William J. Hozack, S. M. Javad Mortazavi, and Camilo Restrepo
DEFINITION
Accurate component positioning and limb alignment is one of the most critical steps in total knee arthroplasty (TKA). It has been shown that errors as small as 3 degrees can significantly affect the rate of loosening and the outcome of the TKA.6 The surgeon's ability to determine component positioning accurately may be influenced by a number of factors, including patient positioning, obesity, variable anatomy, deformity, bone loss, and ligamentous anatomy.
The main aim of computer-assisted TKA is to improve component positioning and limb alignment. Such improvement has been shown to increase longevity of the implant and decrease the need for revision surgery.2–4,7,8
The main function of navigation is to give the surgeon accurate information about the knee anatomy, limb alignment, and knee range of motion during the operation. The navigation system also gives real-time information as the operation proceeds. This dynamic intraoperative feedback regarding the orientation of bone cuts, soft tissue balancing, component positioning, limb alignment, and knee range of motion with the trial component in place should help the surgeon make the appropriate adjustments when needed.2
The navigation system makes more accurate information available for the surgeon, providing data that may help in making better decisions. It is still the surgeon, however, not the computer software, that decides how and where to make the cuts or release soft tissues to achieve the best implant position for the individual patient.
ANATOMY
The success of TKA depends on proper alignment of the prosthesis in the coronal, sagittal, and horizontal planes. The surgical principle for proper alignment in the coronal plane is to restore the mechanical axis to neutral by placing the femoral and tibial components vertical to the mechanical axis of the limb.
The mechanical axis is defined as a line connecting the center of the femoral head to the center of the ankle joint. The anatomic axis of the knee is described as the intersection of the lines drawn parallel to the long axis of the femur and tibia in the coronal plane and typically is between 5 and 7 degrees.
In the standard intramedullary techniques, the anatomic axes are used as guides to estimate the mechanical axis; in navigation-assisted technique, however, the mechanical axes are determined and cuts are made perpendicular to those axes.
When performing a TKA, the sagittal plane is kinematically important because most of the range of motion in the knee occurs in the sagittal plane. The degree of posterior slope of the proximal tibia has been used as the main indicator of proper sagittal alignment.
The mechanical axis of the tibia on the sagittal plane can be determined in different ways. In one method, the midpoint of the tibial plateau is connected to the midpoint of the talus. In another, the midpoints of medial tibial plateau and the tibial plafond are connected in the sagittal plane. Either line can be used as the reference for measuring the tibial plateau slope.7,9
Rotational alignment of the implanted components is of the utmost importance in TKA. When a prosthesis is implanted with the incorrect amount of rotation, poor patellar tracking and anterior knee pain can result.1
The reference axis for rotational alignment of the femur remains controversial: the transepicondylar axis, the anteroposterior axis of Whiteside, and the posterior condylar axis have all been suggested. Each of these axes has flaws, however. For the rotational axis of the tibia, the medial third of the tibial tuberosity, as advocated by Insall, is approved by most surgeons.5
In navigation-assisted TKA, the reference for femoral rotation is the average rotational axis calculated by digitized transepicondylar and Whiteside lines, and the reference for tibial rotation is the digitized tibial anteroposterior axis.
In navigation-assisted TKA with registration, the positions of anatomic landmarks and axes are digitized as references for instrument, bone cuts, and leg alignment. If a mistake in digitization occurs, the computer software usually will not progress. If a mistake is identified, the appropriate landmark should be re-registered.
SURGICAL MANAGEMENT
Preoperative Planning
The most important step in preoperative evaluation is determining that the patient definitely does need the TKA.
Preoperative knee radiographs should include a standing anteroposterior (AP) view, a lateral view, and a skyline view of the patella. A long-leg standing AP radiograph usually is unnecessary, because the navigation system can accurately determine the mechanical axis of the limb intraoperatively, even in the presence of previous deformity secondary to trauma or previous surgical procedures.
