Influence of clearance on the time-dependent performance of the hip following hemiarthroplasty: A finite element study with biphasic acetabular cartilage properties
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Hip hemiarthroplasty is a common treatment for femoral neck fracture. However, the acetabular cartilage may degenerate after hemiarthroplasty leading to postoperative failure and the need for revision surgery. The clearance between the acetabular cartilage and head of the prosthesis is one of the potential reasons for this failure. In this study, the influence of joint clearance on the biomechanical function of a generic hip model in hemiarthroplasty was investigated using biphasic numerical simulation. Both a prolonged loading period of 4000 s and dynamic gait load of 10 cycles were considered. It was found that a larger clearance led to a higher stress level, a faster reduction in load supported by the fluid and a faster cartilage consolidation process. Additionally, the mechanical performance of the acetabular cartilage in the natural model was similar to that in the hemiarthroplasty model with no clearance but different from the hemiarthroplasty models with clearances of 0.5mm and larger. The results demonstrated that a larger clearance in hip hemiarthroplasty is more harmful to the acetabular cartilage and prosthesis heads with more available dimensions (i.e. smaller increments in diameter) could be manufactured for surgeons to achieve a lower clearance, and reduced contact stress in hemiarthroplasty surgeries.Keywords:
Biomechanics
Tribology and wear of articular cartilage is associated with the mechanical properties, which are governed by the extracellular matrix (ECM). The ECM adapts to resist the loads and motions applied to the tissue. Most investigations take cartilage samples from quadrupeds, where the loading and motions are different to human. However, very few studies have investigated the differences between human and animal femoral head geometry and the mechanical properties of cartilage. This study assessed the differences between human, porcine, ovine and bovine cartilage from the femoral head; in terms of anatomical geometry, thickness, equilibrium elastic modulus and permeability. Diameter of porcine (3-6 months old), bovine (18-24 months old), ovine (4 years old) and human femoral heads were measured (n=6). Plugs taken out of the superior region of each femoral head and creep indentation was performed. The human femoral heads were obtained from surgery due to femoral neck fracture. Cartilage thickness was measured by monitoring the resistive force change as a needle traversed the cartilage and bone at a constant feed rate using a mechanical testing machine. The percentage deformation over time was determined by dividing deformation by thickness. A biphasic finite element model was used to obtain the intrinsic material properties of each plug. Data is presented as the mean ± 95% confidence limits. One-way ANOVA was used to test for significant differences (p Significant differences in average femoral head diameter were observed between all animals, where bovine showed the largest femoral head. Human cartilage was found to be significantly thicker than cartilage from all quadrupedal hips. Human cartilage had a significantly larger equilibrium elastic modulus compared to porcine and bovine cartilage. Porcine articular cartilage was measured to be the most permeable which was significantly larger than all the other species. No significant difference in permeability was observed between human and the other two animals: bovine and ovine (Table 1). The current study has shown that articular cartilage mechanical properties, thickness and geometry of the femoral heads differ significantly between different species. Therefore, it is necessary to consider these variations when choosing animal tissue to represent human.
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Biomechanics
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Investigations into tissue-preserving orthopaedic treatments should consider the tribology of articular cartilage; where simulations using animal joints are a predominant choice. However, very few studies have investigated the differences between human and animal cartilage. The aim of the present study was to characterise the differences in geometry and mechanical properties of human, porcine, bovine and ovine articular cartilage. Creep indentation was performed on osteochondral plugs taken from the superior region of femoral heads of all these species. Cartilage thickness was measured via the resistive force change of a needle descending through cartilage and bone. A biphasic finite element model was used to derive equilibrium elastic modulus and permeability. Results showed that human cartilage was significantly thicker than all other species tested. A positive correlation was found between femoral head diameter and cartilage thickness when comparing between species of quadrupeds. Human cartilage had the largest equilibrium elastic modulus, which was significant when comparing against porcine and bovine. However, porcine cartilage had significantly lower permeability. Significant differences in geometry and mechanical properties of articular cartilage were found between all species tested. It is necessary to consider these variations when choosing animal tissue to represent human.
