Biomechanical evaluation of different types of rigid internal fixation techniques for subcondylar fractures.

PURPOSE: The aim of this study was to evaluate the biomechanical stability of various internal fixation systems for subcondylar fractures. MATERIALS AND METHODS: Eighteen identical synthetic mandibles were used. Left condylar processes were cut to mimic perpendicular subcondylar fracture and right sides were mimicked oblique subcondylar fracture. The fixation systems used included single 4-hole mini adaptation plate, double fixation with the same plates, single 4-hole mini dynamic compression plate (DCP), Eckelt lag screw system, Wurzburg lag screw plate system and double 4-hole biodegradable miniplates made of poly L-lactide (PLLA). In oblique fractures, one of the screws fixing plates was used bicortically through bone fragments. The loading vector simulated physiologic forces to the condyle on biting with servohydraulic testing machine until failure was reached. Load-displacement curve, maximum load for failure, and stiffness were measured. RESULTS: In perpendicular fracture, double adaptation plate showed the highest level of tolerance load followed by Eckelt lag and double PLLA plate. In stiffness, double adaptation plate and Eckelt lag screw showed higher level of stiffness, whereas double PLLA was almost at the same level of single DCP. In oblique fracture, double adaptation plate showed the highest strength. CONCLUSIONS: In this laboratory setting, double adaptation plates fixation proved to have superior biomechanical stability in both fracture conditions. Eckelt lag screw showed good stability in the perpendicular fracture, however, it was weak in the oblique fracture.     

Treatment of mandibular-condylar fractures

Particularly with true dislocation fractures, nonoperative treatment with maxillomandibular fixation followed by physiotherapeutic exercises leads to poor results, as was proved with axiography and clinical examinations. The main reason for this is the shortening and scarring of the condyloid process and the lack of function of the lateral pterygoid muscle. The condyle with its insertion of the muscle is usually displaced medially and anterially and nearly in touch with the origin on the pterygoid process so that protrusion by the muscle is no longer possible. The physiologic relationship of the lateral pterygoid muscle is restored after reduction of the condyle and osteosynthesis of the condylar neck fracture and the original distance between origin and insertion of the muscle is re-established and is a fundamental necessity for regaining function (Fig. 40). The anchor screw osteosynthesis is a most effective technique with low limitations for its indication. A comparison with plates shows this technique to be very economic because one anchor screw has the effect of at least one five-hole plate with five plating screws. That means a reduction of osteosynthesis implants of up to 80%, which saves a lot of money. On the other hand, the sophisticated technique of an anchor screw osteosynthesis needs some training on the part of the surgeon to get the best results possible. In general, we could realize that the anchor screw osteosynthesis gives a perfect adaptation of the fracture ends with compression also on the inner cortical layer, which with plates is only possible in rare cases. After an osteosynthesis of mandibular condyle neck fractures with an axial anchor-screw there are a few cases with an absorptive process in the fracture interface where the screw migrates in an axial direction with loosening of the osteosynthesis. This effect can be compared with the effect of a dynamic hip screw, which leads to compression of the callus, which speeds up bony union at the expense of shortening the bone. When the same absorption happens using a plate, the fracture ends cannot become sintered and the plate is in danger of fracturing as a result of metal fatigue. Ceipek evaluated 136 patients with mandibular condylar neck fractures treated with axial anchor screw osteosynthesis. Thirty-six of these screws showed signs of migration, but only 3.7% for more than 4 mm. For the migration process there are some important risk factors: difficult repositioning of the proximal fragment, dorsal luxation fracture, indirect method of anchor screw osteosynthesis, narrow condyle neck, no intercuspation in the molar region, no compliance, and disturbance of bone healing. Another stable technique of osteosynthesis should be used if patients show more risk than one risk factor.     

