Copyright © 2009 by The Journal of Bone and Joint Surgery, Inc.
Commentary & Perspective
Commentary & Perspective by
Douglas A. Dennis, MD*,
Rocky Mountain Musculoskeletal Research Laboratory, Denver, Colorado
Posted August 2009
When Kim et al. compared the clinical and radiographic performance of subjects who received both a standard and a "high-flexion" posterior cruciate-retaining total knee prosthesis during bilateral total knee arthroplasty, they found that, at a mean follow-up time of three years, there were no significant differences between the two groups with regard to range of knee motion, clinical knee scores, or radiographic parameters.
The finding of a lack of difference between the two evaluated designs, especially when the components were tested under weight-bearing conditions, is not unexpected for knees in which the posterior cruciate ligament has been retained. Postoperative flexion following total knee arthroplasty has been directly correlated with the amount of posterior femoral
translation1,2 that is present, which can be limited in posterior cruciate-retaining total knee designs3. Banks et al.1 performed a fluoroscopic kinematic analysis of 121 total knee prostheses representing sixteen differing condylar geometries and observed a 1.4° increase in flexion magnitude for each additional millimeter of posterior femoral translation. In a fluoroscopic kinematic comparison of 200 knee arthroplasties which obtained either high (>110°) or low (<95°) magnitudes of weight-bearing flexion, posterior femoral translation was significantly higher in the cohort with greater knee flexion2. Although the standard design (NexGen posterior cruciate-retaining prosthesis; Zimmer, Warsaw, Indiana) used in the study by Kim et al. has been shown in fluoroscopic kinematic studies to demonstrate some posterior femoral translation, it is typically less than that observed in posterior stabilized designs that incorporate a cam-and-post mechanism4. Other posterior cruciate-retaining total knee designs have often demonstrated paradoxical anterior femoral translation during progressive flexion when tested under weight-bearing conditions. Anterior femoral translation anteriorizes the axis of flexion, resulting in tightening of the extensor mechanism, earlier posterior impingement, and diminished flexion3. In the study by Kim et al., there was a substantial reduction in motion (approximately 15°) in both designs when testing under weight-bearing was compared with the passive range of motion measured under non-weight-bearing conditions. The results of this analysis suggest that simply changing the thickness and radius of curvature of the posterior femoral condyles in a posterior cruciate-retaining design does not result in improved flexion. This may be related to the fact that this design feature does not ensure posterior femoral translation during deep flexion.
It is important to note that the preoperative flexion of both groups (128°) was quite high. This may be related, at least in part, to the ethnicity of the patients analyzed (Asian), as patients from Asian countries are typically engaged in deep-flexion activities much more frequently than patients from Western countries are. One report of subjects who underwent implantation of a high-flexion total knee prosthesis demonstrated that the greatest improvement in postoperative flexion occurred in the subjects who had preoperative flexion of <120°5. This suggests that the current study may not be able to detect differences between standard and high-flexion total knee designs due to the fact that the groups of patients who were analyzed had such high mean flexion preoperatively.
The use of high-flexion prosthetic devices has potential disadvantages. Should a failure occur that requires revision, the additional posterior femoral condylar bone resection that is required with many current high-flexion implants may create difficulty in establishing flexion-gap stability. If high-flexion magnitudes are obtained, increased polyethylene and fixation stresses are incurred that may result in premature failure of the prosthesis. While limited failures have been reported in most evaluations of these devices, Hann et al.6 reported a 38% prevalence of aseptic femoral component loosening at a mean follow-up time of only thirty-two months with use of a fixed-bearing high-flexion total knee design. For these reasons, it is critical to continue to analyze these devices to determine (1) whether higher flexion is routinely obtained, (2) which patients are most likely to benefit from high-flexion total knee implants, and (3) whether there are any long-term detrimental effects from the use of high-flexion total knee designs as compared with the use of traditional total knee designs.
In summary, published kinematic data demonstrate that normal posterior femoral translation is not routinely reproduced following posterior cruciate-retaining total knee arthroplasty. Therefore, the use of high-flexion posterior cruciate-retaining total knee implants may not dramatically improve postoperative flexion, especially when the prosthesis is tested under weight-bearing conditions. Higher weight-bearing flexion has been observed with use of posterior cruciate-substituting prosthetic devices, likely because this design provides better posterior femoral translation secondary to engagement of the cam-and-post stabilizing mechanism7. Additional evaluations of high-flexion total knee implants in differing patient populations are needed to determine exact patient indications, critical design features, and long-term safety of these devices as well as to provide clear evidence of superior postoperative flexion.
*The author did not receive any outside funding or grants in support of his research for or preparation of this work. The author, or a member of his immediate family, received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (DePuy Orthopaedics).
References
1. Banks S, Bellemans J, Nozaki H, Whiteside LA, Harman M, Hodge WA. Knee motions during maximum flexion in fixed and mobile-bearing arthroplasties. Clin Orthop Relat Res. 2003;410:131-8.
2. Sharma A, Dennis DA, Komistek RD, Mahfouz MR. Kinematic difference between subjects having low flexion and high flexion for the same flexion angles: a multicenter study. Orthopaedic Transactions. 2008;33:0139.
3. Dennis DA, Komistek RD, Colwell CE Jr., Ranawat CS, Scott RD, Thornhill TS, Lapp MA. In vivo anteroposterior femorotibial translation of total knee arthroplasty: a multicenter analysis. Clin Orthop Relat Res. 1998;356:47-57.
4. Bertin KC, Komistek RD, Dennis DA, Hoff WA, Anderson DT, Langer T. In vivo determination of posterior femoral rollback for subjects having a NexGen posterior cruciate-retaining total knee arthroplasty. J Arthroplasty. 2002;17:1040-8.
5. Gupta SK, Ranawat AS, Shah V, Zikria BA, Zikria JF, Ranawat CS. The P.F.C. Sigma RP-F TKA designed for improved performance: a matched-pair study. Orthopedics. 2006;29(9 Suppl):S49-52.
6. Hann HS, Kang SB, Yoon KS. High incidence of loosening of the femoral component in legacy posterior stabilised-flex total knee replacement. J Bone Joint Surg Br. 2007;89:1457-61.
7. Dennis DA, Komistek RD, Stiehl JB, Walker SA, Dennis KN. Range of motion after total knee arthroplasty: the effect of implant design and weight-bearing conditions. J Arthroplasty. 1998;13:748-52.
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