General discussion 123 General discussion Sixty years ago, a group of 13 Swiss surgeons founded the Arbeitsgemeinschaft für Osteosynthesefragen [Association of Osteosynthesis] (AO) with the aim of improving fracture care.1 Since the 60 years of its foundation, the AO's impact on science, education, patient care, and the MedTech business has been significant. The main principles of aftercare treatment during this period in surgically treated trauma patients with peri- and intra-articular fractures of the lower extremities has been historically non- or restricted weight bearing for 6-12 weeks.2 This non- or restricted weight bearing protocol was based on clinical reasoning. The reason for this non or restricted weight bearing protocol was the surgeons’ hesitation in allowing early weight bearing after lower extremity fractures in order to limit risks of loss of reduction and implant failure. Studies evaluating this non- or restricted weight bearing protocols are, however, lacking. More recently there has been a renewed clinical interest in earlier start of weight-bearing according to the patient’s tolerated pain, feeling of instability, or redness and swelling at the site of the fracture. The term for this aftercare treatment is “permissive weight bearing”. Biological advantages of a permissive weight bearing regimen over restricted weight bearing are plentiful, both of which have observed in biomechanical human and animal studies.3,4 Wolff’s law5 states that bone responds to the mechanical stresses applied to it, allowing it to strengthen over time. At a cellular level, appropriate strain in the fracture gap leads to optimum cellular proliferation of osteoblasts, which orient themselves according to their mechanical environment and other cell lines involved in bone formation.6 Several theories exist on the mechanical influences on fracture healing. Two more important ones, perhaps somewhat connected to eachother are Perren’s classical strain theory (tissue formed in a fracture gap is dictated by the degree of local motion/strain that occurs between the surfaces of the bone) and Claes and Heigele’s work (different types of bone formation in fracture healing according to local mechanical influences; bone can be formed under tension or compression, but in either case it needs some sort of external mechanical influence/load in order to promote the fracture healing process).6 Numerous studies have shown that an absence of motion at the fracture gap leads to significantly reduced callus volume and slower formation. Kenwright and Gardner7 most notably have demonstrated the importance of interfragmentary motion and loading on callus formation in relation to time, showing that early reduced motion led to both lower volume and quality of callus. These advantages span beyond the sole improvement of bone health. In one prospective randomized study, one year after surgery, patients who had undergone anti-gravity treadmill rehabilitation in the first six weeks postoperatively showed better gait than patients in the control group, and those with tibial plateau fractures had less muscle atrophy.8