Where Is The Infection Going?
Infections tend to develop and travel along the path of least resistance. This implies that an infection will stay within the potential space of a given surgical layer or plantar compartment before extravasation into another layer or compartment. Often this involves proximal extension along the relatively avascular tendon sheaths or fascial planes between muscular layers. The studies that have helped define the number and boundaries of plantar foot compartments have also given us information about relatively consistent fascial clefts where communication between different layers and compartments is likely. These have involved pressurized injection imaging studies in which a contrast medium infiltrates a known compartment and one can map the extravasation into other compartments. The findings of these studies are summarized in “A Guide To Intercompartmental Communications” below.7,8,11-15 These communications are obviously numerous and complex. The important concept to realize is that an infection is likely to develop initially within the potential space of a single layer or compartment. There is a tendency for the infection to move proximally before communicating with another layer or compartment. However, note that patterns of communication are present along known anatomic structures such as tendons and neurovascular structures to each of the other compartments, as well as the dorsum of the foot and plantar superficial fascia.16 Intraoperative investigation of an infection should focus on these structures to trace the extent of plantar involvement. Also note the majority of these communications are found in the forefoot around the metatarsophalangeal (MPJ) level so distal infections have an increased likelihood of multi-compartment involvement.11
Where Will The Healing Come From?
One of the most important lessons the podiatric surgeon can learn is that we do not cure diabetic foot infections. No degree of surgical intervention will actually cure the patient of an invading pathogen. While aggressive surgical debridement will bluntly remove affected tissue, it will not continue to fight remnant infection and it will not heal remaining viable tissue. The patient’s own immune system is required for this with the vascular system as the necessary vehicle. For this reason, a thorough understanding of lower extremity angiosomes is essential in preoperative planning. One cannot overstate the impact of vascular disease on diabetic foot infection. It is vitally important to have a thorough understanding of patients’ vascular status in terms of both macrovascular occlusive disease and microvascular endothelial dysfunction. Standard noninvasive vascular examinations and a formal vascular surgery consultation will provide preliminary information about this and help to optimize this system. Surgeons need to take it a step further in order to garner specific information about which vessels are actually carrying blood into the foot and the collateral circulation. This is information that is readily available with a thorough physical examination using angiosome principles. The term angiosome describes all tissue contained within the surgical layers of dissection supplied by a single source artery. There are five angiosomes in the foot and ankle, and they are supplied by the posterior tibial, anterior tibial and peroneal arteries. One can determine angiosome patency in the presence of macrovascular occlusive disease in the area of infection via a complete physical examination with a handheld Doppler. This information is vital in terms of incision placement, systemic antibiotic tissue penetration and the overall healing potential of infected sites.17-19 Thorough knowledge of angiosome principles is strongly encouraged for any surgeon undertaking diabetic limb salvage.