Collagen architecture and failure processes in bovine patellar cartilage

Cartilage fails by fibrillation and wearing away. This study was designed to identify the microscopic failure processes in the collagen network of bovine cartilage using scanning electron microscopy. Cartilage samples from fibrillated cartilage from the bovine patella were removed from the bone, fixed, digested to remove proteoglycans, freeze-fractured, and processed for SEM. The architecture of the collagen network in the normal cartilage was first defined, and then the failure processes were identified by examining sites of fibrillation and at crack tips. The bovine patellar cartilage was organised with a superficial layer composed of 3-5 lamina, attached to a sub-superficial tissue by angled bridging fibrils. Collagen in the sub-superficial tissue was organised in lamina oriented in the radial direction up to the transition zone. Failure of the system occurred by cracks forming in superficial layer and lamina, creating flaps of lamina that rolled up into the larger 'fronds'. Larger cracks not following the laminar planes occurred in the transition, mid, and deep zones. Failure at the crack tips in the sub-superficial tissue appeared to be by peeling of collagen fibrils, as opposed to breaking of collagen fibrils, suggesting a 'glue' bonding the collagen fibrils in a parallel fashion. Cracks propagated by breaking these bonds. This bond could be a site of disease action, since weakening of the bond would accelerate crack propagation.