This chapter contains discussion and video links that assist the surgeon in performing calipered kinematically aligned (KA) total knee arthroplasty (TKA) with specific manual instruments, a verification worksheet, and a decision tree. The treatment of 4726 primary TKAs performed from 2009 through 2019 with KA to restore the patient’s prearthritic joint lines regardless of knee deformity and without ligament release, by the senior author, provide the background. During this time, 4316 had retention and 410 had an excision or an unintended transection or tibial detachment of the posterior cruciate ligament (PCL). This experience evolved into a highly reproducible surgical technique that works best with the retention of the PCL. The first section discusses tips for exposing the knee. The second section describes the use of caliper measurements and verification checks that set the femoral component coincident to the patient’s prearthritic or native distal and posterior femoral joint lines with minimal flexion of the femoral component. The third section describes the use of caliper measurements and verification checks that set the tibial component coincident to the prearthritic tibial joint line. The fourth introduces a decision tree that, when followed, restores the native tibial compartment forces, laxities (except anterior laxity), and tibiofemoral and patellofemoral kinematics, thereby balancing the calipered KA TKA. The fifth section discusses the management of the severe fixed valgus deformity and the knee with flexion laxity from injury to the PCL. The educational objective is to encourage those surgeons that use manual, patient-specific navigation and robotic instrumentation to retain the PCL, not release the collateral ligaments, measure bone resections with a caliper, perform verification checks, and follow a decision tree to align and balance the calipered KA TKA reproducibly.
Tips for Exposing the Knee to Perform Calipered Kinematically Aligned Total Knee Arthroplasty
The following tips for the surgical exposure of the osteoarthritic knee might be of help when performing calipered KA TKA. The authors favor either a midvastus or a traditional medial parapatellar arthrotomy. With the knee in 90 degrees of flexion, make the incision long enough to clearly view 3 cm of femur proximal to the trochlea and 4 to 5 cm of the proximal tibia. The fat pad can either be excised or retained. The medial tibia is subperiosteally exposed deep to the superficial medial collateral ligament to view the posteromedial corner. A clear view of the posteromedial tibia is useful for setting the slope of the tibia resection to match that of the patient’s prearthritic slope when a cruciate ligament–retaining (CR) implant design is used. The iliotibial band and popliteus tendons are protected and retained. To preserve the posterior cruciate ligament (PCL) during exposure, place a single-prong Hohmann retractor lateral to the PCL. The straddling of the PCL with a two-prong retractor should be done carefully, as pushing the retractor distally can peel the PCL off the insertion on the posterior tibia. Remove all marginal, notch, and posterior osteophytes to restore the prearthritic resting length of the ligaments and full motion arc of the knee. Resurfacing the patella is preferred, as is the use of an anatomic-shaped patella implant. Because KA restores the patient’s prearthritic Q-angle and does not set the femoral component distal to the prearthritic joint line, the patella tracks well without a lateral release, with the exception of the few patients with a chronically dislocated patella. The workflow of KA is “femur first” followed by the tibial resection. The TKA is balanced by fine-tuning the varus-valgus (V-V), slope, and depth of the tibial resections, which restores the native tibial compartment forces without ligament release. ,
Caliper Measurements and Verification Checks That Set the Femoral Component Coincident to the Patient’s Prearthritic Joint Lines With Minimal Flexion
shows the use of intraoperative and verification checks that restore the native distal and posterior femoral joint lines.
Manual, patient-specific navigation and robotic instrumentation use the same KA femoral targets, which are to restore the patient’s prearthritic or native distal (0 degrees) and posterior (90 degrees) femoral joint lines and to limit flexion of the femoral component to less than 5 degrees from the sagittal anatomic axis of the distal femur. Knowledge of the thicknesses of the distal and posterior condyles of the femoral component, patterns of cartilage wear, and the width of the saw blade are required. The surgeon can obtain the thicknesses of the distal and posterior femoral condyles from the implant manufacturer and should verify the thicknesses intraoperatively by measuring them with a caliper. Be aware that some brands increase the thickness of the condyles in the largest size and in the PCL-stabilized designs. Adjust 2 mm for full-thickness cartilage wear and 1 mm for the kerf from the saw blade. The caliper measurement of each of the femoral bone resections should equal that of the condyle of the femoral component after compensation for cartilage wear and kerf. In the osteoarthritic knee, bone wear is rare on the femur at 0 degrees and 90 degrees.
