Understanding The Biomechanics Of The Transmetatarsal Amputation

In a thorough review of the literature on the transmetatarsal amputation in patients with diabetes, these authors discuss keys to proper patient selection, essential biomechanical aspects of the procedure, when adjunctive procedures can have an impact and tips on post-op shoe gear.

Non-traumatic lower extremity amputation (LEA) in the United States is attributed to diabetes more than any other disease with an overall incidence of 195 per 100,000 person-years. This number will likely continue to climb as the number of patients afflicted with diabetes increases and life expectancy continues to rise.1,2 Consequently, the podiatric surgeon will encounter increasing numbers of pedal complications associated with the disease that require surgical intervention to prevent major limb loss and the associated cardiovascular compensatory ramifications.

   Limb preservation in the diabetic population remains challenging and is most effective utilizing a multidisciplinary approach.3 This multidisciplinary team includes the podiatric surgeon, vascular surgeon, infectious disease specialist, internal medicine physician, interventional radiologist, plastic surgeon and rehabilitation specialists among others. Primary objectives include maximizing function; minimizing the risk of tissue breakdown and the need for a more proximal amputation; and avoiding the morbidity and mortality associated with major limb amputation and the need for intensive rehabilitation.

   When surgeons perform the transmetatarsal amputation (TMA) correctly and in combination with adjunctive procedures when necessary, the TMA is a valuable surgery in the limb salvage effort, and preferred over a below- or above-knee amputation when functionally and physiologically reasonable with authors reporting success rates of over 90 percent.4 The TMA enables patients to better sustain a quality of life as most patients will not require the help of others to perform activities of daily living, and the likelihood of patients ambulating independently is greater than those who have had transtibial and transfemoral amputations. Researchers have shown that oxygen consumption increases up to 280 percent of normal during ambulation in patients with major limb amputations.5

   Furthermore, 30-day mortality rates for below- and above-knee amputations are much higher than those for TMAs with authors reporting rates as high as 6.3 percent and 13.3 percent while survival probabilities decrease to 28 percent and 20 percent for below- and above-knee amputations, respectively, at 7.5 years postoperatively.6 Other researchers have reported significantly better 30-day mortality rates for TMA with rates being less than 2 percent.7,8

Keys To The Clinical Exam And Pertinent Perioperative Considerations

During routine clinical examination of the patient with diabetes, it is critical to identify the risk factors that predispose patients to the sequelae associated with abnormal physiology proceeding from longstanding and/or poorly controlled hyperglycemia. These risk factors may be neurological, vascular or biomechanical in nature.

   It is well known that peripheral sensory neuropathy and peripheral vascular disease (PVD) are independent risk factors for lower extremity amputation in patients with diabetes. Peripheral vascular disease is associated with a threefold increased risk of lower extremity amputation and peripheral sensory neuropathy is a risk factor for minor amputation.9 Furthermore, a meta-analysis comprised of 14 studies and 94,640 participants determined the risk of lower extremity amputation to increase by 26 percent with one percentage point increase in HbA1c.10

   The patient with diabetes who requires surgical intervention typically presents with one or more of the following clinical exam findings:

• a full-thickness wound and concomitant osteomyelitis suggested by a fairly reliable positive probe-to-bone test, radiographic evidence of osteolysis and cortical destruction, or with positive MRI and/or bone scintigraphy findings when radiographs are equivocal;
• gross tissue changes consistent with skin necrosis and critical limb ischemia with a superimposed infection (wet gangrene);
• dry gangrene with unrelenting ischemic forefoot pain;
• crepitus in the soft tissue and radiographs indicating subcutaneous emphysema (gas gangrene); or
• fulminant infection, abscess formation and extensive soft tissue necrosis.

   When bone is involved, we maintain that surgical debridement of the infected bone and necrotic tissue supplemented with long-term antibiotic therapy provides the best results in terms of sustainable limb salvage rates. This is consistent with published results.11

   In some instances, such as with a global forefoot infection when extensive tissue loss renders a non-salvageable forefoot or when vascular reconstruction cannot sustain adequate forefoot hemodynamics, a primary TMA becomes the surgery of choice. Surgeons also commonly perform the TMA as a secondary or tertiary procedure. Examples of this include performing a TMA after a local forefoot amputation (digit, metatarsal or ray) when infection has recurred; when insufficient perfusion for healing warrants a more proximal amputation; when ulcers develop after partial ray resections secondary to overload; or as a staged procedure following radical debridement of a diabetic foot infection.

