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Is There a New Way to Measure Knee Alignment in Patellofemoral Pain Syndrome?

There is some convincing evidence that altered kinematics is a major factor in patellofemoral pain syndrome (PFPS). Kinematics refers to patterns of movement -- specifically how the patellofemoral joint and the knee joint rotate and glide in relation to one another during motion.

The patellofemoral joint occurs where the patella (kneecap) glides up and down over the femur (thighbone). Increased pressure from contact between the patella and the femur can lead to PFPS. This is called retropatellar stress -- it means behind the kneecap.

Stress on the patellofemoral joint is made worse by rotations of the lower leg during weight-bearing activities. And repetitive actions with weight-bearing load during running and jumping increase retropatellar stress. The result is PFPS.

In this study, physical therapists attempt to use a two-dimensional (2-D) method of measuring knee alignment. The measurement was called the frontal plane projection angle (FPPA). The hope was to find a simple tool to use in the clinic to measure altered kinematics during weight-bearing activities.

The two planes of kinematics (motion) investigated were frontal and transverse. Frontal refers to a line down the middle of the joint dividing it into the front half and the back half. Transverse is the plane through the knee from side to side.

Physical therapists often work with athletes with PFPS. If they could identify who has altered movement patterns, it might be possible to treat the problem and prevent PFPS from developing. To test their theory, they compared two groups of similar females. One (control) group was made up of healthy, active women between the ages of 18 and 35. They did not have any knee pain or knee problems. The second group were also active in recreational sports requiring running and jumping. But women in the second group had PFPS.

A 2-D method was used to measure the frontal plane projection angle. Other studies have used 2-D measurements of knee angles and kinematics. When compared with 3-D high-speed motion analysis, the results were promising that 2-D would be accurate enough to use in the clinic. Three-dimensional motion analysis is expensive and not usually available in the clinic. If it was successful, the 2-D device could measure leg alignment during weight-bearing activities.

The equipment used was simple, portable, and inexpensive. Everyone wore the same type of shoe (Nike Air Pegasus). The test required only a single-leg squat, which uses less space than testing athletes during full-speed cutting movements. A digital photograph of the leg was taken as the subject bent the knee for the single-leg squat position. The frontal plane projection angle (FPPA) was taken from the image.

At the same time, transverse and frontal plane kinematics of the hip and knee were recorded using a high-tech 3-D (computerized) movement analysis system. The researchers compared the 2-D measurements for FPPA during single-leg squats with 3-D analysis of hip and knee movement for the same squat motion. The 3-D system was also used to record ground reaction forces during running and single-leg jumps. Results were compared for women with PFPS and whose who didn't have PFPS.

The therapists also measured other factors that might be important later when identifying women at risk for PFPS. For example, each woman was asked to rate her pain during squatting, long periods of sitting, running, going up or down stairs, and jumping. This was reported as a number on a scale from zero (no pain) to 10 (worst pain).

Other factors were also evaluated to see if any of these would be able to predict who would develop PFPS. These included: unknown cause for symptoms, pain behind the knee with pressure over the patella, and change in function. Here's what they found:

  • During single-leg squats, women with PFPS rotated the knee outward into external rotation. That motion was measured in the transverse plain. Women in the control group had no particular transverse plane directional rotation.
  • The correlation between 2-D studies and 3-D motion analysis was very poor. Only 20 to 30 percent of the three-dimensional frontal plane projection angles (FPPA) even showed up on the 2-D digital images.
  • Hip rotation was also linked with FPPA values. This suggests that it might be possible to try pelvic stabilization exercises to reduce the amount of external rotation during weight-bearing activities. The result could be to reduce retropatellar stress.

    It appears that females who adduct the hip (knee moves toward the other knee) during single-leg squat motions have a greater tendency to develop PFPS. PFPS is more likely to occur if the knee externally rotates at the same time. The overall pattern of motion is one of medial collapse, sometimes referred to as knee valgus.

    Despite all of the information gleaned from this study, we still don't know why changes occur in leg motions that lead to PFPS. Perhaps hip muscle weakness is a factor. If so, it may be possible to retrain abnormal kinematics in anyone with PFPS who has large negative values measured for FPPA. This supports the idea that focusing on hip and trunk strengthening may make a difference.

    But it's not the only cause and effect because about 20 per cent of the women in this study didn't have a medially collapsed position during squats, jumps, or landings. They were either in a neutral position or they had more positive FPPA values.

    Future studies need to take a look at FPPA values before and after treatment. Is the FPPA value sensitive enough to changes in the lower extremity brought about by changes in lower extremity kinematics? This would help therapists know if the measurement can be used to tell if someone is getting better or worse. Tracking outcomes or changes in alignment of the knee after treatment is another area where further study is needed to refine the process.

    The authors conclude that it's not surprising the 2-D model of a three-dimensional, rotational joint fell short of being a valid and reliable screening measurement for PFPS. However, the relationship between FPPA, knee rotation, and hip adduction is important. When one changes, the others change an equal amount.

    Three-dimensional analysis is expensive and not very practical. But for now,it doesn't look like the 2-D approach is going to work either. It's clear that abnormal kinematics of the lower extremity during weight-bearing activities such as single-leg squats are part of the problem for women with PFPS. Efforts will continue to find ways to change this through rehab and exercise.

    John D. Willson, MSPT, PhD, and Irene S. Davis, PT, PhD. Utility of the Frontal Plane Projection Angle in Females with Patellofemoral Pain. In Journal of Orthopaedic & Sports Physical Therapy. October 2008. Vol. 38. No. 10. Pp. 606-615.


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