title: The Tibial Slope in Patients With Achondroplasia: Its Characterization and Possible Role in Genu Recurvatum Development.
abstract: BACKGROUND
  Genu recurvatum, a posterior resting position of the knee, is a common lower extremity deformity in patients with achondroplasia and has been thought to be secondary to ligamentous laxity. To the best of our knowledge, the role of the tibial slope has not been investigated, and no studies describe the tibial slope in patients with achondroplasia. Our goals were to characterize the tibial slope in children and adults with achondroplasia, explore its possible role in the development of genu recurvatum, and compare the tibial slope in patients with achondroplasia to that in the general population.
  
  
  METHODS
  We reviewed 252 lateral knee radiographs of 130 patients with achondroplasia seen at our clinic from November 2007 through September 2013. Patients were excluded if they had previous lower extremity surgery or radiographs with extreme rotation. We analyzed patient demographics and, on all radiographs, the tibial slope. We then compared the mean tibial slope to norms in the literature. Tibial slopes >90 degrees had an anterior tibial slope and received a positive prefix. Statistical analysis included intraclass and interclass reliability, Pearson correlation coefficient, and the Student t tests (significance, P<0.05).
  
  
  RESULTS
  The overall mean tibial slope for the 252 knees was +1.32±7 degrees, which was significantly more anterior than the normal slopes reported in the literature for adults (7.2 to 10.7 degrees, P=0.0001) and children (10 to 11 degrees, P=0.0001). The Pearson correlation coefficient for mean tibial slope and age showed negative correlations of -0.4011 and -0.4335 for left and right knees, respectively. This anterior tibial slope produces proximal and posterior vector force components, which may shift the knee posteriorly in weightbearing.
  
  
  CONCLUSIONS
  The mean tibial slope is significantly more anterior in patients with achondroplasia than in the general population; however, this difference diminishes as patients' age. An anterior tibial slope may predispose to a more posterior resting knee position, also known as genu recurvatum.
  
  
  METHODS
  Level IV-retrospective case series.
authors.affiliation: *Department of Orthopaedic Surgery, The Johns Hopkins University, Baltimore, MD †Department of Orthopaedic Surgery, Washington University School of Medicine, St Louis, MO.
authors.forename: Jaysson T
authors.initials: JT
authors.lastname: Brooks
authors.forename: David L
authors.initials: DL
authors.lastname: Bernholt
authors.forename: Kevin V
authors.initials: KV
authors.lastname: Tran
authors.forename: Michael C
authors.initials: MC
authors.lastname: Ain
mesh_terms: D000130:Achondroplasia / Q000000981:diagnostic imaging* / Q000503:physiopathology; D000293:Adolescent; D000328:Adult; D000368:Aged; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D007223:Infant; D007719:Knee Joint / Q000002:abnormalities / Q000000981:diagnostic imaging* / Q000503:physiopathology; D038061:Lower Extremity Deformities, Congenital / Q000000981:diagnostic imaging* / Q000503:physiopathology; D008297:Male; D008875:Middle Aged; D011859:Radiography; D015203:Reproducibility of Results; D012189:Retrospective Studies; D012399:Rotation; D013977:Tibia / Q000002:abnormalities / Q000000981:diagnostic imaging*; D016474:Weight-Bearing / Q000502:physiology; D055815:Young Adult