Enabling Technology- How do we achieve an aligned and balanced knee?

Enabling Technology- How do we achieve an aligned and balanced knee?

The Big Updates from BASK 2023

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By Stuart Pearce & Stephen Key

This article is part of a series, you can find the previous two articles here: 
Maintaining Outcomes for Total Knee Arthroplasty – The Big Updates From BASK 2023 

Balance or Alignment – does it make a difference and what should be the priority? Functional and Procedural Outcome Differences 

Enabling Technology- How do we achieve an aligned and balanced knee? 

Following on from BASK, the recent BOA Annual Congress continued the discussion around optimising outcomes of total knee arthroplasty (TKA) with sessions devoted to the use of enabling technologies. Building on the growing interest in non-mechanical alignment, the value of enabling technology in achieving that was emphasised. Furthermore, the point was made that the expansion of robotic surgery has allowed us to evolve from considering 2-d alignment to a detailed appreciation of the 3-d position of the implant and the impact that changes to that will have on not only alignment but also balance. 

In the last episode we discussed whether gold standard care should prioritise strict alignment or accurate balance within the TKA procedure, and the role of soft tissue releases: should we focus on strict alignment targets, accepting that soft tissue release may then be necessary to achieve balance, or primarily target balance with more relaxed alignment goals in order to minimise soft tissue release? At the BOA, experienced users of robotic TKA systems almost universally reported adapting implant position to optimise balance to avoid soft tissue release. 

Looking at the data behind medium to long-term outcomes, differences between the two techniques are evident and small variance in measurements have a significant impact on functional outcome, pain, and long-term survival of the implant. However, in the absence of routine objective measurement of balance it could be argued that we are instead using alignment as a proxy for it. We previously explored how kinematic alignment assumes a normal soft tissue envelope, mechanical alignment often fails to optimise the native soft tissues throughout the full range of movement, surgeon “feel” of balance is inadequate, and that small variations of around 3mm in ligament laxity can have significant negative impact on clinical outcomes following TKA. Within this episode we will discuss the technology enabling us to measure balance directly and the impact that can have on outcomes. 

Gap Measurement 

The use of manual instruments to measure flexion and extension gaps is long-established. However, such techniques require at least some of the bone cuts to have been made prior to the measurement and remain dependent upon the surgeon’s ability to perform accurate cuts. Utilisation of computer assistance to measure gaps and plan the cuts make it highly accurate and controlled. Navigation-based systems facilitate both measured resection and gap balancing techniques. By tracking bone landmarks to define the bone cuts based on thickness and alignment to the limb’s axes, gaps can then be balanced by titrated, and therefore controlled, releases following a measured resection principle. Alternatively, since gaps are predicted prior to making any cuts, the implant position can be adjusted at the planning stage to optimise gaps and balance the knee within its soft tissue envelope, reducing the need for soft tissue release. The impact on alignment can also be established and guide the planned implant position and releases. The addition of robotics to perform the cuts further increases accuracy and precision by removing that aspect of surgeon-dependence, and as a result highly accurate and personalised joints can be achieved.  

Much of the historical literature comparing navigated with manual TKA has focussed on mechanical alignment in both groups, therefore balanced by soft tissue release in many cases, which may explain why navigation failed to achieve clear benefits despite improved accuracy1-3. The growing appreciation of the impact of soft tissue releases has led to approaches deviating from mechanical alignment that are able to achieve balance while minimising releases, yet avoiding extreme alignments4,5; results so far are promising but the outcomes of a number of RCTs are awaited6-9

Pressure Sensors 

In contrast to gap measurement technology, sensors can measure joint compartment pressures directly and so provide an alternative means of quantifying knee balance. Trial components containing sensors can be used to map compartment pressures and tibiofemoral contact points at different positions through the arc of motion. Unlike navigation/robotic gap measurement, sensors generally require cuts to have been made before they can be used so lend themselves more to guiding releases than adapting implant position, although systems exist which can guide femoral rotation to gap-balance the flexion space. They also do not provide any insight into alignment. 

A number of studies using sensor technology have shown improved knee scores and patient satisfaction in objectively balanced knees vs unbalanced groups10-13. However, evidence supporting sensors vs conventional techniques is more mixed; Chow and Breslauer14 have demonstrated some of the possible benefits of the technology with significantly better clinical and functional metrics accompanying sensor-assisted TKA as opposed to manual TKA. Interestingly, other studies have struggled to demonstrate consistent differences between similar sensor aligned and manual techniques15, raising questions around whether we need to understand the process and margin of error better in order to achieve best outcomes. Another criticism has been that open-chain, non-weightbearing intra-operative load assessment may not reflect physiological load-bearing conditions and could mean a lack of balance when moving through a loaded range of motion, in turn potentially returning to a similar endpoint of an unbalanced joint. 

