Chronic changes in the rabbit tibial plateau following blunt trauma to the tibiofemoral joint

The knee is often a site of injury that can often lead to a chronic disease known as osteoarthritis (OA). The disease may be initiated, in part, by acute injuries to joint cartilage and its cells. In a recent study by this laboratory, using Flemish Giant rabbits, an impact compressive load on the tibial femoral joint was shown to cause significant levels of acute damage to chondrocytes in cartilage of the medial and lateral tibial plateaus. In the current study, using the same model, histological and mechanical data from the plateaus were documented at 6 and 12 months post impact, and compared to the unimpacted control limbs and a limb from unimpacted, control animals. The mechanical properties of cartilage were measured with indentation relaxation tests on the medial and lateral plateaus in regions covered and uncovered by the meniscus. The histological studies on impacted limbs showed surface lesions on both plateaus, thickening of the underlying subchondral bone at 12 months and numerous occult microcracks at the calcified cartilage–subchondral bone interface at 6 and 12 months, without significant changes in cartilage thickness or its mechanical properties versus controls. Yet, there was an increase in both the matrix and fiber moduli and a decrease in the permeability of uncovered, medial plateau cartilage in both limbs of impacted animals between 6 and 12 months post impact that was not documented in control animals.     

Osmotic loading to determine the intrinsic material properties of guinea pig knee cartilage

Few methods exist to study cartilage mechanics in small animal joints due to the difficulties associated with handling small tissue samples. In this study, we apply an osmotic loading method to quantify the intrinsic material properties of articular cartilage in small animal joints. Cartilage samples were studied from the femoral condyle and tibial plateau of two-month old guinea pigs. Swelling strains were measured using confocal fluorescence scanning microscopy in samples subjected to osmotic loading. A histochemical staining method was developed and calibrated for quantification of negative fixed charge density in guinea pig cartilage. Site-matched swelling strain data and fixed charge density values were then used with a triphasic theoretical model for cartilage swelling to determine the uniaxial modulus of the cartilage solid matrix. Moduli obtained in this study (7.2 MPa femoral condyle; 10.8 MPa, tibial plateau) compare well with previously reported values for the tensile moduli of human and other animal cartilages determined from uniaxial tension experiments. This study provides the first available data for material properties and fixed charge density in cartilage from the guinea pig knee and suggests a promising method for tracking changes in cartilage mechanics in small animal models of degeneration.     

3D finite element model of meniscectomy: changes in joint contact behavior

The goal of this study is to quantify changes in knee joint contact behavior following varying degrees of the medial partial meniscectomy. A previously validated 3D finite element model was used to simulate 11 different meniscectomies. The accompanying changes in the contact pressure on the superior surface of the menisci and tibial plateau were quantified as was the axial strain in the menisci and articular cartilage. The percentage of medial meniscus removed was linearly correlated with maximum contact pressure, mean contact pressure, and contact area. The lateral hemi-joint was minimally affected by the simulated medial meniscectomies. The location of maximum strain and location of maximum contact pressure did not change with varying degrees of partial medial meniscectomy. When 60% of the medial meniscus was removed, contact pressures increased 65% on the remaining medial meniscus and 55% on the medial tibial plateau. These data will be helpful for assessing potential complications with the surgical treatment of meniscal tears. Additionally, these data provide insight into the role of mechanical loading in the etiology of post-meniscectomy osteoarthritis.     

In vivo tibiofemoral contact analysis using 3D MRI-based knee models

This paper quantified the motion of the tibiofemoral contact points during in vivo weight bearing flexion using MRI- based 3D knee models and two orthogonal fluoroscopic images. The contact points on the medial and lateral tibial plateau were calculated by finding the centroid of the intersection of the tibial and femoral cartilage layers and by using the bony geometry alone. Our results indicate that the medial femoral condyle remains in the central portion of the tibial plateau and the lateral condyle translates posteriorly with increasing flexion. Using the bony contact model increased the total translation of the medial and lateral condyles by 250 and 55%, respectively, compared to the cartilage contact model. These results suggest that using the bony geometry alone may not accurately represent the articular surfaces of the knee. Articular cartilage geometry may have to be used to accurately quantify tibiofemoral contact.     

