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Revised AO/OTA classification (Jan 2018)


A revision of the AO/OTA Fracture and Dislocation Classification was published in the January 2018 issue of the Journal of Orthopaedic Trauma (Compendium and support material). This module is organized according to the 2007 version. Please read through its revised classification (below) before returning to the AO Surgery Reference.

32A1 Simple, spiral fracture

32A1* Simple, spiral fracture

a - Proximal 1/3
b - Middle 1/3
c - Distal 1/3

32A2 Simple, oblique fracture (≥30°)

32A2* Simple, oblique fracture (≥30°)

a - Proximal 1/3
b - Middle 1/3
c - Distal 1/3

32A3 Simple, transverse fracture (<30°)

32A3* Simple, transverse fracture (<30°)

a - Proximal 1/3
b - Middle 1/3
c - Distal 1/3

32B2 Intact wedge fracture

32B2* Intact wedge fracture

a - Proximal 1/3
b - Middle 1/3
c - Distal 1/3

32B3 Fragmentary wedge fracture

32B3* Fragmentary wedge fracture

a - Proximal 1/3
b - Middle 1/3
c - Distal 1/3

32C2 Multifragmentary, intact segmental fracture

32C2* Multifragmentary, intact segmental fracture

i - Proximal diaphyseal-metaphyseal
j - Pure diaphyseal
k - Distal diaphyseal-metaphyseal

32C3 Multifragmentary, fragmentary segmental fracture

32C3* Multifragmentary, fragmentary segmental fracture

i - Proximal diaphyseal-metaphyseal
j - Pure diaphyseal
k - Distal diaphyseal-metaphyseal

Bridge plating


John H Wilber, Friedrich Baumgaertel


Plate fixation of fractures is a form of stabilization with the potential for both load bearing and load sharing properties. Functional treatment of the limb for preservation of muscle strength, coordination, and joint mobility depends on the stability provided by the plate-bone construction. Fracture consolidation is to be expected if the mechanics of fixation and the biology of the fracture are compatible and mutually beneficial.

  • Biological bridge plating uses the plate as an extramedullary splint fixed to the two main fragments. The complex fracture zone is virtually left untouched; however, it is bridged by the plate. Length, alignment, and rotation are restored, but anatomical reduction of each fragment is not attempted.

This concept combines the relative stability provided by the plate with the preservation of natural fracture biology to achieve rapid callus formation and fracture consolidation.

  • Bridge plating techniques are applicable to all long-bone fractures with complex fragmentation and where intramedullary nailing or conventional plate fixation is not suitable.

a  Complex fracture of tibia and fibula (42-C2). There is severe softtissue injury. Proximal extension of the fracture and polytrauma (ISS = 48) precludes intramedullary nailing.

b  Emergency fixation with a unilateral external fixator.

c  Percutaneous bridge plating 3 weeks post injury after the soft tissues had settled. Percutaneous lag screws have been used to reduce some of the severely displaced fracture fragments. The external fixator was retained to provide additional medial support.

d  Clinical aspect 8 weeks after the accident.

e  Fracture union with callus formation at 29 weeks.

With “classical” direct fracture reduction and plate fixation with absolute stability, the viability of soft tissues and bone fragments may be jeopardized. This risk exists to a lesser degree in simple fractures (with less soft-tissue injury) and thus has less consequence on fracture healing. It is the goal of fracture surgery to maintain vascularity at the fracture site. This calls for the use of bridging techniques in fracture patterns with significant fragmentation.

Simple type A diaphyseal fractures can be successfully treated with intramedullary nailing, a technique of relative stability, or by anatomic reduction and compression plate fixation, providing absolute stability.

  • Experience has shown that bridge plating, providing relative stability, has a high risk of nonunion or plate failure in simple type A diaphyseal fractures.

This is because the strain at the fracture site is above the strain tolerance of the granulation tissue within the fracture site and so normal fracture healing will not take place. In complex type C diaphyseal fractures with numerous intermediate fragments, the bridging plate allows micromovement between the different fragments, while tissue strain is within the strain tolerance of granulation tissue, allowing normal callus formation [1]. If a complex, multifragmentary fracture is splinted in a cast, there will be some movement between fragments. However, the system as a whole will tolerate a significant amount of deformation, since it is distributed along the whole distance of the fracture zone. Thus, strain will be low and this allows tissue differentiation to progress. Callus formation between intermediate fragments can occur rapidly, even in the presence of considerable fragment displacement. This is the basis of Perren’s strain theory. The prerequisites for successful bone healing in this situation are optimal preservation of fragment vascularity and a favorable mechanical and cellular environment for the production of callus.