   Prior to performing the TMA, a comprehensive vascular evaluation including ultrasound Doppler is critical to assess perfusion to the six pedal angiosomes to ensure initial wound healing and the ability to eradicate infection, increasing the probability of a successful amputation.12-15 One should carefully plan the incision, keeping in mind the principles of angiosomes and ensuring viable skin availability needed for primary closure. Alternately, surgeons also need to consider the possible need for secondary means of closure, such as skin grafting, for postoperative management.

   Ideally, one should preserve a long plantar musculocutaneous flap for bone coverage and to optimize healing. Surgeons should bevel the osteotomies and make them as distal as possible in a manner that respects the metatarsal parabola to preserve physiologic plantar pressure distribution during gait, and avoids overload and subsequent tissue breakdown. If one knows preoperatively that the extent of tissue necrosis or infection will prohibit primary closure, make the patient aware that he or she will require continued local wound care, or negative pressure wound therapy followed by eventual skin grafting or flap closure.

   Communication is of paramount importance between the surgeon and patient to avoid contrasting expectations. The surgeon must provide the patient with sufficient information of the entire process, particularly in cases when primary closure is not possible, and continued wound care and a second surgery may be required. The patient must be committed to strict adherence during the postoperative course as this can be a time-consuming process, taking weeks or months to achieve a satisfactory outcome. Additionally, the patient must recognize that a more proximal amputation may be necessary in the future.

Pertinent Biomechanical Considerations

The TMA has been successful in increasing limb salvage rates and decreasing the need for major limb amputation. However, a fundamental understanding of foot and ankle biomechanics becomes imperative for limb salvage sustainability, and to avoid preventable postoperative complications.

   With respect to midfoot amputations, one of the goals is to maximize foot length in order to provide optimal biomechanics so that subsequent tissue breakdown does not occur. Potential techniques and modalities to help achieve this goal include skin grafts, pedicled and microsurgical free flaps, and Ilizarov frames.6 By maximizing foot length, one can shorten the lever arm for propulsion to a minimum and maximize the surface area of the plantar foot residuum to distribute pressures.

   Researchers have shown that those with a TMA have a 40 to 48 percent shorter moment arm in comparison to the normal foot. This theoretically requires a 48 percent increase in the ground reaction force to generate a given plantarflexor moment, which would increase localized pressure at the distal aspect of the TMA stump, leading to increased risk of ulcerating.16 When taking this into consideration, the TMA is preferable over more proximal pedal amputations yet it is discouraged when one can confidently ensure forefoot salvage.

   The most common biomechanical complication associated with the TMA is the equinovarus deformity with coinciding gait discrepancies resulting from the elimination of extrinsic musculature and intrinsic tendon alterations inherent to the pathophysiology of diabetes.

   During the swing phase of gait, the anterior muscle group dorsiflexes the foot at the ankle joint and counteracts plantarflexory forces for ground clearance. The anterior muscle group also assists in deceleration of the forefoot at heel strike through eccentric contractions to prevent the forefoot from slapping onto the ground through midstance. However, with all long extensor tendons traversing the ankle joint one is transecting, the posterior muscle group, specifically the gastrocnemius-soleus complex, gains a mechanical advantage. This forces the foot into an equinus position at the ankle joint. Not only is it more difficult for the foot to now achieve ground clearance following propulsion without the help of the hip flexors but the contact phase can be significantly altered. The patient may now first touch ground on his midfoot or even the forefoot when the contraction is severe enough.

