Proof of concept: hip joint damage occurs at the zone of femoroacetabular impingement (FAI) in an experimental FAI sheep model

Statistical Shape Modeling of an Experimental, Induced Cam-Femoroacetabular Impingement Deformity in a Rabbit Model: A Platform to Study Mechanism of Hip Disease

BACKGROUND

Femoroacetabular impingement (FAI) is a common determinant of hip pain in young adults and an established risk factor in the subsequent development of osteoarthritis (OA). The mechanism of hip OA secondary to FAI is unknown. Small-animal models are critical translational tools to understand mechanisms of disease and develop interventional therapies. Kamenaga and colleagues proposed a novel animal model to mimic cam-FAI; however, 3D morphology of the induced deformity has not been objectively investigated.

PURPOSE

To use statistical shape modeling to quantitatively describe the induced proximal femoral head-neck deformity in order to take the necessary step in validating this animal model as a translational model for human cam-FAI.

STUDY DESIGN

Controlled laboratory study.

METHODS

Six-week-old immature New Zealand White rabbits (n = 13) were subject to right femur physis injury, with left femurs serving as controls. Micro-computed tomography images of femurs were taken at minimum 4 weeks after injury. 3D reconstructions were aligned and underwent statistical shape modeling with 2048 particles placed on each femur. Differences between mean shapes were calculated and analyzed using the Hotelling T2 test. Principal component analysis was used to describe shape variation, and parallel analysis was used to determine the statistically significant modes.

RESULTS

Hotelling T2 test demonstrated significant differences between cam-FAI and control mean shapes (P < .01). The cam-FAI mean shape protruded above the control mean by a maximum of 0.8 mm in the anterolateral head-neck junction with sustained protrusions of ~0.6 to 0.8 mm over the anterosuperior aspect and anteroposterior midline of the femoral head-neck junction. Maximum deviations between individual cam-FAI femurs and the mean control femur ranged between 0.1 and 1.6 mm in the same region. The first 6 modes explained 92.1% of the cumulative variation, and the first 13 modes were statistically significant, confirming the deformity.

CONCLUSION

The proposed model resulted in a head-neck cam deformity similar to human cam-FAI.

CLINICAL RELEVANCE

This proposed animal model creates a cam-type deformity similar to that observed in human FAI, helping validate the model as a platform to study mechanisms of hip FAI OA and develop future interventional therapies for this disease.

Experimentally Induced Femoroacetabular Impingement Results in Hip Osteoarthritis: A Novel Platform to Study Mechanisms of Hip Disease

BACKGROUND

We previously established a small animal model of femoral head-neck cam-type hip deformity by inducing physeal injury in immature rabbits. We investigated whether this induced deformity led to hip osteoarthritis (OA) within 4 months.

METHODS

Six-week-old immature New Zealand White rabbits underwent surgery to induce physeal injury in the right femoral head, causing growth arrest and secondary head-neck deformity. Animals were divided into early-pre-OA (4 weeks) and late-OA (16 weeks) groups. Left hips served as (nonsurgical) controls. Radiographs were made to visualize deformities and OA progression. The Beck classification was used to assess macroscopic cartilage damage and OA on the acetabulum and femoral head. Micro-computed tomography (CT), histological scoring, and gene expression were used to evaluate OA progression. The Wilcoxon signed-rank test was used for group comparisons. Significance was set at p < 0.05.

RESULTS

At 16 weeks, the injured hips showed radiographic evidence of joint space narrowing and a higher OA grade than the control hips (p = 0.0002). Micro-CT confirmed degenerative OA changes and a higher femoral head bone volume fraction (BV/TV) and trabecular thickness (Tb.Th) in the injured hips than in the control hips (BV/TV: p = 0.0001, Tb.Th: p = 0.0007). Macroscopically, the injured hips exhibited a greater prevalence and severity of chondral lesions at 4 weeks (83.3%, p = 0.015) and 16 weeks (100.0%, p = 0.002) post-injury compared with the control hips (0%), with worsening over time (4 versus 16 weeks: p = 0.016). The Osteoarthritis Research Society International (OARSI) score and synovitis score increased from 4 to 16 weeks post-injury. Compared with the control hips, the injured hips showed decreased Col2 expression and increased Col10 and MMP13 expression at 16 weeks post-injury (p = 0.062, p = 0.016, p = 0.041, respectively), confirming catabolism and OA progression.

