Can Scaling Theory Aid In Charcot Foot Reconstruction In Obese Patients?

Body mass index is an objective patient finding that is known to correlate with not only the development and outcome of diabetic foot ulceration but also perioperative and long-term patient morbidity and mortality.1 The Centers for Disease Control and Prevention (CDC) reported in 2010 that 35.7 percent of United States adults and almost 17 percent of youth are defined as obese based on the body mass index (BMI).2-4 Put bluntly, obese patients have worse clinical and surgical outcomes following the development of lower extremity pathology.

   Further, we do not expect that this situation and these obesity rates are going to improve over the next several decades. In fact, they are likely to become a progressively worsening problem. Faced with this dilemma, our profession has two options: Continue to work passively within our established paradigm and offer patients conventional interventions, or actively look to other fields of study to evaluate if other research and theories may present unique options for more successful outcomes.

   One possible well-established theory within the field of veterinarian biology is that of scaling theory, which recognizes two important concepts that may be applicable to our current situation. The first is that tissue systems of organisms cannot maintain constant morphology as body mass increases. The second is that constraints of locomotion and support are different in large and small organisms. In other words, the musculoskeletal anatomy, physiology and biomechanics of the lower extremity of an active and healthy patient with a BMI of 22 are not, and probably should not, be the same as the anatomy, physiology and biomechanics of a neuropathic patient with diabetes and a BMI of 48.

   Recent investigations have studied morphologic changes as they relate to scaling theory in the so-called graviportal animals (elephants, rhinoceroses, hippopotami, etc.).5-7 These are evolutionary anatomic and physiologic adaptations that have developed over centuries to assist these animals in withstanding an increased body mass. It is the modest proposal of this investigation to review this work and interpret whether these findings may be related to the surgical reconstruction of the obese patient affected by Charcot neuroarthropathy.

Translating The Dynamics Of Graviportal Soft Tissue To Soft Tissue Loss With The Plantar Calcaneus

On an initial gross macroscopic examination, one can immediately appreciate the thickness of the soft tissues on the plantar aspect of graviportal hind limbs. Similarly, although far advanced in comparison to human feet, graviportal hind limbs consist of a highly structured network of adipose tissue organized into compartments (supporting adjacent osseous structures) and reinforced with collagen, reticulin and elastic fibers. This has the reinforcement of cushions consisting of thick sheets of fibrous connective tissue, which form the chambered adipose compartments.

   Microscopically, there is an intricate and complex neurovascular supply highlighted by numerous Pacinian corpuscles that are embedded within the adipose tissue and Meissner’s corpuscles within the dermal papillae of the thickened plantar skin.
There is an additional unique anatomic structure within the medial metatarsal adipose compartment. This is a cartilaginous rod known as the prehallux or “sixth ray,” and may provide an extra support structure to reinforce this important soft tissue.

   What are the implications for human surgical reconstruction?

   These findings underscore the incredible difficulty we have dealing with soft tissue loss of the plantar calcaneus. This area is challenging to offload and attempts to surgically reconstruct this tissue with autograft and allograft techniques are relatively ineffective unless one can restore sensate and structured plantar soft tissue. Graviportal findings point toward the incredible importance of the structure, function and maintenance of the plantar soft tissue, particularly about the calcaneus. Significant tissue loss from this location in humans may represent a contributing attribute to an “unsalvageable” limb in the obese, neuropathic patient with diabetes.

How Scaling Theory Relates To Beaming Technique For Charcot

In the graviportal osseous structure of animals, an immediate relative equinus deformity is present in comparison to the human osseous structure. Graviportal animals compensate for equinus with a tripod configuration within the metatarsals/forefoot. In comparison to other quadrupeds, all graviportal metatarsals are oriented more horizontally to the weightbearing surface. The second, third and fourth metatarsals serve as the primary supports while the first and fifth metatarsals are oriented slightly more vertically. Interestingly, the third metatarsal is the longest and the lateral metatarsals are relatively thicker in comparison to the medial metatarsals. Additionally, strong soft tissue structures rigidly anchor adjacent metatarsals to each other.

   When it comes to surgical reconstruction, these findings may correspond to the relative recent success of the medial and lateral column beaming technique for surgical deconstruction of midfoot Charcot neuroarthropathy.8,9 This may be particularly true when one performs the procedure in conjunction with arthrodesis of the subtalar joint to further exaggerate a tripod configuration of the human foot, which is inherently present in graviportal animals.

