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Standardised balancing goals are not able to reconstruct individual laxity-functional knee phenotypes in more than 50 % of knees
, Randa Elsheikhb,c, Andreas Schustera, Andrej M. Nowakowskib,c, Michael T. Hirschmannb,c
⁎Corresponding author: Heiko Graichen. heiko.graichen@knee-cat.com
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Received: ,
Accepted: ,
This article was originally published by Reed Elsevier India Pvt. Ltd. and was migrated to Scientific Scholar after the change of Publisher.
Abstract
Abstract
Laxity Phenotype (LP) analysis has revealed a wide range of variability among knees, showing three phenotypes in extension and three in flexion. Based on this variability recently a Laxity-Functional Knee Phenotype (L-FKP) matrix was created. This study aimed (1) to assess how well-standardised balancing goals reproduce individual LPs, and (2) to develop a treatment algorithm to convert critical L-FKP groups into optimal ones.
Eighty-six knees were simulated using a validated alignment simulator to create and analyse the L-FKP matrix. Knees were classified as optimal (LP matched “all gaps equal” or lateral flexion laxity <6 mm), intermediate, and critical (presence of medial laxity in extension and/or flexion). A three-step treatment algorithm was developed for optimizing L-FKP; first, by adjusting bony anatomy within defined boundaries, second by targeting soft tissue releases if correction remained >1 mm, and third, by using additional constraint if option 1 and 2 were not sufficient.
Only 37 % of knees showed optimal L-FKP patterns after restoring the patient-specific B-FKP. Another third of the knees was classified as suboptimal, which could be transferred to optimal L-FKP matrix groups with minor bone adaptions, finally reaching 70 % of optimal L-FKP patterns. Still, 30 % remained within the critical L-FKP matrix groups. The algorithm enabled the structured conversion of these cases into optimal groups.
Standardized balancing goals reconstruct optimal L-FKP groups in less than 40 % of knees, even if a personalized B-FKP workflow is applied. By minor bony modifications this amount can be increased to 70 %. A structured algorithm to restore optimal L-FKP is presented.
Keywords
Knee
Bony phenotype
Laxity phenotype
B-FKP
Alignment
L-FKP
Functional knee phenotypes
Personalized alignment
Mechanical alignment
Kinematic alignment
1 Introduction
Owing to the deeper understanding of bony phenotype variability, personalized alignment techniques have gained increased popularity in recent years.1–4 While most personalized alignment workflows are mainly focused on coronal bony alignment, more or less simplified and reduced to a few anatomical angles such as the medial proximal tibial angle (MPTA) and the lateral distal femoral angle (LDFA),5–7 Hirschmann et al. showed the importance of integrating the sagittal and the axial plane in the concept of personalization.8–11 Consequently, the 3D phenotype concept was developed.1
Importantly, if phenotyping the knee is performed, bony morphology should be assessed in conjunction with soft tissue characteristics, highlighting the equal relevance of laxity phenotype analysis.12 Several authors have described the wide variability in knee laxity.8,13–16 Most recently, Graichen et al. introduced the Laxity-Functional Knee Phenotype (L-FKP) concept.15 This model categorizes the knee into three distinct laxity phenotypes in both extension and flexion, resulting in a 3x3 matrix of nine possible combinations, effectively illustrating the substantial variability in laxity patterns.
With the exception of functional alignment, most personalized alignment techniques aim to restore the individual native bony anatomy based on specific principles. However, when it comes to balancing goals, these techniques rely on standardized targets. For instance, mechanical alignment (MA) typically aims to equalize all flexion and extension gaps within a range of 2 mm. This systematic approach alters the native laxity phenotype in the majority of knees.15 Another frequently applied target is to allow or even aim for increased lateral laxity in flexion, which has been shown to better replicate natural knee kinematics.13,17,18 Yet, despite mimicking physiological patterns, this approach is still systematic, seeking the same outcome in all patients, independent of the individual L-FKP.