Interrelationship of cartilage composition and chondrocyte mechanics after a partial meniscectomy in the rabbit knee joint – Experimental and numerical analysis

Site-specific and depth-dependent properties of cartilage were implemented within a finite element (FE) model to determine if compositional or structural changes in the tissue could explain site-specific alterations of chondrocyte deformations due to cartilage loading in rabbit knee joints 3 days after a partial meniscectomy (PM). Depth-dependent proteoglycan (PG) content, collagen content and collagen orientation in the cartilage extracellular matrix (ECM), and PG content in the pericellular matrix (PCM) were assessed with microscopic and spectroscopic methods. Patellar, femoral groove and samples from both the lateral and medial compartments of the femoral condyle and tibial plateau were extracted from healthy controls and from the partial meniscectomy group. For both groups and each knee joint site, axisymmetric FE models with measured properties were generated. Experimental cartilage loading was applied in the simulations and chondrocyte volumes were compared to the experimental values. ECM and PCM PG loss occurred within the superficial cartilage layer in the PM group at all locations, except in the lateral tibial plateau. Collagen content and orientation were not significantly altered due to the PM. The FE simulations predicted similar chondrocyte volume changes and group differences as obtained experimentally. Loss of PCM fixed charge density (FCD) decreased cell volume loss, as observed in the medial femur and medial tibia, whereas loss of ECM FCD increased cell volume loss, as seen in the patella, femoral groove and lateral femur. The model outcome, cell volume change, was also sensitive to applied tissue geometry, collagen fibril orientation and loading conditions.     

Pathways of load-induced cartilage damage causing cartilage degeneration in the knee after meniscectomy

Results of both clinical and animal studies show that meniscectomy often leads to osteoarthritic degenerative changes in articular cartilage. It is generally assumed that this process of cartilage degeneration is due to changes in mechanical loading after meniscectomy. It is, however, not known why and where this cartilage degeneration starts. Load induced cartilage damage is characterized as either type (1)--damage without disruption of the underlying bone or calcified cartilage layer--or type (2), subchondral fracture with or without damage to the overlying cartilage. We asked the question whether cartilage degeneration after meniscectomy is likely to be initiated by type (1) and/or type (2) cartilage damage. To investigate that we applied an axisymmetric biphasic finite element analysis model of the knee joint. In this model the articular cartilage layers of the tibial and the femoral condyles, the meniscus and the bone underlying the articular cartilage of the tibia plateau were included. The model was validated with data from clinical studies, in which the effects of meniscectomy on contact areas and pressures were measured. It was found that both the maximal values and the distributions of the shear stress in the articular cartilage changed after meniscectomy, and that these changes could lead to both type (1) and type (2) cartilage damage. Hence it likely that the cartilage degeneration seen after meniscectomy is initiated by both type (1) and type (2) cartilage damage.     

Quantitative proteomics at different depths in human articular cartilage reveals unique patterns of protein distribution

The articular cartilage of synovial joints ensures friction-free mobility and attenuates mechanical impact on the joint during movement. These functions are mediated by the complex network of extracellular molecules characteristic for articular cartilage. Zonal differences in the extracellular matrix (ECM) are well recognized. However, knowledge about the precise molecular composition in the different zones remains limited. In the present study, we investigated the distribution of ECM molecules along the surface-to-bone axis, using quantitative non-targeted as well as targeted proteomics.\In a discovery approach, iTRAQ mass spectrometry was used to identify all extractable ECM proteins in the different layers of a human lateral tibial plateau full thickness cartilage sample. A targeted MRM mass spectrometry approach was then applied to verify these findings and to extend the analysis to four medial tibial plateau samples.In the lateral tibial plateau sample, the unique distribution patterns of 70 ECM proteins were identified, revealing groups of proteins with a preferential distribution to the superficial, intermediate or deep regions of articular cartilage. The detailed analysis of selected 29 proteins confirmed these findings and revealed similar distribution patterns in the four medial tibial plateau samples.The results of this study allow, for the first time, an overview of the zonal distribution of a broad range of cartilage ECM proteins and open up further investigations of the functional roles of matrix proteins in the different zones of articular cartilage in health and disease.     