In the standard technique, templates can be used to anticipate approximate component size and bone defects that would have to be treated intraoperatively. In the navigation technique, intraoperative templating is performed by digitization of different anatomic areas.
The preoperative range of motion also is assessed by the navigation system, which is more accurate and helps the surgeon plan different cuts, including femoral flexion and tibial slope.
Anesthesia, venous thromboembolism prophylaxis, and cardiovascular and internal medicine clearance are the same as that for standard TKA techniques.
Positioning
The patient is placed supine on the operating table.
A tourniquet is applied snugly to the upper thigh as far proximally as practical. In very obese patients, fat may be pulled distally from beneath the tourniquet, causing it to bulge from the distal edge of the tourniquet. This prevents it from migrating and ensures that the tourniquet is placed as proximal as possible.
A transverse bar is placed on the table at a level just distal to the joint line. When the knee is fully flexed, the foot engages the bar and can, therefore, be maintained in the flexed position without the use of an assistant.
The navigation system should be placed opposite the surgeon. Before starting with patient registration (ie, landmark digitization), it is recommended that the camera be brought in line with the knee joint so that all instruments can communicate easily with each other during the surgery (FIG 1).
The system must be set up before the operation begins. All trackers and pointing tools should be initialized and validated, and the pointer tip should be calibrated.
Approach
All standard and minimally invasive approaches for exposure of the knee joint can be applied and supported with the navigation system. The standard median parapatellar approached is described here.
The most commonly used skin incision for primary TKA is an anterior midline incision.
FIG 1 • The system should be placed opposite the surgeon to ensure instrument visibility.
TECHNIQUES
EXPOSURE
The incision is made with the knee in flexion to allow the subcutaneous tissue to fall sideways, which eases exposure.
The skin incision should be long enough to avoid excessive skin tension during retraction, because that can lead to areas of skin necrosis.
The medial skin flap should be kept as thick as possible by keeping the dissection just superficial to the extensor mechanism.
The retinacular incision is extended proximally to the length of the quadriceps tendon, leaving a 3- to 4-mm cuff of tendon on the vastus medialis for later closure. The incision then is continued around the medial side of the patella, extending 3 to 4 cm onto the anteromedial surface of the tibia along the medial border of the patellar tendon.
The medial side of the knee is exposed by elevating the anteromedial capsule subperiosteally and elevating the deep medial collateral ligament off the tibia to the posteromedial corner of the knee.
The patella initially is everted to facilitate fat pad removal, but the remainder of the surgery is performed with the patella subluxated but not everted.
The knee is flexed, and the anterior and posterior cruciate ligaments are removed.
PLACEMENT OF TRACKER PINS
All anchoring pins are placed within the incision. Although this requires a slightly longer incision, it greatly simplifies pin insertion and minimizes damage to muscle. It also eliminates the potential for fractures around the pins, because they are not placed in diaphyseal bone.
On the femur, the anchoring pin is positioned medially on the anterior surface just at the proximal aspect of the metaphysis (TECH FIG 1A). The tracker pin must be proximal enough to avoid interfering with the femoral cutting jigs and trial components. Medial placement allows a more distal pin placement (and, therefore, a smaller incision), because the medial portion of the femoral component does not extend as far proximally as the lateral femoral flange.
TECH FIG 1 • A. Position of the tracker pins in the tibia and femur. B. The depth should be measured accurately to make sure the tracker pins will be inserted bicortically.
On the tibia, the anchoring pin should be inserted across the medial tibial plateau parallel to the joint line in the sagittal plane to avoid collision with the tibial cutting guide and the keel of the implant. Placement in this location with the knee flexed also minimizes the risk of injury to the posterior vascular structures.
The anchoring pin should be angled 30 degrees away from the mid-sagittal plane to avoid interfering with tibial cutting guides.
For fixation, a pilot hole is predrilled with a 3.2-Amp drill. The pins should be driven bicortically using the insertion tool, and accurately measured for depth (TECH FIG 1B).