CONCLUSIONS

To our knowledge, we have created the first small animal model of hip OA secondary to experimentally induced head-neck deformity. In this model, the deformity resulted in hip OA at 16 weeks post-injury.

CLINICAL RELEVANCE

This model can be used to test future interventional therapies and study mechanisms of femoroacetabular impingement-mediated hip OA.

In Vitro Ovine Cam Impingement Model and Its Effect on Acetabular Cartilage

Background

Femoroacetabular impingement syndrome is a condition where abnormal contact occurs between the femoral head and the acetabulum, leading to chondral damage and hip osteoarthritis. To better understand and treat femoroacetabular impingement syndrome, it is crucial to establish in vitro models that mimic the condition and assess potential interventions.

Purpose

To establish an in vitro ovine cam impingement model and assess the effectiveness of cam excision in reducing the incidence of type 3 acetabular labrum articular disruption (ALAD) (chondral flap) lesions.

Study Design

Controlled laboratory study.

Methods

Utilizing an ovine in vitro cam impingement model, 40 hips were subjected to testing across 5 groups (n = 8 per group): group 1 (control group), 750 N for 200 cycles; group 2 (cycle decrease), 750 N for 100 cycles; group 3 (load decrease), 500 N for 200 cycles; group 4 (cam excision), cam excision followed by 750 N for 200 cycles; and group 5 (halfway cam excision), 750 N for 100 cycles followed by cam excision under an additional 750 N for 100 cycles loading. Each specimen was subsequently assessed for chondral damage according to the ALAD classification, both macroscopically and microscopically.

Results

The control group (group 1) demonstrated the highest ALAD scores (2.7 ± 0.4, 2.8 ± 0.3) compared with other groups, whereas the cam excision group (group 4) exhibited lower scores (0.5 ± 0.5, 0.7 ± 0.4) than both the cycle decrease group (group 2) (1.6 ± 0.5, 1.6 ± 0.5) and the halfway cam excision group (group 5) (1.8 ± 0.6, 2 ± 0.5) (P < .05) in both macroscopic and microscopic gradings (P < .05). The load decrease group (group 3) (1 ± 0.5) also displayed lower scores compared with group 5 (2 ± 0.5) at histological grading (P < .05).

Conclusion

An in vitro sheep model was established that reliably induces mechanical chondrolabral damage in the hip joint. The findings show that reducing the load results in less chondrolabral damage compared with reducing the number of cycles. Furthermore, this model emphasizes the protective effect of cam excision in the management of chondral flap lesions (ALAD type 3).

Development of a novel rabbit model for femoroacetabular impingement through surgically induced acetabular overcoverage

There is a lack of validated small animal models for femoroacetabular impingement (FAI) that induce intra-articular lesions and cause osteoarthritis (OA) progression. The gene expression profile of articular cartilage in patients with FAI has not been characterized in animal studies. The purpose of this study is to describe a novel rabbit model for FAI with validated induction of intra-articular lesions and OA progression and to characterize the gene expression pattern in impinged cartilage using this model. Thirty 6-month-old New Zealand White rabbits underwent unilateral endobutton implant placement at the acetabular rim to surgically create overcoverage. Radiological assessment confirmed secure placement of endobutton at the acetabular rim for all operated hips with a mean alteration in lateral center-edge angle (ΔLCEA) of 16.2 ± 6.6°. Gross inspection revealed secondary cartilage injuries in the anterosuperior region of the femoral head for the operated hips. Cartilage injuries were shown to exacerbate with increased impingement duration, as demonstrated by the modified Outerbridge scores and Mankin scores. Immunostaining and quantitative real-time polymerase chain reaction revealed elevated expression of inflammatory, anabolic and catabolic genes in impinged cartilage. RNA sequencing analysis of cartilage tissue revealed a distinct transcriptome profile and identified C-KIT, CD86, and CD68 as central markers. Our study confirmed that the novel rabbit FAI model created acetabular overcoverage and produced articular cartilage injury at the impingement zone. Cartilage from the impingement zone demonstrated a heightened metabolic state, corroborating with the gene expression pattern observed in patients with FAI.

