Francisco Ezquerro Juanco

Comparative Biomechanical Study on Contact Alterations After Lateral Meniscus Posterior Root Avulsion, Transosseous Reinsertion, and Total Meniscectomy

PURPOSE To compare the effects of lateral meniscus posterior root avulsion left in situ, its repair, and meniscectomy on contact pressure distribution in both tibiofemoral compartments at different flexion angles. METHODS Eight cadaveric knees were tested under compressive 1000 N load for 4 lateral meniscus conditions (intact, posterior root avulsion, transosseous root repair, and total meniscectomy) at flexion angles 0°, 30°, 60°, and 90°. Contact area and pressure distribution were registered using K-scan pressure sensors inserted between menisci and tibial plateau. RESULTS In the lateral compartment, root detachment decreased contact area (P = .017, 0° and 30°; P = .012, 60° and 90°) and increased mean (P = .012, all angles) and maximum (P = .025, 0° and 30°; P = .017, 60°; P = .012, 90°) pressures relative to intact condition. Repair restored all measured parameters close to intact at 0°, but effectiveness decreased with flexion angle, yielding no significant effect at 90°. Meniscectomy produced higher decreases than root avulsion in contact area (P = .012, 0° and 90°; P = .05, 30° and 60°) and increases in mean (P = .017, 0° and 30°; P = .018, 90°) and maximum pressure (P = .012, 0°; P = .036, 30°). In the medial compartment, lesion changed the contact area at high flexion angles only, while meniscectomy induced greater changes at all angles. CONCLUSIONS Lateral meniscus posterior root avulsion generates significant alterations in contact area and pressures at lateral knee compartment for flexion angles between full extension and 90°. Meniscectomy causes greater disorders than the avulsion left in situ. Transosseous repair with a single suture restores these alterations to conditions close to intact at 0° and 30° but not at 60° and 90°. CLINICAL RELEVANCE Altered contact mechanics after lateral meniscus posterior root avulsion might have degenerative consequences. Transosseous repair with one suture should be revised to effectively restore contact mechanics at high flexion angles.

Synthesis of Planar Mechanisms

Design in Mechanical Engineering must give answers to different requests that are based on two cornerstones, analysis and synthesis of mechanisms. Analysis allows determining whether a given system will comply with certain requirements or not. Alternatively, synthesis is the design of a mechanism so that it complies with previously specified requirements. For instance, mechanism synthesis allows finding the dimensions of a four-bar mechanism in which the output link generates a desired function with a series of precision points or a mechanism in which a point follows a given trajectory. Therefore, synthesis makes it possible to find the mechanism with a response previously defined. Currently, there is a set of methods and rules that makes it possible to find the solution to many mechanism design problems. However, since this is quite a newly-developed discipline, there are still many problems that need to be solved. The concept of synthesis was defined in Chap. 1 as follows: synthesis refers to the creative process through which a model or pattern can be generated, so that it satisfies a certain need while complying with certain kinematic and dynamic constraints that define the problem (Fig. 10.1). Other definitions can be added but all of them will somehow express the idea of creating mechanisms that can carry out a certain type of motion or, in a more general way, mechanisms that comply with a set of given requirements. There are several classifications for different types of synthesis but, basically, most authors agree on grouping the synthesis of mechanisms in two main branches: structural synthesis and dimensional synthesis.

Vibrations in Systems with One Degree of Freedom

A mechanical system can undergo mechanical solicitations that can either be constant or variable in time. The behavior of the system will change widely depending on what type of load is applied. Even loads of similar magnitude will have different effects. Traditionally, time variation of stress in a system was not considered and only calculations considering maximum stress were carried out. It was up to the designer and his experience to choose a higher or lower safety factor according to how variable the load was in order to anticipate any eventuality. Theoretical analysis methods gave the opportunity to study system characteristics and dynamical behavior with precision. These dynamical loads, which are variable in time, provoke deformations in elastic bodies that can give way to vibration processes. The study of vibrations is related to oscillatory motion of non-rigid bodies and the forces related to them. Any system with mass and elasticity can vibrate.

