The entropy law and eye plastic surgery: Risk, uncertainty, and irreversible degradation in the surgical process.

Eye plastic surgeons, and for that matter, most surgeons do not ponder how the laws of thermodynamics might effect the outcome of surgery. This is unfortunate because these physical laws have startling implications for surgeons and their patients. The laws of thermodynamics help us to understand why an unfavorable surgical outcome can occur despite every effort to be as precise and accurate as possible in performing our surgery. These physical laws also allow us to appreciate the important difference between risk and uncertainty. Finally, by understanding the role of the second law of thermodynamics in surgical outcome, we can begin to understand a previously poorly understood class of surgically induced aesthetic and functional complications.

The first law of thermodynamics states that the energy of a closed system is conserved.1 A closed system is one that does not communicate with other systems outside its boundaries. The universe is considered a closed system. However, a less abstract example is that of a hypothetical container filled with a gas of a certain temperature. If at time zero this gas is initially confined to a small portion of the vessel, as time passes, the gas becomes evenly distributed through out the container. Initially, the energy represented by the movement of the gas from this concentrated area might be use to power a small wind turbine to do work. However, once the gas is evenly distributed through out the container, no more useful work can be done by the gas. The total energy of the system is not changed. However, it is no longer available to perform work. The energy of the system has moved from a higher free energy state to a lower free energy state. The directionality of free energy to decrease in time is one form of the second law of thermodynamics. The second law of thermodynamics can also be summarized as the entropy of the universe must increase.

It is a physical fact that the energy of systems moves in one direction from higher to lower free energy states. However, entropy increases as free energy decreases over time. The laws of thermodynamics relate to the statistical behavior of the microscopic particles that make up a macroscopic system. Entropy is a system property representing the microscopic disorder. The lower the entropy, the more ordered is the system. In the case of the gas in the container, the initial conditions where the gas is confine to a small portion of the container is a lower entropy state than the final condition with gas uniformly distributed throughout the container.

The subject of entropy can be confusing because entropy is often related directly to the idea of order, as we understand it in everyday experience. Intuitively we might think of a tidy room as one having lower entropy than a room that is untidy. However, the entropy of these two macroscopic systems is probably the same. To better understand what would represent a change in the entropy of the room, picture the room contents being destroyed by fire. Now the free energy that bound the molecules composing the items of the room have been released in the form of heat. The energy of this system is no longer available to do potential work. The incinerated contents of the room have higher entropy than the room contents before the fire. Lower entropy systems are more "orderly" than high entropy systems in the sense that the microscopic components of the higher entropy system can occupy a larger number of potential states.

This idea of orderliness means that there is greater certainty about what states the low entropy system will be found. The more certain we are, the more information we have. This leads to an important relationship between entropy and information.2 Consider the Maxwell demon from statistical mechanics. The hypothetical demon is positioned at a gate that separates two chambers filled with a gas. The gas molecules move randomly in the chambers with a distribution of velocities determined by the average temperature. Suppose the demon operates the gate to isolate the faster gas molecules in one chamber and the slower molecules in the other. Eventually, the chamber containing the faster gas molecules becomes warmer than the chamber with the slower gas molecules. This temperature gradient could be used to do work and the process repeated to create a perpetual motion machine. This clear violation of the first law of thermodynamics does not occur. The energy gained by this scheme is just cancelled by the energy needed by the Maxwell demon to determine the velocity of the approaching gas molecule and act on this information. This paradox is implicit in the Heisenberg uncertainty principle from quantum mechanics that determining the position or momentum of a particle affects the energy of that particle.3

With these concepts as a foundation, we are ready to consider matters of interest to surgeons. Schrodinger, in his book What is Life, noted that one of life's fundamental aspects was its ability to extract order from the environment to evade the decay to equilibrium.4 He observed that living things seem to defy the second law of thermodynamics and operate far from equilibrium. Living things are complex systems that are open to fluxes of energy and materials and reside in semi-stable states.5 They exist by virtue of increasing the entropy of the larger system in which they are embedded. They operate by extracting high-grade energy and low entropy from the environment and dissipating degraded energy and material into the environment in the form of heat and waste. As we can see then, living systems do not violate the laws of thermodynamics.

Eddington described entropy as time's arrow.6 It is the basis for the psychological perception that time moves in only one direction.7 A discussion of the mechanisms of aging is beyond the scope of this paper.8,9 However, we know that human beings grow up through childhood, maturing into adulthood before becoming senescent. The process never occurs in reverse. While the body possesses significant homeostatic mechanisms, including the ability to repair injuries and fight infection, these mechanisms are not capable of reversing or stopping the aging process. Physiological processes do not cause irreversible changes. In contrast, disease processes can shift the living systems from relative stability toward states of lower stability. 10 Premature destabilization of the steady state of entropy production in these open systems is thought to accelerate the aging process.11 Irrespective of age and disease process, body structure represents highly ordered, low entropy states.

It is the goal of the surgeon to increase the organizational state of the tissues to enhance appearance or improve function. The surgeon must expend energy to achieve this result. This is also not a violation of the laws of thermodynamics, because while the entropy of the tissues being operated on may decrease, it is at the cost of a net increase in the entropy of the universe. We go wrong as surgeons when we assume that a lower entropy state is the only possible outcome of surgery. In fact, surgery can and probably always increases the entropy of the tissues on which we are operating. Just think about what happens to tissues when using an electrocautery during surgery. It is the clinical significance of the entropy change that is relevant to the surgeon. It bears repeating that the change in entropy that follows surgery resides both in the change of appearance or function, and also in the change of free energy expended by the surgeon.

