Design

Design features of surgical instruments are important in determining how they work, and when they don't work, give clues as to how they may be fixed.

Teeth enable the surgeon to hold firmly onto what has been grasped. (important when manipulating tissue that is slippery). Interlocking teeth provide instruments with the ability to "remember" and stay where and how they were placed. Interlocking teeth are incorporated into the design of hemostats as well as into the cogwheel mechanism on self-retaining retractors.

FIGURE Interlocking teeth

Some instruments freeze movements until the surgeon decides to undo them (Weitlaner). When incorporated into a wheel mechanism interlocking teeth hold apart lever arms. By using interlocking teeth oriented in such a way that they are apposed and separated by completely different movements of the hand.

Edges, the design feature underlying scissors (Mayo, metzenbaum, cut and compress tissue along straight line. When the sharp blades are approximated, the scissors cut. Lever arms increase the force applied by arms or blades across a fulcrum. Pincers on hinged arms allow precise application of a grasping force at a site distant from the hinge to which they are connected by arms. Stepping the distal with respect to the proximal portion of an instrument allows the surgeon to work within a hole without losing sight of the working end behind his hand. The Malis bipolar and bayonet forceps incorporate this design feature.

One way to catalogue surgical instruments is according to their use during head trauma operations (Table: Instruments and indications). Soft tissue instruments include those for division, separation, retraction, manipulation, and approximation. Bone is perforated, cut, shaped, and fixed by instruments specifically designed to interact with this tissue.

Power

Power is an essential element in the modern surgical workspace. Gas pressure gradients provided from outlets connected to centrally located pumps (suction) or cannisters (pressure) provide forces for suction of fluid from the operative field and for turning of rotary motors in perforators and craniotomes. Electrical current is converted to the torque that turns blades and drill bits in some motorized instruments, and is the source of the heat between bipolar, or emanating from the monopolar, electrocautery tips.

Field

Surgical instruments that organize instruments on the field and position the patient within it include the operating table and its attachments, sterile drapes, and a variety of instrument-holding devices that can be fixed to them.

Patient

The spatial geometry of the surgical field is determined by the position of the patient with respect to the floor, the surface of the OR table, and to the surgeons. Positioning is the art of arranging the patient's body and its appendages in three-dimensional space to afford optimal surgical visualization and access and the OR table with its attachments, allows for all of the positioning variations necessary to perform the full range of head trauma craniotomies. With the exception of ICU craniostomy and ventriculostomy (discussed in a previous chapter), head trauma surgery should be done on tables, not in beds.

OR table and attachments

        Table tops are rigid and flat. By raising or lowering the head and foot panels of the OR table manipulation of certain physiologic parameters is possible: venous pressure in the dural sinuses is deceased with concomitant reduction in ICP by elevation of the patient's head with respect to his heart. Elevation of the head at 30 degrees is best for the management of intracranial pressure and head trauma surgery should be done on a table with a head panel that raises the head panel to this angle. Tables are so heavy that to them can be securely attached a variety of attachments which support the patient in virtually any conceivable surgical position and can be used to restrain and contain patient movement.

The segmental design of the OR table has important implications for patient positioning. It enables the surgeon to drop the patient's feet below the level of the heart which optimizes venous drainage. Instruments for patient positioning should allow the surgeon to optimize physiology by elevation of the torso and lower extremities with respect to the heart. In general it is desirable to have the dependent lower extremities lower than the heart. Positioning also requires securing the patient in space for safety and to prevent motion of objects in the operative field. If necessary the patient's head can be elevated or rotated from side to side.

The OR table can be rotated to position the surgical field and patient in any orientation with respect to the doors and large equipment in the room. It can be securely looked and will resisted several hundred pounds of laterally exerted force.

FIGURE: OR table

Positioning

Instrument

An important principle of optimizing access to instruments is minimizing clutter by selecting only those instruments which are necessarily on the field. All others should be on a stand or table nearby.

The sterile field is created by a series of drapes that provide the backdrop for positioning of instruments within reach of the patient. Usually several layers of drapes are applied. An added convenience is when the drapes have an attached drainage apron for collection of irrigant solution and blood that overflow the wound.

Instruments that hold instruments on the drapes include a variety of clips that can be strategically placed to minimize obstruction of the surgeon's movements and also of his need to reach back and forth across the wound.

Manipulation

The field itself must be large enough and oriented such that the surgeon can work within it. The planes of activity are defined by tables and stands that hold instruments at different heights with respect to the floor, the patient, and the surgeon.

Visualization of objects and of the consequences of their manipulation within the surgical field require adequate illumination, irrigation, and suction. Operating room lights illuminate, irrigation fluid washes, and accompanying suction aspirates, to maintain the surgeon's visual access to the surgical field filling persistently throughout the course of surgery with smoke, blood, and tissue detritus.

While the patient position and overall layout of the field remains relatively stationary throughout surgery, the surgeon's view of it varies continuously. Objects are viewed from many different angles and illumination must be comprehensive enough that the surgeon can accurately manipulate objects from many different approaches.

Illumination

The ambient room lighting provide diffuse illumination in the environment surrounding the surgical field in which the non-operative members of the surgical field move about throughout the procedure. This lighting must permit ready identification, access, and avoidance as necessary of objects large and small wherever in the room.

    a) Illumination

        In addition to room ambient lighting, OR lights provide illumination to the operative field. They are broader than standard room lights with special reflecting surfaces that focus intense illumination onto a limited field. They may be attached to the ceiling or mobile, but in whatever location the directions of their beams are adjustable.

Because troubleshooting and repair of a failing OR light requires expertise beyond that of the members of the surgical team at least two should be available for every procedure.

Headlights focus light of intensity even greater than that of OR lights into a consequently brighter but more limited field.

FIGURE: Headlight

Headlights not only enable visualization of the depths of intracerebral hematoma evacuation cavities that overhead OR lights cannot reach, but help as well under the bone edges of craniotomies (that may have left unexposed portions of a traumatic extra-axial hematoma.

Complete evacuation of an inadequately exposed lesion may require looking under edges of bone to identify and aspirate hidden residual clot. Any instrument manipulation under bone without headlight-assisted visualization of the cortical surface is done risking damage to pial and deeper vessels and nervous tissue.

FIGURE: Looking under bone edge with headlight

The major disadvantages of headlights is that they tether the surgeon to the light source box and are inconvenient protrusions from the surgeon's forehead that can bump against other surgeon headlights, sterile instruments, or the operative field.

When the aperture of the light source is optimized regulating diameter of light beam, the the headlight should illuminate most of the central portion of the surgeon's visual field. The direction of the beam should be adjusted to focus 1 to 1 ? feet from the surgeon's gaze (where with his face and eyes forward the axes of his central gaze converge).

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