Introduction

Minimally invasive plate osteosynthesis (MIPO) or submuscular plating was first advocated in the late 1990s by individual surgeons [ 1, 2]. Their indications included femur, distal femur, and pilon. Some pioneers [ 3– 5] also suggested performing MIPO on the humeral shaft after having conducted many anatomical studies.

With the introduction of the less invasive stabilization system distal femur (LISS-DF) and the locking compression plate (LCP) increasing numbers of surgeons all over the world have started performing submuscular plating. Since MIPO is a new technique requiring special attention to many technical steps and anatomical details, specific AO Courses have been developed to help interested surgeons. It is a longstanding tradition of the AO Foundation to teach general principles applicable to the specific needs of surgeons. In this case to allow them to perform MIPO in all anatomical regions in a safe and reproducible way.

Definition

Minimally invasive plate osteosynthesis is a form of minimal access osteosynthesis. The minimal invasiveness of MIPO is not defined by the length of the incision as in joint-replacement surgery. Minimally invasive surgery involves the whole process of an osteosynthesis including planning the approach in a safe anatomical corridor, reduction technique, maintenance of reduction, implant insertion, and fixation.

To teach the essence of MIPO in a principle-based manner the following definition of minimal invasiveness in MIPO was proposed by the Expert Group for Minimal Invasive Osteosynthesis (MIOEG) of the AO Technical Commission (AOTK):

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  Access to the bone through a “soft-tissue window”

  
  
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  Minimal additional trauma to the soft tissue and the bone mostly by indirect reduction

  
  
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  Minimal additional trauma at the fracture site if direct (percutaneous or minimally open) reduction is necessary

  
  
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  Application of instruments and/or implants which cause small “footprints”

A soft-tissue window must be as large as necessary to achieve anatomical reduction at the joint level. The window may be short, as for cases of simple incisions of the tibial plateau ( Fig 10-1 ), or large, as for a parapatellar approach for an intraarticular fracture of the distal femur ( Fig 10-2 ). The goal of anatomical reduction on the level of the joint remains the same as for open reduction and internal fixation (ORIF).

At the level of a shaft fracture the soft-tissue window serves two purposes. Firstly, it is the insertion aisle for the plate. Secondly, it allows positioning of the plate in the center of the shaft remote from the fracture zone ( Fig 10-3 ). Indirect reduction is the main type of reduction used in shaft fractures with the goal of achieving correct alignment of the fracture with respect to length, axis, and rotation. The soft-tissue envelope usually aids fracture alignment especially in complex fracture patterns. In simple fracture types, however, direct reduction with a joystick or a percutaneous clamp through a stab incision is often necessary. If an anatomical reduction is needed to obtain a stable fixation, a small soft-tissue window at the fracture site (minimally open access) may also be necessary. Soft-tissue handling and reduction technique are important factors for undisturbed fracture healing. It is not the length of the incision but the “footprint” the surgeon leaves at the site of surgery that is key. Special minimally invasive osteosynthesis (MIO) instruments are available, eg, the collinear forceps or the cerclage passer (see chapter 3 Instruments and chapter 2 New aspects of cerclage: improved technology applicable to MIO with special reference to periprosthetic fractures). Attempts to achieve fracture reduction using conventional reduction clamps and cerclage passers disturb the vascularity of the bone fragments (see Fig 10-6 ). Specialized MIPO instruments facilitate reduction in MIPO without leaving large “footprints” caused by surgery.

Prerequisites for MIPO surgery

A careful evaluation of each fracture should be based not only on the technical feasibility of a MIPO approach but also on the knowledge of the advantages and disadvantages of the technique. Advantages include undisturbed bone healing with reduced need to perform bone grafting, less infection, and less pain. Faster rehabilitation due to reduced blood loss through the limited access to the bone is also likely. Disadvantages of MIPO resulting from the limited view provided by image intensification include the risk of malalignment or malunion. Nonunion may occur due to a gap at the fracture site, or delayed union may occur when a simple fracture is treated with a bridging plate concept [ 6].

As a general rule MIPO is indicated in fractures involving the epiphyseal/metaphyseal area of long bones and at the level of the diaphysis, when an intramedullary nail is not feasible due to a narrow or deformed medullary canal, an open epiphysis, or an implant occupying the medullary canal.

After the case has been critically analyzed preoperative planning is the next mandatory step. Planning should include consideration of patient and C-arm positioning, draping, approaches, reduction techniques and reduction aids, and the type and function of the implant. This step is the same as for any operative procedure in trauma surgery. A step-by-step preoperative plan must be drawn. In addition an alternative plan to extend the incisions to achieve a correct fracture reduction if percutaneous reduction is not adequately possible should be formulated. MIPO should not be an excuse for an inadequate fracture reduction and fixation.

One of the major problems in MIPO surgery is malalignment, which should be prevented by using certain well-described preventative measures. These include the assessment of the axis of the lower limb using the cable method [1]. The un-injured leg should be draped free in lower extremity surgery to facilitate the measurement of length and rotation ( Fig 10-4 ).

In MIPO surgery image intensifier time is usually increased due to the fact that limited approaches do not allow direct visual control of the fracture site. To reduce the amount of radiation to the patient and operating team a standardized technique using a stepwise increase of stability from proximal to distal or from distal to proximal, depending on the fractured area and the fracture type, should be applied. Therefore temporary fixation is applied on one side, then the fracture is reduced by instruments and maintained using K-wires or screws on the other side to increase stability for checking alignment, length, or rotation. Reduction is of paramount importance but maintaining reduction during fixation is also vital. This can be achieved by a variety of techniques including the use of an external fixator, reduction handles, a distractor, or a plate as a reduction tool.

The concept used to achieve definitive fracture fixation depends on the fracture pattern.

In comminuted fractures flexible fixation using the bridge plate concept without touching the fracture zone is preferred. This technique has proven advantages of undisturbed fracture healing when compared to ORIF [1, 7].

In simple fracture types, according to Perren‘s [ 8] strain theory, there may be delayed healing depending on both the amount of gap at the fracture site and the stiffness of the implant. This is influenced by how close the first screw is to the fracture line [ 9]. When a simple fracture pattern is present the surgeon must decide whether to achieve either relative or absolute stability. If absolute stability is chosen the fracture has to be anatomically reduced and compressed using a plate-independent or plate-dependent lag screw. In a MIPO procedure this usually means using direct percutaneous reduction techniques with joysticks, percutaneous reduction clamps, collinear forceps ( Fig 10-5 ), or a MIPO cerclage passer ( Fig 10-6 ). If an anatomical reduction which allows for a stable fixation is not achieved by percutaneous approach, the surgeon should not hesitate to make a soft-tissue window at the fracture site which is large enough to achieve anatomical reduction under visual control (minimally open access) ( Fig 10-7 ). The biomechanical principles of absolute stability in MIPO are the same as for ORIF. However, the surgeon should minimize the length of the skin incision at the fracture site by making a short incision to expose the fracture site if required.