  • Bone fragments, once they have been stripped of their softtissue attachments (periosteum, muscles, etc), will not be incorporated into the early callus, since they will first need to be revascularized.

Perren's strain theory

a  Motion at the fracture results in deformation producing strain in the granulation tissue at the fracture site.

b  A perfectly reduced simple fracture (small gap) stabilized under compression (absolute stability and low strain) heals without external callus (direct healing).

c  A simple fracture (small gap) fixed with a bridging plate (relative stability) is exposed to movement (high strain). Fracture healing is delayed or will not occur at all.

d  In a complex fracture (large gap) fixed with a bridging plate (relative stability) the strain will be low in spite of movement, and fracture healing will occur with callus formation (indirect bone healing). Fracture union with callus formation at 29 weeks.



a  Shotgun injury. Grade IIIc open, complex radial shaft fracture with radial artery disruption and compartment syndrome.

b  Arterial repair and fasciotomy were performed. The bridging plate on the radial shaft has corrected length, axis, and rotation with anatomical reduction of both radio-ulnar joints to maintain forearm function. Additional lag screws were used for indirect reduction.

c  Indirect bone healing with callus formation 3 months after injury.

(With permission by Christoph Sommer.)

In diaphyseal type C fractures, the endosteal blood supply of fragments is, as a rule, interrupted. Preservation of bone vitality relies on periosteal vascularity, which also contributes to fracture healing. In the absence of mechanical continuity between the two main fragments, maintenance of stability entirely lies with the bridging plate. Wide exposure with periosteal stripping to allow precise fragment reduction and fixation by interfragmentary compression and plating further compromises the vascularity and must be avoided, as it increases the risk of bone-healing complications in type C fractures [2-8]. Mechanistic thinking and technique, together with misapplication and misinterpretation of the principles of interfragmentary compression, are probably responsible for the majority of failures and complications.

Simple metaphyseal fractures (type A) that require fixation are best treated with techniques of absolute stability that provide anatomic reduction and compression with screws and plates. In general, the same principle should be applied to simple metaphyseal fractures with simple articular fractures (C1). However, this technique is not suitable for complex metaphyseal fractures (A1 and A3) or those associated with articular fractures (C2 and C3). Anatomical reconstruction and absolute stability of the joint surface is paramount. The metaphyseal bone, given its good healing qualities, will withstand a higher degree of iatrogenic damage from manipulation than will the diaphysis. The critical area is not the metaphysis but its junction with the more compact bone of the diaphysis. These regions of transition remain under significant bending loads and show a tendency to delayed or failed fracture healing. In the past, liberal use of bone grafting was advocated in attempts to restore the biologic activity that was compromised by the injury and the subsequent treatment.

  • Current plating concepts embrace the principle of placing biology before mechanics.

This development has led to a more flexible and individualistic approach to internal fixation, based on the “personality” of a fracture. The surgeon attempting operative stabilization of a complex multifragmentary fracture must be able to reduce the fracture without further interfering with the blood supply and, at the same time, apply a fixation device that provides adequate fixation to maintain length, alignment and rotation, and produce a biological and mechanical environment that stimulates rapid healing by callus.

Indirect reduction techniques

The femoral distractor is an excellent tool for indirect fracture reduction.

  • Biological or bridge plating is usually applied following some form of indirect reduction.

The goal of indirect reduction is to manipulate fragments into the correct position without opening the fracture site, thus minimizing further damage to the bone blood supply [9-12]. The mechanical principle underlying indirect reduction is distraction. This principle applies to diaphyseal as well as to metaphyseal bone. The muscular envelope surrounding the diaphysis of long bones provides the mechanical environment for indirect reduction, since a controlled pull on the muscle and periosteal attachments of any single fragment tends to align it in the desired way. A muscle envelope under distraction exerts concentric (hydraulic) pressure on the shaft, easing fragments into place. This also holds true for metaphyseal and epiphyseal bone, although the distraction required to align fragments is transferred through capsular tissues, ligaments, tendons, and muscular attachments. This phenomenon, regularly seen as part of nonoperative fracture management, is described by the term “ligamentotaxis”, coined by Vidal [13]. Traction applied by a traction table to an entire limb produces indirect reduction of a fracture. However, the use of an implant or large distractor to a single bone controls reduction more effectively and permits subtle adjustments as well. Indirect reduction techniques with distractor or external fixator and plate can sometimes be combined.

