The dotted lines are just there for illustration of how the three compartments take different stresses.
Stressing one compartment
If one of the compartments is overloaded - for example the medial tibiofemoral compartment, the lateral tibiofemoral compartment or the patellofemoral compartment - unicompartmental arthritis may develop in the area of stress. Such stress may be due to joint cartilage breakdown in that one area, or damage to the meniscus in the same area, and this may cause that part of the joint to take extra strain during walking.
"Knee forces are highly significant in osteoarthritis and in the survival and function of knee arthroplasty....the division of the joint into three compartments (two tibiofemoral and one patellofemoral) and the presence of menisci....contribute significantly in the transfer of contact stresses from one articular surface to another..."