AO philosophy and evolution


Thomas P Rüedi, Richard E Buckley, Christopher G Moran

AO philosophy

The philosophy of the AO (Arbeitsgemeinschaft für Osteosynthesefragen/Association for the Study of Internal Fixation) has remained consistent and clear, from its inception in 1958—as a small group of Swiss colleagues and friends—to its current status as a worldwide surgical and scientific foundation and community. Advances in basic science and technology, together with growing clinical expertise, have resulted in many changes in the implants, instruments, and techniques used in trauma surgery. However, the basic philosophy of care remains the same today as in 1958 when AO was founded.

  • Focus is on patients with musculoskeletal injuries and related disorders. The aim is to provide care that allows an early return to function and mobility. This was the guiding principle of the founding group and remains the central mission and goal of today’s AO Foundation.


In the first half of the 20th century, fracture management mainly focused on the restoration of bone union and the prevention of infection. The methods employed to treat fractures, mostly by immobilization in plaster or traction, inhibited rather than promoted function throughout the healing period. The key concept of the AO was to provide safe open reduction and stable internal fixation of fractures while protecting soft tissue and allowing early functional rehabilitation.

Long before the establishment of the AO, the value of operative fixation of fractures had been recognized. Early advocates included the Lambotte brothers, Elie and Albin, Robert Danis, Fritz König, William O’Neill Sherman, William Arbuthnot Lane, Gerhard Küntscher, Raoul Hoffmann, and Roger Anderson. However, their ideas and innovations were not widely adopted because great obstacles needed to be overcome. The list of technical, metallurgical, and biological obstacles was formidable; especially the risk of infection, which in those times usually resulted in amputation. In addition, peer group skepticism often amounted to real hostility. Innovations, such as stable internal fixation by Albin Lambotte, advances in intramedullary nails by Gerhard Küntscher, and the introduction of early motion (albeit in traction) by Lorenz Böhler or Jean Lucas-Championnière and his disciple George Perkins, were restricted by the inability to reconcile two key concepts within one pattern of care: effective splinting of the fracture and controlled mobility of the joints.

a  Albin Lambotte’s (left) first application of his original model of external fixator (1902).

b  Robert Danis (1880–1962).

c  Gerhard Küntscher (1900–1972) instructing Finnish surgeons on a return visit in 1954.

d  Lorenz Böhler receives first AO Manual as a present from Hans Willenegger in Vienna, at his 85th birthday celebration.

The role of the AO

What was needed—and what the AO provided—was a coordinated approach to identify these obstacles, to study the difficulties they caused, and to set about overcoming them. The chosen path was to investigate and understand the relevant biology, to develop appropriate technology and techniques, to document the outcomes and react to the findings, and, through teaching and writing, share whatever was discovered.

This enormous challenge was triggered by an apparently small issue. In the 1940s and 1950s questions were being asked, not least by the Swiss workman’s compensation board insurance company, why it took some fractures 6–12 weeks to heal but 6–12 months for patients to return to work.

Robert Danis, first through his writings and later by a personal visit, inspired Maurice E Müller and the early AO group including Martin Allgöwer, Robert Schneider, and Hans Willenegger. The essence of Danis’ observation was that healing without callus took place if he used a compression device to impart absolute stability to a diaphyseal fracture. During healing, the adjacent joints and muscles could be exercised safely and painlessly.

Inspired by this concept and driven by a determination to apply it clinically, and establish how and why it worked, Müller and the AO group set in motion a process of surgical innovation, technical development, basic research, and clinical documentation. This progressed as a campaign to improve the functional outcome and minimize the problems and complications of fracture care. The group propagated their message by writing and teaching, and by developing innovative courses to teach their principles and surgical techniques. That work continues to this day, involving many specialist groups working for the common goal of improving trauma care worldwide.

a  Early AO Course (1960) with Maurice Müller instructing.

b  First AO operating room personnel (ORP) Course (1960).

Original AO principles

Today the key concepts—the AO principles—are remarkably similar to the early AO publications from 1962 onwards. The essential feature, now as then, is the proper management of the patient and the fracture. This requires a complete understanding of patient and fracture factors that influence treatment and outcome.

The original management objectives were (1) restoration of anatomy, (2) stable fracture fixation, (3) preservation of blood supply, (4) early mobilization of the limb and patient. These were at first presented as the fundamentals of good internal fixation. However, with increased understanding of the importance of soft tissues, the biomechanics of fixation, and how fractures heal, they have undergone certain conceptual changes to become the overall principles of fracture management and not just internal fixation.

