Osteoarthritis is thought to be the result of mechanical factors overlaid on a milieu of baseline factors such as genetics and aging. The medial tibiofemoral compartment tends to be affected much more commonly than the lateral compartment primarily due to the larger forces experienced in the medial compartment during typical bipedal activities such as walking and running. The larger forces generally experienced in this compartment are due to the ground reaction force primarily falling medial to the knee joint center of rotation in the coronal plane.

I am not a clinician, but as far as I know idiopathic osteoarthritis often occurs on the medial side due to a varus deformity, which is observed more often in males and on the lateral side due to a valgus deformity, which is more often seen in females. So I do not agree that osteoarthritis occurs only in the medial compartment.

Because due to center of gravity, the loading line passess through the medial compartment of the knee.

Ageing is not the main reason for knee OA,otherwise it would happen predictably and symmetrically, which it does not. The impact of genetic and environmental factors that lead to OA takes time to develop, and so the disease is more common in the elderly, but it is not ageing pre se. The loads in the medial compartment of the knee are generally greater, as the mechanical axis of the limb (centre of hip to centre of ankle) passes just to the medial side of the midline of the knee joint itself, and that explains why it is often manifest in the medial compartment first. But even in apparently isolated medial OA, there will generally be some changes in lateral and PF compartments. The valgus and varus deformities often seen in association with lateral and medial OA are to a large degree a manifestation of the asymmetric joint space narrowing (patients wit medial OA are aware that they have become more bow-legged with time as the disease progresses).

Dynamic forces in bipedal humans mean that the most commonly affected joints for OA are the knee and hip. Microscopic changes do also occur in the lateral compartment, although due to varus change the medial compartment is usually most severely affected in the knee. Our understanding is that OA is a complex, multifactorial disease influenced by dynamic forces, obesity, genetics and pain preception. This explains why changes are also observed in smaller joints e.g. the hand. Although OA is more prevalent in older people, it is not persay only a disease of aging, since cartilage and bone do show attempts at repair at the microscopic level.

It's all about biomechanics. Higher medial compartment involvement in OA is due to the way forces are generated and the axis of joints. Aging and obesity are the two most potent risk factors for knee OA but there are many other mechanical factors such as previous trauma, excessive loadings, leg length inequality, knee malalignment, muscle weakness etc. There is also genetic link to knee OA.

It seems that the main concern of the cause of medial compartment OA knee has been shifted to the mechanical axis deviation! Did anybody notice that there is a systematic review investigated about this topic? - "Does knee malalignment increase the risk of development and progression of knee osteoarthritis? A systematic review. Arthritis Rheum. 2009 Apr 15;61(4):459-67." (http://www.ncbi.nlm.nih.gov/pubmed/19333985). Only one cohort study was found by this review: "Knee alignment does not predict incident osteoarthritis: the Framingham osteoarthritis study." (http://www.ncbi.nlm.nih.gov/pubmed/17393450). The conclusion of this cohort study said that "We found that baseline knee alignment is not associated with either incident radiographic tibiofemoral (TF) OA or medial TF OA. These results suggest that malalignment is not a risk factor for OA, but rather is a marker of disease severity and/or its progression." If one still thought mechanic axis is the most important factor, how much the force is needed to jeopardize the cartilage in medial compartment of the knee? Why the cartilage in the ankle joint can tolerate much more force? If walking or running can cause human being to get OA knee, does it mean we are not suitable to survive in this planet?

http://www.ncbi.nlm.nih.gov/pubmed/17393450

Aging is strong factor

1-increase age==>increase time for exposure to mechanical stressors and time for cartilage insult

2-decrease mitochondrial functin and number

3-decrease ability for regeneration

4-muscle weakness

5-ligament laxity

6- altered gait and biomechanics

7-associated medical disease as diabetes,,

8-aging==>chondrocalcinosis==>mechanical cartilage injury

9-decrease pain threshold==>more symptoms

10-hormonal female menopause and male as well