Postoperative knee instability is one of the major reasons accounting for unsatisfactory outcomes, as well as a major failure mechanism leading to revision surgery after primary total knee arthroplasty (TKA). However, postoperative knee instability is not well defined clinically, and the boundaries between stable and unstable TKA knees are still poorly understood. This lack of understanding is likely due to the multitude of factors that are associated with knee instability, including inadequate soft-tissue balancing (Hosseini Nasab et al., 2019; Song et al., 2014), loss of ligamentous integrity (Nagle and Glynn, 2020), and improper component sizing (Nagle and Glynn, 2020; Parratte and Pagnano, 2008).

Clinical assessment of joint stability relies on passive laxity examinations and patient-reported outcome measures (PROMs). Manual stress tests, including the anterior/posterior drawer, Lachman, and varus/valgus tests, as well as questionnaires such as the Knee injury and Osteoarthritis Outcome Score (KOOS) (Roos et al., 1998), Oxford Knee Score (OKS) (Dawson et al., 1998), University of California and Los Angeles (UCLA) Activity Scale (Zahiri et al., 1998), and the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) (Bellamy et al., 1988) are widely used in clinical practice to assess knee instability after TKA. However, such clinical assessments are highly subjective with large inter-rater variability (Mears et al., 2022). With the goal to objectively investigate knee instability, methods for quantifying passive knee laxity such as the KT-1000/2000 (White et al., 1991; Ishii et al., 2005), Telos (Murer et al., 2021; Moser et al., 2022; Jung et al., 2006), Rotometer (Moewis et al., 2016; Lorbach et al., 2012), and Rolimeter (Schuster et al., 2011) devices, all generally combined with stress radiography, have been presented. Studies using these techniques have demonstrated that passive knee laxity plays a key role for functional outcomes and patient satisfaction after TKA (Oh et al., 2015; Jones et al., 2006). Nevertheless, it remains unclear whether self-reported instability after TKA is reflected in implant kinematics in vivo during functional activities of daily living. Specifically, both downhill walking and stair descent are considered as challenging tasks for subjects with knee pathologies (Stacoff et al., 2005; Simon et al., 2018), and could therefore present relevant activities for provoking feelings of instability.

As the primary actuators of the locomotion system, muscles play a critical role in guiding motor functionality as well as supporting stability of the knee joint. Muscular deficits are often observed after TKA and closely correlate with physical impairment and limitations in daily activities (Berman et al., 1991; Walsh et al., 1998). Well-coordinated muscular activity is thought to be necessary for normal gait patterns (Berman et al., 1991). Here, adaptation of muscle recruitment patterns in the early postoperative phase, where increased knee extensor (quadriceps) and flexor (hamstrings) co-activation has generally been observed, is thought to enhance knee stability (Lundberg et al., 2016; Benedetti et al., 2003). However, whether such muscular adaptation is temporary and can be reversed in the long term after TKA remains controversially discussed (Benedetti et al., 2003; Thomas et al., 2014). Moreover, it remains unknown whether such adaptive strategies persist in unstable knees, or whether different lower limb muscle synergy patterns develop to compensate for deficits in knee stability.

While the accurate assessment of tibiofemoral kinematics during functional activities is challenging, the recent development of dynamic video-fluoroscopy systems now provides access to implant kinematics with a high level of accuracy and without soft-tissue artefact throughout consecutive cycles of gait activities (Taylor et al., 2017; Schütz et al., 2019b; Schütz et al., 2019a; List et al., 2020). Such systems have recently allowed investigations into the impact of activity (Schütz et al., 2019a) and implant geometry (Schütz et al., 2019b; List et al., 2020) on tibiofemoral kinematics during functional activities such as level walking, downhill walking, and stair descent, and can be combined with electromyography (EMG) to assess muscle activation synergies and its role on modulating joint kinematics (Benedetti et al., 2003; Taylor et al., 2017; Ardestani et al., 2017). As such, the application of these approaches to subjects with stable and unstable knees after TKA could establish whether compensation mechanisms continue to occur, but also elucidate the role of neuromuscular activation and coordination on kinematic adaptations in the joint.

To understand tibiofemoral kinematics in unstable TKA knees and its interaction with muscle activation strategies, the objective of this study was to investigate in vivo tibiofemoral implant kinematics and muscle synergy patterns in subjects with stable and self-reported unstable knees after TKA during functional activities of daily living using dynamic video-fluoroscopy and EMG. Specifically, we aimed to establish whether unstable knees exhibit higher levels of relative tibiofemoral motion and distinct muscle synergy patterns/strategies than their stable counterparts.

