Injuries of the posterior cruciate ligament.

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Abstract
Although the anterior cruciate ligament has received much attention in the orthopaedic literature, the posterior cruciate ligament has not attracted similar interest. With the exception of avulsion fractures [106], injuries of the posterior cruciate ligament have almost universally been treated non-operatively in the general orthopaedic community, which has left this problem outside the mainstream of operative orthopaedics. Recent studies, however, have shed new light on the natural history of injury to the posterior cruciate ligament, as well as on its complex anatomy and functional mechanical behavior. The new information is likely to alter the way that orthopaedic surgeons have traditionally treated injuries to this structure. The posterior cruciate ligament is surrounded by a synovial sleeve. The ligament has an average length of thirty-eight millimeters and an average width of thirteen millimeters, and the cross-sectional area decreases from the proximal to the distal attachments [38,45]. In about 70 per cent of knees, there is either an anterior meniscofemoral ligament of Humphrey or a posterior meniscofemoral ligament of Wrisberg; the latter is more common and is characterized by a femoral origin merging with that of the posterior cruciate ligament [38,47]. These meniscofemoral ligaments may play a minor role as secondary restraints to posterior tibial translation after complete transection of the posterior cruciate ligament [20]. Traditionally, the posterior cruciate ligament has been described as consisting of two parts, or so-called bands, variously called anterior or anterolateral and posterior or posteromedial [1,8,31,49,90,94,110]. A number of authors have noted that the large cross-sectional anterior-fiber group tightens with flexion of the knee and relaxes with extension, while the posterior-fiber group, which has a much smaller cross-sectional area, is lax with flexion and tightens with extension [38,49,53,110]. In a recent study of the functional anatomy of the posterior cruciate ligament, O'Brien et al [81] also described the ligament in terms of two fiber bundles: a posterior-oblique bundle, consisting of a small (approximately 5 per cent of the substance of the ligament) but distinctly recognizable group of fibers that run obliquely across the posterior surface of the ligament, originating at the posterosuperior aspect of the femoral attachment and inserting posterolaterally on the tibia; and an anterior bundle, representing the remaining 95 per cent of the ligament. Characterization of the macroscopic anatomy of the posterior cruciate ligament as comprised of only two reciprocally functioning parts is perhaps too simplistic. Kurosawa et al [64] and Trent et al [107] believed that the ligament is best described in terms of anterior, middle, and posterior fiber bundles according to the location of the femoral insertion. Recent studies of the macroscopic and functional anatomy have characterized the ligament as a continuum of fibers without truly separate bands or bundles [23,36]. In order for surgeons to understand the complex anatomy of this continuum of fibers, an anatomical subdivision scheme, consisting of so-called fiber-regions, has been proposed on the basis of functional as well as morphological criteria [23]. The four consistent geographical fiber-regions (as opposed to separate bands) have been called anterior, central, posterior longitudinal, and posterior oblique, mainly on the basis of the orientation of the fibers, the mechanical behavior during motion of the joint, and the osseous sites of insertion [23]. Most of the substance of the posterior cruciate ligament is made up of the anterior and central fiber-regions, with the combined posterior-longitudinal and posterior-oblique fibers accounting for only 10 to 15 per cent of the mass of the ligament [23] (Fig. 1).