Kinematically Aligned Total Knee Arthroplasty Using Calipered Measurements, Manual Instruments, and Verification Checks
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This chapter presents the philosophy of kinematic alignment (KA) and the surgical technique for setting the positions of the components using ten calipered measurements, manual instruments, and nine verification checks. The adoption of KA is increasing. Four meta-analyses, three randomized trials, and a national multicenter study showed that patients treated with KA total knee arthroplasty (TKA) reported significantly better pain relief, function, and flexion and a more normal feeling knee than patients treated with mechanically aligned TKA [1–8]. Two randomized trials that limited the severity of the preoperative knee deformities showed similar clinical outcomes [9, 10]. KA co-aligns the axes of the femoral and tibial components with the three axes of the native knee without restrictions on the level of preoperative deformities . The surgical goal of restoring the native alignments of the limb, Q-angle, and joint lines unique to each patient depends on accurately setting the components coincident to the native joint lines, which co-aligns the axes. The surgical goal of restoring the laxities, tibial compartment forces, knee adduction moment, and gait to those of the native knee without ligament release balances the TKA and promotes long-term implant survival [12–19]. A description of the calipered technique of KA with manual instruments, the sequence for measuring bone positions and resection thicknesses, the intraoperative recording of these measurements on the verification worksheet (Fig. 24.1), and the use of decision trees for balancing the TKA with the medial pivot CS and CR inserts are shown (Figs. 24.2 and 24.3). Calipered measurements of the thicknesses of the femoral and tibial bone resections restore the native joint lines with high reproducibility when they are adjusted within ±0.5 mm of the femoral and tibial components after compensating for cartilage and bone wear and the 1 mm kerf from the saw cut [20–22]. Because calipered measurements are a basic surgical skill, inexpensive, and highly reliable, they should be a required verification check when performing KA with manual instruments, patient-specific guides, navigation, and robotics. Examples of treatment of patients with severe varus and valgus deformities and flexion contractures treated with kinematically aligned TKA without ligament release are shown. Finally, the reasons for the low risk of tibial component failure, low risk of patellofemoral instability, and high implant survival at 10 years after KA TKA are explained [11, 23, 24].
24.2 Co-aligning the Axes of the Femoral and Tibial Components with the Three Axes of the Native Knee Is the Philosophy of Kinematic Alignment
24.3 First Surgical Goal: Restore the Native Joint Lines, Q-Angle, and Limb Alignments Unique to Each Patient
24.4 Second Surgical Goal: Restore Laxities, Tibial Compartment Forces, and Knee Adduction Moment of the Native Knee Without Ligament Release
Most TKA techniques resect the ACL and replace the articular cartilage and menisci with implants of graduated sizes with conformities and stiffnesses different from the native knee. A study in cadaveric knees showed that kinematic alignment with a posterior cruciate ligament-retaining implant restored 35 of 40 measures of laxity (8 laxities × 5 flexion angles) to those of the native knee. The restoration of most of the native laxities suggests that femoral and tibial components aligned with KA compensate for the articular cartilage, menisci, and ACL .
KA without ligament release limits high compartment forces by restoring those of the native knee [17, 18, 19, 43]. There is no evidence of medial or lateral compartment overload even in the subset of patients with alignment of the tibial joint line and limb in a varus or valgus outlier range according to MA criteria . In contrast, the medial and lateral tibial compartment forces after mechanical alignment and ligament release to a 0° hip–knee–ankle with measured resection and gap-balancing techniques are three to six times higher than those of the native knee at 0°, 45°, and 90° of flexion [17, 19, 42, 44]. Hence, KA without ligament release restores native medial and lateral tibial compartment forces, whereas MA with ligament release does not [17, 18, 19].
