Non-Invasive, Non-Radiological Quantification of Anteroposterior Knee Joint Ligamentous Laxity

Non-invasive, non-radiological quantification of anteroposterior knee joint ligamentous laxity

RUSSELL DF1, DEAKIN AH1, FOGG QA2, PICARD F1

1Golden Jubilee National Hospital, Clydebank, UK 2Laboratory of Human Anatomy, University of Glasgow, Glasgow, UK [email protected]

Introduction: As Computer Assisted Orthopaedic Surgery continues to develop in its range of application and gains support from clinical evidence, it is important to scientifically evaluate new technologies, especially in terms of precision and accuracy. Conventional computer navigation systems using bone fixation of trackers have been validated in measuring anteroposterior (AP) translation of the tibia and are used in computer assisted cruciate ligament reconstruction. Recent developments in non-invasive skin- mounted systems may allow quantification of AP laxity in the out-patient setting. Following a pilot study comparing different methods of non-invasive tracker fixation [1] we have refined an experimental protocol to validate the non-invasive system using fresh cadaveric materials and standardization of force application. The primary aim of this study was to evaluate the precision and accuracy of a non-invasive image-free navigation system by comparison with a validated invasive system routinely used in operative practice. The secondary aim of the study was to analyse the effect of flexion on precision and agreement between the invasive and non-invasive systems. Materials and Methods: 12 lower limbs from 10 cadaveric specimens were used for this study. Average age of the specimens was 81.1y (range 65-91y). During the testing process, the cadaveric specimens remained entirely intact and in a supine position. The lower limb being tested was suspended from the femur and foot to maintain knee flexion at discrete intervals and minimize the soft tissue artefact which occurs when the limb rests on a surface. This was achieved by a pin inserted into the anterior femur and a strap around the foot. A commercial image free navigation system using passive trackers secured by bone screws was used to measure AP translation of the tibia. Registration of each cadaveric limb was carried out using a novel workflow based on existing commercial high tibial osteomy and cruciate ligament reconstruction software. The experimental protocol involved applying 100N of force perpendicular to the longitudinal axis of the tibia using a transducer secured to the tibial tuberosity. This force was applied at 10° knee flexion intervals from full extension to 60° flexion. The experiment protocol was then repeated using the same tracker fixation and registration data for the limb in order to collect a set of repeated measurements from the invasive system. Bone screw mounting for the passive trackers was removed and a non-invasive fabric strap system used to re-attach the same passive trackers to the limb. This method of securing trackers non-invasively has been validated in previous studies [1&2]. The same experiment protocol was repeated to collect a set of repeated measurements using the non-invasive system. Repeatability coefficients were calculated to reflect precision within each system [3]. Limits of agreement were also calculated to convey agreement between the measurements from the invasive and non-invasive systems [3]. The image-free navigation system has a precision of 1˚ when repeatedly measuring a fixed point [4]. We therefore accept a repeatability coefficient of ≤2mm as demonstrating excellent precision, and limits of agreement of ≤2mm as demonstrating excellent agreement between the invasive and non-invasive systems. Acceptable repeatability coefficient and limits of agreement of ≤3mm were set based on diagnostic criteria for ACL insufficiency when using other mechanical devices to measure AP tibial translation [5].

13th Annual Meeting of the International Society for Computer Assisted Orthopaedic Surgery Orlando, FL, USA, June 12-15, 2013

Results: 11 of the 12 lower limbs demonstrated a degree of flexion contracture; mean flexion contracture 6.8˚, range 0˚ - 15˚. Precision within the individual invasive and non-invasive systems measuring AP translation of the tibia was excellent throughout the range of flexion tested (repeatability coefficient ≤1.6mm) (Figure 1a). Agreement between the two systems was acceptable when measuring AP laxity

13th Annual Meeting of the International Society for Computer Assisted Orthopaedic Surgery Orlando, FL, USA, June 12-15, 2013 between full extension and 40° knee flexion (limits of agreement ≤2.9mm). Beyond 40° of flexion, agreement between the systems was unacceptable (limits of agreement >3mm) (Figure 1b). Discussion: This study validated for the first time precision of a non-invasive system for measuring AP laxity in the early functional range of knee flexion, and agreement between the non-invasive and invasive methods in an in-vitro setting. The results highlight the important effect of knee flexion on accuracy of this non-invasive method, despite it displaying acceptable precision levels throughout flexion in this, and in a previous study [1]. This study is limited by use of a cadaver in-vitro model which soft tissue artefact, joint hydration and laxity differs from in vivo studies. The study does however improve on previous work by increasing specimen number, use of fresh cadaveric material with improved tissue quality, and standardisation of force application [1]. Further work should be carried out to validate the non-invasive system in an in-vivo setting. Most authors advocate the Lachman test be carried out at 20˚. The non-invasive device has been demonstrated to be precise and accurate up to 40˚ knee flexion in this in-vitro study. Assessment of cruciate ligament integrity is possible using modalities such as arthroscopy and magnetic resonance imaging, however these methods cannot be used for standardised assessment of AP stability [4]. Non- invasive navigation technology could be used in the out-patient setting in conjunction with a method of quantified force application to standardise diagnostic grading of cruciate ligament laxity, and post- operative evaluation of graft performance & integrity; providing a quantitative means for evaluation of operative techniques. In conclusion, non-invasive, non-radiological quantification of AP tibial translation is precise and accurate in an in-vitro setting in the early range of knee flexion relevant to clinical testing and functional dynamic stability. Definitive measurement of cruciate ligament integrity has powerful diagnostic and follow-up applications in the areas of soft-tissue knee reconstruction and sports medicine, as well as research into the AP stability of the knee in health and disease.

References 1. Russell, D. et al, Proceedings CAOS International 2012, Seoul, pp 275 2. Clarke, J.V. et al, Computer Aided Surgery 2012;17:29-39 3. Bland, J.M. et al, Lancet 1986;327:307-10. 4. Clarke, J.V. et al, Computer Aided Surgery 2010;15:13-18 5. Arneja, S. et al, J Orthop Surg 2009;17:77-9

13th Annual Meeting of the International Society for Computer Assisted Orthopaedic Surgery Orlando, FL, USA, June 12-15, 2013