Measured resection mechanically and kinematically aligned TKA’s often result in an imbalanced knee

Introduction

The majority of total knee replacements are performed using a measured resection technique aiming for a neutral mechanical axis with 3? of external femoral rotation. This technique assumes that all knees are the same. Recent anatomic studies have shown large variation in the bony anatomy of the knee with only 1/1000 knees have a neutral tibia and femur. This maybe one of the factors why up to 20% of TKA’s are unhappy. Recently kinematic alignment TKA has been advocated as this technique enables an anatomic resurfacing of the knee however it doesn’t factor in the variable soft tissue envelope.
The Brainlab 3 CAS system enables the surgeon to evaluate the balance consequence of a mechanically aligned and a kinematically aligned TKA

Method

We carried out a single surgeon prospective study off 340 consecutive patients undergoing a navigated BrainLab 3 TKA between January 2019 and November 2020. After osteophyte removal the native knee was stressed in flexion and extension. We then virtually positioned the implant to achieve neutral mechanical axis tibial and femoral cuts with the posterior femur cut at 3 degrees of external rotation relative to the posterior condylar axis. We then assessed the flexion and extension gaps. TKA’s who had laxity of greater than 2mm in extension, flexion or flexion extension mismatch were determined. The same process was then repeated, virtually positioning the implant kinematically, with zero degrees of rotation on the femur with equal posterior femoral condyle cuts and anatomical distal femoral and tibial cuts.

Results

In the mechanically aligned group 41% were found to have an extension imbalance
(Range -10mm - 7.5mm) while 46% had a flexion imbalance (Range -8mm - 8.5mm). A medial flexion/extension mismatch was present in 42% of patients (Range -12mm - 8.5mm) while 39% demonstrated a lateral flexion/extension imbalance (Range -8.5mm - 6.5mm).
Only 24% had a balanced extension and flexion gap and flexion/extension gap. With trials inserted the imbalance could not be corrected in 40%
In the kinematically aligned group, 20% were found to have an extension imbalance
(Range -6.5mm - 5.5mm) while 34% had a flexion imbalance (Range -10mm -+8 mm). A medial flexion/extension mismatch was present in 22% of patients (Range -10mm - 6.5mm) while 34% demonstrated a lateral flexion/extension imbalance (Range -10mm, +8mm). Only 33% had a balanced extension and flexion gap and flexion/extension gap. With trials inserted the imbalance could not be corrected in 42%

Balanced MA KA
Extension 59% 80%
Flexion 54% 66%
Medial Flexion/Extension 58% 78%
Lateral Flexion/Extension 61% 66%
All Gaps 24% 33%
Uncorrectable 40% 42%

Conclusion

This study shows that is uncommon to achieve an optimally balanced TKA with a mechanical alignment or kinematic alignment surgical technique. This is achieved 24% of the time with MA and 33% of the time with KA if a > 2mm tolerance is used. More importantly balance could not be achieved with selective ligament releases due to the variable imbalance in 40% of MA TKA’s and 42% of KA TKA’s.
For this reason we perform a patient specific alignment CAS ligament guided balanced TKA whereby the prosthesis is implanted as anatomically as possible with small changes made in the implant position to enable an optimally balanced TKA without soft tissue releases.