Knee instability is one of the most common reasons for unsatisfactory outcomes after TKA, accounting for up to 26% of revisions (Parratte and Pagnano, 2008; Wilson et al., 2017). However, the relationships between self-reported assessment of instability and knee functionality in daily living remain unclear. Using moving video-fluoroscopy and EMG, we compared kinematic parameters of knee joint stability, as well as muscle synergy patterns, between subjects who had expressed concern at the stability of their knee, and those who were happy with its functional performance. Our results indicate that kinematic parameters, including A-P translations and knee rotations, as well as their RoMs during functional activities, were generally comparable between stable and unstable TKA knees. However, increased heterogeneity in muscle synergy patterns between subjects and across activities, especially during challenging tasks such as stair descent, as well as prolonged activation of the classified knee flexor synergy, were observed in the unstable group. These differences between groups plausibly reveal long-lasting muscular adaptation strategies that develop as a compensation mechanism for feelings of joint instability and serve to prevent possible unstable events. Despite these modifications in muscular strategies, however, specific cases of acute instability were still reported by some subjects during the measurements, and analysis of their data revealed clear deviations in kinematic patterns from those of both the stable and unstable groups. These observations suggest that accurate movement analysis might be highly sensitive for detecting acute instability events, but might be less robust in identifying general joint instability.

The range of A-P translation in both groups studied here was comparable to previous studies during functional activities in good outcome TKAs with similar UC inlay geometries (Schütz et al., 2019b; Roberti di Sarsina et al., 2022; Cardinale et al., 2020). The so-called ‘paradoxical anterior translation’ pattern that is commonly observed in tibiofemoral kinematics after TKA (Dennis et al., 2003) was also observed in both groups during level walking and became even more evident during downhill walking and stair descent. Interestingly, while the stable and unstable groups showed comparable A-P translation RoMs on both condyles (Table 2), the assessment of subject-specific tibiofemoral kinematics was capable of identifying acute instability events. In vivo kinematics of three unstable knees who reported instability during downhill walking and stair descent showed distinct A-P motion patterns, with an early, rapid condylar posterior translation followed by an additional anterior translation in the early/mid-swing phase (Figure 3). Such kinematic patterns stand out from those of both the stable cohort and even the unstable knees that did not report instability during the measured activities. Although our findings suggest that the accurate assessment of tibiofemoral kinematics has the potential to identify acute instability events, we have not been able to detect a common distinct kinematic pattern throughout the cohort of unstable knees.

Extensive efforts have been made to understand kinematic adaptations in knee pathologies including osteoarthritis (OA) and posterior cruciate ligament deficiency (PCLD). However, no significant differences have yet been found between PCLD and healthy knees in abduction/adduction or in internal/external rotation (Fontboté et al., 2005), nor in knee flexion/extension between OA knees and healthy controls (Na et al., 2018) during level gait. During more challenging activities such as stair descent, the differences in knee flexion angle and extensor torque between PCLD and healthy knees have been found to be statistically significant but presented no outstanding clinical consequences (Hooper et al., 2002). Interestingly, the structural deficits presented in such cases did not seem to result in clear kinematic deviations. Similarly in our study of knee joint instability, it seems that tibiofemoral movement patterns become well-compensated/controlled, plausibly through altered muscular control mechanisms.

It was previously found that unsatisfactory TKA knees present fewer muscle synergies for locomotion compared to satisfactory TKA knees and healthy controls (Ardestani et al., 2017). However, our results suggest that more heterogenous synergy patterns were developed in the unstable compared to the stable knees in the long term, albeit with a comparable number of synergies extracted from both groups in all activities. A higher number of non-classifiable, subject-specific muscle synergies was identified in the unstable compared to the stable group during level walking, and this number further increased during the more demanding activities of downhill walking and stair descent. This finding suggests that more challenging locomotor activities have a greater potential for provoking knee instability, which is plausibly linked to the requirement for a specific or individual response to each unstable situation or musculoskeletal deficit. Higher FWHM in the knee-/dorsiflexor-controlled activation patterns were observed in the unstable knees compared to their stable counterparts during stair descent, which further indicates that the feeling of instability can permanently trigger prolonged activation of flexor muscle groups. This finding is in line with previous studies reporting the widening of muscle activation patterns in pathologies (Cappellini et al., 2016; Martino et al., 2015; Janshen et al., 2020), early developmental stage (Cappellini et al., 2016; Dominici et al., 2011), ageing (Santuz et al., 2020; Dewolf et al., 2021), and presence of external mechanical perturbations (Santuz et al., 2018a). A possible explanation for this peculiar behaviour can be found in mouse studies, where similar temporal modifications of muscle activity have been associated with a lack of sensory feedback from proprioceptors (Akay et al., 2014; Mayer and Akay, 2018; Santuz et al., 2019). Interestingly, our results showed that knee extensor–flexor co-activation was present throughout the stance phase of the gait cycle in both TKA groups in all activities. However, this co-activation pattern was generally not reported in healthy stable knees (Santuz et al., 2018a; Cappellini et al., 2006; Lacquaniti et al., 2012). This finding shows that co-activation of knee extensors and flexors appears after TKA and persists in the long term. Moreover, we found a major involvement in TKA knees of knee flexors in the propulsion/early-swing phase during all activities, contrary to what is typically found in healthy subjects (Santuz et al., 2020; Santuz et al., 2018a; Cappellini et al., 2006; Lacquaniti et al., 2012). These activation bursts of the knee flexors possibly serve as an additional adaptive strategy to ensure joint stability during daily activities (Benedetti et al., 2003; Hirokawa et al., 1991).

