Cruciate retaining and cruciate substituting ultra-congruent insert

Introduction

The role of posterior cruciate ligament (PCL) conservation and the choice of the level of constraint of polyethylene insert in total knee arthroplasty (TKA) are still debated in literature. The PCL is considered one of the primary stabilizers of the joint (1) and its retention may influence knee stability, kinematic, proprioception and it may reduce the shear forces on the tibia. However, cruciate retaining (CR) implants have some disadvantages, first of all the possible difficulty to obtain a good balancing of the PCL. For those reasons, the so-called postero-stabilized (PS) implants were introduced (2,3).

Theoretically the PS insert should prevent posterior dislocation of the tibia through a cam-mechanism, allowing also an increased range of motion (ROM), reproducing the physiological posterior femoral rollback and increasing the moment arm of the quadriceps. There are evidences that PS implants have less physiological kinematics, but soft tissue balancing and joint stabilization is simplified (4-6). In literature, similar outcomes are reported for CR and PS implants (7,8).

However, the PS implants have some disadvantages, mainly due to the cam-mechanism. This mechanism guarantees the posterior stability of the implant, resulting in more stresses on the insert, and high risk for polyethylene wear at the level of the cam-mechanism, most of all in fixed bearing (FB) implants (9). Furthermore, PS implants need a greater amount of bone cut in the intercondylar box. To minimize the polyethylene wear of the cam-mechanism and the bone sacrifice due to the intercondylar box, different types of inserts were developed, trying to increase the implant conformity and to reduce stresses transmitted to the bone-implant interface (10). Mobile bearing (MB) inserts were developed to overcome this issue, in order to provide a more physiological motion of the implant and to correct small tibial rotational misalignment, in order to minimize polyethylene wear (11,12). Nowadays no differences in clinical outcomes or survivorship were found between FB and MB (13-15).

However, the introduction of those new inserts did not solve the problem: how contact stresses transferred on the polyethylene and resulting polyethylene wear could be reduced without sacrificing the stability and the physiological kinematic? How can the implant design avoid the problem of cam-mechanism wear? In this scenario it has been hypnotized that increasing the congruence of the implant may guarantee the stability avoiding the use of the cam-mechanism. For this reason ultra-congruent (UC) inserts were recently developed. Those inserts are characterized by a higher anterior wall and deeper trough compared to the standard PS inserts. These inserts should assure stability avoiding the posterior cam-mechanism, potentially diminishing polyethylene wear rates and consequently possible TKA aseptic loosening.

The state of art and clinical results of these UC inserts will be described in this manuscript.

Background: the cruciate retaining (CR) and postero-stabilized (PS) inserts

The choice between CR and PS inserts is still debated. CR inserts theoretically guarantee a more physiological proprioception and control of knee flexion, resulting in good knee stability and high functional outcomes in daily activities such as kneeling and climbing stairs. Furthermore the CR design does not require the sacrifice of an intercondylar box, preserving femoral bone-stock (16-18). The physiological kinematics of the PCL is not easy to reproduce in the joint arthroplasty. Furthermore, in cases of PCL laxity or flexion contracture to use the CR inserts can be extremely challenging, because of the difficult ligament balancing (19). In these cases a PS insert may be useful. PS inserts are characterized by a cam-mechanism engaging in a femoral intercondylar box, which stabilizes the posterior translation of the tibia. PS implants, because of the post-cam mechanism, allow an easier ligament balancing compared to CR implants, in which it is mandatory to achieve a good PCL balancing (20,21). Furthermore, the mechanical enforcement of femoral rollback seems to modestly increase the ROM in posterior-stabilized implants (22-24).

Despite to the original advantages hypnotized for the PS insert, a clear supremacy towards the CR inserts is still not established. A recent meta-analysis concluded that the use of CR or PS depends mostly on the surgeon choice (25). The 2005 Cochrane systematic literature review by Jacobs et al. failed to prove a clear consensus in prosthesis design choice, except for enhancing a slight better knee flexion in PS TKA and a higher Hospital for Special Surgery score compared to CR implants (26). Recently another meta-analysis of randomized controlled trials carried out by Li et al., shows similar results with a comparable clinical efficacy and prosthesis survival of the two inserts (8).

