Abstract
The main aim of this study was to analyze how screws affect the strain concentration induced on the mandibular condyle during implantation, screwing, and drilling, as well as after condylar loading. A clean cadaveric mandible was analyzed experimentally in the intact state and was then implanted with a Biomet/Lorenz Microfixation temporomandibular joint (TMJ) implant with seven bicortical self-tapping screws. The external surface of the mandible was instrumented with three strain gauges. A load of 500 N on the TMJ was applied to the condyle before and after implantation. The results showed a strain concentration of −1500 μɛ near the screws due to their implantation on the external surface of the mandible. The drilling process induced up to 80 μɛ near the hole. The strain concentration did not change when there were more than six screws. Loading on the TMJ before and after implantation presented only a 10% difference in maximum principal strain. This study demonstrates the importance of the strain concentration induced by the screws. The process of implanting screws shows the importance of lateral surface preparation for a good fit in the condyle. Strain distribution after implantation and loading of the Biomet implant was found to be similar to that in the intact condyle.
Various diseases affecting the temporomandibular joint (TMJ) have been described in the literature, and their prevalence is reported to be between 15% and 18%. These conditions may result in clinical effects such as pain and reduced maximum mouth opening. Since the outcomes of the first cases of TMJ reconstruction surgery in young patients (age 40.9 ± 10.3 years) were reported, these problems have gained even greater importance.
The total TMJ replacement system is indicated for arthritic conditions, such as osteoarthritis (which represents up to 48% of all replacements according to several well established national reports ), traumatic arthritis, rheumatoid arthritis, ankylosis, avascular necrosis, fracture, functional deformity, and degenerated or resorbed joints; it is also used to resolve other failure conditions (alloplastic reconstruction, autogenous grafts). The total TMJ replacement is a surgical option to be considered in patients who require mechanical improvements in this articulation.
A total TMJ replacement can significantly improve mandibular function. There are several different TMJ prostheses available on the market, both custom-made and stock models. Currently, only one stock TMJ prosthetic systems are approved by the US Food and Drug Administration (FDA; a regulatory authority): TMJ Concepts, TMJ Implants/Christensen, and the Biomet/Lorenz Microfixation TMJ Replacement System. These systems present very similar condylar geometry and function. However, results indicate that patients require revision surgery when commercial implants fail, mainly in the fossa component.
Several reports have been published in recent years on the performance of the Biomet/Lorenz Microfixation TMJ Replacement System ; however, these have presented only short-term clinical results. Improvements were found in the first 3 years and stabilization after 4 and 6 years. The main advantage is a reduction in pain, which decreases significantly only 6 months after surgery.
The TMJ implant solutions available on the market use screws to attach the condyle to the fossa components. Fixation systems have presented different numbers of screws and holes, and different displacements between screw positions and angles with respect to the ramus. In the past, some implant systems have positioned the fixation screws in the same direction (in a line), but more recently certain breakthrough designs have presented geometries in which the screws are positioned in different directions (non-oriented position). The fixation usually uses titanium alloy screws of 2.7 mm in diameter in the condyle component and of 2 mm in diameter in the fossa component. Screw fixation has been studied experimentally in sheep, but no study has been conducted (there are no published results) on a cadaveric mandible, or using the fixation positioned with a TMJ implant in place.
Some finite element studies have been conducted to simulate the behaviour of a mandible incorporating a TMJ implant ; however, the screws were considered as pins with a straight geometry so that the finite element models and the computation process could be simplified. Other experimental studies have analyzed the behaviour of the mandible, but none has analyzed the behaviour of the screws experimentally in a cadaveric mandible.
The aim of this study was to analyze how screws affect the strain concentration induced on the mandibular condyle during the different implantation steps, using a TMJ alloplastic model.
Materials and methods
A clean cadaveric mandible without teeth, from a 45-year-old woman, was provided by the Laboratoire d’Anatomie Fonctionnelle (University of Bordeaux). This was cut into two symmetrical parts. Three strain gauge rosettes (KFG-1-120-D17-11 L3M2S; Kyowa Electronic Instruments Co. Ltd, Tokyo, Japan) were glued to the external (lateral) and internal (medial) cortical surfaces of the right condyle ( Fig. 1 ). Two of the rosettes were positioned on the external surface and the other on the inner surface of the mandible (strain gauge 3). The strain gauges were glued to the condyle and positioned close to the planned TMJ implant position ( Fig. 1 ) in order to analyze the influence of the screws on the condyle. Strain gauge 2 was positioned in the centre of the implant, between screw positions. The strain gauge rosette model was used to measure the maximum and minimum principal strains. A PXI system (National Instruments, Austin, TX, USA) was used to collect and analyze the results. This strain gauge rosette model was the smallest that could be acquired on the market. Due to its dimensions (1 mm), it was not possible to glue one of these onto the implant.
The condyle was fixed in the tooth region (molars 37–38), and kept in the anatomical position ( Fig. 2 ). In the fixed system, the mandibular condyle was glued in a box using polymethylmethacrylate (PMMA) bone cement.
The maximum and minimum principal strains were calculated for each rosette position according to Eq (1) :
ε max / min = 0.5 ε a + ε c ± 2 { ( ε a − ε b ) 2 + ( ε b − ε c ) 2 }
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