There are some limitations to this retrospective observational study. Firstly, only small sample sizes were considered, which consisted of 70% male in the stable group but >60% female in the unstable group. Since subjective differences in soft-tissue properties are known to exist between males and females, it is possible that gender imbalance could have influenced the results of the passive laxity tests presented. Even though no statistical differences were found between our study groups, a more balanced cohort should be investigated in future to confirm the findings of this study. Moreover, self-reported instability is highly subjective and activity dependent, which is visible in the high inter-subject kinematic variability, and not all patients in the unstable group reported a feeling of instability during measurement. Our analyses have, therefore, not been able to capture all the characteristics of self-reported instability, but have rather been able to make progress in understanding the relationships between several important confounding factors, as well as the individual compensation mechanisms that have developed due to joint instability. The unstable knees in this study demonstrated highly heterogenous, unclassifiable muscle synergies with variable activation patterns, suggesting patient-specific muscular strategies. However, it remains unclear whether the observed synergy patterns are a hallmark for classifying knee stability. Lastly, only a single implant type was examined in this study. Here, we acknowledge that considerable further investigation would be required to establish whether the same or similar results are observed in subjects with other implants. Moreover, it is likely that more challenging tasks (e.g. stop-and-go activities) could expose the subjects to more frequent instability events than the daily activities investigated in our study, but such tasks are hard to standardize for reliable assessment of kinematic instability.

Source Data files and related codes have been provided for all Figures and Tables in the supplementary and can be found here: https://doi.org/10.3929/ethz-b-000584582.

1. 1. Rao L

   (2022) ETH Library research collection

   Comparable in vivo joint kinematics between self-reported stable and unstable knees after TKA can be explained by muscular adaptation strategies.

   https://doi.org/10.3929/ethz-b-000584582

1. 1. Akay T
   2. Tourtellotte WG
   3. Arber S
   4. Jessell TM

   (2014) Degradation of mouse locomotor pattern in the absence of proprioceptive sensory feedback

   PNAS 111:16877–16882.

   https://doi.org/10.1073/pnas.1419045111

   • PubMed
   • Google Scholar
2. 1. Ardestani MM
   2. Malloy P
   3. Nam D
   4. Rosenberg AG
   5. Wimmer MA

   (2017) TKA patients with unsatisfying knee function show changes in neuromotor synergy pattern but not joint biomechanics

   Journal of Electromyography and Kinesiology 37:90–100.

   https://doi.org/10.1016/j.jelekin.2017.09.006

   • PubMed
   • Google Scholar
3. 1. Bellamy N
   2. Buchanan WW
   3. Goldsmith CH
   4. Campbell J
   5. Stitt LW

   (1988)

   Validation study of WOMAC: a health status instrument for measuring clinically important patient relevant outcomes to antirheumatic drug therapy in patients with osteoarthritis of the hip or knee

   The Journal of Rheumatology 15:1833–1840.

