[Biomechanical properties (compressive strength and compressive pressure at break) of hyaline cartilage under axial load]

Bioinspired Bottlebrush Polymers for Aqueous Boundary Lubrication

An extremely efficient lubrication system is achieved in synovial joints by means of bio-lubricants and sophisticated nanostructured surfaces that work together. Molecular bottlebrush structures play crucial roles for this superior tribosystem. For example, lubricin is an important bio-lubricant, and aggrecan associated with hyaluronan is important for the mechanical response of cartilage. Inspired by nature, synthetic bottlebrush polymers have been developed and excellent aqueous boundary lubrication has been achieved. In this review, we summarize recent experimental investigations of the interfacial lubrication properties of surfaces coated with bottlebrush bio-lubricants and bioinspired bottlebrush polymers. We also discuss recent advances in understanding intermolecular synergy in aqueous lubrication including natural and synthetic polymers. Finally, opportunities and challenges in developing efficient aqueous boundary lubrication systems are outlined.

Phospholipids and Hyaluronan: From Molecular Interactions to Nano- and Macroscale Friction

Phospholipids and hyaluronan are two key biomolecules that contribute to the excellent lubrication of articular joints. Phospholipids alone and in combination with hyaluronan have also displayed low friction forces on smooth surfaces in micro- and nanosized tribological contacts. In an effort to develop aqueous-based lubrication systems, it is highly relevant to explore if these types of molecules also are able to provide efficient lubrication of macroscopic tribological contacts involving surfaces with roughness larger than the thickness of the lubricating layer. To this end, we investigated the lubrication performance of hyaluronan, the phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and mixtures of these two components using glass surfaces in a mini-traction machine. We compared our data with those obtained using flat silica surfaces in previous atomic force microscopy studies, and we also highlighted insights on hyaluronan–phospholipid interactions gained from recent simulations. Our data demonstrate that hyaluronan alone does not provide any lubricating benefit, but DPPC alone and in mixtures with hyaluronan reduces the friction force by an order of magnitude.

Effects of focal metallic implants on opposing cartilage - an in-vitro study with an abrasion test machine

Background

For focal cartilage defects, biological repair might be ineffective in patients over 45 years. A focal metallic implant (FMI) (Hemi-CAP Arthrosurface Inc., Franklin, MA, USA) was designed to reduce symptoms. The aim of this study was to evaluate the effects of a FMI on the opposing tibial cartilage in a biomechanical set-up. It is hypothesized that a FMI would not damage the opposing cartilage under physiological loading conditions.

Methods

An abrasion machine was used to test the effects of cyclic loading on osteochondral plugs. The machine applied a compressive load of 33 N and sheared the samples 10 mm in the anteroposterior direction by 1 Hz. Tibial osteochondral plugs from porcine knees were placed in opposition to a FMI and cycled for 1 or 6 h. After testing each plug was fixed, stained and evaluated for cartilage damage.

Results

After 1 h of loading (n = 6), none of the osteochondral plugs showed histologic signs of degradation. After 6 h of loading (n = 6) three samples had histologic signs of injury in the tangential zone (grade 1) and one had signs of injury in the transitional and deep zones (grade 2). Exploration for 6 h resulted in significant more cartilage damage compared to the shorter exploration time (p = 0.06). However, no significant difference between saline and hyaluronic acid was evident (p = 0.55).

Conclusion

Under physiologic loading conditions, contact with a FMI leads to cartilage damage in the opposing articular cartilage in six hours. In clinical practice, a thorough analysis of pre-existing defects on the opposing cartilage is recommended when FMI is considered.

Biolubrication synergy: Hyaluronan - Phospholipid interactions at interfaces

The manner in which nature has solved lubrication issues has fascinated scientists for centuries, in particular when considering that lubrication is achieved in aqueous media. The most outstanding system in this respect is likely the synovial joint, where close to frictionless motion is realized under different loads and shear rates. This review article focuses on two components present in the synovial area, hyaluronan and phospholipids. We recapitulate what has been learned about their interactions at interfaces from recent experiments, with focus on results obtained using reflectivity techniques at large scale facilities. In parallel, modelling experiments have been carried out and from these efforts new detailed knowledge about how hyaluronan and phospholipids interact has been gained. In this review we combine findings from modelling and experiments to gain deeper insight. Finally, we summarize what has been learned of the lubrication performance of mixtures of phospholipids and hyaluronan.

Influence of Sutures on Cartilage Integrity: Do Meniscus Sutures Harm Cartilage? An Experimental Animal Study

PURPOSE

To evaluate whether different suture materials in meniscal repair may harm cartilage.