Recently, Graichen et al. demonstrated that a wide range of individual L-FKPs exist, suggesting that laxity phenotype (LP) reconstruction may also require personalization rather than uniform application of balancing goals.8–10,12,15,17 However, attempting to recreate the full spectrum of native laxity phenotypes is not always advisable, as implant design, especially insert conformity, significantly influences joint stability and might limit the feasibility of reconstruction. For example, medial instability is poorly tolerated in primary cruciate-retaining (CR) or posterior-stabilized (PS) designs.
To date, there are no clearly defined boundaries for how much laxity should be restored. In this study, the concept of LP patterns and the L-FKP matrix is applied to develop a treatment algorithm for personalized LP restoration. The specific aims of this study are: (1) to evaluate how well standardized balancing targets restore individual LP, and (2) to develop a treatment algorithm that defines a standardized pathway for transforming critical L-FKP matrix groups into more optimal phenotypes.
2 Material and methods
2.1 Definition and calculation of the L-FKP matrix
The L-KFP matrix was developed using existing data from 86 consecutive knees. This matrix incorporates individualized extension (L-FKPext) and flexion (L-FKPflex) gap measurements.
While the L-FKP matrix can be applied across various alignment workflows, this study specifically employed the boundary-guided functional knee phenotype (B-FKP) workflow.
In this approach, key anatomical angles, namely medial proximal tibial angle (MPTA), lateral distal femoral angle (LDFA), and tibia slope, were extracted from long-leg and lateral radiographs and entered into the Knee-CAT simulation software (SOS GmbH; Germany). Intraoperative gap sizes obtained through standardized stress testing were subsequently adjusted based on the preoperative radiographic inputs. Due to the restricted nature of the B-FKP workflow, defined boundaries were established. For MPTA reconstruction, the boundary was set at 85–92°, at 86–92° for LDFA, at 2–8° for tibia slope, and for HKA it was set at 175–183°. For extension gap analysis, the individual MPTA and LDFA values were set, while for flexion gap simulation, the individual MPTA and posterior condyle axis (PCA) at 0° rotation were used. Following virtual bone resection using the B-FKP workflow, gap sizes from all 86 cases were visualized on the planning screen, and the corresponding L-FKPext and L-FKPflex values were recorded for matrix generation (Fig. 1).
Based on the individual L-FKPext and L-FKPflex, all 86 data sets were categorized in the L-FKP matrix displaying the nine different potential LP-subgroups (Fig. 1).
2.2 Development of the L-FKP-treatment algorithm
In a next step, a treatment algorithm based on the L-FKP matrix was developed. To enable this, the relative distribution of all nine LP matrix subgroups was calculated. Each LP matrix subgroup was then classified into one of three categories based on its biomechanical characteristics: optimal (green), intermediate (yellow), or critical (red). This classification was guided by whether a given LP matrix pattern aligned with one of the two accepted balancing goals for total knee arthroplasty (TKA).
Goal No. 1: All gaps equal, corresponding to balanced extension and flexion gaps.
Goal No. 2. Slight lateral flexion gap laxity up to 6 mm; with a neutral extension gap.
Two LP matrix subgroups (type IV and V) met these criteria and were rated as optimal, meaning the LP could be completely reconstructed by applying personalized B-FKP. In LP-matrix IV, the extension gap is neutral, while the flexion gap exhibits physiological lateral laxity (L-FKPflex-latlax). This matches the second accepted goal of kinematic alignment. The acceptable degree of lateral laxity depends on implant and insert design, typically ranging between 2 and 6 mm.13
In LP-matrix type V, the individual L-FKPext and L-FKPflex are balanced within 1 mm (the balancing goal in this case was all gaps are equal).