Dynamic contact mechanics on the tibial plateau of the human knee during activities of daily living

Despite significant advances in scaffold design, manufacture, and development, it remains unclear what forces these scaffolds must withstand when implanted into the heavily loaded environment of the knee joint. The objective of this study was to fully quantify the dynamic contact mechanics across the tibial plateau of the human knee joint during gait and stair climbing. Our model consisted of a modified Stanmore knee simulator (to apply multi-directional dynamic forces), a two-camera motion capture system (to record joint kinematics), an electronic sensor (to record contact stresses on the tibial plateau), and a suite of post-processing algorithms. During gait, peak contact stresses on the medial plateau occurred in areas of cartilage–cartilage contact; while during stair climb, peak contact stresses were located in the posterior aspect of the plateau, under the meniscus. On the lateral plateau, during gait and in early stair-climb, peak contact stresses occurred under the meniscus, while in late stair-climb, peak contact stresses were experienced in the zone of cartilage–cartilage contact. At 45% of the gait cycle, and 20% and 48% of the stair-climb cycle, peak stresses were simultaneously experienced on both the medial and lateral compartment, suggesting that these phases of loading warrant particular consideration in any simulation intended to evaluate scaffold performance. Our study suggests that in order to design a scaffold capable of restoring ‘normal’ contact mechanics to the injured knees, the mechanics of the intended site of implantation should be taken into account in any pre-clinical testing regime.     

Regional differences of tibial and femoral cartilage in the chondrocyte gene expression,immunhistochemistry and composite in different stages of osteoarthritis

The function of articular cartilage as an avascular tissue is mainly served by collagen type II and proteoglycan molecules. Within this matrix homeostasis between production and breakdown of the matrix is exceptionally sensitive.The current study was conducted to identify regional differences in specific alterations in cartilage composition during the osteoarthritic process of the human knee joint. Therefor the changes in the expression of the key molecules of the extracellular matrix were measured in dependence of the anatomical side (femoral vs tibial) and associated with immunohistochemistry and quantitative measurement.60 serial osteochondral femoral condyle and the tibial plateau samples of patients undergoing implantation of total knee endoprosthesis of areas showing mild (Group A, macroscopically ICRS grade 1b) respectively advanced (Group B, macroscopically ICRS grade 3a/3b) (30 each) osteoarthritis according to the histological-histochemical grading system (HHGS) were compared with 20 healthy biopsies with immunohistochemistry and histology. We quantified our results on the gene expression of collagen type I and II and aggrecan with the help of real-time (RT)-PCR. Proteoglycan content was measured colorometrically.In group A slightly increased colour intensity was found for collagen II in deeper layers, suggesting a persisting but initially still intact repair process. But especially on the medial tibia plateau the initial Col II increase in gene expression is followed by a decrease leading to the lowest over all Col II expression on the medial plateau, here especially in the central part. There in late stage diseases the collagen type I expression was also more pronounced. Markedly decreased safranin O staining intensity was observed in the radial zone and less reduced intensity in the transitional zone with loss of zonal anatomy in 40% of the specimens in group A and all specimens in group B. Correlation between colorometrically analysed proteoglycan GAG content and aggrecan Real Time PCR is mainly weak.Tibial and femoral cartilage in contrast to patellar cartilage both are preferential exposed to compressive stresses, but presence of menisci affects the load distribution at the tibial side, which creates varying conditions for the different cartilage surfaces in the knee.As directly measured Poissońs ratio in tibial cartilage is higher but Youn?s modulus is lower than in femoral cartilage, different resulting feedback amplification loops interact with proceeding cartilage damage. The initial loss of aggrecan may support Matrix metalloproteinases (Mmps) in the access to the collagen network and the considerably differing mechanical properties at both joint surfaces result in varying increased synthesis and release of matrix degrading enzymes.The present study has identified a selection of events which reflect the response of cartilage structure and composite, chondrocytes itself and their productivity to changes in mechanical stress depending on the anatomical site.     