Future Prospects 

Could there be a place where sensors and robotic systems combine robotic planning and precision, gap measurement, and pressure analysis to provide us with even more superior TKA outcomes? Such combinations are being explored. Now we can quantify balance accurately, future work needs to define what our gold standard balance targets should be in specific patient groups. If we could consistently optimise both alignment and balance using enabling technology, could this unlock better outcomes, providing longer lasting and more functional joints, in turn reducing reintervention and downstream healthcare burden, and drive down the waiting lists that seem to be ever-increasing? It has been long established that reintervention only leads to poorer outcomes, so investing in the first intervention could make both clinical and economic sense. This topic will be investigated further within the next episode… 



Stuart PearceStuart Pearce PhD

Medical Affairs Johnson & Johnson MedTech

Stuart joined the organisation in 2020 following the end of his Doctorate studies at Barts and the London, University of London in 2019. Publishing on Aneurysms, Atherosclerosis and Medical Teaching during his studies he has continued to develop his knowledge of Medical Devices, Medical Technologies and their impact on the wider healthcare system. 


Stephen KeyStephen Key

UK and Ireland Medical Lead for DePuy Synthes

Formerly a consultant hip and knee surgeon in London, having completed undergraduate medical training at the University of Cambridge in 2004, specialist Orthopaedic training on The Royal London rotation, and fellowship training in hip and knee arthroplasty and soft tissue knee surgery in Australia and the UK.



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  1. Lee DH, Park JH, Song DI, et al. Accuracy of soft tissue balancing in TKA: comparison between navigation-assisted gap balancing and conventional measured resection. Knee Surg Sports Traumatol Arthrosc. 2010;18(3):381-387. 

  2. Han SB, Nha KW, Yoon JR, et al. The reliability of navigation-guided gap technique in total knee arthroplasty. Orthopedics. 2008;31(10)(Suppl 1). 

  3. Singh VK, Varkey R, Trehan R, et al. Functional outcome after computer-assisted total knee arthroplasty using measured resection versus gap balancing techniques: a randomised controlled study. J Orthop Surg (Hong Kong). 2012;20(3):344-347. 

  4. Chang JS, Kayani B, Wallace C, et al. Functional alignment achieves soft-tissue balance in total knee arthroplasty as measured with quantitative sensor-guided technology. Bone Joint J. 2021;103-B(3):507-514.  

  5. Clark G, Steer R, Wood D. Functional alignment achieves a more balanced total knee arthroplasty than either mechanical alignment or kinematic alignment prior to soft tissue releases. Knee Surg Sports Traumatol Arthrosc. 2023;31(4):1420-1426. 

  6. Clark GW, Esposito CI, Wood D. Individualized Functional Knee Alignment in Total Knee Arthroplasty: A Robotic-assisted Technique. Techniques in Orthopaedics. 2022;37(3):185-191. 

  7. Steer R, Tippett B, Khan RN, et al. A prospective randomised control trial comparing functional with mechanical axis alignment in total knee arthroplasty: study protocol for an investigator initiated trial. Trials. 2021;22(1):523.  

  8. Kayani B, Konan S, Tahmassebi J, et al. A prospective double-blinded randomised control trial comparing robotic arm-assisted functionally aligned total knee arthroplasty versus robotic arm-assisted mechanically aligned total knee arthroplasty. Trials. 2020;21(1):194.  

  9. Young SW, Zeng N, Tay ML, et al. A prospective randomised controlled trial of mechanical axis with soft tissue release balancing vs functional alignment with bony resection balancing in total knee replacement-a study using Stryker Mako robotic arm-assisted technology. Trials. 2022;23(1):580. 

  10. Gustke KA, Golladay GJ, Roche MW, et al. A new method for defining balance: promising short-term clinical outcomes of sensor-guided TKA. J Arthroplasty. 2014;29(5):955-960. 

  11. Gustke KA, Golladay GJ, RocheM, et al. Increased satisfaction after total knee replacement using sensor guided technology. Bone Joint J. 2014;96-B(10):1333-1338. 

  12. Anderson C, Roche M, Golladay G, et al. Bony cuts or soft-tissue release? Using intra-operative sensors to refine balancing techniques in TKA. Orthop Proceedings. 2016;98-B(SUPP_1):16. 

  13. Golladay GJ, Bradbury TL, Gordon AC, et al. Are patients more satisfied with a balanced total knee arthroplasty? J Arthroplasty. 2019;34(7S):S195-200. 

  14. Chow JC, Breslauer L. The Use of Intraoperative Sensors Significantly Increases the Patient-Reported Rate of Improvement in Primary Total Knee Arthroplasty. Orthopedics. 2017;40(4):e648-e651.  

  15. Sava MP, Hara H, Alexandra L, et al. Verasense sensor-assisted total knee arthroplasty showed no difference in range of motion, reoperation rate or functional outcomes when compared to manually balanced total knee arthroplasty: a systematic review. Knee Surg Sports Traumatol Arthrosc. 2023;31(5):1851-1858.