LP matrix subgroups I and II were rated as intermediate. In both groups, the extension gap exhibited varus alignment (L-FKPext-varus), with the lateral compartment being laxer than the medial. These subtypes may be acceptable if the lateral-medial difference is ≤ 2 mm. However, if the gap asymmetry exceeds 2 mm, the subgroup is reclassified as critical, requiring correction to one of the optimal subgroups, depending on the amount of gap difference and the individual anatomical parameters.
Corrections in the B-FKP workflow, being a personalized alignment strategy, prioritize adjustments through modifications of bone resections. If the pre-set boundaries (e.g. MPTA, LDFA, tibial slope, or HKA) are reached, then soft tissue releases are considered as a secondary option to achieving a more acceptable LP. In contrast, in MA, gap differences can only be minimized by soft tissue release as bone cuts are fixed and standardized.
The final group consists of LP matrix subtypes marked as critical (red), comprising five subgroups (Types III, and VI-IX). These patterns are characterized by medial laxity, either in extension (L-FKPext-valgus) and/or in flexion (L-FKPflex-medlax), which is a well-described failure mode in TKA and is associated with inferior outcomes.18–20 Therefore, transitioning these knees to a safer LP matrix is recommended. The specific treatment options for such a transfer depend on the type and amount of medial laxity.
Treatment of these critical subgroups follows a stepwise approach.1.Modifications of bone resections, respecting the defined anatomicalboundaries;2.Soft tissue releases, if bone modifications alone are insufficient;3.A higher amount of constraint as a final option if both, bone and soft tissue adjustment fail to achieve a stable and acceptable ligament phenotype.
This structured algorithm allows for individualized, reproducible planning within the B-FKP workflow while maintaining safe alignment and soft tissue balance in TKA.
3 Results
Laxity phenotype distribution (Table 1)
| Lateral laxity | Neutral | Medial laxity | Total | |
| Varus | I (26 %) | II (7 %) | III (2 %) | 35 % |
| Straight | IV (23 %) | V (14 %) | VI (7 %) | 44 % |
| Valgus | VII (7 %) | VIII (10 %) | IX (4 %) | 21 % |
| Total | 56 % | 31 % | 13 % | 100 % |
The most common LP matrix subgroups observed were Groups I and IV. Notably, 56 % of the knees showed L-FKPflex-latlax, while nearly one-third showed a neutral flexion gap (L-FKPflex-neutral). This indicates that, in 85 % of the cases, the individual L-FKP in flexion aligns with one of the two defined balancing goals (lateral flexion laxity or equal medial-lateral flexion gaps). In B-FKP workflow the L-FKPext patterns did not correlate with the amount of preoperative deformity. Instead, extension gap configurations were more evenly distributed, with 21 % valgus, and 44 % showing neutral (straight) alignment.
Interestingly, only ≈40 % of all knees fell into LP-matrix Groups IV and V, indicating that complete LP reconstruction is achieved in less than half of the knees without any bony cut modifications or soft tissue release. Groups I and II, which are close to optimal, can typically be balanced towards LP-matrix groups IV and V through minor bone cut modifications. This means that, overall, 70 % of all knees can be completely balanced either without changes or with only minor bone cut adaptions. Contrastingly, the remaining 30 % of cases were categorized into critical LP-matrix Groups III and VI-IX, demonstrating that all these need relevant modifications to their original LP to achieve a stable and balanced joint.
Treatment algorithm for L-FKP matrix (Fig. 2)
3.1 Optimal phenotypes
Two LP-matrix groups (IV and V) are marked in green (Fig. 2), as their LP aligns with the goals for optimal final TKA balancing. In both groups, the extension gap is balanced (≤1 mm difference; L-FPKext-neutr), requiring no further modifications to the individual LP in either extension or flexion. In L-FKP Group IV, the extension gap is neutral and the flexion gap is lateral lax, whereas in Group V, both extension and flexion gaps are neutral (Fig. 2).