Regional assessment of articular cartilage gene expression and small proteoglycan metabolism in an animal model of osteoarthritis

Osteoarthritis (OA), the commonest form of arthritis and a major cause of morbidity, is characterized by progressive degeneration of the articular cartilage. Along with increased production and activation of degradative enzymes, altered synthesis of cartilage matrix molecules and growth factors by resident chondrocytes is believed to play a central role in this pathological process. We used an ovine meniscectomy model of OA to evaluate changes in chondrocyte expression of types I, II and III collagen; aggrecan; the small leucine-rich proteoglycans (SLRPs) biglycan, decorin, lumican and fibromodulin; transforming growth factor-β; and connective tissue growth factor. Changes were evaluated separately in the medial and lateral tibial plateaux, and were confirmed for selected molecules using immunohistochemistry and Western blotting. Significant changes in mRNA levels were confined to the lateral compartment, where active cartilage degeneration was observed. In this region there was significant upregulation in expession of types I, II and III collagen, aggrecan, biglycan and lumican, concomitant with downregulation of decorin and connective tissue growth factor. The increases in type I and III collagen mRNA were accompanied by increased immunostaining for these proteins in cartilage. The upregulated lumican expression in degenerative cartilage was associated with increased lumican core protein deficient in keratan sulphate side-chains. Furthermore, there was evidence of significant fragmentation of SLRPs in both normal and arthritic tissue, with specific catabolites of biglycan and fibromodulin identified only in the cartilage from meniscectomized joints. This study highlights the focal nature of the degenerative changes that occur in OA cartilage and suggests that altered synthesis and proteolysis of SLRPs may play an important role in cartilage destruction in arthritis.     

Effect of superficial collagen patterns and fibrillation of femoral articular cartilage on knee joint mechanics-a 3D finite element analysis

Collagen fibrils of articular cartilage have specific depth-dependent orientations and the fibrils bend in the cartilage surface to exhibit split-lines. Fibrillation of superficial collagen takes place in osteoarthritis. We aimed to investigate the effect of superficial collagen fibril patterns and collagen fibrillation of cartilage on stresses and strains within a knee joint. A 3D finite element model of a knee joint with cartilage and menisci was constructed based on magnetic resonance imaging. The fibril-reinforced poroviscoelastic material properties with depth-dependent collagen orientations and split-line patterns were included in the model. The effects of joint loading on stresses and strains in cartilage with various split-line patterns and medial collagen fibrillation were simulated under axial impact loading of 1000 N. In the model, the collagen fibrils resisted strains along the split-line directions. This increased also stresses along the split-lines. On the contrary, contact and pore pressures were not affected by split-line patterns. Simulated medial osteoarthritis increased tissue strains in both medial and lateral femoral condyles, and contact and pore pressures in the lateral femoral condyle. This study highlights the importance of the collagen fibril organization, especially that indicated by split-line patterns, for the weight-bearing properties of articular cartilage. Osteoarthritic changes of cartilage in the medial femoral condyle created a possible failure point in the lateral femoral condyle. This study provides further evidence on the importance of the collagen fibril organization for the optimal function of articular cartilage.     