Comparative evaluation of ten different condylar base fracture osteosynthesis techniques

The aim of this study on the mandibles of minipigs was to compare the biomechanical stability of different methods of osteosynthesis that are used in the operative treatment of fractures of the base of the condyle. Ten different systems of osteosynthesis were used to fix 164 fractures, which were tested by a two-point bending test after repositioning and fixing. This stress test was applied in four directions: lateral to medial, anterior to distal, distal to anterior, and medial to lateral. The Eckelt lag screw, one or two 2.0 mm miniplates, one miniplate with bar (KLS Martin), minicompression plates (Medicon), zygoma compression plates (Medartis), condylus fracture plates (Medartis), square 4-hole plates (KLS Martin), and either one or two resorbable 4-hole miniplates (Resorb-X, Martin) were used for osteosynthesis. A total of 164 tests were done using a universal test machine that measured forces until the osteosynthesis failed. Advantages in mechanical load capacity were also measured for the Eckelt lag screw when force was applied from medial to lateral. Fixation with one resorbable miniplate was not functionally stable. Irrespective of the direction of force applied, two miniplates were the most stable technique. There were pronounced differences depending on the direction of force applied. The results suggest that treatment with a single resorbable miniplate is not functionally stable.     

Development and biomechanical testing of a new osteosynthesis plate (TCP) designed to stabilize mandibular condyle fractures.

INTRODUCTION: In this paper, the different steps of development and experimental validation of a new type of three-dimensional (3-D) trapezoidal osteosynthesis plate (Modus TCP 2.0, Medartis, Basel, Switzerland) is described. These plates have been designed to stabilize sub-condylar and condylar neck fractures of the mandible. MATERIAL AND METHODS: In order to apply the principles of functionally stable osteosynthesis to the mandibular condyle, i.e. to put the plate as close as possible to the tensile strain lines occurring during function, two new 4- and 9-hole 3-D trapezoidal plates were designed. Tests were conducted on fresh human mandibles before and after osteosynthesis of a standardised unilateral sub-condylar 'fracture', and a static biting exercise between the ipsilateral first molars was reproduced on a test bench. The resulting condylar fragment displacement in the sagittal plane was measured and the alterations of the condylar tensile strain lines induced by the osteosynthesis were investigated by using photoelastic strain tests. RESULTS: None of the plates broke. No macroscopic condylar displacement was noted when assessing the quality of the primary stabilization. Strain analysis showed the ability of these 3-D plates to transmit physiological strains across the fracture line and the absence of potentially damaging strains around the plate. DISCUSSION: These results were accredited to the 3-D and trapezoidal features of the plates. CONCLUSION: The Modus TCP plates experimentally fulfil the principles of functionally stable osteosynthesis in the condylar region and are able to resist physiological strains.     

Plate osteosynthesis of condylar fractures: a retrospective study of 45 patients.

OBJECTIVE: This retrospective study presents the type of osteosynthesis used for the fixation of condylar fractures and the postoperative results and complications observed. METHOD AND MATERIALS: Forty-five patients with fractures of the mandibular condyle underwent open reduction and osteosynthesis with plates and screws. The surgical approach was, in most cases, via a submandibular incision. Stabilization was achieved in the majority of the cases with a 2.0-mm single or double miniplate, but 2.0-mm mini dynamic compression plates were also used. RESULTS: The complications were mainly inadequate reduction, screw loosening, and limitation of mouth opening. No plate fractures or infections were observed. CONCLUSION: The use of a single plate (miniplate or dynamic compression plate) often produces inadequate stability and reduction postoperatively. The placement of two 2.0-mm zygomatic dynamic compression plates demands great tissue dissection and detachment and can lead to limitation of mouth opening. The use of two 2.0-mm miniplates seems to produce better stability and fewer complications.     