Implant considerations

In biological or bridge plating, the surgeon must study the fracture morphology, carefully plan reduction, and finally choose a plate appropriate to the anatomical location and the configuration of the fracture.

  • The majority of plates can function as a bridging plate.

a  Subtrochanteric fracture (32-C1.1).

b  Indirect reduction with angled blade plate and bridging of fracture zone. Large intermediate fragment deliberately left unreduced. No bone graft.

c  Indirect healing with callus formation 21 weeks postoperatively.

d  Complete reconstitution of cortical continuity and massive stable callus after implant removal at 2 years.

Wave plate allowing for grafting of lateral defect.

If an angled blade plate is used, one has the option of first placing it underneath the muscle and then inserting it into the metaphyseal fragment prior to reduction. Reduction can then be obtained using the plate as a reduction tool.

The common denominator in all bridge plating is the use of a very long plate as a splint on the outside of a bone, in the same way a nail splints the bone from within or an external fixator spans the fracture and holds the bone from the outside. Splinting of complex fractures has been a principle applied by surgeons for many years, but it has only recently been accepted as a principle of plate fixation. The wave plate, with its central curved segment, is a special type of bridging plate and provides three advantages for the treatment of fractures:

  • It reduces interference with the vascular supply of the fracture site by avoiding bone contact.
  • It provides excellent access to the fracture site for application of a bone graft.
  • It alters the load to pure tension forces on the plate.

If there is a fracture gap, stress concentration on one screw hole may lead to fatigue failure.

In practice, both ends of a bridging plate are solidly fixed to a main fragment and the strength of fixation to each main fragment should be balanced. Long plates (about 3 times the length of the fracture zone) bridging an extensive zone of fragmentation with only short fixation on either end of the bone will undergo considerable deformation forces.

  • As bending stresses are distributed over a long segment of the plate, the stress per unit area is correspondingly low, which reduces the risk of plate failure.


In simple fractures, repetitive bending stresses will be concentrated and centered on a short segment of a plate with a high risk of failure. If stress is concentrated on a screw hole, it may break more easily due to fatigue. The incidence of mechanical failure can be considerably reduced if longer plates are used despite short zones of comminution, so that stresses are deliberately distributed over a proportionately longer section of the plate. This is accomplished by fixing the end of the plate over longer segments, well away from the fracture, producing an elastic construction [14, 15]. Furthermore, the bridging concept using plates has been aided by the new principles established for the internal fixator and the developments in plate design, such as LCP and LISS. The internal fixator principle is based on the locking head screws providing angular stability and minimizing the area of contact between plate and bone, thus interfering less with periosteal blood supply while enhancing axial stability. The LCP and LISS also display an even distribution of strength throughout the plate, thereby eliminating stress risers at a screw hole.

Demonstration of “stress concentration” on a strip of plywood.

Demonstration of “stress distribution” on a strip of plywood.

Soft-tissue considerations

Biological plating provides relative stability, preserves vascularity around the fracture and allows controlled micromotion, resulting in more rapid and abundant callus formation, as observed in intramedullary nailing or in nonoperative fracture treatment. However, the success of this operative approach greatly depends on how the surgeon handles the soft tissues and on how well the anatomical characteristics of any given fracture have been taken into consideration during the planning and execution of surgery.

If required, the muscle envelope over the fracture site may be elevated from the intermuscular septum by gentle blunt dissection. The periosteum is left untouched and the perforating vessels are only ligated, if needed. The plate is pushed through a tunnel between muscle and bone. The exposure can safely be extended to control plate position and fracture alignment at either end of the long bridging plates where close contact between bone and plate is necessary. Using implants with locking head screws (eg, LISS or LCP), long incisions are avoided by restricting the exposure to where the plate is anchored to bone [16-18].

  • It is most important not to damage the soft-tissue envelope around the fracture site.  

In the tibia, a plate can be introduced subcutaneously on the medial side. However, care should be taken not to place excessive tension on the delicate overlying skin [19]. For placement lateral to the tibial crest, somewhat more dissection with a sharp elevator is necessary. Screws are easily introduced through stab incisions.

