Lateral force–displacement behaviour of the human patella and its variation with knee flexion — a biomechanical study in vitro

Abstract

This study measured the patellar lateral force–displacement behaviour at a range of knee flexion angles in normal human cadaver specimens. The knee extensor muscles were loaded in proportion to their physiological cross-sectional areas, the tensions being applied in physiological directions along the separate quadriceps muscles. Knee extension was blocked at a range of knee flexion angles from 0 to 90°, and patellar lateral displacement versus force characteristics were measured. This experiment was repeated with three total muscle forces, 20, 175 and 350 N, which were held constant at all flexion angles. It was shown that similar stability variation was obtained with the different total muscle loads, and also the forces required to produce a range of patellar displacements (1, 5, 9 mm) were examined. A 5 mm lateral patellar displacement required a constant displacing force (i.e. the patella had constant lateral stability) up to 60° knee flexion, and then a significant increase at 90°. The results were related to surgicaland anatomical observations.

Introduction

Lateral instability of the human patella is a very common clinical problem, and occurs most frequently in adolescent females. Although symptoms often subside spontaneously with maturity, they persist in some patients, who may be candidates for corrective surgery. The disabling symptoms of this syndrome are related to the tendency of the patella towards lateral maltracking, when the patella may be subluxed from its normal position of stability in the base of the femoral trochlear groove, through to complete lateral dislocation. This can occur over a spectrum of frequency, from rarely, e.g. during sports activity, through to constantly with knee flexion motion. Fulkerson and Hungerford (1990) reported that in 1959, 137 different surgical techniques for treating patellofemoral stability disorders had been listed, and few have been refuted since. The proliferation of surgical procedures suggests a lack of reliable success for any one of them. We have hypothesised that this lack of success relates to a lack of detailed biomechanical understanding of the function of the patellofemoral joint, which would be a logical basis for rational design of surgical procedures to correct a mechanical dysfunction.

Previous biomechanical studies (Heegaard et al., 1994; Huberti and Hayes, 1988; Lewallen et al., 1990) have examined capsular procedures such as lateral retinacular release, in which lateral soft tissue attachments are transected from the patella, but found no significant changes in tracking or contact pressures in cadaver knees with ‘normal’ loads applied. Similarly, Huber et al. (1994) studied tibial tubercle medialisation, which acts to reduce the Q angle, but they, too, did not simulate ‘pathological’ conditions and they found no consistent effect.

The studies cited above did not actually measure patellar stability, which may be quantified in terms describing the tendency of the patella to return to the position of stable equilibrium in the base of the femoral trochlear groove after being perturbed. Instead, stability changes have been inferred in these studies from changes in patellar kinematics, contact patterns, or contact pressures (Ahmed et al., 1983) at the articular surfaces. The surgical literature commonly uses ‘instability’ and ‘stability’ to describe the presence or absence of a patient’s functional symptoms, and so ‘laxity’ is usually employed when reporting force–displacement or ‘stiffness’ tests, to avoid confusion. Lateral patellar displacement stiffness was measured in vitro in full knee extension by Conlan et al. (1993), who found the contributions of the medial soft tissue restraints by sequential cutting. Similarly, lateral patellar displacement in response to an applied force has been studied in vivo at full knee extension (James, 1989) and at 30° flexion (Fithian et al., 1995), when it was found that symptomatic patients had significantly greater lateral patellar mobility.

We did not find any published study of lateral patellar force–displacement behaviour across a range of knee flexion angles. Some patients report that patellar instability episodes occur towards 30° knee flexion, when the patella is engaging the trochlear groove. Many authors have stated that the patella becomes more stable as the knee flexes further, since it settles into a deepening trochlear groove. However, in a recent anatomical study (Farahmand et al., 1998), we found that the trochlear groove had a constant cross-section, or ‘skyline’, from the anterior aspect contacted by the patella near knee extension through to the distal aspect contacted at 90° knee flexion. This finding suggests that the patella is not stabilised by a deepening groove as the knee flexes. We felt that it was appropriate to investigate patellar stability by lateral force–displacement, or stiffness, tests. The results of these could be presented as laxity at a given displacing force, or as ‘restraining force’, the tendency of the patella to return to the position of stable equilibrium from a chosen lateral displacement.

Patellar stability depends on a complex interplay between a range of factors: joint surface geometry, magnitudes and lines of action of the muscle tensions, and passive soft tissue stabilising structures. If the effects of abnormalities in these factors are to be understood, the changes which they cause away from normal knee behaviour must be found. Thus, the objective of this biomechanical study was to quantify lateral patellar force–displacement behaviour in the normal human cadaver knee across a range of knee flexion angles. It was intended that this study would provide a datum from which future studies of the effects of various pathologies, such as changes to the factors noted above, and corrective surgical procedures could be studied.