What is the pathophysiology of patellofemoral arthritis?

Updated: Dec 11, 2019
  • Author: Dinesh Patel, MD, FACS; Chief Editor: Thomas M DeBerardino, MD  more...
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Articular cartilage in the patella differs from that of other joints in several ways. The patellar cartilage is not necessarily congruent with the contours of the underlying subchondral bone. In 60% of patellae, the thickest area of articular cartilage is located lateral to the thickest area of underlying bone.

Some biomechanical studies have also indicated that this cartilage is less stiff and, thus, more compressible than that of other joints. One cadaveric comparison of femoral and patellar articular cartilage showed that patellar cartilage had a 66% higher permeability, a 30% lower compressive aggregate modulus, and was 23% thicker. [5] The proteoglycan content was 19% higher in the femoral cartilage than in the patellar cartilage. Whether these differences help prevent or promote arthritic changes in the patellofemoral joint is not yet clear.

Using magnetic resonance imaging, Herberhold and colleagues studied the relationship of static loading to cartilage deformation in fresh-frozen cadaveric knees. [6, 7] After 214 minutes of static loading with 150% body weight, the thickness of the articular cartilage was reduced by 44% in the patella and 30% in the femur. The changes in the thickness of articular cartilage of the patella were greatest at the lateral facet, the area of thickest cartilage.

Of note, 7% of the final deformation occurred during the first minute, and 25% occurred in the first 8 minutes. In the initial response to loading, the cartilage appeared to be stiffer than it was in response to prolonged loading. In normal loading, fluid in the cartilage is thought to support the applied load and to prevent cartilage deformation from occurring.

The main function of the patella is thought to be improving the mechanical advantage of the quadriceps extensor mechanism by increasing the lever arm of the muscle. The patella also acts to dissipate the forces generated in the patella tendon during knee flexion and extension.

The angle of force of the quadriceps muscle group (ie, the Q angle) is thought to be a factor in the development of knee injuries and arthritis. However, no findings conclusively support this assertion. The Q angle is measured as the angle between a line connecting the patella to the tibial tubercle and a second line between the anterior superior iliac spine and the center of the patella. A larger Q angle is thought to increase the lateral tracking of the patella mechanism. [8]

Huberti and colleagues concluded that, at normal Q angles, pressure is evenly distributed across the patella. [9] Increases in this angle, however, result either in a shifting of pressure to the lateral facet or a change in the distribution of force. Cadaveric studies demonstrate that with an increasing Q angle, the patella shifts more laterally and rotates medially as the knee is flexed. This change is thought to increase lateral contact at the patellofemoral joint and, possibly, to increase the incidence of patella subluxation and dislocation.

In closed-chain exercises with the foot planted on the floor (eg, squatting), contact forces increase with progressive degrees of knee flexion. However, in open-chain exercise in which the foot is off the floor (eg, hamstring curls), no corresponding increase in patella contact force occurs as the knee progresses through a range of motion.

The portion of the patella that is in contact with the trochlea also changes during range of motion. [10] With the knee extended, only the distal aspect of the patella contacts the trochlear groove. With progressive flexion, the contact shifts to the proximal aspect of the patella. At greater than 90° of flexion, the contact area is predominantly in the center of the patella, which corresponds to the thickest area of articular cartilage.

The medial facet of the patella articulates with the trochlea only during positions of full flexion. The force on the patella increases with knee flexion from 0-60°. However, no consensus exists among researchers regarding the relative amount of force generated with progressive flexion. The contact forces likely are related to the amount of force being generated by the quadriceps muscles during deep flexion exercises. Gait lab analysis has shown that walking with the foot plantar flexed, as occurs when high-heeled shoes are worn, increases the forces in the patellofemoral joint and in the medial compartment of the knee.

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