   In the frontal plane, the supinatory forces of the tibialis posterior and triceps surae pull the foot into a varus orientation at the subtalar joint. The tibialis anterior, another subtalar joint supinator, pulls up on the medial ray and acts relatively unopposed as the extensor digitorum longus tendons have been transected and the peroneus brevis cannot alone counteract the overwhelming supinatory forces.17

   The foot now assumes an equinovarus position. This will have many implications in terms of function but also comes into play when surgeons consider initial incision healing, ulcer occurrence or recurrence, infection and the need for a more proximal amputation. One can also attribute the pathogenesis of the equinus contracture to the non-enzymatic glycosylation of collagen.

   Grant and colleagues studied the Achilles tendon specimens of patients with diabetes using electron microscopy and found several abnormalities. These abnormalities included an increased fibrillar density, irregularity of the four individual collagen fibrils, smaller fibril diameters and foci of collagen disruption and disorganization. This results in loss of tendinous elasticity and limited joint mobility.18 Given that intrinsic musculature and the plantar fascia are also sacrificed during a TMA, the foot loses its rigid lever effect during late midstance into propulsion. This causes increased pressures to exert at the distal aspect of the stump.17

   Due to the muscular imbalance, decreased joint mobility, deformity and aforementioned gait alterations, many of the measurable gait parameters change and become pathologic. Researchers have hypothesized that the major difference in the gait cycle is an increase in the amount of time the foot contacts the ground during midstance and because of the decreased surface area of the plantar foot in the post-TMA patient, one should expect the total force exerting on the plantar foot to rise significantly.17

   Garbalosa and colleagues demonstrated a statistically significant difference in peak mean plantar pressures during the gait cycle when comparing post-TMA feet with intact feet. There was a trend toward higher pressures in the forefoot in post-TMA feet and a frequency of peak pressures occurring mostly laterally and medially with none occurring at the heel.19

   In studying patients with diabetes mellitus and TMA, Mueller and co-workers found that they displayed a decreased plantarflexion range of motion, took shorter steps, and walked slower in comparison to age-matched controls.20

   Kelly and co-workers found that peak plantar pressures consistently occur at approximately 80 percent of the stance phase of the gait cycle in patients with diabetes and TMA. When it comes to this segment of the gait cycle in these patients, the study authors concluded that one should consider ambulation strategies and shoe gear options to help manage forefoot ulcers.21

   Researchers have also shown that patients with triceps surae contractures will transfer pressure distribution from the hindfoot to the midfoot and forefoot, increasing the pressure on the latter by 38 percent and 59 percent respectively.22 The limited mobility at the ankle joint combined with increased peak plantar pressures puts the foot at a considerable risk for tissue breakdown and re-amputation.23 When the mobility of other pedal joints, such as the subtalar joint, are affected, the loss of the shock absorption mechanisms, such as pronation, become amplified.24,25 With continuous microtrauma to the forefoot in patients with peripheral neuropathy, the risk of tissue breakdown becomes a primary concern, particularly to the plantar lateral aspect of the stump.

   To compensate for the equinus contracture, the body implements many mechanisms in the foot, knee and hip. When the foot cannot dorsiflex through the stance phase of gait, the tibia cannot move anteriorly over the talus. One mechanism of compensation occurs with early heel lift, which theoretically can induce toe walking.22 The hip flexors must now activate earlier and pull the leg forward during swing rather than rely on momentum that can no longer be generated, increasing energy expenditure.

Restoring Anatomical Alignment

Studies have shown that up to 44 percent of patients will fail to heal or require a more proximal amputation following a midfoot amputation while many others may require additional debridements or stump revision to obtain skin closure.26,27 Although the reason for re-amputation may not always be biomechanically induced, there are a variety of methods, both conservative and surgical, the podiatric surgeon can implement to obviate the complications associated with the biomechanically flawed foot. The primary objective is to achieve a functional, stable foot residuum that can accommodate weightbearing and ambulation, and avoid future tissue breakdown and major limb amputation. Surgeons have proposed both tendon balancing procedures and skeletal stabilization operations to meet these goals.

   Various authors have described the tendo-Achilles lengthening (TAL), tenotomy and gastrocnemius recession in the literature for patients with an equinus ankle deformity. The Silfverskiold test can help differentiate between the muscle responsible for the contracture. When one supplements this test with clinical findings as to the degree of contracture, he or she can choose an appropriate surgical technique. In doing so, the podiatric surgeon can help reduce the risk of ulcer recurrence, re-infection and additional amputations.

