Introduction: A common outcome measure for Total Knee Replacement patients is the measurement of their knee extension angle. In theatre the surgeon usually ensures that the replacement knee can achieve full extension with the patient supine. However patients frequently comment that they are unable to reach full extension while seated. This is due to the flexed hip tightening the hamstrings.

The aim of this study was to deduce the effect of Hip flexion on the knee’s full extension angle (popliteal angle) in a control group of subjects with normal knees.

Method: An electromagnetic motion analysis system (Polhemus Fastrak) was used to assess twenty knees in ten normal subjects. The groups mean age, height and weight were 35 years (SD=7), 1.82 m (SD=0.05) and 83.9 kg (SD=12.9) respectively. Each subject was placed in a supine position on an examination couch with their legs hanging over the end and their knees in 90 degrees of flexion. The electromagnetic source was then positioned 50 cm from the knee joint. Two electromagnetic sensors were then taped to the lateral side of the leg, one over the femoral midpoint and one over the tibial mid point. A recording was then made as the subject extended their knee to full extension. The subjects hip was then flexed to 90 degrees with the knee in 90 degrees flexion. The subject was the asked to extend their knee as far as possible, while keeping their hip flexed. This processed was repeated 3 times for each knee to give average knee extension angles, with the hip straight and flexed.

Results: In the supine hip-straight position the mean extension angle for the fully extended knees was 1.2° (SD=2.7°). In the supine hip-flexed position the mean extension angle for the fully extended knees was 23.8° (SD=12.6°). This gave a mean difference of 22.6°. A paired t test of the extension angles for the two hip positions yielded a significant difference with p=0.0000001.

Conclusion: These results indicate that hip flexion significantly reduces the amount of full knee extension, in normal subjects.