Ultra-congruent (UC) inserts: rationale and biomechanics

The rationale of modern designs is to minimize polyethylene wear by improving its conformity and reducing stress forces transmitted to the metal interface. The cam-mechanism of the PS inserts may lead different problems. This mechanism allows for more stability, but torque forces around it are enhanced, leading to polyethylene debris of the cam-mechanism. MB TKA is also characterized by a similar problem: those inserts are able to correct tibia rotational alignment, but their rotation on the tibial tray may increase the polyethylene debris (backside wear). Since long-term survival of implants is the primary aim in TKA, with polyethylene wear and implant loosening recognized as major causes of current TKA late failure (27,28), to reduce the risk related to the cam-mechanism wear, UC inserts were developed. These inserts are characterized by an elevated anterior lip and deep-dish through, in order to theoretically prevent anterior subluxation of femoral condyles during flexion (19,29). Furthermore, this increased congruence should theoretically avoid contact stress peaks providing better stress forces distribution.

Using these inserts, the stability of the knee is guaranteed by a more conforming articulation in conjunction with a correct soft-tissue tension. These implants do not need a cam-mechanism, avoiding the risk of cam impingement, wear or breakage, and reducing the risk of condylar fracture and excessive bone resection due to the intercondylar box (30,31).

Firstly UC FB inserts were introduced, but the early loosening due to the high congruence and low mobility of the implant suggested the development of a MB model (32). The UC MB inserts are characterized by the same benefits of a classical MB design, with shrunk shear stress on tibial surface and rotational freedom of the femur, in association to theoretical improved clinical outcomes and more physiological joint kinematic (33,34).

Little studies are published about UC insert clinical outcomes at short to mid-term follow-up. Similar outcomes at short term follow-up are reported when comparing UC MB and standard PS MB inserts. Concerning the radiographic analysis, lesser radiolucent lines in UC MB were evidenced at short-term follow-up compared to PS MB inserts, with similar survivorship reported at mid-term follow-up (35). However, flexion reduction using UC inserts was also reported compared to PS TKA (36). Moreover, fluoroscopic studies investigating the in vivo kinematics during active knee motion, demonstrated not perfect femoral rollback in both UC and PS inserts (37).

Biomechanics

There are few papers analyzing UC inserts kinematic. Most of those studies compared standard PS or CR implants to PS UC inserts, to evaluate hypothetical advantages concerning ROM and stability. In PS TKA the function of the PCL is substituted by the cam-mechanism that should provide a more physiological femoral rollback and wider ROM (38). Comparing fixed-bearing UC and PS inserts, similar kinematics patterns were found. Both the inserts showed reduction of the rotation of the femur on the tibia and slightly increase of shear stress forces, potentially correlating to higher risk of aseptic loosening (29,33). To improve kinematics and reduce the risk of polyethylene wear, mobile-bearing UC inserts were developed. A recent randomized controlled trial (RCT) compared MB PS UC and standard MB PS inserts. The UC group showed more anterior femoral translation from 80° to 120° of knee flexion compared with the PS group. Furthermore, the UC groups showed lower paradoxical internal rotation and greater external rotation from 40° to 120° of knee flexion compared with the PS group. No statistical differences were detected between the two groups in terms of coronal alignment. The authors concluded that, even if small intra-operative kinematic differences between mobile UC and mobile PS TKA can be detected, both designs are not able to fully reproduce physiologic knee kinematics (39).

Another recent study compared CR standard inserts to UC inserts in 39 patients. The authors analyzed both intra-operative stability and ROM after implantation of a CR insert and after the resection of the PCL and substitution with an UC insert. All data were collected using a navigation system. The authors demonstrated similar stability between the inserts, concluding that UC may be useful to preserve bone stock in case of PCL deficiency, but they do not increase the ROM (40).