Central to the AO’s concepts was the understanding that articular fractures and diaphyseal fractures have very different biological requirements and the recognition that the type and timing of surgical intervention must be guided by the degree of injury to the soft-tissue envelope as well as the physiological demands of the patient.

Progress and development

The AO principles relating to anatomy, stability, biology, and mobilization still stand as fundamentals. However, it is now accepted that the pursuit of absolute stability, originally proposed for almost all fractures, is mandatory only for joint and certain related fractures, provided it can be obtained without damage to the blood supply and soft tissues. Within the diaphysis, length, alignment, and rotation must be restored but anatomical reduction is not necessary. When fixation is required, nails are often used to provide relative stability and this usually leads to union by callus. Even when the clinical situation favors the use of a plate, proper planning and meticulous surgical technique should minimize any insult to the blood supply of bone fragments and soft tissues. New developments in minimal access surgery will take this a step further.

a-b  Complex, high-energy forearm fracture.

c-d  Fixation of the ulna with a bridging plate providing relative stability, and fixation of the radius with a compression plate providing absolute stability. Length, alignment, and rotation are restored with anatomical reduction of the radioulnar joints. The ulna will heal with callus formation and there will be primary bone healing of the radius. Normal forearm function should be restored.

It is now appreciated that simple diaphyseal fractures react differently to plating and to nailing: If plating is employed, absolute stability must be achieved. In contrast, multifragmentary fractures can be treated by splinting—providing relative stability—either with an intramedullary nail, external fixation, or bridge plating using the internal fixator principle. Articular fractures demand anatomical reduction and absolute stability to enhance the healing of articular cartilage and make early motion possible. The principle of the internal fixator, which was introduced by Stephan Perren and Slobodan Tepic with the PC-Fix in 1985, has now evolved to the locking compression plate (LCP). The use of locking head screws, providing angular stability and preventing the plate from being pressed against the bone surface, has radically changed the concept of plating but requires a new understanding and interpretation of the AO principles.

a  Diaphyseal fracture of the femur with associated partial articular fracture of the lateral condyle.

b  Locked femoral nail providing relative stability and restoring length, alignment, and rotation. Anatomical reduction and lag screw fixation of the articular fracture.

c  Relative stability of the femoral shaft results in healing by callus. Absolute stability of the articular fracture results in bone healing without callus.

Good soft-tissue care is essential. This preserves the blood supply to the bone and must be addressed in every phase of fracture management. A thorough assessment of the fracture pattern and related soft-tissue injuries will lead to the formulation of a preoperative plan, including the surgical approach, reduction technique (direct or indirect), the type of fixation, and the choice of implant, compatible with the biological and functional demands of the fracture and the patient.

AO philosophy and principles: today and in future

In earlier days, the AO principles appeared in a succinct, even dogmatic format with the goal of improving functional outcome. Fracture care has become a structured process, based upon good science, sound technology, and supported by research and clinical studies. With this foundation, the future of trauma care is promising. Locking plates and the internal fixator principle will evolve rapidly over the next few years. Computer-aided navigation may revolutionize trauma surgery, increase accuracy and safety, and lead to further expansion of minimal access surgery. Biotechnology will allow surgeons to influence fracture healing, perhaps reducing the time to union and preventing nonunion due to “biological failure”. However, even with these advances, the AO principles remain as valid today as they were nearly 50 years ago when the AO group was formed. These principles form the basis of this textbook and will remain the key principles of fracture care for the foreseeable future.

AO principles:

  • fracture reduction and fixation to restore anatomical relationships;
  • fracture fixation providing absolute or relative stability as the “personality” of the fracture, the patient, and the injury requires;
  • preservation of the blood supply to soft tissues and bone by gentle reduction techniques and careful handling;
  • early and safe mobilization and rehabilitation of the injured part and the patient as a whole.

Suggestions for further reading

Böhler L (1957) Technik der Knochenbruch Behandlung 12.-13. Auflage. Wien: W. Maudrich.

Danis R (1947) Théorie et pratique de l’ostéosynthèse, Paris: Masson Ed.

Lambotte A (1913) Chirurgie opératoire des fractures. Paris: Masson Ed.

Lucas-Championnière J (1907) Les dangers de l’immobilisation des membres—fragilité des os—altérnation de la nutrition de la membre— conclusions pratiques. Rev Med Chur Pratique; 78:81–87.

Müller ME, Allgöwer M, Willenegger H (1965) Technique of Internal Fixation of Fractures. Berlin Heidelberg New York: Springer-Verlag.

Schlich T (2002) Surgery, Science and Industry; A Revolution in Fracture Care, 1950s–1990s. Hampshire and New York: Palgrave Macmillan.



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