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The cell density and incorporation of 35SO4 and 3H-glycine into human articular cartilage from 8 osteoarthrotic and 7 normal (subcapital fracture) femoral heads were studied. It was found that osteoarthrotic cartilage incorporates on a per cell basis about twice as much 35SO4 and 2--5 times as much 3H-glycine as normal cartilage. There was no relationship between the intensity of incorporation and either the location of the cartilage (weight-bearing versus non weight-bearing areas) in normal cartilage or the degree of damage (normal-like, fibrillated, and ulcerated) in osteoarthrotic articular cartilage. In the latter tissue the increased synthetic capacity of the cells seems to be a diffuse rather than a localised process, for it was also found in cartilage from peripheral osteophytes. Histo-autoradiographic studies showed that the osteoarthrotic chondrocytes are metabolically hyperactive all over the femoral head, including wedge-shaped margins of the zone of exposed bone. These results support the hypothesis that much of the articular cartilage from osteoarthrotic femoral heads is of an immature chondroblastic type. It is suggested that de-novo synthesis of articular cartilage occurs during the process of regional remodelling of the femoral head, which would account for the observed hyperactivity.
Subchondral bone
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This report describes an animal model of chondrolysis, i.e., acute necrosis of the articular cartilage. The blood circulation of rats' femoral heads was disrupted by incising the periosteum at the base of the femoral neck and cutting the ligamentum teres. The joint cartilage was necrotic in 18 of the 30 and 21 of the 39 osteonecrotic femoral heads of rats killed on the 30th and 42nd postoperative day, respectively. Chondrolysis was limited to the superficial cartilage layer in mild cases. It involved the entire cartilage in the more severely affected instances. Chondroclasts abutted on the necrotic cartilage where fibrous tissue replaced the subchondral bone plate. At the most advanced stage, the cartilage was segmentally absent, polished bone constituting the articular aspect. Chondrolysis, rarely detected in otherwise healthy children, commonly occurs in patients with a slipped capital femoral epiphysis, a disorder commonly associated with osteonecrosis of the femoral head. The herein presented model may serve the study of necrosis of the cartilaginous and osseous compartments of the femoral head in a small laboratory animal.
Periosteum
Avascular Necrosis
Epiphysis
Chondropathy
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The present study evaluated the tribological properties of the articular cartilage surface of the human femoral head with postcollapse stage avascular necrosis (AVN) using atomic force microscopy. The cartilage surface in the postcollapse stage AVN of the femoral head was reported to resemble those of disuse conditions, which suggests that the damage could be reversible and offers the possibilities of success of head-sparing surgeries. By comparing the tribological properties of articular cartilage in AVN with that of osteoarthritis, the authors intended to understand the cartilage degeneration mechanism and reversibility of AVN. Human femoral heads with AVN were explanted from the hip replacement surgery of four patients (60–83 years old). Nine cylindrical cartilage samples (diameter, 5 mm and height, 0.5 mm) were sectioned from the weight-bearing areas of the femoral head with AVN, and the cartilage surface was classified according to the Outerbridge Classification System (AVN0, normal; AVN1, softening and swelling; and AVN2, partial thickness defect and fissuring). Tribological properties including surface roughness and frictional coefficients and histochemistry including Safranin O and lubricin staining were compared among the three groups. The mean surface roughness Rq values of AVN cartilage increased significantly with increasing Outerbridge stages: Rq = 137 ± 26 nm in AVN0, Rq = 274 ± 49 nm in AVN1, and Rq = 452 ± 77 nm in AVN2. Significant differences in Rq were observed among different Outerbridge stages in all cases (p < 0.0001). The frictional coefficients (μ) also increased with increasing Outerbridge stages. The frictional coefficient values were μ = 0.115 ± 0.034 in AVN0, μ = 0.143 ± 0.025 in AVN1, and μ = 0.171 ± 0.039 in AVN2. Similarly to the statistical analysis of surface roughness, significant statistical differences were detected between different Outerbridge stages in all cases (p < 0.05). Both surface roughness and frictional coefficient of cartilage, which were linearly correlated, increased with increasing Outerbridge stages in postcollapse AVN. The underlying mechanism of these results can be related to proteoglycan loss within the articular cartilage that is also observed in osteoarthritis. With regard to the tribological properties, the cartilage degeneration mechanism in AVN was similar to that of osteoarthritis without reversibility.