Daniilidis et al. evaluated 31 patients (50 knees) who received a fixed-bearing CR TKA, 22 who received a flat polyethylene inlay (PE), 9 who received a deep dished PE and 19 patients with healthy knees were used as a control group. The authors concluded that highly conforming polyethylene inserts improved antero-posterior stability. No significant differences were observed between antero-posterior translation and femoral rollback. However, those inserts did not allow to restore a physiological kinematic, and the little differences between the inserts did not influence the outcomes (37).

Analyzing the existing literature on UC inserts compared to both CR and PS inserts, a greater antero-posterior translation can be achieved using UC inserts. However, despite those little differences in kinematics between the inserts, no differences on clinical outcomes between UC, CR and PS inserts can be detected.

Literature evaluation and clinical outcomes of ultra-congruent (UC) inserts

There are few papers focused on UC inserts clinical outcomes, and most of them are level IV studies (Table 1). This is due mainly to the recent introduction of those inserts.

In 2009 Wajsfisz et al. evaluated intra-operative flexion achieved with three different TKA designs (UC, PS and PS Flex), concluding about a slight superiority of PS models over the UC designs in terms of maximum flexion reached (36). More recently Kim et al. evaluated the intra-operative motion and mid-term clinical outcomes of MB UC TKAs vs. standard PS inserts. The authors observed a more physiological intra-operative kinematic of the PS inserts, without significant differences in clinical outcomes at three years of follow up (39).

Lützner et al. in 2013 compared the intra-operative stability and ROM before and after PCL resection, using a standard CR insert and a UC PS insert in the same TKA. The authors observed an about comparable medio-lateral and antero-posterior stability, as well as ROM between the inserts (40). Massin et al., in their kinematic analysis, observed a lower antero-posterior translation and femorotibial rotation with UC inserts, without any differences in terms of ROM compared to standard PS implants. However, the authors underlined some kind of posterior impingement in absence of physiologic rollback using an UC inserts (42).

The retrospective studies comparing UC TKA with standard PS or CR models (19,29,35,41,43,46,47) analyzed a big number of variables, trying to assess the superiority of one design vs. another. Implants survivorship, ROM, clinical scores (mostly Modified Hospital for Special Surgery Knee Score), radiologic results, patient satisfaction scores, incidence of adverse events, revision and complication rates were the most studied outcomes. However, none of those studies demonstrated significative differences regarding the analyzed variables, showing comparable outcomes between the inserts (19,29,35,41,43,46,47).

Conversely, two studies were able to detect some significant differences between UC and standard inserts. Recently Machhindra et al. retrospectively reviewed 281 TKAs, concluding about a slight reduced ROM reached with UC models and an interesting different recovery pattern. The UC TKAs reached the improvement peak of all variables studied at one year after surgery and then remained stable, while PS TKAs had a constant improvement for 2 years after surgery (44).

A smaller cohort of 50 FB CR TKAs was evaluated by Daniilidis et al., who compared a highly conforming insert with the standard flat one. The authors demonstrated that either the congruent or flat inserts cannot restore a physiological kinematic movement, resulting both in an amount of abnormal anterior-posterior translation. In the same study the authors concluded that the high congruency of one group of insert may increase the metal/bone interface stresses, potentially reducing implant survival rates (37).

Heyse et al. focused on patello-femoral pain using different inserts, concluding that higher patello-femoral peak pressure and mean contact pressure can be achieved in UC TKA compared with CR implants (48).

Nowadays the only study that directly compared the CR and PS models of UC MB TKA interestingly found that the preservation of PCL was not helpful in improving kinematics and clinical outcomes. No differences in terms of ROM, functional scores and radiographic results were noted. However, the observation of less physiological kinematics, including more varus rotation and anterior dislocation of the femoral component preserving the PCL, leads to the recommendation of using posterior stabilized UC inserts (45).

Table 1 summarizes the existing literature on UC inserts.

