7 Principles of angular stable fixators


7 Principles of angular stable fixators

Staubli, Alex E

1 Surgical principles

In recent years there has been rapid development of the so-called “biological” internal fixation of metadiaphyseal multifragmentary and comminuted fractures and of techniques in osteotomies around the joints. The term “biological” refers to the preservation of bone and soft-tissue vitality that, combined with biomechanical stability, is essential for bone healing [1]. Titanium plates were introduced about 30 years ago [2], and due to their biomechanical properties and good biocompatibility, became standard devices in addition to the existing steel implants [2].

For a long time exact reduction and interfragmentary compression were considered the main principles of internal fixation to stabilize bone fragments in the diaphyseal and metaphyseal region of long bones. Osteosynthesis with an internal plate fixator is based on the so-called “splinting principle”, ie, bridging of unstable fracture and osteotomy zones. This stimulates the consolidation of loose fragments. Within this concept, the “splinting” in comminuted diaphyseal and metaphyseal zones, and preservation of tissue vitality have priority over interfragmentary compression. Furthermore, bone healing is stimulated in this type of osteosynthesis by alternating load induced by functional postoperative exercises leading to micromotion in the fracture and/or osteotomy zone [3]. On the basis of mechanical testing of the “splinting principle”, the relevant fixation techniques and implants were further developed in biomechanical studies at the AO Research Institute in Davos [1]. Principles of internal fixation apply equally to fracture healing and healing of osteotomies around the joints (see chapter 12 “Radiological examination of bone healing after open-wedge tibial osteotomy”). Exceptions to the treatment concept of “splinting” are intraarticular fractures, for which interfragmentary reduction without step-off and stable fixation remain the basic principles of fixation.

2 Development of open-wedge valgization osteotomy of the proximal tibia

The aim of valgization correction osteotomy of the proximal tibia is to unload a degenerated medial compartment, whereby the correction should heal without secondary loss of correction.

The following procedures are commonly performed in tibial head osteotomy:

  • Open-wedge (additive) osteotomies

  • Closed-wedge (subtractive) osteotomies

  • Focal dome or gable-shaped osteotomies

The afore mentioned procedures differ in size and shape of the osseous contact zones. The largest bone interface is achieved in closed-wedge technique with removal of a lateral-based bone wedge, a technique that is still practised very frequently world-wide [4].

Open-wedge high-tibial osteotomy (HTO) was rather unpopular at first because common implants could not counteract the high axial compression and torsional forces at the proximal tibia. Complications due to failure of fixation often occured [57].

However, this surgical procedure gained greater acceptance as new implants were developed. The method of open-wedge tibial valgization osteotomy commonly performed now, creates a wedge-shaped gap with a medial base between the two main fragments, which results in an unstable situation. The size and volume of the gap depend on the amount of correction required.

The first results of open-wedge tibial correction osteotomy were published approximately 25 years ago. These cases were immobilized in a plaster until the bone has completely healed. In subsequent years, the osteotomy was stabilized by interposition of two or three bicortical iliac crest bone grafts that were transplanted into the osteotomy gap [6]. This procedure was suboptimal regarding precision and mechanical stability, not to mention significant morbidity at the iliac crest donor site. Far better results were obtained by combining internal fixation with the interposition of cement blocks [5, 6].

In recent years, various implants have been designed for internal stabilization of open-wedge tibial osteotomies, where-by a particular spacer plate (Puddu plate) [8] gained the most popularity. Nevertheless, a high failure rate was recorded, especially in osteotomy gaps greater than 10 mm, severely obese patients, and patients with manifest osteoporosis. Lobenhoffer et al reported a failure rate of 6% with corresponding loss of correction in 101 open-wedge osteotomies stabilized with this implant [9, 10]. Spahn conducted a study to compare this so-called spacer plate with the angular locking C-plate that he had developed himself [11]. The overall complication rate for the spacer plate was 43.6%. 11.7% of the patients required an additional lateral fixation. In this study, loss of correction was only observed with the Puddu plate.

At the end of the 1990s the author was using the 95° titanium angled blade plate at his hospital for closed- and open-wedge osteotomies of the tibia. Even corrections greater than 10° were easily achieved with this implant without filling the osteotomy gap ( Fig 7-l ).

Fig 7-1a-d Proximal correction osteotomy of the tibia with an angled blade plate. a-b Closed-wedge tibial head osteotomy of the right leg with osteotomy of the fibular diaphysis. c-d Open-wedge tibial head osteotomy with a 95° titanium angled blade plate without bone graft in the osteotomy gap.

