Current Considerations In Performing Transmetatarsal Amputations
With the recognition of limb salvage as a key to decreasing mortality and increasing the quality of life in patients with chronic non-healing ulcerations, the transmetatarsal amputation (TMA) has become a common procedure.1-4 The current epidemic of diabetes mellitus affects 7.8 percent of the population in the United States. Moreover, an additional 57 million individuals have been diagnosed with “pre-diabetes.”5
One sequela of the disease is chronic non-healing neuropathic ulcerations of the lower extremities that put these individuals at risk for lower extremity amputation. In order to help preserve the quality of life in these patients, limb salvage is an obvious goal.
In the current literature, there is a trend showing an increased mortality rate the more proximal the level of an amputation of a diseased limb.1,2 One study has shown one-year survival rates at 34 percent after above-the-knee (AKA) amputations and 60 percent after below-the-knee amputations (BKA).1 The five-year survival rates are even more dismal for AKA (10 percent) and BKA (28 percent).1 On the contrary, minor amputations consisting of digital amputations and other foot sparing amputations have improved survival rates to 81 percent at one year and 59 percent at five years.1
The Strong Heart study has shown comparably poor outcomes after all types of amputation with the mortality rate being 76 percent at an average of 8.7 years of follow-up.2 As with previous studies, a trend for an increased mortality rate existed with proximal amputations in comparison to that of patients with minor amputation. Patients without amputation had a mortality rate of 24.6 per 1,000 person years whereas mortality rates for those who had digital amputations and BKA procedures were 114.8 and 143.7 per 1,000 person years respectively.
Furthermore, not only does performing a minor amputation decrease the mortality rates in patients but it can also increase the quality of life by increasing independence, mobility, and the ability to return home sooner.1,4 The metabolic demand of walking with these procedures is increased in comparison to more proximal amputations that lead to a decreased speed of gait and decreased distance that can be achieved before reaching the maximum cardiopulmonary limit.6
When a previously ambulating patient presents with a limb-threatening condition, one should attempt to preserve as much of the limb as possible.3 While the TMA is an ideal procedure for limb salvage, successful healing rates of TMA have ranged from 39 percent to 93.3 percent.7
In addition, many studies have shown that even when authors have reported high rates of healing, additional procedures are often required secondary to wound dehiscence and infection.3,7 Accordingly, surgeons should educate potential TMA candidates on the risks involved and emphasize careful attention to details such as smoking cessation and tight blood glucose control. Researchers have shown that these are independent risk factors with failure of the procedure.8,9
Emphasizing The Importance Of Adequate Vascular Assessment And Appropriate Referrals
There are other predictors of wound healing that can assist in determining which patients are most likely to heal a TMA. Vascular studies are the most important and the workup should include Doppler examination of the foot itself to determine which, if any, arteries are viable to guide incision and flap planning.10
If severe vascular disease exists and the previous vascular tests are inconclusive, one may pursue transcutaneous oxygen (TcO2) studies and angiogram studies.11,12 A TcO2 reading of 30 mmHg or higher has been associated with a 90 percent chance of healing, readings between 20 to 29 mmHg predict a 70 percent chance of healing and readings of less than 20 mmHg suggest a 50 percent rate of healing.12 It should be noted, however, that transcutaneous oxygen studies in the face of infection may lead to a falsely elevated tissue perfusion reading. Accordingly, one should exercise caution in using these studies.13
If vascular studies show results that are incompatible for healing, a referral for limb revascularization is indicated. Many institutions have interventional radiologists and vascular surgeons who can open occluded and stenosed arteries, and place stents while performing angiograms. One may also consider further intervention by the way of vascular bypass to restore vascular flow to the distal extremity and assist with healing and preservation of the extremity. In limbs without stents in place, intermittent pneumatic compression may be another means to temporarily treat critical ischemia.14
Failing to address vascularity prior to performing a non-urgent TMA can negatively impact outcome and require a more proximal amputation.
