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Range of Motion of Standard and High-Flexion Posterior Cruciate-Retaining Total Knee Prostheses*A Prospective Randomized Study
Young-Hoo Kim, MD1; Yoowang Choi, MD1; Jun-Shik Kim, MD1
1 The Joint Replacement Center of Korea at Ewha Womans University MokDong Hospital, 911-1, MokDong, YangCheon-Gu, Seoul 158-710, South Korea. E-mail address for Y.-H. Kim: younghookim@ewha.ac.kr
View Disclosures and Other Information
Read in part at the Annual Meeting of the American Academy of Orthopaedic Surgeons, Las Vegas, Nevada, February 26, 2009.
Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.
A commentary is available with the electronic versions of this article, on our web site (www.jbjs.org) and on our quarterly CD-ROM/DVD (call our subscription department, at 781-449-9780, to order the CD-ROM or DVD).
Investigation performed at The Joint Replacement Center of Korea at Ewha Womans University MokDong Hospital, Seoul, South Korea

The Journal of Bone and Joint Surgery, Inc.
J Bone Joint Surg Am, 2009 Aug 01;91(8):1874-1881. doi: 10.2106/JBJS.H.00769
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Abstract

Background: The main goals of total knee arthroplasty are pain relief and improvement in function and the range of motion. The purpose of this study was to compare the ranges of motion of the knees of patients treated with a standard posterior cruciate-retaining total knee prosthesis in one knee and a high-flexion posterior cruciate-retaining total knee prosthesis in the other.

Methods: Fifty-four patients (mean age, 69.7 years) received a standard posterior cruciate-retaining total knee prosthesis in one knee and a high-flexion posterior cruciate-retaining total knee prosthesis in the contralateral knee. Five patients were men, and forty-nine were women. At a mean of three years postoperatively, the patients were assessed clinically and radiographically with the knee-rating systems of the Knee Society and the Hospital for Special Surgery and with the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) score.

Results: The mean postoperative Knee Society and Hospital for Special Surgery knee scores were 93.7 and 89 points, respectively, for the knees with a standard posterior cruciate-retaining prosthesis, and they were 93.9 and 90 points, respectively, for the knees with a high-flexion posterior cruciate-retaining prosthesis. The mean postoperative WOMAC score was 22 points. Postoperatively, the mean ranges of motion without and with weight-bearing were 131° (range, 90° to 150°) and 115° (range, 75° to 145°), respectively, in the knees with a standard prosthesis and 133° (range, 90° to 150°) and 118° (range, 75° to 145°), respectively, in those with a high-flexion prosthesis. Patient satisfaction and radiographic results were similar in the two groups. No knee had aseptic loosening, revision, or osteolysis.

Conclusions: After a minimum duration of follow-up of three years, we found no significant differences between the two groups with regard to the range of knee motion or the clinical or radiographic parameters.

Level of Evidence: Therapeutic Level I. See Instructions to Authors for a complete description of levels of evidence.

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    References

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    Young-Hoo Kim, MD
    Posted on August 14, 2009
    Dr. Kim and colleagues respond to Mr. Malviya
    Joint Replacement Center of Korea at Ewha Womans University School of Medicine, Seoul, South Korea

    We appreciate Dr. Malviya's comments regarding our article, "Range of Motion of Standard and High-Flexion Posterior Cruciate-Retaining Total Knee Prostheses A Prospective Randomized Study" (1).

    1. The NexGen CR-Flex total knee prosthesis was designed to increase the contact area between the posterior femoral condyles and the tibial polyethylene liner at high flexion angles and thereby potentially decrease polyethylene wear and osteolysis. Furthermore, it was designed to enhance knee flexion and to prevent so-called paradoxic translational motion by providing asymmetric femoral condyles as a result of a 2 mm increase in the thickness of the posterior condyles (on both the medical and lateral sides) of the femoral component; this may allow posterior femoral rollback with increasing knee flexion (2).

    The NexGen CR-Flex prosthesis was not designed to provide an increased posterior condylar offset by 2 mm. As Dr. Malviya pointed out, the amount of bone resected from the posterior femoral condyles was 2 mm greater when the knee was to be treated with the NexGen CR-Flex prosthesis. However, the 2 mm greater bone resection is replaced with 2 mm thicker posterior femoral condyles of NexGen CR-Flex femoral component. As a result, the final posterior femoral condylar offset would be similar in both NexGen CR-Flex and NexGen standard CR prostheses.

    2. An a priori power calculation was performed with 5° considered to be a clinically relevant improvement in knee flexion. The standard deviation of 9° is not used for 5° relevant improvement in flexion, but it is used for the amount of knee flexion (eg. 131°±13.9°). If the power analysis was performed using an independent t-test, Dr. Malviya's comments are right. However, we did power analysis using paired t-test [(using Simple Interactive Statistical Analysis) calculator online with "pairwise analysis", the power was 1 (100%)]. Therefore, there is somewhat of a discrepancy between two test methods in terms of sample size.

