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Chapter 2C - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles

The Choice Between Limb Salvage and Amputation: Major Limb Amputation for End-Stage Peripheral Vascular Disease: Level Selection and Alternative Options

Peter T. McCollum, B.A., M.B., B.Ch. 
Michael A. Walker, M.B., Ch.B., M.D.,M.Ch., F.R.C.S.I. F.R.C.S.Ed. 

Major amputations of the limbs are essentially disfiguring operations that carry a fairly high perioperative mortality and morbidity in elderly, debilitated patients suffering from critical limb ischemia (CLI). Estimated incidence rates of major amputations (Table 2C-1.) suggest that in the United Kingdom, as in other parts of Europe, the amputation rate is likely to be between 10 to 15 per 100,000 per year, up to half of whom may be considered unfit for referral to a limb-fitting service because of widespread chronic arterial disease. These figures, taken in conjunction with recent advances in both limb prosthetics and surgical techniques, highlight the need for further critical appraisal of available options open to all involved in the care of patients with a limb that may require amputation. Although trauma, tumor, and infection are significant disease entities that can require primary or secondary amputation, over 90% of all limb amputations in the Western world occur as a direct or indirect consequence of peripheral vascular disease (PVD) and/or diabetes. This chapter seeks to explore the moral and ethical dilemmas faced by both the patient and medical team presented with such a problem and describes investigation and treatment options open to those faced with a decision whether to amputate a limb or to attempt some form of limb salvage procedure.

LIMB SALVAGE OR PRIMARY AMPUTATION-GENERAL CONSIDERATIONS

The presence of a chapter on alternative options to amputation in a book on amputation and prosthetics highlights the difficulties and importance of decisions confronting physicians when presented with end-stage PVD. As a consequence of newer techniques and a more aggressive and enlightened approach to treatment, many patients who in the past would have undergone an amputation may now be offered the chance of a limb salvage procedure. However, it is also clear that advances in prosthetics and the concurrent development of limb-fitting services have meant that a functioning prosthesis is now the likely outcome of major limb amputation rather than a wheelchair existence. Nevertheless, this assumes that access to such facilities is available and that the patient is fit enough to manage an artificial limb. Furthermore, despite these advances, the prognosis of those undergoing an amputation for CLI secondary to end-stage PVD or diabetes is very poor. In many published studies amputation itself carries a significant mortality. Although this has been reported as up to 30% in some series, the use of spinal and regional anesthesia combined with improved perioperative management has been responsible for a greatly improved perioperative mortality. In addition to perioperative deaths, up to 40% will have died within 2 years of amputation, and a further 30% may develop CLI in the remaining limb that will require either a second amputation or limb salvage surgery.

While there can be little doubt that saving a limb and retaining its function should always be the primary goal of the physician, there are many situations in which limb salvage is neither feasible nor indeed desirable. Although a high perioperative morbidity attends an unsuccessful revascularization procedure, primary amputation should only be offered when revascularization is deemed inappropriate. Specific examples of this include a functionally useless limb, necrosis of a major part of the limb, life-threatening toxemia, absolute absence of distal vessels (rare), or instances in which revascularization is inadvisable due to severe coexistent medical disease.

The overall desire for limb salvage should not therefore overshadow the primary aim of all those dealing with end-stage vascular disease; the aim should be to decrease mortality and morbidity and improve the quality of life in both the short and medium term. It is evident that in approaching this problem consideration must be given to the possible morbidity and mortality of the revascularization or amputation procedure itself.

Whatever the primary disease, each individual case must be reviewed and investigated thoroughly and then dealt with on its own merits. Therefore a multidisci-plinary team approach is crucial to provide the best results. Several basic considerations and principles must be followed in each case where findings suggest that either limb salvage or amputation is deemed appropriate. Clearly, the single most important factor in this process is the projected quality of life following the particular action taken.

QUALITY OF LIFE AND PATIENT EXPECTATIONS

Quality of life is very much dependent on the individual expectations of the patient and the sometimes more realistic expectations of the attending staff. In a patient with a poor prognosis, perhaps because of a stroke, it may be felt that the morbidity of a salvage procedure is too great to achieve salvage of an already functionless limb. Conversely, in a unilateral amputee, extra effort may be necessary to ensure that the remaining limb is salvaged in order to keep the patient mobile. Professional judgement must also be exercised as to whether the patient's expectations are realistic. This can be a difficult and emotional issue. There is also no doubt that the expectations of relatives and paramedical and nursing staff can also influence the situation. This may prove especially important and useful if the patient has an unrealistic expectation of his future, whether it be overly optimistic or otherwise.

