Abstract
Introduction: Outcome following intra-articular fractures is dependent on a myriad of variables, including the extent of the initial osteochondral and soft tissue injury. In the clinical setting it is impossible to control such variables, and studies are largely confined to radiographic and outcome based assessments. Therefore the effect of fracture line orientation has not been widely examined. Theoretically an incongruent intra-articular fracture results in a “low” side that is relatively unloaded, at the expense of a “high” side exposed to increased loads. Furthermore the orientation of the fracture may give rise to a narrow or broad swathe of wear on the opposing articular surface.
Aim: To evaluate the effect of an incongruent intra-articular fracture of the medial femoral condyle on subsequent loading and wear patterns, using an in vitro model.
Materials and Methods: 15 porcine stifle (knee) joints were harvested within three hours of death. Three groups of five joints were evaluated. Group S underwent a sagittal osteotomy of the medical femoral condyle that was then fixed in an incongruent position. In Group C the osteotomy was performed in the coronal plane. The third group acted as a control group and had no oseotomy performed. In all cases great care was taken to prevent injury to the menisci, articular surfaces and Ligamentous structures. The size of the step-off was documented using a contour-mapping machine (CMN). In addition the surface roughness of the femoral condyles was documented using a laser interferometry device (UBM, Germany). The specimen was mounted on a custom-made electro-pneumatic rig, and pressure mapping of the articular surfaces performed with pressure sensitive film (SPI, New Jersey, USA). Following mapping, each specimen underwent 10,000 cycles of flexion and extension over a three-hour period. When testing was complete, pressure mapping was again performed, the size of the step-off re-measured using the CNM, and surface roughness of the menisci, femoral condyles and tibial plateau assessed. Data was restored on a laptop for subsequent statistical analysis.
Results: Pressure mapping documented an unloaded area on the low side of the step-off in both Group C and S. This extended up to 8mm, and was mirrored by an area of increased load on the high side. Following testing, the area exposed to altered loads on both the high and low side of the osteotomy had diminished. On ANOVA testing the uncovered tibial articular surfaces in test subjects were significantly rougher than control specimens, though no difference was noted between Groups C and S (Mean Ra value GC: 101.83+22.78, GS: 93.52+17.89, ns. vs. Con 53.45+25.8,p< 0.05). Meniscal surface roughness was greater in the test groups, though this did not reach statistical significance. No significant difference in femoral condyle surface roughness was noted following testing. Nor was any difference noted in surface roughness in the submeniscal areas of the tibial articular surface.
Discussion: The displaced femoral osteotomy resulted in an area of increased wear on the opposing tibial articular surface. However no significant difference was noted between the coronal and sagittal group. It is probable that the menisci negated the effect of fracture line orientation. We suggest that they minimized secondary articular damage by decreasing the area of direct tibiofemoral contact. Furthermore the elasticity of the menisci, in addition to their ability to move in the anteroposterior plane further decreased stress transmission between joint surfaces. Further studies will be performed on the hip joint to determine the effect of articular incongruity in the absence of such a fibrocartilage buffer.
The abstracts were prepared by Raymond Moran. Correspondence should be addressed to him at the Irish Orthopaedic Assocation, c/o Cappagh National Orthopaedic Hospital, Finglas, Dublin 11, Ireland.