   • PubMed
   • Google Scholar
4. 1. Benedetti MG
   2. Catani F
   3. Bilotta TW
   4. Marcacci M
   5. Mariani E
   6. Giannini S

   (2003) Muscle activation pattern and gait biomechanics after total knee replacement

   Clinical Biomechanics 18:871–876.

   https://doi.org/10.1016/S0268-0033(03)00146-3

   • Google Scholar
5. 1. Berman AT
   2. Bosacco SJ
   3. Israelite C

   (1991) Evaluation of total knee arthroplasty using isokinetic testing

   Clinical Orthopaedics and Related Research 271:106.

   https://doi.org/10.1097/00003086-199110000-00015

   • Google Scholar
6. 1. Burckhardt K
   2. Székely G
   3. Nötzli H
   4. Hodler J
   5. Gerber C

   (2005) Submillimeter measurement of cup migration in clinical standard radiographs

   IEEE Transactions on Medical Imaging 24:676–688.

   https://doi.org/10.1109/TMI.2005.846849

   • PubMed
   • Google Scholar
7. 1. Cappellini G
   2. Ivanenko YP
   3. Poppele RE
   4. Lacquaniti F

   (2006) Motor patterns in human walking and running

   Journal of Neurophysiology 95:3426–3437.

   https://doi.org/10.1152/jn.00081.2006

   • PubMed
   • Google Scholar
8. 1. Cappellini G
   2. Ivanenko YP
   3. Martino G
   4. MacLellan MJ
   5. Sacco A
   6. Morelli D
   7. Lacquaniti F

   (2016) Immature spinal locomotor output in children with cerebral palsy

   Frontiers in Physiology 7:478.

   https://doi.org/10.3389/fphys.2016.00478

   • PubMed
   • Google Scholar
9. 1. Cardinale U
   2. Bragonzoni L
   3. Bontempi M
   4. Alesi D
   5. Roberti di Sarsina T
   6. Lo Presti M
   7. Zaffagnini S
   8. Marcheggiani Muccioli GM
   9. Iacono F

   (2020) Knee kinematics after cruciate retaining highly congruent mobile bearing total knee arthroplasty: an in vivo dynamic RSA study

   The Knee 27:341–347.

   https://doi.org/10.1016/j.knee.2019.11.003

   • PubMed
   • Google Scholar
10. 1. Dawson J
    2. Fitzpatrick R
    3. Murray D
    4. Carr A

    (1998) Questionnaire on the perceptions of patients about total knee replacement

    The Journal of Bone and Joint Surgery. British Volume 80:63–69.

    https://doi.org/10.1302/0301-620x.80b1.7859

    • PubMed
    • Google Scholar
11. 1. Dennis DA
    2. Komistek RD
    3. Mahfouz MR

    (2003) In vivo fluoroscopic analysis of fixed-bearing total knee replacements

    Clinical Orthopaedics & Related Research 410:114–130.

    https://doi.org/10.1097/01.blo.0000062385.79828.72

    • Google Scholar
12. 1. Dewolf AH
    2. Sylos-Labini F
    3. Cappellini G
    4. Ivanenko Y
    5. Lacquaniti F

    (2021) Age-Related changes in the neuromuscular control of forward and backward locomotion

    PLOS ONE 16:e0246372.

    https://doi.org/10.1371/journal.pone.0246372

    • PubMed
    • Google Scholar
13. 1. Dominici N
    2. Ivanenko YP
    3. Cappellini G
    4. d’Avella A
    5. Mondì V
    6. Cicchese M
    7. Fabiano A
    8. Silei T
    9. Di Paolo A
    10. Giannini C
    11. Poppele RE
    12. Lacquaniti F

    (2011) Locomotor primitives in newborn babies and their development

    Science 334:997–999.

    https://doi.org/10.1126/science.1210617

    • PubMed
    • Google Scholar
14. 1. Fontboté CA
    2. Sell TC
    3. Laudner KG
    4. Haemmerle M
    5. Allen CR
    6. Margheritini F
    7. Lephart SM
    8. Harner CD

    (2005) Neuromuscular and biomechanical adaptations of patients with isolated deficiency of the posterior cruciate ligament

    The American Journal of Sports Medicine 33:982–989.

    https://doi.org/10.1177/0363546504271966

    • PubMed
    • Google Scholar
15. Book

    1. Foresti M

    (2009)

    In Vivo Measurement of Total Knee Joint Replacement Kinematics and Kinetics During Stair Descent

    ETH.

