Kinematically Versus Mechanically Aligned Total Knee Arthroplasty

Transcription

1 Feature Article Kinematically Versus ly Aligned Total Knee Arthroplasty H. GENE DOSSETT, MD, MBA; GEORGE J. SWARTZ, MD; NICOLETTE A. ESTRADA, RN, PHD; GEORGE W. LEFEVRE, MD; BERTRAM G. KWASMAN, MD TKA abstract Full article available online at ORTHOSuperSite.com. Search: The purpose of this study was to compare 2 alignment methods for total knee arthroplasty (TKA): kinematic alignment with the use of patient-specific guides and mechanical alignment with conventional instruments. A randomized, controlled trial of 41 kinematically aligned and 41 mechanically aligned patients was conducted with the patient, radiographic evaluator, and clinical evaluator blinded to the alignment technique. Radiographic measurements were made from long-leg computer tomography scanograms. Clinical outcome scores and motion were measured preoperatively and 6 months postoperatively. The hip knee ankle angle (0.3 difference; P.693) and anatomic angle of the knee (0.8 difference; P.131) were similar for both groups. In the kinematically aligned group, the angle of the femoral component was 2.4 more valgus (P.000) and the angle of the tibial component was 2.3 more varus (P.000) than the mechanically aligned group. At 6 months postoperatively, the Western Ontario and McMaster Universities Osteoarthritis Index score was 16 points better (P.000), Oxford Score was 7 points better (P.001), combined Knee Society Score was 25 points better (P.001), and flexion was 5.0 greater (P.043) in the kinematically aligned group than in the mechanically aligned group. Our findings suggest that the risk of early failure related to limb or knee alignment should be similar in kinematic and mechanically aligned TKA. More anatomic alignment of the implant was associated with better flexion and better clinical outcome scores in the kinematically aligned group. Drs Dossett, Swartz, Estrada, LeFevre, and Kwasman are from Phoenix VA Health Care System, Phoenix, Arizona. Drs Dossett, Swartz, Estrada, LeFevre, and Kwasman have no relevant fi nancial relationships to disclose. Biomet, Inc (Warsaw, Indiana) provided funding for the patient-specifi c guides and magnetic resonance imaging scans used in this study. The views expressed herein are those of the authors and should not be construed as offi cial policy of the Department of Veterans Affairs or the United States government. This material is based upon work supported by the Department of Veterans Affairs. Correspondence should be addressed to: H. Gene Dossett, MD, MBA, Phoenix VA Health Care System, 650 E Indian School Rd, Phoenix, AZ (gdossettmd@me.com). doi: / A B Figure: Composite long-leg scanogram of 2 representative patients, 1 with a kinematically aligned total knee arthroplasty (A) and the other with a mechanically aligned total knee arthroplasty (B). The line connecting the centers of the femoral head and ankle pass through the center of the knee in both total knee arthroplasties. The obliquity and level of the joint line in the kinematically aligned total knee arthroplasty are similar to the contralateral knee. However, the joint line in the mechanically aligned total knee arthroplasty is in more varus and more proximal than the contralateral knee. The obliquity of the joint line in the kinematically aligned total knee arthroplasty replicates the anatomic alignment of the joint line in the normal limb noted by Hungerford and Krackow and Krackow, which was associated with better clinical outcome scores and better flexion than the mechanically aligned total knee arthroplasty. e160 ORTHOPEDICS ORTHOSuperSite.com

2 KINEMATICALLY VS MECHANICALLY ALIGNED TKA DOSSETT ET AL Figure 1: Schematic showing the parallel and perpendicular relationships between the 3 kinematic axes of the knee. The transverse axis in the femur about which the tibia fl exes and extends passes through the center of the medial and lateral femoral condyles, which are symmetric in the varus and valgus knee. 16 The transverse axis in the femur about which the patella fl exes and extends is proximal, anterior, and parallel to the transverse axis in the femur about which the tibia fl exes and extends. 17 Both transverse axes in the femur are perpendicular to the longitudinal axis about which the tibia internally and externally rotates on the femur. Shape-matching a single radius femoral component to the restored articular surface of the femur preserves the normal orientation of the 2 transverse axes in the femur. ly aligned total knee arthroplasty (TKA) relies on restoring the hip knee ankle angle of the limb to neutral or as close to a straight line as possible. 1 This principle is based on studies that suggest limb and knee alignment is related to long-term survival and wear. 2-4 Measurement of the alignment of normal limbs in the coronal plane made with the use of a computed tomography (CT) scanogram at the hip, knee, and ankle have shown that 98% of normal limbs do not have a neutral mechanical axis, and that 76% of normal limbs have a deviation of 3 from neutral. 