Principles of MIPO technique

The amount of bone devitalization and damage to the soft tissue in the injury zone are major factors leading to complications, such as infection, delayed union, and nonunion. Minimally invasive surgery helps to reduce the iatrogenic trauma to the fracture ends and the fracture fragments. Theoretically, preservation of the soft-tissue envelope around the fracture zone has biological advantages. There should be less blood loss with less soft-tissue dissection resulting in fewer infections and healing complications and less chance of secondary bone grafting.

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  MIPO for articular fractures requires a soft-tissue window, which is large enough to achieve a precise anatomical reduction. After anatomical reduction the principle of absolute stability is applied using compression.

  
  
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  MIPO for diaphyseal fractures may involve indirect closed or percutaneous direct reduction and soft-tissue windows away from the fracture site. These should be large enough for implant insertion and visualizing and palpating the plate on the bone.

  
  
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  The main reduction method used in MIPO is indirect reduction. For diaphyseal fractures the restoration of length, axis, and rotation is needed. The fracture zone is preferably bridged with a locked internal fixator.

Principles of bridge plating

Using long plates to bridge the fracture zone is known as bridging plate osteosynthesis. In contrast to internal fixation using anatomical reduction and a compression plate, the bone in this procedure does not contribute to the mechanical stability of the fracture, or only contributes to it partially ( Table 4-2 ). Bridging plate fixation reduces movement between fracture fragments, but does not completely stop it. Micromotion in the fracture zone promotes indirect healing through callus formation.

Internal fixators

The introduction of the internal fixator has made MIPO a more practical proposition and extended its scope and range of application.

The internal fixator is in essence a subcutaneous (MIPO on the medial side of the tibia), submuscular or epiperiosteal external fixator. The unique design feature of the internal fixator is the locking head screw (LHS)—the screw head has a double conical thread for secure fixation into a corresponding conical thread in the plate hole. This feature imparts a degree of angular stability to the construct, as the locked screw head can no longer toggle in the plate hole. Also, since the screw head is locked in the plate hole, it does not press the plate against the underlying bone as the screw is tightened, unlike conventional screws, such as cortex or cancellous bone screws ( Fig 4-1, Fig 4-2 ). Hence, the internal fixator has features making it suitable for MIPO, for example:

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  • LHSs prevent the plate from being pressed against the underlying bone, thus sparing the periosteal blood supply.

  
  
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  The bone is not pulled against the plate by LHSs as the screws are tightened so there is no loss of primary reduction if the fracture has already been reduced.

  
  
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  Consequently, accurate contouring of the plate is not necessary, which is a definite advantage in MIPO because the bone is not exposed for templating.

  
  
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  Angular stability of the construct prevents secondary loss of fracture reduction when placed under load.

  
  
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  As LHSs are either self-drilling and self-tapping, or only self-tapping, screw application is made easier in the MIPO setting since drilling and/or tapping is no longer required, which is not the case with the application of conventional screws.

The first internal fixator specifically designed for use with MIPO technique was the less invasive stabilization system (LISS) for the distal femur. When the advantages of LISS became apparent, the demand for a more versatile system increased. This led to the development of the locking compression plate (LCP) with a specially designed combination hole. Half of this combination hole is designed as a dynamic compression unit that allows the use of conventional screws for interfragmentary or axial compression, while the other half is threaded to allow the use of LHSs. Thus the LCP can function as a compression plate when conventional screws are used or as an internal fixator when only LHSs are used.

In theory, no contouring of the LCP is necessary when used as an internal fixator, but in practice some degree of contouring is usually needed, especially in the epiphyseal/metaphyseal segments of the bone. Otherwise the plate may stand out and become prominent subcutaneously or cause irritation of the adjacent soft tissues. To overcome this problem, specially designed metaphyseal plates have been introduced ( Fig 4-3 ). There are two special features of these plates. Firstly, the juxtaarticular end of the plate is thinned out to facilitate contouring. Secondly, the two distal holes in this thinned area of the plate are angled 11° toward the center of the plate allowing insertion of LHSs in the epiphyseal area whilst avoiding penetration of the articular surface.

A further refinement of this technique is the development of the anatomically preshaped LCP for use in specific epiphyseal/metaphyseal parts of the skeleton. The metaphyseal end of such a plate allows for insertion of a cluster of LHSs in a divergent or convergent manner to improve their pull-out strength. Furthermore, no contouring of the plate is usually needed. An additional advantage of these anatomically preshaped LCPs is that they can be used as an aid for indirect fracture reduction when used with conventional screws. Conventional screws draw the bone toward the plate and thus the bone fragments adapt to the shape of the plate. Examples of anatomically preshaped LCPs are the locking proximal humeral plate, LCP distal humerus, LCP distal radius, LCP distal femur, LCP proximal lateral tibia, and LCP distal tibia.