Two trackers are used, one green and one blue. The green one is affixed to the femoral anchoring pin and the blue to the tibial anchoring pin. All femoral points are referenced off the green tracker and all tibial points off the blue tracker.
DETERMINATION OF FEMORAL HEAD CENTER
The center of the hip joint is identified by rotating the hip with both the hip and knee flexed. The software geometrically produces the center of femoral head within 1 mm of accuracy (TECH FIG 2). This is the most accurate way of identifying the center of rotation of the femoral head.
During hip rotation, pelvic movement should be minimized. If the pelvis moves, an assistant should stabilize the pelvis and digitization should be repeated for location of the femoral head center.
TECH FIG 2 • A. Hip center identification by rotating the leg with hip and knee in flexion. B. Navigation system screen shot of digitized point during hip center identification.
DISTAL FEMUR MAPPING
The medial and lateral epicondyles and the knee center are digitized by placing the tip of the pointer at each point and pressing the Select button to record that point (TECH FIG 3A,B).
For the femoral AP axis (Whiteside's line), the axis of the pointer should be aligned with the most anterior point of the intercondylar groove (TECH FIG 3C,D). The Select button then is pressed for recording. The computer software also averages the digitalized AP axis and transepicondylar axis, which can be used as an alternative reference for rotational alignment of the femoral component.
TECH FIG 3 • A. Digitization of the medial epicondyle using a navigation system pointer. B. Corresponding navigation system screenshot. C. Determination of Whiteside's line. D. Corresponding navigation system screenshot. E. Digitizing the anterior cortex. F. Corresponding navigation system screenshot. G. Digitizing the medial condyle. H. Corresponding navigation system screenshot.
Surface mapping of the distal femur is determined by digitization of the anterior cortex, the distal and posterior surfaces of the medial condyle, and the distal and posterior surfaces of the lateral condyle.
For each surface, the tip of the pointer is located on that surface and digitizing is begun by pressing the Select button. The pointer's tip should be moved on the surface in a painting fashion.
The computer automatically progresses to the next reference point when the number of selected points is enough for mapping.
In anterior surface mapping, the least prominent anterior region, which usually is located along the lateral border of the femur, should be included in the mapping (TECH FIG 3E,F) to minimize oversizing of the femoral component.
When mapping the distal surface, the most distal aspect of the femoral condyle should be included (TECH FIG 3G,H).
During digitization, the pointer should never leave the surface.
PROXIMAL TIBIA MAPPING
The surfaces of medial and lateral compartments of the tibia are mapped and registered in the computer (TECH FIG 4A,B). The lowest point on each condyle must be digitized.
The center of the tibial plateau and the anteroposterior axis also are digitized (TECH FIG 4C,D). The center of the insertion of the anterior cruciate ligament seems to be the most accurate landmark to use.
TECH FIG 4 • A. Digitizing the medial compartment of the tibia. B. Corresponding navigation system screenshot. C. The AP axis of the tibia is identified. D. Corresponding navigation system screenshot.
DETERMINATION OF THE CENTER OF THE ANKLE
The medial and lateral malleoli are digitized, and the computer determines the center of the ankle as a reference for the anatomic axis of the tibia and the mechanical axis of the limb (TECH FIG 5).
TECH FIG 5 • A. Digitizing the medial and lateral malleoli. B. Corresponding navigation system screenshot.
ASSESSMENT OF INITIAL ALIGNMENT AND DEFORMITY
The trackers are attached to the anchoring pins, and the initial alignment, deformity, and range of motion are recorded. This information is extremely helpful in determining soft tissue releases that must be performed, bone cuts, and actual component selection.
MAKING THE BONE CUTS
In this section we describe an anterior referencing system. Posterior referencing software also is available. The technique described here involves cutting the femur first, before the tibia, but the software is flexible and also allows tibia-first approaches.
For all bone cuts, the green tracker will be located proximal to the blue tracker. For example, during all femoral cuts, the green tracker will be on the femoral anchoring pin and the blue tracker on the cutting jig. For all tibial cuts, the blue tracker will be on the anchoring pin and the green tracker on the cutting jig.