1 A recent study of normal limbs with weight-bearing radiographs showed that 32% of men and 17% of women had constitutional varus knees with a natural mechanical alignment of 3. 5 Because of the large variation in limb alignment from neutral and the fact that 98% of normal limbs do not have a neutral limb alignment, the surgical correction of the arthritic knee to establish a straight mechanical axis does not represent a correction to normal. 1,5,6 Because mechanically aligned TKA strives to correct limb alignment to a straight line, the kinematics of the knee can be altered. 1,7 There is universal agreement that mechanically aligned TKA improves the quality of life of patients with endstage knee arthritis. However, international arthroplasty registries in the United Kingdom, Canada, and New Zealand have shown that 20% to 25% of patients with mechanically aligned TKA are dissatisfied Accordingly, kinematically aligned TKA was implemented in 2006 as an alternative alignment strategy with the goal of reducing the prevalence of unexplained pain, stiffness, and instability and improving the rate of recovery, kinematics, and contact forces An alternative alignment method that attempts to replicate the kinematics of the knee is kinematic alignment. 6,15,16 In contrast to the 2-dimensional principle of mechanical alignment, kinematic alignment considers the 3-dimensional alignment of the components with respect to the knee, instead of the 2-D alignment of the components with respect to the centers of the femoral head, knee, and ankle. The principle behind kinematic alignment is placement of the components so that the orthogonal 3-D orientation of the 3 axes that describe normal knee kinematics is restored to that of the prearthritic knee (Figure 1). In TKA, the 2 parallel transverse axes in the femur about which the tibia and patella flex and extend are theoretically reestablished by surface fitting a femoral component with symmetric condyles on the articular surface of the femur after correcting for wear and compensating for the kerf or bone removed by the saw blade. 6,11-13,15,16 A method for kinematically aligning a TKA with the use of patient-specific femoral and tibial cutting guides designed from a magnetic resonance imaging (MRI) of the arthritic knee became available for use in However, we were unable to find any randomized control trials that evaluated alignment in kinematically aligned compared with mechanically aligned TKA. Accordingly, we performed this double-blind, randomized, controlled trial to determine whether kinematically aligned TKA using patient-specific guides has different limb, knee, and component alignment than mechanically aligned TKA with conventional instruments. Our secondary outcome measure compared clinical outcome scores (Western Ontario and McMaster Universities Osteoarthritis Index [WOMAC] score, Oxford Score, and Knee Society Score) and range of motion (ROM) at 6 months postoperatively between the 2 alignment methods because 1 FEBRUARY 2012 Volume 35 Number 2 e161

3 Feature Article we wanted assurance that any change in alignment detected in the kinematic group did not result in worse clinical scores. MATERIALS AND METHODS The hospital research committee and Institutional Review Board approved the study protocol, the patients provided informed consent for participation in the study, and the study was registered at (identifier NCT ). Inclusion criteria included substantial pain and loss of function due to arthritis of the knee. Patients with a history of fracture of the tibia or femur, infection, previous joint replacement, previous osteotomy about the knee, or medical condition precluding surgery were excluded. Patients who needed bilateral TKA and patients who could not undergo an MRI study of the knee were also excluded. One hundred twenty patients scheduled for TKA at our institution were enrolled in the study and were randomized into kinematically aligned TKA with patient-specific guides or mechanically aligned TKA with conventional instruments (Figure 2). A sealed envelope assigned each recruited patient to a treatment group. Forty-four patients were treated with kinematically aligned TKA and 44 patients were treated with mechanically aligned TKA by 2 senior attending surgeons (H.G.D., G.J.S.) who acted as co-surgeons. Perioperative management was identical for both groups. Each patient was blinded to the treatment by undergoing a preoperative MRI of the operative knee. A posterior cruciate-retaining prosthesis with patellar resurfacing was inserted with cement (Vanguard; Biomet, Inc, Warsaw, Indiana). SURGICAL TECHNIQUE Kinematically Aligned TKA Patient-specific femoral and tibial cutting guides were used to set all 6 degreeof-freedom positions for each component in the operating room The guides Figure 2: Participant fl ow diagram showing the numbers of patients randomly assigned to each group, the number of patients who received the intended treatment or were excluded after randomization with reasons, the number of patients lost to follow-up after the intended treatment with reasons, and the number of participants analyzed at 6 months postoperatively. were sterilized per company instructions and opened on the sterile field