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Three-Dimensional Tibiofemoral Articular Contact Kinematics of a Cruciate-Retaining Total Knee Arthroplasty
Guoan Li, PhD1; Jeremy Suggs, MS1; George Hanson, BS1; Sridhar Durbhakula, MD2; Todd Johnson, PhD3; Andrew Freiberg, MD4
1 Massachusetts General Hospital, 55 Fruit Street, GRJ 1215, Boston, MA 02114. E-mail address for G. Li: gli1@partners.org
2 6080 Falls Road, Suite 203, Baltimore, MD 21209
3 P.O. Box 708, 1800 West Center Street, Warsaw, IN 46581
4 Massachusetts General Hospital, Yawkey Center for Outpatient Care, 32 Fruit Street, YAW-3-3B, Boston, MA 02114
View Disclosures and Other Information
Note: The authors thank Dr. Louis DeFrate, Ramprasad Papannagari, and Elizabeth DeSouza for their technical assistance.
In support of their research for or preparation of this manuscript, one or more of the authors received grants or outside funding from Zimmer. In addition, one or more of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity (Zimmer). Also, a commercial entity (Zimmer) paid or directed, or agreed to pay or direct, benefits to a research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.
Investigation performed at the Bioengineering Laboratory, Department of Orthopaedic Surgery, Massachusetts General Hospital/Harvard Medical School, Boston, Massachusetts

The Journal of Bone and Joint Surgery, Incorporated
J Bone Joint Surg Am, 2006 Feb 01;88(2):395-402. doi: 10.2106/JBJS.D.03028
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Background: Accurate knowledge of the location of tibiofemoral articular contact following total knee arthroplasty is important in order to understand polyethylene wear and the mechanisms of component failure. The present study was performed to determine the three-dimensional tibiofemoral articular contact patterns of a posterior cruciate ligament-retaining total knee replacement during in vivo weight-bearing flexion.

Methods: Nine osteoarthritic patients who were managed with a single design of a posterior cruciate ligament-retaining total knee implant were investigated with the use of an innovative dual orthogonal fluoroscopic imaging system. The position of the components during in vivo weight-bearing flexion was measured from full extension to maximum flexion in 15° intervals. Tibiofemoral articular contact was determined by the overlap of the tibiofemoral articular surfaces. The centroid of the surface intersection was used to report the point of contact location. The average tibiofemoral contact points on both the medial and lateral tibial component surfaces were reported as a function of flexion.

Results: The average maximum weight-bearing flexion angle was 113.3° ± 13.1° (range, 96° to 138°). In the anteroposterior direction, the contact location was relatively constant in the medial compartment and moved posteriorly by 5.6 mm in the lateral compartment as the knee flexed from full extension to 90° of flexion. The range of the contact location in the mediolateral direction was 3.7 mm in the medial compartment and 4.8 mm in the lateral compartment. For both compartments, posterior translation of the contact point was significant from 90° to maximum flexion, but the contact point at maximum flexion was not observed to reach the posterior edge of the polyethylene tibial insert articular surface.

Conclusions: While the minimum anteroposterior translation of the contact point on the medial side might be interpreted as a medial pivot rotation during knee flexion, the contact point did move in the mediolateral direction with flexion. Beyond 90°, both medial and lateral contact points were shown to move posteriorly but stopped before reaching the posterior edge of the polyethylene tibial insert articular surface. It seemed that the current component design did not allow the femoral condyle to roll off the polyethylene edge at high degrees of flexion because of the geometry at the posterior lip.

Clinical Relevance: These three-dimensional tibiofemoral contact data may provide new insight for determining polyethylene tibial insert wear patterns in vivo and for designing the articulating surfaces by accounting for contact location in both the anteroposterior and mediolateral directions.

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    These activities have been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the American Academy of Orthopaedic Surgeons and The Journal of Bone and Joint Surgery, Inc. The American Academy of Orthopaedic Surgeons is accredited by the ACCME to provide continuing medical education for physicians.
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    Guoan Li, Ph.D.
    Posted on March 27, 2006
    Dr. Li responds to Dr. John
    Massachusetts General Hospital, Yawkey Center for Outpatient Care, Boston, MA 02114

    I thank Dr. John for his interest in our work involving in-vivo total knee arthroplasty (TKA) contact kinematics(1). His questions and comments are important and timely. We believe that a scientific discussion will definitely help readers of the Journal better understand in-vivo knee joint kinematics, especially tibiofemoral articular contact kinematics.

    It is true that the term “posterior femoral translation” is not synonymous with “femoral rollback.” We use “posterior femoral translation” exclusively when referring to our own data on femoral condylar motion or contact point motion. However, we also noticed in the literature that a precise description of the tibiofemoral kinematics is not a trivial task, especially when the medial and lateral compartments are involved. “Femoral rollback” has been sometimes used as a "short cut" term when describing “posterior femoral translation”. If any of these terms are used, a clear definition has to be given.

