A Guide To Surgical Offloading In The Neuropathic Foot

      The neuropathic foot presents unique challenges when treating and preventing chronic wounds. One of the most difficult challenges is offloading the neuropathic foot without compromising function or causing a transfer of pressure that leads to further ulceration. When performing a limb salvage procedure, the goal is to provide the patient with a stable, plantargrade foot while still allowing for ambulation.1       In choosing the appropriate procedure to offload the foot, it is important to consider minimal bone resection versus a partial pedal amputation. In addition, one should consider specific tendon balancing procedures to maintain a functional limb. Accordingly, let us take a closer look at common pedal ulcerations and their corresponding surgical management.

How To Offload Hallux And Lesser Digital Ulcerations

      As hammer digit syndrome, including hallux hammertoe, is inherent to the progressively insensate foot, so are the distal clavi and tuft ulcerations. For these patients, the anatomical consideration is the combination of increased distal pressure, the rigidity of the deformity and subcutaneous fat protection of the distal phalanx. One may extrapolate the stepwise process of reduction in these deformities — which has been useful in normal hammertoe correction (as popularized by Green) — to prevention and cure in the patient with neuropathy.       Often, simple arthroplasty procedures, tenotomy and capsulotomy, or more aggressive treatment such as distal interphalangeal joint disarticulation will be definitive in solving this issue. While flexor tenotomy is one of the last considerations in the stepwise approach, one should consider it as a viable treatment option for hammer digit syndrome in the progressively insensate foot. When choosing this alternative, take care to avoid extension contractures or a lack of toe purchase.       Hallux IPJ ulcerations are also frequently present in the neuropathic foot. Neuropathy combined with limited joint mobility at the metatarsophalangeal joint (leading to increased plantar pressure) becomes the aggravating factor. Keller arthroplasty is warranted when conservative treatment fails to heal or maintain healing of ulcers of the great toe.2 One might also consider the range of options in the surgical management of hallux limitus and rigidus as other alternatives. The success of the procedure depends on increasing the range of motion of the hallux.3       A recent retrospective review reported on the healing rates of hallux ulcerations following a Keller procedure.4 In this retrospective study, researchers examined 11 patients with 13 ulcerations after a modified Keller procedure. At the six-month follow-up, all hallux ulcerations were healed. However, transfer ulcerations developed in five patients as a late complication. Other reported complications of the Keller procedure include lack of hallux purchase, dorsal toe contracture and lesser metatarsal stress fractures.5 Occasionally, an IPJ sesamoid might be the culprit for ulcer formation. Always obtain radiographic studies to determine the best course of action in surgical planning.

Keys To Addressing Plantar Ulceration At The Metatarsal Heads

      Building upon the effects of digital deformities, we find ulcerations inferior to a metatarsal head to be a recurrent issue in patients with neuropathic feet. Retrograde force from contracted digits lead to prominent, plantarflexed metatarsal heads. Again, the rigidity of the bony deformity, gait abnormality and the presence of fat pad atrophy lead to ever increasing plantar pressures and eventual soft tissue breakdown. Surgical options might include: addressing the digital deformity as previously discussed in order to reduce retrograde force; soft tissue balancing; single or multiple metatarsal head resection; or an elevation osteotomy.       Jacobs originally implemented multiple metatarsal head resections for patients with rheumatoid arthritis (RA).6 Multiple or pan-metatarsal head resections are increasingly being utilized for the treatment of longstanding ulcerations on the plantar forefoot. The logic in preventing transfer lesions by the redistribution of pressure holds true for neuropathic feet just as in the RA group. However, the danger in transfer lesions in the neuropathic population is not painful calluses but the risk for limb loss.       One complication reported is the regrowth of the metatarsal head with recurrent ulcer formation.7 Armstrong has also suggested that the angle of resection of the metatarsal can have late untoward effects for ulcer formation.8 One should avoid plantargrade skiving of the metatarsal shaft as regrowth will naturally be directed to the weightbearing surface. It is also important to keep in mind that it is not necessary to resect all the metatarsals for every forefoot ulcer.       Resection of a single metatarsal head has been shown to correlate with increased wound healing. Wieman, et. al., showed that 88 percent of ulcers healed following resection of a prominent metatarsal head.9 More recently, in a retrospective cohort limited to fifth metatarsal head resections, researchers state that the surgical group healed significantly faster and had a deceased risk of recurrence in comparison to the non-surgical group.8 One may compare this success to the success (and reported low risk of transfer lesions) of fifth metatarsal head resection in treating tailor’s bunions. Occasionally, one should consider isolated central metatarsal (second to fourth) head resections.

