Medacta’s GMK Sphere knee prosthesis, MyKnee Patient Matched Technology, and GMK Efficiency Single Use Instruments provide an accurate, reproducible, and stable solution to TKA. With the understanding this system is different from traditional PS and CR knees currently on the market, the Medacta knee marketing team reached out to surgeons who have extensive experience with this TKA system for them to share some of their ‘tips and tricks’. In this article, you will learn about surgical pearls which are used by Tyler Goldberg, MD, of Texas Orthopedics, Sports & Rehabilitation Associates in Austin, TX, Coleman Fowble, MD, of Midlands Orthopaedics & Neurosurgery in Columbia, SC, John Maltry, MD, of Tucson Orthopaedic Institute in Tucson, AZ, and Ryan Molli, DO, of Orthopedic Association of Meadville in Meadville, PA in order to obtain optimal outcomes for their TKA patients.
RM: Probably one of the biggest advantages of the GMK Sphere is the exceptional patellofemoral kinematics. The deeper trochlear groove is also slightly lateralized to help promote better patellofemoral tracking. The concept of medial stabilized knees relies upon medial stability with coupling and congruency with lateral rollback with knee flexion and more laxity which is very similar to a native knee. When the knee demonstrates lateral rollback of the femur on the tibia during knee flexion, the tibia is in essence internally rotating relative to the femur which causes the tibial tubercle to medialize which in turn decreases the functional Q-angle during knee flexion. This helps to drive the patella into the trochlea for more kinematically pleasing patellofemoral motion. A final component which helps with better patellofemoral kinematics is the anatomic 3-peg patellar button which is available, although the kinematics are so pleasing that many surgeons are moving away from patellar resurfacing unless there is profound patellofemoral disease or patellofemoral mal-tracking.
TG: The GMK Sphere is designed to reproduce normal knee kinematics through a medially congruent surface. Thus, classically trained orthopedic surgeons will need to change their mindset regarding balance, technique, and outcomes as they begin their Sphere journey. For me, this aspect was understanding the laxity encountered in both flexion and extension once the implants are placed. This is what I like to call “symmetrical lateral laxity". The image below shows the GMK sphere implant demonstrating space on the lateral side at extension and flexion with the bovie being able to be inserted into the joint space. This implant attempts to achieve normal knee kinematics in its design and thus will require surgeons to understand and accept lateral laxity.
RM: Since both cruciates are removed during the operation (which is obligatory for the sphere to function properly), it is important to not over-resect the posterior femoral condyles when preparing the femur. With the sphere being a medially stabilized implant, it relies on a fairly snug or better termed, "appropriately tensioned" medial compartment with regards to both the flexion and extension gaps. I feel that posterior referencing with the Sphere is very helpful to ensure avoiding over-resection of posterior femoral condyles. This is applicable for both conventional and MyKnee users. I generally avoid resecting more than 9mm off the posterior femoral condyles unless I absolutely have to for sizing purposes to avoid notching anteriorly.
RM: Just as making sure to not over-resect the posterior condyles, it is important to avoid flexion instability with the sphere implant. An equally important technical point is to also avoid increasing posterior tibial slope excessively. This is another way that many cruciate retaining surgeons help to "loosen" a tight flexion space in addition to "recessing" the PCL. In the sphere technique, the PCL is resected and it is critical to maintain a stable flexion space so keeping posterior slope typically between 2-3 degrees is key in accomplishing this. Anything beyond 3 degrees in addition to the obligatory PCL resection can lead to flexion instability.
TG: I have come to trust the MyKnee guides—which can aid in setting rotation of the tibia—so much that when I am trialing a GMK Sphere knee, I am really only interested in the gaps and poly thickness. Consequently, I impact the keel punch and remove it immediately. I then articulate my trial poly to the tibia baseplate and place them on the flat surface of the tibia. I will then place the femoral trial while flexing the knee. By placing the poly first, I ensure that my locking mechanism of the poly is secured perfectly. Second, I do not have the struggle of placing the poly in midflexion. Occasionally, I will change my poly thickness after my final implants are placed. In those situations, I must remove and place the poly in midflexion.
JM [Conventional GMK Efficiency]: Lubricate all plastic parts with joint fluid or fat scraps prior to assembly and use.....makes things MUCH easier! Use Stryker Precision blades. They don't beat up or dislodge the cutting blocks. Be careful when punching the tibia. The bone on one side of the tibial plateau will be much harder than the other and can break the punch blades if you are not careful. [Medacta recommends you score the bone for the fins of the keel with a saw blade before punching with the trial tibial keel.]
TG: One small detail that I really like about Efficiency is the alignment rod for the tibia to check the varus/valgus. There is a small hole at the top of the rod that will lock the rod at 90° and thus I am able to check the slope. A common risk with the MyKnee resection guide is to add slope to the tibia. This is easily checked with the fixed angle drop rod from Efficiency.
JM [Conventional GMK Efficiency]: Be sure to cut enough tibia. [In my opinion] cut at least 5-7mm from the affected side. Use an "angel wing" through the [conventional] tibial cutting block to adjust and check tibial slope. Additionally, through my experience, I always add 2 mm to the distal femur cut depth. This [allows me] to achieve full extension and balance. This is a PCL sacrificing knee design, and it acts like it!
CF: It is helpful to mount the custom cutting guides on the femur, outline the footprint of the custom cutting guide, and then compare this to the model before taking the articular cartilage off the femur. This will allow it to sit better on the femur as well as minimize the time it takes to take down the articular cartilage.
CF: Do the distal femoral cut and then the tibial cut in knees with more significant angular deformity. Check the flexion and extension gaps before using 4-in-1 cutting block on the femur.
RM: I typically use MyKnee Patient Specific Instrumentation (PSI) cutting blocks for the majority of my TKA's to help aid in more accurate component placement with regards to neutral mechanical alignment as well as more accurate femoral component positioning with regards to rotation in the axial plane. A huge advantage of the blocks, is you can study them carefully to identify cumbersome osteophytes that may hinder exposure. One common example are the large superiorly projecting osteophytes off the posterior tibia that can make anteriorization of the tibia challenging and sometimes impossible until the osteophyte is removed. Assess the femoral mold for any large posterior femoral condylar osteophytes that may be hindering complete extension and encouraging persistent flexion contractures after surgery. The extent of these is often quite obvious on the molds and helps to delineate what needs to be removed to help restore motion.
RM: It is important to check each step of the proposed plan before osseous resection and use either an angel wing, reference lines (AP/Whiteside's lines and/or epicondylar axis reference lines) or drop rods to assess what the MyKnee blocks are suggesting. Having a logical and reproducible flow with checks and double checks is important to validate the technology as well as avoid any potential engineering or operator errors.
