**3. Artifact-user affordances versus artifact-artifact affordances**

Gibson demonstrated how animal perception and action is continuous, with interactions with inanimate objects or surfaces [14]. The affordances of a product are what it provides, offers, or furnishes to a user. Gibson's 'system theory' of perception corresponds to an open system, which is rather different from the view of isolated artifacts [29]. For engineering design, an affordance can be defined as the relationship between person and artifact from which the behavior emerges. These affordances between artifacts and the people that use them are called artifact-user affordances (AUA).

For example, the gear pair (**Figure 2(a)**) is referred to as an artifact-artifact affordance (AAA) for uniform motion transmission between two parallel axes, and it is possible only if the line of action passes through a fixed point, known as the pitch point. Moreover, assuming that gear 1 rotates with constant angular velocityω<sup>1</sup> , the motion is transferred by direct contact at points K1 and K2. The objective is to determine whether or not the angular velocity ω<sup>2</sup> will remain constant or present uniform motion transmission. Kennedy's theorem identifies the fundamental property of two interacting rigid bodies in motion [30], such that three instantaneous centers shared by three rigid bodies in relative motion to one another, all lie on the same straight line. Uniform motion transmission between two parallel axes is possible only if the line of action passes through an invariant point, known as the pitch point. The pitch point is the instantaneous center of velocity for the two gears. For the gear teeth to remain in contact, the two-component velocities along the common normal must be equal. The absolute velocities along the line of action must

**55**

**function**

*The Knee Proprioception as Patient-Dependent Outcome Measures within Surgical…*

be identical; otherwise, bodies 1 and 2 become separated. It was shown that if the involute profile describes *the gear profile*, the common normal does not change its direction because it is an invariant of the structure. However, this may not be the case

*(a) The artifact-artifact conjugate action, in the form of two interacting gears, demonstrating the uniform motion transmission between two parallel axes as can be found in a knee joint. (b) the knee joint synergy as represented by six constraints* ( ) \$ 16 *<sup>i</sup> ,i ,..,* ′ = *, which are conjointly reciprocal to the instantaneous knee screw (\$) as indicated by their intersections (at the* ⊗ *'s). A balance of forces happens when the virtual coefficient* 

Gibson claims that some affordances are beneficial while others are injurious, such as maintaining a line of action versus veering off course [14]. These benefits and disadvantages, safeties and dangers, positive and negative affordances are properties of events taken with reference to an observer, and not properties of the observer's experiences; they are not personal values or feelings of pleasure or pain added to neutral perceptions [14]. For example, physical properties of the tibial tunnel and the intercondylar notch roof are not affordances in and of themselves, but they do determine what affordances are offered to a surgeon depending on a patient's anatomic features. Thus, the characteristics that affect positive AUA are the same as those affecting negative AUAs. The artifact only has one set of characteristics, which is a customization of the tunnel placement, and this is all that the designers or surgeons can act upon. As a consequence of such mutuality, affordances do not exist in the patient's tunnel or intercondylar roof, but in what they offer to the surgeon. Importantly, AUAs may conflict with one another when the graft becomes slack or loose (i.e., loss of extension in the graft at full extension, or the graft being trapped in the notch), indicating a negative affordance or an increase in the potential for injury. Thus, when a surgical designer identifies a functional range in which a joint is not allowed to fail, they need to constrain the target bounds for that same joint to enhance positive affordances and avoid negative

*DOI: http://dx.doi.org/10.5772/intechopen.94887*

if we consider it in terms of affordances.

*vanishes, being it the necessary and sufficient condition for knee equilibrium.*

**Figure 2.**

affordances. This approach is addressed in the section below.

**4. The affordance-based design applied to reconstructed knee joint** 

A joint 'gear' cannot perceive itself or its joint gear since gears are inanimate. Gears simply conjugate uniform motion transmission by virtue of their tensegrity *The Knee Proprioception as Patient-Dependent Outcome Measures within Surgical… DOI: http://dx.doi.org/10.5772/intechopen.94887*

#### **Figure 2.**

*Proprioception*

similar in both groups [19].

perceive stairs differently than young people. However, the ratio between step height and the distance between the stepping foot and the top edge of the step was

difficulty of identifying affordances with engineering [27].