Group IV: Treatment algorithm
No adjustment of the individualized LP is required. Lateral laxity up to 6 mm is generally acceptable.13,21 However, to enhance stability, especially in patients with increased activity or instability concerns, the use of a higher-conformity insert (e.g., medial stabilized or medial pivot design) may be considered. Up to 2–4 mm laxity, standard CR/PS inserts have been shown to perform well.22–24
Group V: Treatment algorithm
This group presents with balanced gaps in both extension and flexion and thus requires no further modification of the individualized LP. Standard CR/PS inserts are appropriate.
3.2 Critical phenotypes
LP-matrix groups III and VI through IX are classified as critical (Fig. 2) due to a pattern of medial laxity in either or both flexion and extension. For each of these groups, specific treatment recommendations were defined based on the individual bony anatomy and predefined boundaries for correction.
Group III—Medial laxity in flexion and L-FkPext-varus: Treatment algorithm
This is a very rare subgroup (2 %) and as the medial laxity is obvious only in flexion but not in extension, the surgeon should first control the integrity of the deep MCL. This phenotype can also be seen in revisions from medial unicompartimental knee to total knee arthroplasty. The following should be considered.A)Minor medial laxity (<2 mm):•As the extension gap also needs balancing, increase the LDFA within anatomical boundaries (92°).•Consider reducing femoral external rotation, without going beyond 1.5° of internal rotation as a boundary.•Ensure that patellofemoral tracking remains acceptable.•These adaptions will lead the subgroup from Group III to Group V.B)Medial laxity >2 mm:•If after performing the potential adaptions as described in A) are not leading to a balanced joint and is still showing a medial laxity > 2 mm the use of a VVC (varus-valgus constrained) insert rather than performing additional soft tissue releases is recommended.
Group VI—Medial laxity in flexion and L-FKPext-straight: Treatment algorithm
This is a small group (7 %) and, like in LP-matrix Group III, the surgeon should first verify the integrity of the deep MCL.A)Minor medial laxity (<2 mm):•Reducing femoral external rotation, like in LP-subgroup III, without going beyond 1.5° of internal rotation and without leading to conflicts with patella tracking and patellofemoral kinematics.•No further changes are needed as the extension gap is balanced.•The individual LP classification will shift to Group V.B)Medial laxity ≥2 mm:•If after performing the potential adaptions as described in A) are not leading to a balanced joint and is still showing a medial laxity > 2 mm the use of a VVC (varus-valgus constrained) insert rather than performing additional soft tissue releases is recommended.
Group VII—Medial laxity in extension and L-FKPflex-latlax: Treatment algorithm
This is a small group (7 %) and, as the medial laxity occurs only in extension but not in flexion, the MCL and medial laxity are likely intact.A)Treatment strategy:•To correct valgus imbalance, reduce the LDFA (up to the boundary of 86°).•If this does not solve the imbalance completely, a lateral release of the ilio-tibial band (ITB) and posterior capsule is recommended as the next step.•MPTA should not be altered, as this would increase the lateral laxity in flexion, further exacerbating flexion imbalance.•With these changes, LP-matrix Group VII will shift to Group IV.
Group VIII—Medial laxity in extension and L—FKPflex-neutr: Treatment algorithm
This group includes 10 % of all knees. As medial laxity is only in extension and not in flexion, MCL is likely intact.A)Treatment strategy:•Similar to LP-matrix Group VII, the LDFA should be reduced (up to the boundary of 86°).•In contrast to Group VII, if residual medial imbalance is present, MPTA reduction can be considered as an additional option. With this change, the L-FKPflex might be altered from balanced to lateral lax (i.e., from Group V to Group IV).•If both maneuvers do not solve the imbalance completely, a lateral release of the ITB and posterior capsule is recommended as the next step.