It is essential not to overestimate the long-term prognosis in patients with end-stage PVD requiring limb salvage or amputation. Life expectancy clearly depends on the natural history of any diseases afflicting the patient. With mortality rates of at least 30%, 50%, and 70% after 5, 10, and 15 years in those with CLI, a 20% mortality rate for those undergoing amputation,and a 2-year survival rate of 60% in those surviving initial amputation, it is clear that the long-term outcome in the group with CLI is poor, despite an advanced age group, whatever action is taken.

Further important factors affecting the decision to amputate or reconstruct include not only the availability of medical facilities but also the standard of medical and paramedical care. Plainly, the expertise and experience of a physician are vital, but equally important is the availability of quality facilities. These include resources that are readily accessible for ancillary care, diagnostic investigations, operative backup, postoperative support, and other treatment options such as prosthetic and rehabilitation infrastructures. Crucial among these is the availability of a good vascular service. In well-developed countries, it is likely that an expert vascular opinion can be obtained within 12 hours of being sought, even if this means transfer of the patient.

ETHICS

Quality of life is quite closely linked with the medical ethics surrounding a patient faced with end-stage vascular disease. An important consideration in decision making relates to the practical aspects of intervention in end-stage PVD. Is it always justifiable to subject a patient to prolonged hospitalization and perhaps suffering in order that limb salvage can be attempted? If so, what is the real return for the misery that can occasionally be caused in some patients due to multiple reconstructive procedures when eventual limb salvage is not guaranteed? Moral considerations are thus very important, and each case must be examined on its own individual merits. There can be no standard doctrine in these difficult situations, although, in general, a reduction in a patient's morbidity should remain the prime objective when considering the options in a particular case.

FINANCIAL CONSIDERATIONS

No discussion on the dilemma would be complete without some mention of the financial implications of treatment in these patients, although whether it is ethically important is a different and debatable issue. While the immediate cost of lower-limb bypass surgery and postoperative treatment is considerable, it does not compare with the cost to a patient or health system of major amputation, especially where there is no family backup and the patient is dependent upon outside support. Indeed, any treatment that can limit this situation and retain quality of life is potentially cost-effective, although it must be remembered that some patients undergoing reconstructive surgery will inevitably come to major amputation later and therefore incur double expenses.

INFORMED CONSENT

It is clear that patients in this difficult position must be made fully aware of the available options and the consequences of a particular course of action. Limb salvage procedures can be fraught with many problems and a prolonged hospital stay without any guarantee of eventual success. Patients should therefore be fully informed as to the regimen they are proposing to embark upon. Whereas it might well be considered a failure of medical technique and practice if an amputation is ultimately required, this feeling should not be conveyed to the patient, who must be persuaded to feel that all possible limb salvage options have been explored and, if these have not been possible or vascular surgery has been unsuccessful, the next stage of amputation and fitting of a functional prosthesis is a logical and necessary progression. Thus, patients must never feel that proceeding to an amputation is an admission of failure in management. A positive attitude to prosthetics is similarly vital if the patient is to later come to terms with his amputation.

The major etiologies that may lead to possible amputation can be grouped as follows:

1. Peripheral vascular disease (PVD) and diabetes mellitus (DM)
2. Trauma
3. Infection
4. Tumor

Each of the above can lead to possible early or late amputation during the natural history of the disease. Trauma, infection, and tumor are specific entities that are covered meticulously elsewhere in this chapter and therefore will not be discussed in more detail here.

PERIPHERAL VASCULAR DISEASE

Of those patients requiring amputation, over 90% are a direct or indirect consequence of CLI. With an increasingly elderly population and a significant increase in life expectancy (males, 73 years; females, 78 years in the United Kingdom) over the past 10 years, it is clear that the number of patients presenting with critically ischemic limbs is likely to increase substantially into the 21st century. This large group is therefore likely to continue to provide all but a few of the patients who require major limb amputation. Most patients presenting with CLI present with severe and debilitating pain, often at rest, that prevents them from sleeping in a bed at night. Indeed, many patients will sit up all night rather than suffer the pain that results from lying prone in bed.

The prevalence of intermittent claudication increases from 0.2% in men aged 45 to 55 to 0.5% in men 55 to 65 years of age. However, there is a large group of patients with significant hemodynamic distal arterial disease who remain asymptomatic-perhaps up to 30% show no symptoms or do not present to a physician.

Work done within the United Kingdom suggests that anywhere between 10% and 50% of those suffering from claudication will present to physicians, with a greater percentage of men, but with only a few deemed serious enough to merit referral to hospital practice. Fortunately, up to 75% of those with claudication will stabilize spontaneously over a few months, which leaves 25% of individuals with increasing trouble from deteriorating symptoms. Of this group, fewer than 5% will eventually come to some form of limb salvage revascularization procedure. Thus, fewer than 2% of all patients who present with intermittent claudication ever come to amputation.