PLGA Conical Nail Fixation for Acetabular Chondrolabral Delamination in Femoroacetabular Impingement Promotes Cartilage and Labrum Regeneration in a Porcine Model

BACKGROUND

Acetabular chondrolabral delamination (ACD) is one of the most common hip cartilage injuries. However, there are very limited clinical treatments for this injury.

PURPOSE

To evaluate the effectiveness of poly (lactic-co-glycolic acid) (PLGA) conical nail fixation in the treatment of acute and chronic ACD in a porcine model.

STUDY DESIGN

Controlled laboratory study.

METHODS

In this study, 24 pigs underwent surgically induced delamination of the chondrolabral junction. Pigs were randomly divided into 3 groups: the control group (delaminated chondrolabral junction without treatment), ACD acute refixation (ACDA) group (delaminated chondrolabral junction fixed with a PLGA nail), and ACD chronic refixation (ACDC) group (placement of a nonabsorbable spacer at the stripped chondrolabral junction for 6 weeks before fixation with a PLGA nail). Porcine specimens underwent magnetic resonance imaging (MRI), hematoxylin and eosin staining, safranin O/fast green (SO/FG) staining, immunohistochemistry examination (collagen 1, collagen 2, and collagen 10), and immunofluorescence examination (SOX9 and aggrecan) to evaluate the chondrolabral regeneration at 6 and 12 weeks postoperatively.

RESULTS

MRI showed focal discontinuity of cartilage and fluid located between the acetabular cartilage and subchondral bone plate in the control group. The acetabular cartilage stained with SO/FG showed significantly more proteoglycan deposition at 12 weeks in the ACDA group than in the control group (P = .0109) and ACDC group (P = .0484). In accordance with the results of the SO/FG and collagen 10 staining, the aggrecan of the femoral head at 6 and 12 weeks was upregulated in the ACDA group (P < .0001) and downregulated in the ACDC group (P < .0001).

CONCLUSION

PLGA conical nail fixation achieved a good treatment outcome on MRI and histological evaluations. Early treatment upregulated the expression levels of SOX9 and aggrecan and promoted proteoglycan deposition.

CLINICAL RELEVANCE

The PLGA conical nail fixation technique may be a viable and effective treatment approach for patients with ACD in clinical practice.

Current understanding of articular cartilage lesions in femoroacetabular impingement syndrome

The concept of femoroacetabular impingement syndrome (FAIS) has received much attention over the past 20 years. Currently, it is believed that FAIS can lead to intra-articular pathologies such as labral tears and articular cartilage lesions, resulting in clinical symptoms and subsequent poor clinical outcomes. FAIS-related articular cartilage lesions are common but unique, and their natural course always leads to early osteoarthritis of the hip. However, despite these cartilage lesions having gradually gained considerable attention, limited consensus has been reached on key aspects, such as diagnosis, mechanisms, classification, and management strategies, which limits clinical and research advances. Hence, an intensive comprehensive overview based on the existing evidence is necessary. The purpose of this review was to introduce the general consensus, controversial issues, and recent advances in FAIS-related articular cartilage lesions.

A Novel Model of Hip Femoroacetabular Impingement in Immature Rabbits Reproduces the Distinctive Head-Neck Cam Deformity

BACKGROUND

Femoroacetabular impingement (FAI) is a leading cause of hip pain in young adults and often leads to degenerative osteoarthritis (OA). A small animal model of hip deformities is crucial for unraveling the pathophysiology of hip OA secondary to FAI.

