A preliminary study of joint surface changes after an intraarticular fracture: A sheep model of a tibia fracture with weight bearing after internal fixation

Objective: To evaluate the changes in the articular cartilage and subchondr al bone after an osteotomy designed to simulate an articular fracture. Design: The contribution of the cartilage and subchondral bone was evaluate d twelve weeks after creating a 1.0-millimeter step-off in the medial plate au of the tibia of twelve adult domestic sheep. All animals surviving were labeled with fluorescent markers for bone production, oxytetracycline (fift y milligrams per kilogram), and calcein (twelve milligrams per kilogram) ni ne and 11.5 weeks after surgery. The knees were loaded in compression using an Instron materials tester with pressure-sensitive film to record joint c ontact pressures above and below the medial meniscus. Setting: The studies were performed in the research laboratories of the Ort hopaedic Laboratory at Harborview Medical Center in Seattle, Washington and the Madigan Army Medical Center in Tacoma, Washington. Animals: Twelve adult domestic sheep. Intervention: An intraarticular osteotomy of the medial tibial plateau with 1.0 millimeter of displacement was performed. The osteotomy was stabilized with 3.5-millimeter lag screws. Main Outcome Measures: The contact pressures of the knee joint and articula r histology were evaluated twelve weeks after surgery. Samples of the artic ular cartilage were analyzed by light microscopy and electron microscopy to evaluate the response of the articular cartilage and subchondral bone of t he differential joint lending because of the irregularity in the articular surface caused by the osteotomy, Results: The knees with an intraarticular step-off had two major contact ar eas with an intervening zone of reduced load corresponding to the edge of t he depressed fragment. Coronal histologic sections through the articular su rface showed the presence of thinning and fibrillation on the high side of the step-off and some compensatory hypertrophy of the cartilage. The subcho ndral bone was not responsible for restoring articular congruity because th e rate of bone production was similar between the low side of the articular fracture (1.85 micrometers per day) and the high side of the fracture (1.6 7 micrometers per day). Scanning electron microscopy showed partial cartila ge remodeling by deformation of the high side cartilage with bending of the vertical collagen fibrils, even in the unloaded state. Conclusions: In this model with a small fracture displacement (1.0 millimet er), which was less than the thickness of the articular cartilage (1.5 mill imeters), the contour of the joint improved despite residual articular surf ace incongruency after the fracture healing.