   Through a minimally invasive approach, it is reasonable that many podiatric surgeons will perform a TAL. This will somewhat restore normal ankle joint kinematics, maintain an anatomically acceptable foot-ankle relationship that will hopefully translate into a more acceptable gait pattern, and reduce the risk of tissue breakdown although this procedure does not come without risk. Researchers have proven that percutaneous TAL dramatically reduces peak plantar pressures by approximately 27 percent while also providing a significant increase in ankle joint dorsiflexion and facilitating healing of the TMA incision.15,28

   However, Maluf and co-workers demonstrated that the decrease in plantarflexory power and plantar forefoot pressures is transient, and both variables increase significantly eight months postoperatively. This renders forefoot ulceration an imminent concern that warrants vigilant postoperative surveillance by both the surgeon and patient.29 More than half of patients undergoing percutaneous TAL may develop new or recurrent foot ulcers, either in the forefoot or heel, substantiating the need to address other factors that may be pivotal in tissue breakdown.30 These factors could be unaddressed osseous prominences that one sees with Charcot deformity or inappropriate shoe gear.

   Accordingly, surgeons must perform the TAL with great caution and take care not to completely rupture or over-lengthen the tendon. Extreme dorsiflexion (>15 degrees) can generate a calcaneal gait, increasing the risk of developing limb threatening heel ulcers. One should perhaps avoid extreme dorsiflexion in patients with an insensate heel pad as up to 47 percent of patients may develop acute transfer ulcers to the heel.31

   The gastrocnemius recession is also an option for patients with TMA-induced equinus and has reported advantages over a TAL. The lengthening with this surgery is more controlled. Therefore, there are reduced risks of over-lengthening and iatrogenic tendon rupture. As a result, there is less of a chance of creating a calcaneal gait and the risk of a transfer heel ulcer remains low.

   One can perform the gastrocnemius recession endoscopically. Doing so minimizes the need for a long incision and avoids injuring an already pathologic tendon. There is less loss of plantarflexory muscle strength with the gastrocnemius recession than one would see with the TAL. Studies have shown that this surgery can decrease forefoot pressures and promote more efficient wound healing, and may reduce the risk of developing transfer ulcers postoperatively.32-34 Surgeons can also perform an Achilles tenotomy to correct the equinus deformity. However, we only consider this an option when performing a tibiotalocalcaneal, tibiocalcaneal or pantalar arthrodesis.

   Surgeons have advocated adjunctive tendon procedures for balancing the post-TMA foot. These procedures include a peroneus longus to brevis tendon transfer to augment pronatory forces; split tibialis anterior transfers to reduce inversion forces; and flexor hallucis longus and extensor digitorum longus transfers to normalize frontal plane deformity and reduce the risk of ulcer development at the plantar lateral aspect of the stump.35,36 All have demonstrated the ability to achieve deformity correction.

   However, one must take the need for an additional incision into account and avoid the aforementioned procedures in the dysvascular foot, an immunocompromised patient or when an active infection is present.36 When infection is present, we recommend eradicating the infection before performing any of the aforementioned tendon balancing procedures and posterior muscle group lengthenings in order to prevent bacterial seeding.

   Skeletal stabilization interventions can also effectively realign the foot and are probably longer lasting than tendon procedures, but are prone to more postoperative complications intrinsic to implanting hardware.

   Schweinberger and Roukis propose a method to correct the deformed TMA stump in the dysvascular foot when tendon transfers and additional incisions are contraindicated.37 Using a large diameter screw extending from the first metatarsal residuum into the talus and occasionally a lateral column stabilization screw, they have been able to achieve a stable, functional foot although they recognize that infection of hardware and screw migration pose concerns.

   Rearfoot arthrodeses, including tibiotalocalcaneal, tibiocalcaneal and pantalar arthrodesis, can also achieve an anatomically correct TMA stump posture. However, they require additional incisions and internal or external fixation. Therefore, surgeons should reserve these procedures for select patients and as a last resort to limb salvage.