Much of the concern over management of patients with severe PVD relates to the fact that arteriosclerosis is a generalized disease and consequently these patients have significant cardiac and cerebrovascular disease. Evidence suggests that approximately 15% will develop a myocardial infarction and at least 5% a cerebrovascular accident (CVA) over a 5-year period. The mortality rate is actually at least threefold greater than that observed in an age-matched population. It has been further noted that 50% of deaths are cardiac related, 15% related to a CVA, and 10% from other vascular causes. The most important coexisting risk factors in this group are smoking, diabetes, and hypertension.

Figure 2C-1highlights the outcome of those suffering from claudication and clearly shows that the prognosis in this group is poor. The actual life expectancy is summarized in Figure 2C-2. Work by Szilagyi et al., who examined the long-term outcome following lower-limb arterial bypasses, suggests that after the fifth postoperative year the annual mortality rate of patients is greater than the graft occlusion rate.

The natural history of the generalized atherosclerosis that coexists in sufferers of claudication and CLI shows that these groups, especially those with CLI, must be considered to represent a population with a poorer-than-average prognosis, even in the short to medium term. Thus, the likely length of hospitalization for the treatment option selected should be gauged against the likely life expectancy for the patient. Most would agree that it is not in patients' best interests to spend the majority of their remaining months in a hospital environment, although occasionally this is an inevitable consequence of whatever course of action is taken.

MANAGEMENT OPTIONS

The decision to perform either a major amputation or major reconstructive surgery is based largely upon clinical parameters that are undoubtedly influenced by past experience and current prejudice. It is therefore important to be aware of the developments in other specialties that may, it is hoped, modify these decisions. It is clear that vascular surgery, in particular, has evolved as a fully fledged specialty in many countries and has made enormous strides in several specific areas. Nowhere is this more apparent than in the management of the patient with CLI. While aortofemoral bypass grafting has become well established over the past 40 years as a reliable method of providing adequate femoral inflow, distal arterial bypass has now also become a routine procedure in specialist vascular centers. Bypass grafting to the tibial or pedal arteries can now be expected to salvage the limb, even in patients with tissue necrosis and ulceration. Vein graft patency to the tibial vessels is about 80% at 1 year, with limb salvage rates somewhat higher. Indeed, the current situation is such that few if any patients should undergo a major amputation for CLI without first having been seen by a specialist vascular surgeon with experience in distal bypass grafting. This is not unreasonable because most physicians in Western countries can obtain access to such services within 12 hours of patient referral. This allows more than sufficient time for CLI investigations and intervention to be instigated without significant deterioration in the patient's condition.

Nonsurgical Management of Critical Limb Ischemia

There has been much interest in the use of pharmacologic agents that could be used in the management of CLI. This is because many patients are high-risk surgical candidates and any method that might reduce the number of patients requiring surgery, either revascularization or amputation, would be welcome. The mainstay of medical management is to postpone amputation either by relieving pain or by sufficiently improving the local blood flow in the limb to render it viable until further collateral circulation develops. A broad range of agents has been studied, including anticoagulants (heparin and warfarin), thrombolytics (streptokinase, urokinase, tissue plasminogen activator [t-PA]), antiplatelet agents (aspirin, dipyridamole), and various vasoactive agents (naftidrofuryl, pentoxifylline). Of these, the use of intravenous prostacyclines and their synthetic analogues (e.g., prostaglandin I₂ [PGI₂] analogues such as iloprost) appears to offer the most hope for the future. Recent work with these prostacycline analogues suggests that a small number of limbs (generally Fontaine stage III, i.e., with rest pain) may be salvaged by prolonged intravenous infusion. Unfortunately, many limbs in these studies would not fall under the definition of true CLI because Doppler pressure measurements are too high in most instances. However, it is clear that some critically ischemic limbs (perhaps 10%) are salvaged by such infusions and further work in this area is urgently needed. In particular, the use of PGI₂ analogues appears to convey some definite benefits.

Interventional Radiology

A further, small number of patients may be suitable for percutaneous angioplasty (PTA), although it is our experience that angioplasty alone rarely "salvages legs." PTA was first described by Dotter and Judkins in 1964and later developed by Gruntzig and Hopffin 1974. In essence, it involves placing a sausage-shaped balloon catheter across a stenosis or occlusion (over a guide wire) and inflating this to restore continuity by ablating the underlying lesion. Used extensively for patients with claudication, it has obvious advantages for the patient with CLI, who is often unfit and usually has coexistent cardiac and other diseases (Fig 2C-3.). In particular, because it is performed under local anesthetic and without long incisions, it carries a lower morbidity and mortality than vascular surgery does. It is quite possible to negotiate short arterial occlusions (5 to 6 cm) with good expectation of success, and as in surgery, the best results are obtained in the larger vessels (iliac and femoral).

