Introduction The deep and proximal face of the patellar tendon is the most frequently injured site. Up to now, we have been thinking that this could be a result of two factors: mechanism of compression (impingement) produced by inferior patellar pole, and a hypovascularized site. We have elaborated a new biomechanical model to show that this pathology may be caused by a traction mechanism and not by compression. The insertion area of tendon on patella is a weak transition site between tendon and bone; while the anatomical tendon-to-bone insertion in the proximal end shows a perpendicular angle, at the distal end at the tibial level, it shows an oblique angle.

Methods With ultrasound, we have measured the maximum length of the superficial and deep faces, bone to bone. These measurements were done in two positions: in maximal extension and 90° flexion. All samples were right knees, and there were 10 patients. Measurements were taken by the same technician (figure 1).

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Figure 1.

Sagittal knee view.

Results Measurements were as follows: in maximal extension for the deep face, the mean value was 4.15cm (range 3.40–5.05) and for the superficial face, the mean value was 4.857cm (range 4.57–5.20). Alternatively, in 90° flexion, the deep face showed a mean value of 3.938 cm (range 2.77–4.64) while for the superficial face it was 4.916 (range 4.27–5.11). Variations in face lengths when we changed the knee position were 1.21% for the superficial face and 5.38% for deep face.

Discussion These results have demonstrated that the deep face had a greater percent increase in length than the superficial face when the knee changed position from extension to flexion. This study has some limitations- it was performed at only 90° flexion because the longitudinal probe is limited, and there are a low number of subjects. In conclusion, traction forces are probably more intense in the deep face than in the superficial face.