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Development of a mechanical ‘pivot shift' knee stability test
  1. M P Sena,
  2. D Coughlin,
  3. R Dellamaggiora,
  4. J C Lotz,
  5. B T Feeley
  1. University California San Francisco, San Francisco, California, USA

Abstract

Anterior cruciate ligament (ACL) injuries are common in sports, with 135 000 ACL tears leading to over 95 000 annual reconstructions in the US. The manual ‘pivot shift' test is the most specific clinical examination for diagnosis of ACL injuries. The presence of a ‘pivot' (anterior/internal tibial subluxation followed by a rapid reduction or ‘clunk') postoperatively correlates with poor patient outcome. Unfortunately, the pivot shift is semi-quantitative and difficult to perform reproducibly. We developed a mechanical pivot shift device (MPSD) that mimics this test through the application of standardised dynamic loads. Manual and mechanised pivot shift tests were performed on right and left cadaveric knees (n=4) in intact and ACL-deficient states. The resultant tibiofemoral motion was recorded using the Optotrak® navigation system (Manual and MPSD tests produced similar kinematic trajectories, with subluxation/reduction occurring at ∼20° /35° for ACL-deficient knees. Following ACL transection, both tests produced increased instability metrics: maximum tibial internal rotation (IRmax), anterior displacement, and posterior translational velocity (PTV). The coefficient of variation (cv=σ/μ) for these metrics was significantly lower using the MPSD (cv=0.03) in comparison to the manual test (cv=0.16). Between right and left knees, MPSD metrics differed by less than 1.5° and 2 mm. These results demonstrate that our device can produce a realistic ‘pivot shift' with a high degree of reproducibility. Logistic regression revealed IRmax as the best single predictor of intact versus deficient ACL state (R2=0.41, p=0.0001), while IRmax and PTV combined predicted knee state with complete accuracy (R2=1, p=0.0001). These data suggest that measurements made using the MPSD are sensitive to clinically relevant ACL condition and may provide kinematic signatures specific to graded soft tissue defects of the knee joint, or to different ACL reconstruction techniques.

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