In fact, the role of PTA in tibial vessel disease is controversial and as yet unproved. However, because the majority of limbs with true CLI have widespread, mul-tisegment arterial occlusions, it is usually not possible to apply this technique to this group of patients. Disappointingly, further new developments in balloon angioplasty have not been accompanied by improved results. In particular, the present vogue for laser-assisted balloon angioplasty does not yet offer any advantage over simple PTA, although it is very much more expensive. Similarly, atherectomy devices have yet to really demonstrate that they confer a significant advantage over PTA or (simple) femoropopliteal bypass grafting in terms of outcome. The one present area of definite interest is the use of local thrombolysis in association with PTA. This technique relies upon the fact that many "acute-on-chronic" ischemic legs have had a recent fresh thrombosis that has precipitated their symptoms. By lysing this thrombus with streptokinase or t-PA and performing an angioplasty upon the underlying stenosis, the vessel may once again be rendered patent.

Although several studies have reported a greater than 75% "limb salvage" rate in CLI patients suitable for angioplasty, the patients concerned in most studies have not all had CLI as currently defined. Data from Sheffield suggest that the clinical success rate from PTA in patients with CLI (and whose disease pattern is potentially treatable with PTA) is only about 50%, which therefore represents fewer than 10% of all patients presenting with CLI.

Sympathectomy

Lumbar sympathectomy was often used to try to improve the blood flow of the lower portion of the leg in CLI. Unfortunately, although a perception of increased flow is achieved in some patients by an apparently warmer foot, this warmth is primarily secondary to opening of nonnutritional arteriovenous shunts and does nothing for the flow in the nutritional capillary bed unless the perfusion pressure is already reasonably good. In a small number of patients with isolated rest pain, chemical or operative lumbar sympathectomy may help to relieve this pain by a direct inhibitory effect on pain perception pathways. This effect has also been achieved with direct spinal cord stimulation, an area of interest that may offer more in the future because there is some evidence of an increased total limb perfusion in addition.

Vascular Reconstruction

The optimal type of reconstructive vascular procedure varies according to the level of the disease process. There is no doubt that PTA is appropriate to short occlusions (<6 cm) and to stenoses in the iliac and superficial femoral arteries. On occasion, correction of such a lesion is sufficient to relieve rest pain in some patients. In the case of longer occlusions that are unsuitable for PTA, bypass surgery can confidently be expected to do likewise. This type of surgery may involve an aortoiliac or aortofemoral bypass using Dacron or polytetrafluoroethylene (PTFE) grafts. If the patient is grossly unfit, an axillofemoral or femorofemoral bypass may be easily accomplished. These "extra-anatomic" bypasses are invaluable in the management of such cases, and although they do not carry the same excellent graft patency rates as aortic procedures, they carry less risk to the patient and are easy to perform in high-risk cases. The absence of a patent common femoral artery is no bar to these approaches because excellent results can be achieved by bypassing directly to the profunda femoris artery, with or without an extended profunda endarterectomy and patch angioplasty. The aim of all of these procedures is to provide an adequate "inflow" to the leg with a good resultant femoral pulse. While such procedures may be sufficient for Fontaine stage III patients (rest pain), they rarely suffice alone where there is tissue necrosis. In these stage IV patients, there is usually superficial femoral occlusion and/or tibial vessel disease, although in the case of diabetics, there may be only tibial vessel occlusions. Such patients nearly always require a femoropopliteal, femo-rotibial, or even femoropedal bypass to achieve healing of the foot in patients with limited outflow tracts (Fig 2C-4.). In diabetics, a popliteal-tibial or popliteal-pedal bypass will often be required.

Critical ischemia of the limb is rarely if ever seen with isolated superficial femoral occlusion. More often, there is multilevel disease. Where there is a reasonable femoral pulse, there will invariably be superficial femoral occlusion with tibial vessel disease also. This usually necessitates a distal bypass with in situ vein or re-versed-vein graft techniques (Fig 2C-5A and Fig 2C-5B ). In such patients, good results (>70% rate of 1-year patency) can be expected where the optimal conduit (autologous vein) is available. However, if prosthetic grafts are used to the tibial vessels, this falls dramatically to perhaps only 25% at 1 year, although there is some evidence that the technique of an interposition vein cuff will improve these results. In general, provided that there is autologous vein present and there is at least one tibial artery present (this may often be the peroneal), good limb salvage rates can be expected from vascular surgeons well versed in these techniques. Unfortunately, this is a specialized area, and consistently good results tend to occur only in dedicated vascular units. In planning surgical outcome, it should be remembered that a patient with digital gangrene and rest pain will often require a prolonged admission with initial distal revascularization, later digital amputation, and subsequent rehabilitation.

