MAXFRAME Multi-Axial Deformity Correction System
Theodor F. Slongo, J. Spence Reid
Fig 1 The MAXFrame hardware is a hexapod ring fixator system. While hexapod systems are not a new technology, MAXFrame hardware introduces some new features that improve handling by surgeons and patients. Introduction to MAXFrame technology
One of the unique features of the MAXFrame deformity correction system (Fig 1) is that it required software development and a new approach to planning and teaching. The MAXFrame is the first and only technology in the AO/DPS portfolio of products in which the hardware provided is directly used as an input for the software planning, calculating the treatment plan. Furthermore, MAXFrame technology provides the first planning software capable of transforming x-ray images based on identified hardware components into a real-time 3-D reconstruction. The MAXFrame system can also be used as a temporary reduction tool in case
of acute correction and plate or nail fixation. In this case, the reduction can be checked using a C-arm or, for exact planning and positioning, the MAXFrame 3-D software is used with perspective frame matching.
The MAXFrame specific components of the hexapod frame are the rings and footplates, and the struts. The rings and footplates are mounted onto bone fragments using wires and half-pins similar to the DO Ring Fixator system. The components are available in several sizes and can be used in multiple configurations in order to provide the stability and range of motion required for the correction. Surgeon preference drives the use of the MAXFrame hardware to ensure optimal correction and patient comfort throughout the treatment.
Six struts connected to two rings in a specified order build a hexapod frame. Through changing the length of the struts, the rings move in a calculated relationship to each other. As this calculation between the rings is basically dependent on an 8th degree trigonometrical system of equations, the MAXFrame 3-D software is required to provide controlled change of strut length and movement, in accordance with a treatment plan with defined limiting factors for patient safety. The standard method (manual measuring and data input) workflow in the MAXFrame 3-D software provides all of the parameterization required to calculate the treatment plan for any 3-D correction based on well-established knowledge. In addition to the standard method commonly used in hexapod systems, the MAXFrame 3-D software provides the perspective frame matching (PFM) method and the acute intentional deformation (AID) method. The PFM workflow is briefly described later on. The AID workflow is a straight forward method usually adopted when the strut values required to enable a simple transition from the deformed to corrected state is known and calculated.
Surgeons language used to define parameters/values
The MAXFrame 3-D software does not use a special technical language. Deformation parameters are not defined according to your chosen reference ring as with other hexapod systems, but rather in the same way clinical deformity is determined. Input values are defined in sentences rather than in abstract numbers in order to avoid misunderstanding or misinterpretation of the definition of the value.
3-D animation a comparison to reality
One of the highlights of the MAXFrame 3-D software is the 3-D animation of the frame and bone segments in all scenarios during the treatment plan. This animation is a very helpful tool used to check the parametrization and view the structure from any angle. If the 3-D animation looks different to what is viewed when looking at the frame, input values should be checked for errors and amended (Fig 2).
Fig 2 Development of 3-D animations. Perspective Frame Matching (PFM) a new planning tool based on 3-D reconstruction
A 3-D reconstruction of a clinical situation is commonly used in navigation. One of the prerequisites for 3-D reconstruction based on two x-rays is the matching of characteristic landmarks or markers of a known body. The perfectly known body in a MAXFrame is the frame construct itself. As we are already aware, the geometry of a MAXFrame is perfectly defined through specific placement of rings and struts. The perspective frame matching method workflow uses this defined frame geometry to produce a perfect 3-D reconstruction of the clinical situation in the individual case. The struts as seen in the x-ray images are matched by their joints and/or axis and the 3-D reconstruction is calculated based on this matching on both images. The images do not need to be rectangular or perfectly shot along the axis. The only prerequisite is that all struts, preferably with the joints, are visible in both x-ray images (Fig 3).
The primary advantage of this method is that artefacts of X-ray imaging are no longer a problem for getting true and exact values. An x-ray is, by definition, only a projection of the real structure like the shadow of a pencil on a piece of paper. As the size of the shadow is dependent on the distance between the object and the source of the light, all structures are magnified on an x-ray and exact values cannot really be measured. In addition, if the structure is in an oblique as opposed to parallel plane when compared to the x-ray, the size of the object is shortened in the direction rectangular to the axis that is common to both planes. The challenges presented by x-ray can be solved with a 3-D reconstruction, enabling the definition of points and axis in space. After the PFM is complete with acceptable precision, the points of reference, the bone axis, and any structure of interest can easily be identified by marking them in both x-ray images. All the distances and angles between the structures of interest are directly calculated based on these markings and no value needs to be measured in the traditional way.
Fig 3 Planning through the 3-D reconstructions. Case: Tibial malunion
Case provided by J Spence Reid, Hershey, USA
A 54-year-old man suffered bilateral tibial fractures 20 years earlier, both treated in a cast. He now experiences pain in the medial right knee. Images taken showed that both legs had substantial malunion, but the right knee caused pain because it was out of mechanical axis (Fig 4). The patient was successfully treated with the MAXFrame system (Figs 5 to 8).
Fig 4 Preoperative images showing malunion and deformity. 
Fig 5 The AP and medial postoperative images show the MAXFrame in position. 
Fig 6 Postoperative AP and medial images used for perspective frame matching. 
Fig 7 Images after MAXFrame correction according to initial treatment plan. As a residual deformity, a translation is visible in the lateral view (b). 
Fig 8 Final correction after replanning the lateral translation with the standard method. Complex Deformity Corrections in Long Bones Using External Fixation
3D Deformity correction: DO-Ring system and next generation of HEXAPOD system
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