皮质骨螺钉内固定技术在颏部骨折中的应用

目的：对颏部骨折应用皮质骨螺钉内固定的效果进行评价。方法：对12例颏部单线骨折采用皮质骨螺钉内固定术进行治疗，术后观察3-6个月。结果：12例患者中，10例应用1枚螺钉同定，2例应用2枚螺钉固定。术后伤151均一期愈合，咬合关系良好，张口运动正常。结论：颏部骨折应用皮质骨螺钉固定，可达到较为理想的加压和稳定效果。皮质骨螺钉技术简单，操作方便，术中避免了弯制接骨板以适应复杂的下颌骨外形的步骤，缩短了手术时间，并可在骨折线上产生压应力，促进骨折愈合。     

Compression osteosynthesis.

Nearly all types of mandibular fractures have been treated successfully by compression osteosynthesis. Special instruments are required, the bone must be treated correctly, the fragments must be reduced exactly, and the plate must be adapted exactly. Basically, only lag screws are used when there is a lamellar fracture and when a large surface permits support; the dynamic compression plate with oblique lateral screw holes is indicated when the fracture runs transversely. The dynamic compression plate with oblique lateral holes and the additional inlay portion is indicated in oblique fractures and when the alveolar process is weak.     

Axial 'anchor' screw (lag screw with biconcave washer) or 'slanted-screw' plate for osteosynthesis of fractures of the mandibular condylar process.

Almost a quarter of all mandibular fractures are located in the condylar neck region and generally lead to disturbances of occlusion. Because they still possess active growth centres, children and adolescents can take advantage of the joint's remodelling capacity following conservative treatment of these fractures. Fractures with displacement of the condylar head in adults can interfere with function if they are not surgically reduced (Krenkel and Strobl, 1989). In addition, a compensatory overloading of the non-fractured side, which originally goes unnoticed, can bring about disc pathology and chronic pain years later. For this reason, a surgical technique was developed for the management of mandibular condylar neck fractures. A new axial/oblique-axial lag screw (anchor screw) with biconcave washers (anchor washer) makes it possible to carry out standardized osteosynthesis in the region of the thin mandibular condyle neck. Functional exercises can be initiated immediately after the operation. There are three operative procedures using an extraoral approach (1-3) and two using an intraoral approach (4-5), depending on the type and severity of the fracture: 1. Direct anchor screw osteosynthesis with closed gliding hole. 2. Indirect anchor screw osteosynthesis with open gliding groove and safety plates. 3. Osteosynthesis with a 'slanted-screw' plate for longer oblique fractures. 4. Intraoral anchor screw method. 5. Intraoral anchor screw method with intraoral 'slanted-screw' plate. The functional long-term results of conservatively and surgically treated mandibular condylar neck fractures were objectively documented by means of mechanical and electronic axiography. The functional long-term results of the condylar neck fractures treated surgically were significantly better than those treated conservatively.(ABSTRACT TRUNCATED AT 250 WORDS)     

An experimental study of the biomechanical stability of ultrasound-activated pinned (SonicWeld Rx+Resorb-X) and screwed fixed (Resorb-X) resorbable materials for osteosynthesis in the treatment of simulated craniosynostosis in sheep

We compared a conventional resorbable screw osteosynthesis with a resorbable, ultrasound-activated pin osteosynthesis, and studied mechanical load capacity and operative handling. This new form of osteosynthesis aims to reduce operation times, and to avoid torque loads and screw fractures to achieve stability. A sheep craniotomy model simulated an operation for dysmorphia on an infant skull. Two rectangular craniotomies of equal size were created in 13 lamb skulls, and each refixed by different means: the first by mesh and 20 screws, and the second by mesh with 20 pins inserted with ultrasound activation. All osteosynthesis material consisted of resorbable amorphous poly-(d,l)-lactide (PDLLA) (Resorb-X, KLS Martin, Tuttlingen, Germany). The insertion time was recorded. The animals were killed at different times, and areas of the healing skull including the plates and pins or screws were removed and divided into sections, which were then tested. In total 74 pin-fixed and 77 screw-fixed samples were obtained. Bending and tensile tests were used to simulate different forms of loading. The time required for the insertion of pins was significantly shorter than for screws. The mechanical tests showed differences in the stability of the bond between the osteosynthesis plate and bone that depended on the osteosynthesis system and the length of time it was in the animal. The pin osteosynthesis gave a stable mechanical load capacity, which was significantly different from that of screw osteosynthesis. Advantages of ultrasound-assisted, resorbable, pin osteosynthesis, include optimum operative handling, reduced insertion time, avoidance of fractures of the fixation elements and higher three-dimensional load capacity.     