    • Google Scholar
16. 1. Grood ES
    2. Suntay WJ

    (1983) A joint coordinate system for the clinical description of three-dimensional motions: application to the knee

    Journal of Biomechanical Engineering 105:136–144.

    https://doi.org/10.1115/1.3138397

    • PubMed
    • Google Scholar
17. 1. Hirokawa S
    2. Solomonow M
    3. Luo Z
    4. Lu Y
    5. D’Ambrosia R

    (1991) Muscular co-contraction and control of knee stability

    Journal of Electromyography and Kinesiology 1:199–208.

    https://doi.org/10.1016/1050-6411(91)90035-4

    • PubMed
    • Google Scholar
18. 1. Hooper DM
    2. Morrissey MC
    3. Crookenden R
    4. Ireland J
    5. Beacon JP

    (2002) Gait adaptations in patients with chronic posterior instability of the knee

    Clinical Biomechanics 17:227–233.

    https://doi.org/10.1016/s0268-0033(02)00002-5

    • PubMed
    • Google Scholar
19. 1. Hosseini Nasab SH
    2. Smith CR
    3. Schütz P
    4. Postolka B
    5. List R
    6. Taylor WR

    (2019) Elongation patterns of the collateral ligaments after total knee arthroplasty are dominated by the knee flexion angle

    Frontiers in Bioengineering and Biotechnology 7:323.

    https://doi.org/10.3389/fbioe.2019.00323

    • PubMed
    • Google Scholar
20. 1. Ishii Y
    2. Matsuda Y
    3. Ishii R
    4. Sakata S
    5. Omori G

    (2005) Sagittal laxity in vivo after total knee arthroplasty

    Archives of Orthopaedic and Trauma Surgery 125:249–253.

    https://doi.org/10.1007/s00402-004-0712-3

    • Google Scholar
21. 1. Janshen L
    2. Santuz A
    3. Ekizos A
    4. Arampatzis A

    (2020) Fuzziness of muscle synergies in patients with multiple sclerosis indicates increased robustness of motor control during walking

    Scientific Reports 10:7249.

    https://doi.org/10.1038/s41598-020-63788-w

    • PubMed
    • Google Scholar
22. 1. Jones DPG
    2. Locke C
    3. Pennington J
    4. Theis J-C

    (2006) The effect of sagittal laxity on function after posterior cruciate-retaining total knee replacement

    The Journal of Arthroplasty 21:719–723.

    https://doi.org/10.1016/j.arth.2005.08.019

    • PubMed
    • Google Scholar
23. 1. Jung TM
    2. Reinhardt C
    3. Scheffler SU
    4. Weiler A

    (2006) Stress radiography to measure posterior cruciate ligament insufficiency: a comparison of five different techniques

    Knee Surgery, Sports Traumatology, Arthroscopy 14:1116–1121.

    https://doi.org/10.1007/s00167-006-0137-3

    • Google Scholar
24. 1. Lacquaniti F
    2. Ivanenko YP
    3. Zago M

    (2012) Patterned control of human locomotion

    The Journal of Physiology 590:2189–2199.

    https://doi.org/10.1113/jphysiol.2011.215137

    • PubMed
    • Google Scholar
25. 1. List R
    2. Postolka B
    3. Schütz P
    4. Hitz M
    5. Schwilch P
    6. Gerber H
    7. Ferguson SJ
    8. Taylor WR

    (2017) A moving fluoroscope to capture tibiofemoral kinematics during complete cycles of free level and downhill walking as well as stair descent

    PLOS ONE 12:e0185952.

    https://doi.org/10.1371/journal.pone.0185952

    • PubMed
    • Google Scholar
26. 1. List R
    2. Schütz P
    3. Angst M
    4. Ellenberger L
    5. Dätwyler K
    6. Ferguson SJ
    7. Writing Committee

    (2020) Videofluoroscopic evaluation of the influence of a gradually reducing femoral radius on joint kinematics during daily activities in total knee arthroplasty

    The Journal of Arthroplasty 35:3010–3030.

    https://doi.org/10.1016/j.arth.2020.05.039

    • PubMed
    • Google Scholar
27. 1. Lorbach O
    2. Brockmeyer M
    3. Kieb M
    4. Zerbe T
    5. Pape D
    6. Seil R

    (2012) Objective measurement devices to assess static rotational knee laxity: focus on the rotameter

    Knee Surgery, Sports Traumatology, Arthroscopy 20:639–644.

    https://doi.org/10.1007/s00167-011-1876-3

    • PubMed
    • Google Scholar
28. 1. Lundberg HJ
    2. Rojas IL
    3. Foucher KC
    4. Wimmer MA

    (2016) Comparison of antagonist muscle activity during walking between total knee replacement and control subjects using unnormalized electromyography

    The Journal of Arthroplasty 31:1331–1339.

    https://doi.org/10.1016/j.arth.2015.12.006

    • PubMed
    • Google Scholar
29. 1. Martino G
    2. Ivanenko YP
    3. d’Avella A
    4. Serrao M
    5. Ranavolo A
    6. Draicchio F
    7. Cappellini G
    8. Casali C
    9. Lacquaniti F

