Computer-Assisted Navigation: Minimally Invasive Surgery for Total Knee Replacement

Chapter 123 Computer-Assisted Navigation

Minimally Invasive Surgery for Total Knee Replacement

Much of the hesitation in using minimally invasive surgery (MIS) in total knee replacement (TKR) evolves from coping with the complexities of a smaller incision without visual cues. When using a constrained exposure, reliance on instruments rather than direct visual reference is paramount. Second, sequencing to augment exposure becomes important to afford better access. If one adds deformity or increased body mass to the equation the traditional visual references, angular and translational position is compromised. Such dependence on traditional instruments, which rely on some degree of visually estimating cuts, may result in less dependable accuracy.35,10,12,16

Computer-assisted surgery (CAS), also known as surgical navigation, becomes necessary to reclaim lost visual reference and confirm accuracy lost in MIS exposure.17 If the surgeon avoids the use of small incisions because of lost accuracy, the addition of CAS can allay concerns about this deficiency. Although doubt still exists in regard to the efficacy of CAS to improve performances, this accuracy ensures the surgeon that the extra step of precision has been taken.* The confidence gained in achieving accuracy with documentation easily overcomes any reticence of a short learning curve in adoption of this technology.

However, there are other factors in addition to the time required to learn the systems. Lack of confidence in believing what the instruments are reading can hinder surgical progress.15 Although some readings are often not realistic or believable, the computer is usually smarter than the surgeon. Once reliance and confidence in CAS have been achieved, less time is spent using traditional instruments to confirm readings and more rapid surgery can be performed.

I have used every system on the market in an effort to provide a global review of the advantages and disadvantages of all systems discussed in this chapter. This will be based on the latest available software and not what is under development. The system that might be most efficacious in an institution is dependent on factors such as TKR systems, incision size, preference in operating room configuration, and data desired be used on the CAS monitor. Just by reading this chapter identifies you as a surgeon willing to consider using technology to afford a better result, and for that there is no wrong answer.

Choices in Systems

Infrared Computer-Assisted Surgery

Infrared (IR) CAS not only holds the competitive edge for acceptance, it also represents the bulk of available products available worldwide. In terms of signal strength and stability, IR CAS is clearly the most advantageous. These advantages include an abundance of proprietary choices, software versatility for multiple TKR systems, and ease of use.

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.

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.

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.

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.

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 image-inch inserted on top. This provides protection to the tibia to prevent penetration into the soft cancelous bed from the distraction maneuver. The image-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).

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.

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Aug 26, 2016 | Posted by in ORTHOPEDIC | Comments Off on Computer-Assisted Navigation: Minimally Invasive Surgery for Total Knee Replacement

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