   The length of the third metatarsal and relative thicknesses of the lateral metatarsals are also interesting when considering the human metatarsal parabola. We tend to think of human biomechanics as a transfer of weight across the metatarsals in a lateral to medial orientation with the first being the thickest and extending the farthest distally as weight transfers through the first metatarsophalangeal joint during propulsion. This graviportal construct appears to accept and distribute more weight laterally and centrally during this process.

What The Graviportal Tendinous Structure Reveals About Offloading The Diabetic Foot

Further differences are present within the tendinous structures of graviportal animals. The tendons of pronation/supination are reduced with decreased thicknesses and insertions that are actually more consistent with flexion/extension. Another interesting finding is that the Achilles tendon is of a very small size and represents a small relative percentage of lower leg musculature in comparison to other animals.

   These findings may further support arthrodesis of the rearfoot and midfoot complexes with elimination of motion and the creation of a relatively stiff plantigrade structure. The pronatory/supinatory movements of the subtalar and midfoot joints may be paradoxically less necessary as body mass increases if motion transitions more closely to purely flexion/extension.

   It is also interesting to consider these tendinous findings when studying the relative successes and failures of offloading devices for the diabetic foot. Research has found that devices such as total contact casting and below-knee cast boots that immobilize the ankle and are better able to decrease subtalar joint range of motion are superior to those devices (surgical shoes and diabetic shoes) that allow for more motion at these joints.10

In Conclusion

Despite a national awareness of obesity and widespread efforts to educate the public, our nation is still gaining weight at an alarming rate. It may be time to view this weight gain as inevitable and evolutionary, and subsequently attempt to mimic and incorporate scaling theory findings that have occurred in other graviportal species into our practices.

   Charcot neuroarthropathy represents a situation in which so-called “normal” human anatomy has failed. The musculoskeletal tissue and support structures in a patient with a normal BMI may not be sufficient to support our patients with larger BMIs. Although we do not consider these findings to be definitive or conclusive, we hope it does present at least a different paradigm looking forward into treatment algorithms for this challenging group of podiatric patients.

   Dr. Meyr is an Assistant Professor in the Department of Podiatric Surgery at the Temple University School of Podiatric Medicine in Philadelphia.

   Dr. Pirozzi is a fourth-year resident with the Temple University Hospital Podiatric Surgical Residency Program.

   Dr. Creech is a second-year resident with the Temple University Hospital Podiatric Surgical Residency Program.

References
1. Vela SA, Lavery LA, Armstrong DG, Anaim AA. The effect of increased weight on peak pressures: implications for obesity and diabetic foot pathology. J Foot Ankle Surg. 1998; 37(5):416-20.
2. Ogden CL, Carroll MD, Kit BK, Flegal KM. Prevalence of obesity in the United States, 2009–2010. NCHS data brief, no 82. National Center for Health Statistics, Hyattsville, MD, 2012.
3. Sohn MW, Budiman-Mak E, Lee TA, Oh E, Stuck RM. Significant J-shaped association between body mass index (BMI) and diabetic foot ulcers. Diabetes Metab Res Rev. 201; 27(4):402-9.
4. Stuck RM, Sohn MW, Budiman-Mak E, Lee TA, Weiss KB. Charcot arthropathy risk elevation in the obese diabetic population. Am J Med. 2008; 121(11):1008-14.
5. Miller CE, Basu C, Fritsch G, Hildebrandt T, Hutchinson JR. Ontogenetic scaling of foot musculoskeletal anatomy in elephants. J R Soc Interface. 2008; 5(21):465-75.
6. Weissengruber GE, Egger GF, Hutchinson JR, Groenewald HB, Elsasser L, Famini D, Forstenpointner G. The structure of the cushions in the feet of African elephants (Loxodonta africana). J Anat. 2006; 209(6):781-92.
7. Fisher RE, Scott KM, Adrian B. Hind limb myology of the common hippopotamus, Hippopatamus amphibius (Artiodactyla: Hippopotamidae). Zoological Journal of the Linnean Society. 2010; 158: 661-682.
8. Grant WP, Garcia-Laven S, Sabo R. Beaming the columns for Charcot diabetic foot reconstruction: a retrospective analysis. J Foot Ankle Surg. 2011; 50(2):182-9.
9. Assal M, Stern R. Realignment and extended fusion with use of a medial column screw for midfoot deformities secondary to diabetic neuropathy. J Bone Joint Surg Am. 2009; 91(4):812-20.
10. Armstrong DG, Nguyen HC, Lavery LA, van Schie CH, Boulton AJ, Harkless LB. Off-loading the diabetic foot wound: a randominzed clinical trial. Diabetes Care. 2001; 24(6):1019-22.

   For further reading, see “Limb Salvage And The Charcot Foot: What The Evidence Shows” in the March 2011 issue of Podiatry Today.