Resilience to height loss of articular cartilage of osteoarthritic stifle joints of old pigs, compared with healthy cartilage from young pigs in a tribological pin-on-plate exposure, revealing similar friction forces

Introduction

We saw a lack of data on the biomechanical behavior of degenerated articular cartilage (OA) compared with that of healthy cartilage, even though the susceptibility to wear and tear of articular cartilage plays a key role in the progression of osteoarthritis (OA). Therefore, we performed a comparison between naturally occurring OA and healthy cartilage from pigs, before and after tribological stress.

Aim

The aim of the study was to compare OA-cartilage with healthy cartilage and to analyze the resilience to tribological shear stress, which will be measured as height loss (HL), and to friction forces of the cartilage layers. The findings will be substantiated in macro- and microscopical evaluations before and after tribological exposure.

Methods

We assessed stifle joints of fifteen old and sixteen young pigs from the local abattoir radiologically, macroscopically and histologically to determine possible OA alterations. We put pins from the femoral part of the joints and plates from the corresponding tibial plateaus in a pin-on-plate tribometer under stress for about two hours with about 1108 reciprocating cycles under a pressure of approximately 1 MPa. As a surrogate criterion of wear and tear, the HL was recorded in the tribometer. The heights of the cartilage layers measured before and after the tribological exposure were compared histologically. The condition of the cartilage before and after the tribological exposure was analyzed both macroscopically with an adapted ICRS score and microscopically according to Little et al. (2010). We assessed the friction forces acting between the surfaces of the cartilage pair–specimens.

Results

Articular cartilage taken from old pigs showed significant degenerative changes compared to that taken from the young animals. The macroscopic and microscopic scores showed strong alterations of the cartilage after the tribological exposure. There was a noticeable HL of the cartilage specimens after the first 100 to 300 cycles. The HL after tribological exposure was lower in the group of the old animals with 0.52 mm ± 0.23 mm than in the group of the young animals with 0.86 mm ± 0.26 mm (p < 0.0001). The data for the HL was validated by the histological height measurements with 0.50 mm ± 0.82 mm for the old and 0.79 mm ±0.53 mm for the young animals (p = 0.133). The friction forces measured at the cartilage of the old animals were 2.25 N ± 1.15 N and 1.89 N ± 1.45 N of the young animals (p = 0.3225).

Conclusion

Unlike articular cartilage from young pigs, articular cartilage from old pigs showed OA alterations. Tribological shear stress exposure revealed that OA cartilage showed less HL than healthy articular cartilage. Tribological stress exposure in a pin–on–plate tribometer seemed to be an appropriate way to analyze the mechanical stability of articular cartilage, and the applied protocol could reveal weaknesses of the assessed cartilage tissue. Friction and HL seemed to be independent parameters when degenerated and healthy articular cartilage were assessed under tribological exposure in a pin–on- plate tribometer.

Report on a large animal study with Göttingen Minipigs where regenerates and controls for articular cartilage were created in a large number. Focus on the conditions of the operated stifle joints and suggestions for standardized procedures

The characterization of regenerated articular cartilage (AC) can be based on various methods, as there is an unambiguous accepted criterion neither for the natural cartilage tissue nor for regenerates. Biomechanical aspects should be considered as well, leading to the need for more equivalent samples. The aim of the study was to describe a large animal model where 8 specimens of regenerated AC can be created in one animal plus the impact of two surgeries on the welfare of the animals. The usefulness of the inclusion of a group of untreated animals (NAT) was to analyzed. Based on the histological results the conditions of the regenerates were to be described and the impact on knee joints were to be explored in terms of degenerative changes of the cartilage. The usefulness of the statistical term “effect size” (ES) will be explained with histological results. We analyzed an animal model where 8 AC regenerates were obtained from one Göttingen Minipig, on both sides of the trochleae. 60 animals were divided into 6 groups of 10 each, where the partial thickness defects in the trochlea were filled with matrices made of Collagen I with or without autologous chondrocytes or left empty over the healing periods of 24 and 48 weeks. One additional control group consisting of 10 untreated animals was used to provide untouched “external” cartilage. We harvested 560 samples of regenerated tissue and “external” controls, besides that, twice the number of further samples from other parts of the joints referred to as “internal” controls were also harvested. The animals recovered faster after the 1^st operation when the defects were set compared to the 2^nd operation when the defects were treated. 9% of all animals were lost. Other complications were for example superficial infections, seroma, diarrhea, febrile state and an injury of a claw. The histological results of the treatments proved the robustness of the study design where we included an “external” control group (NAT) in which the animals were not operated. Comparable significant differences between treated groups and the NAT group were detected both after ½ year and after 1 year. Spontaneous regenerated AC as control revealed differences after an observation time of nearly 1 year. The impact of the treatment on cartilage adjacent to the defect as well as the remaining knee joint was low. The ES was helpful for planning the study as it is shown that the power of a statistical comparison seems to be more influenced by the ES than by the sample size. The ranking of the ES was done exemplarily, listing the results according to their magnitude, thus making the results comparable. We were able to follow the 3 R requirements also in terms of a numerical reduction of animals due to the introduction of a group of untreated animals. This makes the model cost effective. The presented study may contribute as an improvement of the standardization of large animal models for research and regulatory requirements for regenerative therapies of AC.

