S. Martínez-Villalobos Castillo¹, M.A. Sicilia Gutiérrez², L.M. Capitán Cañadas²,
A. Labrot Moleon², D. Sánchez López¹, E. Valencia Laseca³

1 Médico Adjunto.
2 Médico Residente.
3 Jefe de Servicio.
Servicio de Cirugía Oral y Maxilofacial.
Hospital Universitario "Virgen de las Nieves". Granada, España.

Correspondencia:
Sergio Martínez-Villalobos Castillo
Servicio de Cirugía Oral y Maxilofacial. HRT
Hospital Universitario "Virgen de las Nieves"
Avda. Fuerzas Armadas 2. Granada 18014
smvillalobos@terra.es

Introduction

Biomechanical principles

The locking principle, also called the "internal-external fixator" is a biomechanical principle that is well-known and widely accepted because of the undoubted advantages that it has regarding the viability of certain osteosyntheses, fundamentally those requiring rigid fixation to guarantee adequate stability following the repair of serious fractures (multiple, comminuted…) or mandibular defects (traumatic or oncologic). By virtue of this principle, the required stability of the system, which consists of the implant and the bone, does not depend on intimate contact between them, that is to say on the plate adapting perfectly to the morphology bone that has been repaired. Technically this is possible because the fixations screws are anchored to the holes of the plate in such a way that the loading forces are transmitted directly from the bone to the screws and from the screw heads down the whole length of the plate, without the need for the latter to be completely fixed to the bone (Fig. 1a). Without prejudicing the efficiency of biomechanics, this is in principle a "relief" given the complexity of three-dimensional molding of conventional plates (especially if these are very long and with a high profile). Evidently this does not mean to say that the implant can be fixed in any way, as other aspects may be affected such as facial symmetry, or palpability -always undesiredcould increase.

With conventional techniques, the insertion of screws compresses the plate onto the bone, which is a necessary condition for reducing the fragments anatomically and for obtaining adequate primary stability, but this can compromise the blood supply to the external cortex (Fig. 1b). On the other hand, if the implant is not perfectly molded to the anatomy of the subjacent bone, primary loss of reduction occurs because of the traction of the screw on the bone needed for securing it to the plate. Secondary loss of reduction is also more frequent in conventional systems, as the resulting loading forces and micro-movements may lead to loosening of the screws and instability. In both cases the consequence tends to be malocclusion. Finally, with locking systems over tightening the screws is avoided during insertion, as before this can take place they are anchored into the holes of the plate. Consequently, loosening and secondary instability (with malocclusion and/or infection) is prevented, as occurs for example when a screw is inserted into a fracture site.

In short, locking systems provide greater stability while vascular supply to the bone is less impaired. Possible errors in standard osteosynthesis techniques are minimized and complication and failure rates, compared with conventional systems, are lowered.^1-7 In addition, postoperative intermaxillary fixation can be avoided, or reduced to the minimum, if there is no condyle and/or associated mid-face fracture.

"Plate-screw head" anchoring systems have evolved becoming simpler, from the original 4.0 Thorp system (in disuse) to the current 2.4 and 2.0 Unilock system (Fig. 2).

Technical characteristics

There are three types of 2.0 Unilock plates that perform the functions of many other conventional systems. The Mini, with a 1 mm profile performs the same functions as the standard 2.0 miniplate. The Medium with a 1.3 mm profile performs the same functions as the 2.0 DCP, and the Large with a 1.5 mm profile is the equivalent of the universal 2.4 fracture [plates]. Unlike the 2.4 Unilock plate, protective screws are not needed for folding and contouring. The three plates are fixed with the same screws with a diameter of 2 mm, available in lengths ranging between 5 and 18 mm, allowing bicortical fixation in basal osteosyntheses. These are titanium- aluminum-niobium screws, which are self-tapping, with a self-retentive recess for the Stardrive screwdriver. It has locking screws (with a threaded head) and normal screws, which can be inserted obliquely so that the fracture site can therefore be penetrated from cortex to cortex. Finally, the fact that a drill guide is not needed should be emphasized as the design of the screw means that it self-centers (Fig. 3).

Material and method

Over the last 24 months a prospective observational study was carried out in our Service of a cohort consisting of 36 patients treated with the 2.0 Unilock plate system. The large majority had fractures of the mandible and the rest had differing situations requiring mandibular osteosynthesis. Fortyeight fixations were carried out in total. The patients were evaluated intraoperatively and postoperatively in order to determine the benefits of the system: characteristics of the mandibular defects, ease of use and complications (Table 1).

Results

Nearly half the patients presented simple mandibular fractures and non-displaced fractures, even if they were double, were considered as such. The rest were complex fractures (multiple and/or displaced) with specific situations that will be discussed later. In all patients, except for one edentate patient, osteosynthesis was carried out with occlusal control and intermaxillary fixation, either with IMF screws or with bimaxillary splints, depending on the dental characteristics and on the presence or not of an associated condylar fracture. The approach chosen depended on the localization of the fracture and the degree of displacement, although the approach of choice was intraoral and transbuccal when possible. Plate election depended on the biomechanical needs of each case, these being wider and longer the greater the degree of displacement, being used for young people with completed dentition and in those cases in which a lack of adequate postoperative sociosanitary collaboration was assumed. In cases of "reconstructive osteosynthesis" a large plate was always used (1.5 mm profile). In all cases the anatomical reduction of the fragments and the primary stability achieved were considered very good or excellent, and the occlusal assessment satisfactory (Fig. 4).