Since the concept of 'affordance' was introduced almost 40 years ago, it has been used in a variety of fields, including child psychology [20], the design of graphical user interfaces [21], mobile robots [22], control room interfaces [23], and more recently, in engineering design [24–26]. The impetus for any design project can be understood in terms of creating and changing affordances. The design process is the construction of an artifact that offers specific affordances, but not certain undesired affordances. An artifact with more positive affordances is considered better, while an artifact with more negative affordances is considered worse. However, this approach does not follow ecological psychology, but instead, it addresses the

Maier and Fadel coined the term artifact-artifact affordance (AAA) [24, 25]; however, AAA has not been properly incorporated within the larger theory of affordances. Although AAA was developed as a new concept, the idea that inanimate objects offer action possibilities in an organism is a foundational concept known since Gibson's work in ecological psychology. The ecological approach demonstrates how animal (including human) perception and action is continuous with interactions between inanimate physical systems, or the world in general. The entrainment of separate limbs during biological coordination, for example, follows the same physical laws as entrainment between two pendulum clocks or other purely mechanical (inanimate) systems [28]. The fact that interactions between inanimate and animate systems are continuous precludes the need to identify AAA as a distinct category. In short, these concepts should be used with great care if knowledge is to be gathered. In the present study, we used a surgical technique as an example of how

the theory of affordances may be utilized for affordance-based design.

**3. Artifact-user affordances versus artifact-artifact affordances**

Gibson demonstrated how animal perception and action is continuous, with interactions with inanimate objects or surfaces [14]. The affordances of a product are what it provides, offers, or furnishes to a user. Gibson's 'system theory' of perception corresponds to an open system, which is rather different from the view of isolated artifacts [29]. For engineering design, an affordance can be defined as the relationship between person and artifact from which the behavior emerges. These affordances between artifacts and the people that use them are called artifact-user

For example, the gear pair (**Figure 2(a)**) is referred to as an artifact-artifact affordance (AAA) for uniform motion transmission between two parallel axes, and it is possible only if the line of action passes through a fixed point, known as the pitch

ω

ω<sup>1</sup> , the

<sup>2</sup> will remain constant or present

point. Moreover, assuming that gear 1 rotates with constant angular velocity

determine whether or not the angular velocity

motion is transferred by direct contact at points K1 and K2. The objective is to

uniform motion transmission. Kennedy's theorem identifies the fundamental property of two interacting rigid bodies in motion [30], such that three instantaneous centers shared by three rigid bodies in relative motion to one another, all lie on the same straight line. Uniform motion transmission between two parallel axes is possible only if the line of action passes through an invariant point, known as the pitch point. The pitch point is the instantaneous center of velocity for the two gears. For the gear teeth to remain in contact, the two-component velocities along the common normal must be equal. The absolute velocities along the line of action must

**54**

affordances (AUA).

*(a) The artifact-artifact conjugate action, in the form of two interacting gears, demonstrating the uniform motion transmission between two parallel axes as can be found in a knee joint. (b) the knee joint synergy as represented by six constraints* ( ) \$ 16 *<sup>i</sup> ,i ,..,* ′ = *, which are conjointly reciprocal to the instantaneous knee screw (\$) as indicated by their intersections (at the* ⊗ *'s). A balance of forces happens when the virtual coefficient vanishes, being it the necessary and sufficient condition for knee equilibrium.*

be identical; otherwise, bodies 1 and 2 become separated. It was shown that if the involute profile describes *the gear profile*, the common normal does not change its direction because it is an invariant of the structure. However, this may not be the case if we consider it in terms of affordances.

Gibson claims that some affordances are beneficial while others are injurious, such as maintaining a line of action versus veering off course [14]. These benefits and disadvantages, safeties and dangers, positive and negative affordances are properties of events taken with reference to an observer, and not properties of the observer's experiences; they are not personal values or feelings of pleasure or pain added to neutral perceptions [14]. For example, physical properties of the tibial tunnel and the intercondylar notch roof are not affordances in and of themselves, but they do determine what affordances are offered to a surgeon depending on a patient's anatomic features. Thus, the characteristics that affect positive AUA are the same as those affecting negative AUAs. The artifact only has one set of characteristics, which is a customization of the tunnel placement, and this is all that the designers or surgeons can act upon. As a consequence of such mutuality, affordances do not exist in the patient's tunnel or intercondylar roof, but in what they offer to the surgeon. Importantly, AUAs may conflict with one another when the graft becomes slack or loose (i.e., loss of extension in the graft at full extension, or the graft being trapped in the notch), indicating a negative affordance or an increase in the potential for injury. Thus, when a surgical designer identifies a functional range in which a joint is not allowed to fail, they need to constrain the target bounds for that same joint to enhance positive affordances and avoid negative affordances. This approach is addressed in the section below.