Osteoarthritis in the Knee Joints of Göttingen Minipigs after Resection of the Anterior Cruciate Ligament? Missing Correlation of MRI, Gene and Protein Expression with Histological Scoring

Introduction

The Göttingen Minipig (GM) is used as large animal model in articular cartilage research. The aim of the study was to introduce osteoarthritis (OA) in the GM by resecting the anterior cruciate ligament (ACLR) according to Pond and Nuki, verified by histological and magnetic resonance imaging (MRI) scoring as well as analysis of gene and protein expression.

Materials and Methods

The eight included skeletally mature female GM were assessed after ACLR in the left and a sham operation in the right knee, which served as control. 26 weeks after surgery the knee joints were scanned using a 3-Tesla high-field MR tomography unit with a 3 T CP Large Flex Coil. Standard proton-density weighted fat saturated sequences in coronal and sagittal direction with a slice thickness of 3 mm were used. The MRI scans were assessed by two radiologists according to a modified WORMS-score, the X-rays of the knee joints by two evaluators. Osteochondral plugs with a diameter of 4mm were taken for histological examination from either the main loading zone or the macroscopic most degenerated parts of the tibia plateau or condyle respectively. The histological sections were blinded and scored by three experts according to Little et al. Gene expression analysis was performed from surrounding cartilage. Expression of adamts4, adamts5, acan, col1A1, col2, il-1ß, mmp1, mmp3, mmp13, vegf was determined by qRT-PCR. Immunohistochemical staining (IH) of Col I and II was performed. IH was scored using a 4 point grading (0—no staining; 3-intense staining).

Results and Discussion

Similar signs of OA were evident both in ACLR and sham operated knee joints with the histological scoring result of the ACLR joints with 6.48 ± 5.67 points and the sham joints with 6.86 ± 5.84 points (p = 0.7953) The MRI scoring yielded 0.34 ± 0.89 points for the ACLR and 0.03 ± 0.17 for the sham knee joints. There was no correlation between the histological and MRI scores (r = 0.10021). The gene expression profiles as well as the immunohistochemical findings showed no significant differences between ACLR and sham knee joints. In conclusion, both knee joints showed histological signs of OA after 26 weeks irrespective of whether the ACL was resected or not. As MRI results did not match the histological findings, MRI was obviously unsuitable to diagnose the OA in GM. The analysis of the expression patterns of the 10 genes could not shed light on the question, whether sham operation also induced cartilage erosion or if the degeneration was spontaneous. The modified Pond-Nuki model may be used with reservation in the adult minipig to induce an isolated osteoarthritis.

A Porcine Animal Model for Early Meniscal Degeneration - Analysis of Histology, Gene Expression and Magnetic Resonance Imaging Six Months after Resection of the Anterior Cruciate Ligament

BACKGROUND/OBJECTIVE

The menisci of the mammalian knee joint balance the incongruence between femoral condyle and tibial plateau and thus menisci absorb and distribute high loads. Degeneration processes of the menisci lead to pain syndromes in the knee joint. The origin of such degenerative processes on meniscal tissue is rarely understood and may be described best as an imbalance of anabolic and catabolic metabolism. A standardized animal model of meniscal degeneration is needed for further studies. The aim of the current study was to develop a porcine animal model with early meniscal degeneration.

MATERIAL AND METHODS

Resection of the anterior cruciate ligament (ACLR) was performed on the left knee joints of eight Göttingen minipigs. A sham operation was carried out on the right knee joint. The grade of degeneration was determined 26 weeks after the operation using histology and magnetic resonance imaging (MRI). Furthermore, the expression of 14 genes which code for extracellular matrix proteins, catabolic matrix metalloproteinases and inflammation mediators were analyzed.

RESULTS

Degenerative changes were detected by a histological analysis of the medial meniscus after ACLR. These changes were not detected by MRI. In terms of their gene expression profile, these degenerated medial menisci showed a significantly increased expression of COL1A1.

CONCLUSION

This paper describes a new animal model for early secondary meniscal degeneration in the Göttingen minipig. Histopathological evidence of the degenerative changes could be described. This early degenerative changes could not be seen by NMR imaging.