    (2015) Neuromuscular adjustments of gait associated with unstable conditions

    Journal of Neurophysiology 114:2867–2882.

    https://doi.org/10.1152/jn.00029.2015

    • PubMed
    • Google Scholar
30. 1. Mayer WP
    2. Akay T

    (2018) Stumbling corrective reaction elicited by mechanical and electrical stimulation of the saphenous nerve in walking mice

    The Journal of Experimental Biology 221:jeb178095.

    https://doi.org/10.1242/jeb.178095

    • PubMed
    • Google Scholar
31. 1. Mears SC
    2. Severin AC
    3. Wang J
    4. Thostenson JD
    5. Mannen EM
    6. Stambough JB
    7. Edwards PK
    8. Barnes CL

    (2022) Inter-Rater reliability of clinical testing for laxity after knee arthroplasty

    The Journal of Arthroplasty 37:1296–1301.

    https://doi.org/10.1016/j.arth.2022.03.044

    • Google Scholar
32. 1. Moewis P
    2. Duda GN
    3. Jung T
    4. Heller MO
    5. Boeth H
    6. Kaptein B
    7. Taylor WR

    (2016) The restoration of passive rotational tibio-femoral laxity after anterior cruciate ligament reconstruction

    PLOS ONE 11:e0159600.

    https://doi.org/10.1371/journal.pone.0159600

    • PubMed
    • Google Scholar
33. 1. Moser LB
    2. Koch M
    3. Hess S
    4. Prabhakar P
    5. Rasch H
    6. Amsler F
    7. Hirschmann MT

    (2022) Stress radiographs in the posterior drawer position at 90° flexion should be used for the evaluation of the PCL in Cr TKA with flexion instability

    Journal of Clinical Medicine 11:1013.

    https://doi.org/10.3390/jcm11041013

    • PubMed
    • Google Scholar
34. 1. Murer M
    2. Falkowski AL
    3. Hirschmann A
    4. Amsler F
    5. Hirschmann MT

    (2021) Threshold values for stress radiographs in unstable knees after total knee arthroplasty

    Knee Surgery, Sports Traumatology, Arthroscopy 29:422–428.

    https://doi.org/10.1007/s00167-020-05964-z

    • PubMed
    • Google Scholar
35. 1. Na A
    2. Piva SR
    3. Buchanan TS

    (2018) Influences of knee osteoarthritis and walking difficulty on knee kinematics and kinetics

    Gait & Posture 61:439–444.

    https://doi.org/10.1016/j.gaitpost.2018.01.025

    • Google Scholar
36. 1. Nagle M
    2. Glynn A

    (2020) Midflexion instability in primary total knee arthroplasty

    The Journal of Knee Surgery 33:459–465.

    https://doi.org/10.1055/s-0039-1678537

    • PubMed
    • Google Scholar
37. 1. Oh CS
    2. Song EK
    3. Seon JK
    4. Ahn YS

    (2015) The effect of flexion balance on functional outcomes in cruciate-retaining total knee arthroplasty

    Archives of Orthopaedic and Trauma Surgery 135:401–406.

    https://doi.org/10.1007/s00402-015-2159-0

    • PubMed
    • Google Scholar
38. 1. Parratte S
    2. Pagnano MW

    (2008)

    Instability after total knee arthroplasty

    The Journal of Bone and Joint Surgery. American Volume 90:184–194.

    • PubMed
    • Google Scholar
39. 1. Pataky TC
    2. Robinson MA
    3. Vanrenterghem J

    (2016) Region-of-interest analyses of one-dimensional biomechanical trajectories: bridging 0D and 1D theory, augmenting statistical power

    PeerJ 4:e2652.

    https://doi.org/10.7717/peerj.2652

    • Google Scholar
40. 1. Roberti di Sarsina T
    2. Alesi D
    3. Di Paolo S
    4. Zinno R
    5. Pizza N
    6. Marcheggiani Muccioli GM
    7. Zaffagnini S
    8. Bragonzoni L

    (2022) In vivo kinematic comparison between an ultra-congruent and a posterior-stabilized total knee arthroplasty design by RSA

    Knee Surgery, Sports Traumatology, Arthroscopy 30:2753–2758.

    https://doi.org/10.1007/s00167-021-06629-1

    • PubMed
    • Google Scholar
41. 1. Roos EM
    2. Roos HP
    3. Lohmander LS
    4. Ekdahl C
    5. Beynnon BD

    (1998) Knee injury and osteoarthritis outcome score (KOOS) —development of a self-administered outcome measure