What Studies Reveal About Tendo-Achilles Lengthening And TMAs

      Lengthening the Achilles tendon alone or in combination with other procedures can be critical in decreasing forefoot pressure during the gait cycle. As we know, increased plantar pressure in a neuropathic foot commonly causes the formation, stabilization and recurrence of ulceration. Achilles tendon lengthening is indicated in patients with partial foot amputations, forefoot deformities or chronic ulcerations.10       Increased plantar pressure is a result of limited dorsiflexion at the ankle joint from a short Achilles tendon or ankle equinus.11,12 Moreover, intrinsic motor neuropathy may exacerbate ankle joint equinus.12 Veves, et. al., found that plantar ulcerations occurred in 35 percent of patients with diabetes with high plantar foot pressure but there were no plantar ulcerations with normal pressure.13 Boulton reported that 51 percent of patients investigated who had neuropathic ulcerations had higher than normal peak plantar pressures on the forefoot.14       Studies have stated that ankle joint dorsiflexion increases in a range from 9 degrees to 18 degrees following lengthening of the Achilles tendon.10-12 A recent study by Mueller, et. al., showed that 100 percent of ulcerations healed following Achilles tendon lengthening after a mean duration of 58 days.15 Holstein, et. al., found that 91 percent of feet with ulcerations healed following tendon lengthening. One major complication was the fact that 47 percent of patients suffered acute transfer ulcerations to an insensate heel pad.16 If the surgeon overlengthens the Achilles tendon, a patient may develop a calcaneal gait. As the patient increases pressure in the insensate heel, ulceration is the most likely outcome.       Achilles tendon lengthening can play an integral role in healing when it comes to patients with a transmetatarsal amputation (TMA). McKittrick, et. al., first described the TMA in 1944.17 Since that time, researchers have shown that the TMA decreases mortality and increases healing rates up to 92 percent.18 As a result, the TMA is regarded as a successful distal amputation in comparison to a more proximal amputation such as a below the knee amputation.       However, the TMA continues to challenge physicians in regard to recurrent ulceration. Recurrent ulceration is triggered in those patients with a TMA when they resume gait activity on an insensate foot with ankle equinus leading to increased plantar pressure. A TMA sacrifices the function of the extensor hallucis longus and the extensor digitorum longus across the ankle joint. This contributes to the development of an ankle equinus deformity. The supinatory force of the posterior tibial tendon may overpower eversion, leading to combined equinovarus.       Barry, et. al., reports an overall healing rate of 91 percent following Achilles tendon lengthening procedures in patients with transmetatarsal amputations.19 Commonly, surgeons perform the lengthening procedure at the time of the amputation.

