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Application of the Transcutaneous Oxygen Monitor to Amputees Undergoing Early Fitting of Below-Knee Patellar Tendon Bearing Prostheses

Keith Linge
Dalton A. Boot, F.R.C.S., M.Ch.Orth
Suresh Keetarut, F.R.C.S.

Introduction

The availability of new materials for the fabrication of prostheses and the use of the transcutaneous oxygen monitor as a clinical tool in the vascular laboratory have led to the possibility for earlier prosthesis fitting together with an accurate prediction of stump healing. Patients who require lower limb amputation for atherosclerosis are often elderly and too weak to make use of an above-knee prosthesis due to the extra effort required to propel the leg following loss of the knee joint. Therefore, saving the knee is of paramount importance.

Pre-operative objective assessment of the diseased limb by Burgess et al.¹ and Dowd et al.² using the transcutaneous oxygen monitor allows the optimal level of amputation to be determined, leading to amputations being performed at the most distal level suitable for fitting of the prosthesis. It is generally accepted that if the transcutaneous oxygen tension (PtcO₂) is greater than 40mm Hg pre-operatively at the site of amputation, then the stump will have an excellent chance of healing primarily. Of course, many factors govern wound healing, but prime importance must be given to an adequate, well-oxygenated peripheral blood supply. Although pre-operative PtcO₂ measurement has now been widely researched, little is known of just what happens to the peripheral circulation around the amputation wound in the postoperative period, especially when subjected to weight-bearing.

It is often felt that following amputation, patients presenting at the Artificial Limb and Appliance Center (ALAC) should not be prescribed a Patellar Tendon Bearing (PTB) prosthesis until stump healing is nearly complete, since fitting a stump too early may result in additional wound breakdown. In the meantime, the patient may nevertheless remain mobile using walking aids. Alternatively, it is possible to provide the patient with an above-knee/below-knee temporary prosthesis which is an above-knee pylon adapted for the below-knee limb. This device consists of a metal socket to bear weight proximally on the ischial tuberosity and a felt sleeve distally to contain the stump without undue pressure. The patient can then achieve mobility with a bipedal gait until the stump is healed, after which the PTB prosthesis can be considered. However, neither a pair of crutches nor an above-knee/below-knee temporary prosthesis is as satisfactory as a PTB prosthesis; and the earlier the amputee is fitted with such a prosthesis, the better the mobility of the patient.

Our past observations of pre-operative PtcO₂ levels suggest that patients with high levels of PtcO₂ at their site of amputation went on to rapid wound healing, while those only approximating to the viability level of 40mm Hg took longer for wound healing to occur and were more subject to wound infection and healing by secondary intention. Similarly, the determination of a postoperative threshold level for PtcO₂ would be a valuable asset in selecting those patients most suitable for early PTB fitting.

Materials and Method

Patient Selection

Patients who participated in the trial were capable of walking at least 100 yards six months prior to amputation. The non-amputated leg was not to be affected by medical conditions such as claudication, hemiplegia, polio or flexion contractures greater than 100 at the hip or 150 at the knee. Patients with a history of breathlessness that was capable of restricting normal walking pace were excluded. Mental alertness and the ability to regularly attend the local limb fitting center were vital. At the initial appointment to the center, the Medical Officer needed to decide whether the stump was of a configuration compatible with the use of a PTB prosthesis and that the wound incision was no greater than 5mm in width.

Stump Wound Categorization

At each visit the Medical Officer was assigned to assess the condition of the stump wound and to categorize it according to the following types:

1. Fully healed stump

2. Narrow crust of width less than 5mm (irrespective of length)

3. Wide crust of width greater than 5mm (irrespective of length)

4. Narrow open wound of width less than 5mm (irrespective of length)

5. Wide open wound of width greater than 5mm (irrespective of length)

The Temporary Patellar Tendon Bearing (PTB) Prosthesis

On admission to the trial at the second week postoperatively, the prosthetist at the ALAC took a plaster of paris negative cast of the stump in 300 of knee flexion. Bony prominences and other landmarks were imprinted in the cast and then transposed onto a positive male cast. The positive cast was "rectified" in the manner described by Radcliffe and Foort³ by "building-up" the pressure sensitive areas of the stump and by "reduction rectification" of the pressure tolerant areas. The socket was fabricated on the modified or "rectified" cast using a thermoplastic inner liner made of Pelite™ material with a rigid polypropylene ex ternal socket. Two side steels were riveted to the socket and fitted distally to a pointed wooden foot rocker which was carefully aligned. The prosthesis (Figure 1) was fitted to the patient one week following casting, i.e., three weeks following amputation.

