Excessive laxity in one or multiple anatomic planes is related to increased risk of graft rupture following ACL reconstruction . However, the physical examinations used to assess knee laxity, which include uniplanar tests of anterior-posterior (AP), varus-valgus (VV), and internal-external rotation (IER) laxities and the multiplanar pivot shift exam, are qualitative and subjective and therefore are examiner dependent and lack precision . Therefore, we have designed and developed a novel measurement instrument to address this clinical need. This study had three goals: 1) to describe the design of this instrument; 2) to assess its safety; and 3) to quantify its reliability in both uniplanar assessments of AP, IER and VV laxities and multiplanar assessments of knee laxity simulating the clinical pivot shift exam.
Regarding goal one, the custom-designed arthrometer utilized an instrumented linkage with five degrees of freedom (DOF) and a six-axis force-torque sensor (Mini58, ATI, Inc). To conduct a test, the subject sat reclined in a chair, the leg was aligned in the arthrometer at 20° of flexion, the femur was fastened to the chair, and the tibia was affixed to the arthrometer. The examiner manually applied AP forces (-50 N posterior to 135 N anterior), VV moments (±4 Nm), and IER moments (±2.5 Nm) to the tibia and the corresponding translations and rotations were recorded. The examiner also performed a simulated clinical pivot shift exam, beginning with a 2 Nm external rotation moment, followed by a 6 Nm valgus moment, a 2 Nm internal rotation moment, and, finally, a 50 N anterior force. Anterior tibial translation (ATT) was measured in response to these multiplanar loads. With IRB approval, 15 healthy, uninjured volunteers (8 males, 7 females; mean age: 28 ± 6 years) were consented into the study. Concerning goal two, pain was assessed on a visual analog scale from 0 (no pain) to 10 (excruciating pain). Regarding goal three, two examiners conducted two independent tests on both knees of each volunteer. Reliability within (intra) and across (inter) examiners was characterized via intra-class correlation coefficients (ICC).
All volunteers successfully completed the protocol; each leg required an average of 8 ± 3 minutes to test. Pain was minimal averaging 0.2 ± 0.1. For AP, IER, and VV laxity, intra-examiner reliabilities ranged from good (0.66 in AP) to excellent (0.92 in VV). In all three directions, inter-examiner reliabilities were good (all ICC’s = 0.62). For the simulated pivot shift, intra- and inter-examiner reliabilities ranged from fair (0.56) to good (0.75).
We successfully designed and developed a safe and reliable arthrometer to assess laxity in one or more planes. Our novel measurement instrument will allow us to more precisely quantify the association between laxity and risk of ACL graft tear, to more accurately diagnose injury to the ACL and surrounding soft tissues, to refine indications for adjunctive treatments (such as lateral augmentation surgery), and to objectively assess clinical success of reconstructive surgery.
REFERENCES:  Magnussen 2016 AJSM  Opstelten 2007 Australian J Physiotherapy