    Journal of Orthopaedic & Sports Physical Therapy 28:88–96.

    https://doi.org/10.2519/jospt.1998.28.2.88

    • Google Scholar
42. 1. Santuz A
    2. Ekizos A
    3. Janshen L
    4. Baltzopoulos V
    5. Arampatzis A

    (2017) On the methodological implications of extracting muscle synergies from human locomotion

    International Journal of Neural Systems 27:1750007.

    https://doi.org/10.1142/S0129065717500071

    • PubMed
    • Google Scholar
43. 1. Santuz A
    2. Ekizos A
    3. Eckardt N
    4. Kibele A
    5. Arampatzis A

    (2018a) Challenging human locomotion: stability and modular organisation in unsteady conditions

    Scientific Reports 8:2740.

    https://doi.org/10.1038/s41598-018-21018-4

    • PubMed
    • Google Scholar
44. 1. Santuz A
    2. Ekizos A
    3. Janshen L
    4. Mersmann F
    5. Bohm S
    6. Baltzopoulos V
    7. Arampatzis A

    (2018b) Modular control of human movement during running: an open access data set

    Frontiers in Physiology 9:1509.

    https://doi.org/10.3389/fphys.2018.01509

    • PubMed
    • Google Scholar
45. 1. Santuz A
    2. Akay T
    3. Mayer WP
    4. Wells TL
    5. Schroll A
    6. Arampatzis A

    (2019) Modular organization of murine locomotor pattern in the presence and absence of sensory feedback from muscle spindles

    The Journal of Physiology 597:3147–3165.

    https://doi.org/10.1113/JP277515

    • PubMed
    • Google Scholar
46. 1. Santuz A
    2. Brüll L
    3. Ekizos A
    4. Schroll A
    5. Eckardt N
    6. Kibele A
    7. Schwenk M
    8. Arampatzis A

    (2020) Neuromotor dynamics of human locomotion in challenging settings

    IScience 23:100796.

    https://doi.org/10.1016/j.isci.2019.100796

    • Google Scholar
47. 1. Santuz A

    (2022) MusclesyneRgies: factorization of electromyographic data in R with sensible defaults

    Journal of Open Source Software 7:4439.

    https://doi.org/10.21105/joss.04439

    • Google Scholar
48. 1. Schuster AJ
    2. von Roll AL
    3. Pfluger D
    4. Wyss T

    (2011) Anteroposterior stability after posterior cruciate-retaining total knee arthroplasty

    Knee Surgery, Sports Traumatology, Arthroscopy 19:1113–1120.

    https://doi.org/10.1007/s00167-010-1364-1

    • PubMed
    • Google Scholar
49. 1. Schütz P
    2. Postolka B
    3. Gerber H
    4. Ferguson SJ
    5. Taylor WR
    6. List R

    (2019a) Knee implant kinematics are task-dependent

    Journal of the Royal Society, Interface 16:20180678.

    https://doi.org/10.1098/rsif.2018.0678

    • PubMed
    • Google Scholar
50. 1. Schütz P
    2. Taylor WR
    3. Postolka B
    4. Fucentese SF
    5. Koch PP
    6. Freeman MAR
    7. Pinskerova V
    8. List R

    (2019b) Kinematic evaluation of the gmk sphere implant during gait activities: a dynamic videofluoroscopy study

    Journal of Orthopaedic Research 37:2337–2347.

    https://doi.org/10.1002/jor.24416

    • PubMed
    • Google Scholar
51. 1. Simon JC
    2. Della Valle CJ
    3. Wimmer MA

    (2018) Level and downhill walking to assess implant functionality in bicruciate- and posterior cruciate-retaining total knee arthroplasty

    The Journal of Arthroplasty 33:2884–2889.

    https://doi.org/10.1016/j.arth.2018.05.010

    • PubMed
    • Google Scholar
52. 1. Song SJ
    2. Detch RC
    3. Maloney WJ
    4. Goodman SB
    5. Huddleston JI

    (2014) Causes of instability after total knee arthroplasty

    The Journal of Arthroplasty 29:360–364.

    https://doi.org/10.1016/j.arth.2013.06.023

    • PubMed
    • Google Scholar
53. 1. Stacoff A
    2. Diezi C
    3. Luder G
    4. Stüssi E
    5. Kramers-de Quervain IA

    (2005) Ground reaction forces on stairs: effects of stair inclination and age

    Gait & Posture 21:24–38.

