ï¿½ Knee: Probably the most extensively studied area of prosthetic design. Because there are no muscles across the knee, the amputee must create knee hyperextension throughout the stance phase to prevent the knee from collapsing. A number of different methods are used to methods are used to assist in this – constant friction, manual locking, variable friction, weight activated, hydraulic, four-bar linkage. The stability in stance phase has to be weighed against the initiation of swing phase flexion.
ï¿½ Foot: Similar to the BKA – depending on the amount of torque that the amputee is able to generate, the foot component of the prosthesis can be made rigid (Solid Ankle Cushion Heel – SACH), single-axial, multi-axial.
AMPUTATION LEVEL SELECTION
Generally, the more distal the amputation, the better the quality and chances of rehabilitation. Especially important in the elderly, already debilitated vascular patient. The selection of level is a balance between most distal level of amputation feasible within the confines of wound healing potential.
ï¿½ Palpable pulses are not necessarily a good indication of healing level – the absence of popliteal and ankle pulses in diabetics does not condemn a BKA to fail. The absence of femoral pulses however, is more prognostic for a poor outcome with a BKA.
ï¿½ not a method of level selection, but rather determines candidates for revascularization.
ï¿½ A patent profunda femoris is important for BKA healing. (Roon, et al. 1977, Am J. Surg)
DOPPLER PRESSURE MEASUREMENTS
ï¿½ BKA’s will heal with lower thigh pressures greater than 70 mmHg.
ï¿½ radiated heat from the skin is picked up by a special camera and translated into a colour image – quantification of which is considered a reflection of skin blood flow.
ï¿½ In theory, a good relationship exists between blood flow and thermography, and this would give excellent indication for skin flap viability.
ï¿½ Expensive, and difficult to interpret the scans and quantitate actual nutritive flow.
ï¿½ Identifies viable skin by its uptake of fluorescein injected intravenously.
ï¿½ Cheap, safe, easy to perform.
ï¿½ Correlation with wound healing has not been well demonstrated.
SKIN BLOOD FLOW MEASUREMENTS
ï¿½ Measurement of the washout of an injected radioisotope tracer is used to calculate a result that is a function of capillary blood flow.
ï¿½ Measures nutritional blood flow; most accurate measurement to assess skin viability around the knee joint (less reliable in the foot).
ï¿½ invasive, slow, and uses a radioactive substance.
TRANSCUTANEOUS OXYGEN MEASUREMENTS
ï¿½ In theory, assessing the availability of oxygen at the skin would predict the degree of oxygen delivery at a cellular level.
ï¿½ Fraught with many technical and problems, as the tcpO2 is affected by PaO2, skin thickness, skin blood flow, electrode response.
ï¿½ Despite limitations, considered the gold standard (cutoff of 25-30 mnmHg)
AMPUTATION LEVEL SELECTION
Rate of metabolic energy expenditure (VO2, ml/kg per min)
At any ambulation speed, amputees will consume metabolic energy at a higher rate and will reach the anaerobic threshold at slower walking speeds then normal. In addition, they will exhibit a greater heart rate and increased cardiac work compared to normals walking at the same speed.
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