KA restores the native joint line obliquity [7, 12, 45], which reduces the peak knee adduction moment during gait and better restores normal gait when compared to MA TKA [12, 13]. A low knee adduction moment is one explanation for the negligible risk of varus failure of the tibial component 2–10 years after KA TKA [11, 23]. Hence, KA is a promising option in limbs with constitutional varus alignment and large coronal bowing of the tibial shaft as the low knee adduction moment and more normal gait lowers the risk of medial compartment overload .
24.5 Calipered Technique for Setting the Femoral Component Coincident to the Native Femoral Joint Line with Verification Checks
The following sequence of surgical steps, calipered measurements, and adjustments and the intraoperative recording of these measurements on a verification worksheet set the proximal–distal position and varus–valgus orientation of the femoral component coincident to the native distal joint line at 0° and the anterior–posterior position and internal–external orientation of the femoral component coincident to the native posterior joint line at 90° with high reproducibility (Fig. 24.4) [21, 24, 32]. The femoral mechanical axis, trans-epicondylar axis, and anterior–posterior axis (Whiteside’s line) are not of interest or use when kinematically aligning the femoral component [26, 31, 39, 40, 46, 47].
Verification Check 1: Record the offset measurement on an electronic or paper version of the verification worksheet (Fig. 24.1). During final balancing before cementation of the components, adjustments are made to the slope of the tibial resection and insert thickness until the offset is matched within 0 ± 1 mm, which restores the native laxities and tibial compartment forces of the flexion space (Fig. 24.7) [15, 16, 48].
Verification Check 2: Keeping a 5–10 mm bridge of bone between the posterior rim of the drill hole and the top of the intercondylar notch limits flexion of the femoral component to within 1° ± 2° with respect to the anatomic axis of the distal femur resulting in a negligible risk of patellofemoral instability [49, 50, 51].
Correct a 1 or 2 mm underresection of the distal femoral condyles by removing more bone from the distal femur with use of a 1 mm distal recut guide or by repositioning the distal femoral resection guide 2 mm more proximal.
Correct a 1 or 2 mm overresection of a distal femoral condyle by filling the gap by placing a 1 or 2 mm-thick washer on the corresponding fixation peg of the 4-in-1 block.
Verification Check 3: Record the calipered measurements on the verification worksheet (Fig. 24.1). The calipered measurements restore the varus–valgus orientation of the femoral component to the contralateral native limb in 97% of subjects .
Size the femoral component by positioning the stylus on the anterior femur. Drill the holes for the 4-in-1 chamfer block. Insert the 4-in-1 chamfer block remembering to place a 1 or 2 mm-thick washer on the corresponding fixation peg to correct for a 1 or 2 mm overresection of a distal femoral condyle. Make the posterior resections before making the anterior and chamfer cuts. Measure the thicknesses of the distal medial and lateral bone resections with a caliper. Adjust the resections of the posterior femur until their thicknesses match the posterior condyles of the femoral component within ±0.5 mm after compensating for 2 mm of cartilage wear when present and a 1 mm kerf from the saw cut. When a posterior femoral resection is 1–2 mm too thick or too thin, elongate the pin hole in the direction of the correction and translate the 4-in-1 chamfer block as needed. Insert the oblique compression screws and secure the reposition of the chamfer block. Make the anterior and chamfer femoral resections.
Verification Check 4: Record the calipered measurements on the verification worksheet (Fig. 24.1).The calipered measurements reproducibly restore the internal–external orientation of the femoral component within 0° ± 1.1° of the posterior joint line and the flexion–extension plane of the native knee .
24.6 Calipered Technique for Setting the Tibial Component Coincident to the Native Tibial Joint Line with Verification Checks
The following sequence of surgical steps, calipered measurements, and adjustments verify the proximal–distal position and the varus–valgus, flexion–extension, and internal–external orientations of the tibial component are coincident to the native tibial joint line. The tibial mechanical axis, intramedullary canal, and tibial tubercle are not of interest or use when KA the tibial component [11, 21, 40, 47, 52].
Verification Check 5: Record the calipered measurements on the verification worksheet (Fig. 24.1).
