The envelope of motion and Protaper Next

They found that utilisation of standard instruments in either reaming and/or filing produced preparations that were irregular in shape, and were not continuously tapering. The narrowest part of the canal, the so-called ‘elbow’, was located at a point coronal to the apex or foramen. In addition, the foramen often displayed transportation, which was called the ‘apical zip’. These characteristics were felt to result from the elastic memory of instruments and a predilection to straighten as they are migrated around curves. To alleviate this problem, Weine suggested removing the flutes from the outer surface of pre-curved files. Coffae and Brilliant corroborated the work of Schilder, demonstrating that tapering preparations were more efficacious in the removal of debris from the root canal system when compared to parallel preparations (1976). They also demonstrated that the serial use of files in a step-back modality were more effective in producing tapering shape. Abou Rass et al (1980) also engaged in a discussion of anti-curvature filing to minimise the problems described by Weine. This method, however, advocated the removal of conspicuous amounts of tooth structure from the outer walls of the curve of a root canal system, which, arguably, would weaken the outer wall. In another attempt to maintain the contour of the canal without transporting the apical foramen, Roane, Sabala and Duncanson described a technique for root canal preparation called ‘balanced force’ (1985). The technique was a variation of reaming, which included ‘back-turning’ the file in a counter-clockwise direction. Purportedly the restoring force or elastic memory of the file, as described by Weine, was overcome when pitted against dentinal resistance. However, Blum, Machtou and others found that these techniques were a predisposing factor to instrument breakage (1997). Walia, Brantley and Gerstein were the first experimenters to discuss the use of nickel-titanium rotary instruments in endodontics, which has changed the landscape of endodontic cavity preparation immeasurably (1988). The earliest investigators including Glosson and colleagues and Esposito and Cunningham, suggested that nickel-titanium rotary instruments were superior to hand instrumentation in maintaining the original anatomy and required fewer instruments (1995; 1995). However, Schäfer (1999) found that nickel-titanium instruments with traditional crosssections and sizes left all curved canals poorly cleaned and shaped, whereby tooth structure was removed almost exclusively from the outer-wall of the curve (1999). Later, more studies stated that the greatest failing of current Herbert Schilder was the first clinician to provide a detailed discussion of the root canal preparation, referring to the procedure as cleaning and shaping, and outlined specific design objectives, which included a continuously tapering shape, maintenance of the original anatomy, an apex that is as small as practical, and conservation of tooth structure (1974). This continuously tapering space was acquired using hand instrumentation with alternating reamers and files. Each instrument was pre-curved, which dictated alternate or intermittent contact with the canal walls and created what Schilder called an ‘envelope of motion’. This intermittent contact produced not only continuously tapering shapes, but minimised both the transportation of the original canal and the opportunity for instrument breakage. Close examination of Schilder’s envelope of motion reveals that although these instruments rotated axially, they cut along a precessional axis, much like a spinning top (Figure 1). Weine, Kelly and Lio used clear acrylic blocks to evaluate the effectiveness of various instrumentation techniques, but their conclusions were somewhat disconcerting (1975).