The dotted lines are just conceptually dividing the Two compartment disease
Bicompartmental arthritis, for example, could involve one tibio-femoral compartment and the patello-femoral compartment, or both tibio-femoral compartments but not the patello-femoral compartment.
Bicompartmental means involving two of the three compartments of the knee.
Bicompartmental arthritis, for example, could involve one tibio-femoral compartment and the patello-femoral compartment, or both tibio-femoral compartments but not the patello-femoral compartment.