Flex the knee to 90°. Insert the tightest-fitting spacer block (choose from 10, 11, 12, 13, and 14 mm) between the femur and tibia. Recut the tibia using the 2 mm recut guide when the flexion space is too tight for a 10 mm spacer.
Verification Check 6: With the knee in 90° of flexion, internally and externally rotate the spacer and assess the relative tightness between the medial and lateral compartments. Confirm the spacer fits tighter in the medial compartment, fits looser in the lateral compartment, and pivots about the medial compartment, which restores a trapezoidal flexion space like the native knee (Fig. 24.7) .
When the lateral compartment is 2 mm tighter, recut the tibia using the 2° valgus recut guide.
When the medial compartment is 2 mm tighter, recut the tibia using the 2° varus recut guide.
When a 1 mm correction is required, place the ~1 mm-thick angle wing between the recut guide and the tibia resection and make a 1° recut.
Verification Check 7: Negligible varus–valgus laxity restores the native rectangular space in full extension with a negligible mean varus–valgus laxity of <±1° and tibial joint line, knee, and limb alignment (Fig. 24.7) [14, 20, 21, 32, 43].
Verification Check 8: Setting the internal–external orientation of the anatomic tibial baseplate to within 0° ± 4° of the flexion–extension plane of the knee restores high-level knee function [32, 33]. Because the mediolateral location of the tibial tubercle varies, the medial border and medial one-third of the tibial tubercle are unreliable landmarks for setting the rotation of the tibial component on the tibia .
Finally, insert trial components and assess the varus–valgus laxities with the knee in full extension and 15–20° of flexion and the anterior offset of the tibia on the medial femur, internal–external rotation, and posterior and distraction translation of the tibia with the knee in 90° of flexion while referring to the corrective measures in the Sphere CR and Sphere CS decision trees (Figs. 24.2 and 24.3). The common principle of these decision trees is that fine-tuning the proximal–distal position and the varus–valgus and flexion–extension (slope) orientations of the tibial resection balances the knee. Balancing is accomplished without ligament release.
24.6.1 Final Verification with Trial Components Check 9
- Place the knee in full extension: Retract the soft tissues and visually examine the varus–valgus laxity between the femoral component and tibial insert, which should be negligible like the native knee (Fig. 24.7) [14, 15].
Correct a 1° varus or 1° valgus instability because this degree of laxity is greater than the native knee and is associated with instability in extension .
- Place the knee in 15–20° of flexion: Check varus–valgus laxity. The medial side should open ~1 mm and the lateral side ~2–3 mm and be looser than in full extension (Fig. 24.7).
When the lateral side opens more than ~3–4 mm, verify the tibial resection is not in excessive valgus by remeasuring the tibial resection at the base of the tibial spines.
- Place the knee in 90° of flexion:
When the posterior cruciate ligament is intact and the CR insert is used, adjust the slope of the tibial resection and thickness of the insert until the anterior offset of the tibia from the distal medial femoral condyle matches the knee at the time of exposure. A 2° increase in the posterior slope and a 2 mm decrease in the insert thickness translates the tibia ~3 mm posterior [17, 53]. Confirm the tibia internally and externally rotates ~±14° like the native knee (Figs. 24.2 and 24.7) [14, 48].
When the posterior cruciate ligament is resected and the sphere CS insert for a medial ball and socket implant is used, check the posterior drawer and distract the tibia. When the insert rides too posterior on the femoral component and the flexion space is slack, use a thicker insert and tighten the flexion space. When the thicker insert limits knee extension, recut 1–2 mm more bone from the distal femur. Refer to the corrective steps in the fourth column of the Sphere CS decision tree (Fig. 24.3).