Proteoglycan 4 downregulation in a sheep meniscectomy model of early osteoarthritis

Osteoarthritis is a disease of multifactorial aetiology characterised by progressive breakdown of articular cartilage. In the early stages of the disease, changes become apparent in the superficial zone of articular cartilage, including fibrillation and fissuring. Normally, a monolayer of lubricating molecules is adsorbed on the surface of cartilage and contributes to the minimal friction and wear properties of synovial joints. Proteoglycan 4 is the lubricating glycoprotein believed to be primarily responsible for this boundary lubrication. Here we have used an established ovine meniscectomy model of osteoarthritis, in which typical degenerative changes are observed in the operated knee joints at three months after surgery, to evaluate alterations in proteoglycan 4 expression and localisation in the early phases of the disease. In normal control joints, proteoglycan 4 was immunolocalised in the superficial zone of cartilage, particularly in those regions of the knee joint covered by a meniscus. After the onset of early osteoarthritis, we demonstrated a loss of cellular proteoglycan 4 immunostaining in degenerative articular cartilage, accompanied by a significant (p < 0.01) decrease in corresponding mRNA levels. Early loss of proteoglycan 4 from the cartilage surface in association with a decrease in its expression by superficial-zone chondrocytes might have a role in the pathogenesis of osteoarthritis.     

Long-term results of deep defects in articular cartilage. A scanning electron microscope study.

Core defects produced in the medial femoral condyle of the rabbit were studied by scanning electron microscopy and light microscopy over a period of 2 years. In some cases the defect was filled by hyaline articular cartilage with a fairly smooth surface, but in others the tissue was markedly fibrillated and resembled fibrous tissue and fibrocartilage. Appearances suggesting disintegration of the newly formed cartilage were seen in some cases. It would appear that a continuation of this process can lead to the exposure of subchondral bone. In one instance no repair tissue or new cartilage could be identified but the surrounding old cartilage had formed a shelf over the defect. The cartilage surrounding the defect was either normal or showed superficial fibrillation, and/or flow formation, and/or fissures.     

Pathology of the Calcified Zone of Articular Cartilage in Post-Traumatic Osteoarthritis in Rat Knees

Objectives

This study aimed to investigate the pathology occurring at the calcified zone of articular cartilage (CZC) in the joints afflicted with post-traumatic osteoarthritis (PTOA).

Methods

Rats underwent bilateral anterior cruciate ligament (ACL) transection and medial meniscectomy to induce PTOA. Sham surgery was performed on another five rats to serve as controls. The rats were euthanized after four weeks of surgery and tibial plateaus were dissected for histology. The pathology of PTOA, CZC area and the tidemark roughness at six pre-defined locations on the tibial plateaus were quantified by histomorphometry.

Results

PTOA developed in the knees, generally more severe at the medial plateau than the lateral plateau, of rats in the experimental group. The CZC area was unchanged in the PTOA joints, but the topographic variations of CZC areas that presented in the control knees were reduced in the PTOA joints. The tidemark roughness decreased in areas of the medial plateau of PTOA joints and that was inversely correlated with the Mankin’s score of PTOA pathology.

Conclusion

Reduced tidemark roughness and unchanged CZC area differentiate PTOA from primary osteoarthritis, which is generally believed to have the opposite pathology at CZC, and may contribute to the distinct disease progression of the two entities of arthropathy.     

Extent and distribution of tibial osteochondral disruption during simulated landing impact with axial tibial rotation restraint

Post-traumatic knee osteochondral injuries are often coupled with anterior cruciate ligament (ACL) injury mechanisms during landing. However, it is not well understood whether restraining axial tibial rotation during landing would influence the extent and distribution of osteochondral disruption. Using ski landing as an example, this study subjected knee specimens to simulated landing impact without and with axial tibial rotation restraint, and investigated the extent and distribution of osteochondral disruption at the tibial plateau. Twenty-one porcine knee specimens were randomly divided into three test conditions, namely: (1) control, (2) impact only (I), and 3) impact with restraint (IR). Simulated landing impact was applied to the specimens based on a single 10 Hz haversine. Osteochondral explants were obtained from anterior, middle and posterior regions of medial and lateral tibial compartments. The extent of cartilage and trabecular disruption in these explants was examined based on histology, SEM and microCT. Only specimens in unrestrained condition incurred ACL failure upon impact. Restraining axial tibial rotation during simulated impact generally inflicted cartilage damage and deformation, and further caused trabecular disruption. Axial tibial rotation restraint did not necessarily restrict anterior tibial translation, as indicated by the presence of relative posterior femoral translation and osteochondral disruption at anterior–posterior tibial regions. While the results obtained in the current study may not be completely translatable to human models, there is likelihood that restraining axial tibial rotation during landing may help to prevent ACL failure, but will also induce osteochondral disruption in most tibial regions.     