Trauma and Acute Vascular Insufficiency

In dealing with any serious limb insult in which vascular compromise is evident, it must be realized that even urgent treatment is barely soon enough! Rapid but careful assessment by the primary-care team and assessment of the order of treatment priorities, first of the patient as a whole and then in relation to the injured limb, are paramount. The level of priority with respect to the limb in trauma cases is generally vessels, nerves, bones, and then soft tissues, although at the time of surgery it is often necessary to splint the bones first prior to attempting vascular reconstruction in the interests of stability.

Any limb that is regarded as showing signs of acute CLI should, if possible, be rapidly referred to a specialist in this field. Time in such cases is vital. Full and careful evaluation by an experienced vascular surgeon followed by appropriate investigation and subsequent treatment will provide the greatest chance of limb salvage. There is little place for treatment of these patients by the occasional vascular surgeon if acceptable results are to be obtained, although this may be necessary on occasion in the absence of immediate specialist resources.

Of all those admitted with acute CLI, those with an otherwise normal vascular tree are most at risk because there will be little if any collateral circulation present, unlike those with previous underlying PVD. Of all those admitted with evidence of acute CLI of the legs, about 60% to 70% will leave the hospital with an intact limb. Of those surviving, up to 15% will require amputation. Operative treatment depends upon circumstance, but general principles include the use of autologous vein as a bypass medium if at all possible and generous decompression fasciotomies to reduce the risk of reperfusion injury. Amputation level selection, where necessary in this group of patients, is defined by the available viable tissue present and depends entirely upon clinical assessment with the emphasis on preserving limb length.

Brief mention may be given to one specific problem that, although fortunately rare, can cause considerable difficulties with management. The increasing growth in the number of drug abusers is inevitably leading to an increasing number of addicts presenting with arterial injury following intra-arterial puncture and consequent microembolization following injection of a variety of substances. Although at first sight these injuries may not seem serious, after a few hours the injected limb can show severe signs of ischemia that is not always reversible. In this situation, opening the diseased artery will often show massive intraluminal reaction and edema causing a significant hemodynamic stenosis in the artery. In general, the treatment of choice is decompression fasciotomies, intravenous anticoagulation, and prostacycline infusions, with a "wait-and-see" policy usually justified. Vascular surgical exploration has a very limited role, although major amputation is often the final outcome. In any dealings with these patients, it should be remembered that a large percentage are human immunodeficiency virus (HIV)-positive and some will have full-blown acquired immunodeficiency syndrome (AIDS).

Infection

In patients with PVD, "dry" gangrene is a result of reduced arterial inflow or stasis in the circulation of the limb or digit. Demarcation develops early and is usually clear-cut, with resultant mummification showing few signs of infection. Patients with dry gangrene will sometimes autoamputate the affected limb or digit with few if any systemic effects, especially if the blood flow is improved to the limb by vascular reconstruction. In contrast, "wet" gangrene may be a consequence of both arterial and venous obstruction and is also seen in diabetics. There is always infection and putrefaction present in this process. Early revascularization may help to reduce the volume of tissue lost in wet gangrene, although there is a risk of graft infection in bypass cases. The decision to amputate primarily or to attempt limb preservation can be extremely difficult, but in the case of life-threatening sepsis, primary amputation will usually be indicated.

AMPUTATION LEVEL SELECTION

For the minority of patients with CLI in whom revascularization has failed or in whom there is some other reason why it cannot be attempted, the alternative is major limb amputation. For most practical purposes, there are three common "levels" of amputation to examine in relation to PVD in the lower limb; these are transfemoral, transtibial, and digital or partial foot. With the conspicuous exception of diabetics, digital and partial-foot amputation for end-stage vascular disease is generally disappointing in the absence of prior successful revascularization. When consideration is being given to one of these distal amputations, the question of a distal bypass must always be recognized because it is almost certain that a successful bypass will result in foot healing. Similarly, if a leg amputated at the Syme or Chopart level shows signs of healing, it is virtually certain that there were patent tibial arteries at the ankle to which a distal bypass could have been placed. The foot therefore might have been salvaged with perhaps only a minor digital amputation. The only exception to this rule is the diabetic patient without serious PVD who may sometimes heal a partial-foot amputation without revascularization. Therefore, the main deliberation in amputation patients is whether they will heal a transtib-ial amputation or not. Most diabetic and vasculitic patients will require a more detailed assessment of foot viability. This is because local excision of dead tissue can often be effected successfully if accompanied by vigorous antibiotic therapy and careful debridement and protection of neurotrophic ulcers and skin with appropriate plaster slippers and splintage.