Perform the following steps with manual instruments to kinematically align and set the proximal-distal (P-D) and V-V rotation of the femoral component coincident with the prearthritic distal femoral joint line. With the exposed knee in 90 degrees of flexion, determine the locations of cartilage wear on the distal femur. Measure the anterior offset (i.e., displacement) of the anterior tibia from the distal medial femur with a caliper. Subtract 2 mm when the cartilage is missing on the medial femoral condyle. Record the measurement on the verification sheet. Remove the medial and lateral femoral osteophytes and those that fill the intercondylar notch. Remove partially worn cartilage down to subchondral bone with a ring curette. Start the drill hole for insertion of the 10-cm long positioning rod into the femoral metaphysis midway between the apex of the intercondylar notch and the anterior cortex ( Fig. 5.1 ). Leave a 5- to 10-mm bone bridge between the posterior edge of the hole and the top of the femoral notch to reduce the risks of flexing the femoral component and patellofemoral instability. , In the axial plane, orient the drill perpendicular to the distal femoral joint line. In the sagittal plane, orient the drill parallel to the anterior and posterior femoral cortex. Insert the positioning rod 8 to 10 cm into the distal femur. Select the distal referencing cutting guide with a 2-mm offset to compensate for complete cartilage wear on either the distal medial or lateral femoral condyle in the knee with varus or valgus osteoarthritis, respectively. Insert the distal referencing guide assembly over the positioning rod and insert distal pins so the guide sits flush against the distal femur. Insert anterior pins in the guide. After making the distal resections, use the caliper to measure the thicknesses. Each resection should equal the thickness of the femoral component after adding 1 mm thickness for the kerf of the saw blade and 2 mm for worn cartilage when present. Record the thicknesses of the distal femoral resections on the verification sheet ( Fig. 5.2 ). When a resection is less than the target thickness or underresected, remove additional bone by either (1) redirecting the saw blade through the cutting block without flexing the guide, (2) using a 1- to 2-mm recut guide, or (3) free-handing the cut until the resection is within ± 0.5 mm of the target. When a resection is 1 or 2 mm greater than the target thickness or overresected, place a 1- or 2-mm washer over the stabilizing pin of the 4-in-1 cutting block. The washer displaces the 4-in-1 cutting block distally, which creates a shallow anterior and posterior chamfer cut that maintains a gap between the overresected distal femoral resection and the femoral component that is filled with cement.
When a manual distal femoral cutting guide designed specifically for KA is not available, consider two options. One is to make 2-mm thick × 10-mm × 5-mm pieces of metal as a spacer to compensate for worn cartilage that is inserted between the subchondral bone and the distal femoral resection guide. The other is to remove all cartilage from the distal femur and adjust the angles of the insertion of the intramedullary rod and the distal referencing guide until the distal femoral cutting block sits flush against the distal femur.
Perform the following steps with manual instruments to kinematically align the anteroposterior (A-P) position and internal-external (I-E) rotation of the femoral component coincident with the patient’s prearthritic posterior femoral joint line. Insert the tip of a knife and check the thickness of the cartilage of the posterior condyles. When the cartilage is intact, use a posterior referencing guide set at 0 degrees of rotation. When the cartilage is worn, insert a 2-mm shim between the foot of the referencing guide and the posterior condyle. Many sets have around a 1.5-mm thick angel wing with markings that can be used as a shim and to measure the width of the distal femur to help select the size of the femoral component. Knees with varus osteoarthritis rarely need the posterior shim unless chronically ACL deficient, whereas 40% of knees with valgus osteoarthritis require the shim to compensate for posterolateral cartilage wear. Be aware that the lateral femoral condyle is not hypoplastic in those knees with valgus osteoarthritis, so the restoration of the prearthritic joint line does not require an adjustment on the lateral femoral condyle. Measure the width of the femur with the angel wing and use the manufacturer’s sizing guide to choose the correct size posterior referencing 4-in-1 cutting block. Secure the 4-in-1 cutting block. Insert a retractor between the femoral condyle and the popliteus tendon to reduce the risk of injury to the tendon. Make the posterior femoral resections and measure the thicknesses with the caliper. Record them on the verification sheet. When necessary, adjust the thickness of the posterior resection to within ±0.5 mm of the target. To correct for a 1- to 2-mm underresection, rotate and refix the 4-in-1 cutting block 1 or 2 degrees and fine-tune the posterior resection. When the rotation of the 4-in-1 cutting block is posterior to the initial resection, a gap results between the posterior resection and the femoral component that is filled with cement. At this point, some surgeons prefer to insert the trial femoral component before making the tibial resection and assess knee extension, V-V laxity, and patella tracking. Often the flexion contracture disappears, and when it does not, apply a gentle hyperextension moment and plastically stretch the posterior capsule. The knee should be stable during V-V laxity testing throughout the range of motion, with the exception of those knees with substantial tibial bone wear. Removing any osteophytes, and not releasing any collateral, retinacular, and posterior cruciate ligaments.
