Non-invasive quantification of lower limb mechanical alignment in flexion

Lower limb alignment and laxity measures before, during and after total knee arthroplasty: A prospective cohort study

BACKGROUND

This study compared knee alignment and laxity in patients before, during and after total knee arthroplasty, using methodologically similar procedures, with an aim to help inform pre-operative planning.

METHODS

Eighteen male and 13 female patients were recruited, mean age 66years (51-82) and mean body mass index of 33 (23-43). All were assessed pre- and postoperatively using a non-invasive infrared position capture system and all underwent total knee arthroplasty using a navigation system. Knee kinematic data were collected and comparisons made between preoperative clinical and intraoperative measurements for osteoarthritic knees, and between postoperative clinical and intraoperative measurements for prosthetic knees.

FINDINGS

There was no difference in unstressed coronal mechanical femoral-tibial angles for either osteoarthritic or prosthetic knees. However, for sagittal alignment the knees were in greater extension intraoperatively (osteoarthritic 5.2° p<0.001, prosthetic 7.2° p<0.001). For osteoarthritic knees, both varus and valgus stress manoeuvres had greater angular displacements intraoperatively by a mean value of 1.5° for varus (p=0.002) and 1.6° for valgus (p<0.001). For prosthetic knees, only valgus angular displacement was greater intraoperatively (0.9°, p=0.002).

INTERPRETATION

Surgeons performing total knee arthroplasties should be aware of potential differences in alignment and laxity measured under different conditions to facilitate more accurate operative planning and follow-up.

Assessment of knee alignment with varus and valgus force through the range of flexion with non-invasive navigation

In image-free total knee arthroplasty (TKA) navigation, infra-red markers are attached to bony landmarks to provide kinematic data intra-operatively, with the aim of improving the precision of implant placement. In non-invasive navigation, infra-red markers are attached to the skin surface, with recent evidence suggesting that this can give repeatable measurements of lower limb mechanical alignment. The aim of our study was to evaluate the use of a non-invasive navigation system in the assessment of mechanical alignment with applied coronal force through the range of flexion. A previously validated non-invasive system (Physiopilot™) was tested on 23 volunteers with healthy knees. Two users performed two registrations of the software workflow on each participant's right and left knees. A force was manually applied to the end-point of varus and valgus knee laxity and the measured change in mechanical alignment was recorded. Force was applied with the knee positioned in increments of flexion from 0 to 90°. In keeping with previous studies, satisfactory values of coefficient of repeatability (CR) of 1.55 and 1.33 were found for intra-observer repeatability in measurement of supine mechanical femoro-tibial angle (MFTA) in extension, with a good inter-observer correlation of intraclass correlation coefficient (ICC) .72. However, when flexion was introduced, intra-observer and inter-observer reliability fell out with acceptable limits. Therefore, the trial did not support use of the Physiopilot™ system as a measure of MFTA when flexion is introduced. It was felt that learning-curve, soft tissue artefacts and lack of force standardisation equipment may have accounted for significant levels of error, with further studies required to address these issues.

Current state of the art in total knee arthroplasty computer navigation

PURPOSE

Computer-assisted surgery in orthopaedics is passing through the initial adapter phase of technology adoption. It started more than 20 years ago, but the uptake of technology is still not widespread. The purpose of this article is to introduce the reader to the basic technology and familiarize with the terminology used in the computer navigation.

METHODS

During this time, the technology has matured and we have the evidence to prove its benefits for patients. Not only does it help placing the prosthetic components in correct orientation, it also helps with other parameters like blood loss and fat embolism reduction. In addition to being a teaching and training tool, it has also opened new areas of research which now question the traditional practices. Since it is not in commonly used, the basic aspects of computer navigation are not very well known.

RESULTS

This paper outlines some important definitions and restates the classification of navigation within the spectrum of computer-assisted technologies; it then elaborates on the key principles behind navigation in knee arthroplasty and goes through some of the differences between navigation systems. Finally, it describes in some detail the surgical steps with an image-free knee navigation system.

CONCLUSIONS

Computer-assisted navigation is not mainstream yet, but this article should help readers unfamiliar with the technology to understand the basic terms and how it actually works.

LEVEL OF EVIDENCE

III.

Quantitative measurement of lower limb mechanical alignment and coronal knee laxity in early flexion

BACKGROUND

Non-invasive quantification of lower limb alignment using navigation technology is now possible throughout knee flexion owing to software developments. We report the precision and accuracy of a non-invasive system measuring mechanical alignment of the lower limb including coronal stress testing of the knee.

METHODS

Twelve cadaveric limbs were tested with a commercial invasive navigation system against the non-invasive system. Coronal mechanical femorotibial (MFT) alignment was measured with no stress, then 15 Nm varus and valgus applied moments. Measurements were recorded at 10° intervals from extension to 90° flexion. At each flexion interval, coefficient of repeatability (CR) tested precision within each system, and limits of agreement (LOA) tested agreement between the two systems. Limits for CR & LOA were set at 3° based on requirements for surgical planning and evaluation.

RESULTS

Precision was acceptable throughout flexion in all conditions of stress using the invasive system (CR ≤ 1.9°). Precision was acceptable using the non-invasive system from extension to 50° flexion (CR ≤ 2.4°), beyond which precision was unacceptable (> 3.4°). With no coronal stress applied, agreement remained acceptable from extension to 40° (LOA ≤ 2.4°), and when 15 Nm varus or valgus stress was applied agreement was acceptable from extension to 30° (LOA ≤ 2.9°). Higher angles of knee flexion had a negative impact on precision and accuracy.

CONCLUSION & CLINICAL RELEVANCE

The non-invasive system provides reliable quantitative data in-vitro on coronal MFT alignment and laxity in the range relevant to assessment of collateral ligament injury, pre-operative planning of arthroplasty and flexion instability following arthroplasty. In-vivo validation should be performed.