Transcutaneous Oxygen Monitor

The model used was a Radiometer TCM1 transcutaneous oxygen monitor having a Clark type polarographic electrode which was oper ated at 44°C. Prior to use, the electrode was calibrated by immersion in two separate solutions of known oxygen tension. Oxygen passing through the covering Mylar membrane enters an underlying solution, causing an electro-chemical reaction. Free electrons formed in the reaction cause a flow of current between a platinum cathode and a silver anode. The amount of current flowing is directly related to the amount of oxygen permeating through the membrane. Following calibration, the electrode was attached to the skin surface by means of a double sided adhesive ring. A heater controlled by a thermistor in the electrode causes capillaries in the skin directly below it to be elevated to its operating temperature, giving complete vasodilation approximately 20 minutes after attaching the electrode. At this point, a steady oxygen tension is achieved and recorded. All PtcO₂ results taken throughout the trial were known only to the technician, hence, decisions taken by the medical staff regarding level of amputation, fitness for prosthesis fitting, and weight-bearing were unbiased and made on clinical judgment alone.

Chronology

Except in cases of emergency surgery, transcutaneous oxygen measurements were performed one or two days pre-operatively at 10cm below the knee joint line lateral to the anterior tibial border, then a further 10cm distal on the posterior aspect of the leg relating to the site of a future posterior skin flap of a below-knee stump. Two weeks following amputation, the dressings were removed and further transcutaneous oxygen tension measurements were performed immediately proximal to the suture line, immediately distal to the suture line, and on the base of the stump.

At this stage, the patient was referred to the Medical Officer at the local ALAC as described above and if suitable for the trial was passed on to the prosthetist for socket fabrication. Daily physiotherapy at the hospital involved the patient walking with a Zimmer frame (walker) or stick(s) between parallel bars. Whenever possible, the same physiotherapy, technical, medical, and prosthetic staff were maintained throughout the trial.

The PTB pylon was available after one week, i.e., three weeks following amputation and the patient was asked to bear weight daily on the prosthesis before returning for reassessment at the ALAC one week later, at which stage repeat transcutaneous oxygen measurements were performed.

Subject to suitable progress and available domestic support, the patient was discharged from the hospital at this stage with instructions to continue weight-bearing through the prosthesis. This activity was reinforced twice a week by visits to the physiotherapy department at the hospital.

A visit to the ALAC was arranged at the sixth postoperative week for further assessment and possible socket or prosthesis refinement. A repeat visit took place at the eighth week post-operatively when transcutaneous oxygen tension measurements were carried out for the final time on those patients with stumps that had achieved complete healing (type A). Those patients having wounds inferior to type A had further measurements taken at the twelfth week postoperatively when normally the final visit to the ALAC took place. Following a satisfactory report from the Medical Officer, the patient was considered to have completed the trial.

Results

Twenty-one patients with an age range of 15 to 74 years (mean 63 years) were included in the trial. Fourteen went on to completion, of whom 13 were admitted for peripheral vascular disease, while one, a 15 year old child, suffered from a vascular deficiency secondary to congenital cardiac failure, resulting in unilateral ulceration around the ankle. Seven patients were withdrawn from the trial due to various complications (Table 1) .

The variation in the PtcO₂ around the site of amputation during postoperative period is shown in Figure 2 .

The degree of variation was compared to that found in normal volunteers as reported by Coleman et al.⁴ , but no statistical significance was found between the patient and normal groups at the 1% level using a Wilcoxon Rank Sum test.

Analysis of the PtcO₂ results of the two patients withdrawn for revision amputations revealed that in each case, the mean level in their stumps was below 40mm Hg at the time of withdrawal. One patient had 49mm Hg pre-operatively, which subsequently deteriorated to 4lmm Hg and 3lmm Hg in the second and fourth postoperative weeks, respectively. The second patient who was admitted for emergency surgery had no pre-operative PtcO₂ measurements taken and achieved only 12mm Hg before undergoing revision surgery.

The two patients not completing the trial due to wound infection had PtcO₂ levels in the stump of 4lmm Hg and 6lmm Hg at the time of withdrawal.