Other Keys To Facilitate Healing
Finally, researchers have cited serum album, a simple measure of nutrition, at levels of >3.0 gm/dL and immunocompetence, measured by a total lymphocyte count of >1,500, as required thresholds for healing an amputation.11,15,16
In order to help achieve the goals of a high quality of life and decreasing mortality, one needs to ensure the salvaged limb is functional and resistant to excessive tissue breakdown. This requires that the remaining foot be plantigrade in nature with concurrent balancing of the foot and addressing ankle equinus at the time of amputation.
Addressing The Impact Of Peripheral Neuropathy
Peripheral neuropathy affects all three divisions of the peripheral nervous system including the sensory, autonomic and motor divisions. The cumulative effects of diabetes mellitus on the peripheral nervous system lead to an insensate, xerotic and biomechanically unbalanced foot, which is more prone to tissue breakdown.17 The loss of protective sensation to the feet leads to an increased risk of ulceration.18 Advanced glycation end products produced in uncontrolled diabetes mellitus lead to decreased joint mobility by way of tendinopathy and loss of elasticity due to alterations in collagen fibers.19 The rigid foot often has increased peak plantar pressure, which also correlates with increases the risk for plantar ulceration.
While sensory loss due to most peripheral neuropathies is non-reversible at this time, one can modify pressure with offloading by accommodative orthotic devices or surgical modification.17,18,20-25
What The Literature Reveals About Adjunctive Surgical Procedures
Traditionally, the tendo-Achilles lengthening has been a successful procedure for relieving ankle equinus and associated abnormal forefoot pressures.26-28 However, recent literature suggests that performing gastrocnemius recession for the equinus can be an alternative option.29
The gastrocnemius recession to correct equinus avoids an incision through a potentially diseased Achilles tendon secondary to diabetes mellitus and maintains the integrity of the tendon near the insertion of the triceps surae.19,30-33 In addition, the gastrocnemius recession decreases the risks that are associated with the percutaneous tendo-Achilles lengthening such as over-lengthening and risk of tendon rupture leading to a calcaneal gait and with possible subsequent heel ulceration.17,34,35
In addition to ankle equinus, a varus rotation of the foot often occurs. To correct a varus forefoot when performing a TMA, the surgeon can transfer the peroneus brevis tendon attachment to the peroneus longus and anchor the flexor hallucis longus to the remaining first metatarsal base to aid in resisting the forces of the tibialis tendons.36-38
Lateral loading of the foot can often lead to increased pressure over the prominent base of the fifth metatarsal leading to a common site of tissue breakdown. If the fifth metatarsal requires removal, one can transfer the peroneus brevis tendon to the cuboid to prevent inversion. Also, a split tibialis anterior tendon transfer or complete transfer helps to reduce the varus component of the forefoot.3,37,39
While not ideal, in certain cases involving an open “guillotine” type TMA, negative pressure wound therapy can assist as a bridging therapy to prepare the wound for closure and results in fewer secondary amputations.40
Diabetic peripheral neuropathy can lead to chronic non-healing ulcerations that increase the risk of limb loss. Closing these wounds and preserving a functional limb from major amputation are the ultimate goals to prevent early mortality and subsequent morbidity. The TMA is a foot sparing procedure that can keep a patient active with a limb. With careful attention to patient selection, healing criteria and surgical planning, the TMA can provide limb salvage from major amputations and significantly increase the quality of life in this patient population.
Dr. Johnson is a Chief Resident at the Hennepin County Medical Center in Minneapolis.
Dr. Rogers is the Associate Medical Director for the Amputation Prevention Center at Valley Presbyterian Hospital in Los Angeles. Dr. Rogers is the Chair of the Foot Council for the American Diabetes Association (ADA).
Dr. Steinberg is an Associate Professor in the Department of Plastic Surgery at the Georgetown University School of Medicine in Washington, D.C. He is a Fellow of the American College of Foot and Ankle Surgeons.
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