Non-invasive computer-assisted measurement of knee alignment

Abstract

The quantification of knee alignment is a routine part of orthopaedic practice and is important for monitoring disease progression, planning interventional strategies, and follow-up of patients. Currently available technologies such as radiographic measurements have a number of drawbacks. The aim of this study was to validate a potentially improved technique for measuring knee alignment under different conditions. An image-free navigation system was adapted for non-invasive use through the development of external infrared tracker mountings. Stability was assessed by comparing the variance (F-test) of repeated mechanical femoro-tibial (MFT) angle measurements for a volunteer and a leg model. MFT angles were then measured supine, standing and with varus-valgus stress in asymptomatic volunteers who each underwent two separate registrations and repeated measurements for each condition. The mean difference and 95% limits of agreement were used to assess intra-registration and inter-registration repeatability. For multiple registrations the range of measurements for the external mountings was 1 larger than for the rigid model with statistically similar variance ( p ¼ 0.34). Thirty volunteers were assessed (19 males, 11 females) with a mean age of 41 years (range: 20-65) and a mean BMI of 26 (range: 19-34). For intra-registration repeatability, consecutive coronal alignment readings agreed to almost AE1, with up to AE0.5 loss of repeatability for coronal alignment measured before and after stress maneuvers, and a AE0.2 loss following stance trials. Sagittal alignment measurements were less repeatable overall by an approximate factor of two. Inter-registration agreement limits for coronal and sagittal supine MFT angles were AE1.6 and AE2.3 , respectively. Varus and valgus stress measurements agreed to within AE1.3 and AE1.1 , respectively. Agreement limits for standing MFT angles were AE2.9(coronal) and AE5.0 (sagittal), which may have reflected a variation in stance between measurements. The system provided repeatable, real-time measurements of coronal and sagittal knee alignment under a number of dynamic, real-time conditions, offering a potential alternative to radiographs.

Standardising the clinical assessment of coronal knee laxity

“…However, in terms of the accuracy of the IR system on larger individuals, it has been shown to be highly repeatable on a group of subjects of BMI up to 34, with clinical examination not affecting alignment measurements. 27 This had been taken to indicate that the tracker mountings are very stable. The additions to the assessment, including the use of the FAD, should not have affected the accuracy of the IR system in any way, so it is felt that the previous results give confidence that this set-up would work with the higher BMI population seen for total knee replacements.…”

“…To validate the repeatability of the system for knee centre to ankle centre measurement, 20 separate registrations were performed by a single clinician on both a leg model with rigidly fixed tracker mounting pins and the right lower limb of a female volunteer (age 37, body mass index (BMI) 19), with the IR trackers removed and reapplied each time. To further assess repeatability, the same clinician carried out repeated measurements on 29 (18 male, 11 female) healthy volunteers of mean age 42 years (20-65) and mean BMI of 26 (19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34).…”

“…This system had been validated for coronal alignment 27 but to verify this technology for recording flexion, comparison was made with a validated flexible electrogoniometer (EG) (Biometrics Ltd, Cwmfelinfach, Gwent, UK). 28 A single volunteer was used for simultaneous measurements of both systems.…”

“…Non-invasive infrared (IR) tracking technology, with kinematic registration of joint centres to give mechanical lower limb alignment, 27 was utilised for the real-time measurement of both coronal and sagittal mechanical tibiofemoral alignment. This system had been validated for coronal alignment 27 but to verify this technology for recording flexion, comparison was made with a validated flexible electrogoniometer (EG) (Biometrics Ltd, Cwmfelinfach, Gwent, UK).…”

Current state of computer-assisted trauma surgery

“…Using pivoting mechanisms to determine the center of the hip joint and algorithms to define the ankle and knee center of rotation, a navigated measurement of the mechanical limb axis is possible. Additional application of predetermined landmarks allows a combined image free visualization of the hip or knee joint, including the mechanical limb axis in relation to the desired implant position [9,10].…”