Group IX—Medial laxity in both flexion and extension: Treatment algorithm
This subgroup includes a very small number of knees (4 %). Nonetheless, this is a very problematic LP subgroup as medial laxity exists in both extension and flexion gaps. Therefore, the integrity of MCL needs to be assessed first. If MCL is deficient, a higher constraint such as rotating-hinge designs is recommended. However, if the MCL is intact the following step should be undertaken.•The MPTA should be increased, as this positively affects the extension and flexion gap.•If the increase is within the defined boundaries for MPTA (92°) and internal rotation (IR) of the femur, and the final gap difference is within 1 mm, a standard insert is sufficient.•If the difference is > 1 mm and <2 mm, medial-constrained inserts should be considered.•Larger differences (>2 mm) require higher constraint or hinged implants, depending on the degree of laxity.•Successful correction may reclassify the case into LP-matrix Group V.
3.3 Suboptimal phenotypes
The last two groups, defined as suboptimal phenotypes (Figs. 2 and 3), show lateral laxity in extension (L-FKPext-varus) of more than a 1 mm difference between lateral and medial gaps.
Group I—Lateral laxity in extension, L-FKPflex-latlax: Treatment algorithm
LP-matrix Group I is the biggest in the matrix with 26 % of all patients. To correct this lateral laxity in extension and flexion the following steps should be followed.•MPTA should be reduced unless the boundary of 85° is already achieved.•If the boundary is reached and the imbalance persists, medial capsule release is recommended.•These manoeuvres can reclassify a Group I case into Group IV.
Group II—Lateral laxity in extension, l-FKPflex-neutral: Treatment algorithm
LP-matrix Group II is another important LP-matrix group (14 %). It shows lateral laxity in extension only, while the flexion gap remains neutral. Therefore, the following treatment algorithm is recommended.•LDFA increase, rather than MPTA increase, is preferred to correct extension imbalance without affecting flexion.•If insufficient, the posterior capsule should be released.•These manoeuvres typically shift the LP classification to Group V.
Finally, the treatment algorithm is summarized in Fig. 3.
4 Discussion
The most important findings of this study were: 1. Significant variety in LP exists, with only 40 % displaying an optimal LP-matrix consistent with one of the two standard TKA balancing goals after setting in B-FKP parameters; 2. Applying a one-size-fits-all balancing goal disrupts native laxity by altering the L-FKPflex in over 40 % of the cases. 3. 30 % of the knees exhibit critical L-FKP-matrix groups, requiring conversion into optimal LP groups via adjustment of B-FKP and/or performance of classical release steps. In rare cases, increased constraint is necessary to achieve balance.
Historically, personalized TKA approaches have focused on individual restoration of bony parameters. Measured resection techniques assume that replicating the natural bony anatomy automatically restores natural knee laxity. However, this concept often results in LP configurations incompatible with modern implant balancing goals. In this study, 63 % of the knees displayed non-optimal LP matrix groups after restoration, in line with the findings from Shatrov et al..25
Gap-balanced techniques, such as inverse kinematic alignment (iKA) or functional alignment (FA) show a high success rate in achieving the predefined balancing goal.26–30 However, enforcing a single predefined balancing goal for all knees alters the native L-FKP in a substantial number of cases. In this study, we found that after shifting problematic L-FKP matrix groups into optimal ones, the “all gap equal” balancing goal matched the individual L-FKP in only 40 % of knees. This underlines that, when this balancing goal was applied, ligament tension was altered in over 50 % of cases.
Studies by Bellemans et al.,31 McEwen et al.,18 and Jimenez-Soto et al.21 suggest that preserving native flexion laxity can lead to higher functional outcome scores than when the “all gaps equal” concept is applied. Nonetheless, this balancing goal has been shown to alter the individual L-FKP in 40 % of knees, highlighting that this goal should not be standardized for all knees. While this might be preferable to “all gaps equal”, it still represents a major deviation for many patients and might be associated with persistent discomfort.
An important factor that needs to be considered when leaving the lateral flexion gap laxer is the amount of acceptable laxity. While some studies suggest that up to 4° may be tolerable with posterior-stabilized (PS) implants,13,24 the acceptable limit for medial-stabilized or medial pivot-type designs remains unclear.
