Tracker Mounting Options

With the exception of one system (Aesculap, Center Valley, Pa), all now have gone to a dual- or multiple-pin fixation format positioned out of the incision opposite the surgeon’s side. Pins are placed at dedicated distances so that they will accommodate the tracker mounting block. Only one system (Stryker Orthopaedics, Mahwah, NJ) has the provision for a third pin to be inserted divergently or nonparallel. For most systems, parallel is the norm, using -inch self-drilling or self-tapping threaded pins in varied lengths.

The surgeon’s preferences of the operative side is the usual mandate for where to place the reflective tracker array. Whether using active or passive reflections, transmissions require constant visual reference to CAS data or the transmitter tower while working. Therefore, if the surgeon operates from the opposite side, as is my preference (stands on the left of the patient for a right TKR), trackers should face to the right of the patient. The other option is to face the trackers over the shoulder of the surgeon, which creates the dilemma of constantly dodging the receiver tower. Although trackers are traditionally inserted on the lateral tibia and femur, my preference is to insert the tracker array on the medial tibia and distal femur to view the CAS screen from the opposite side. This limits the choice of systems or may mandate modification in the system’s software for side prioritization and CAS programming.

Lateral Monitor and Medial Pin Insertion Option

The tibia insertion point is best started at or approximately 4 cm below the tibial tubercle to avoid cutting jig interference and remain free of the incision access. The pins are inserted with the first pin bicortically applied at a 45- to 60-degree angle off the anterior crest of the anteroposterior (AP) plane of the tibia and the second pin rotated sufficiently to allow just enough room for the tracker array to clear the drapes and the crest of the tibia (Fig. 123-1). Some tracker pin blocks allow multiaxial rotation, so the exact rotation may not be necessary. However, fixed tracker blocks require some attention to rotation and elevation.

Figure 123-1 IR trackers in place for a left medial approach, with surgeon positioned to right of patient. Note the dorsal fin configuration for best reception in any leg position.

The femoral tracker may be applied intraincisionally by first installing the distal pin 2 cm above the medial epicondyle at a 10- to 20-degree angle off the midsaggital plane. This first pin is the best bicortical pin fixation and therefore is the first inserted—plus, it uses two single cortical screws, not pins. Pins are inserted at 45-degree knee flexion so as to minimize tethering forces from the vastus medialis and skin edges. This first pin should also capture a supracondylar ridge of bone above the lateral epicondyle on the lateral femur. The second pin can be applied by single cortical or bicortical penetration. If the first pin is firm, the need for bicortical fixation is not so important. However, if the patient is osteoporotic or there is a suspicion of less firm fixation on the distal pin, the stabilization is achieved through the second pin and bicortical fixation is imperative. Care must be taken to allow for optional exposure to prevent the vastus medialis oblique (VMO) and skin flaps from deforming the pins, which would cause aberrant values from the CAS system. If tethering occurs, such as in muscular males, one must extend the incision or readings will be in error because of pin deflection in flexion.

Medial Monitor and Lateral Pin Option

Some systems (e.g., ORTHOsoft, Zimmer, Warsaw, Ind), have no provisions for medial mounting of the tracker arrays and therefore the surgeon is compelled to affix them to the lateral side. Although tibial pins are not generally a problem, the femoral side can be a potential source of vascular injury. The femoral vessels coursing into the adductor canal have not reached the posterior position on the lower femur until the lower third of the femur; therefore, mounting trackers in an interiorly rotated position may place the upper pins in an intersection with the femoral artery. To avoid, this remember to proceed in a more anterior saggital direction on the upper pin and midsaggital on the lower pins while taking care not to penetrate the cortices by any pins. As noted, you may only need one bicortical pin because the second often serves as a derotation pin. There is little to disturb fixation on the lateral thigh, so these pins rarely lose fixation.

In all systems, tracker arrays should be adjusted to face the emitter, similar to the dorsal fin of a fish. Hence, the reflection surface will be planar to the AP axis with a 10-degree convergence and a 20-degree elevation for the best view.

Intraincisional Considerations

The temptation for almost every MIS-trained surgeon is to see how small one can make the incision and still get the job done. With reliance and confidence in CAS as the primary source of navigation, the surgeon can be more accurate in achieving that goal. The next question is whether one can keep the tracker pins inboard to the incision, or at least include one tracker assembly inside the wound margins.

Although it is possible to keep all pins inbound, there are two prerequisites. The system must have a medial pin application and a pin footprint of no more than a 2-cm separation between pins (Galileo Plus and Medtronics; see later). Anything with a wide tracker base will likely require a second small percutaneous incision to maintain the tissue-sparing look of a MIS incision. A small pinhole can be used, which may be just the right mix for many surgeons.