During the postoperative [period] intermaxillary elastic traction was maintained only in those patients with an associated condylar fracture, none of whom were given open reduction for osteosynthesis as a second choice (7 of the 15 simple fractures, 5 of the 9 complex fractures and 1 of the 2 comminuted fractures) in the NVBG (non-vascularized bone graft) case and in the panfacial fracture, during a period which varied between 10 and 21 days. The rest of the patients left the operating room with mouths open. The only case presenting malocclusion, even though there was no condylar fracture, required intermaxillary fixation for 30 days. The patient did not wish to be reoperated, preferring "conservative" treatment that did not rectify the disocclusion perfectly, and she needed an additional orthodontic adjustment. The radiological controls, which were carried out immediately and which included a TC scan of the occasional complex fracture and comminuted case, were considered satisfactory as fracture sites were correctly aligned with the exception of the previously mentioned malocclusion (Fig. 5). The follow- up, with the exception of the oncologic cases, varied between 8 weeks for simple fractures and 12 months for more complex ones, and consolidation was considered definitive in all cases. (Fig. 6).

With regard to complications, not including those attributed to associated condylar fractures such as aperture limitation and lateral deviation, these are reflected in table 1: one malocclusion (previously mentioned), one infection and one plate exposure. In one of the open comminuted fractures delayed infection developed (more than 1.5 months post-op.) secondary to an avascular necrosis of a fragment from the mandibular base that had been fixed with traction screws; this was resolved by removing it, and stability of the remaining osteosyntheses was observed together with normocclusion (Fig. 7). The other complication that arose two months after intervention, was the intraoral exposure of a mini plate on a symphyseal level, in the other comminuted fracture treated with this system. This was a patient was nearly edentulous having an open fracture from a gun wound (bullet), and exposure was considered secondary to scar retraction of soft tissues. It was resolved by removing (at 6 months) the osteosynthesis material that had become exposed, and removal of the consolidated bone fragment was not needed (Fig. 8).

Discussion

The design of the 2.0 Unilock system encompasses two concerns for most maxillofacial surgeons concerned with the use of osteosynthesis techniques: the use of implants with the lowest possible profile and simplification of application with maximum guarantees of stability.^8-14 This "clinical pragmatism", while being necessary and tightening the collaboration with engineering and industry, has made its development possible (Synthes Maxillofacial, Paoli, PA-USA) even though in the multicentric study previous to its commercialization (more than 200 cases) central European hospitals have also participated. The aim is to combine the benefits of locking systems (fundamentally the 2.4 Unilock) with implants of a lower profile that can be more easily applied intraoperatively. As we have already mentioned, it represents a considerable relief regarding threedimensional molding of long implants, but it is also very interesting in relation to the application of small implants for osteosynthesis in areas with difficult access such as the m a n d i b u l a r condyle. The experience collected in this pilot study initially defines the indications and limitations of this system (Table 2).

Even if the indications appear to be well-established (load sharing osteosynthesis), the same cannot be said of the contraindications; indeed, it is been well-known that the resistance of these plates is not adequate for supporting load bearing osteosynthesis conditions (the only exception would be the fixation of microvascularized bone grafts) such as bridging osteosynthesis or the fixation of an NVBG. (Fig. 9) In comminuted fractures cases, the implant of choice would be the 2.4 system. In our series there are two cases in which the election of the 2.0 system was not a whim; in the patient that was nearly edentulous due to a bullet, the condition of his soft tissues led us to take this decision in an effort to minimize the possibility of exposure, given the reduced profile of the plates. In the second case, the presence of a condyle fracture and the resulting need for postoperative intermaxillary fixation encouraged us to use the 2.0 system with good results. In both cases the complications can be considered to be independent of the system used, appearing later following complete consolidation and normocclusion.

With regard to the NVBG case, this was a young irradiated patient with a small face and fine skin, with reduced biomechanical requirements (mastication/ collaboration). The result was very satisfactory even though she in fact underwent "preventative" intermaxillary fixation for three months that may not have been needed had we used a 2.4 system. (Fig. 10). In cases of approaches involving osteotomies, they are a reliable alternative that can on occasion reinforce marginal mandibulectomies (Fig. 11) although with radiotherapy the 2.4 system should first be considered.

Finally, even though this system has been designed specifically for mandibular osteosynthesis, it can be particularly useful for cases involving panfacial fractures as with the same set, with identical drills and screws, we can perform osteosynthesis of the mid-face and orbit (with mini plates) making instrumentation much simpler (Fig. 12).

Conclusions cannot evidently be made of these isolated cases, but the aim is to show that the limits are not clearly defined. For example, there is not enough experience that allows evaluating cases of atrophic mandibles (where the 2.0 profile would be highly preferable to the 2.4 profile for elderly patients with lax skin and little musculature) or of established focal osteomyelitis. In any event, we should individualize our decisions according to each concrete situation, and these should be based on our knowledge of the advantages and disadvantages of each system that we should be adequately familiar with. As a general rule we advise that for "borderline" cases the 2.4 system should be used as, although it is more cumbersome to use, its performance has been contrasted.

Conclusions

The 2.0 Unilock implant system covers the existing gap between the miniplate and the rigid plate. It incorporates the advantages of the locking system allowing mandibular osteosynthesis to be carried out 90% of the time. These are thinner plates with a lower profile meaning that, on many occasions, the introduction of a larger rigid plate is avoided. Soft tissues are less irritated and vascular supply to the bone is greater. The presentation of the hardware in a small set and its refined design, allow simple manipulation. Surgical times are therefore speeded up, and instrument errors are minimized. A prospective and comparative study will be required in order to define precisely the indications, limitations and benefits of this system with regard to conventional systems: miniplates and reconstruction plates.

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