Making the Distal Femoral Cut
The reference for the distal femoral cut resection level is the most distal point of the digitized condyles. The system calculates the length of perpendicular distance, from the most distal point to the resection plane, thereby establishing the depth of cut (TECH FIG 6A).
The freehand horseshoe guide is put on the distal femoral surface and is fixed with two pins (TECH FIG 6B). Then the distal femoral cutting guide is assembled on the horseshoe guide while the tracker is attached to the tracker interface (TECH FIG 6C).
In the reactive workflow, the software automatically opens the Resect Distal Femur dialog box (TECH FIG 6D). On the screen, the yellow disc visualizes the actual cutting block position. At the same time, flexion–extension alignment and medial and lateral resection depth are numerically displayed.
The distal and lateral screws on the cutting device are adjusted until the cutting guide corresponds exactly to what the surgeon thinks is the most appropriate distal femoral resection for that patient (TECH FIG 6E).
After fixation and position verification, the distal femur can be resected (TECH FIG 6F,G). For cut verification and documentation, a plane probe is held flush against the cut surface while the tracker is attached to it (TECH FIG 6H,I). Adjustments to this cut can be made as deemed necessary by the surgeon.
TECH FIG 6 • A. The system calculates the perpendicular distance, from the most distal point to the resection plane (depth of cut). B. The freehand horseshoe guide is fixed on the distal femoral surface. C.The distal femoral cutting guide is assembled on the horseshoe guide. D. In the Resect Distal Femur dialog box, the yellow disc is visualizing the actual cutting block position. E. The amount of femoral resection, flexion–extension, and varus–valgus orientation can be set up by the surgeon. F,G. Resection and verification of distal femoral cut. H. The femoral rotation guide and the blue tracker are placed on the distal femoral cut. I. Corresponding navigation system screenshot.
Making the Femoral Rotational Cut
The blue tracker is attached to the rotation guide and placed on the distal femoral cut, and the Align Femoral Rotation menu is selected in reactive workflow. The yellow lines represent femoral rotation. Rotation is displayed numerically with respect to the average rotational axis, as well as the digitized AP and transepicondylar axes (TECH FIG 7A). The surgeon decides which rotational reference to use.
The stylus is then attached to the rotational guide to prevent anterior notching (anterior referencing; TECH FIG 7B).
Once proper alignment is obtained, the anterior cut is made. Again, the surgeon must determine the actual depth of this cut with respect to the anterior cortex. The plane probe can be used to check the rotational accuracy of this cut.
TECH FIG 7 • A. A stylus is attached to the rotational guide to determine the level on the anterior cut. B. The 4:1 cutting block is adjusted by the anterior cut surface and remaining femoral bone resections.
TECH FIG 8 • Horseshoe cutting guide is fixed on the proximal tibia.
Finishing Cuts on the Femur
The 4:1 cutting block is adjusted with the anterior cut surface, and the remaining femoral bone resections are completed (TECH FIG 8).
Computer digitization has suggested a femoral size based on points chosen by the surgeon, but the size of the actual component chosen depends on many other factors that the surgeon must take into account when choosing the appropriate 4:1 cutting block.
If it is not clear which size to choose, it is best to err initially on a larger size.
Making the Proximal Tibial Cut
The horseshoe guide is fixed on the proximal tibia using two pins (TECH FIG 9A–C). The tibial cutting guide is then assembled on the horseshoe guide while the green tracker is attached.
The Resect Proximal Tibia interface is selected on the workflow. The yellow line on the screen is the actual cutting block position. The varus–valgus alignment, slope, and mediolateral resection depth are displayed numerically.
The depth of resection, slope, and alignment are adjusted using adjusting screws, and then the cutting block is fixed to the tibial bone with two pins (TECH FIG 9D,E). Again, the surgeon decides on the depth, angle, and slope of the cut. The navigation system merely gives accurate numerical information to assist with this decision.