A Closer Look At Soft Tissue Balancing Procedures

      Mid- and rearfoot amputation are more likely among patients who have undergone forefoot amputation or who have severe foot deformity related to flatfoot or Charcot. Contrary to the primary focus upon osseous procedures to offload and cure forefoot ulceration, mid- and rearfoot ulceration should rely more on soft tissue correction. As previously discussed, one would prevent an equinus deformity in the patient with a transmetatarsal amputation by performing Achilles tendon lengthening.       However, as the TMA level progresses proximally and as one considers other proximal level amputations, other tendon balancing procedures may be required in addition to the Achilles tendon lengthening as the biomechanics of the foot have been altered. The goal of the surgeon is to create a foot that has mechanically sound function during the gait cycle.20 Sanders has suggested transferring the tibialis anterior tendon to the neck of the talus to provide a dorsiflexory force that counteracts the plantarflexory force of the Achilles tendon.21       Not only is it important to maintain a balance between dorsiflexion and plantarflexion as mentioned above, one also needs to maintain a balance between inversion and eversion. For example, if one must remove the base of the fifth metatarsal in a transmetatarsal amputation patient, then the remaining stump will develop an adductovarus deformity. The peroneus brevis muscle is a strong pronator of the subtalar joint during the contact phase of gait. The pronatory force of the peroneus brevis balances out the supinatory force of the tibialis posterior muscle. Accordingly, if one sacrifices the peroneus brevis tendon, the result will be a breakdown of the plantar lateral stump.21       Otis, et. al., found that the peroneus brevis muscle is a significantly stronger evertor of the subtalar joint than the peroneus longus muscle.22 Therefore, to maintain balance across the subtalar joint, one should transfer the peroneus brevis tendon to a more proximal location. Clark, et. al., report that the surgeon should transfer the peroneus brevis tendon to the cuboid to oppose the inversion action of the tibialis anterior and tibialis posterior.1       In addition to transferring the peroneus brevis tendon in a TMA, a split tibialis anterior tendon transfer reportedly reduces the supinatory force across the subtalar joint and increases the pronatory force to help prevent lateral plantar breakdown. One may split and transfer the tibialis anterior tendon to the lateral cuneiform or to the cuboid. If the surgeon decides to perform a Lisfranc amputation, he or she needs to address the salvage of the peroneal, tibialis anterior and tibialis posterior tendons. Take care to maintain the attachment of the tibialis anterior tendon to the medial cuneiform. The tibialis posterior tendon attachment to the bases of the second and third metatarsals will be lost in a Lisfranc amputation but the main attachment to the navicular will remain.       For the Chopart amputation in which the attachment of the tibialis anterior on the medial cuneiform is sacrificed, one should transfer the tibialis anterior tendon to the neck of the talus.23 This will allow the tibialis anterior to maintain function as an ankle dorsiflexor.24 One may also transfer the extensor tendons to the neck of the talus in a Chopart amputation. Also recall that the tibialis posterior retains attachment to the sustentaculum tali at the Chopart level. This deforming force can still cause equinovarus contractures if it is not addressed. The senior author advocates tenotomy of the posterior tibial tendon in balancing the Chopart amputation.

What About Osseous Procedures?

Osseous procedures with an evidence basis for success in the middle and rearfoot are virtually nonexistent. One must address prominences of the plantar, lateral and medial foot individually for isolated bony resection. Counsel patients regarding the potential for failure of a bumpectomy to achieve long-term resolution of a plantar ulceration.       Weakening of the strong interosseous ligamentous support, which is sometimes unavoidable in these undertakings, can lead to further deformation of the foot.       While there is new progressive thought regarding reconstruction in active phase Charcot patients to restore the arch and redistribute plantar pressure, do not be misled that this is osseous correction. Charcot reconstruction, as described by Wang, relies on the windlass mechanism and Achilles lengthening to restore normal architecture even though the primary work is placement of pins into bone and fabrication of an external fixation construct.25

In Conclusion

      Achieving a cure, maintaining a functional limb and preventing future ulceration are the primary goals in the surgical offloading of the neuropathic foot. The caveat in limb salvage is extrapolation of commonly used elective procedures for the forefoot while soft tissue balancing should remain the focus of our proximal level work.       Understanding the importance of maintaining a functional plantargrade foot for ambulation through these procedures will lead to increasing success rates and widespread interdisciplinary adoption of pedal amputations. By providing the patient with a functional limb, the physician is giving the patient a more functional lifestyle.       Dr. Grieder is a second-year resident at Forest Park Hospital in St. Louis, Mo.       Dr. McMurray is a second-year resident at Forest Park Hospital in St. Louis, Mo.       Dr. Claxton is in private practice in Belleville, Ill.
 

References:

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