All those patients going on to successful completion of the trial and whose wounds healed primarily did so with skin oxygen tensions in the stump greater than 40mm Hg at the time of discharge.

The patient suffering from cardiac insufficiency showed signs of delayed wound healing and subsequently a decision was made to limit weight-bearing during the early postoperative period. Wound healing was achieved by secondary intent and it was subsequently revealed that the pre-operative PtcO₂ was 43mm Hg with 25, 25, and 32mm Hg at the second, fourth, and eighth postoperative weeks, respectively.

The relationship between the wound condition (as previously categorized) and the postoperative period in the 14 patients completing the trial are shown in Table 2 .

One case of temporary wound deterioration was recorded, that of a type B at two weeks to type C at four weeks. This result was despite satisfactory PtcO₂ values of 43mm Hg and 58mm Hg at the second and fourth weeks respectively. This case of a 35 year old male was the only incidence of obesity amongst the trial group. The wound subsequently went on to heal, but by secondary intent following limited weight-bearing.

Table 3 shows the relationship between the wound condition and the mean PtcO₂ level.

The increase in PtcO₂ related to wound healing would seem to confirm the observations of Romano and Burgess⁵ who reported that the circulation in the below-knee amputation stump improved with wound healing.

The overall mean PtcO₂ value measured in the below-knee stumps of the trial patients during their period of recovery was 53mm Hg compared to 70mm Hg as observed by Dowd et al.⁶ in the legs of healthy normal volunteers. The significant difference in these values would appear to reflect the latent reduction in the vascularity of the stumps of peripheral vascular amputees.

Discussion

The rehabilitation of the below-knee amputee is partly dependent upon the hospital to which the patient is referred. Standardization of a rehabilitation program which would be applicable to the majority of amputees would be most desirable from the point of view of both the hospital and patient, but at present there is no method of objective assessment available to the clinician capable of identifying the most suitable time for weight-bearing following fitting of a prosthesis.

Without doubt, early mobility improves the patient's morale and reduces the risks of medical complications associated with prolonged bed rest.

Financial savings are also a consideration in early discharge from hospital. Therefore, an objective means of predicting wound healing not just at the pre- but at the postoperative stage would be a great asset in determining the optimal level for limb amputation and the most suitable rehabilitation regime.

The results of this trial substantiate the previous findings of Dowd et al.⁷ who found that, if the skin at the site of incision had a pre-operative PtcO₂ of greater than 40mm Hg, then the incision wound would stand a good chance of healing. While one patient in the trial was shown to be initially above this level, a deterioration in the vascularity of the limb to below the threshold level predictably led to its subsequent loss. The need for a minimum PtcO₂ level to afford primary healing is further sup .ported by the two peripheral vascular patients who were below 40mm Hg and required revision surgery at a more proximal level.

The case of the congenital heart failure patient, who achieved wound healing by secondary intent only following continually low PtcO₂ readings, highlights the risks of amputation at sites with poor skin oxygen perfusion pressure. Such patients would be unsuitable for early weight-bearing on their prostheses.

Since no statistically significant variation of skin oxygen tension nor wound deterioration were noticeable in the vast majority of below-knee stumps when subjected to regular weight-bearing on a PTB prosthesis, it appears that following the third postoperative week, wound healing along the suture line had progressed sufficiently to withstand compressive and shear forces caused by contact with the PTB socket. These findings support those of Christie working at the artificial limb center in Edinburgh which suggest that early mobilization of the below-knee amputee fitted with a PTB socket, despite having an unhealed wound, was a sensible alternative treatment. It is important that regular assessment of the stump condition at the ALAC, monitoring of the PtcO₂ levels, and an exercise regime need to be adhered to strictly.

Therefore, routine observations of skin oxygen tension would appear to be a useful indication of which wounds are most suited to early PTB fitting and which are more at risk of delayed healing or breakdown. We observed that maintenance in the postoperative period of skin oxygen perfusion pressures above 40mm Hg around the suture line and base of the stump resulted in progressive wound healing, despite early weight-bearing in all but the case of the obese patient. However, more work remains to be done in this field, especially the monitoring of contact pressures and shear forces between the stump and PTB socket. Such findings may indicate the maximum safe contact pressures to which the unhealed stump may be subjected without detriment to the healing process. This information would further the successful rehabilitation therapy of the below-knee amputee.