The proximal tibia is cut, and the cutting surface is verified and documented using Resection Plane Probe with the tracker on it (TECH FIG 9F,G).
Tibial Rotation
Tibial rotation is set using the appropriate tibial template assembled to the alignment handle and tracker. On the reactive workflow, the Tibial Rotation screen is selected. The yellow cross shows the rotational alignment of the handle, which also is shown numerically (TECH FIG 10).
The tibial template should be aligned in the proper position, as determined by the surgeon, and pinned into the tibia.
TECH FIG 9 • A. Assembled tibial cutting guide. B. Corresponding navigation system screenshot. C. The surgeon is able to set the depth, varus–valgus orientation, and slope of the tibial cut. D. Verification of proximal tibial cut. E. Corresponding navigation system screenshot. F. Tibial rotation is determined by the surgeon using the appropriate tibial template and tracker. G. Corresponding navigation system screenshot.
TECH FIG 10 • The space for the tibial component keel is prepared using a power burr (A) and impactor (B).
Tibial Component Insertion
At this stage, osteophytes along the medial or lateral margins of the knee can be removed to anatomic contours. The space for the tibial component keel is prepared using a power burr and impactor (TECH FIG 11A).
If a PCL-substituting design is chosen, the intercondylar box is removed to accommodate the housing for the post and cam mechanism (TECH FIG 11B–F).
TECH FIG 11 • A. Preparation of the intercondylar box for PCL-substituting prosthesis. B. The overall limb alignment and knee motion are assessed while trackers are attached. C–F. Corresponding navigation system screenshots.
LIMB ALIGNMENT AND SOFT TISSUE BALANCE
Trial components are placed, and the trackers are attached to the anchoring pins. Overall limb alignment and knee motion are assessed.
Soft tissue then is selectively released according to the residual deformity present. (Specific details of how to balance the limb are beyond the scope of this chapter.)
PATELLA
The patella is cut and balanced according to standard techniques.
IMPLANTATION OF COMPONENTS AND CLOSURE
The technique described in this section uses standard techniques for final implantation \of components and closure.
We remove the anchoring pins before implanting the components. However, if the surgeon prefers, the anchoring pins can be left in place during implantation of components to check for accuracy of final component position and limb alignment.
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POSTOPERATIVE CARE
Postoperative care after navigation TKA is the same as that after the standard techniques.
Important perioperative interventions including prophylactic antibiotic, and deep vein thrombosis prophylaxis should be administered according to standard protocol. The limb is put in a compression bandage at the conclusion of the operation. Pain is controlled according to the selected pain management protocol.
On the day of surgery, both passive and active range of motion is begun, and the patient sits on the side of the bed, stands with assistance, and walks if able. The importance of active and passive extension is emphasized to the patient.
The expected length of stay in the hospital is 3 or 4 days.
On the second and third postoperative days, the patient transfers to and from the bed and chair, sits up in a chair, and ambulates with weight bearing as tolerated using a walker or crutches.
At the same time, the physical therapist starts daily rehabilitation programs to increase knee range of motion and to strengthen the operated leg.
On the third or fourth postoperative day, the patient should achieve flexion of at least 70 degrees and can be discharged with a walker or crutches.
In the first 2 weeks, a patient should be visited at home by a nurse and physical therapist to check the wound and continue rehabilitation.
At 2 weeks, sutures or staples are removed, and the patient should be sent to an outpatient physical therapy facility if needed.
The patient is then seen at the office at 6 weeks and 6 months after surgery and then routinely followed every 3 years.
OUTCOMES
Total knee arthroplasty has shown results that are both durable and consistent, with over 90% survivorship into the second decade. This long-term success has been related to patient characteristics and the accuracy with which the prosthesis is implanted.
The navigation system, unlike standard technique, makes it possible to significantly improve the mechanical alignment of the limb, sagittal and frontal alignment of the femoral and tibial components, and knee range of motion without increased short-term complications. This more accurate and precise positioning and alignment of the components should reduce the rate of long-term complications and revisions.
REFERENCES
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