Graphical and Analytical Methods for Dynamic Analysis of Planar Linkages

We can basically distinguish two types of problems in the study of forces acting on a mechanism. On one side, we can determine the type of motion produced by a known system of forces (direct problem). On the other side, and provided that we know the variables defining the motion of the links, we can find the forces producing it (inverse problem). In this chapter we will focus on the second problem. Forces acting on a mechanism can be due to several reasons, such as the weight of the links (gravity forces), external forces, friction forces, acceleration of the links (inertial forces) and so on. However, the weight of the links is usually a negligible force compared to the other forces mentioned. Furthermore, if the mechanism is well lubricated, it is possible to neglect friction forces and obtain sufficiently accurate results, simplifying the problem significantly. For this reason we will only consider external and inertial forces in this book. We will first study the action of external forces applied to a mechanism in static equilibrium. We will begin by developing a method for the static analysis of linkages which, in addition to facilitating the comprehension of force transmission in mechanisms, will be useful to explain methods that will subsequently be applied when considering inertial forces of the links, that is, when performing a dynamic analysis. We will only study the action of forces in articulated mechanisms with planar motion and assume that all forces occur on the same plane, not on parallel ones. When the neglected moments are considerable, it will be necessary to make a second analysis on a plane perpendicular to the one on which forces are acting.

Kinematic Analysis of Mechanisms. Relative Velocity and Acceleration. Instant Centers of Rotation

Kinematic analysis of a mechanism consists of calculating position, velocity and acceleration of any of its points or links. To carry out such an analysis, we have to know linkage dimensions as well as position, velocity and acceleration of as many points or links as degrees of freedom the linkage has. We will point out two different methods to calculate velocity of a point or link in a mechanism: the relative velocity method and the instant center of rotation method.

Balancing of Machinery

In the previous chapter, we have studied forces that act on rigid bodies due to accelerations. We named them inertia forces and studied the effect they have on a mechanism. In most situations, these forces have a negative effect on the machine, as they cause parasite time-periodic stress with negative consequences for the machine. Vibration is an important issue that will be addressed further ahead in this book. Usually, the best way to handle vibrations in a machine is to equilibrate inertia forces and torques with other forces. The process of studying and making changes in a machine in order to reduce or eliminate shaking forces and torques is called balancing. A link in a planar linkage can have two basic types of motion, rotation and translation, which give raise to two different balancing processes: the balancing of rotating masses and the balancing of masses with reciprocating linear motion. Both methods will be analyzed in this chapter.

Analytical Methods for the Kinematic Analysis of Planar Linkages. Raven’s Method

Graphical methods have played an essential role along the history of Machine and Mechanism Theory. However, mathematical methods of mechanism analysis have gained major importance mainly due to the appearance of programmable calculators and personal computers. Above all, graphical methods have a great educational value and are interesting when it comes to solving mechanisms in one position, since, as well as being simple, they give us a clear view of their operation. Finding the mathematical solution to a mechanism is more complicated, as it requires a higher investment of time and errors are easier to make and more difficult to detect. However, the ability of the computer to save and re-use all operations implies that mathematical analysis can save a lot of time when we have to study a mechanism by varying parameters such as the length of the bars or the position, velocity and acceleration of the input link. It is also possible to easily obtain variable diagrams along one revolution. Mathematical methods basically try to obtain an analytical expression of the variables that we want to determine, such as position, velocity and the acceleration of any link in terms of the dimensions of the mechanism \( (r_{1} ,r_{2} ,r_{3} , \ldots ) \) and the position, velocity and acceleration of the motor link (\( \theta_{2} \), \( \omega_{2} \) and \( \alpha_{2} \)). Along this chapter, several mechanisms will be solved using Raven’s method in order to foment comprehension.