Introduction: The meniscus plays an important role in protecting the articular surfaces of the tibia and femur from excessive wear due to aberrant forces across the knee joint. While the biochemical changes associated with cartilage and meniscal wear have been well documented, little data exists in the literature describing the ultrastructural events associated with such a degenerative process.

Aim: To develop an in vivo model to evaluate the effects of joint incongruity on meniscal wear.

Materials and Methods: Six New Zealand White rabbits underwent an arthrotomy of the right knee joint. A sagittal osteotomy of the medial femoral condyle was then performed on four of these animals while the remaining two served as controls. Post operatively all animals were allowed to mobilise ad librium and were sacrificed after 20 weeks. The medial meniscus was harvested and processed for electron microscopy by routine methods. Survey light microscopy sections (1um) were examined and adjacent ultra-thin sections (50nm) were assessed in an electron microscope at magnifications from 1,500 to 30,000.

Results: On gross examination of the menisci there was no evidence of any significant wear. Neither was there noticeable damage on light microscopy. There was no significant difference in the numbers of chondrocytes and fibroblasts in the superior and inferior surfaces of both control and test samples, indicating a lack of cellular response in the test specimens. On electron microscopy, the superficial electron dense layer was markedly attenuated in test subjects (control 246 – 305 nm, test 109 – 167nm). The superior surfaces of two test samples were markedly roughened, while chondral fragments were noted in craters on the surface. Numerous cystic lesions were present within the superficial collagen stroma of test subjects. Interestingly cleavage planes were noted in the superior and inferior aspects of one of the four test subjects. No difference in collagen fibril diameter was observed between control and test subjects.

Discussion: These results represent the early stages of meniscal degeneration. The process appears to one of microfibrillation, with degeneration occurring within the bundles of collagen fibrils rather than within the substance of the fibrils themselves. The absence of a healing response suggests that this is an intractable process, a finding which accords well with findings in the clinical setting.

Introduction: The magnitude of the initial chondral injury and the residual articular step-off are amongst prognostic factors implicated in outcome following intra-articular fractures. The alignment of an intra-articular fracture line may be an as yet unrecognised prognostic variable.

Hypothesis: That fractures in the coronal plane of the medial femoral condyle result in worse outcomes than those in the sagittal plane.

Aim: To compare the effect of displaced intra-articular osteotomies (ie simulating fractures fixed in an incongruent position) of the medial femoral condyle – in one group performed in the sagittal plane, in the other in the coronal plane.

Materials and Methods: The study was conducted in two arms: in vitro and in vivo.

In vitro study: A pneumo-electric rig was designed and built. Ten freshly harvested porcine knee joints underwent osteotomy (test specimens: 5 sagittal, 5 coronal). 5 control specimens underwent no osteotomy. Specimens were mounted on the rig and subjected to cyclical flexion and extension under load (40,000 cycles over 11 hours). Transarticular pressure measurements were performed before and after testing. Surface roughness was measured following testing using laser interferometry.

In vivo study: Three groups (A to C), each comprising 15 New Zealand white rabbits were utilised. Rabbits from each group were consigned to a control (5), coronal osteotomy (5) or sagittal osteotomy (5) group. Rabbits in group A were sacrificed at 3 weeks (early outcome), group B at 10 weeks (immediate) and group C at 20 weeks (long term). The knee was then harvested en bloc and prepared for light microscopy. A further 10 specimens underwent electron microscopy of the medial meniscus.

Results:

In vitro study: A significant difference in loading patterns was noted between the sagittal, coronal and control groups. Specimens from the sagittal group sustained significantly more wear on the apposing medial tibial articular surface (p=0.04), with the meniscus having a protective effect on the underlying articular surface.

In vivo study: Light microscopy confirmed degenerative changes in the apposing tibial articular cartilage, being more marked in sagittal specimens. On the femoral side of the knee, the healing response of the femoral osteotomy was significantly better in sagittal test specimens than coronal (p< 0.05).

Conclusion: In contrast to the hypothesis, sagittal femoral step-offs gave rise to more tibial wear. This can be explained by the short duration of exposure of the coronal incongruity to the apposing joint during the flexion extension cycle. The sagittal step-off was constantly exposed, giving rise to persistently elevated tibial joint loading pressures opposite the high side of the step-off.

In contrast, the coronal femoral osteotomies had a worse healing response. The alignment of the fracture line perpendicular to the plane of motion of the joint exposes the repair tissue within it to increased shear and tensile stresses. This may play a negative role in the repair of these coronal defects when compared to sagittal osteotomies, which are relatively protected from the high transarticular pressures and showed a greater tendency to remodel their articular surface.