Background: While stress radiography has been used to objectively determine the limits of posterior tibial displacement in knees with posterior cruciate ligament tears, the magnitude and distribution of posterior tibial translation has not been defined in a large population of patients with this injury.

Methods: A retrospective diagnostic study of 1041 consecutive patients with posterior cruciate ligament tears was done. Posterior tibial displacement values that were obtained with use of instrumented stress radiography with the knee held in 90° of flexion in the Telos device were evaluated and compared with the values from relevant cadaveric dissection studies.

Results: The mean amount of posterior tibial displacement on stress radiographs was -11.58 ± 4.31 mm (range, -5 to -30 mm). There was a displacement peak in the range of -9 to -12 mm, with 37.9% of patients exhibiting posterior laxity within this range. Traffic-related injuries were associated with significantly greater displacement values than were sports-related injuries (p < 0.001). Grade-I or II instability (12 mm of posterior tibial displacement) occurred in association with 68.7% of the sports-related injuries, compared with 54.1% of the traffic-related injuries (p < 0.001). The mean amount of posterior tibial displacement on the intact side was -1.31 ± 1.85 mm (range, -6 to 4 mm).

Conclusions: Instrumented stress radiography is a useful testing method for objectively determining the amount of posterior tibial displacement of the knee in adults with a posterior cruciate ligament injury. Absolute posterior tibial displacement in excess of 8 mm is indicative of complete insufficiency of the posterior cruciate ligament. With tibial displacement exceeding 12 mm, additional injury of secondary restraining structures should be considered. We recommend the use of stress radiography to grade and classify posterior knee laxity.

Level of Evidence: Diagnostic Level IV. See Instructions to Authors for a complete description of levels of evidence.