New Technology

THE MATRIX METHOD® FOR MINIMALLY INVASIVE/NAVIGATED TOTAL KNEE REPLACEMENT

Craig M. McAllister, M.D. and Jeff Stepanian, P.A.

Computer Navigation and the Matrix Method®

The ability of computer navigation to improve component position and alignment after knee replacement is accepted. Computer navigation eliminates the need for an IM rod, decreases blood loss, and decreases the potential for fat emboli. Computer navigation also decreases the need for exposure, and greatly facilitates MIS surgical techniques. The Matrix® System combined with the Triathlon MIS instruments allows the orthopedic surgeon to comfortably introduce MIS surgical techniques and navigation in an intuitive, incremental, efficient, and safe manner.

The MIS Method
In the MIS technique, modified retractors, down-sized instruments, and an altered sequence of osteotomies allow total knee replacement without eversion of the extensor mechanism. Specialized Matrix® Retractors have been designed and shaped to eliminate abrupt margins and optimize exposure without obstructing visualization. These retractors must be placed strategically and used dynamically to optimize visualization and protect the MCL, PCL, and LCL. The assistant uses them to relax one side of a mobile soft-tissue window while maximizing exposure on the other side. This method decreases the need for more extensile soft tissue dissections without sacrificing exposure.

Skin incision: the size of the skin incision is of cosmetic interest only and does not impact postoperative range of motion or knee function. The typical incision required for comfortable completion of the Matrix Method® is 10 – 14 cm. The incision starts at the superior pole of the patella and is swept slightly medial to the medial border of the tibial tubercle. Some subcutaneous dissection helps to free up the skin and create a mobile skin window.

A medial parapatellar arthrotomy is done with the knee in extension. The arthrotomy is initially extended to 3 cmabove the patella. It can and should be lengthened as needed to comfortably accommodate the instruments and achieve adequate visualization. The medial release is done with knee in extension and in the figure-of-four position with the patella reduced (fig. 1). By utilizingSpecialized Matrix® Retractors, reducing the patella, and leaving the tibia in a physiologic position,a less extensive medial release is required. A release of the lateral meniscus and the menisco-tibial ligament from anterior and lateral tibial plateau facilitates dislocation of the extensor mechanismand exposure of the lateral tibia (fig 12A). Little or no supracondylar debridement is required. A provisional patellar osteotomy and removal of femoral osteophytes are done to “debulk” the knee and reduce the need for exposure.

The Computer Navigation Survey
Placement of the Kirkland Bullet®

The computer survey begins with placement navigation trackers using the Kirkland Bullet® (fig 2). The tapered tip of the trochar bores though the near cortex slowly. As the the threaded portion engages the cortex, the pin is pulled at its own speed to the distant cortex. It is important to stop turning the pin as this occurs. The outer sleeve is then tightened gently against the near cortex. The outer sleeve adds stiffness and controls rotation.

Digitizing the Leg
Digitizing the leg begins with the identification of the center of the femoral. This is done by rotating the hip in smooth, concentric motions. Femoral and tibial marks are identified, and range of motion, ligament function, and kinematics are evaluated.

Altered sequence of osteotomies
In the MIS technique, emphasis is placed on avoiding nonphysiologic positions of the knee, and patella eversion is not done with an extensor mechanism under tension. The osteotomies are sequenced to take advantage of the “relaxing effect” of decompressing the joint, and full subluxation of the tibia has been decreased or eliminated. This technique requires that the dissection be done with the knee in a variety of positions. It also involves a dynamic approach to retraction where the assistant utilizes a mobile soft-tissue window by relaxing on one side of the knee while exposing the other. Ideally, the tibia is osteotomized first and is done with the joint located.

Osteotomizing the tibia first allows the joint space to decompress making the rest of the exposure much easier. However, this step puts the MCL, the LCL, the PCL and the patellar tendon at risk for injury from the oscillating saw. It is, therefore, critical that Matrix Retractors® be placed precisely as shown in figure 5. Specialized saws also increase the safety of this step.

Tibia-Cut-First Technique
FLEXION WITH THE PATELLA DISLOCATED BUT NOT EVERTED

The tibial-cut-first technique establishes the tibial osteotomy based on pre-operative templating and anatomic landmarks (fig 6). The tibial osteotomy creates a platform, from which the rest of the joint can by built systematically utilizing the balancer method and computer navigation. The proximal tibial osteotomy is performed using an external tibial quide system in conjunction with computer navigation. The Matrix External Tibial Guide® references the tibial eminence and is navigation compatible (fig 7). Precise measurement of the correct level of the tibial osteotomy is critically important, since this osteotomy will determine both the flexion gap and the extension gap.

