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Recently, personalized medicine applied
here as individualized surgery, which represents the customization in insertion
sites, size of the graft, tunnel placement and graft tension for each
individual patient, has been proposed. A question is whether the surgeon’s task
defines the level of analysis. The alternative presented here proposes the
viewpoint of understanding the affordances of the knee as a means to influence
the user. This study sought to offer a design method supporting knee
reconstruction surgery, affording an approach for the user/surgeon/therapist.
This explains how perceivers judiciously
pick up the instantaneous knee screw (IKS) as haptic information for
perceiving affordances. For shank and thigh, must compound into a screw on IKS,
whence walkers perceive affordances of the cylindroid. Thus the cylindroid must
become a familiar conception with the students of the affordance-based surgical
design.
Keywords: Self turning system of the knee,
ACL impingement, Kneesynergy, Instantaneous knee screw (IKS), Invariant screw,
Information pick-up, Gibsons affordance-based surgical design
INTRODUCTION
The
anterior cruciate ligament (ACL) is a critical knee joint, bone-to-bone
connected, stability ligament, which is attached from an anterior location of
the proximal tibia to a posterior location of the distal femur. Proper
positioning of the ACL graft has been proven to be of paramount importance for
graft longevity. Malposition of the ACL graft, in the anterior placement of the
tibial tunnel, was associated with roof impingement. Roof impingement of the
reconstructed ACL graft was recognized early as a significant factor that leads
to graft failure [1].
Recently,
personalized medicine applied here as individualized surgery, which represents
the customization in insertion sites, size of the graft, tunnel placement and
graft tension for each individual patient has been proposed [2]. A critical decision-making issue pertains
to the surgeon’s placement of a tibial-femoral tunnel so as to mimic the native orientation of the ACL attachment [2]. Thus,
the surgeon’s concern has to do with particular aspects of the local anatomy
and, thus, by extension, the particular
biomechanical artifacts that can be manipulated by the surgeon. A question is
whether the surgeon’s task defines the level of analysis. The alternative
presented here proposes the viewpoint of understanding the affordances of the
knee as a means to influence the user.
The affordances of a product are what it provides, offers, or furnishes to a user or another product. Affordance links perception to action, as it links a creature to its environment. The concept of affordance implies a special approach to psychology, particularly to perception-the ecological approach [3]. We applied Gibson’s concept of affordance to the design of artifacts in general and, in particular, to the domain of anatomic artifacts having a biomechanical influence. Gibson demonstrated how human (and animal) perception and motor control is continuous with interactions between inanimate physical systems [3]. The point of view of this paper is functional, in the old sense, but also in a modern sense that incorporates systems theory.
In the context
of engineering design, an affordance was defined as a relationship between two
subsystems in which a potential behavior can occur
that would not be possible with either subsystem acting in isolation [4]. Maier
and Fadel coined the term artifact-artifact affordance (AAA). These affordances
between artifacts and the people that use them are called artifact-user
affordances (AUA). For example, the gear pair (Figure 1(a)) is referred
to as an artifact-artifact affordance (AAA) for uniform, continuous motion
transmission between two parallel axes, which we will call “resonance” or
“tuning”, and it is possible only if the line of action passes through a fixed
point, known as the pitch point. The trajectory and resting pitch point are not
monitored and adjusted (Recall that neither joint ‘gear’ perceives the other
gear) but simply conjugate uniform motion transmission by virtue of their
structure, the principle of structural influence behavior [5].
However,
the limbs and extremities are, of course, motor organs as well as sensing
organs, but the function of motor performance can be subordinated to the task
of exploratory adjustment in the case of the limbs. This “resonance” or
“tuning” of the perceptual system to environmental information is different
from the resonance of a gear pair, as shown in Figure 1(a) since these
are fixed and will only resonate to a specific kind of their structure-a
particular gear profile.
The aim of this study is to identify the
measurable invariants as a basis for structural placement of the tunnel during
ACL reconstruction as a positive affordance-based design. We drew on a surgical
technique as an example of how the theory of affordances may be utilized for
high-level affordance-based design by customizing the placements of the tunnel
during the reconstruction of the anterior cruciate ligament.
MATERIALS & METHODS
It is
not often realized that it is the function of a joint to not merely permit
mobility of the articulated bones, but also to register the relative position
and movement of the bones. Information about things and events exists in
ambient arrays of energy. Actions have consequences that turn up new
information about the environment. They also provide information about the
actor-about where he is, where he is going, what he is doing. All actions have
this property, but it is useful to distinguish executive action from action
that is information-gathering. Other scientists also made an extension of a
distinction James Gibson made between two modes of activity: exploratory and
performatory. Without this distinction, psychologists will forever be
separating into camps–one a group of objectivists, the other a group with
subjectivist sympathies.
