Quantitative measurement of patellofemoral joint stability: Force–displacement behavior of the human patella in vitro

Abstract

Patellofemoral joint instability is a common clinical problem. However, little quantitative data are available describing the stability characteristics of this joint. We measured the stability of the patella against both lateral and medial displacements across a range of knee flexion angles while the quadriceps were loaded physiologically. For eight fresh-frozen knee specimens a materials testing machine was used to displace the patella 10 mm laterally and 10 mm medially while measuring the required force, with 175 N quadriceps tension. The patella was connected via a ball-bearing patellar mounting 10 mm deep to the anterior surface to allow natural tilt and other rotations. Patellar force-displacement behavior was tested at flexion angles of O", lo", 20", 30°, 45", 60", and 90". Significant differences were found between the lateral and medial restraining forces at 10 mm displacement. For lateral displacement, the restraining force was least at 20" of knee flexion (74 N at 10 mm displacement), rising to 125 N at 0" and 90" of knee flexion. The restraining force increased progressively with knee flexion for medial patellar displacement, from 147 N at 0" to 238 N at 90". With quadriceps tension, the patella was more resistant to medial than lateral displacement. Our finding that lateral patellar displacement occurred at the lowest restraining force when the knee was flexed 20" agrees with clinical experience of patellar instability.

Patellofemoral kinematics during knee flexion-extension: An in vitro study

“…12 This ligament slackens as the patella moves medially in the first 208 of flexion and the patella is still not fully engaged in the trochlear groove, so that here the patella is least stable, which means that it has least resistance against lateral displacing forces. 11 The patella engages initially with the trochlear groove by contact at the proximal-lateral edge of the trochlea, which guides it distally and medially into full engagement, after which it is carried progressively laterally in relation to the femoral shaft axis as the knee flexes. This corresponds primarily to the axis of the trochlear groove being inclined distal-lateral.…”

“…They were prepared using methods described previously. 11,12 The skin, underlying fat, and muscles other than the distal quadriceps were removed. Care was taken to preserve the retinaculae and the fascia of the quadriceps muscles.…”

“…The relationship between patellofemoral joint stability and maltracking is unclear, but presumably they are related; if the patella is stable in the trochlear groove, then it is difficult to move the patella away from this position, and so it will track normally. Work in vitro has quantified both ''normal'' stability 11 and also the effects of abnormalities of joint geometry, lack of tension in vastus medialis, or ruptured medial retinaculae. 12 A number of studies of patellar tracking have been conducted, both in vitro and in vivo.…”

Effects of lateral retinacular release on the lateral stability of the patella

“…The methods and apparatus used are described in full in the literature [16][17]. Seven fresh-frozen cadaveric knees, with no history of knee surgery or disease, were used for this study (mean age 75 years, range 65 -82).…”

“…A ball bearing was cemented inside the patella, at the geometric centre of, and 10 mm deep to the anterior surface ( Figure 1). This allowed natural tilt and other patellar rotations when the patella was displaced laterally [16][17].…”