Clinical and radiological evaluation following surgical treatment of condylar neck fractures with lag screws.

It was the purpose of this study to evaluate the outcome of lag screw osteosynthesis in severely displaced fractures or fracture dislocations of the mandibular condyle as well as intra- and postoperative complications of this technique. From 1980 to 1996 a total of 492 patients with condylar fractures were treated with lag screw osteosynthesis. Clinical and radiological follow-ups were carried out in 230 patients with severely displaced fractures or fracture dislocations of the mandibular condyle. The period between surgery and follow-up was between 6 months and 2 years. The majority of the clinical results were satisfactory presumably due to the anatomically exact reduction of the fragment (93.4%). Extreme anatomic variations of the mandible (5.8%) and incorrect application of the technique (8.8%) resulted in reduced functional stability of lag screw osteosynthesis. Insufficient fragment reduction and postoperative complications (such as wound infection) entailed restriction of mandibular mobility and arthrotic deformations were seen as persistent radiological irregularities of the mandible. It has been possible to avoid complications and to achieve favourable functional results in complicated fractures of the mandibular condyle by applying strict indications for lag screw osteosynthesis and by considering the individual anatomical peculiarities.     

Fixation of mandibular angle fractures: in vitro biomechanical assessments and computer-based studies

Purpose

The purpose of this study was to review the literature regarding the evolution of current thoughts on fixation of mandibular angle fractures (MAFs), based on in vitro biomechanical assessments and computer-based studies.

Methods

An electronic search in PubMed was undertaken in August 2012. The titles and abstracts from these results were read to identify studies within the selection criteria. Eligibility criteria included studies from the last 30 years (from 1983 onwards).

Results

The search strategy initially identified 767 studies. Thirty-one studies were identified without repetition within the selection criteria. Two articles showing significance in the development of treatment techniques was included. Additional hand searching yielded five additional papers. Thus, a total of 38 studies were included.

Conclusions

The osteosynthesis positions as well as the plating technique play important roles in the stability of MAF repair. The only in vitro study evaluating the use of wire osteosynthesis concluded that wires placed through the lower border approach would provide greater stability than those at the upper border. Many studies indicate that the use of two miniplates avoids (or decreases) lateral displacement of the lower mandibular border and opening of the inferior fracture gap. Some studies even suggest that the use of two miniplates may be considered a more “rigid” fixation technique for MAFs than the use of a reconstruction plate. When using two miniplates, the biplanar plate orientation provides greater biomechanical stability than the monoplanar one. However, despite its greater biomechanical stability, the two-miniplate technique has some disadvantages that should also be taken into account. Studies with biodegradable plates suggest the use of at least two plates for each MAF. There are few studies with compression plates, and they have not yet reached a consensus. The solitary lag screw proved to withstand the functional loading of the mandible; however, only few biomechanical assessments were performed. In vitro studies have shown good biomechanical stability with the use of 3-D grid plates. The use of malleable miniplates alone is not sufficient to withstand the early postoperative bite force. Some studies suggest that the segment of the tension band miniplate located at the distal fragment of the MAF should be fixed with three screws. The studies also showed some limitations. None considered the stabilization of the fracture site afforded by the masseter–pterygoid muscle pouch. Most of the studies did not evaluate plating system strength in the long term and therefore did not observe the effect of resorption on the strength of the different biodegradable plating systems. Another limitation of many studies is the absence of a control group. A confounding factor that could not be tested in in vitro investigations is the additional resistance to displacement of jagged fracture margins present in the human fracture.     

The development of plate osteosynthesis for the treatment of fractures of the mandibular body - a literature review.