    3. The mean preoperative range of knee flexion is very high in the Asian patients group (1,3-5) compared to the Western patients group. As we discussed in the Discussion section of our article, a high degree of flexion was achieved with both NexGen CR-Flex and NexGen standard CR prostheses, which may have clouded the possible advantage of the NexGen CR-Flex implant. Several factors may have played a role in the achievement of a high degree of flexion in both groups in the current and previous series (3,4), including the preponderance of women, the low body-mass index of the patients, the use of a less-invasive approach, the relatively good preoperative range of motion, and the effective restoration of the joint line and the posterior femoral condylar offset.

    References

    1. Kim YH, Choi Y, Kim JS. Range of motion of standard and high-flexion posterior cruciate-retaining total knee prostheses a prospective randomized study. J Bone Joint Surg Am. 2009;91:1874-81.

    2. 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.

    3. Kim YH, Sohn KS, Kim JS. Range of motion of standard and high-flexion posterior stabilized total knee prostheses. A prospective, randomized study. J Bone Joint Surg Am. 2005;87:1470-5.

    4. Kim YH, Choi Y, Kwon OR, Kim JS. Functional outcome and range of motion of high-flexion posterior cruciate-retaining and high-flexion posterior cruciate-substituting total knee prostheses. A prospective, randomized study. J Bone Joint Surg Am. 2009;91:753-60.

    5. Kim YH, Kim JS, Choi Y, Kwon OR. Computer-assisted surgical navigation does not improve the alignment and orientation of the components in total knee arthroplasty. J Bone Joint Surg Am. 2009;91:14-9.

    Ajay Malviya
    Posted on August 08, 2009
    Range of Motion of Standard and High-Flexion Posterior Cruciate-Retaining Total Knee Prostheses
    Wansbeck General Hospital, Ashington, United Kingdom

    To the Editor:

    Kim et al. (1) have published the three-year results of the outcome following NexGen CR and NexGen CR-Flex knee prostheses. They have found no significant difference between the range of flexion between the two groups of patients.

    This may be because of several reasons:

    1. The NexGen CR-Flex prosthesis aims to provide an increased posterior condylar offset by 2 mm. In the Methods section, the authors state that, “The amount of bone resected from the posterior femoral condyle was 2 mm greater when the knee was to be treated with the NexGen CR-Flex prosthesis than when it was to be treated with the standard NexGen CR prosthesis”. If this is done, then the final posterior condylar offset will be the same for both the groups (as noted in the study, Table V), and the aim of achieving the extra 2 mm offset would not be realized.

    2. A post hoc power analysis with the figures quoted in Table III (Gp 1 Mean Flexion 1310 sd 13.9; Gp 2 Mean Flexion 1330 sd 10.4; Sample size 54 each group) shows a one-tail test power of 21.2% and a two-tail test power of 13.5%. This obviously shows that the study is clearly underpowered to detect a difference in between the two groups. The authors did do a power analysis prior to the study using 5° as clinically relevant difference in flexion, with a standard deviation of 9°. Can they please clarify why they used 9° as the standard deviation which is more than the difference they were aiming for?

    3. The mean amount of flexion preoperatively was 128°, which is obviously quite high as compared to a typical group with arthritis requiring knee replacement (2). This may contribute to the limited improvement of flexion noted in this study.

    The role of posterior condylar offset in improving range of motion following knee replacement has been proven in several independent studies (3,4,5). It may well be that the current study does not show any difference because of technical, methodological and epidemiological issues.

    The author did not receive any outside funding or grants in support of his research for or preparation of this work. Neither he nor a member of his immediate family received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the author, or a member of his immediate family, is affiliated or associated.

    References

    1. Kim YH, Choi Y, Kim JS. Range of motion of standard and high-flexion posterior cruciate-retaining total knee prostheses a prospective randomized study. J Bone Joint Surg Am. 2009;91:1874-81.

    2. Lizaur A, Marco L, Cebrian R. Preoperative factors influencing the range of movement after total knee arthroplasty for severe osteoarthritis. J Bone Joint Surg Br. 1997;79:626–9.

    3. Bellemans J, Banks S, Victor J, Vandenneucker H, Moemans A. Fluoroscopic analysis of the kinematics of deep flexion in total knee arthroplasty. Influence of posterior condylar offset. J Bone Joint Surg Br. 2002;84:50-3.

    4. Massin P, Gournay A. Optimization of the posterior condylar offset, tibial slope, and condylar roll-back in total knee arthroplasty. J Arthroplasty. 2006;21:889-96.

    5. Malviya A, Lingard EA, Weir DJ, Deehan DJ. Predicting range of movement after knee replacement: the importance of posterior condylar offset and tibial slope. Knee Surg Sports Traumatol Arthrosc. 2009;17:491-8. Epub 2009 Jan 13.

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