Clinical Assessment

It is not adequate to merely perform angiography in patients with CLI who are being considered for amputation and/or limb salvage revascularization. This is simply because the absence of visible vessels does not necessarily portend failure for surgery. In many instances, it is simply a failure of radiologic technique where contrast is not seen at the foot level. In such cases, exploration of the ankle or pedal vessels will generally reveal an adequate recipient artery. This quite common failure of angiography to show distal vessels is well recognized and has led to the development of other methods to demonstrate patent ankle and foot vessels.

If, despite all this, amputation is felt to be required, further investigations may be needed to aid in selection of the optimum level of amputation. While it is generally agreed that there are certain clinical factors (such as a severe flexion contracture of the knee) that precipitate an amputation at a particular level, the physician's ability to select the optimum level of amputation based upon clinical appreciation of tissue viability is poor. Criteria such as poor skin edge bleeding at the time of surgery and absence of pulses do not correlate well with failure to heal despite the fact that they are still commonly used standards. Although clinical judgement alone will produce reasonable results when it is carried out by an experienced amputation surgeon, it is clear that this form of assessment is not very objective and is probably ineffective in the majority of patients. This is particularly so where attempts are being made to save the knee joint in cases of marginal viability.

Angiography

As already stated, arteriography ought to have been performed already in almost all preamputation patients as a preliminary to possible arterial reconstruction. It has, however, been shown to be of little value in deciding the optimum level of amputation. Robbs and Ray found no difference between healing and failure to heal in terms of the number of major patent vessels present in a study of 84 amputees. Other workers have also found arteriography to be of little value, although Roon et al. maintained that a patent profunda femoris was of major importance in the presence of a superficial femoral occlusion and should be ensured before attempting a transtibial amputation. Certainly, occlusion of both profunda and superficial femoral vessels would seem to indicate a poor prognosis for healing in transtibial amputations.

Segmental Systolic Pressure Measurements

Although they are necessarily an indirect assessment of tissue perfusion and not reliable in situations where arteries are calcified, Doppler-derived segmental pressure measurements have been utilized in the assessment of CLI for many years. Indeed, the working party of the International Vascular Symposium set out a definition of CLI based upon segmental pressure criteria in 1983, although this definition has now undergone further refinement in a consensus conference. While these are useful moves aimed at identifying patients with CLI and allowing reasonable comparison of patient groups between different studies, the criteria are of little help in deciding upon the level at which to amputate. There was considerable initial enthusiasm for ankle Doppler pressure measurements, but this was tempered by the failure of later studies to substantiate initial reports. In particular, ankle pressure measurements appear to be of little or no value in partial-foot amputations and also in diabetics. This is probably largely because of arteriovenous shunting in the foot giving rise to nonnutritional blood flow and also because of the difficulty in compressing calcified vessels with consequent artificially high "systolic pressures" in diabetics. In contrast, thigh pressure measurements appear to be predictive of success in transtibial amputation surgery. Several workers have demonstrated that transtibial amputation healing is likely to occur at thigh pressures greater than 70 mm Hg and possibly between 50 and 70 mm Hg. The widespread adoption of this baseline standard alone would certainly reduce the number of transfemoral amputations currently being performed, with very little, if any, consequent increase in morbidity.

Infrared Thermography

Infrared thermography has been used to delineate areas of nonviability and to try to identify specific skin flaps for lower-limb amputations over the past two decades. Although there is a good relationship between skin blood flow and thermography, recent work suggests that much of the "heat" seen on the thermogram derives from two main sources: convective heat where the transfer is primarily from arterioles >50 µm in diameter and conducted heat where there is a temperature gradient from the deeper structures to the skin.Good results have been obtained from specialized units using this technique, but the cost and difficulty of interpretation of such images combined with a difficulty in accurately quantitating the image other than by additional skin blood flow measurements render it useful mainly as a development tool in specialist centers. It is, however, one of the few tests that provides an indication of specific skin flap viability.

Skin Fluorescence

As with thermography, the principle of being able to outline nonviability of specific skin flaps has been used by several workers. The absence of uptake of fluorescein when injected intravenously has been used to examine healing in amputations, and more recently, a technique of quantitative fluorometry has been developed in an effort to provide more objective criteria for the method. Since it is both fairly cheap and reasonably easy to perform, there is some promise in this methodology, and it is to be hoped that other units will validate the data found by Silverman and his colleagues. Despite the technique being available for many years, however, it has not found widespread popularity, and there is still some difficulty in interpretation of results, especially where there is associated inflammation.

