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

BASK raised many questions for us to dig further into, within this series we will be discussing the hot topics emanating from the conference, providing an overview of current evidence, and giving examples of how this can inform your practice. 

A pain-free, well-functioning knee and longevity of the joint replacement are the overarching goals when any TKA is undertaken, although how these outcomes are best achieved still divides people. Two concepts that are often discussed are alignment of the joint, whether that’s mechanical or kinematic alignment (or something in between), and balance of the soft tissue that supports it.  

The optimal alignment philosophy has been debated for years, but opinion remains divided on whether we should be aiming for restoration of pre-disease native joint anatomy, kinematic alignment (KA)¹, providing a joint with a neutral hip-knee-ankle (HKA) axis, and joint line perpendicular to the mechanical axes, mechanical alignment (MA)², or something in between. The issue is that while MA has demonstrated good longevity, KA provides more natural movement, less reported pain, and better patient satisfaction as a result^3,4. 

Mechanical alignment (MA) has been seen as the gold standard since Freeman et al. described the technique in 1973⁵. However, MA replicates only 0.1% of the human population’s native anatomy⁶. Consequently, MA techniques often lead to alterations in the position of the articular surface which then requires soft tissue release to accommodate the altered articular anatomy; the mean change in distal joint line position is 3.3mm laterally and 2.1mm medially in varus knees, while imbalance ≥ 3mm was seen in up to 51% of varus and 82% of valgus knees  before soft tissue release⁷. This may account for at least some of the reported pain and instability of MA patients.  Furthermore, joint line elevation has a significant negative correlation with functional outcome scores8, while instability after TKA, often caused by changes in joint line position and subsequent abnormal ligament tension, accounts for around 22% of revisions and is the second most common reason for early revision in some series^9,10.

KA aims to overcome the disadvantages of MA by restoring the articular surface to its native pre-disease position, aligning the implant with the axes of rotation of the knee so that the soft tissues are restored to their pre-disease tension and the joint is inherently balanced without the need for soft tissue release¹¹. However, this assumes the soft tissue envelope remains unchanged by disease, injury, or the surgical procedure itself, and with disease progression comes increasing uncertainty about the pre-disease joint position relative to that found intra-operatively. Additionally, the limb may be restored to what would traditionally be considered an outlier alignment, with concerns over the longevity of the joint, although evidence increasingly suggests that tolerance of implants to deviation from mechanical alignment is considerably greater than previously thought^12,13.

For a joint to function correctly, ligament balance is a prerequisite. Gutske et al. found that balanced joints have better outcomes irrespective of alignment¹⁴.  Fehring et al. concluded that 27% of early revision was due to ligament instability¹⁵, with variables such as loosening, tibial wear, and pain being significantly higher within unbalanced groups¹⁶. The more a TKA replicates native joint anatomy, the better the ligament balance and consequently better patient satisfaction and functional scores. This is highlighted by medial ligament stability and lateral laxity in extension significantly correlating with these scores¹⁷. Aunan et al. amongst others, have highlighted the importance of measuring and controlling balance, with small changes in medial laxity (>2mm) in extension having a significant negative impact on function and pain measures¹⁸. Surgeon ‘feel’ has already been shown to be inaccurate and inadequate for producing reproducible high-quality joints^19,20. While more anatomic alignment will tend to lead to better ligament balance, it may or may not be adequately achieved regardless of alignment technique, and without the routine use of objective intra-operative assessment tools to assess and ensure a balanced knee one might postulate that outcomes will always be mixed.

Clark et al. used functional alignment (FA), where the pre-resection balancing of the joint occurs so bone cuts are positioned to achieve the primary goal of a balanced knee without releases, while alignment is a secondary consideration, and variations of it are accepted, to achieve more consistent balance than either KA or MA²¹.  With the increasing use of technology in TKA enabling precise, accurate implant placement and objective balance assessment, should we now be refocussing on ensuring optimal balance and accepting that, to some extent, the alignment target can be relaxed? Classification systems such as Coronal Plane Alignment of the Knee (CPAK)22 provide a starting point for the alignment goal, but maybe more consistent clinical outcomes and implant survivorship will be achieved by deviating from that in pursuit of balance as the primary goal. Would future efforts to refine what that optimal balance goal is, and how best to achieve it, be more fruitful than chasing a “gold standard” alignment? Maybe instead we need to seek a “gold standard” balance.

How do you balance or align your TKA procedures? Is it patient-specific and how do you aim to optimise the long-term outcomes for your patients? Can it be one method fits all, or do we need to be more personalised in how we treat to minimise reinterventions further down the line? With waiting lists at record levels, surely better surgery now is preferable to more challenging revisions in a few years?

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