The depth of the tibial cut is referenced from the tibial prominence, just posterior to the ACL footprint and anterior to the PCL insertion. Typically, 9mm from a normal lateral plateau corresponds to 15 mm from the tibial prominence (range from 13mm to 17 mm depending on the size of the patient, soft-tissues, and type of knee replacement).

The proximal tibia is osteomized in situ. This can be done comfortably and safely, as long as the Matrix® retractors are placed precisely and consistently to protect the MCL, the LCL, and the PCL (fig 8). The knee is then hyperflexed, and the proximal tibia is partially subluxated. The joint is not actually dislocated. On occasion, the osteotomy is completed in sections to protect the patellar tendon and other soft tissues. Modified saw blades should also be utilized and the surgeon should take special care to avoid trauma to the patellar tendon.

After the proximal tibia is osteotomized, the extension space is evaluated with the combined use of the balancer and a soft tissue torque wrench (fig 9). The size and symmetry of the extension space is checked with the balancer set at 30-foot pounds of tension. Ideally, the “pre-cut gap” should be close to 14mm in flexion and extension. Appropriate adjustments can be made in either the level of the tibial osteotomy or the upcoming femoral osteotomy depending on the pre-cut gaps.

The Distal Femoral Osteotomy using The Matrix Femoral Guide®
The Matrix Femoral Guide® can be utilized as either an IM device or a navigated device or both.This allows the surgeon to comfortably transition from IM techniques to navigated techniques. It provides a familiar method for validating the accuracy of navigation. It also provides a backup methodology in the case of computer failure. This instrument allows independent, stable, and precise control of all three resection planes.

The block is placed on the distal end of the femur, just as is done with an IM based system. An IM Rod can be used or eliminated, based on surgeon preference.
A Matrix® threaded pin is placed in the slot that corresponds to the deficient femoral condyle and the fixed hole on the other side.
The varus/valgus correction is achieved (fig 10A) and a third threaded pin is placed to fix the block in this position.
1 to 2 degrees of femoral flexion is introduced
The depth of the cut is set (fig 10B)
Matrix threaded pins® are used to fix the Matrix Cannulae® (fig 10C)

The Matrix Cannulae® combined with threaded pins increases the precision and consistency of cutting block placement and fixation. This helps to eliminate the frustration of seeing changes in position as the surgery progresses. The Matrix Cutting Blocks® are positioned and shaped to decrease the need for exposure. They allow for after-cut correction. They are available in a disposable, magnetic version that is more accurate but smaller than traditional closed blocks.

Computer Navigation and the Sentinel Moment
The distal femoral osteotomy is done with the knee in flexion and with the extensor mechanism is in a dislocated position. This osteotomy sets the level of the joint line, the varus-valgus angle, and the flexion of the femoral component. It also completes the expension space. However, it does not commit the femoral component rotation, size, AP position, or the flexion space. The transepicondylar axis and Whiteside’s line are marked. The balancer is then placed in the flexion space and measurements are taken at the 30-foot pounds of tension (fig 9). The size and shape of the flexion space can be evaluated prior to completing rest of the femoral osteotomies. This information is used along with Whiteside’s line, the transepicondylar axis, femoral sizing information, and femoral rotation. Femoral rotation (most commonly 3 degrees of external rotation) and A/P position can then be selected to optimize the size and shape of the flexion gap, patellofemoral mechanics, and component sizing.

Down-sized A/P sizer: A number of modifications have been made to the A/P sizer (fig11). Its medial-to-lateral size has been decreased. The posterior condylar feet have been debulked to allow the knee to be somewhat extended for this step. This improves exposure of the anterior femur. Drill holes accommodate 1/8th inch drill bits that are used to confirm sizing. This improves the accuracy of sizing while decreasing the need for exposure. The MIS Triathlon A/P sizer references the posterior condyles. However, because it is navigation ready, it also references the anterior cortex. This A/P sizer allows the surgeon to independently adjust femoral rotation and A/P position, thereby obviating the dichotomy between anterior versus posterior referencing systems.

Once femoral sizing, rotation, and A/P position has been selected, it is advisable to check for the potential for notching using navigation. If in doubt, simply choose a four-in-one block that is one size greater than predicted and perform a provisional anterior osteotomy. It is easy to then down-size to the next smaller four-in-one block and redo the anterior osteotomy.

Matrix® retractors and then placed to protect the MCL and the LCL and the rest of the femoral osteotomies are completed.

Summary

Stiffness and the long weeks and months of rehabilitation that typically follow knee replacement contribute to patient dissatisfaction. MIS techniques can help ease the postoperative rehabilitation for these patients. Surgical technique that standardizes for flexion and extension gap combined with computer navigation allows for precision in gap management, component position, and alignment. The Matrix Method® allows these techniques to be integrated in a comfortable, incremental, and safe fashion.