In appraising the literature, however, it is important to distinguish between mobile-bearing and fixed-bearing unicompartmental implants. Mobile-bearing implants have a fully congruent articulation that moves freely on a polished tibial baseplate, and the theory of this design has been previously published15. Fixed-bearing implants have a relatively nonconforming articular surface, which can be inserted as an all-polyethylene tibial component, or as a non-modular or modular metal-backed tibial component. Proponents of mobile-bearing unicompartmental implants believe this design has improved wear characteristics over fixed-bearing implants, and thus may offer a survival advantage in the long-term16.naudiefigure_1a.jpg
naudiefigure_1b.jpg

 

 

 

 

 

 

 

 

Figures 1-A and 1-B. Preoperative anteroposterior and lateral radiographs of the left and right knees of a 67-year old male patient demonstrating arthritis confined to the medial compartment of each knee.

The reality is that both designs have demonstrated excellent ten-year survivorship free of revision. Cartier and associates report a ten-year survivorship using the Marmor fixed-bearing prosthesis of 93%5. Recently, ORourke and associates report continued excellent survivorship with the same implant at a minimum of twenty-one year follow-up6. Argenson and associates report a 94% ten-year survivorship with the Miller-Galante fixed-bearing implant7. Using the same fixed-bearing implant, we report 90% ten-year survivorship; and Berger and associates report a 98% ten-year survivorship8,9.

On the mobile-bearing side, Murray and associates report ten-year survival rates free of revision of 98% using the Oxford prosthesis10. Price and associates also report an excellent ten-year (95%) survivorship with the Oxford prosthesis11. Other studies have not reproduced these numbers, however, and have actually reported lower ten-year survivorship rates than fixed-bearing implants. Keblish and Briard report 82% prosthetic survivorship at 11 years using the Oxford and Low Contact Stress (LCS) mobile-bearing implants12. Similarly, Vorlat and associates report a cumulative survival rate of 82% at ten years with the Oxford mobile-bearing implant13.

There have also been studies directly comparing the results of mobile-bearing and fixed-bearing implants. Emerson and associates showed a 99% survival for the mobile-bearing Oxford implant compared to a 93% survival for the fixed-bearing Brigham implant at 11 years16. However, an initial comparison of the Oxford unicompartmental arthroplasty implant to the fixed-bearing Marmor implant revealed a significantly higher cumulative revision rate with the Oxford implant17. Another recent study by Gleeson and associates also found that the Oxford mobile-bearing prosthesis had a higher re-operation rate and significantly less pain relief than the St. Georg sled fixed-bearing implant18.

The case for a fixed-bearing rather than a mobile-bearing unicompartmental implant, therefore, can be made on the basis of more reproducible long-term survivorship. In fact, Swedish registry data does not support the widespread use of a mobile-bearing unicompartmental implant because of the increased rates of revision observed in Sweden17,19. Fixed-bearing unicompartmental implants are probably also less technically demanding than mobile-bearing implants, which require exact balancing and intact cruciate ligaments15. Dislocation of the bearing has been described, for example, as a unique complication in association with the Oxford knee20,21.

naudiefigure_1c.jpg
naudiefigure_1d.jpg

Figures 1-C and 1-D. Standing anteroposterior and lateral radiographs of both knees of the same patient taken ten years after bilateral medial unicompartmental arthroplasties.

A fixed-bearing implant is also more versatile than a mobile-bearing implant. A fixed-bearing implant can be successfully employed as a lateral compartment arthroplasty with excellent long-term results recently reported22. When the Oxford mobile-bearing arthroplasty is used in the lateral compartment of the knee, ten percent of the bearings dislocate21. A fixed-bearing implant also permits use of an all-polyethylene tibial component, allowing for a thicker polyethylene insert without resection of additional host bone. Excellent results using an all-polyethylene tibial component have recently been reported23. Finally, a fixed-bearing implant is less expensive than a mobile-bearing implant, particularly when an all-polyethylene tibial component is employed.

naudiefigure_2a_1.jpg
naudiefigure_2b.jpg
Figures 2-A and 2-B. Standing anteroposterior and lateral radiographs of a 65-year-old female patient taken two years after lateral unicompartmental arthroplasty with an all-polyethylene tibial component.

In summary, unicompartmental knee arthroplasty remains an excellent treatment alternative for osteoarthritis confined to the medial or lateral part of the knee joint. Fixed-bearing implants have been shown to provide good pain relief and restoration of function, and have reproducibly demonstrated excellent survivorship into the second decade. This author makes the case for a fixed-bearing over a mobile-bearing implant because they are technically easier, avoid the complication of bearing dislocation, are more versatile, and less expensive.