Table 1 Summary of literature analysis about ultra-congruent (UC) inserts
Full table

Conclusions

UC inserts were recently introduced basically to avoid the cam-mechanism wear and femoral intercondylar bone sacrifice related to PS inserts. Few literature regarding both kinematic and clinical outcomes of UC inserts is available. Clinical, functional and radiological outcomes, as well as kinematic findings are similar between UC MB and standard PS MB inserts at short to mid-term follow-up. However, those studies are characterized by small samples of patients and short follow-up: further studies are needed to confirm their findings. Considering that UC inserts were recently introduced as an alternative to PS inserts, the lack of studies comparing PS and CR UC inserts is not surprising. At our best knowledge, only one study directly compared CR and PS UC inserts, with no differences in radiological or clinical short-term outcomes. However, a less physiological kinematic was found in CR UC compared to PS UC inserts, including more varus rotation and anterior dislocation of the femoral component.

In conclusion there are few evidences on PS UC inserts, which may be a good alternative to standard PS implants, reducing the problems related to the cam-mechanism wear or breakage, and to the bone losses due to the intercondylar box. There are no clear advantages of preserving the PCL in UC inserts in terms of clinical outcomes, but less favorable kinematic was demonstrated, leading those authors to recommend PS UC instead of CR UC inserts.

Acknowledgements

None.

Footnote

Conflicts of Interest: The authors have no conflicts of interest to declare.