Proteoglycans contribute locally to swelling,but globally to compressive mechanics,in intact cervine medial meniscus

Loss of charged proteoglycans in the knee meniscus, which aid in the support of compressive loads by entraining water, is an effect of degeneration and is often associated with osteoarthritis. In healthy menisci, proteoglycan content is highest in the inner white zone and decreases towards the peripheral red zone. We hypothesized that loss of proteoglycans would reduce both osmotic swelling and compressive stiffness, spatially localized to the avascular white zone of the meniscus. This hypothesis was tested by targeted enzymatic digestion of proteoglycans using hyaluronidase in intact cervine medial menisci. Mechanics were quantified by creep indentation on the femoral surface. Osmotic swelling changes were assessed by measuring collagen fiber crimp period in the radial-axial plane in the lamellar layer along both the tibial and femoral contacting surfaces. All measurements were made in the inner, middle, and outer zones of the anterior, central, and posterior regions. Mechanical measurements showed variation in creep behavior with anatomical location, along with spatially uniform decreases in viscosity (average of 21%) and creep stiffness (average of 15%) with hyaluronidase treatment. Lamellar collagen crimp period was significantly decreased (average of 27%) by hyaluronidase, indicating a decrease in osmotic swelling, with the largest decreases seen in locations with the highest proteoglycan content. Taken together, these results suggest that while proteoglycans have localized effects on meniscus swelling, the resulting effect on compressive properties is distributed throughout the tissue.     

Knee meniscal extrusion in a largely non-osteoarthritic cohort: association with greater loss of cartilage volume

We conducted a longitudinal study (duration 2 years), including 294 individuals (mean age 45 years, 58% female), in order to examine associations between meniscal extrusion, knee structure, radiographic changes and risk factors for osteoarthritis (OA) in a largely non-osteoarthritic cohort. Meniscal extrusion, tibiofemoral cartilage defect score and cartilage volume, and tibial plateau bone area were determined using T1-weighted fat-saturated magnetic resonance imaging. At baseline the presence of medial meniscal extrusion was significantly associated with body mass index (odds ratio [OR] per kg/m2 = 1.13, 95% confidence interval [CI] = 1.02-1.25), past knee injury (positive versus negative history: OR = 3.73, 95% CI = 1.16-11.97), medial tibial bone area (OR per cm2 = 1.37, 95% CI = 1.02-1.85), and osteophytes (OR per grade = 4.89, 95% CI = 1.59-15.02). Two-year longitudinal data revealed that medial meniscal extrusion at baseline was associated with a greater rate of loss of medial tibiofemoral cartilage volume (extrusion versus no extrusion: -1.4%/year; P < 0.05) and greater risk for increased medial femoral cartilage defects (OR = 2.59, 95% CI = 1.14-5.86) and lateral tibial cartilage defects (OR = 2.64, 95% CI = 1.03-6.76). However, the latter two associations became nonsignificant after adjustment for tibial bone area and osteophytes. This study suggests that increasing body mass index and bone size, past knee injury, and osteophytes may be causally related to meniscal extrusion. Most importantly, meniscal extrusion at baseline is associated with greater loss of knee cartilage over 2 years, and this seems to be mediated mostly by subchondral bone changes, suggesting extrusion represents one pathway between bone expansion and cartilage loss.     

