Infection in an ulcer can be difficult to treat because of:
- polymicrobial infection
- deficient immunity
- ischaemia
- spread of infection along deep tissue planes
- development of ostoemyelitis with dead bone
The usual signs of swelling, rubor, heat and discharge may be obvious, but in an ischaemic foot with deficient neutrophil function the diagnosis may be difficult. If a probe inserted into the ulcer goes directly down to bone, osteomyelitis is probably present. Ultimately, surgical exploration and biopsy may be necessary to make the diagnosis.
All standard imaging modalities can play a part in diagnosing infection, and normal pathways should be followed. There is a significant false negative rate. In the presence of Charcot arthropathy all imaging is particularly difficult to interpret and labelled antibody fragments may be the most accurate method.
An infected foot should be elevated and broad-spectrum antibiotics administered intravenously. Each unit needs to develop their own antibiotic protocol with the microbiologist according to local organisms and sensitivities. Improvement of the vascular inflow may improve the chance of limb salvage.
Negative-pressure dressings may improve healing rates over conventional dressings, although the evidence is not yet strong (Evans et al 2001, Eginton et al 2003). The technique is useful in wounds which have failed other attempts at healing (Clare et al 2002).
Established deep infection may require surgical debridement and this may include amputation at various levels:
- Single or multiple toes
- Ray(s)
- Transverse amputations at the Lisfranc or midtarsal levels
- Hindfoot amputations such as Syme or Pirogoff
- Trans-tibial
- Trans-femoral
Distal amputations have a good healing rate overall and can maintain mobility in appropriate shoes and orthoses. However, losing one toe is a major risk factor for further amputation (Murdoch et al 1997). Loss of the great toe significantly increases pressure under the remaining forefoot (Armstrong and Lavery 1998) and there is a high incidence of subsequent lesser toe deformity (Quebedeaux et al 1996). Midfoot amputations are probably best when a good plantar flap can be used for durable closure without tension. Midtarsal (Chopart) amputations require motor balancing by transfer of all available dorsiflexors into the talar neck to balance the Achilles tendon and prevent equinus. Hindfoot amputations have the advantage that they can usually be walked on a short distance without a prosthesis (useful for going to the toilet in the middle of the night) (Pinzur et al 2003). However, prosthetic fitting for outdoor mobility is more difficult than a below-knee stump. More than at most sites, the decision to do a hindfoot or BK amputation should be taken in close collaboration with the prosthetist.
Peters et al (2001) found that Sickness Impact Profile scores in diabetics with forefoot or midfoot amputations were no different from those without amputations, but diabetics with trans-tibial amputations scored significantly worse. It is worth trying to amputate distally.
Our own policy is to do forefoot procedures, up to the level of the Lisfranc joint, where possible. Wounds are left open and the patient mobilises in a Scotchcast boot, TCC or walker as soon as the wound has settled. Patients with wounds which cannot be closed at the Lisfranc joint or distal would usually be offered a proximal amputation, trans-tibial if possible.
A few resistant ulcers around the hindfoot can be closed by excision and grafting or flap closure in collaboration with a plastic surgeon (Attinger et al 2002, Musharrafieh et al 2003). However, ischaemia and infection limit what is feasible.