Skin Blood Flow Measurements

The application of "point source" measurements to assess tissue viability is epitomized by the use of skin blood flow measurements. The principle of skin blood flow techniques is to measure the washout of an intra-dermally injected radioisotope tracer and from this to derive a result that is a function of capillary skin blood flow. By using an original principle devised by Kety,the precise skin blood flow can be calculated. The advantages of this technique are that it appears to measure nutritional blood flow and it provides an absolute measurement for blood flow. The problems with the technique principally relate to both its invasive methodology and the need for a radioactive tracer. Testing is also relatively slow to perform and requires multiple measurements in order to avoid the pitfall of finding and relying upon an isolated high or low result. In addition, it is not easy to perform repeated measurements because the isotope dosage is cumulative.

A number of different tracers have been used, including ¹³³Xe,¹³¹I, and ¹²⁵I-iodoantipyrine. Xenon is more difficult to use because of its biphasic clearance and affinity for adipose tissue; nevertheless, excellent results have been achieved, principally by Moore's group in San Francisco. These results have been reproduced by using ¹²⁵I-iodoantipyrine more recently and a good correlation between healing and mean skin blood flow levels greater than 2.5 mL/100 g/min was achieved. While providing good and reproducible data, especially in the lower limb, skin blood flow measurements in the foot are very variable, and a high skin blood flow value does not necessarily predict a successful outcome to amputation. The logical explanation for this is that a local point measurement of skin blood flow is not representative of the region as a whole, and this is particularly so in the foot. In any event, the healing of an amputation depends upon the skin blood flow after the amputation, not upon that measured prior to the procedure. Despite these drawbacks, the skin blood flow technique is probably the single most accurate measurement to assess skin viability that is currently available, particularly around the knee joint level.

A development of the skin blood flow technique is the measurement of skin perfusion pressure, pioneered in Denmark. The principle of the method is to note the blood pressure at which the capillary return to the skin is abolished. The detection technique may be the clearance of a radioisotope or the use of a photo-spectrometer. The method has the advantage of ignoring specific values for skin blood flow but has the disadvantage that there can be great difficulty in deriving absolute pressure values at the very low clearance rates found in critically ischemic skin. There is some debate as to what absolute skin perfusion pressure should be used to predict healing, and this may reflect the methodology involved.

Transcutaneous Oxygen Measurements

Since transcutaneous oxygen pressure (TcPO₂) measurements were first used to aid in the assessment of tissue viability, there has been an explosion of published data discussing its value in PVD, especially in relation to amputation level. Although the transcutaneous electrode was originally designed to reflect arterial po₂ in the newborn, since then its application has been widened to include the evaluation of ischemic tissue. Unfortunately, electrode design has not been modified to allow for the skin characteristics of the adult, and there is a great variation in results between different workers using different machines. Variables affecting TcPO₂ measurements in normal skin such as Pao₂, skin thickness, electrode response, skin blood flow, local oxygen availability, and others are overshadowed by a maximal vasodilatory response to the integral heater. Fortunately, ischemic skin does not respond in the same way and is much more affected by other variables, particularly by the underlying perfusion pressure.

The great attraction of the method is its simplicity and ease of use. However, the difficulties of calibration and interpretation are usually greatly underestimated or not considered and result in data that at best are insensitive. In early reports of the method, there were wide variations in conclusions as to what TcPO₂ value reliably predicted amputation healing at the transtibial level. This was highlighted by several reports that healing took place in a number of amputations where the TcPO₂ was zero. The addition of 100% oxygen inhalation undoubtedly improved the sensitivity of the technique immeasurably but may be inconvenient and time-consuming.

It therefore appears that TcPO₂ levels greater than 35 mm Hg at the calf reliably predict transtibial amputation healing but that values below this give an unpredictable outcome. If oxygen inhalation is added to the test, then discrimination between healers and nonheal-ers is improved dramatically. TcPO₂ values in the foot are of little predictive value.

SUMMARY

Several other techniques have also been tried in an attempt to aid in amputation level selection. These include laser Doppler studies, ^99mTc pertechnetate scanning, muscle pH studies, and others. Of these, the laser Doppler has the most potential because it offers the ability to interrogate the microcirculation in a noninvasive mode.

There are several major problems that can be identified when attempting to analyze data relating to amputation level selection. Principal among these is the enormous variation in approach on the part of physicians. Specifically, if the benefits of a certain test to predict healing in transtibial amputations are to be examined, then the optimum conditions under which that amputation is to be performed must be provided. This is certainly not the case in many of the published series, which renders much of the accumulated data of dubious value. One obvious example of this issue is the problem of who performed the amputation. In many cases, it is undoubtedly not the most experienced surgeon available. The second major area concerns clinician prejudices. Where there is a partiality for trans-femoral amputations in a unit, the value of ancillary methods to predict amputation outcome is considerably diminished because it is likely that those amputations that would be considered most "at risk" would automatically have transfemoral resections. This is a particularly important area because transfemoral/transtibial amputation ratios vary enormously from region to region and country to country. Again, if there is a predilection for primary amputation rather than attempted reconstructive surgery, a better amputation healing rate may be attained because many of these patients would not have strictly "end-stage" PVD. In other words, some would undoubtedly be technically reconstructable with one or more patent tibial vessels. In these instances, any attempt to perform noninvasive vascular tests is largely irrelevant because most amputations in such patients will heal. It is only when surgery is being performed at the edge of tissue viability that sophisticated evaluation techniques have a valuable role to play. Therefore, all published amputation data for CLI should include some reference to the number of distal vascular reconstructions (and failures) currently being performed in the unit.