We do
not simply gait, we walk. The knee system is a motor system, as well as a
sensory one. Perception occurs over time and is active. Action participates in
perception. Active adjustments in the sensory systems are essential. But action
itself may be informative, too. It was developed in detail by Gibson ̶ e.g. in
his experiments on active touch [6]. Perception is a self-tuning process, in
which the pick-up of environmental information is intrinsically reinforcing so
that the system self-adjusts so as to optimize its resonance with the
environment: “A system `hunts' until it achieves clarity”, wrote Gibson, a
little like the scanning of a modern digital tuner [7]. We perceive affordance
of the ground to be walked on.
The
affordances of the environment are what it offers the animal, what it provides
or furnishes, either for good or ill. The verb to afford is found in the
dictionary, but the noun affordance is not. I have made it up. I mean by it
something that refers to both the environment and the animal in a way that no
existing term does. It implies the complementarity of the animal and the
environment [3].
We proposea
self-tuning system of the perceptual organ of the knee where six constraints
($) are collectively reciprocal to the instantaneous knee screw (IKS or$)
indicated by ⊗ (Figure
1(b)).
Recently,
the self-tuning system of the perceptual organ of the knee approach proposed
herein was validated experimentally [8]. That study experimentally described
the invariant of the self-tuning system of the knee (STSK) by measuring whether
all the lines of action intersect at the IKS ($) following natural knee motion,
resulting in the mean distances between each constraint line of action and the
IKS staying below 3.4 mm and 4.5 mm for ex vivo and in vivo assessment,
respectively (Figure 2(b)). At issue is how an affordance level of
analysis is referred to as the STSK?
RESULTS
As
described, we identified the measurable second-order invariant of knee synergy
and proposed it as a new view of the basis of tunnel placement (Figure 1(b)).
The knee synergy approach identifies the information as a means to perceive the
affordance of uniform motion transmission. To apply the described approach and
identify the invariant, we characterized the shank to the thigh (the tibia to
the femur) relative motion, i.e., the second-order invariance of the knee
synergy. These results were then compared with experimental data for validation
as provided by the “Grand Challenge Competition to Predict In Vivo Knee Loads” as part of the Symbiosis project funded by the
National Institutes of Health [9]. The IKS is defined in terms of the
second-order invariant by a linear combination of the two screws of the
first-order invariant, S and T, instantaneous screw axes of the shank and thigh
(Figure 2(a)) [10]. Then the IKS must be a screw that has been picked up
from the many candidate screws on the cylindroids [11], which is reciprocal to
STSK (Figure 1(b)). Hence, the ratio of the amplitudes about S and T,
(S, T) may be determined (Figure 2(a)), which manifests the fact that
the sensitivity of the knee joint to its disposition is of crucial importance
in picking up perceptual information. Figure 3 shows that the
deterministic character of (S, T), the ratio of the angular velocity (unit of
rad/0.05 sec) of the two limb segments, the shank and thigh, involving at the
knee joint.
To
validate our ecological approach to perception and motor control during the
stance phase of a gait, we used previously published experimental datasets [9]
and compared their measured medial and lateral contact forces with our
predicted datasets [12]. Predicted and measured values of constraint forces at
the lateral contract are compared with the RMS error being 148.1 N [12].
Available data included limb motion capture, fluoroscopy images, GRFs,
electromyo graphical readings determining muscle forces, as well as medial and
lateral knee contact forces derived from GRFs. Data were collected from an
instrumented right knee reconstruction in an adult male subject (65 kg mass and
1.7 m height). When the variations in the ground contact patterns (magnitudes and
direction) are shown along with the variations of knee movement in terms of
IKS, the STSK-is uniquely determined by the two corresponding pairs (Figure
4).
The IKS
was determined by a linear combination of two instantaneous screw axes of the
shank and thigh (Figure 3). The IKS is shown to nearly coincide with a
reciprocal screw of the GRF, as indicated in a magnified inset image (Figure
4). Expressing this differently, the DOF of instantaneous screws and the
constraints of impulsive screws are projective, where virtual work is done on
the IKS should vanish and merge into the STKS directly or instantaneously. The
statement that there are no constraints to such a system is merely a different
way of asserting the obvious proposition that when a body is perfectly free, it
cannot remain in equilibrium if the forces which act upon it have a resultant.