骨关节炎软骨细胞发生内质网应激

目的：研究骨关节炎软骨细胞是否发生内质网应激现象。方法：对关节置换术后的人类骨关节炎软骨标本和正常关节软骨标本切片进行内质网应激标志分子免疫球蛋白重链结合蛋白（BiP）的免疫组织化学检测;对小鼠膝关节进行半月板切断术诱发实验性骨关节炎,在术后1、3和6周取材,对组织切片进行番红花“O”染色、Mankin评分及BiP的免疫组织化学检测。结果：所有人类骨关节炎标本中软骨细胞BiP的表达明显升高。番红花“O”染色结果表明,在小鼠骨关节炎模型中,全部手术侧关节表面发生磨损,且随着术后时间延长关节表面磨损范围逐步扩大,手术侧Mankn分值显著高于对照侧;此外,手术侧的软骨细胞内BiP呈阳性表达,且表达量随术后时间延长而增加。结论：在人类骨关节炎标本和实验性小鼠骨关节炎模型中,关节软骨细胞均发生明显的内质网应激现象。     

Comparison of nonlinear mechanical properties of bovine articular cartilage and meniscus

Nonlinear, linear and failure properties of articular cartilage and meniscus in opposing contact surfaces are poorly known in tension. Relationships between the tensile properties of articular cartilage and meniscus in contact with each other within knee joints are also not known. In the present study, rectangular samples were prepared from the superficial lateral femoral condyle cartilage and lateral meniscus of bovine knee joints. Tensile tests were carried out with a loading rate of 5 mm/min until the tissue rupture. Nonlinear properties of the toe region, linear properties in larger strains, and failure properties of both tissues were analysed. The strain-dependent tensile modulus of the toe region, Young's modulus of the linear region, ultimate tensile stress and toughness were on average 98.2, 8.3, 4.0 and 1.9 times greater (p<0.05) for meniscus than for articular cartilage. In contrast, the toe region strain, yield strain and failure strain were on average 9.4, 3.1 and 2.3 times greater (p<0.05) for cartilage than for meniscus. There was a significant negative correlation between the strain-dependent tensile moduli of meniscus and articular cartilage samples within the same joints (r=−0.690, p=0.014). In conclusion, the meniscus possesses higher nonlinear and linear elastic stiffness and energy absorption capability before rupture than contacting articular cartilage, while cartilage has longer nonlinear region and can withstand greater strains before failure. These findings point out different load carrying demands that both articular cartilage and meniscus have to fulfil during normal physiological loading activities of knee joints.     

Direct contribution of axial impact compressive load to anterior tibial load during simulated ski landing impact

Anterior tibial loading is a major factor involved in the anterior cruciate ligament (ACL) injury mechanism during ski impact landing. We sought to investigate the direct contribution of axial impact compressive load to anterior tibial load during simulated ski landing impact of intact knee joints without quadriceps activation. Twelve porcine knee specimens were procured. Four specimens were used as non-impact control while the remaining eight were mounted onto a material-testing system at 70° flexion and subjected to simulated landing impact, which was successively repeated with incremental actuator displacement. Four specimens from the impacted group underwent pre-impact MRI for tibial plateau angle measurements while the other four were subjected to histology and microCT for cartilage morphology and volume assessment. The tibial plateau angles ranged from 29.4 to 38.8°. There was a moderate linear relationship (Y=0.16X; R²=0.64; p<0.001) between peak axial impact compressive load (Y) and peak anterior tibial load (X). The anterior and posterior regions in the impacted group sustained surface cartilage fraying, superficial clefts and tidemark disruption, compared to the control group. MicroCT scans displayed visible cartilage deformation for both anterior and posterior regions in the impacted group. Due to the tibial plateau angle, increased axial impact compressive load can directly elevate anterior tibial load and hence contribute to ACL failure during simulated landing impact. Axial impact compressive load resulted in shear cartilage damage along anterior–posterior tibial plateau regions, due to its contribution to anterior tibial loading. This mechanism plays an important role in elevating ACL stress and cartilage deformation during impact landing.