PURPOSE: Today plate and screw osteosynthesis of mandibular fractures is a standard procedure in routine clinical practice. In this review, the breakthroughs and drawbacks of the development of this important aspect of maxillofacial surgery are followed-up. METHODS: Medline search of relevant English and German literature. RESULTS: In 1886, Carl Hansmann was the first who applied steel screws and plates. Until today the material, the types of plates and applications have been continually improved. Over the last two decades miniplate osteosynthesis has induced a revolution in mandibular fracture treatment. The modern systems provide better handling, higher stability and less pressure on the bone. CONCLUSION: Modern miniplates have great advantages, like the intra-oral approach and the easy adaptability. In addition, it is no longer necessary to expose bone as extensively.     

Minimally invasive fracture treatment with cannulated lag screws in intracapsular fractures of the condyle.

PURPOSE: We examined the use of cannulated lag screw osteosynthesis for the treatment of fractures of the mandibular condylar head in providing a high-quality durable fixation, while at the same time reducing the trauma necessary for an open approach to the fracture site. PATIENTS AND METHODS: A preauricular approach was used for exposure, reduction, and osteosynthesis in 5 cases of type B condylar fractures. A cannulated screw system was used that allowed optimum placement of the self-cutting cannulated lag screw following insertion of a guiding wire and using clinical control to ensure its correct position. The joints were submitted to functional exercises immediately following surgery and postoperative radiologic, axiographic, and clinical follow-ups were performed. RESULTS: Radiologic follow-up revealed correct reduction and fixation in all 5 cases. Axiographic and clinical follow-up showed an initial limitation, but normal mobility of the condyles was achieved within 3 months postoperatively, with a maximum mouth opening of 41.2 +/- 9.4 mm after 6 months. There were no occlusal disturbances, no trismus, no lateral deviations of the mandible, and no nerve lesions. Intraoperatively, the method applied shortened the time necessary for and simplified the procedure of reduction and osteosynthesis. CONCLUSION: By using a cannulated lag screw, it was shown that the major factor in the extent of the trauma relating to surgical access was the reduction of the fracture fragments. The method ensured stable fixation of the fracture with a minimum of osteosynthesis material, while reducing the operative time. In combination with intraoperative imaging techniques it can also successfully be applied to other fractures in maxillofacial surgery.     

Mechanical aspects of a multidirectional, angular stable osteosynthesis system and comparison with four conventional systems.

BACKGROUND: Following open reduction, internal fixation of fractures of the mandible is predominantly achieved using plates and screws. Today, a multitude of osteosynthesis systems are available on the market. One therapy modality, primarily developed for orthopaedic surgery, is using angular stable osteosynthesis plate systems. The dominating principle of these is the bond between screw and plate following insertion. This principle of an "internal fixateur" results in a more stable fixation of the fragments associated with less compression of the bone surfaces. MATERIAL: A new multidirectional osteosynthesis system (TiFix=Smartlock, Hamburg - Germany) was modified to fit the maxillofacial region and compared with four other well established osteosynthesis systems developed by Mondial, Medicon, Synthes, Leibinger-Stryker, one of these (Unilock by Synthes) being also angular stable. The resistance to deformation in varying directions was investigated following fixation in four different materials. RESULTS: The TiFix system proved more resistant to deformation even when mounted with fewer screws than the non-angular stable systems. CONCLUSION: This system results in greater stability even when fewer screws are used. For the clinician this means smaller access incisions, less soft tissue trauma, better aesthetic results, decreased duration of operation and a reduction of costs.     

Resorbable fixation techniques for genioplasty.