24.7 Kinematic Alignment Corrects Severe Varus Deformities Without Ligament Release
Since 2006, all patients suitable for a primary total knee replacement were treated following the principles of kinematic alignment which are to co-align the axes and joint lines of the components with the three “kinematic” axes and joint lines of the pre-arthritic or native knee without placing restrictions on the preoperative deformity and postoperative correction and without ligament release. During these 13 years, there were over 5000 primary KA TKAs from which all patients with severe deformities secondary to post-traumatic arthritis, progressive osteoarthritis post high tibial osteotomy, and patients with multiple-level deformity were included.
Surprisingly, intrinsic contracture and stretching of the collateral and posterior cruciate ligaments were exceedingly uncommon. Preoperatively, the AP radiographs of chronic varus or valgus deformities often showed a joint space larger than typical suggesting intrinsic stretching or laxity of the lateral or medial collateral ligament, whereas intraoperatively these ligaments were not lax. The AP radiograph of a knee with a fixed flexion contracture explains the inconsistency. The lateral and medial laxity of a flexed knee is several millimeters more than the extended knee, which is why flexion is the preferred position for performing an arthroscopic meniscectomy. When treating a patient with extrinsic laxity of a collateral or posterior cruciate ligament secondary to trauma, components are still aligned coincident with the native joint lines with use of the kinematic principles, and added constraint with use of implants that offer a box in the femoral component and a post on the tibial insert compensates for the extrinsic laxity. The use of cones and short stem extensions enables positioning of components coincident with the native joint line with a low risk of stem impingement of the femoral and tibial cortex.
24.7.1 Case Example, History
24.7.2 Postoperative Result
KA with use of a posterior cruciate ligament substituting implant because of the torn PCL corrected this severe varus deformity of 20° and flexion contracture of 15° without ligament release. Postoperatively, the patient had a 6° varus hip–knee–ankle angle. The 6° angle between the transverse axes of the components was less than 106°, which is compatible with high function [24, 32]. At 2 years, the patient ambulated without difficulty or pain, range of motion improved to 0°–115°, and the Oxford Knee Score increased from 11 to 45 points, the Knee Society Score increased from 31 to 98 points, and Knee Society Function Score increased from 40 to 70 points.
24.8 Kinematic Alignment Corrects Severe Valgus Deformities Without Ligament Release
24.8.1 Case Example, History
24.8.2 Postoperative Result
KA with use of a posterior cruciate ligament retaining implant corrected this severe valgus deformity and flexion contracture without ligament release. Postoperatively, the patient had a 3° valgus hip–knee–ankle angle. The transverse axes of the femoral and tibial components were within 3° of parallel, which is compatible with high function [24, 32]. At 2 years, the patient ambulated without difficulty or pain, range of motion improved to 0°–119°, and the Oxford Knee Score increased from 13 to 44 points, Knee Society Score increased from 41 to 98 points, and Knee Society Function Score increased from 30 to 70 points.
24.9 Kinematic Alignment Has a Low Risk of Tibial Component Failure, Low Risk of Patellar Instability, and High Implant Survival at 10 Years
Three biomechanical advantages explain the negligible risk of varus tibial loosening after kinematically aligned TKA. First, KA provides more physiological strains in the collateral ligaments than MA TKA by restoring the native joint lines and constitutional alignment without releasing ligaments . Second, KA provides medial and lateral tibial compartment forces comparable to those of the native knee with no evidence of tibial compartment overload even when the postoperative alignments of the limb, knee, and tibial component are within the varus or valgus outlier range according to mechanical alignment criteria [17, 18, 19]. Third, KA is an especially promising option for patients with large varus coronal bowing of the tibia because the knee adduction moment and risk of varus overload are lower than after MA TKA .