Analysis of Gene Expression and Ultrastructure of Stifle Menisci from Juvenile and Adult Pigs

The origin of the age-associated degenerative processes in meniscal tissue is poorly understood and may be related to an imbalance of anabolic and catabolic metabolism. The aim of the current study was to compare medial menisci isolated from juvenile pigs and degenerated medial menisci from adult pigs in terms of gene expression profile and ultrastructure. Medial menisci were isolated from the knee joints of juvenile and adult pigs (n = 8 for each group). Degeneration was determined histologically according to a scoring system. In addition, the gene expression profiles of 14 genes encoding extracellular matrix proteins, catabolic matrix metalloproteinases and mediators of inflammation were analyzed. Changes in the ultrastructure of the collagen network of the meniscal tissue were analyzed by using transmission electron microscopy. The histologic analysis of menisci showed significantly higher grade of degeneration in tissue isolated from adult porcine knee joints compared with menisci isolated from juvenile knee joints. In particular, destruction of the collagen network was greater in adult menisci than in juvenile menisci. Degenerated menisci showed significantly decreased gene expression of COL1A1 and increased expression of MMP2, MMP13, and IL8. The menisci from adult porcine knee joints can serve as a model for meniscal degeneration. Degenerative changes were manifested as differences in histopathology, gene expression and ultrastructure of collagen network.

Instruments for reproducible setting of defects in cartilage and harvesting of osteochondral plugs for standardisation of preclinical tests for articular cartilage regeneration

Background

Standardisation is required in research, so are approval procedures for advanced therapy medical products and other procedures for articular cartilage therapies. The process of creating samples needs to be reproducible.

The aim of this study was to design, create and validate instruments (1) to create reproducible and accurate defects and (2) to isolate samples in the shape of osteochondral cylinders in a quick, reliable and sterile manner.

Methods

Adjustable instruments were created: a crown mill with a resolution of 0.05 mm and a front mill to create defects in articular cartilage and subchondral bone. The instruments were tested on knee joints of pigs from the slaughterhouse; 48 defects were created and evaluated. A punching machine was designed to harvest osteochondral plugs. These were validated in an in vivo animal study.

Results

The instruments respect the desired depth of 0.5 and 1.5 mm when creating the defects, depending on whether the person using the instrument is highly experienced (0.451 mm; confidence interval (CI): 0.390 mm; 0.512 mm and 1.403 mm; CI: 1.305 mm; 1.502 mm) or less so (0.369 mm; CI: 0.297 mm; 0.440 mm and 1.241 mm; CI: 1.141 mm; 1.341 mm). Eighty samples were taken from knee joints of Göttingen Minipigs with this punching technique. The time needed for the harvesting of the samples was 7.52 min (±2.18 min), the parallelism of the sides of the cylinders deviated by −0.63° (CI: −1.33°; 0.08°) and the surface of the cartilage deviated from the perpendicularity by 4.86° (CI: 4.154°; 5.573°). In all assessed cases, a sterile procedure was observed.

Conclusions

Instruments and procedures for standardised creation and validation of defects in articular cartilage and subchondral bone were designed. Harvesting of samples in the shape of osteochondral cylinders can now be performed in a quick, reliable and sterile manner. The presented instruments and procedures can serve as helpful steps towards standardised operating procedures in the field of regenerative therapies of articular cartilage in research and for regulatory requirements.

In vitro mapping of 1H ultrashort T2* and T2 of porcine menisci

In this study, mapping of ultrashort T2 and T2* of acutely isolated porcine menisci at B0 = 9.4 T was investigated. Maps of T2 were measured from a slice through the pars intermedia with a spin echo-prepared two-dimensional ultrashort-TE T2 mapping technique published previously. T2* mapping was performed by two-dimensional ultrashort-TE MRI with variable acquisition delay. The measured signal decays were fitted by monoexponential, biexponential and Gaussian-exponential fitting functions. The occurrence of Gaussian-like signal decays is outlined theoretically. The quality of the curve fits was visualized by mapping the value δ = abs(1 - χ(2) red). For T2 mapping, the Gaussian-exponential fit showed the best performance, whereas the monoexponential and biexponential fits showed regionally high values of δ (δ > 20). Interpretation of the Gaussian-exponential parameter maps was found to be difficult, because a Gaussian signal component can be related to mesoscopic (collagen texture) or macroscopic (slice profile, shim, sample geometry) magnetic field inhomogeneities and/or residual (1) H dipole-dipole couplings. It seems likely that an interplay of these effects yielded the observed signal decays. Modulation of the T2* signal decay caused by chemical shift was observed and addressed to fat protons by means of histology. In the T2 measurements, no modulation of the signal decay was observed and the biexponential and Gaussian-exponential fits showed the best performance with comparable values of δ. Our results suggest that T2 mapping provides the more robust method for the characterization of meniscal tissue by means of MRI relaxometry. However, mapping of ultrashort T2, as performed in this study, is time consuming and provides less signal-to-noise ratio per time than the mapping of T2*. If T2* mapping is used, pixel-wise monitoring of the fitting quality based on reduced χ(2) should be employed and great care should be taken when interpreting the parameter maps of the fits.