PURPOSE: This study evaluated the capability and effectiveness of resorbable bone fixation devices in genioplasty surgery. MATERIALS AND METHODS: Twenty patients underwent different genial movements that were stabilized with either 2.5-mm polylactic-polyglycolic acid lag screws or 2.0-mm polylactic-polyglycolic acid plates and screws. RESULTS: Twenty-one anterior mandibular osteotomies were performed in 20 patients. Sixteen patients had advancement (80%), 2 had horizontal setback (10%), and 2 had vertical reduction (10%). The average advancement was 7.6 mm (range, 4 to 14 mm), the average horizontal setback was 6.0 mm (range, 4 to 8 mm), and the average vertical reduction was 7.0 mm (range, 5 to 9 mm). Fixation was done using the lag screw technique in 13 patients (65%) and plate and screw fixation in 7 patients. (35%) Intraoperative stability was satisfactory in all cases. There were no postoperative infections or segmental instability up to 6 months after surgery. CONCLUSION: Resorbable polylactic-polyglycolic acid lag screw and plate and screw fixation is a viable alternative for fixation of anterior horizontal osteotomies of the mandible.     

Lag screw osteosynthesis of fractures of the mandibular condyle: potential benefit of preoperative planning using multiplanar CT reconstruction

OBJECTIVE: The purpose of the study was to develop a method of multiplanar CT reconstruction in fractures of the mandibular condyle in order to evaluate the feasibility of lag screw osteosynthesis. STUDY DESIGN: Angulated axial and sagittal images of the mandibular ramus were calculated using collected CT data. In 183 mandibular rami the optimal position of the screw axis was determined retrospectively. The thickness of the covering bone as well as the degree of screw exposure were measured. RESULTS: Application of the method led to medial (28.4% of cases) or lateral (3.8% of cases) correction of the usual screw axis. In 1.6% of cases an extraosseous position of screws could not be avoided by this correction. CONCLUSIONS: Evaluation of the feasibility of lag screw osteosynthesis by this method is possible. Medial or lateral change of the axis ensures the optimal intraosseous screw position. Intraosseous screw placement is almost always possible.     

In vitro mechanical test of grid plates for mandibular angle fractures

This study evaluated, by mechanical testing, the strength of four-hole grid plates used for mandibular angle fracture fixation. Much has been discussed about the treatment of mandibular fractures with straight or curved plates. However, there are few studies about the use of grid plates. The geometry of such plates provides three-dimensional stability, resistance against torque associated with a low profile. Ninety polyurethane hemimandibles were used as substratum. The hemimandibles were divided into nine groups and fixation of the fractures was performed varying the type and position of plate and type of screw. An intact hemimandible group was used as control. It was possible to observe that plates positioned at the tension zone present the highest load values, both for dislocations of 3 and 5 mm. There was no statistical difference when plates with or without an intermediate bar were tested. The locking screws were more efficient than the non-locking screws when the plates were positioned at the neutral zone. Results allowed us to conclude that grid plates installed at the tension zone presented greater mechanical efficiency. If grid plates are positioned at the neutral zone, they should have an intermediate bar and be fixed with locking screws.     

Biomechanical testing of different osteosynthesis systems for segmental resection of the mandible.

PURPOSE: This investigation assessed the mechanical behavior of 3 different locking and nonlocking reconstruction systems-Unilock 2.4, Reconstruction 2.4, and Reconstruction 2.7-with regard to plate and screw fracture. MATERIALS AND METHODS: Five different plate/screw configurations (Unilock 2.4-locking screws, Unilock 2.4 -conventional screws, Reconstruction 2.4-conventional screws, Reconstruction 2.7-conventional screws, and Unilock 2.4-locking screws with a 1-mm gap; Synthes, Umkirch, Germany) were tested on synthetic mandibles. All mandibles were resected on the left side between the canine and third molar, reconstructed, and loaded cyclically between 30 and 300 N up to 250,000 cycles or until screw or plate failure occurred. RESULTS: No screw fractures were observed. All plates fractured close to the distal fragment. The Unilock plates fixed with locking screws withstood significantly more cycles until failure than the Reconstruction plates 2.4 fixed with conventional MF-Cortex screws. No significant differences were found in the other groups. Only 2 of the 34 plates tested, both of the Reconstruction 2.7 system, reached the runout limit. CONCLUSIONS: Unilock plates fixed with locking screws have a higher long-term stability than the Reconstruction 2.4 system. A 1-mm gap between the plate and mandible does not lead to early screw failure in the Unilock 2.4 system with locking screws. The Reconstruction 2.7 system seems superior if well contoured, because 2 of those plates reached the runout limit; however, this system is not as easy to handle as the 2.4 systems, and good contouring is difficult to achieve. Therefore, we consider the Unilock 2.4 system with locking screws the best choice.     