The greatest obstacle comes with the reduced room for accessing cutting jigs. This commits one to a freehand cut on the femur and tibia or modifying current cutting jigs to lie over and behind the view of the trackers to make the cuts. My preference is to use a freehand cut with a small two-pin block on the femur (Fig. 123-2). This is done by holding the cutting block in one hand, with a paddle array in the cutting slot. The other hand holds a pin or screw on a drill, which provides a means of fine correction of varus-valgus once resection depth is established.

Figure 123-2 Freehand navigation of distal femoral cut. The drill acts as a stabilizer for less drastic fine tuning of varus-valgus flexion-extension.

On the tibial side, some cutting jigs set up in an elevated position (BrainLAB, Munich, Germany; ORTHOsoft). This affords just enough maneuvering room that a three-axis adjustment is possible over the top of the tracker array. However, the space is tight, thereby making the importance of proper tracker placement paramount to prevent the frustration of inadequate room for cutting jigs. Most importantly, the cutting jig needs firm fixation on the tibia to reduce saw blade drift and the need for corrective cuts.

Sequencing Changes

When MIS TKR was first introduced, femoral preparation followed by the tibia was taught. There are three reasons why this is no longer applicable. First, bone removal of the tibia-first cut is sometimes difficult without the hard cortical surface of the femur to pry against to remove it in osteoporotic patients. Second, many surgeons prefer basing all future cuts on the rectangular flexion-extension gap of their tibial cut. This facilitates the ability of the surgeon to achieve some visual confirmation if one doesn’t want to take the time for cumbersome tensometer or gap distance measures. Finally, and most importantly, without the tibial cut removed, there is no easy way to see the lateral posterior femoral condyle to assess femoral waypoints or wear asymmetry to acquire valid computer baseline data adequately. Many systems lack the software to proceed without this waypoint identified or the posterior-based cutting blocks must be forcibly placed, which is subject to error. My suggestion is that unless the surgeon is firmly committed to a femur-first resection, he or she should become comfortable with the tibia-first approach, because this will be the direction toward which most systems will be moving.

Surgical Technique

With the incision completed and trackers in place, the femoral head center is acquired. This is done by concentric circle overlap software or multiple cone intersection points added, depending on the system. Next, waypoints for the tibia are acquired and the tibia is resected.

This is undoubtedly the most difficult part of the procedure, but all the following steps hinge on access gained from the proximal tibial approach. When performing this, the knee is flexed to 90 degrees and tactile sensation provides the primary end point of saw blade excursion. A wide patellar retractor (fat fork) is placed laterally, a forked retractor posteriorly, and a cruciate or Hohmann retractor on the medial side to protect the medial collateral ligaments. In large patients, 80 mm is the furthest excursion of the saw blade to the poster lateral corner, although 60 mm is adequate for women. The posterior retractor protects the popliteal structures. Use caution not to abrade the patellar tendon anteriorly with the blade oscillations because it can cut as easily by the edge as it does on the cutting end. When sawing is complete, a twist of the saw is usually enough to pop the remaining capsule free of any bound bony tissue. For large patients with marginal osteophytes or for osteoporotic individuals, blade twisting to elevate the respected bone may not be sufficient, or worse yet can mash the tibial metaphases. I use a 1.5-inch osteotome gently inserted across the osteotomy and then a second -inch inserted on top. This provides protection to the tibia to prevent penetration into the soft cancelous bed from the distraction maneuver. The -inch osteotome is twisted with a pliers or the back of a locked Cocker, to dislodge the bone enough which aids in the initially freeing of the fragment (Fig. 123-3).

Figure 123-3 Retractors are in place in the posterior proximal fragment to pry it forward. Once free, the sweetheart clamp is used to apply added pressure to role out the fragment laterally while freeing posterior meniscal attachments.

Next, the fragment is elevated with a sweetheart (Lewin) clamp) on the anterior medial edge while a Hohmann retractor resting on the posterior condyle is levered forward, thereby freeing the medial portion enough to see the posterior capsule, cruciate, and soft tissue. Cutting free these structures leaves the last remaining hold, which is the posterior lateral meniscus. Once the popliteal meniscal complex is cut, the entire tibial fragment can be rotated free in a progressive role-out manner rather than an anterior drawer removal.

