Letter to The Editor: Raising the Joint Line in TKA is Associated with Mid-flexion Laxity: A Study in Cadaver Knees

To the Editor,

In a bid to unwind the perplexities of mid-flexion laxity in TKA, Luyckx and colleagues [7] examined the biomechanical effect of raising the medial joint line. I have read their study with great enthusiasm and would like to acknowledge their wonderful work and novel approach. I would also like to add a few comments of my own regarding this complicated topic.

While we know that flexion instability following TKA can lead to revision surgery, uncertainty remains as to whether mid-flexion laxity is a manifestation of flexion instability or a separate entity that merits serious consideration. If the flexion and extension gaps are not well balanced, this could cause laxity early in flexion. I believe that mid-flexion instability may be the result of an uncorrected flexion contracture. Although some suggest that mid-flexion instability may be caused by joint line elevation [8], we should keep in mind that joint line elevation to balance the flexion and extension gaps is an approach we use to treat flexion instability [10].

Luyckx and colleagues [7] performed their research in cadaver knees with no osteoarthritis, and with no deformity or ligamentous laxity. It is unclear whether elevating the joint line by only 2 mm or 4 mm has any meaningful clinical relevance on pain, functional outcome, overall implant stability, or long-term survivorship. Clinically, surgeons often encounter substantial knee flexion contractures, which call for increased amounts of distal femoral resection, and these additional resections are made without any apparent clinical sequelae. That being so, an important and as-yet unanswered question is, “How much laxity after TKA implantation is desirable?” Unfortunately, the current study [7] cannot answer this question. While the authors found that increasing the flexion space by 2 mm resulted in an increase in mid-flexion instability in cadavers, I am not sure this will be clinically problematic, and it may be advantageous in some patients to improve deep flexion [3]. It would be ideal if future studies could answer these questions in patients (or at least cadavers) with osteoarthritis.

The current study nicely details why joint line elevation would lead to mid-flexion laxity using a two-dimensional (2-D) analytical model that depicts slackening of superficial medial collateral ligament in the mid-flexion (30^° to 60^°) range after raising and anteriorizing the joint [7]. However, other authors have refuted this finding by noting that joint line elevation increases the distance between collateral ligament attachment sites, which increases ligament constraints, and thus maintains knee stability [6]. Hence, the joint line elevation is unlikely to be a major contributor to mid-flexion instability [5]. Amid these contradictions, it is conceivable that mid-flexion laxity with joint line elevation might not be solely attributable to collateral ligament length change.

Kinematic alignment in TKA is another controversial topic. Luyckx and colleagues [7] did not find any difference in coronal plane stability between the knees implanted with kinematic alignment versus mechanical alignment. However, Incavo and colleagues [2] were skeptical of the potential for mid-flexion laxity after anatomic alignment. The findings of their cadaveric study demonstrated inconsistent spacing patterns after anatomic alignment (lateral joint space opening in mid-flexion [50^˚-70^˚], but medial joint space opening in late flexion) [2], which is in contrast to consistently larger lateral space throughout ROM in mechanical alignment.

It remains unclear whether the type of implant has any appreciable effect on mid-flexion stability and joint line. Luyckx and colleagues implanted single-radius posterior-stabilized TKAs in all knees but did not investigate whether different implant designs might generate different results [7]. The multiradius femoral design is often criticized for inducing mid-range instability [4]. However, one in vitro kinematic study did not find any mid-range instability due to multiradius design and concluded that mid-range instability may be related to unrecognized ligament laxity during surgery rather than a specific implant design feature [9]. While laxity in mid-flexion often is attributed to posterior stabilized designs [1], this claim has not, to my knowledge, been substantiated.

While numerous methods are employed to measure the joint line on 2-D radiographs, we must also think about the three-dimensional (3-D) nature of knee joint; does radiographic measurements corroborate with the actual level of joint line? Future studies should explore whether 3-D CT and MRI imaging could supplement older methods to more precisely distinguish the level of the joint line.

Most controversies about mid-flexion instability relate to our lack of well-designed in vivo clinical studies, as well as an absence of standardized methods of quantifying laxity and measuring the joint line. How much of mid-flexion laxity is tolerable without any substantive short- or long-term effect? How much laxity is acceptable during surgery? In the presence of well-balanced soft-tissue sleeve after TKA, does mid-flexion laxity has any tangible effect to be wary of? Should surgeons performing TKA routinely assess for laxity in mid-flexion range?

These lingering issues indicate that we remain far from any conclusive concrete guidelines.