Point/Counterpoint: Are Custom Implants the Future of Knee Replacement?

    At ICJR’s Pan Pacific Congress, Dr. William Kurtz and Dr. John Callaghan debated the use of custom implants for patients undergoing total knee arthroplasty. Below are the abstracts from their presentations.

    Custom Implants: Pro
    William B. Kurtz, MD, Tennessee Orthopedic Alliance, Nashville, Tennessee

    Ever since the first hinged replacement was performed by Leslie Gordon Percival Shiers in the 1950s, total knee replacement designs have moved away from artificially imposed constraints and toward efforts to mimic the native knee anatomy and biomechanics.

    Improvements in 3D imaging over the past 40 years have shown there are large variations in patients’:

    • Knee size
    • Knee shape
    • J curve shapes
    • Varus joint line

    Through the 1980s and 1990s, surgeons tried various alignment techniques with mechanical jigs – namely gap balancing and measured resection – to resolve these anatomic differences.

    Over the last decade, some surgeons have added computer navigation and pre-navigation with patient-specific jigs in an attempt to restore the varus joint line and achieve more normal knee kinematics through kinematic alignment. The symmetric implants utilized with kinematic alignment, however, require complicated bone cuts that are non-perpendicular to the mechanical axis to restore the varus joint line.

    Some off-the-shelf implants have been designed specifically for female patients, and others have been designed with a set amount of varus built into the knee design. Both of these are improvements, but they do not accommodate the variations presented within the patient population.

    Studies conducted on the efficacy of patient-specific instrumentation have shown mixed results, with some reports showing similar results to conventional techniques, while others have shown higher percentage of correctly aligned knees, albeit with a significant amount of outliers.

    Improvements in manufacturing and imaging have finally advanced enough that in addition to patient-specific instrumentation, the patient can now receive a customized knee implant designed to accommodate the the patient’s knee size, knee shape, J curve shape, and varus joint line.

    Use of patient-specific instrumentation, in conjunction with customized, individually made implants seems to be a promising technology to consistently achieve mechanical axis alignment, optimum fit, and restoration of the patient’s native geometry.

    Dr. Kurtz’s presentation can be found here.

    Custom Implants: Con
    John J. Callaghan, MD, University of Iowa and the VA Medical Center, Iowa City, Iowa

    In more than 30 years of practice, I have utilized the traditional approach to knee alignment in reconstruction of the arthritic knee:

    • Creation of a neutral mechanical axis
    • Femoral tibial angle of 5° to 9° of valgus
    • Joint line parallel to the floor

    This is done by resecting the proximal tibial bone perpendicular to the shaft of the tibia and resecting an amount of femoral bone to place the distal femur in 5° to 7° of valgus.

    The amount of bone resected from the distal femur, proximal tibia, and posterior femoral condyles is determined in a way that the flexion gap is equal to the extension gap. The posterior femoral condylar resection is determined using the transepicondylar axis and is parallel to the cut surface of the tibia. Symmetry of the two gaps medially and laterally is obtained by performing ligament releases rather than ligament tightening.

    In 20-year follow-up studies from our institution, in which we used posterior cruciate retaining and posterior cruciate sacrificing knee designs with cement fixation and the technique described above, the results were extremely durable, even with the use of gamma in air polyethylene for the cruciate retaining knee. The revision prevalence for loosening with the cruciate retaining knee was 6%, and for the cruciate sacrificing component, the prevalence of revision for loosening was 0%.

    This occurred even though all knees were not aligned within 3° of mechanical axis. A system I and many other surgeon had experience with in the early 1980s (PCA, Howmedica) was designed to resect the tibia so as to leave it in 3° of varus; it provided less durable results.

    Therefore, I recommend reconstructing the knee in an alignment that we know provides durable results rather than simply replace the knee in the previously aligned position. My feeling is that if the patient’s articular cartilage wore out in this position, the components will similarly wear out over a time of extensive use.

    Dr. Callaghan’s presentation can be found here.