Is there any test that the present-day amputation surgeon can apply to help in selecting the optimum amputation level? Most of the available techniques are expensive and difficult to perform correctly, and results are difficult to reproduce. Therefore, while they may provide useful data for a particular specialist unit, the methods often prove quite difficult to duplicate effectively elsewhere. In particular, no one method has proved completely reliable for the purposes of partial-foot amputations, although skin perfusion pressure techniques appear to be most predictive. Much more work needs to be carried out on the critically ischemic foot to ascertain why current techniques are insensitive as predictors of foot healing.

For general purposes, the single most readily available (and cheapest) test is undoubtedly Doppler-de-rived systolic pressure measurement. Although relatively insensitive, there is considerable evidence to support the use of a cutoff range of 50 to 70 mm Hg at the lower part of the thigh for transtibial amputations in the absence of arterial wall stiffening. With this reference, many transfemoral amputations could probably be avoided, with few transtibial failures as a consequence. It is probably only in those limbs with marginal skin flap viability that other techniques of evaluation have a major role to play. Of these, skin blood flow measurements would seem to offer the most precise objective criteria. Although skin blood flow is a local measurement and therefore not necessarily representative of the blood flow in an entire region, multiple skin blood flow measurements appear to be more dependable and correlate well with outcome. A mean skin blood flow of less than 2.5 mL/100 mg/min may be regarded as likely to result in flap failure. It is probably worth corroborating these data with some other regional methodology to look for specific well-vascular-ized flaps. Fluorescein angiography or thermography offers the best methods for this approach, with fluorom-etry perhaps being much cheaper although more difficult to perform and interpret.

It is clear that clinical assessment alone is inadequate for the majority of surgeons in decision making for amputation surgery, and therefore adjunctive preoperative laboratory testing has become a crucial part of any amputation service that is committed to preserving limb length.

CONCLUSIONS

The dilemma facing the present-day physician regarding critically ischemic limbs is increasing rather than disappearing. This is because we live in an age of increasing life expectancy coupled with the ever-present patient and relatives' notion that something can and ought to be done for the leg. Although this prejudice can be unreasonable at times, successful surgical revascularization of a critically ischemic limb is the optimum management in the vast majority of such patients. There may be a good case to be made for nonintervention in a small minority of elderly patients whose CLI is merely a manifestation of total-body failure and whose inevitable conclusion will be early death. These patients are, in general, obvious to the clinician and require only good nursing and analgesia to keep them comfortable. The main difficulty lies in the group of patients in whom it is not clear whether a primary amputation or revascularization should be attempted.

There are several rules of thumb that may be used in this difficult group, however. Where tissue necrosis has impinged upon the midtarsal area or hindfoot, a good transtibial amputation will provide a much better chance of early rehabilitation and a shorter inpatient care than will distal vascular surgery followed by a midtarsal amputation or Syme ankle disarticulation. Occasionally, it may be justified to revascularize a limb with the object of attaining healing at the transtibial or Syme level in order to cater for a specific circumstance, but these cases are very few. Indeed, it is likely that unless partial-foot amputation can be confined to the digital or transmetatarsal level after revascularization, primary transtibial amputation should be undertaken. Fortunately, the vast majority of limbs with CLI may be salvaged with no amputation or else "minor" partial-foot amputation. These patients have a very limited life span, and prolonged reconstruction, amputation, and rehabilitation should be avoided if they are not to spend most of the rest of their lives in hospital.

Conversely, it is obvious that there are still many legs being lost to major amputation without sufficient thought being given to revascularization. In the absence of prior vascular reconstruction or pure diabetic microangiopathy, the lowest level that should ever be contemplated for primary amputation is the transtibial level. This is simply because if a more distal level is to heal, there must be at least one reasonable tibial or pedal vessel patent and therefore distal revascularization should be possible.

Patient expectations and improvements in therapeutic options have changed our approach to the patient with end-stage lower-limb vascular disease today. It is important for us all to be aware of the wide variety of options that are at our disposal before deciding upon a particular strategy. The approach to these patients must of necessity involve a multidisciplinary team, with each member realizing the limits of his own expertise. Inevitably, the patient can only benefit from such a team approach.

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Chapter 2C - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles

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