What does a face or eyelid look like when its entropy increases? The possibilities are limitless. These states are hard to describe but clinically easy to recognize. Essentially any state that is worse after surgery represents a clinically significant increase in the entropy of that system. Any feature that does not function as well or is more dysmorphic represents degradation in the low entropy state. I call this surgical degradation. If the changes cannot be corrected, they are irreversible. To combine surgical and thermodynamic parlance, this would be a change of state complication. However, the truth is that a properly done facelift or upper eyelid ptosis surgery also represents an increase in entropy. Anyone revising an upper eyelid ptosis surgery knows that the tissues planes are never the same after the first surgery irrespective of whether the original surgery was underdone, overdone, or correctly performed. The difference is that when the clinical result is the desired one, we are satisfied with the outcome. The point is that the surgeon cannot necessarily predict the exact outcome associated with this increase in system entropy no matter how precise and accurate the surgery.

It is easy to understand how excessive upper blepharoplasty and resulting lagophthalmos can interfere with eyelid closure and lead to corneal break down. It may be less obvious how aesthetic alterations of the face and eyelids affect the information conveyed by the appearance. It is a mistake to consider that changes caused by aesthetic surgery are culturally relative. The adage that "beauty is in the eye of the beholder" would suggest that aesthetic judgments relate to cultural or possibly ethnic preferences rather than something more fundamental. Research on facial attractiveness demonstrates that aesthetic judgment of facial shapes is similar across different cultural backgrounds. This suggests that facial aesthetic values are rooted in evolutionary selection pressures and as such are broadly shared by humans.12,13 This immediately relates to the concept of negative entropy. In its simplest form, information represents a decrease in uncertainty or negative entropy.14 Noise in a communication channel degrades the information in that channel and creates uncertainty. The facial appearance is an important means of communicating social and sexual fitness and therefore plays an important functional role in reproductive and social success.15 We clearly understand that the goal of aesthetic surgery is to alter some of the stigmata of aging and that successful aesthetic surgery is often associated with a more youthful appearance.16 When these surgeries are less than successful in creating the verisimilitude of sexual or social fitness, confusion and uncertainty in the mind of the beholder results. This is where we can actually perceive the increase in entropy that results following surgery.

We have difficulty understanding and describing change of state complications because we lack a scientific vocabulary for talking about them. The absence of a vocabulary reflects a deficiency in comprehending the nature of these problems.17 For example we do not have a term to describe the stretch back of eyelid skin into the eyebrow when too much upper eyelid skin is resected as part of an upper blepharoplasty. We lack an agreed upon set of terms to describe inferior scleral show and rounding of the lateral canthal angle following transcutaneous lower eyelid surgery. We have no term to describe the inability of the eyelids to close due to a lack of force generation when too much pretarsal orbicularis oculi muscle has been resected. There is no scientific term that describes the wind swept look in the face after an over-done face lift or what happens to skin quality following excessive laser resurfacing.

We lack these terms because little thought is given to irreversible surgical degradation and change of state complications. I propose that the iatrogenic transformation of a lower entropy state into a higher entropy state is the quintessential, although by no means, the sole reason that surgery evolves. The more frequent the complication, the more urgent the pressure to improve. Explicitly studying this problem should accelerate the evolution of surgical progress.

Our surgeries converge to a finite set of procedures performed in relatively specific ways. Uncertainty in outcomes and dissatisfaction with results creates pressure to evolve technique. Our patients don’t like surgery that increases clinically significant facial or eyelid entropy. As surgeons we don’t like it either. Consider the evolution of surgery for lower eyelid laxity or frank ectropion. We no longer do the Bick procedure, which results in a lower eyelid margin contour abnormality. Instead the lower eyelid is shortened with a lateral tarsal strip. However, when there is relative proptosis, shortening the lower eyelid often causes the lower lid margin to be pulled under the curvature of the globe to follow a geodesic, the shortest distance between to points on a curved surface.18 Having recognized and defined this unexpected problem, lower eyelid tightening in cases of relative proptosis is now often combined with midface surgery and the placement of a spacer graft to vertically lengthen the eyelid while at the same time horizontally shortening it. This combination of procedures has made the surgical outcome more predictable.19

We discuss surgical risk with our patients but how often do we discuss irreversible surgical degradation and potential change of state complications? We only allude to them in the most heuristic of ways. Risk applies to calculating probabilities when the exact outcome is not fixed but will be one of several known possibilities. Uncertainty attaches when we don’t know the outcome. How many of our patients would agree to a surgery if we told them we don’t know what will be? Instead of performing a clinically therapeutic surgery, we are in fact conducting more of an experiment. In truth, all surgeries carry some uncertainty because in each case, the outcome can be unique. As surgeons, we prefer the most predictable procedures. That is why a muellerectomy is often a better choice than a levator resection ptosis surgery when a patient is an appropriate candidate.20

By recognizing that surgical interventions can introduce irreversible degradations and that the face possesses only a finite capacity to maintain a low entropy state, surgeons should rethink their role in altering the appearance and dysfunctional states of their patients. Each surgical intervention is associated with a definite irreversible cost. In thinking about and performing surgery, we must incorporate this paradigm in approaching our patients. Surgical choices must be based on a fundamental understudying that our patients have only finite resources to look good and natural with normal function in their lifetimes. The entropy law tells us that untoward and inappropriate surgical choices can result in irreversible degradation in our patients. The task remains to catalog and describe change of state complications and improve surgical technique to reduce these outcomes.

References

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