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 subchondral bone after an osteotomy designed to simulate an articular fracture.


METHODS
The contribution of the cartilage and subchondral bone was evaluated twelve weeks after creating a 1.0-millimeter step-off in the medial plateau of the tibia of twelve adult domestic sheep. All animals surviving were labeled with fluorescent markers for bone production, oxytetracycline (fifty milligrams per kilogram), and calcein (twelve milligrams per kilogram) nine and 11.5 weeks after surgery. The knees were loaded in compression using an Instron materials tester with pressure-sensitive film to record joint contact pressures above and below the medial meniscus.


METHODS
The studies were performed in the research laboratories of the Orthopaedic Laboratory at Harborview Medical Center in Seattle, Washington and the Madigan Army Medical Center in Tacoma, Washington.


METHODS
Twelve adult domestic sheep.


METHODS
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.


METHODS
The contact pressures of the knee joint and articular histology were evaluated twelve weeks after surgery. Samples of the articular cartilage were analyzed by light microscopy and electron microscopy to evaluate the response of the articular cartilage and subchondral bone of the differential joint loading because of the irregularity in the articular surface caused by the osteotomy.


RESULTS
The knees with an intraarticular step-off had two major contact areas with an intervening zone of reduced load corresponding to the edge of the depressed fragment. Coronal histologic sections through the articular surface showed the presence of thinning and fibrillation on the high side of the step-off and some compensatory hypertrophy of the cartilage. The subchondral bone was not responsible for restoring articular congruity because the 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 (1.67 micrometers per day). Scanning electron microscopy showed partial cartilage 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 millimeter), which was less than the thickness of the articular cartilage (1.5 millimeters), the contour of the joint improved despite residual articular surface incongruency after the fracture healing.
Trumble
T
T
Department of Orthopaedic Surgery, University of Washington Medical Center, Seattle, WA 98195, U.S.A.
Allan
C H
CH
Miyano
J
J
Clark
J M
JM
Ott
S
S
Jones
D E
DE
Fernacola
P
P
Magnusson
M
M
Tencer
A
A
D000818:Animals; D002356:Cartilage / Q000378:metabolism*; D005593:Fracture Fixation, Internal / Q000379:methods*; D017102:Fracture Healing; D008855:Microscopy, Electron, Scanning; D008953:Models, Anatomic; D010027:Osteotomy; D011859:Radiography; D012756:Sheep; D013978:Tibial Fractures / Q000000981:diagnostic imaging / Q000601:surgery*