Accurately setting the flexion of the femoral component in the sagittal plane results in negligible patellofemoral instability after KA [49, 50, 51]. At 1–10 years of follow-up, there is a 0.4% incidence of patellofemoral instability (13 of 3212 prostheses) in patients treated with kinematically aligned TKA. In KA, flexion of the femoral component greater than 10° with respect to the anatomic axis of the distal femur increased the risk of patellofemoral instability by downsizing the femoral component ~1–2 sizes, reducing the cross-sectional area of the trochlea, reducing the proximal reach of the flange by ~8 mm, and delaying the engagement of the patella during early flexion [49, 51]. A change in the native Q-angle does not cause patellofemoral instability as KA restores the native Q-angle, whereas mechanical alignment increases or decreases the native Q-angle in limbs with varus or valgus constitutional alignment, respectively (Figs. 24.5 and 24.6) . The design of the femoral component does not cause patellofemoral instability as KA more closely restores the groove location and the sulcus angle of the native trochlea and trochlea morphology without overstuffing than mechanical alignment [57, 58]. Internal rotation about the center of the femoral component of ~3 relative to mechanical alignment does not cause patellofemoral instability as the ~1.5 mm increase in the distance between the lateral prosthetic trochlea and lateral femur is negligible . The use of a distal referencing guide attached to an intraosseous positioning rod limits flexion of the femoral component to 1 ± 2° with respect to the femoral anatomic axis, which is 9° less than patients with patellofemoral instability (Fig. 24.9) . Hence, limiting flexion of the femoral component lowers the risk of patellofemoral instability when performing kinematically aligned TKA .
The 10-year implant survivorship of a single-surgeon series of KA TKAs performed without restricting the degree of preoperative varus–valgus and flexion deformity is comparable if not higher than two single-surgeon series of MA TKAs. Using aseptic revision at 10 years as the end point, the 98.5% implant survival after 220 KA TKAs was 5.5% higher than the ~93% implant survival after 398 MA TKAs in the United States  and 4.5% higher than the ~94% implant survival after 270 MA TKAs in the United Kingdom . The estimated number of revisions for 1000 patients is 15 for KA TKA and 70 and 60, respectively, for the US and UK studies of MA TKA. In the study of KA, four of seven revisions were associated with excessive flexion of the femoral component (N = 3) and reverse slope of the tibial component (N = 1) in the sagittal plane. Limiting flexion of the femoral component and restoring the slope of the native tibia could have lowered the incidence of these revisions [23, 49, 50, 51]. The postoperative alignment of the tibial component, knee, and limb in varus and valgus outlier ranges according to mechanical alignment criteria does not adversely affect the 10-year implant survival, yearly revision rate, and level of function as measured by the Oxford Knee and WOMAC scores . Hence, restoring the native joint lines, Q-angle, and limb alignments unique to each patient results in high long-term implant survival regardless of the degree of preoperative varus-valgus and flexion deformity and postoperative alignment.
This chapter presented the philosophy of calipered KA and the surgical technique for setting components coincident to the native joint lines using ten calipered measurements, manual instruments, and nine verification checks. KA co-aligns the axes of the femoral and tibial components with the three axes of the native knee without ligament releases and without restricting the level of preoperative deformities and postoperative correction. The surgical goals are (1) restoration of the native alignments of the limb, Q-angle, and joint lines unique to each patient and (2) restoration of the laxities, tibial compartment forces, knee adduction moment, and gait of the native knee without ligament release. Measurement of the thicknesses of the femoral and tibial bone resections with a caliper and adjustment of the resections until they match those of the components after compensating for cartilage and bone wear and the 1 mm kerf from the saw cut restores the native joint lines with high reproducibility. These measurements are recorded intraoperatively on a worksheet, which verifies kinematic positioning of the components before cementation. Decision trees for balancing the TKA with CR and CS medial pivot tibial inserts balance the knee by adjusting the varus–valgus and slope of the tibial resection and not by releasing ligaments. Finally, the restoration of native alignment and tibial compartment forces lowers the risks of tibial component failure and patellofemoral instability and results in high implant survival at 10 years regardless of the level of preoperative deformity and whether the postoperative alignments of the tibial component, knee, and limb are within varus and valgus outlier ranges according to MA criteria.
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