Computer-Assisted Knee Arthroplasty: A Critical Audit

Arthroplasty Expertise vs. Automated Navigation in Knee Replacement

Computer-assisted navigation was designed to improve the reproducibility of surgical cuts by registering anatomical landmarks and recommending precise resection planes. However, this technology fails to incorporate the critical factor of surgical experience and has not demonstrated superiority over a high-volume knee replacement surgeon. Developed through the surgical practice of Dr. Saminathan Suresh Nathan, this analysis examines why generalized navigation protocols often fall short when applied to the unique morphology of the Asian knee.

Anatomical Principles and Western Normative Data

At the core of computer-aided surgery is the understanding of mechanical and anatomical axes. Historically, these indices were based on Western populations. We increasingly recognize significant anatomical variations in Asia Pacific and Australasian patients that challenge these "normal" Western standards.

The Mechanical Axis: A line from the center of the femoral head to just medial of the knee, extending to the center of the ankle. This is the weight-bearing axis and sits 3 degrees from the line of the center of gravity.
The Anatomical Axis: The femoral anatomical axis sits 6 degrees from the mechanical axis and 9 degrees from the center of gravity. The mechanical axis of the tibia should align with its anatomical axis.
Navigation: then attempts to make cuts perpendicular to the the mechanical axis.

Figure 1. In navigation the critical measurement is alignment in the frontal plane.This is generally made in relation to the mechanical axis of the lower limb. Depending on the shape of the femur however (which varies from person to person) the cut which is made in relation to a mechanical axis can be in too much valgus (as on the right).

Pitfalls and Anatomical Variants in Asia

A failure to account for individual anatomy results in suboptimal outcomes. Research published by our group (Singh et al., Bone Joint J 2013) found that Asian patients possess a greater posterior tibial slope than Western populations.

Frontal (Coronal) Plane: Asian patients frequently exhibit lateral femoral bowing. A computer-dictated cut perpendicular to the mechanical axis—while true to the weight-bearing axis—may be too valgus for the knee prosthesis's tolerance, leading to a "knock-kneed" result. (Figure 2)
The Sagittal Plane: Navigation protocols poorly account for the sagittal plane, increasing the risk of femoral notching and periprosthetic fractures. (Figure 3)

The Rotational Paradox: Femoral implant rotation is the most critical factor for success. Establishing this requires full-limb MRI or CT scans—adding a 20% cost increase and additional radiation. Present navigation protocols lack the ability to reliably reproduce correct implant rotation. (Figure 4)

Tibia Vara: Asian patients often develop significant tibial bowing (tibia vara). This deformity causes the tibial axis to exit on the outside of the knee. Navigation registration is frequently misled here, rendering rotational and sagittal data unusable. (Figure 5)

Figure 2. In navigation the critical measurement is alignment in the frontal plane.This is generally made in relation to the mechanical axis of the lower limb. Depending on the shape of the femur however (which varies from person to person) the cut which is made in relation to a mechanical axis can be in too much valgus (as on the right).

Figure 3. The side plane or sagittal plane is poorly understood in navigation. Blindly following the instructions of the computer can result in notching or taking too much off the front of the femur.

Figure 4. The rotational axis of the the femur is poorly reproduced in traditional navigational protocols.

Figure 5. Again in Asian patients, the tibia tends to be bowed. This results in the process of navigation being misled (yellow line) and causes implants to be put in the wrong position.

Variations on the Theme

Studies demonstrate that the primary metric of success in navigation—correction within 3 degrees of a neutral 0 degrees tibio-femoral axis—is clinically irrelevant. While the data generated has improved our understanding of arthritic pathomechanics and led to the development of robotics, the fundamental pitfalls remain.

In centers where robotics are utilized, the surgeon registers landmarks and hands the procedure to the robot for the bone cuts. While intriguing, this is merely an execution branch of navigation and is subject to the same navigational validity errors.

Minimally Invasive and Partial Knee Options

The combination of partial knee replacements (unicompartmental) with navigation is an enticing idea, but carries significant risk:

Underestimated Disease: Most patients needing replacement have multi-compartment involvement. Partial replacements in these cases are prone to long-term failure due to advancing disease.
The Track Record Gap: While methods like MAKOplasty have improved short-term results for partial replacements, they are unlikely to outperform the 30-year gold-standard track record of traditional total knee replacements.

Summary

Navigation achieved the goal of axial modification, but it has not translated to clinical superiority. It introduces additional risks, such as fractures at pin placement sites. We utilize navigational data only as a reference, maintaining the surgical authority to override the computer when it conflicts with the unique morphology of the Asian knee.

Page updated

Google Sites

Report abuse