Addressing Shoe Gear After A TMA

Although many surgical techniques have proven to anatomically and biomechanically normalize the deformed TMA stump, the astute podiatric surgeon must acknowledge possible extrinsic factors that can lead to complications and be prepared to address them postoperatively. Following a TMA and after one has eradicated any infection, obtained an acceptable foot/leg, and healed incisions and wounds, we must consider the impact shoe gear and orthoses may have on the diabetic foot. Keep in mind that we need to ensure the physiological distribution of plantar pressures without overloading any aspect of the foot. Other areas of concern include instability and imbalance during gait for those with a shortened foot length and peripheral neuropathy.

   There are a number of options for patients with a TMA. These options include toe fillers to aid in wearing normal shoe gear; below-ankle and combined below- and above-ankle orthoses; prostheses with carbon fiber plating; rigid rocker bottom soles; short shoes (length of residuum); ankle-foot orthoses; and total contact inserts. Various authors have proposed that these modalities aid in ambulation, including improved stability, increased gait velocity, a decrease in peak plantar pressures and improved function.38-40

   Mueller and Strube advocate the full-length shoe, total contact insert and a rigid rocker bottom sole for most patients with diabetes and a TMA to optimize physical performance and walking speed.39 They report that patients do not tolerate the ankle-foot orthoses well and that short shoes are cosmetically unpleasing to some patients although they did not interfere with activities such as stair climbing as did the front of the full-length shoe.

In Conclusion

Transmetatarsal amputation is a valuable surgery that can prevent major limb loss and minimize loss of function, optimizing the quality of life for patients who require limb salvage procedures. Optimizing foot length is ideal but one should not do this at the expense of insufficient debridement and poor cutaneous blood supply.

   The biomechanical consequences of TMA are well known and podiatrists must address these in order to optimize healing, normalize gait and function, and minimize tissue breakdown and the need for subsequent surgeries, including re-amputation. Achieving initial healing and avoiding the need for subsequent debridement correlates significantly with the patient’s ability to ambulate and overall limb salvage.15

   Dr. Gambardella is a Chief Resident in Podiatric Medicine and Surgery at the Yale-New Haven Hospital in New Haven, Ct.

   Dr. Blume is an Assistant Clinical Professor of Surgery in the Department of Surgery and an Assistant Clinical Professor of Orthopaedics and Rehabilitation in the Department of Orthopaedics, Section of Podiatric Surgery at the Yale University School of Medicine in New Haven, Ct. Dr. Blume is a Fellow of the American College of Foot and Ankle Surgeons.

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35. Schweinberger MH, Roukis TS. Balancing of the transmetatarsal amputation with peroneus brevis to peroneus longus tendon transfer. J Foot Ankle Surg. 2007;46(6): 510-514.
36. Roukis TS. Flexor hallucis longus and extensor digitorum longus tendon transfers for balancing the foot following transmetatarsal amputation. J Foot Ankle Surg. 2009;48(3):398-401.
37. Schweinberger MH, Roukis TS. Intramedullary Screw Fixation for Balancing of the Dysvascular Foot Following Transmetatarsal Amputation. J Foot Ankle Surg. 2008 Nov-Dec;47(6):594-597.
38. Mueller M, Strube M, Allen B. Effect of six types of footwear on peak plantar pressures in patients with diabetes and transmetatarsal amputation. Clin Biomech (Bristol, Avon). 1997;12(3):S3.
39. Mueller MJ, Strube MJ. Therapeutic footwear: enhanced function in people with diabetes and transmetatarsal amputation. Arch Phys Med Rehabil. 1997;78(9):952-6.
40. Spaulding SE, Chen T, Chou LS. Selection of an above or below-ankle orthosis for individuals with neuropathy and transmetatarsal amputation: a pilot study. Prosthet Orthot Int. 2012;36(2):217-24.

   For further reading, see “Current Considerations In Performing Transmetatarsal Amputations” in the February 2011 issue of Podiatry Today, “How To Determine The Appropriate Level Of Amputation” in the January 2005 issue or “A Guide To Transmetatarsal Amputations In Patients With Diabetes” in the July 2006 issue.