preoperatively. Before the onset of the procedure, patient initials, birth date, and implant size and side (left or right) were confirmed by comparing the information on the guides with the patient information and the surgical plan. One tray of TKA instruments and correct size trials was required for each procedure. The anterior fat pad was excised in all cases. The femoral patient-specific guide was placed on the distal femur by sliding the trochlear portion of the guide distally and posteriorly until the guide locked into place. The guide was then compressed against the distal femur and secured by drilling 2 pins through the pin holes in the distal surface of the guide into the femoral articular surface, and by drilling 2 pins through the pin holes in the anterior surface of the guide. The distal cut on the femur was made through the saw slot of the patient-specific guide. The conventional 2 e162 ORTHOPEDICS ORTHOSuperSite.com

4 KINEMATICALLY VS MECHANICALLY ALIGNED TKA DOSSETT ET AL 4-in-1 cutting block that matched the size of the planned femoral component was placed into the 2 pin holes in the distal femoral articular surface, and the anterior, posterior, and chamfer femoral cuts were made. A posterior cruciate retractor was used to sublux the tibia anteriorly on the femur, and the patellar tendon was retracted gently with a collateral ligament retractor. The tibial patient-specific guide was then placed, checking to be sure it was seated both medially and laterally, compressed axially, and secured by drilling 2 pins through the pin holes on the proximal surface on the tibial guide, and by drilling 2 pins through the pin holes in the anterior surface of the guide. The tibial cut was made through the slot in the guide, and marginal osteophytes on the tibia and femur were removed with an osteotome. Posterior osteophytes were removed and a posterior capsular release was performed when there was a flexion contracture. Medial and lateral osteophytes were removed to restore length to the collateral ligaments. Trial components were placed, and knee ROM, joint stability, implant rotation, posterior cruciate ligament (PCL) tension, and patellar tracking were checked. Stability, motion, and limb alignment were achieved in each kinematically aligned knee without release of the medial or lateral collateral ligaments or the PCL. The internal external rotation of the tibial component was aligned parallel to the pinholes drilled through the proximal surface of the tibial guide. The definitive implants were cemented, with care to remove all excess cement. The wounds were closed in layers after final irrigation. ly Aligned TKA ly aligned TKA was performed as described in the technique manual provided by the manufacturer (Biomet, Inc). Eight standard instrument trays were used for the procedure. The anterior fat pad was excised in all cases. The PCL was left intact. The distal femoral bone cut was made with an intramedullary alignment system with the angle of the distal resection set at 5 of valgus. The posterior femoral bone cuts were made with a posterior referencing guide set at 3 of external rotation. The tibial bone cut was made with an intramedullary alignment system (n 39) or with use of an extramedullary alignment in cases of severe varus bowing of the tibia (n 2). Significant posterior osteophytes were removed at this time with a three-quarterinch curved osteotome. Trial components were placed, and knee ROM, joint stability, PCL tension, and patellar tracking were checked. Release of the collateral and retinacular ligaments was performed when necessary at the discretion of the co-surgeons. The definitive implants were cemented, with care to remove all excess cement. The wounds were closed in layers after final irrigation. Postoperative Management Postoperative management was identical for both groups. The patient, physical therapist, and clinical evaluator (B.G.K.) who collected the clinical data and examined the patients were blinded to each patient s alignment method. Scanogram Evaluation of Limb, Knee, and Component Alignment Using standardized protocol, anteroposterior and lateral CT scanograms of the affected limb were obtained to determine the coronal alignment of the limb, knee, and components and the sagittal alignment of the components. The rotation of the knee was established in the coronal plane by centering the flange between the posterior condyles of the femoral component and in the sagittal plane by superimposing the femoral condyles. 