METHODS

A preloaded linear friction testing setup including porcine knees with porcine cartilage, porcine meniscus, and different suture materials (braided nonabsorbable, absorbable monofilament) was used. Five groups with different tribological pairs were tested: cartilage on meniscus (control), cartilage on cartilage (control No. 2), and cartilage on different meniscus sutures (3 groups). Cartilage integrity was analyzed macroscopically by the India ink method and histologically using Giemsa-eosin-stained undecalcified methyl methacrylate sections. Cartilage lesions were classified by using a quantitative scoring system.

RESULTS

The control groups did not show cartilage damage, either macroscopically or histologically. Loading cartilage with sutured menisci led to significant damage of the superficial radial and transitional zones with braided nonabsorbable (P = .03) and absorbable monofilament (P = .02) sutures at final examination. Menisci sutured with braided nonabsorbable material resulted in deeper damage to the cartilage. However, there were no significant differences between the suture materials. Sutures oriented perpendicular to surface motion led to a larger defect than parallel-oriented sutures.

CONCLUSIONS

Braided nonabsorbable and absorbable monofilament suture materials cause significant damage to cartilage during long-term cyclic loading in vitro. The extent of damage depends on suture orientation.

CLINICAL RELEVANCE

This study provides data on the extent to which different suture materials in meniscus repair may harm cartilage.

Modeling underwater hearing and sound localization in the frog Xenopus laevis

Animals that are small compared to sound wavelengths face the challenge of localizing a sound source since the main cues to sound direction-interaural time differences (ITD) and interaural level differences (ILD)-both depend on size. Remarkably, the majority of terrestrial vertebrates possess internally coupled ears (ICE) with an air-filled cavity connecting the two eardrums and producing an inherently directional middle-ear system. Underwater, longer wavelengths and faster sound-speed reduce both ITD and ILD cues. Nonetheless, many animals communicate through and localize underwater sound. Here, a typical representative equipped with ICE is studied: the fully aquatic clawed frog Xenopus laevis. It is shown that two factors improve underwater sound-localization quality. First, inflated lungs function as Helmholtz resonator and generate directional amplitude differences between eardrum vibrations in the high-frequency (1.7-2.2 kHz) and low-frequency (0.8-1.2 kHz) range of the male advertisement calls. Though the externally arriving ILDs practically vanish, the perceived internal level differences are appreciable, more than 10 dB. As opposed to, e.g., lizards with thin and flexible eardrums, plate-like eardrums are shown to be Xenopus' second key to successfully handling aquatic surroundings. Based on ICE, both plate-like eardrums and inflated lungs functioning as Helmholtz resonators explain the phonotaxis performance of Xenopus.

Synergies in lubrication

In living organisms the aqueous medium is used for providing low friction forces. This is achieved by synergistic actions of different biomolecules that together accomplish a high load bearing capacity and sustain an easily sheared water layer.

Effect of porosities of bilayered porous scaffolds on spontaneous osteochondral repair in cartilage tissue engineering

Poly(lactide-co-glycolide)-bilayered scaffolds with the same porosity or different ones on the two layers were fabricated, and the porosity effect on in vivo repairing of the osteochondral defect was examined in a comparative way for the first time. The constructs of scaffolds and bone marrow-derived mesenchymal stem cells were implanted into pre-created osteochondral defects in the femoral condyle of New Zealand white rabbits. After 12 weeks, all experimental groups exhibited good cartilage repairing according to macroscopic appearance, cross-section view, haematoxylin and eosin staining, toluidine blue staining, immunohistochemical staining and real-time polymerase chain reaction of characteristic genes. The group of 92% porosity in the cartilage layer and 77% porosity in the bone layer resulted in the best efficacy, which was understood by more biomechanical mimicking of the natural cartilage and subchondral bone. This study illustrates unambiguously that cartilage tissue engineering allows for a wide range of scaffold porosity, yet some porosity group is optimal. It is also revealed that the biomechanical matching with the natural composite tissue should be taken into consideration in the design of practical biomaterials, which is especially important for porosities of a multi-compartment scaffold concerning connected tissues.

The effect of radial head implant length on radiocapitellar articular properties and load transfer within the forearm

BACKGROUND

The effect of radial head implant length on forearm biomechanics is not well understood. This study examined the influence of an increase or a decrease in radial head implant length on forearm load transfer as measured by interosseous membrane (IOM) tension and changes in radiocapitellar joint contact properties.

METHODS

An upper extremity simulator was used to examine 6 cadaveric specimens with 5 different radial head implant lengths (-4 mm, -2 mm, anatomically correct, +2 mm, and +4 mm). A load-sensing device was woven into the fibers of IOM to quantify its tension. An interpositional pressure measurement sensor was used to determine radiocapitellar joint contact area and pressure. Axial loads of 160 N were applied to the forearm through active pronation and supination with the elbow fixed at 90 degrees of flexion.

