tential morbidities. For example, when a distal vascular bypass graft gets infected, the limbs of many patients cannot be salvaged at the transtibial level, and often require transfemoral amputation.6
WOUND-HEALING PARAMETERS Healing of surgical wounds in individuals with diabetes poses several special concerns due to the metabolic deficiencies associated with the disease, the disease-specific deficiencies in the patients’ immune system, the oft-associated relative malnutrition and the ever-present micro- and macroscopic central and peripheral vascular disease.
Specifically, healing of an amputation wound requires a combination of arterial blood supply, tissue nutrition and immuno- competence.
The development of peripheral vascular disease is a slow process, allowing patients to develop collateral blood flow during the period of time when the major vessels are slowly occluding.
Unlike the vascular surgeon or interventional radiologist who is looking for a ‘pressure head’ or a reconstituted vessel, surgical wound healing simply requires a thresh- old level of oxygenated blood.
Both Yao and Wagner have shown that one needs approximately one-half the normal blood flow to support wound healing in the dysvascular limb.4,5,7 The most common method of measuring arterial inflow is accomplished by measuring ultrasound Doppler arterial pressures.
In the normal arterial waveform, the area under the waveform curve represents blood flow.
The ankle, brachial index (ABI) is the ratio of the ul- trasound Doppler pressure, taken at the level of the dorsalis pedis or posterior tibial artery, to the brachial pressure.
Wagner described the ‘ischaemic index’ as the ratio of the ultrasound Doppler pressure at the level of interest, i.e. popliteal for healing of a transtibial amputation, to the brachial pressure.
The threshold measure of arterial inflow sufficient to support wound healing in a dysvascular limb is an ischaemic index of approximately 0.5.4,5,7 Unfortunately, calcification of the leg arteries makes the measured ultrasound Doppler falsely elevated in at least 15% of patients with longstanding diabetes.8,9 This has prompted many to use transcuta- neous oximetry as a measure of the oxygen-delivering capacity of the cardiovascular system to the area in question.8,10 When transcutaneous oxygen measurement is not locally available, the vascular laboratory can measure toe pressure as an indicator of arterial inflow to the foot.
This is due to the observation that arteries of the hallux do not seem to be calcified, as do the vessels of the leg.11−13 The accepted threshold toe pressure is 30 mm Hg.
There is a significant impairment to wound healing in diabetic individuals for several rea- sons, in addition to the presence of peripheral vascular disease.
Diabetic patients with foot infection tend to exhibit parameters of malnutrition due to a combination of their inherent metabolic disease and the co-morbidity contribution of impaired renal function.14,15 While the nutrition literature has accepted a standard serum albumin of 3.5 g/dl as the lower limits of normal, it is currently accepted that the threshold serum albumin necessary to support wound healing in the diabetic dysvascular limb is 3.0 g/dl.2,14,15 In addition to these factors, individuals with diabetes have a multifactorial impairment in their immune system.
This may explain why the offending organisms seen in diabetic
JWBK089-26 JWBK089-Boulton April 22, 2006 8:25 Char Count= 0
310 AMPUTATIONS IN THE DIABETIC FOOT
foot infection are so unusual, and seemingly non-‘pathogenic’. Immunoglobulin production and competence are impaired, as well as leucocyte function. Affected individuals are often incapable of mounting a white blood cell response, demonstrating low or ‘normal’ white blood cell counts in the face of life-threatening infection. The accepted threshold marker for lymphocyte function is a total (absolute) lymphocyte count of 1500.
SURGERY FOR INFECTION AND/OR GANGRENE When patients present with clinical signs of sepsis and wound-healing thresholds below the ac- cepted wound-healing parameters, surgery should involve debridement (removal) of all infected or gangrenous tissue, combined with open wound management.
Broad-spectrum parenteral an- tibiotic therapy can be initiated until culture-specific antibiotic therapy is instituted (Chapters 13 and 14).
The overall metabolic and medical condition of the patient can be optimised, and nutritional support should be initiated.2,14 When both the systemic medical condition and the nutritional environment have been clinically optimised, a definitive amputation or wound closure can be performed (Plate 5).
Osteomyelitis, diabetic foot abscess and gangrene are surgical diseases.
All dead and in- fected tissue must be removed.
Resolution of these disease processes cannot be accomplished with antibiotic therapy alone.
Antibiotic therapy augments wound management, following removal of all infected and non-viable tissue.
Swab cultures are inadequate.
