Right-angled drilling and screw driving instruments are an alternative to transbuccal systems. In comparison to the transbuccal system, any transcutaneous access is avoided and the use of a rectangular or 90° drill/screwdriver system via a strictly transoral approach is considered as a less invasive procedure. The use of a drill/screwdriver in anatomical sites that are difficult to access is usually monitored by endoscope or minicamera equipment.
In principle the 90° drill/screwdriver provides allows burr screw holes and insert screws to be burred perpendicular to the bony surface without a direct route for instrumentation. The procedure diminishes the loading force along the drill bit or screw because the activation is exerted from a remote pivot point over the handle and not in line with the insertion axis.
The drill /screwdriver is a precision-built mechanical instrument analogous to an angled dentist drill that transmits the force of a longitudinal input rotation axis into a smaller output axis and which carries the tool, ie, the drill bit or a screw driver blade. The drill/ screw driver is construed of several main components:
The back of the handle has a quick coupling connection to attach an electric motor or a handgrip for manual screw insertion. The mechanics inside are continuous to the interface with the shaft.
Cont’d The shaft contains the input axis and is equipped at the outside with a screw-holding slider that prevents the screw from falling off the screw driver blade into the wound. The screwdriver head houses a small gear box linking the two rotational axes in a rectangular order. The dimensions of the head (flat, low overall height) should be minimal. This is problematic however, as screw insertion into the mandibular cortex requires high torques, which must be withstood by the gearwheels that cannot therefore be below a certain size. Long drill bits limit the maneuverability of the screwdriver head and bicortical screw insertion is commonly excluded in surgical cavities of restricted size. The end of the head has an insert slot to exchange screw drill bits and screw driving blades. For fast and comfortable surgery it is recommendable to use two drill/screwdrivers at a time to avoid the need for exchange.
There are several drill/screwdriver instruments on the market, each with particular specifications. A careful evaluation and selection is advisable.
The most commonly used screw dimensions together with 90° screwdrivers are 1.5, 2.0 and 2.4.
The screwdriver shafts and drill bits are inserted into the head of the shaft and have a click coupling. When being inserted, the distinctive click should be heard in order to safely engage the bit to the shaft. For removal of the inserts, use either the removal instrument or the insert removal bolt in the mini module.
The indications for the use of the 90° drill/screwdriver are fractures extending into the lower border of the mandibular angle and the ascending ramus including the caudal and low parts (subcondylar division) of the condylar process.
A note of caution: A bicortical screw insertion is usually difficult to administer. With increasing screw diameter and length, the procedure becomes more difficult due to cam out of the screw driver blade and high driving torques. The use of locking head screws cannot be unconditionally recommended, since the burr holes cannot be centered appropriately into the plate holes. The 90° drill/screwdriver can also be used for plate fixation of sagittal split osteotomies.
Typical application of the angled drill/screw driver for plate fixation of a condylar fragment.
6. Pitfalls and back-up strategies
Endoscopically monitored surgery and the use of 90° drills and screwdrivers require team effort and training prior to clinical application. Limited space in the upper region of the ramus and relatively high torque requirements for screw insertion in the mandible are challenges for predrilling, plating, and screw insertion in these regions. Endoscopically monitored subcondylar fracture repair can be highly complex and technically demanding. If the planned procedure cannot be successfully completed, a back-up strategy is mandatory and must be agreed on with the patient in the informed consent. Have a transbuccal set ready for back-up.
7. Pearl: direct percutaneous digital pressure
Direct percutaneous digital pressure onto the screwdriver head helps to improve axial loading but will obscure the vision into the surgical site and wear out the mechanical gear.