Hemicallotasis is another suitable procedure for unilateral and unidirectional correction. The medial distraction is achieved by applying a unilateral or ring fixator over several weeks or months. This method is frequently applied in Eastern Europe, in Anglo-Saxon and Scandinavian countries, and in Italy [12].

The advantages of the hemicallotasis method are nonextensive surgery, the possibility of secondary axial correction, and good applicability even for complex deformities. The disadvantages include the long wearing time, patient discomfort, pain during distraction, and, above all, the high rate of pin-track infections.

3 Application of angular stable plate fixators in open-wedge tibial osteotomy

3.1 The internal plate fixator TomoFix

The locking compression plates (LCPs) that have been developed over the past 20 years, eg, PC-Fix and the less invasive stabilization system (LISS), consist of an angular stable plate and a locking screw system, whereby the screw head locks into the threaded plate hole [1315]. This internal fixation technique has become well established in fracture treatment [16, 17].

Early in the year 2000, the LCP system for the fixation of open-and closed-wedge correction osteotomies around the knee was first integrated into the new generation of plates and introduced to the market as the TomoFix ( Fig 7-2 ). The idea behind the development of this implant was the adaptation of the LISS plate for the lower extremity to meet the demands of proximal tibial osteotomy [18]. The results were critically analyzed by the members of the Knee Expert Group (KNEG) of the AO in multicenter studies [9, 10]. Experience over the past 3 years has shown that the application of this implant offers great potential for the correction of both uniplanar and multiplanar deformities. Good results are also achieved with this implant in revision surgery for failed osteosynthesis with other implant systems. These good results did not depend on the application of bone graft in the osteotomy gap.

Fig 7-2 The TomoFix angular stable plate fixator applied to stabilize a proximal medial open-wedge tibial osteotomy.

3.1.1 Implant design

The design and construction of the plate fixator are shown in Fig 7-3 . For a better and consistent understanding of plate design and surgical technique, the four plate holes in the proximal segment are marked A, B, C, and D and the four combination holes in the distal segment are marked 1, 2, 3, and 4. The plate design described here has proven to be of value over the last 8 years and is still applied in unmodified form today.

The T-plates are manufactured from pure titanium and are identical for the right and left side. The TomoFix plate fixator cannot be substituted by the proximal tibial fracture plate LCP 4.0/4.5 which is unsuitable for open-wedge HTO. The plate is made from a punched blank, whereby the shape of the plate is matched to the anatomy of the proximal tibia to correspond to an average valgus correction of 10°. The 38° curvature of this plate is equivalent to that of the average radius of the proximal tibia. The T-shape (T-arm = 32 mm) of the implant and its buttress, with three angular stable bolts oriented from antero-medial towards posterolateral and the continuous plate thickness of 2.8 mm over a length of 11.5 cm, ensure best possible elastic adaptation to the proximal tibial epiphysis, metaphysis, and diaphysis. This construction distributes bending forces over a long distance and neutralizes torsion avoiding punctual stress concentration ( Fig 7-4 ).

The conical screw holes with their 2.5 double threads in the T-arm (holes A, B, and C) have an inclination of 4° caudally to prevent perforation of the tibial articular surface. The holes are aligned convergently and stabilize the lateral hinge by spring suspension of applied forces. Plate holes 1, 2, 3, and 4 in the longitudinal arm are arranged in staggered alignment to avoid longitudinal diaphyseal fissures. Holes A, B, and C are 360° threaded holes, whereas hole D and holes 1, 2, 3, and 4 are 270° combination holes.

Fig 7-3 Design of the TomoFix internal plate fixator with three threaded holes in the T-arm and 5 combination holes on the longitudinal arm.

The 5.0 mm locking head screws are self-cutting (green) and self-tapping/self-drilling (blue). The two 3 mm distance plugs for holes D and 3 or 4 act as spacers (see chapter 9 “High-tibial open-wedge valgization osteotomy with plate fixator”, Fig 9-2d-e , and chapter 12 “Radiological examination of bone healing after tibial open-wedge osteotomy”). The screw heads all have a low profile and are locked with the torque screwdriver of 4 Nm limitation.

Fig 7-4 Balanced biomechanical stress distribution. The bending forces are distributed over a long distance during axial loading.

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Jun 30, 2020 | Posted by in ORTHOPEDIC | Comments Off on 7 Principles of angular stable fixators
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