Functional alignment minimizes changes to joint line obliquity in robotic-assisted total knee arthroplasty: a CT analysis of functional versus kinematic alignment in 2,116 knees using the Coronal Plane Alignment of the Knee (CPAK) classification

Listen to Simon and Amy discuss the paper 'Functional alignment minimizes changes to joint line obliquity in robotic-assisted total knee arthroplasty: a CT analysis of functional versus kinematic alignment in 2,116 knees using the Coronal Plane Alignment of the Knee (CPAK) classification' published in the December 2024 issue of Bone & Joint Open.

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Transcript

[00:00:00] Welcome back to another episode of AI Talks with Bone & Joint from the publishers of Bone & Joint Open. Today we're discussing the paper 'Functional alignment minimizes changes to joint line obliquity in robotic-assisted total knee arthroplasty: a CT analysis of functional versus kinematic alignment in 2,116 knees using the Coronal Plane Alignment of the Knee (CPAK) classification', published in December 2024 by V. A. van de Graaf, G. W. Clark, D. Colopy, J. A. Wood, D. B. Chen and S. J. MacDessi. 

I'm Simon and I'm joined by my co-host Amy. 

Hello, Simon. I'm looking forward to diving into this. The study tackles a crucial aspect of knee arthroplasty, comparing functional and kinematic alignments using robotic assistance. It's a rather topical subject in orthopaedics at the moment. 

Quite right, Amy. To begin, let's consider why this research was done. Total knee arthroplasty, or TKA, is a common procedure to relieve pain and restore function in [00:01:00] those with severe knee arthritis. However, the process involves complex decisions about implant positioning, which can significantly alter surgical outcomes.

Precisely, Simon. The researchers compared functional alignment, FA, and kinematic alignment, KA, in total knee arthroplasty, focusing on changes to joint line obliquity, JLO. They used the CPAK classification to categorize knee alignments. The primary hypothesis was that FA would not significantly alter the arithmetic hip-knee-ankle angle, aHKA, and JLO from their kinematic positions.

Indeed, the main method in this study was a retrospective analysis of 2,116 robotic-assisted FA TKAs. They measured angles such as the lateral distal femoral angle, LDFA, and the medial proximal tibial angle, MPTA to calculate the aHKA and JLO, both pre-resection and post-implantation. 

One fascinating finding was that 71.6% of knees [00:02:00] had an aHKA change of two degrees or less, and 87.0% had a JLO change of 3 degrees or less. This suggests that FA significantly minimizes changes in joint line obliquity, preserving the native anatomy like kinematic alignment does, while better providing a balanced TKA. 

Another interesting result was that the CPAK type was maintained in 58.1% of knees, with the highest proportions in Types I, II, and IV. However, constitutional valgus knees, which are Types III and VI, often shifted into a neutral alignment. This implies that while FA maintains anatomical integrity, it may still alter Coronal Plane Alignment in some cases. 

It's also noteworthy that the study examined secondary outcomes, comparing alignment changes between sexes. Males generally started with more varus alignment compared to females. Despite this, the alignment changes from KA to FA were quite similar across both sexes. 

It's worth highlighting that FA was particularly effective [00:03:00] in avoiding significant changes to joint line obliquity, which is likely to be crucial for patient satisfaction post-surgery. Only 0.1% of the studied knees were returned to a postoperative apex proximal JLO, for example. This may point to the precision of robotic-assisted TKA with FA, especially versus MA, which may be associated with significantly more unintentional alterations to the JLO. 

The FA group involved initial implant positioning based on kinematic alignment, followed by adjustments within predefined boundaries to achieve balanced extension and flexion gaps. This approach reduces the need for soft-tissue releases and improves gap balance while still preserving the knee's natural laxities. A key takeaway here is the potential application of FA as a middle ground strategy. It offers a balanced compromise between unrestricted kinematic alignment and fixed mechanical alignment, making it a promising option for personalised TKA.

Indeed. This combined strategy of [00:04:00] starting with kinematic alignment and finalising with functional positioning shows great promise for optimising surgical outcomes and enhancing patient satisfaction. 

So, to summarise, functional alignment in robotic-assisted TKA reduces changes to joint line obliquity and preserves native anatomy, whilst providing an optimally balanced knee replacement better than kinematic alignment. This could mark a significant step forward in personalising TKA procedures. 

Quite so, Amy. That's all for today's episode of AI Talks with Bone & Joint. We hope you enjoyed this deep dive into the latest advancements in knee arthroplasty. Don't forget to check out the full paper in Bone & Joint Open for a more detailed read.