The adjunctive antibiotic therapy should be determined by surgically obtained tissue cultures.
Following re- moval of infected and gangrenous tissue, and clinical resolution of the infection, a terminal end organ of weight bearing must be created from the remaining tissue.
The optimal goal is to provide an ulcer-free plantar grade foot that will allow the patient to bear weight and maintain walking independence with commercially available therapeutic footwear3,16 (Plate 5).
AMPUTATION LEVEL SELECTION The first step in determination of specific surgical amputation level is to establish reasonable goals for the patient.
In patients with severe peripheral vascular disease, the vascular surgeon can advise on the probability of providing improved vascular inflow.
Consultation with phys- ical medicine and rehabilitation can help determine the patient’s ambulatory potential.17,18 Distal bypass surgery, endovascular surgery and foot and ankle amputation should generally be performed only in patients with the potential to ambulate.
When the reasonable goal is chair sitting and wheelchair ambulation, more proximal amputation at the transtibial or knee disarticulation level should be performed.
The questionable benefit of partial limb retention to assist in wheelchair transfer does not justify the surgically associated morbidity.
The second step is an assessment of wound-healing capacity.
The patient must possess an adequate bony platform and soft tissue envelope sufficient to create a functional terminal organ of weight bearing.
The roles of plastic surgical soft tissue transfer and free tissue transfer and the use of vacuum-assisted wound healing provide interesting potential to expand the surgeon’s capacity for function-sparing limb salvage.
Their roles will be better defined with more experience.
Wound-healing parameters should be above threshold to predict acceptable wound failure rates.
When patients have adequate vascular inflow, serum albumin above 2.5, 3.0 g/dl and a total lymphocyte count greater than 1500, one can expect a wound-healing rate
JWBK089-26 JWBK089-Boulton April 22, 2006 8:25 Char Count= 0
FUNCTIONAL AMPUTATION LEVELS 311
of greater than 90%. When one of the parameters is below threshold values, the wound-healing rate drops to 60%.2,8,14,15 The clinical examination of the patient and an assessment of the wound-healing param- eters provide the surgeon with a biological amputation level. A realistic assessment of the patient’s rehabilitation potential will then provide guidance for the surgeon on the appropriate rehabilitation amputation level.
THE END ORGAN OF WEIGHT BEARING The human foot is uniquely adapted for its role as a terminal end organ of weight bearing.
The engineer refers to this interaction as load transfer, and quantifies that load with a measure of the vertical ground reaction force (GRF).
During the period of time that the GRF is applied, the multiple joints of the foot allow optimum positioning of the foot to maximise the surface area and the ability to distribute the load.
The plantar skin and underlying connective tissue are uniquely evolved to manage the load without breaking down.
One must understand both these concerns, and the availability of specialised therapeutic footwear to distribute forces and reduce plantar pressure.16 The residual limb needs to be positioned in a plantar grade orientation to optimise its capacity as a platform.
There should be no bony prominences, as the insensate foot ulcerates when shear forces are applied over a bony prominence with an adherent or poorly cushioned soft tissue envelope.
The soft tissue envelope needs to be sufficiently cushioned and covered with durable skin to tolerate the loads associated with daily living.
Split-thickness skin graft and bioengineered skin substitutes are rarely sufficiently durable to achieve these tasks.3,19
FUNCTIONAL AMPUTATION LEVELS Toe Amputation The hallux (great toe) acts as the stabiliser of the medial column of the foot during the terminal stance phase.
If possible, the proximal metaphysis of the proximal phalanx should be preserved in order to maintain the insertion of the flexor hallucis brevis.
If this insertion cannot be maintained, important stability during the terminal stance phase will be lost and such patients will become relatively apropulsive.
Some (a minority of) authors suggest that these patients would be better served with midfoot amputation (Figures 26.1 and 26.2).
The lesser toes serve very little function.
Preservation of some proximal phalanx of the second toe will prevent lateral migration of the hallux, preventing a severe ‘bunion’ deformity that may be prone to ulceration (Figure 26.3).
A smooth contour of remaining toes will avoid potential pressure concentration (Figure 26.4).
Ray Resection Longitudinal amputation of a toe and its corresponding metatarsal is generally performed only for infection. One would not expect healing following resection of longitudinal gangrene, as peripheral vascular disease worsens with distance from the core. As a primary procedure,
JWBK089-26 JWBK089-Boulton April 22, 2006 8:25 Char Count= 0
312 AMPUTATIONS IN THE DIABETIC FOOT
(a)
(b)