    https://doi.org/10.1016/j.gaitpost.2003.11.003

    • PubMed
    • Google Scholar
54. 1. Taylor WR
    2. Schütz P
    3. Bergmann G
    4. List R
    5. Postolka B
    6. Hitz M
    7. Dymke J
    8. Damm P
    9. Duda G
    10. Gerber H
    11. Schwachmeyer V
    12. Hosseini Nasab SH
    13. Trepczynski A
    14. Kutzner I

    (2017) A comprehensive assessment of the musculoskeletal system: the CAMS-knee data set

    Journal of Biomechanics 65:32–39.

    https://doi.org/10.1016/j.jbiomech.2017.09.022

    • PubMed
    • Google Scholar
55. 1. Thomas AC
    2. Judd DL
    3. Davidson BS
    4. Eckhoff DG
    5. Stevens-Lapsley JE

    (2014) Quadriceps/hamstrings co-activation increases early after total knee arthroplasty

    The Knee 21:1115–1119.

    https://doi.org/10.1016/j.knee.2014.08.001

    • PubMed
    • Google Scholar
56. 1. Walsh M
    2. Woodhouse LJ
    3. Thomas SG
    4. Finch E

    (1998) Physical impairments and functional limitations: a comparison of individuals 1 year after total knee arthroplasty with control subjects

    Physical Therapy 78:248–258.

    https://doi.org/10.1093/ptj/78.3.248

    • PubMed
    • Google Scholar
57. 1. White S
    2. O’Connor J
    3. Goodfellow J

    (1991) Sagittal plane laxity following knee arthroplasty

    The Journal of Bone and Joint Surgery. British Volume 73-B:268–270.

    https://doi.org/10.1302/0301-620X.73B2.2005152

    • Google Scholar
58. 1. Wilson CJ
    2. Theodoulou A
    3. Damarell RA
    4. Krishnan J

    (2017) Knee instability as the primary cause of failure following total knee arthroplasty (TKA): a systematic review on the patient, surgical and implant characteristics of revised TKA patients

    The Knee 24:1271–1281.

    https://doi.org/10.1016/j.knee.2017.08.060

    • PubMed
    • Google Scholar
59. 1. Zahiri CA
    2. Schmalzried TP
    3. Szuszczewicz ES
    4. Amstutz HC

    (1998) Assessing activity in joint replacement patients

    The Journal of Arthroplasty 13:890–895.

    https://doi.org/10.1016/s0883-5403(98)90195-4

    • PubMed
    • Google Scholar

Author details

1. Longfeng Rao

   Laboratory for Movement Biomechanics, Institute for Biomechanics, Zurich, Switzerland

   Contribution

   Conceptualization, Data curation, Formal analysis, Funding acquisition, Validation, Investigation, Visualization, Methodology, Writing - original draft, Writing – review and editing

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-2139-4972

2. Nils Horn

   Department of lower extremities, Schulthess Clinic Zurich, Zurich, Switzerland

   Contribution

   Conceptualization, Resources, Data curation, Supervision, Investigation, Methodology, Writing – review and editing

   Competing interests

   No competing interests declared

3. Nadja Meister

   Laboratory for Movement Biomechanics, Institute for Biomechanics, Zurich, Switzerland

   Contribution

   Data curation, Formal analysis, Validation, Investigation, Writing – review and editing

   Competing interests

   No competing interests declared

4. Stefan Preiss

   Department of lower extremities, Schulthess Clinic Zurich, Zurich, Switzerland

   Contribution

   Conceptualization, Resources, Supervision, Investigation, Methodology, Project administration, Writing – review and editing

   Competing interests

   No competing interests declared

5. William R Taylor

   Laboratory for Movement Biomechanics, Institute for Biomechanics, Zurich, Switzerland

   Contribution

   Conceptualization, Resources, Supervision, Validation, Visualization, Methodology, Project administration, Writing – review and editing

   For correspondence

   bt@ethz.ch

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0003-4060-4098

6. Alessandro Santuz

   1. Laboratory for Movement Biomechanics, Institute for Biomechanics, Zurich, Switzerland
   2. Max Delbrück Center for Molecular Medicine, Berlin, Germany

   Contribution

   Software, Formal analysis, Validation, Visualization, Methodology, Writing – review and editing

   Competing interests

   No competing interests declared

7. Pascal Schütz

   Laboratory for Movement Biomechanics, Institute for Biomechanics, Zurich, Switzerland

   Contribution

   Conceptualization, Resources, Software, Supervision, Validation, Investigation, Visualization, Methodology, Writing – review and editing

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0003-1711-7881

• 431

  views

• 64

  downloads

• 2

  citations

Views, downloads and citations are aggregated across all versions of this paper published by eLife.