Although the incidence data on the occurence of hallux valgus deformity found in literature vary, it can still considered as collectively a common occurance, with between 2% (children) and 49,5% (European adults) of the population being affected [1-3]. An internal fixation is required at least in adults to correct the hallux deformity permanently, an operation that has been described in more than 130 variations over the last 100 years [4]. The choice to operate and what technique to apply depends on the severity of the deformity [5, 6]. In accordance with current guidelines proximal operations of the first metatarsal are reserved for severe deformities with pronounced intermetatarsal - I - angles over 15° - 20° and hallux valgus - angles over 40° [7].

A problem of a proximal intervention is the achievement of a biomechanically competent internal fixation in order to realize an early functional treatment, as well as to avoid the possible drawbacks associated with a plaster cast immobilisation of longer duration. The commonly used screw fixation is biomechanically incapable of bearing load and thus not considered weightbearing [8]. In theory, this results in an risk of a secondary correction loss, which may have particularly serious consequences when considering the long lever acting at the distal first metatarsal after a proximal osteotomy. This problem of possible secondary loss of correction has led to the development of advanced locking plate systems with the aim of allowing the partial load bearing without loss of correction, even after a proximal first metatarsal proceedure.

The aim of this study was thus to examine resistance to angulation and displacement of the internal fixation of a proximal first metatarsal lateral displacement osteotomy as described by Thomas [9], using a locking plate system (TOM^® plate) compared with a conventional crossed screw fixation. The Thomas technique was chosen on the one hand because the TOM^® plate was especially designed for this lateral displacement osteotomy (Fig. 2b). In addition to this it represents an easily understandable technique with reliable results.

Fig. (2b). Preoperative X-ray, schematic drawing and postoperative X-ray of a proximal metatarsal lateral displacement osteotomy with a TOM® plate osteosynthesis.

The hypothesis was that locking plate fixation provides more stability than srew fixation.

We found larger stability about all three axes of movement analyzed for the PLATE than the CSO, i.e. the relative movements observed were lower along the x - axis distal to proximal, the y - axis parallel to the osteotomy and the z - axis perpendicular to the plane defined by the markers 1- 4 for PLATE than CSO. A striking observation hereby is the superiority of the locking plate especially with high plantar load measurements (Fig. 3). When observing translational relative movements of the osteotomy fragments at a load application of 200 N, the maximum values for CSO were 2.71 mm, 3.9 mm, and 1.71 mm and along the x, y, and z - axis respectively. In contrast, the PLATE shows maximum relative movements of only 0.66 mm, 0.9 mm and 0.35 mm, again along the x, y and z - axis respectively. Correspondingly, the PLATE shows maximal relative rotational movement at 200 N load of 0.9 °, 2.36 °, and 2.28 °, and a 7.1 °, 9.1°, and 2.71° for the CSO about the three axes respectively.

Fig. (3). Relative fracture fragment motion measured for the PLATE and CSO. Shown is a boxplot diagram for relative movements of the osteotomy fragments in millimeter rsp. degree (mean indicated by small black line).

The two - way analysis of variance by ranks done with the results of the x,y and z groups for PLATE and CSO showed statistical significance at a level of a = 0.05 for all groups and both translational and rotational movements for a load from 20 N to 200 N for all three axes (Fig. 3).

The osteosythesis with crossed cancellous screws failed macroscopically in one case when the specimen was loaded with more than 200 N. Therefore this specimen was not included in the statistical analysis. In all other cases (seven specimens) an osteosynthesis failure at the plate-screw interface, as a failure of the fixed threaded screw head or as a patent screw or plate fracture was not observed.

This study demostrates the superior stability of the locking plate fixation compared to the crossed cancellous screw osteosynthesis.

Both in terms of translational and rotational relative movement of the osteotomy fragments, the locking plate was observed to be more stable under the application of a ground reaction force under the first metatarsal head of 200 N. The locking plate resulted in a significantly less relative movement of the osteotomy fragments with respect to each other.

Using proximal first metatarsal osteotomies, a very high potential of correction can be realized so that severe hallux valgus deformities can also be corrected and sufficiently stabilized. Essentially four different methods are mentioned in literature: closing - wedge osteotomy, opening - wedge osteotomy, crescentic osteotomy with curved cutting incision and proximal Chevron - osteotomy [13, 14]. For all of these methods, it could be shown that conventional methods of osteosynthesis like screw, plate or wire fixation are much less stable than for distal correction procedures [15]. A stable internal fixation however is essential to prevent a pseudarthrosis or secondary loss of correction.

Campbell et al., as well as Rosenberg et al., demostrated that a conventional plate osteosynthesis is superior on stability than screw fixation [16, 17].