References

1. Harner CD, Xerogeanes JW, Livesay GA, et al. The human posterior cruciate ligament complex: an interdisciplinary study. Ligament morphology and biomechanical evaluation. Am J Sports Med 1995;23:736-45. [PubMed]
2. Insall JN, Binazzi R, Soudry M, et al. Total knee arthroplasty. Clin Orthop Relat Res 1985.13-22. [PubMed]
3. Maruyama S, Yoshiya S, Matsui N, et al. Functional comparison of posterior cruciate-retaining versus posterior stabilized total knee arthroplasty. J Arthroplasty 2004;19:349-53. [PubMed]
4. Argenson JN, Parratte S, Ashour A, et al. The outcome of rotating-platform total knee arthroplasty with cement at a minimum of ten years of follow-up. J Bone Joint Surg Am 2012;94:638-44. [PubMed]
5. Fantozzi S, Catani F, Ensini A, et al. Femoral rollback of cruciate-retaining and posterior-stabilized total knee replacements: in vivo fluoroscopic analysis during activities of daily living. J Orthop Res 2006;24:2222-9. [PubMed]
6. Colizza WA, Insall JN, Scuderi GR. The posterior stabilized total knee prosthesis. Assessment of polyethylene damage and osteolysis after a ten-year-minimum follow-up. J Bone Joint Surg Am 1995;77:1713-20. [PubMed]
7. Sando T, McCalden RW, Bourne RB, et al. Ten-year results comparing posterior cruciate-retaining versus posterior cruciate-substituting total knee arthroplasty. J Arthroplasty 2015;30:210-5. [PubMed]
8. Li N, Tan Y, Deng Y, et al. Posterior cruciate-retaining versus posterior stabilized total knee arthroplasty: a meta-analysis of randomized controlled trials. Knee Surg Sports Traumatol Arthrosc 2014;22:556-64. [PubMed]
9. Maniar R. Rationale for the posterior-stabilized rotating-platform knee. Orthopedics 2006;29:S23-7. [PubMed]
10. Carothers JT, Kim RH, Dennis DA, et al. Mobile-bearing total knee arthroplasty: a meta-analysis. J Arthroplasty 2011;26:537-42. [PubMed]
11. Buechel FF, Pappas MJ. The New Jersey Low-Contact-Stress Knee Replacement System: biomechanical rationale and review of the first 123 cemented cases. Arch Orthop Trauma Surg 1986;105:197-204. [PubMed]
12. Ranawat CS, Komistek RD, Rodriguez JA, et al. In vivo kinematics for fixed and mobile-bearing posterior stabilized knee prostheses. Clin Orthop Relat Res 2004.184-90. [PubMed]
13. Bistolfi A, Massazza G, Lee GC, et al. Comparison of fixed and mobile-bearing total knee arthroplasty at a mean follow-up of 116 months. J Bone Joint Surg Am 2013;95:e83. [PubMed]
14. Jacobs W, Anderson P, Limbeek J, et al. Mobile bearing vs fixed bearing prostheses for total knee arthroplasty for post-operative functional status in patients with osteoarthritis and rheumatoid arthritis. Cochrane Database Syst Rev 2004.CD003130. [PubMed]
15. Van der Bracht H, Van Maele G, Verdonk P, et al. Is there any superiority in the clinical outcome of mobile-bearing knee prosthesis designs compared to fixed-bearing total knee prosthesis designs in the treatment of osteoarthritis of the knee joint? A review of the literature. Knee Surg Sports Traumatol Arthrosc 2010;18:367-74. [PubMed]
16. Chalidis BE, Sachinis NP, Papadopoulos P, et al. Long-term results of posterior-cruciate-retaining Genesis I total knee arthroplasty. J Orthop Sci 2011;16:726-31. [PubMed]
17. In Y, Kim JM, Woo YK, et al. Factors affecting flexion gap tightness in cruciate-retaining total knee arthroplasty. J Arthroplasty 2009;24:317-21. [PubMed]
18. Conditt MA, Noble PC, Bertolusso R, et al. The PCL significantly affects the functional outcome of total knee arthroplasty. J Arthroplasty 2004;19:107-12. [PubMed]
19. Laskin RS, Maruyama Y, Villaneuva M, et al. Deep-dish congruent tibial component use in total knee arthroplasty: a randomized prospective study. Clin Orthop Relat Res 2000.36-44. [PubMed]
20. Insall JN, Lachiewicz PF, Burstein AH. The posterior stabilized condylar prosthesis: a modification of the total condylar design. Two to four-year clinical experience. J Bone Joint Surg Am 1982;64:1317-23. [PubMed]
21. Rossi R, Bruzzone M, Bonasia DE, et al. Evaluation of tibial rotational alignment in total knee arthroplasty: a cadaver study. Knee Surg Sports Traumatol Arthrosc 2010;18:889-93. [PubMed]
22. Victor J, Banks S, Bellemans J. Kinematics of posterior cruciate ligament-retaining and -substituting total knee arthroplasty: a prospective randomised outcome study. J Bone Joint Surg Br 2005;87:646-55. [PubMed]
23. Harato K, Bourne RB, Victor J, et al. Midterm comparison of posterior cruciate-retaining versus -substituting total knee arthroplasty using the Genesis II prosthesis. A multicenter prospective randomized clinical trial. Knee 2008;15:217-21. [PubMed]
24. Yoshiya S, Matsui N, Komistek RD, et al. In vivo kinematic comparison of posterior cruciate-retaining and posterior stabilized total knee arthroplasties under passive and weight-bearing conditions. J Arthroplasty 2005;20:777-83. [PubMed]