References

1. Insall J., Aglietti P. A five to seven-year follow-up of unicondylar arthroplasty. J Bone Joint Surg Am. 1980;62(8):1329-37.

2. Laskin R.S. Unicompartmental tibiofemoral resurfacing arthroplasty. J Bone Joint Surg Am. 1978;60(2):182-5.

3. Kozinn S.C., Scott R. Unicondylar knee arthroplasty. J Bone Joint Surg Am. 1989;71(1):145-50.

4. Corpe R.S., Engh G.A. A quantitative assessment of degenerative changes acceptable in the unoperated compartments of knees undergoing unicompartmental replacement. Orthopedics. 1990;13(3):319-23.

5. Cartier P., Sanouiller J.L., Grelsamer R.P. Unicompartmental knee arthroplasty surgery. 10-year minimum follow-up period. J Arthroplasty. 1996;11(7):782-8.

6. ORourke M.R., Gardner J.J., Callaghan J.J., et al. The John Insall Award: unicompartmental knee replacement: a minimum twenty-one year follow-up study. Clin Orthop Relat Res. 2005;440:27-37.

7. Argenson J.A., Chevrol-Benkeddache Y.C., Aubaniac J. Modern unicompartmental knee arthroplasty with cement: A three to ten-year follow-up study. J Bone Joint Surg Am. 2002;84(12):2235-37.

8. Naudie D., Guerin J., Parker D.A., et al. Medial unicompartmental knee arthroplasty with the Miller-Galante prosthesis. J Bone Joint Surg Am. 2004 Sep;86-A(9):1931-5.

9. Berger R.A., Meheghini R.M., Jacobs J.J., et al. Results of unicompartmental knee arthroplasty at a minimum of ten years of follow-up. J Bone Joint Surg Am. 2005;87(5):999-1006.

10. Murray D.W., Goodfellow J.W., O'Connor J.J. The Oxford medial unicompartmental arthroplasty: a ten-year survival study. J Bone Joint Surg Br. 1998;80(6):983-9.

11. Price A.J., Waite J.C., Svard U. Long-term clinical results of the medial Oxford unicompartmental knee arthroplasty. Clin Orthop. 2005; 435:171-180.

12. Keblish P.A. and Briard J.L. Mobile-bearing unicompartmental knee arthroplasty: a 2-center study with an 11-year (mean) follow-up. J Arthroplasty. 2004; 19(7-S2):87-94.

13. Vorlat P., Putzets G., Cottenie D., et al. The Oxford unicompartmental knee prosthesis: an independent 10-year survival analysis. Knee Surg Sports Traumatol Arthrosc. 2006; 14(1): 40-5.

14. Goodfellow J., O'Connor J. The mechanics of the knee and prosthesis design. J Bone Joint Surg Br. 1978;60-B(3):358-69.

15. Price A.J., Short A., Kellett C., et al. Ten-year in vivo wear measurement of a fully congruent mobile bearing unicompartmental knee arthroplasty. J Bone Joint Surg Br. 2005;87(11):1493-7.

16. Emerson R.H., Hansborough T., Reittman R.D., et al. Comparison of a Mobile with a Fixed-Bearing Unicompartmental Knee Implant. Clin Orthop Relat Res. 2002; 404:62-70.

17. Lewold S., Goodman S., Knutson K., et al. Oxford meniscal bearing knee versus the Marmor knee in unicompartmental arthroplasty for arthrosis. A Swedish multicenter survival study. J Arthroplasty. 1995;10(6):722-31.

18. Gleeson R.E., Evans R., Ackroyd C.E., et al. Fixed or mobile bearing unicompartmental knee replacement? A comparative cohort study. Knee. 2004;11(5):379-84.

19. Robertsson O., Knutson K., Lewold S., Lidgren L. The routine of surgical management reduces failure after unicompartmental knee arthroplasty. J Bone Joint Surg Br. 2001;83(1):45-9.

20. Verhaven E., Handelberg F., Casteleyn, et al. Meniscal bearing dislocation in the Oxford knee. Acta Orthop Belg. 1991;57(4):430-2.

21. Robinson B.J., Rees J.L., Price A.J., et al. Dislocation of the bearing of the Oxford lateral unicompartmental arthroplasty. A radiological assessment. J Bone Joint Surg Br. 2002;84(5):653-7.

22. Pennington D.W., Swienckowski J.J., Lutes W.B., et al. Lateral unicompartmental knee arthroplasty: survivorship and technical considerations at an average follow-up of 12.4 years. J Arthroplasty. 2006;21(1)13-7.

23. Hyldahl H.C., Regner L., Carlsson L., et al. Does metal backing improve fixation of tibial component in unicondylar knee arthroplasty: a randomized radiostereometric analysis. J Arthroplasty. 2001;16(2):174-179.