Effect of tibial plateau fracture on lubrication function and composition of synovial fluid.

Traumatic intra-articular fractures, such as those of the tibial plateau, are at risk of joint degeneration and posttraumatic arthritis even when treated according to traditional orthopaedic principles to restore articular congruency and anatomic alignment1-7. The consequences of poor results can be life-changing—painful weight-bearing, restricted activity, and lost time in the work force1,8-10. The costs of posttraumatic arthritis are a component of the estimated >$100 billion annual burden of osteoarthritis on the U.S. economy, with 9.8% of the cases of knee osteoarthritis estimated to be of posttraumatic etiology; furthermore, patients with posttraumatic arthritis and its chronic consequences tend to be young, with an additional impact on employment9. Compared with those without posttraumatic arthritis, patients with posttraumatic arthritis have worse clinical outcomes after arthrodesis or arthroplasty11,12. Posttraumatic arthritis and its consequences have not been eliminated by employing traditional orthopaedic principles, suggesting a possible role for additional biological and biomechanical factors. The pathogenesis of posttraumatic arthritis is complex and multifactorial. Recent studies have focused on articular chondrocyte death and cartilage damage due to direct trauma, enzyme-mediated cartilage degradation, and the role of reactive oxygen species13-16. Deficient lubrication may also contribute to cartilage deterioration after trauma. Lubrication typically allows articular cartilage to bear a load and slide with low friction and low wear17, and is mediated by high levels of proteoglycan-4 and hyaluronan in synovial fluid18. Synovial fluid lubricant molecules are secreted by chondrocytes and synoviocytes lining the joint, and are concentrated through selective retention by synovium. However, after an intra-articular fracture, soft tissues that normally produce and retain synovial fluid lubricants are damaged, and blood and cellular components from damaged tissues and bone marrow infiltrate the joint space13,19-22. Such alteration of synovial fluid may disrupt its lubrication functions. Alteration in the lubricating function and lubricant composition of synovial fluid appears to be involved in cartilage deterioration after anterior cruciate ligament (ACL) injury, as well as other injuries of the knee joint. After ACL injury of human knees, the level of proteoglycan-4 in synovial fluid was reduced, while levels of degradative enzymes, cartilage matrix degradation products, and inflammatory markers were increased for up to six to eighteen months after injury15,23. Deficient lubrication after an injury has similarly been detected in the synovial fluid from acutely injured equine joints, in association with a decreased concentration of hyaluronan24. In guinea pig25 and rat26,27 models of knee injury, synovial fluid lubrication function and lubricant levels are also diminished. These studies suggest that human knee trauma, and specifically intra-articular tibial plateau fracture, may lead to a deficiency in synovial fluid lubrication. The hypothesis tested in the present study was that tibial plateau fractures impair the friction-lowering lubrication function of human synovial fluid in association with changes in the concentrations of proteoglycan-4 and hyaluronan lubricant molecules. The objectives of this study were to compare the synovial fluid from joints after acute tibial plateau fracture and the synovial fluid from normal knees in terms of (1) friction-lowering boundary lubrication function and (2) biochemical composition, including hyaluronan and proteoglycan-4 concentrations. In addition, the possible biomechanical basis for impaired lubricant function was assessed by correlating friction coefficient and lubricant concentration.