Axial ‘anchor’ screw (lag screw with biconcave washer) or ‘slanted-screw’ plate for osteosynthesis of fractures of the mandibular condylar process

Almost a quarter of all mandibular fractures are located in the condylar neck region and generally lead to disturbances of occlusion. Because they still possess active growth centres, children and adolescents can take advantage of the joint's remodelling capacity following conservative treatment of these fractures. Fractures with displacement of the condylar head in adults can interfere with function if they are not surgically reduced (Krenkel and Strobl, 1989). In addition, a compensatory overloading of the non-fractured side, which originally goes unnoticed, can bring about disc pathology and chronic pain years later.For this reason, a surgical technique was developed for the management of mandibular condylar neck fractures. A new axial/oblique-axial lag screw (anchor screw) with biconcave washers (anchor washer) makes it possible to carry out standardized osteosynthesis in the region of the thin mandibular condyle neck. Functional exercises can be initiated immediately after the operation.There are three operative procedures using an extraoral approach (1–3) and two using an intraoral approach (4–5), depending on the type and severity of the fracture:

1. Direct anchor screw osteosynthesis with closed gliding hole.

2. Indirect anchor screw osteosynthesis with open gliding groove and safety plates.

3. Osteosynthesis with a ‘slanted-screw’ plate for longer oblique fractures.

4. Intraoral anchor screw method.

5. Intraoral anchor screw method with intraoral ‘slanted-screw’ plate.

The functional long-term results of conservatively and surgically treated mandibular condylar neck fractures were objectively documented by means of mechanical and electronic axiography. The functional long-term results of the condylar neck fractures treated surgically were significantly better than those treated conservatively. The compensatory overload on the non-fractured side in the conservatively treated group, confirmed by axiography, predisposes this joint to disc pathology and secondary joint damage due to overstretching of the capsular ligaments. Therefore, osteosynthesis of condylar neck fractures not only serves to restore anatomical conditions, but also acts as a preventive measure against overloading the non-fractured side.     

Self-adapting washer system for lag screw fixation of mandibular fractures. Part II: In vitro mechanical characterization of 2.3 and 2.7 mm lag screw prototypes and in vivo removal torque after healing.

The aim of this study was to mechanically characterize self-tapping 2.3 and 2.7 mm titanium lag screw prototypes which are part of the newly developed 'self-adapting washer' maxillofacial lag screw osteosynthesis system. In vitro in a screw testing machine the insertion torque, maximum locking torque and axial force and the ultimate torsional strength were assessed. In vivo in six miniature pigs using a mandibular symphyseal fracture model, the removal torques after 3 and 6 months of healing were measured. Additionally the bone-metal contact (BMC) of the screws was assessed histometrically. The maximum insertion torque (0.185 Nm) was far below the mechanical limits of the screws (2.3 mm = 0.96 Nm, 2.7 mm = 1.6 Nm). A tightening of the 2.7 mm screw with an axial force of 1000 N and of the 2.3 mm screw with 500-550 N leaves a safety margin of approximately one-third on the ultimate torsional strength. Clinically these values permit the use of two 2.3 mm lag screws or one 2.7 mm lag screw in mandibular symphysis fractures since 1000 N tensile axial force are required in this indication. During screw removal after 6 months healing, torque levels close to the mechanical limits of the screws were recorded and screw failures were observed. This failure rate may have been due to the BMC of 49.8% which was in the range of titanium dental implants. Accordingly the screw heads were reinforced to prevent fractures.