Avascular Necrosis
Articular surface
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Using a new roentgenographic technique for measuring cartilage deformation in intact joint specimens, twenty-eight normal human hip joints from subjects twenty-five to eighty-five years old were loaded with a force of five times body weight in a testing machine. The initial unloaded thickness of the articular cartilage of the femoral head and the changes in thickness of this cartilage under load were measured roentgenographically at seven to twelve sites on each femoral head. These measurements showed that the deformations of femiral-head articular cartilage under load in the intact joint are non-uniform and increase greatly with age. In twelve specimens measurements were also made of the increase in cartilage deformation with time when the load of five times body weight was maintained on the joint. A single osteoarthrotic joint was also studied. The experimental findings imply changes in the fundamental mechanical properties of the cartilage with age, which probably result from age-related alterations in cartilage microstructure and chemical composition.
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Open reduction and internal fixation was performed on 50 displaced acetabular fractures in 49 patients by nine different attending surgeons over a 10-year period. At an average follow-up of 38 months, poor results were noted clinically in 38% and radiographically in 40%. The incidence of short- and long-term complications was greater than in other studies. Particularly distressing was the 58% incidence of heterotopic ossification (HO). Twenty-four percent had grade III or IV; five hips were autofused and the remainder had 40-60% loss of motion. There was no correlation of HO with age, sex, fracture type, degree of comminution, associated femoral head fracture or dislocation, delay to surgery, or operative time. However, 26 of 28 patients who had a trochanteric osteotomy as part of the operative exposure developed HO. Other complications included wound infection (12%), avascular necrosis of the femoral head (10%), nerve palsy (8%), and deep vein thrombosis/pulmonary embolism (8%). The data suggest formulation of specific treatment protocols, an awareness of surgical risks, and that staff specialization may reduce complications and improve outcome. Avoiding a trochanteric osteotomy at surgery and using prophylactic postoperative irradiation or indomethacin are suggested to reduce HO.
Avascular Necrosis
Acetabular fracture
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Viability and function of the articular cartilage must be considered when a femoral head-sparing approach is used to treat osteonecrosis.The cartilage,cortical end plate,and subchondral bone are a functional and structural unit.Osteonecrosis of trabecular bone under articular cartilage,changes the load conduction and contribution of the articular cartilage,as well as affect articular cartilage to acquire nutrition and eliminate metabolic product.The bone defect of osteonecrosis had effect on the structure and metabolism of the articular cartilage near the bone defect, and sugest osteonecrosis at the stage of pro-collapse may affect the cartilage overlying it.
Subchondral bone
Bone remodeling
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Objective In order to expound the effect of MMP in articular cartilage destroyed of ANFH.Methods In clinical we collected articular cartilage of GANFH(glucoticorticoid-induced avascular necrosis of the femoral head),OA of prime hip joint,and fresh fracture of head/neck of femur,After sliced,the expression of MMP-1、MMP-13 and TIMP-1 was detected.Results That cartilage of ANFH and OA of prime hip joint had higher expression of MMP-1、MMP-13,but lower expression of TIMP-1.Conclusion MMP has the important effects in articular cartilage destroyed of ANFH.
Avascular Necrosis
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