Disorders of the patellofemoral joint are a common cause of knee pain throughout adult life. For teenagers and those in early adult life the disorders present as non-specific anterior knee pain for which there can be a multitude of causes and can be a cause of troublesome disability. The exact incidence is unknown but with increasing age and progressive damage to the articular surfaces, established osteoarthritis of the patellofemoral joint develops. The condition is usually symptomatic causing moderate levels of pain and disability and particularly restricts the function of the knee. Paradoxically some patients can have well established osteoarthritic change present on radiographs without significant symptoms.

     
Typical isolated Patellofemoral arthritis

Increased awareness of these disorders has occurred as a result of more detailed investigation of the patellofemoral joint firstly with Merchant or Ficat’s tangential plain radiograph and secondly with the more widespread use of transverse images with CT and MR. An investigation by Rogers (1) of medieval burial sites has revealed that disease of the patellofemoral joint was extremely common at that time. More recently the studies of Noble (2), McAlindon (3) and Davies (4) have revealed a significant incidence of symptomatic patellofemoral disease in the general population. Davies’ study of 208 patients attending an orthopaedic clinic for arthritic pain in the knee showed that nearly 10% had definite evidence of isolated patellofemoral disease. These studies have demonstrated patterns of disease which include isolated changes in either medial or lateral facets of the patellofemoral joint, or symmetrical disease. They lead to the inescapable conclusion that isolated patellofemoral disease exists in a proportion of patients with osteoarthritis of the knee, and this process can remain localised for many years before progressing to the other compartments.

There are many causes of patellofemoral disorders and most are fundamentally related to the mechanical stability of the extensor mechanism. This is a complex articulation transmitting force from the large extensor musculature over a narrow pulley to the tibial tubercle. Efficient function of this mechanism is essential for effective locomotion in the upright posture. Goodfellow and Hungerford (5) described the contact bearing areas of the seven separate articulating facets of the patella and emphasised the complex overlap that occurs with the tibio femoral joint, unique in the animal kingdom. Minor degrees of incongruity with the trochlea will lead to considerable increase in the forces at the articular surfaces. Thus the patellofemoral joint is particularly vulnerable to minor degrees of incongruity and to the effects of injury. Mismatches in the articulation can result from a range of developmental malformations of either the patella or the trochlea. Alterations in the overall alignment of the limb both in varus, valgus or rotation can lead to incongruities and overload of the surfaces.

(Reproduced by permission of the Journal of Bone and Joint Surgery)

Contact points - Goodfellow and Hungerford (5)

The stability of the patello-femoral joint is largely determined by the complex muscular and retinacular arrangements of the proximal extensor mechanism. Muscular imbalance of the quadriceps mechanism is a common cause of knee dysfunction particularly when there are underlying mal-alignments or incongruities. The factors determining muscular control are as yet ill understood and quadriceps dysfunction is sometimes difficult to correct. Once it has occurred, usually through the effects of injury, it often persists.

These mechanical factors can lead on to articular cartilage damage with a progressive cycle of attrition, which is not easy to reverse.

Studies (6) of the pathology of advanced patellofemoral disease show that the lateral facet of the patella and trochlea are most commonly affected with over 70% of cases demonstrating this pattern of lateral mal-alignment with subluxation. A small number of cases show medial facet and trochlea disease, (<10%). The precise aetiology of this condition is obscure but it would suggest that there may be an equivalent and opposite medial overload syndrome. The remaining cases demonstrate symmetrical patterns of wear affecting equally both the medial and lateral facets of the patella and trochlea groove.

    
Lateral facet OA                           Medial facet OA

References

1. Rogers J, Dieppe P. Is tibiofemoral osteoarthritis in the knee joint a new disease? Ann Rheum Dis 1994; 53: 612-613.
2. Noble J, Hamblen DL. The Pathology of the degenerate meniscus lesion. J Bone Joint Surg (Br) 1975; 57-B: 180-186.
3. McAlindon TE, Snow S, Cooper C, Dieppe PA. Radiographic pattern of osteo-arthritis of the knee joint in the community; The importance of the patellofemoral joint. Ann Rheum Dis 1992; 51: 844-849.
4. Davis AP, Vince AS, Shepstone L, Donell ST, Glasgow MM. The radiologic prevalence of patellofemoral osteoarthritis. Clin Orthop 2002; 402: 206-12.
5. Goodfellow J, Hungerford DS, Zindel M. Patellofemoral joint mechanics and pathology. J Bone Joint Surg (Br) 1976; 58-B: 287-290.
6. Tauro B, Ackroyd CE, Newman JH, Shah NA. The Lubinus patellofemoral arthroplasty. (J Bone Joint Surg (Br) 2001; 83-B: 696-701.