    Dr. John is correct in his reservations about using a nearest point methodology to determine point of contact on articular surfaces, especially when attempting to report contact between conforming articulating surfaces. This method has been used in previous studies and has provided much of the initial data on knee kinematics. We have published an article in the Journal of Biomechanics that compared the contact locations determined using the nearest point methodology with contact determined using intersecting surfaces in human knee joints [2]. This comparison indicated that articular contact kinematics are better measured from the intersection of the articulating surfaces. When determining the tibiofemoral contact of a cruciate-retaining TKA in our study [1], the intersection of the articular surfaces was measured for both the medial and lateral compartments. The resulting intersection between the femoral component and the proximal tibial polyethylene insert produced an area of contact. As described in the Materials and Methods section, the area centroid was calculated and used to compare the locations of the contact areas at each flexion angle. Therefore, the contact points reported in our study represent the center of the contact areas. We agree that this contact point may not be the location of peak pressure. However, this data analysis can provide a quantitative and consistent way to report contact kinematics. Peak contact location can only be obtained through a 3D finite element calculation in our cases.

    In our current manuscript [1], we did a general comparison of the contact kinematics of patients after cruciate retaining TKA to the cartilage contact kinematics of normal, healthy subjects [2,3]. Dr John is right that if the data are to be compared quantitatively, the same reference should be used. In our work, we plotted the contact points directly on the tibial plateau surfaces to demonstrate the actual contact locations. Therefore, qualitative comparison can be done, as discussed in our paper.


    1. Li et al. Three-dimensional articular contact knee kinematics of a cruciate-retaining TKA. JBJS 2006; 88-A(2): 395-402.

    2. DeFrate et al. In vivo tibiofemoral contact analysis using 3D MRI- based knee models. J Biomech. 2004; 37(1):1499-504.

    3.Li et al. In vivo articular cartilage contact kinematics of the knee: an investigation using dual-orthogonal fluoroscopy and magnetic resonance image-based computer models. Am J Sports Med. 2005; 33(1): 102- 7.

    Joby John
    Posted on February 25, 2006
    Three-Dimensional Tibiofemoral Articular Contact Kinematics of a Cruciate-Retaining Total Knee Arthr
    Robert Jones Agnes Hunt Hospital, Oswestry, England.

    To The Editor:

    I read with interest the article by Li, et al, (1). The paper confirms that it is not possible to have asymmetric posterior translation of the lateral and medial femoral condyles without some mediolateral translation. However, the paper raises a few questions.

    There seems to be a tendency to use posterior translation of tibiofemoral contact synonymously with femoral roll-back. Femoral roll back is the posterior translation of the variable instant centres of rotation of the respective femoral condyles, especially the lateral condyle. This would certainly correlate and probably mirror the posterior translation of tibiofemoral contact points but only in a stable knee. In an unstable knee with the loss of the ACL, where anteroposterior tibiofemoral translation is possible even without flexion, to attribute any posterior translation of tibiofemoral contact point to the changes in instant centres of rotation of the femoral condyles and hence flexion of knee alone would certainly seem misplaced. Even if it were to be argued that posterior translation would be governed by the presence of the PCL alone, the initial contact point would certainly be anterior in the absence of the ACL.

    There seems to be a huge disparity in the methodology used for measuring femoral rollback. Patil, et al, (2) reported results in unicompartmental knees using the midpoint of the transepicondylar line as the reference point to measure femoral roll-back. The authors have used the nearest points between the metal tibial tray and femoral component to deduce the point of contact of the components with the plastic tray. This methodology would be feasible if the polyethylene were flat as some of the PCL retaining designs were. However,in the presence of a dished polyethylene component as illustrated in the figures presented in this article,the accuracy of their technique would be questionable since there is no point of contact anymore, but only an area of contact. It would seem that in the presence of conforming articulating surfaces, the geometry of the surfaces would bear a big influence on the closest points between the femoral and tibial components and that a specific point may not necessarily represent the point of maximum compression. The observation that the posterior lip of the polyethylene tray impinges at the end of flexion may possibly be due to this.

    The authors have mentioned using the methodology in normal patients and have reported a similar kinematic profile in them, but they have not elaborated on how the contact points were determined. They have not defined the stationary point from which posterior translation was defined. In contrast, in knees with a prosthesis implanted, specific points on the metal tray can be identified to allow for reproducible measurements.

    I believe that validation of methodolgy with observer errors is important. It is also important not to ascribe all posterior tibiofemoral translation to have been caused secondary to changes in instant centres of rotation of the femoral condyles (Femoral roll-back) and hence flexion at the knee, although I agree that differentiation of the individual contributions may be difficult if not entirely impossible.

    It would be very helpful to readers of the Journal if there were a consensus on terms and methodology used in measuring tibiofemoral kinematics.


    1. Li, et al. Three-Dimensional Tibiofemoral Articular Contact Kinematics of a Cruciate-Retaining Total Knee Arthroplasty. J Bone Joint Surg Am. 2006; 88:395-402

    2. Patil, et al. Can Normal Knee Kinematics Be Restored with Unicompartmental Knee Replacement? J Bone Joint Surg Am. 2005;87: 332-338.

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