This reciprocity is captured by the concept of a mutuality relationship between
the constraints and the DOF [13, 14]. Information about the subject accompanies
information about the environment. Here it is shown that ego reception
accompanies exteroception; subjective and objective information is available to
specify both characteristic poles. One pole perceives the environment, and
other pole co-perceives itself [3].
DISCUSSION
In this
study, we present the positive affordance-based design objective on graft
placement while in continuous tension, rather than designing against the
negative affordance by preventing impingement. For the ACL-patient to engage
the IKS directly, clinicians have to measure the tunnel placement relative to
the posture and behavior of the person being considered, making continuous
graft tension possible. If the path of an ACL graft is so selected that it cuts
the IKS of the self-tunning system of the knee (STSK) then the line becomes a
member of the STSK, which ensures the isokinetic graft placement related to
trans-tibial-femoral tunneling.
If a
self-tuning system of the perceptual organ of the knee can be pre-stressed to
get the same configuration as if external loads were applied, then this feature
allows the pre-stress to be chosen to yield the same configuration in the swing
phase (external forces are absent), as will be achieved in the stance phase
where the external forces are present [15]. It has also been noted that
preparedness is not only a reactive aspect of the movement apparatus, it also
relates to anticipatory adjustments that predispose a system to behave in a
particular way [16]. Bruner studied the “attainment of competence”. “In the
growth of such competence in infants, three themes are central-intention,
feedback and the patterns of action that mediate between them” [17]. Bruner’s
description of an infant’s actions in capturing an object differed from earlier
descriptions of reaching and grasping because he emphasized the intentional,
unified character of the action. Bruner quoted Bernstein’s model [18] for
programming an action, one that emphasizes neither reflexes nor random
responses but “future requirements” [6].
The
Russian physiologist Bernstein [18] defined coordination as a problem of
mastering the very many degrees of freedom (DOF) involved in the particular
movement by reducing the number of independent variables to be controlled [19].
However, in order to reduce the number of musculoskeletal DOFs upon which the
nervous system must operate, we have adopted the proposition that the nervous
system controls muscle synergies, or groups of co-activated muscles, rather
than individual muscles [20, 21]. Bernstein (1967) saw that this meant that
actions must be planned at a very abstract level because it is impossible for
the central nervous system to program all of these local, contextually varying,
force-related interactions specifically and ahead of time. Indeed, once a
decision to move has been made, the subsystems and components that actually
produce the limb trajectory are softly assembled (to use a term introduced
later by [22]) from whatever is available and best fits the task. This type of
organization allows the system great flexibility to meet the demands of the
task within a continually changing environment while maintaining a movement
category suited to the goal in mind [23].
CONCLUSION
It will
greatly facilitate our further surgical design process to introduce how
information is picked up, which will clearly exhibit the determinate character
of the (S, T). It must thus be apparent the sensitivity of the knee to its disposition,
IKS depends upon one parameter (S, T) family, and that consequently, the
different position of IKS would form a single infinite series, known in linear
geometry as a cylindroid proposed [11]. Then walkers must pick up IKS uniquely
from the screw on the cylindroid, which is reciprocal to the GRF. This explains
how perceivers judiciously pick up the IKS as haptic information for perceiving
affordances. For S and T, must compound into a screw on IKS, whence walkers
perceive affordances of the surface. It demonstrates that a unique combination
of invariants, a compound invariant, is just another invariant [3].
Walkers
explore the cylindroid (S, T), which contains these two screws. Since (S,T) are
appropriated to two different body segments of the knee joint, no kinematic
significance or meaning can be attached to the composition of the two twists on
(S,T). If however, the two twists on (S,T), having the proper ratios of
amplitudes, had been applied to a single rigid body, the displacement produced
is one which could have been effected by a single twist about a single screw on
the cylindroid (S,T). If this intermediate screw, the IKS, be given, the ratio
of the amplitude of the twists on the given screw (S, T) is determined.
This
study sought to offer a design method supporting knee reconstruction surgery,
affording an approach for the user/surgeon/therapist. Thus the cylindroid must
become a familiar conception with the students of the theory of affordance
based design.
ACKNOWLEDGMENT
Author
WK extends thanks to Ms. Flavia Yazigi for her hard work with the radiography
and a long recruitment process. WK also thanks to his mother-in-law, Ms. Sun
Lee, for her continuous encouragement for this research. The experimental data
used for validation were provided by the “Grand Challenge Competition to
Predict In Vivo Knee Loads” as part
of the Symbiosis project funded by the US National Institutes of Health via the
NIH Roadmap for Medical Research (Grant # U54 GM072970).
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