Discussion

A common complication of implant prosthetics is with components. The abutment screw connects the abutment to the fixture and can fracture under occlusal functional load.

In preparation for treatment of a patient presenting with an abutment screw fracture, a radiograph should be taken to ensure osseous support quality of the supporting implant. The fixture screw hole should be flushed with a dilute solution of sodium hypochlorite (0.5%) and rinsed with water to ensure removal of any organic or particulate debris that may inhibit removal of the fragment.

Fragment removal is accomplished with a festooned #557 burr in a reversed, counterclockwise, handpiece. When the coronal of the fragment is accessible, then a #33 1/3 burr in right-hand rotation (normal, clockwise) is touched against the fragment. This will rotate the fragment completely out of the fixture, and this should be held fast with a small-orifice aspirator tip. It is this author's experience that continuing the reverse turn #557 causes wobbling of the fragment that may cause it to actually turn back into the fixture. Thus, a #33 1/3 is used to complete the removal of the fragment.

The crown containing the abutment and coronal screw fragment is placed in a porcelain oven (Ney Co, Bloomfield, Conn) and slowly heated to 1000°C at 100°/minute. This rate is done to preclude any temperature gradients that could induce a crack or fracture of the porcelain. 1000°C is well below the fusion temperature of feldspathic porcelain. The oven temperature is held at 1000°C for 20 minutes to ensure that the cement is thermally destroyed; resin and resin-modified glass ionomer cements at 1000°C will turn into carbon ash products.

Rinsing the screw hole with 10:1 dilute aqueous sodium hypochlorite is an effective dissolving agent and not harmful to intact epithelium.

The crown/abutment/fragment is then removed from the oven, and the abutment is easily separated from the intaglio of the crown. Cement ash residue may cause a frictional interface between the abutment and crown intaglio that can make separation difficult; however, this can be overcome by grasping the PFM crown with cushioned forceps and the abutment hex with a mosquito forceps and pulling the pieces apart. Then, ultrasonic cleaning can dislodge any interfering debris from the crown intaglio and the abutment surface.

The metal surfaces of the crown and abutment are polished to remove surface tarnish. The new abutment screw replaces the fractured screw. The new abutment screw/abutment/crown is then seated into the fixture and evaluated for functional relationships. The interproximal contacts and occlusal scheme are evaluated. Any occlusal overloading should be relieved by selective grinding until the crown functions with relief in excursive movements. Off-axial loads are the most detrimental. The new abutment screw is torqued, and the crown can then be recemented with an appropriate luting agent. Care must be taken to ensure there is no remnant marginal cement, which may induce marginal bone loss or peri-implantitis. This may be especially problematic with resin cements.

When occlusal overload is considered, the patient's ability to generate an excessive load should be measured in the area of a proposed implant site. The range for human bite load ability is 50–1600 N. Most patients fall into the 200–400 N range. Patients who are at the upper range may overload an implant component and induce a fracture. Thus, clinical measurement of bite force capability is a parameter that can be measured and considered in treatment planning.

A patient with a diet of hard foods, such as raw vegetables, may incur high stress loads on implants. The chronic cyclic loads of a hard food diet may induce an overload of an abutment screw.

Different brands of implants have different abutment screw fracture rates. Various alloys are used among manufacturers that may affect fracture toughness. One study by Seetoh and coworkers tested three systems. There were failures of at least one component in every system: the abutment screw, the abutment, and/or the implant neck. There were significant differences noted among systems. Various abutment fixture connection designs also affect fracture resistance. The Morse taper abutment-fixture connection may provide more fracture resistance than do external and internal hex designs. Many of these hex designs also include a Morse-type taper connection.

Importantly, after an abutment screw has fractured, there needs to be an assessment of the occlusal scheme. Cyclic loading can induce stresses that cause an abutment screw fracture. An occlusal scheme that minimizes off-axial loads to the components is important in preventing fracture complications. Plastic and porcelain dentures wear after many years of service, and this may cause a pattern that increases off-axial loads. Thus, the worn occlusal scheme needs to be addressed by adjustment or repair or replacement.

While not well studied, one retainer design may produce less stress than another. Finite elemental analysis has been used to indicate occlusal stresses in systems.