6,11 An examiner blinded to the alignment method measured each scanogram. In the coronal plane, the following anatomical landmarks were determined in absolute spatial coordinates (x and y) with a public domain software program (Scion Image; Scion Corporation, Frederick, Maryland): (1) the center of the femoral head was the center of a circle fit to the articular surface of the femoral head; (2) the center of the knee joint at the distal end of the femoral component; and (3) the center of the ankle at the proximal end of the talus. The mechanical axis of the femur was a line connecting the center of the femoral head and the center of the knee. The mechanical axis of the tibia was a line connecting the center of the knee and the center of the ankle. In the coronal and sagittal planes, the anatomic axis of the femur was a line between a point that bisected the femur at the proximal end of the distal fourth of the femur and the center of the distal end of the femoral component. The anatomic axis of the tibia was a line between a point that bisected the distal end of the proximal fourth of the tibia and the center of the proximal end of the tibial component. From the spatial relationship between the femoral and tibial components and the femoral and tibial mechanical axes, the following angles were determined: the hip knee ankle angle of the limb, the anatomic angle of the knee formed by the intersection of the anatomic axis of the femur and tibia, the angle of the femoral component relative to the femoral mechanical axis, the angle of the tibial component relative to the tibial mechanical axis, the angle between the femoral component and the line from the center of the femoral head and center of the ankle, the extension flexion of the femoral component in relation to the anatomic axis of the femur, and the extension flexion of the tibial component in relation to the anatomic axis of the tibia (posterior slope). Statistical Methods The arithmetic mean, standard deviation, and 95% confidence interval (CI) were determined for each measure in each treatment group. The difference in the means of the primary and secondary outcome measures between the treatment FEBRUARY 2012 Volume 35 Number 2 e163

5 Feature Article groups were determined with use of the nonparametric Wilcoxon signed-rank test for non-normally distributed data, an unpaired t test for normally distributed data, and the chi-square test using statistical software. Significance was set at P.05. A power analysis was conducted to estimate the minimum sample size needed to observe a clinically meaningful difference in the primary outcome variables of angular alignment of the limb, knee, and components between the kinematically and mechanically aligned treatment groups. The current study, comprising 82 patients, was adequately powered to be able to detect a 2 difference in alignment (.05) with 80% power assuming a standard deviation of 3. That degree of power would be achieved with study groups comprising at least 73 knees. RESULTS The number of patients assessed for enrollment, excluded, allocated, treated, lost to follow-up, and analyzed in each treatment group is shown in Figure 2. Forty-one patients in each treatment group completed the radiographic and clinical outcome analysis at 6 months patients. Comparison of Preoperative Demographic, Intraoperative, and Hospitalization Parameters Patient age, sex, body mass index (BMI), American Society of Anesthesiologists (ASA) score, and preoperative extension, flexion, and clinical outcome scores (WOMAC score, Oxford Score, Knee Society Score, and Knee Function Scores) are shown in Table 1. The change in hemoglobin (P.344), distance walked prior to discharge (P.098), ROM prior to discharge (P.737), were not significantly different between the 2 methods of alignment. The operative time in the kinematically aligned group was 21 minutes less than the mechanically aligned group (P.000). The length of the incision was 5.9 cm in the kinematically aligned group and 15.8 cm in the mechanically aligned group, a difference of 0.1 cm, which was not significantly different (P.710). Primary Outcome Measures: Limb, Knee, and Component Alignment In the coronal plane, the average hip knee ankle angle was 0.3 varus in the kinematically aligned group, compared with 0.0 in the mechanically aligned group, a difference of 0.3 (P.693) (Table 2). The anatomic angle Primary Outcome Measure Coronal plane b Table 1 Baseline Demographic and Clinical Characteristics Kinematic Alignment (n 41) Alignment (n 41) Mean age, y Males, % Mean BMI, kg/m ASA score, % Extension, deg Flexion, deg WOMAC score a Oxford Score b Knee Society Score c Knee Function Score d Abbreviations: ASA, American Society of Anesthesiologists; BMI, body mass index; WOMAC, Western Ontario and McMaster Universities Osteoarthritis Index. a 0 is best, 96 is worst. b 0 is best, 48 is worst. c 100 is best, 0 is worst. d 100 is best, 0 is worst. Table 2 Primary Outcome Measures Mean SD, deg Kinematic Alignment (n 41) Alignment (n 41) Difference (95% CI) a P Hip knee ankle angle ( 0.9 to 1.3) Anatomic knee angle ( 1.8 to 0.2) Angle between femoral component and mechanical axis of femur Angle between femoral component and weight-bearing line from center of femoral head to center of ankle Sagittal plane c Angle between femoral component and anatomic axis of femur Angle between tibial component and anatomic axis of tibia ( 3.4 to 1.4) (1.1 to 3.1) Abbreviations: CI, confi dence interval; SD, standard deviation. a Difference kinematic mechanical. b varus/ valgus. c component fl exed/ component extended (2.9 to 7.6) ( 4.3 to 0.1) e164 ORTHOPEDICS ORTHOSuperSite.com

