Non-invasive assessment of lower limb mechanical alignment and assessment of knee laxity using navigation technology is now possible during knee flexion owing to recent software developments. We report a comparison of this new technology with a validated commercially available invasive navigation system.

We tested cadaveric lower limbs (n=12) with a commercial invasive navigation system against the non-invasive system. Mechanical femorotibial angle (MFTA) was measured with no stress, then with 15Nm of varus and valgus moment. MFTA was recorded at 10° intervals from full knee extension to 90° flexion. The investigator was blinded to all MFTA measurements. Repeatability coefficient was calculated to reflect each system's level of precision, and agreement between the systems; 3° was chosen as the upper limit of precision and agreement when measuring MFTA in the clinical setting based on current literature.

Precision of the invasive system was superior and acceptable in all conditions of stress throughout flexion (repeatability coefficient <2°). Precision of the non-invasive system was acceptable from extension until 60° flexion (repeatability coefficient <3°), beyond which precision was unacceptable. Agreement between invasive and non-invasive systems was within 1.7° from extension to 50° flexion when measuring MFTA with no varus / valgus applied. When applying varus / valgus stress agreement between the systems was acceptable from full extension to 20° & 30° knee flexion respectively (repeatability coefficient <3°). Beyond this the systems did not demonstrate sufficient agreement.

These results indicate that the non-invasive system can provide reliable quantitative data on MFTA and laxity in the range relevant to knee examination.

Conventional computer navigation systems using bone fixation have been validated in measuring anteroposterior (AP) translation of the tibia. Recent developments in non-invasive skin-mounted systems may allow quantification of AP laxity in the out-patient setting.

We tested cadaveric lower limbs (n=12) with a commercial image free navigation system using passive trackers secured by bone screws. We then tested a non-invasive fabric-strap system. The lower limb was secured at 10° intervals from 0° to 60° knee flexion and 100N of force applied perpendicular to the tibial tuberosity using a secured dynamometer. Repeatability coefficient was calculated both to reflect precision within each system, and demonstrate agreement between the two systems at each flexion interval. An acceptable repeatability coefficient of ≤3mm was set based on diagnostic criteria for ACL insufficiency when using other mechanical devices to measure AP tibial translation.

Precision within the individual invasive and non-invasive systems measuring AP translation of the tibia was acceptable throughout the range of flexion tested (repeatability coefficient ≤1.6mm). Agreement between the two systems was acceptable when measuring AP laxity between full extension and 40° knee flexion (repeatability coefficient ≤2.1mm). Beyond 40° of flexion, agreement between the systems was unacceptable (repeatability coefficient >3mm).

These results indicate that from full knee extension to 40° flexion, non-invasive navigation-based quantification of AP tibial translation is as accurate as the standard invasive system, particularly in the clinically and functionally important range of 20° to 30° knee flexion. This could be useful in diagnosis and post-operative follow-up of ACL pathology.

Introduction

The superficial anterior vasculature of the knee is variably described; most of our information comes from anatomical literature. Descriptions commonly emphasise medial-dominant genicular branches of the popliteal artery. Quantifying the relative contribution of medial and lateral vessels to the anastomotic network of the anterior knee may help provide grounds for selecting one of a number of popular incisions for arthrotomy.