Cartilage‐on‐cartilage versus metal‐on‐cartilage impact characteristics and responses

Abstract

A common in vitro model for studying acute mechanical damage in cartilage is to impact an isolated osteochondral or cartilage specimen with a metallic impactor. The mechanics of a cartilage-on-cartilage (COC) impact, as encountered in vivo, are likely different than those of a metal-on-cartilage (MOC) impact. The hypothesis of this study was that impacted in vitro COC and MOC specimens would differ in their impact behavior, mechanical properties, chondrocyte viability, cell metabolism, and histologic structural damage. Osteochondral specimens were impacted with either an osteochondral plug or a metallic cylinder at the same delivered impact energy per unit area, and processed after 14 days in culture. The COC impacts resulted in about half of the impact maximum stress and a quarter of the impact maximum stress rate of change, as compared to the MOC impacts. The impacted COC specimens had smaller changes in mechanical properties, smaller decreases in chondrocyte viability, higher total proteoglycan content, and less histologic structural damage, as compared to the impacted MOC specimens. If metal-on-cartilage impact conditions are to be used for modeling of articular injuries and post-traumatic osteoarthritis, the differences between COC and MOC impacts must be kept in mind.

Measuring microscale strain fields in articular cartilage during rapid impact reveals thresholds for chondrocyte death and a protective role for the superficial layer

“…Instead, the unconfined compression and small-diameter impacting rod were used to impose a wide range of strains in the field of view, enabling strain and cell death correlations to be rigorously investigated. Translating loading from the bulk scale to the microscale will depend strongly on the geometry and boundary conditions, potentially affecting cartilage response (Heiner et al, 2013; Jeffrey and Aspden, 2006). However, as a physical property, the microscale strain thresholds would not be expected to change with loading and geometry.…”

Establishing a live cartilage-on-cartilage interface for tribological testing

“…The senior author and several groups have utilized multi-axial loading devices and complex motion apparatus to evaluate tissue engineered constructs[4,37,39,54,55], yet these systems have been less frequently applied in the context of cartilage tissue. Second, other cartilage-on-cartilage models exist within the study of friction and lubrication[25,29,56] or for supraphysiological impaction[16], but these models may use simple motion patterns or not evaluate the influence of the biological response on wear. Further, the applied load of 45N results in a contact stress in excess of 2 MPa to the cartilage discs, in rage with in vivo findings[40].…”

“…Physiological loads experienced by cartilage at the hip[57,58] or knee[59] have been measured to be about 1 MPa during standing with peaks of 5 – 10 MPa during walking. It is also important to note that MoC models have been shown to experience higher contact stresses and frictional forces[19,56], as well as higher impact stress[16], when compared to CoC models. The stress differences highlight the need to use CoC for understanding natural joint wear.…”

“…The biomechanics of natural joints includes rolling and gliding motion at the cartilage surface with compressive, shear, and tensile stresses[5]. Models of cartilage mechanics apply loading conditions such as static or dynamic compression[6–9], shear forces[10–13], hydrostatic pressure[14], or supraphysiological impaction forces[15,16]. These models provide an incomplete picture of the tribological stresses at play in a joint by not simulating shear forces at the surface.…”

“…Second, in tribological testing, the biological cartilage-cartilage interface of a joint is often replaced by an interface of cartilage against glass or other biomaterials[22–26], neglecting the native tissue response to motion and loading against a naturally soft counterface. The few studies that do utilize a cartilage-on-cartilage interface do so for studies of friction and/or lubrication[27,28], for comparison against chondroplasty materials[29,30], or for supraphysiological impaction[16]. Third, a final biological concern with many studies is the limited understanding of living cartilage response to tribological stresses as frozen or dead tissue has been typically used[21,31].…”