RESULTS

Increasing radial head implant length by 4 mm unloaded the IOM in all cases. Decreasing implant length by 4 mm significantly increased the IOM tension (P = 0.005). No significant differences were found in IOM tension between the correct head implant length and the -2 mm implant (P = 0.29). Contact pressure significantly increased with increasing radial head implant length (P = 0.021) and contact area diminished with both an increase and a decrease in radial head implant length, but this was not statistically significant (P = 0.051).

CONCLUSIONS

Increasing radial head implant length decreased IOM tension and increased radiocapitellar joint contact pressure.

CLINICAL RELEVANCE

These findings illustrate the importance of precise restoration of radial length when performing a radial head replacement. If the native radial head length is difficult to accurately assess, avoid increasing the length of the radial head to prevent detrimental changes in the biomechanics of the forearm and the potential for clinically important radiocapitellar joint pathology.

Cartilage labelling for mechanical testing in T-peel configuration

PURPOSE

The purpose of this study was to find a suitable method of labelling cartilage samples for the measurement of distraction distances in biomechanical testing.

METHODS

Samples of bovine cartilage were labelled using five different methods: hydroquinone and silver nitrate (AgNO3), potassium permanganate (KMnO4) with sodium thiosulphate (Na2S2O3), India ink, heat, and laser energy. After the labelling, we analysed the cartilage samples with regard to cytotoxity by histochemical staining with ethidiumbromide homodimer (EthD-1) and calcein AM. Furthermore, we tested cartilages labelled with India ink and heat in a T-peel test configuration to analyse possible changes in the mechanical behaviour between marked and unlabelled samples.

RESULTS

Only the labelling methods with Indian ink or a heated needle showed acceptable results in the cytotoxity test with regard to labelling persistence, accuracy, and the influence on consistency and viability of the chondrocytes. In the biomechanical T-peel configuration, heat-labelled samples collapsed significantly earlier than unlabelled samples.

CONCLUSION

Labelling bovine cartilage samples with Indian ink in biomechanical testing is a reliable, accurate, inexpensive, and easy-to-perform method. This labelling method influenced neither the biomechanical behaviour nor the viability of the tissue compared to untreated bovine cartilage.

The evaluation of a biphasic osteochondral implant coupled with an electrospun membrane in a large animal model

Conventional clinical therapies are unable to resolve osteochondral defects adequately; hence, tissue engineering solutions are sought to address the challenge. A biphasic implant that was seeded with mesenchymal stem cells (MSCs) and coupled with an electrospun membrane was evaluated as an alternative. This dual phase construct comprised of a polycaprolactone (PCL) cartilage scaffold and a PCL-tricalcium phosphate osseous matrix. Autologous MSCs were seeded into the entire implant via fibrin and the construct was inserted into critically sized osteochondral defects located at the medial condyle and patellar groove of pigs. The defect was resurfaced with a PCL-collagen electrospun mesh, which served as a substitute for periosteal flap in preventing cell leakage. Controls without either implanted MSCs or resurfacing membrane were included. After 6 months, cartilaginous repair was observed with a low occurrence of fibrocartilage at the medial condyle. Osteochondral repair was promoted and host cartilage degeneration was arrested as shown by superior glycosaminoglycan maintenance. This positive morphological outcome was supported by a higher relative Young's modulus, which indicated functional cartilage restoration. Bone ingrowth and remodeling occurred in all groups, with a higher degree of mineralization in the experimental group. Tissue repair was compromised in the absence of the implanted cells or the resurfacing membrane. Moreover, healing was inferior at the patellar groove when compared with the medial condyle and this was attributed to the native biomechanical features.

Mechanical behavior of intact and low-grade degenerated cartilage / Mechanische Eigenschaften von intaktem und niedriggradig geschädigtem Knorpel

Young's modulus, elastic and plastic deformation, mechanical hardness and load at failure were determined for low-grade degenerated hyaline cartilage in a porcine model. Osteochondral plugs from the medial condyle of 30 female pigs were used. Cartilage defects were classified using the International Cartilage Repair Society (ICRS) protocol. Mechanical hardness was measured using a Shore A testing device. Total stiffness and plastic deformation was evaluated in the range 50-200 N using a 5-mm indenter. The load at failure was then determined. ICRS grade I specimens showed significantly lower stiffness than grade 0 specimens. ICRS grade 0 specimen showed no significant plastic deformation within the load range 25-100 N. In degenerated cartilage, plastic deformation started at a significantly lower load (50 N). The Young's modulus at 25 N in ICRS grade 0 specimens (18.8 MPa) was significantly higher than in grade I (11.1 MPa) or grade II (10.5 MPa) specimens. Intact cartilage showed significantly higher tension at failure and mechanical Shore A hardness. Young's modulus and tension at failure showed strong correlation. Cartilage degeneration is associated with a significant loss of elasticity and mechanical stress resistance. Shore hardness measurement is an adequate method for rapid biomechanical evaluation of cartilage specimens.