Dr. Krackow was a stickler for precision, and he was relentlessly thinking about how to make his bone-cutting techniques more reproducible and precise. This chapter on bone-cutting technique and accuracy is inspired from much of the teaching and direction he provided to countless residents and fellows over the years.
Accurate, safe, and efficient use of a power bone saw has become a basic task in modern orthopedic surgery. This section discusses those aspects of equipment and technique that relate to the development of an ability to make accurate and efficient bone cuts. The use of slotted cutting jigs will assist in limiting the error margin for saw cuts and can dictate the thickness of the saw blade. It is useful for the surgeon to understand the basic principles of cutting on an open block versus a slotted jig and the technical considerations for selecting this power tool, which is intimately involved throughout the procedure and has a direct effect on the achievement of the success of the case. For the novice, placing the saw at full power to observe the excursion distance of the tip of the saw is worthwhile to understand the width of the effective cutting arc compared with the physical saw blade. For similar reasons, observation of the distance between the saw’s cutting edge and the handpiece to understand changes in the angle along the length of the sawblade due to lifting off the cutting jig that would affect over- or underresection is a worthwhile exercise.
At least five characteristics of saw blade design and manufacture relate to cutting properties at total knee arthroplasty: metal quality, tooth arrangement, blade length, blade width, and blade thickness. Although intuitively obvious, it is worth pointing out that the oscillating saw cuts at the end not at the sides of the blade, and a reciprocating blade cuts along the edge of the blade and can cut at the tip depending on the configuration of the saw blade. Selection of the saw blade will have a direct effect on the technique used to complete the surgical procedure.
Metal hardness, the metal’s ability to take and hold a good edge with respect to maintaining sharp saw teeth, is a characteristic that is difficult to assess, especially as it is not routinely supplied by manufacturers. Certain saw tooth configurations provide relatively sharp cutting action but introduce interesting control problems, see Fig. 8.1 . If one approaches the bone eccentrically or asymmetrically using principally one side of the saw blade, then this blade strongly tends to kick toward the opposite direction because of the angular orientation of the saw teeth. The saw tooth arrangement creates a form of ratchet or barb effect. Avoidance of this blade behavior is achieved by approaching with the saw so that the blade is centered within the arc of travel over the bone encountered by the blade.
The next point concerning saw tooth design relates to a superior/inferior splaying of saw teeth and the significance of this for some press-fit applications. Fig. 8.2 shows how this aspect of tooth arrangement causes the cut surface to be produced some distance “deep” to the surface of a cutting guide. This is not a realistic problem in the vast majority of cases. It is, however, a detail that should be recognized, especially if one is shown new saw blades with greater variation in the angulation and/or greater length to the angled teeth, a condition that also would accentuate this deep cutting effect.
For the same tooth design and sharpness, functionally the thinner and narrower blade will appear to be sharper and will cut more easily. Note that at this point no consideration has been given to the control of the direction of the cut, hence the blade’s accuracy or its tendency for any skiving is due to the blade’s potential for increased flexibility. However, the blade may seem to be more difficult to control when trying to achieve a flat cut and the saw blade flexibility has caused a nonuniform flat cut due to the arching of the blade. Consideration of completing the saw cut and then returning to the cut with a heavier less flexible blade as described below and creating a new edge for the bone cut under direct visualization can solve this problem.
The last size characteristic to analyze is blade length. One can regard either the size of the bone front or the issue of moment arm to see that although a long blade may reach farther, it is less powerful in its cutting role and has a wider excursion at the cutting end. Because of the fixed arc of oscillation, the longer blade traverses a greater linear distance of arc and therefore can encounter a proportionately larger area of bone front. The resistance to oscillation applied at the saw blade edge has a proportionately greater moment because of the longer moment arm of the longer blade itself. Thus sclerotic bone, due to its increased hardness, can at times be more effectively cut using a thinner, shorter blade.
One can expect the sharpest, most effective cutting of hard bone while using short, thin, and narrow blades. However a shorter and narrow blade does not create the smoothest and most accurate cut and can affect the accuracy of the bone cut.
Several variables regarding bone density, direction of cut, and pressure of cut may be altered to improve the efficiency of cutting.
An overall factor to be regarded is cutting pressure, that is, how hard one leans on or pushes the saw into the bone. Developing the sensitivity to manual feedback with the saw will allow the surgeon to continually adjust both pressure and direction. Tactile sensitivity is necessary when one is making blind cuts, such as when cutting the lateral condylar surface of the tibia or the posterior condyles of the femur. The most obvious sense of ineffective bone cutting is imparted as greater saw pressure leads to a greater sense of vibration in the handpiece with a decreased advancement forward of the saw; this can bind the blade against the bone. Excessive vibration in the handpiece means that the saw blade is not moving fully at the cutting, bone front, and the oscillation of the motor is then being transferred to the handpiece rather than to the saw tip. This is generally accompanied by a change in the sound of the saw similar to when a reamer is in contact with cortical bone when placed in a tight intramedullary shaft. The opposite extreme occurs when the saw is working at full oscillatory rate and inadequate vibration is felt in the handpiece as minimal; this may indicate the presence of the saw tip within soft tissue. The sense of pushing the blade forward and “feeling” the bone is also important from the standpoint of protecting surrounding soft tissues and producing complete cuts. Feeling the bone refers to the practice of using the end of the moving saw blade as a gauge to determine that it is still against bone, as opposed to being outside the bony margin or having cut through the bone so that the blade is working against soft tissue with the potential for soft tissue damage. The sense is analogous to feeling the distal cortex of diaphyseal bone when drilling holes for fixation screws. One drills through the near cortex and then carefully advances the drill deeper, varying the forward pressure until the far cortex is palpated. Then one responds quickly to the giving-way sensation, anticipating it and guarding against it as the drill goes through the distal cortex.
This sense with a bone saw is similar to what one encounters in removing a plaster cast. An oscillating saw is used to cut completely through the cast. With experience, care is taken to “feel” when the cut is through the cast to avoid plunging the saw blade unnecessarily deep and, at the same time, to be certain that the cast is cut entirely.
The last aspect of bone-sawing efficiency to be considered is what can be thought of as the “whittle” principle. In wood carving as a wider, relatively flat surface develops, the carver finds themselves trying to cut a lot larger front of wood, and it becomes difficult, perhaps impossible, to make headway. Cutting becomes more effective when the board is approached at an angle approximating 45 degrees; this allows the entire energy of the sawing effort to be directed toward a relatively smaller dimension of wood. Even as the cut is deepened, a much smaller surface is attacked than would be encountered if the saw blade was laid flat against the longer side of the board. In surgery entrapping the sawblade will decrease the oscillating movement of the saw and will not allow movement of bone debris away from the cutting teeth. This can be avoided with the whittling technique. This front becomes more formidable as it gets wider and/or the bone is harder. One can use an awareness of these factors as a more effective means to facilitate the task of cutting hard bone. Fig. 8.3 indicates how one can use this change in direction to diminish the size of the bone front and increase the effectiveness of the cutting. The directional change should not be made in a haphazard way but with a specific eye toward achieving the whittle effect.