Intra-articular fractures are complicated injuries. These fractures can damage the articular cartilage and subchondral bone and lead to articular incongruity, step-off and instability. All of these factors are believed to play a role in the overall outcome of these fractures and may vary in their individual importance among different joints.
Fractures that have intra-articular extent cause damage to the articular cartilage. This injury can be isolated to the articular cartilage or extend into the subchondral bone. The repair response will vary depending on the extent of the injury. Injuries that only involve damage to the avascular hyaline articular cartilage will attempt to mount a healing response that is short lived and unable to completely fill the cartilage defect. Injuries that extend into to the vascular subchondral bone will heal with a type of cartilage that is intermediate in structure to hyaline cartilage and fibrocartilage. This is due to the hemorrhage and fibrin clot released from the bone that stimulates an inflammatory response which promotes healing. Ultimately, mesenchymal stem cells will begin to differentiate into chondrocytes and synthesize type II collagen matrix.
There are a number of factors that contribute to the overall outcome and development of post-traumatic arthritis. Studies indicate that the severity of injury to the articular cartilage is one important factor related to the development of post-traumatic arthritis and outcome. Additionally, the severity of injury has also been shown to directly correlate with the quality of reduction.
Articular step-off is another factor that has a complex relationship to outcome. A step-off can have varying effects on outcome depending on the joint involved and even the location of the step-off within the joint. It has been shown that the sensitivity to step-offs in relation to contact pressure is inversely related to the cartilage thickness. Thus, joints with thick cartilage may be better able to tolerate joint incongruity without development of post-traumatic arthritis or symptoms. Additionally, animal studies have shown the ability of articular step-offs to remodel with time, possibly diminishing the effect of the articular incongruity.
Joint alignment and stability have also been shown to be important predictors of outcome for certain joints. Malalignments and instability can lead to abnormal contact pressures unrelated to step-off and appear to important factors in the development of post-traumatic arthritis of the large weight-bearing joints of the lower extremity.
As articular step-off is an important factor for predicting outcome and post-traumatic arthritis for some joints, the ability to measure step-off on plain films has been studied. The analysis of plain radiographs to assess articular step-off has demonstrated poor accuracy and inter-observer reliability. Thus, small articular step-offs cannot be reliably measured with plain radiographs. CT scan provides much better detail in regard to articular step-off and now is commonly used to assess intra-articular fractures and post-operative reductions.
As discussed previously, the importance of certain factors in predicting outcome varies for each joint. Articular step-off and incongruity is likely more important for joints with thinner, less resilient cartilage, while joint alignment and stability may play a more important role for the weight-bearing joints of the lower extremity with thick articular cartilage, such as the tibial plateau.