Nano-Scale Mechanical Properties of the Articular Cartilage Zones in a Mouse Model of Post-Traumatic Osteoarthritis  
[PERSON]  
1 Center for Applied Tissue Engineering and Regenerative Medicine, Munich University of Applied Sciences, 80533 Munich, Germany; [EMAIL_ADDRESS] (L.F.); [EMAIL_ADDRESS] (A.A.); [EMAIL_ADDRESS] (H.C.-S.)  
2 Department of Orthopaedics and Trauma Surgery, Musculoskeletal University Center Munich (MUM), University Hospital, Ludwig-Maximilians-University (LMU), 82152 Planesg, Germany; zsuzs  
date, no cure or disease-modifying drug is available for OA and therapeutic interventions aim to relieve symptoms or eliminate the problem via joint replacement [5]. Therefore, understanding cartilage physiology and pathophysiology is of paramount importance to develop better therapeutic modalities for the treatment of OA. Articular cartilage is composed of a specialized ECM and chondrocytes, a unique cell type that makes up only 5-10% of the total tissue volume. The cartilaginous ECM consists primarily of proteoglycans (mainly aggrecan) and type II collagen fibrils, which are interconnected with adaptor proteins and build networks to withstand compressive and tensile forces [6; 7]. Topically, the ECM of AC is composed of a narrow pericellular matrix (PCM) immediately surrounding the chondrocytes and the extensive territorial/inter-territorial matrix (TM/ITM), each of which contribute in a unique way to the chemical and mechanical features of the AC [8]. Based on its structural and functional properties, healthy articular cartilage can be divided into four distinct vertical zones, namely the superficial, the medial, the deep and the calcified zones. Each zone is characterized by different chondrocyte shape, ECM composition and organization, which ultimately endow the tissue with specific zonal biomechanical properties [9]. The interplay between ECM composition, biomechanics and OA is still not fully understood, although it is generally accepted that impaired mechanical properties of the articular cartilage drive its degeneration. Thus, it is of utmost importance to investigate the biomechanics of the cartilaginous ECM in the different zones during the development of spontaneous or injury-related OA. The destabilization of the medial meniscus (DMM) is a surgical model in mouse, which was developed to mimic human post-traumatic osteoarthritis (PT-OA) caused by joint instability [10]. The DMM model has been widely used to assess OA pathophysiology, but little consideration has been given to the biomechanics of AC upon DMM [11]. The fine details of the ultrastructural and mechanical properties of the AC have been mostly evaluated using the gold standard, indentation-type atomic force microscopy (IT-AFM) [12]. Utilizing a nanometer-sized tip, it is possible to distinguish between the stiffness of collagen fibers and proteoglycans [13; 14]. However, ambiguous results on AC stiffness during OA progression are reported in the literature, probably depending on different experimental set-up variables, e.g., the geometry and size of the indenter, the anatomical location of the indentation test, and measurements on tissue surfaces or cross-sections, as well as the diverse OA models investigated. For example, an AFM-based indentation study using a micrometer-sized spherical tip on native mouse femoral condyle cartilage reported reduced stiffness of the AC surface at the onset and during the progression of PT-OA [15]. Another study has assessed mouse tibial AC on cross-sections and observed reduced micro-scale modulus of the PCM in the middle/deep zone during DMM-induced PT-OA [16]. Furthermore, surface indentation studies on human cartilage from patients undergoing total knee arthroplasty have shown a decrease in stiffness with increasing severity of OA [17; 18]. In contrast, a study using nano-scaled AFM indentation on cross-sections of human AC revealed stiffening of collagen fibrils with increasing OA grade in all zones [19], whereas another study showed a decrease in elasticity in both the ITM and PCM [20].