If the patient is large or the fragment stubborn, just bisect it at the notch, using an osteotome to remove the medial and then the lateral fragment; this is the easiest method (see earlier). If even this leaves an incomplete resection, I often will extend the knee to get slack in the collaterals to remove any remaining large fragments. However, this is not the final clean-out, so don’t waste time looking for a little debris. All you want is a metaphyseal platform big enough to support a paddle tracker to confirm proper resection angles and heights. If you are within approximately 1 degree, you can proceed. If not, I usually make a corrective sanding or blade pass with the saw and do not verify the accuracy after the correction. Remember, you are already three times better with navigation than with traditional instruments, when there was no valid way to check cuts except cumbersome alignment rods, which are subject to rotation.

This now makes the femoral waypoint acquisition ready by viewing the posterior condyles or using the posterior referencing rotation jigs. Note first any abrasive wear or skid marks in relation to Whiteside’s line. This can be an important clue to an aberrant trajectory of true knee flexion rather than anatomic points. Once the computer has inputted waypoints on the femoral rotation, anterior cortex, and distal condylar surface, the distal femur can be resected. A freehand jig is my preference, but it takes some practice to keep the three axes in perspective on CAS monitors (see Fig. 123-3). As noted, be sure that your jig tracker is in the dorsal or saggital plane and not oblique to prevent composite angle changes. I start with the on-screen crosshairs to get varus-valgus roughed in. Then, while holding the cutting guide on the medial side of the femur in one hand, move it to the approximate calculated femoral resection (10 mm in most systems). I use the formula of 10 mm + = 1 mm resection for every 4-degree lag of extension noted preoperatively to achieve adequate flexion contracture correction. I have found this to be an accurate guide for providing adequate extension in patients with flexion contractures not corrected by osteophyte removal, similar to other authors.²

Orient the cutting block first by using the graphic lines on AP and lateral monitor pictures to line up the block crudely. Then, slide the block to the proper resection level and push (not drill) a pin to hold the rough position of the same capture. While monitoring numbers to fine-tune all angles try not to fall into the trap of “perfect numbers.” Choose your own ideal target but try not to chase numbers and hence waste time. Remember, CAS is capable of improving accuracy by 3 fold (+/− 3 degrees human site VS. +/− 1 degree CAS). However even CAS has accuracy limits which are not improved by spending excessive time on small degree variations of 1 degree off target values. My personal limits are 0-1 degree accept, 2 degrees think about, and >3 degrees fix or change. If, when anchoring the block, the drill deviates the saw capture, put a second pin in while overcorrecting in the plane opposite to the pin deflection. The spring effect usually results in a stabilization to the target value originally sought. Remember, large femurs invariably result in excessive varus over the CAS value because the saw blade drift undercuts the lateral and more distant condyle. This should achieve a first-time correct cut, but I still verify my cuts. If the monitor CAS value is still off by 1 or 2 degrees, don’t unpin the guide. Instead, force-feed the saw blade to achieve the ideal angles; one can generally correct for small errors.

Next, the femoral finishing block is applied and rotated to the proper 3- to 4-degree external rotation of posterior condyles (or 0 degrees if using Whiteside’s line). I use the computer to reference the anterior notch status. This is faster than using posterior reference guides, and most systems have on-screen notch and rotation values available. If size was predicted, I prefer to undersize if between sizes. Edge to edge coverage without overhang is the goal, which is best based on visual reference.

Once the block is positioned with convergent or multiple pins, a check for proper midline positioning is done. If the block somehow is too far medial or lateral, I skip drilling peg holes and cut the notch to the side minus the correction desired to side-slip the block to correct any miscentered placement. In this way, you still obtain precise rotational and AP cuts while providing a final centering of the implant in the medial lateral plane.

The knee is now ready for final clean-up in extension (Fig. 123-4). I prefer a large laminar spreader to separate the bone ends if a more elaborate foot holder is not available. I pay particular attention to three problem bleed areas. The first is the inferior genicular vessel to the lateral meniscus, just adjacent to the popliteus tendon. Second is the posterior cruciate found in the top of the box cut or in the notch. The final active vessel is the medial inferior genicular vessel to the medial meniscus and capsule. Even if I see no active bleeding, I generally use a Bovie or Bipolar (high-frequency bipolar unit) in these regions while the tourniquet is down. Then the capsule is injected. Avoid the popliteal corner because of the proximity of the peroneal nerve. I use an injection cocktail of 15 mL of 0.5% ropivacaine (Naropin), 100 mL of 0.2% ropivicaine, 0.5 mL of epinephrine 1/1000, a 30-mg solution of ketorolac (Toradol), and 10 mg of morphine sulfate. As a natural depot, the pes anserinus is injected with 10 to 15 mL while the remaining 25-mL allotment is placed in the muscle tissue and medial capsule, near the pes anserinus insertion.

Figure 123-4 Laminar spreader in place allows for less obstructed view of the posterior joint and does not interfere with trackers.

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