retroversion is a form of dysplasia where the acetabulum is tilted excessively in the anterior plane with a loss of posterior coverage. Excessive anterior coverage may cause impingement, and reduced acetabular coverage may increase cartilage contact pressures in the posteroinferior region, thereby causing hip osteoarthritis (OA). Treatment of retroversion is controversial: patients receive debridement of the anterior acetabular rim or posteriorly directed reorientation of the acetabulum via peri-acetabular osteotomy (PAO). Improved methods to quantify pre- and post-operative cartilage mechanics could be used to standardize treatment.Copyright

Finite Element Predictions of Joint Contact Mechanics and Acetabular Curvature in Dysplastic Human Hips

Acetabular dysplasia is characterized by a shallow acetabulum and is believed to cause accelerated hip osteoarthritis (OA) via altered mechanics (Fig. 1 A, B) [1]. Dysplastic acetabula are also described as more flat and less congruent than normal acetabula [2]. However, the relative contributions of altered labrum and cartilage contact mechanics, as well as quantitative differences in curvature are poorly understood in the dysplastic hip compared to the normal hip. Clinical observations of hypertrophied and damaged labra indicate altered labrum mechanics in dysplastic hips [3]. Finite element (FE) studies demonstrate abnormal cartilage mechanics in dysplastic hips, but are limited to models that omit the labrum or utilize idealized joint geometry (e.g., [4]). Therefore, the objectives of this study were to compare contact mechanics and curvature between normal and dysplastic hips using subject-specific FE models.Copyright

An Enhanced Discrete Element Model for Predicting Hip Contact Stresses

Chronic exposure to excessive stress on articular cartilage in the hip joint predicts the progression and onset of osteoarthritis (OA) [1]. Discrete element analysis (DEA) has been used to predict cartilage contact stresses [2, 3]. Because of its low computational expense and relative ease of application, DEA could be an effective alternative to the finite element (FE) method for the study of contact stresses in the hip. Previous DEA models have assumed concentric hip joint geometry and constant cartilage thickness. These assumptions lead to underestimates for cartilage contact stress and predict unrealistic, simplified contact patterns [2, 4]. It is possible that DEA could provide more realistic predictions of cartilage contact stress if subject-specific bone and cartilage geometry were used. The objectives of this study were to develop a DEA model that accommodates subject-specific bone and cartilage geometry, and to compare DEA model predictions of cartilage contact stresses with predictions from validated FE models.Copyright