Other areas of application for minimal access plating include the distal femur and the proximal and distal tibia [16]. These locations have distinct anatomical characteristics requiring precision not only in positioning, but also in contouring the plate. New anatomically designed plates, in combination with locking head screws, have improved the ability to apply these techniques in complex areas. The surgeon may find it necessary to combine direct open reduction of the articular components with indirect reduction and submuscular positioning of the plate for associated metaphyseal or diaphyseal fractures. If difficulties occur, a conventional approach is advisable, which still allows careful handling of the soft-tissue cover and minimal exposure of the bone itself. Even when using biological techniques, the surgeon must always be mindful of soft-tissue damage caused by the initial trauma. Hohmann retractors and reduction tools such as Verbrugge forceps should not be used since they leave large tracks and can cause significant soft-tissue stripping and crushing. We recommend pointed reduction forceps, ball spikes, picks, and awls as instruments for bone manipulation, and Langenbeck retractors for the soft tissues.

Bridge plating with the LISS-PLT using MIPO.

a-b  Complex proximal tibial fracture (41-C3) extending into the tibia shaft.

c  Intraoperative photograph showing the limited exposure for the articular reconstruction and the submuscular plate insertion as well as the small incision for the more distal locking head screws.

d-e  Open reconstruction of articular components with independent lag screws. Bridging of the meta-/diaphyseal fracture zone with 14-hole tibia LISS fixed with locking head screws to distal main fragment. The large intermediate butterfly fragment has been reduced by two additional lag screws in AP direction.

f-g  The patient was immediately allowed to freely move the limb and start partial weight bearing of 15–30 kg after 3 weeks. Follow-up x-rays after 1 year.

(With permission by Christoph Sommer.)

In grade III open fractures or closed injuries with considerable soft-tissue contusion, bridge plating is not the first choice for fixing a multifragmentary fracture in an emergency situation. Here, a bridging external fixator or the use of intramedullary nailing may be indicated. Bridge plating techniques are often applied later, when soft-tissue injury has stabilized. The management of difficult fractures is demanding and requires experience, as well as careful planning of options and tactical steps. Major pitfalls are the correct axial and rotational alignment as these can only be judged indirectly.

Closed fracture of the left lower leg (42-B1).

a-b  AP and lateral preoperative x-rays. Length and alignment of the bone are satisfactory. There is minor malrotation.

c-d  AP and lateral postoperative x-rays. Using the distractor for indirect reduction, percutaneous bridge plating of the tibia and intramedullary splinting of the fibula were carried out. No attempt was made to achieve anatomical reduction. Intermediate screws are placed in the main fragments only.

e-f  AP and lateral views at 5 months. The fracture has healed by indirect bone healing. Length and alignment are correct.

g-h  AP and lateral views at 1 year. Remodeling is complete.

Experimental verification

Numerous clinical studies have demonstrated excellent results when applying the biological or bridging technique of plating [10, 16, 18, 20–22]. Animal experiments have been performed to study the effect of biological plating in a multifragment subtrochanteric fracture model in sheep. This was used to compare anatomical reduction to various forms of indirect reduction with subsequent bridge plating [20]. As a result, the group of indirect reduction healed with greater production of bone mass and higher resistance to breaking. This caused a decrease in failure rate.


 Techniques of indirect reduction in combination with bridge plating have proven, experimentally and clinically, to optimize healing in complex, multifragmentary fractures. Direct anatomical reduction and interfragmentary compression to provide absolute stability should be reserved for simple type A fractures with minimal soft-tissue injuries that are not considered suitable for intramedullary nailing. With locking plates , the trend to minimal access surgery continues, so that submuscular tunnelling and plate introduction will be facilitated by new reduction tools, instruments for radiographic and endoscopic viewing and navigated surgery.

  • A prerequisite for successful biological plating is a sound knowledge supported by practical experience in the art of conventional compression plating.


[1] Perren SM (1991) The concept of biological plating using the limited contact-dynamic compression plate (LC-DCP). Scientific background, design and application. Injury; 22(Suppl 1):1–41. Review

[2] Lies A, Scheuer I (1981) Die mediale Abstützung—Bedeutung und Möglichkeiten der Wiederherstellung bei Osteosynthesen. Hefte Unfallheilkunde; 153:243–248.