These use of caliper measurements and verification checks align the femoral component coincident to the prearthritic distal and posterior femoral joint lines with high reproducibility (i.e., root mean square error of 1.4‒1.5 mm or degrees). This level of reproducibility does not require the expense of high-cost imaging modalities such as magnetic resonance imaging (MRI) and computed tomography (CT). Calipered KA reliably coaligns the flexion-extension (F-E) axis of the femoral component closely to the cylindrical axis. The reproducibility of hitting the KA targets is greater than the use of mechanical alignment (MA) targets such as the transepicondylar axis, anterior-posterior (A-P) axis, femoral mechanical axis, and intramedullary canal, which are nonkinematic and hard to identify.
Caliper Measurements and Verification Checks That Set the Tibial Component Coincident to the Patient’s Prearthritic Tibial Joint Line
shows the use of intraoperative and verification checks that restore the native proximal tibial joint line.
For manual, patient-specific navigation and robotic instrumentation, the tibial targets for kinematic alignment are to restore the V-V plane and posterior slope of the native knee while preserving the PCL. Be aware of factors that risk injury to the PCL, which include an overly thick tibial resection, posterior slope greater than prearthritic slope, the use of too wide a saw blade in small knees, and soft bone. To deliver the tibia anteriorly and preserve the PCL, place a single-prong Hohmann retractor lateral to the PCL. Straddling the PCL with a two-prong retractor should be done carefully, as pushing the retractor distally can peel the PCL off the posterior tibial insertion.
Perform the following steps with manual instruments to KA the orientation and position of the trial tibial baseplate. Draw an A-P line that bisects the tibial spines and another that parallels the major axis of the elliptical-shaped lateral tibial condyle. On the anterior tibia, extend the tibial spine line distal toward the medial border of the tibial tubercle to use later as a secondary target for setting I-E rotation of the tibial component. , Apply an extramedullary guide with a proximal tibial resection guide to the ankle. Set the provisional V-V plane for the saw slot of the tibial resection guide parallel to the proximal tibial joint plane, which typically requires a 15-mm translation of the long arm of the tibial resection guide from the center at the ankle. Set the thickness of the tibial resection by positioning the offset stylus on the unworn tibial condyle at the base of the tibial spine and secure the guide to maintain the adjustment. Check that the offset of the stylus makes a conservative tibial resection with the target of using an insert 1-mm thicker than the thinnest one available. Insert an angel wing into the medial side of the saw slot and adjust the slope of the tibial resection plane parallel to the patient’s prearthritic slope while visually compensating for cartilage and bone wear and secure the guide to maintain the adjustment. Fine-tune the V-V orientation of the proximal tibial resection guide until the resection plane is parallel to the proximal tibia compensating visually for cartilage and bone wear, and secure the guide to maintain the adjustment. Next, internally or externally rotate the proximal tibial resection guide so that it is parallel to the two A-P lines drawn previously on the tibia, and pin the guide. Insert a retractor between the femoral condyle and the popliteus to reduce the risk of injury to the tendon. Perform the tibial resection.
Measure the thickness of the medial and lateral tibial resections at the base of the tibial spine with a caliper and record them on the verification sheet ( Fig. 5.2 ). Visually inspect the medial tibial resection and determine whether the slope matches the patient’s prearthritic slope. When necessary, fine-tune the V-V plane and slope of the tibial resection. Remove residual osteophytes from the tibial margin, the posterior femur, the femoral condyles, and the intercondylar notch.
The use of caliper measurements and verification checks set the tibial component to the prearthritic V-V plane and posterior tibial slope with high reproducibility. These steps restore the native left to right symmetry of the proximal medial tibial angle within 3 degrees in nearly all patients with a normal contralateral limb with a negligible risk of varus alignment of the tibial component with respect to the patient’s prearthritic tibial joint line. A study that used three-dimensional shape registration showed that these steps consistently corrected the varus deformity.
Follow the Decision Tree to Restore the Native Tibial Compartment Forces, Ligament Lengths, and Laxities That Balance the Calipered Kinematically Aligned Total Knee Arthroplasty
shows the use of the decision tree to restore native tibial compartment forces, ligament lengths, and laxities.
Filling out the verification worksheet and following the decision tree restore the native tibial compartment forces, ligament lengths, and laxities, with the exception of anterior, unless a medial ball-in-socket insert design is used, and native tibiofemoral and patellofemoral kinematics ( Fig. 5.3 ). , , A precondition for consistently fulfilling these balancing objectives is the setting of the femoral and tibial components coincident with the patient’s prearthritic joint lines and the retention of the PCL and collateral ligaments without release. MA does not restore native tibial compartment forces because the orientations and positions of the femoral and tibial components change the patient’s prearthritic joint lines in most knees. ,