PURPOSES

To (1) characterize a new minimally invasive surgical technique to create a proximal femoral head-neck deformity in a skeletally immature rabbit model and (2) document the effect of an injury to the medial proximal femoral epiphysis on head-neck morphology at 28 days after the injury.

STUDY DESIGN

Controlled laboratory study.

METHODS

Six-week-old New Zealand White rabbits (n = 10) were subjected to right hip surgery, with the left hip used as a control. An epiphyseal injury in the medial femoral head was created using a 1.6-mm drill. Hips were harvested bilaterally at 28 days after surgery. Alpha and epiphyseal shaft angles were measured on radiographs. Alpha angles at the 1- and 3-o'clock positions were measured on the oblique axial plane of micro-computed tomography images. Bone bar formation secondary to growth plate injuries was confirmed using alcian blue hematoxylin staining.

RESULTS

All hips in the study group showed a varus-type head-neck deformity, with lower epiphyseal shaft angles on anteroposterior radiographs versus those in the control group (133°± 8° vs 142°± 5°, respectively; P = .022) and higher epiphyseal shaft angles on lateral radiographs (27°± 12° vs 10°± 7°, respectively; P < .001). The mean alpha angles in the study group were higher at both the 1- (103°± 14° vs 46°± 7°, respectively; P < .002) and 3-o'clock (99°± 18° vs 35°± 11°, respectively; P < .002) positions than those in the control group. Alcian blue hematoxylin staining of all hips in the study group indicated that the injured physis developed a bony bar, leading to growth plate arrest on the medial femoral head.

CONCLUSION

The proposed model led to growth arrest at the proximal femoral physis, resulting in a femoral head-neck deformity similar to human FAI.

CLINICAL RELEVANCE

Our novel small animal model of a femoral head-neck deformity is a potential platform for research into the basic mechanisms of FAI disease progression and the development of disease-modifying therapies.

How Does Chondrolabral Damage and Labral Repair Influence the Mechanics of the Hip in the Setting of Cam Morphology? A Finite-Element Modeling Study

BACKGROUND

Individuals with cam morphology are prone to chondrolabral injuries that may progress to osteoarthritis. The mechanical factors responsible for the initiation and progression of chondrolabral injuries in these individuals are not well understood. Additionally, although labral repair is commonly performed during surgical correction of cam morphology, the isolated mechanical effect of labral repair on the labrum and surrounding cartilage is unknown.

QUESTION/PURPOSES

Using a volunteer-specific finite-element analysis, we asked: (1) How does cam morphology create a deleterious mechanical environment for articular cartilage (as evaluated by shear stress, tensile strain, contact pressure, and fluid pressure) that could increase the risk of cartilage damage compared with a radiographically normal hip? (2) How does chondrolabral damage, specifically delamination, delamination with rupture of the chondrolabral junction, and the presence of a chondral defect, alter the mechanical environment around the damage? (3) How does labral repair affect the mechanical environment in the context of the aforementioned chondrolabral damage scenarios?

METHODS

The mechanical conditions of a representative hip with normal bony morphology (characterized by an alpha angle of 37°) and one with cam morphology (characterized by an alpha angle of 78°) were evaluated using finite-element models that included volunteer-specific anatomy and kinematics. The bone, cartilage, and labrum geometry for the hip models were collected from two volunteers matched by age (25 years with cam morphology and 23 years with normal morphology), BMI (both 24 kg/m2), and sex (both male). Volunteer-specific kinematics for gait were used to drive the finite-element models in combination with joint reaction forces. Constitutive material models were assigned to the cartilage and labrum, which simulate a physiologically realistic material response, including the time-dependent response from fluid flow through the cartilage, and spatially varied response from collagen fibril reinforcement. For the cam hip, three models were created to represent chondrolabral damage conditions: (1) "delamination," with the acetabular cartilage separated from the bone in one region; (2) "delamination with chondrolabral junction (CLJ) rupture," which includes separation of the cartilage from the labrum tissue; and (3) a full-thickness chondral defect, referred to throughout as "defect," where the acetabular cartilage has degraded so there is a void. Each of the three conditions was modeled with a labral tear and with the labrum repaired. The size and location of the damage conditions simulated in the cartilage and labrum were attained from reported clinical prevalence of the location of these injuries. For each damage condition, the contact area, contact pressure, tensile strain, shear stress, and fluid pressure were predicted during gait and compared.