Some of these procedures show well known disadvantages: shortening of the first ray with the risk of developing a transfer metatarsalgia (closing wedge osteotomy) or an increase in the already pathologically enlarged DMMA angle (opening wedge osteotomy) what requires an additional distal metatarsal osteotomy. Crescentic osteotomies last but not least are technically demanding and require high surgical skill [5]. All this is avoided by a lateral displacement osteotomy in the Thomas technique, especially since a wide variability of possible saw cuts is tolerated by the procedure, both perpendicular to the shaft as well as descending or ascending [9]. A stable internal fixation, however, is mandatory as well.

The failure of internal fixation may be realized either in a single event with maximum load application, e.g. in a fall, or in terms of a gradual failure of load application in the sense of fatigue. The latter can either lead to a pseudarthrosis or displacement of the distal fragment, especially in dorsiflexion [18, 19]. A displacement rate as high as 28% of the patients with a first ray dorsiflexion after a crescentic osteotomy is reported in literature [19, 20].

For internal fixation most often crossed screws, plate osteosynthesis or additional Kirschner wires are recommended [17, 21-23]. Compared to these techniques, a locking plate system has several advantages. It protects the blood supply of the bone and offers a much higher stability of the fixation of the osteotomy fragments one to each other [24]. So it was only obvious that the principle of locking plate fixation was applied on first metatarsal osteotomies as well. Nevertheless Cohen found no benefits of the locking plate system compared to crossed screw fixation in the first tarsometatarsal joint arthrodesis [25]. These findings however may be due to the specific design of the locking plate consisting of a relatively soft titanium alloy without possibility of interfragmentary compression and obligatory fixation at the dorsal side of the first metatarsal with the risk of plantar osteotomy gapping. The TOM^® plate used in our study is made of a more solid titanium alloy, facilitates interfragmentary compression and is placed at the medial side of the MT 1. The benefits of interfragmentary compression with increased bone contact and maximum friction between the bone surfaces can thus be combined with the increased rigidity and stability of the locking plate system.

In this investigation, the stability of the TOM^® plate was compared to CSO in a complete human cadaver-foot model, which does not rely on isolated bone specimens or artificial bone substitute materials. A complex plantar dissection necessary to improve the stability by a plantar plate placement [26] was not necessary. Test models described in literature often use feet disarticulated in the ankle, arranged in an unphysiological position, measuring only translational forces or rely on synthetic bone [25, 27-29]. In some studies the locking plate lacked the possibility of interfragmentary compression and three - dimensional aspects were not taken into consideration in contrast to our study [30, 31].

To our knowledge, our study is the only one in which a biomechanical model was established which does not rely on simplified representations and where the complete anatomical human specimen was employed and a physiological position and loading were applied simulating the in vivo loading situation. The maximum loading condition of the first metatarsophalangeal joint applied in this study has for instance been documented by means of dynamic pedobarography, where this phase of the stance-phase of gait is correlated to peak pressures and loading [10, 32-35]. By using the same specimens to test both osteosynthesis techniqes the bias resulting from specimen variability could be reduced and the stability of the implants compared in a paired experimental design. As mentioned above, care was taken to avoid that the standard screws were crossing the screw holes of the TOM^® plate leading to reduced stability of the osteosynthesis. To grant full bone contact the standard screws were applied directed against each other and crossing the metatarsal osteotomy before the plate fixed with locking screws was taken off. The locking screws fixed to the plate were just staying in the first metatarsal when the standard screws were appplied. Therefore the standard screws were placed in independent holes.

The authors‘ decision to compare the stiffness of locking plate fixation with that of standard screw fixation was done because they wanted to refer to a basic and well established method of osteosynthesis for metatarsal osteotomies [13, 16, 23, 25].

The measuring equipment and methods had been validated before the actual tests in a pre-test setting. In contrast to the studies mentioned above a three - dimensional testing model was used. Thus the focus of measurement was not fixed to a single plane whereas a possible failure of the fixation in one of the other planes occurred. Rather the three - dimensional orientation of the ostoetomy and the osteosynthesis was analysed with our testing model taking into consideration both rotational and translational forces.

The situation of measurement corresponds to the commonplace situation before surgery is done. Normally a bone mineral density of the foot is not available for the patient. Therefore the possible results cannot be taken into consideration resulting in the requirement of the most stable osteotomy fixation in respect of the assumption of poor bone quality.

The results of our study confirmed that the lateral proximal first metatarsal displacement osteotomy in combination with a locking plate fixation delivers several advantages at once. On the one hand, it avoids the drawbacks of other first metatarsal osteotomies, such as shortening of the first ray or a technically difficult exposure of the ROI with a high skilling factor and corresponding risks. On the other hand, the TOM ^® locking plate, which also allows internal fragment compression, is a possibility of a technically simple internal fixation of superior stability compared to other osteosynthesis devices.

The results of this study suggest that a permanent deformity correction without postoperative relative movements can be realized, which is prerequisite for reliable bone healing. In our view, the locking plate device investigated thus provides due to its design with the combination of interfragmentary and locking screw fixation the high internal stability necessary for an early functional treatment after hallux valgus correction, while avoiding the otherwise necessary period of prolonged immobilization.

Declared none.