25. Bercik MJ, Joshi A, Parvizi J. Posterior cruciate-retaining versus posterior-stabilized total knee arthroplasty: a meta-analysis. J Arthroplasty 2013;28:439-44. [PubMed]
26. Jacobs WC, Clement DJ, Wymenga AB. Retention versus removal of the posterior cruciate ligament in total knee replacement: a systematic literature review within the Cochrane framework. Acta Orthop 2005;76:757-68. [PubMed]
27. Callaghan JJ, Insall JN, Greenwald AS, et al. Mobile-bearing knee replacement: concepts and results. Instr Course Lect 2001;50:431-49. [PubMed]
28. Dennis DA, Komistek RD. Mobile-bearing total knee arthroplasty: design factors in minimizing wear. Clin Orthop Relat Res 2006.70-7. [PubMed]
29. Hofmann AA, Tkach TK, Evanich CJ, et al. Posterior stabilization in total knee arthroplasty with use of an ultracongruent polyethylene insert. J Arthroplasty 2000;15:576-83. [PubMed]
30. Lombardi AV Jr, Mallory TH, Vaughn BK, et al. Dislocation following primary posterior-stabilized total knee arthroplasty. J Arthroplasty 1993;8:633-9. [PubMed]
31. Lombardi AV Jr, Mallory TH, Waterman RA, et al. Intercondylar distal femoral fracture. An unreported complication of posterior-stabilized total knee arthroplasty. J Arthroplasty 1995;10:643-50. [PubMed]
32. Blunn GW, Joshi AB, Minns RJ, et al. Wear in retrieved condylar knee arthroplasties. A comparison of wear in different designs of 280 retrieved condylar knee prostheses. J Arthroplasty 1997;12:281-90. [PubMed]
33. Dennis DA, Komistek RD, Mahfouz MR, et al. Mobile-bearing total knee arthroplasty: do the polyethylene bearings rotate? Clin Orthop Relat Res 2005.88-95. [PubMed]
34. Hamai S, Miura H, Higaki H, et al. Kinematic analysis of mobile-bearing total knee arthroplasty using a 6-DOF knee simulator. J Orthop Sci 2008;13:543-9. [PubMed]
35. Chavoix JB. Functionality and safety of an ultra-congruent rotating platform knee prosthesis at 5.6 years: more than 5- year follow-up of the e.motion ((®)) UC-TKA. Open Orthop J 2013;7:152-7. [PubMed]
36. Wajsfisz A, Biau D, Boisrenoult P, et al. Comparative study of intraoperative knee flexion with three different TKR designs. Orthop Traumatol Surg Res 2010;96:242-8. [PubMed]
37. Daniilidis K, Skwara A, Vieth V, et al. Highly conforming polyethylene inlays reduce the in vivo variability of knee joint kinematics after total knee arthroplasty. Knee 2012;19:260-5. [PubMed]
38. Fitzpatrick CK, Clary CW, Cyr AJ, et al. Mechanics of post-cam engagement during simulated dynamic activity. J Orthop Res 2013;31:1438-46. [PubMed]
39. Kim TW, Lee SM, Seong SC, et al. Different intraoperative kinematics with comparable clinical outcomes of ultracongruent and posterior stabilized mobile-bearing total knee arthroplasty. Knee Surg Sports Traumatol Arthrosc 2015. [Epub ahead of print]. [PubMed]
40. Lützner J, Firmbach FP, Lützner C, et al. Similar stability and range of motion between cruciate-retaining and cruciate-substituting ultracongruent insert total knee arthroplasty. Knee Surg Sports Traumatol Arthrosc 2015;23:1638-43. [PubMed]
41. Argenson JN, Boisgard S, Parratte S, et al. Survival analysis of total knee arthroplasty at a minimum 10 years' follow-up: a multicenter French nationwide study including 846 cases. Orthop Traumatol Surg Res 2013;99:385-90. [PubMed]
42. Massin P, Boyer P, Sabourin M. Less femorotibial rotation and AP translation in deep-dished total knee arthroplasty. An intraoperative kinematic study using navigation. Knee Surg Sports Traumatol Arthrosc 2012;20:1714-9. [PubMed]
43. Ko YB, Jang EC, Park SM, et al. No difference in clinical and radiologic outcomes after total knee arthroplasty with a new ultra-congruent mobile bearing system and rotating platform mobile bearing systems after minimum 5-year follow-up. J Arthroplasty 2015;30:379-83. [PubMed]
44. Machhindra MV, Kang JY, Kang YG, et al. Functional outcomes of a new mobile-bearing ultra-congruent TKA system: comparison with the posterior stabilized system. J Arthroplasty 2015;30:2137-42. [PubMed]
45. Roh YW, Jang J, Choi WC, et al. Preservation of the posterior cruciate ligament is not helpful in highly conforming mobile-bearing total knee arthroplasty: a randomized controlled study. Knee Surg Sports Traumatol Arthrosc 2013;21:2850-9. [PubMed]
46. Peters CL, Mulkey P, Erickson J, et al. Comparison of total knee arthroplasty with highly congruent anterior-stabilized bearings versus a cruciate-retaining design. Clin Orthop Relat Res 2014;472:175-80. [PubMed]
47. Uvehammer J, Kärrholm J, Regnér L, et al. Concave versus posterior-stabilized tibial joint surface in total knee arthroplasty: randomized evaluation of 47 knees. J Arthroplasty 2001;16:25-32. [PubMed]
48. Heyse TJ, Becher C, Kron N, et al. Patellofemoral pressure after TKA in vitro: highly conforming vs. posterior stabilized inlays. Arch Orthop Trauma Surg 2010;130:191-6. [PubMed]