RESULTS

The cartilage in the hip with cam morphology experienced higher stresses and strains than the normal hip. The peak level of tensile strain (25%) and shear stress (11 MPa) experienced by the cam hip may exceed stable conditions and initiate damage or degradation. The cam hip with simulated damage experienced more evenly distributed contact pressure than the intact cam hip, as well as decreased tensile strain, shear stress, and fluid pressure. The peak levels of tensile strain (15% to 16%) and shear stress (2.5 to 2.7 MPa) for cam hips with simulated damage may be at stable magnitudes. Labral repair only marginally affected the overall stress and strain within the cartilage, but it increased local tensile strain in the cartilage near the chondrolabral junction in the hip with delamination and increased the peak tensile strain and shear stress on the labrum.

CONCLUSION

This finite-element modeling pilot study suggests that cam morphology may predispose hip articular cartilage to injury because of high shear stress; however, the presence of simulated damage distributed the loading more evenly and the magnitude of stress and strain decreased throughout the cartilage. The locations of the peak values also shifted posteriorly. Additionally, in hips with cam morphology, isolated labral repair in the hip with a delamination injury increased localized strain in the cartilage near the chondrolabral junction.

CLINICAL RELEVANCE

In a hip with cam morphology, labral repair alone may not protect the cartilage from damage because of mechanical overload during the low-flexion, weightbearing positions experienced during gait. The predicted findings of redistribution of stress and strain from damage in the cam hip may, in some cases, relieve disposition to damage progression. Additional studies should include volunteers with varied acetabular morphology, such as borderline dysplasia with cam morphology or pincer deformity, to analyze the effect on the conclusions presented in the current study. Further, future studies should evaluate the combined effects of osteochondroplasty and chondrolabral treatment.

Acetabular Cartilage Thickness Differs Among Cam, Pincer, or Mixed-Type Femoroacetabular Impingement: A Descriptive Study Using In Vivo Ultrasonic Measurements During Surgical Hip Dislocation

OBJECTIVE

To investigate acetabular cartilage thickness among (1) 8 measurement locations on the lunate surface and (2) different types of femoroacetabular impingement (FAI).

DESIGN

Prospective descriptive study comparing in vivo measured acetabular cartilage thickness using a validated ultrasonic device during surgical hip dislocation in 50 hips. Measurement locations included the anterior/posterior horn and 3 locations on each peripheral and central aspect of the acetabulum. The clock system was used for orientation. Thickness was compared among cam (11 hips), pincer (8 hips), and mixed-type (31 hips) of FAI. Mean age was 31 ± 8 (range, 18-49) years. Hips with no degenerative changes were included (Tönnis stage = 0).

RESULTS

Acetabular cartilage thickness ranged from 1.7 mm to 2.7 mm and differed among the 8 locations (P < 0.001). Thicker cartilage was found on the peripheral aspect at 11 and 1 o'clock positions (mean of 2.4 mm and 2.7 mm, respectively). At 5 out of 8 locations of measurement (anterior and posterior horn, 1 o'clock peripheral, 12 and 2 o'clock central), cartilage thickness was thinner in hips with pincer impingement compared to cam and/or mixed-type of FAI (P ranging from <0.001 to 0.031). No difference in thickness existed between cam and mixed-type of impingement (P = 0.751).

CONCLUSION

Acetabular cartilage thickness varied topographically and among FAI types. This study provides first baseline information about topographical cartilage thickness in FAI measured in vivo. Thinner cartilage thickness in pincer deformities could be misinterpreted as joint degeneration and could therefore have an impact on indication for hip preserving surgery.