Biomecanica de rodilla

MINI-SYMPOSIUM: SOFT TISSUE SURGERY IN THE KNEE

(i) Biomechanics of the knee joint
S D Masouros A M J Bull A A Amis

Abstract
The knee joint has biomechanical roles in allowing gait, flexing and rotating yet remaining stable during the activities of daily life, and transmitting forces across it. Geometrical, anatomical and structural considerations allow the knee joint to accomplish thesebiomechanical roles. These are addressed and discussed in this article.

range of motion. Similarly, the PF joint has a complex, threedimensional range of motion across TF joint flexion in order to allow for minimal quadriceps contraction to extend the knee. This complex mechanism of knee joint motion means that the geometry itself is not adequate to maintain stability, requiring input frompassive soft tissue (e.g. ligaments) and muscle tensions. It also means that large forces acting on small articulating areas generate high articular stresses, commonly called joint contact pressure. The complexity of knee joint behaviour is a result of the individual behaviour of and interaction between three different factors:  static stability e geometry and anatomy of the joint surfaces.  activestability e muscle contraction.  passive stability e ligaments, menisci and retinacula.

Knee joint kinematics
The primary motion of the TF joint is rotation in the sagittal plane (flexioneextension); therefore a simplified description is that it acts as a hinge joint in the sagittal plane. The complex three-dimensional motion of the TF joint can be described through six degrees of freedom (threerotations and three translations) in a clinical joint coordinate system1 (Figure 1). Full extension (i.e. 0 flexion) is usually defined when the long axes of the tibia and femur are aligned in the sagittal plane. Active knee flexion is possible primarily through hamstring contraction and usually reaches 130 , whereas passive flexion can reach 160 . Some individuals can hyperextend (i.e. negativeflexion) to up to À5 flexion. When at full extension, the knee allows for optimized weight support and stability. If the body was at rest in some other angle of flexion, then the vertical line of action of the ground reaction force would pass posterior to the knee joint, which would mean that the quadriceps muscles would need to do work to maintain posture; this would be energetically

Keywordsbiomechanics; joint; knee; kinematics; ligaments; tissue
mechanics

Introduction e basic function of the knee joint
The knee joint comprises two distinctly separate joints/articulations; the tibiofemoral (TF) joint e the articulation of the femur over the tibia e and the patellofemoral (PF) joint e the articulation of the patella over the femur. The main roles of the knee joint complex are (1) toallow locomotion with (a) minimum energy requirements from the muscles and (b) stability, accommodating for different terrains, and (2) to transmit, absorb and redistribute forces caused during the activities of daily life. Clinical interest in the knee joint stems from the frequency of injury and pain resulting from degenerative changes, and there are mechanical factors that are associated withthis. The aim of this article is to review the knee joint from a clinically driven mechanics viewpoint. The knee joint acts as a pivot between the two longest bones in the human body whilst the strongest muscles in the body (the quadriceps muscles) act across it. The TF joint has a wide range of motion, reaching up to 160 of flexion (rotation in the sagittal plane), with coupled rotations in theother two planes; this leads to incongruency between articulating surfaces across part of the

S D Masouros PhD DIC DiplEng Research Associate in the Department of Bioengineering, Imperial College London, London SW7 2AZ, UK. Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, UK. A M J Bull PhD DIC ACGI CEng FIMechE Professor of Musculoskeletal Mechanics in the...