There is still a limited understanding of which L-FKP variations are clinically acceptable and which require correction. However, existent evidence shows that medial laxity, whether in extension or flexion, correlates with poorer clinical outcomes and faster implant failure than a balanced knee.32–34 Accordingly, 30 % of the knees were categorized as critical in the present study and were corrected to optimal groups using the developed treatment algorithm.
A possible reason for the high proportion of medial lax knees can be attributed to the potential technical errors in bone angle measurements on long-leg radiographs due to factors like focus-to-film distance (FFD) or rotation, which might result in up to 3° of errors.35–38 Even such small errors can shift a knee from a potentially optimal group to a critical one. Nevertheless, minor corrections in bone angles can often restore these knees to optimal alignment.
Importantly, bone-cut boundaries inherently limit the full restoration of native bony anatomy. Except in unicompartmental knee arthroplasty (UKA), all workflows enforce angle boundaries to avoid reconstructing a pathological alignment and reduce the risk of implant failure of polyethylene wear. As Hirschmann et al. evidenced, only a few implants are designed for such varus/valgus positions.6,39
Introducing boundaries for bone cuts automatically affects L-FKPs. For instance, in severe varus deformities, a boundary will limit bony reconstruction as well as complete L-FKP reconstruction in extension and flexion. Contrastingly, the LDFA boundary in severe valgus deformities will affect L-FKPs in extension only. In addition to considering boundaries in the coronal plane during extension, it is equally important to account for limitations in the flexion gap and the sagittal plane. In this study, we established 1.5 degrees of internal femoral rotation and 8° degrees of posterior tibia slope as the upper boundaries for bony corrections needed to safeguard anatomical and biomechanical outcomes. However, further longitudinal studies are needed to validate these parameters and to correlate them with the optimization of implant survival and long-term functional outcomes.
In this study, a treatment algorithm tailored to each L-FKP matrix group was developed, providing a stepwise approach: bone cuts modification, followed by soft tissue releases, and finally, constraint use if needed. These principles are consistent across all workflows. Personalized workflows (e.g., KA, FA) allow both bone and soft tissue modifications. In contrast, systematic workflows (e.g., MA, anatomical alignment) often fix bone geometry, requiring more reliance on soft tissue releases, which are inherently harder to fine-tune, especially for isolated gap imbalances.
A key limitation of this study is the sample size, as it is based on a simulation cohort. However, the structure of the L-FKP matrix and the treatment algorithm are conceptually robust and not sample-size dependent. Another limitation is the homogeneity of the assessed cohort, with all patients being Caucasian, which may affect the distribution in the LP-FKP matrix groups but the validity of the treatment approach remains unaffected.
5 Conclusion
The L-FKP matrix comprises nine distinct groups. Only two correspond to standardized TKA balancing goals. Neither “all gaps equal” nor “lateral flexion gap laxity” succeed in reproducing the individual L-FKP matrix group in over 60 % of knees. Therefore, a personalized workflow, combining restoration of both B-FKP and L-FKP, is essential for true patient-specific TKA. Approximately 30 % of knees require reclassification of their L-FKP matrix group to achieve optimal balance. The treatment algorithm proposed in this study provides a structured, group-specific pathway that begins with bony corrections, followed by soft-tissue release, and finally the use of higher constraints when necessary. While applicable to any workflow, the algorithm is most effectively implemented within personalized frameworks that permit both bone and soft tissue adjustments.
Patient consent statement
Not applicable.
Ethical approval
Not applicable.
Author's contributions according to CRediT taxonomy
HG: conceptualization, formal analysis, writing-original draft, writing-review and editing; GMA: writing-review and editing, formal analysis; RE: writing-review and editing, visualization; AS: writing-review and editing, visualization; AMN: writing-review and editing; MTH: writing-original draft, writing-review and editing, formal analysis.
Funding statement
No funding was provided for this work.
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