[3] Loomer RL, Meek R, De Sommer F (1980) Plating of femoral shaft fractures: the Vancouver experience. J Trauma; 20(12):1038–1042.

[4] Lüscher JN, Rüedi T, Allgöwer M (1979) Experiences with plate fixation in 131 femoral shaft fractures. Helv Chir Acta; 45(1-2):39–42.

[5] Magerl F, Wyss A, Brunner C, et al (1979) Plate osteosynthesis of femoral shaft fractures in adults. A follow-up study. Clin Orthop; (138):62–73.

[6] Merchan EC, Maestu PR, Blanco RP (1992) Blade-plating of closed displaced supracondylar fractures of the distal femur with the AO system. J Trauma; 32(2):174–178.

[7] Tscherne H, Trentz O (1977) [Technique of internal fixation and results in comminuted and multifragment fractures of the femoral shaft (collective study by the German Section of AO International) (author’s transl)]. Unfallheilkunde; 80(5):221–230.

[8] Wagner R, Weckbach A (1994) [Complications of plate osteosynthesis of the femur shaft. An analysis of 199 femoral fractures]. Unfallchirurg; 97(3):139–143.

[9] Bolhofner BR, Carmen B, Clifford P (1996) The results of open reduction and internal fixation of distal femur fractures using a biologic (indirect) reduction technique. J Orthop Trauma; 10(6):372– 377.

[10] Baumgaertel F, Gotzen L (1994) [The “biological” plate osteosynthesis in multi-fragment fractures of the para-articular femur. A prospective study]. Unfallchirurg; 97(2):78–84.

[11] Baumgaertel F, Perren SM, Rahn B (1994) [Animal experiment studies of “biological” plate osteosynthesis of multi-fragment fractures of the femur]. Unfallchirurg; 97(1):19–27.

[12] Tepic S, Remiger AR, Morikawa K, et al (1997) Strength recovery in fractured sheep tibia treated with a plate or an internal fixator: an experimental study with a two-year follow-up. J Orthop Trauma; 11(1):14–23.

[13] Vidal J (1979) External Fixation: Current State of the Art. Brooker HS, Edward CC (eds), Treatment of articular fractures by “ligamentotaxis” with external fixation. Baltimore: Williams & Walkins.

[14] Schmidtmann U, Knopp W, Wolff C, et al (1997) [Results of elastic plate osteosynthesis of simple femoral shaft fractures in polytraumatized patients. An alternative procedure]. Unfallchirurg; 100(12):949–956.

[15] Stürmer KM (1996) [Elastic plate osteosynthesis, biomechanics, indications and technique in comparison with rigid osteosynthesis]. Unfallchirurg; 99(11):816–817.

[16] Krettek C, Schandelmaier P, Miclau T, et al (1997) Transarticular joint reconstruction and indirect plate osteosynthesis for complex distal supracondylar femoral fractures. Injury; 28(Suppl 1):A31–A41.

[17] Krettek C, Schandelmaier P, Miclau T, et al (1997) Minimally invasive percutaneous plate osteosynthesis (MIPPO) using the DCS in proximal and distal femoral fractures. Injury; 28(Suppl 1):A20–A30.

[18] Wenda K, Runkel M, Degreif J, et al (1997) Minimally invasive plate fixation in femoral shaft fractures. Injury; 28(Suppl 1):A13–A19.

[19] Helfet DL, Shonnard PY, Levine D, et al (1997) Minimally invasive plate osteosynthesis of distal fractures of the tibia. Injury; 28(Suppl 1):A42–A48.

[20] Heitemeyer U, Kepmer F, Hierholzer G, et al (1987) Severely comminuted femoral shaft fractures: treatment by bridging-plate osteosynthesis. Arch Orthop Trauma Surg; 106(5):327–330.

[21] Kregor P, Stannard J, Zlowodzki M, Cole P (2004) [Treatment of distal femur fractures using the Less Invasive Stabilization System: Surgical experience and early clinical results in 103 fractures]. J Ortho Trauma; 18(8):528-520.

[22] Cole P, Zlowodzki M, Kregor P (2004) [Treatment of proximal tibia fractures using the Less Invasive Stabilization System: Surgical experience and early clinical results in 77 fractures]. J Ortho Trauma; 18(8):528-535.



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