Posterolateral stabilizers of the knee: anatomy and injuries assessed with MR imaging.

T he posterolateral aspect of the knee contains several ligamentous and tendinous structures that are the primary restraints against varus angulation and external rotation of the tibia. These structures are the fibular collateral ligament (FCL), the arcuate ligament, the popliteus tendon, the popliteofibular ligament, and the fabellofibular ligament [1]. Hughston et al. [2] refer to the first three structures collectively as the arcuate complex. In addition to these static stabilizers, dynamic stabilization of the posterolateral corner of the knee is provided by the popliteus, biceps femoris, and lateral gastrocnemius muscles [2]. Injury to the posterolateral structures may lead to posterolateral rotatory instability of the knee, in which the lateral tibial plateau rotates externally and subluxates posteriorly [3]. Acutely, patients may complain of pain in the posterolateral aspect of the knee and chronically may describe the sensation of the knee giving way in extension [1]. Such injuries often accompany anterior cruciate ligament (ACL) and posterior cniciate ligament (PCL) tears [1] but may be subfle and miSsed by the examiner [2]. Clinically unrecognized posterolateral injury has been suggested as a cause of ACL graft failure [4] as well as instability after PCL repair [5]. Materials and Methods We dissected the posterolateral aspect of an embalmed cadaveric knee obtained from our medical school’s department of anatomy. After identifying and photographing the posterolateral ligaments, we coated them with 120% w/v barium paste (E-ZPaste; E-Z-EM, Westbury, NY) and radiographed the specimen using a detail extremity film-screen combination with 6.4 mA and 60 kV. We also reviewed the reports of all MR imaging examinations of the knee peiformed at our institution over a 2-year period, looking forcases ofinjury to the posterolateral Stabilizers. All Studies were performed on l.5-T units (Signa; General Electric Medical Systerm, Milwaukee, WI, or Gyroscan; Philips Medical Systems, Shelton, CI’) using dedicated extremity coils. The routine protocol consisted of sagiual Spinecho Tl-weighted images (TR rangeiTE range, 500600/15-20), gradient-echo T2-weighted images (600-875/15-20; flip angle, 30#{176}), axial spin-echo Ti-weighted images (500-600/15-20), fat-suppressed fast spin-echo 12-weighted images (17004500/50-90; echo train length, eight), and coronal fat-suppressed fast spin-echo proton densityweighted images (1700-3500/18-40; echo train length, eight). Fat suppression was achieved by frequency-selective presaturation on the Signa unit and spectral inversion recovery on the Gymscan unit. Field of view was 16-20 cm and one to two excitetions were performed. Slice thickness for the sagittal and T2-weighted axial sequences was 3 mm, with no interslice gap on the Sigma unit and a gap ofO3 mm on the Gyroscan unit; the slice thickness on the coronal images was 3-4 mm for both units, with no gap on the Signa unit and a gap ofO.3-0.4 mm on the Gyroscan unit. Matrix size was 256 x 192-256 and 205 x 256 for the Signa and Gyroscan units, respectively.