Kinematically aligned medial pivot knee implant duplicates natural alignment, stability
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Total knee arthroplasty has offered great benefit to patients in recent decades. Although many patients are satisfied, studies show about 20% of them are not.
Presented is a philosophy of restoring the patient’s joint line and stability by implant choice and alignment choice to make the patient’s knee feel more natural. The surgical technique and specialized instruments are discussed.
The best way to restore function is to restore, as best possible, the patient’s normal anatomy and kinematics. Kinematic alignment (KA) restores the patient’s femoral joint surface and therefore the specific flexion-extension axis of the knee. The femoral joint surface and the ligaments work as a functional unit, so the normal ligament tension can be used to balance the knee with subtle changes to the tibial resection. This contrasts with mechanical alignment techniques in which the implant does not match the natural joint line and ligaments are often released to balance the knee.
Implant choice
The natural knee demonstrates medial pivot kinematics. The Evolution medial pivot implant (MicroPort) restores the medial pivot kinematics and rotational axis of the knee. Traditional TKA implants have been categorized as cruciate retaining (CR) or posterior stabilized (PS) based on how the implants try to restore posterior stability to the knee.
The Evolution medial pivot implant provides anterior and posterior stability through range of motion. A medial pivot design has been shown to be more stable than either a CR or PS design. Studies by James W. Pritchett, MD, show a medial pivot knee is preferred by patients three to one vs. CR or PS knees. The predecessor of the medial pivot knee implant discussed herein showed 98.9% survivorship at 17 years.
Surgical technique, instruments
The Kinematic Alignment Set (MicroPort) for the medial pivot knee implant allows the surgeon to restore the patient’s individual anatomy. The femoral surface is placed in the pre-arthritic position. Normal ligament tension is restored. The knee is balanced by a subtle adjustment of the tibial resection rather than by releasing ligaments. KA prioritizes restoring the patient’s joint line (Figure 1).
Source: Robert N. Steensen, MD, FAAOS
A cartilage thickness gauge is used to measure the cartilage thickness on the distal femoral condyles so that the amount of wear can be extrapolated. The femoral guide has medial and lateral adjustable pads that account for the cartilage wear. Use of the pads achieves a resection that places the surface of the implant at the pre-arthritic level of the femur (Figure 2). For example, if there is 2 mm of medial wear and no lateral wear, for a 10-mm thick femur, the femoral guide will resect 8 mm medially and 10 mm laterally.
Source: MicroPort
The posterior referencing sizer sets the femoral rotation. This is typically set at 0° in KA to duplicate anatomy because there is usually not much posterior wear. However, this can be adjusted for any wear or deformity that is present.
Once the femoral resections have been made, the femoral trial is placed. The medial and lateral extension gaps between the femoral trial and the native tibia are measured. This is done with gap guides or “spoons” that are used to balance the knee (Figure 3). The native knee is stable in extension, so the gap determination is done in extension. Because the femoral implant also matches the posterior femoral condyles, the flexion gaps/stability will be natural.
Source: MicroPort
The extension gap measurements are used to set the Dual Stylus/Variable Angle Tibial Guide (MicroPort) (Figure 4). The guide has a medial stylus and a lateral stylus that are set independently. The stylus for each side of the tibial plateau adjusts to the desired resection. For example, with 3 mm of medial wear and no lateral wear, the guide is set to resect 7 mm from the medial plateau and 10 mm from the lateral plateau to create the space for a 10-mm tibial implant. It would also indicate the angle of the resection, which the surgeon can override, if desired.
Source: MicroPort
Conclusion
The medial pivot implant KA set discussed follows a joint line restoration philosophy to place the femoral TKA component so that it matches the native femur (Figure 5). This is important as the knee rotates around the femoral surface. This restores the normal flexion-extension axis in individual patients. The femoral joint surface and the ligaments are a functional unit, so the knee is balanced by using ligament tension to set the resection of the tibia. The procedure is reproducible, but it is individualized for each patient. The medial pivot TKA implant restores the normal medial pivot axis and provides anteroposterior stability. By matching both axes of the knee, the patient’s kinematics are restored.
Source: Robert N. Steensen, MD, FAAOS
Studies show a benefit to a medial pivot implant and to KA. Combining both the implant and KA creates synergy, with the goal of increasing patient satisfaction.
- References:
- Dossett HG, et al. Bone Joint J. 2014;doi:10.1302/0301-620X.96B7.32812.
- Macheras GA, et al. Knee. 2017;doi:10.1016/j.knee.2017.01.008.
- Pritchett JW. J Arthroplasty. 2011;doi:10.1016/j.arth.2010.02.012.
- Pritchett JW. J Bone Joint Surg Br. 2004;doi:10.1302/0301-620x.86b7.14991.
- Wautier D, et al. Knee Surg Sports Traumtol Arthrosc. 2017;doi:10.1007/s00167-016-4038-9.
- For more information:
- Robert N. Steensen, MD, FAAOS, an orthopedic surgeon at Orthopedic One, can be reached at 3777 Trueman Ct., Hilliard, OH 43026; email: ebeerbower@ampublicrelations.com.
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