6 KINEMATICALLY VS MECHANICALLY ALIGNED TKA DOSSETT ET AL Table 3 Secondary Outcome Measures 3A 3B Figure 3: Composite long-leg scanogram of 2 representative patients, 1 with a kinematically aligned total knee arthroplasty (A) and the other with a mechanically aligned total knee arthroplasty (B). The line connecting the centers of the femoral head and ankle pass through the center of the knee in both total knee arthroplasties. The obliquity and level of the joint line in the kinematically aligned total knee arthroplasty are similar to the contralateral knee. However, the joint line in the mechanically aligned total knee arthroplasty is in more varus and more proximal than the contralateral knee. The obliquity of the joint line in the kinematically aligned total knee arthroplasty replicates the anatomic alignment of the joint line in the normal limb noted by Hungerford and Krackow 20 and Krackow, 21 which was associated with better clinical outcome scores and better flexion than the mechanically aligned total knee arthroplasty. Secondary Outcome Measure Perioperative outcomes Kinematic Alignment (N 41) of the knee was 3.6 valgus in the kinematically aligned group compared with 2.8 valgus in the mechanically aligned knee, a 0.8 difference that was not significant (P.131). The angle between the femoral component and the mechanical axis of the femur was 1.4 valgus in the kinematically aligned group and 1.0 varus in the mechanically aligned group, a difference of 2.4, which was significant (P.000) (Figure 3). The angle between the tibial component and the mechanical axis of the tibia was 2.4 varus in the kinematically aligned group and 0.1 varus in the mechanically aligned group, a difference of 2.3, which was significant (P.000). The angle between the femoral component and weight-bearing line from the center of the femoral head to the center of the ankle was 2.1 valgus in the kinematically aligned group compared with 0.0 in the mechanically aligned group, a difference of 2.1, which was significant (P.000). In the sagittal plane, the angle between the femoral component and the anatomic axis of the femur was 9.8 flexed in the kinematically aligned group, and 4.6 Mean SD Alignment (N 41) Difference (95% CI) a P Operative time, min ( 31 to 11) Change in hemoglobin, g/dl ( 0.9 to 0.3) Distance walked prior to discharge, feet ( 8 to 100) ROM prior to discharge, deg ( 7 to 7) Incision length, cm ( 0.7 to 1.0) Clinical outcome scores b WOMAC score c ( 23.0 to 8.4) Oxford Score d ( 10.9 to 3.1) Knee Society Score e (4.6 to 19) Knee Function Score f (4 to 22) Combined Knee Society Score (10 to 39.3) ROM b Extension, deg ( 1.0 to 0.71) Flexion, deg (0.5 to 9.7) Abbreviations: CI, confi dence interval; ROM, range of motion; SD, standard deviation; WOMAC, Western Ontario and McMaster Universities Osteoarthritis Index. a Difference kinematic mechanical. b At 6 2-month follow-up. c 0 is best, 96 is worst. d 0 is best, 48 is worst. e 100 is best, 0 is worst. f 100 is best, 0 is worst flexed in the mechanically aligned group, a difference of 5.2, which was significant (P.000). The angle between the tibial component and the anatomic axis of the tibia was 5.0 extended (posterior slope) in the kinematically aligned group, compared with 3.0 extended in the mechanically aligned group, a difference of 2.0, which was significant (P.035). FEBRUARY 2012 Volume 35 Number 2 e165

7 Feature Article Secondary Outcome Measures: Clinical Outcome Scores and Motion At 6 months postoperatively, the WOMAC score (0 is best and 96 is worst) was 12 in the kinematically aligned group and 28 in the mechanically aligned group, a difference of 16 points, which was significant (P.000) (Table 3; Figure 4). The Oxford Score (0 is best and 48 is worst) was 8 in the kinematically aligned group and 15 in the mechanically aligned group, a difference of 7 points, which was significant (P.001). The Knee Society Score (100 is best and 0 is worst) was 90 in the kinematically aligned group and 79 in the mechanically aligned group, a difference of 11 points, which was significant (P.001). The Knee Function Score (100 is best and 0 is worst) was 84 in the kinematically aligned group and 70 in the mechanically aligned group, a difference of 14 points, which was significant (P.004). The combined Knee Society Score (200 is best, and 0 is worst) was 174 in the kinematically aligned group and 149 in the mechanically aligned group, a difference of 25 points, which was significant (P.001). Flexion was 120 in the kinematically aligned group and 115 in the mechanically aligned group, a difference of 5, which was significant (P.043). Extension was similar between the 2 treatment groups (P.734). Complications Seven complications occurred. In the kinematically aligned group, 1 patient required evacuation of a hematoma, 2 patients required a manipulation under anesthesia, and 1 patient underwent operative treatment for patellar subluxation at 6 months postoperatively. In the mechanically aligned group, 1 patient sustained a hematoma and skin slough treated with serial dressing changes and local debridement, 1 patient required evacuation of a hematoma, and 1 patient had a patella fracture requiring open reduction and internal fixation. 4 Figure 4: Quantile box plot summarizing the distribution of the Western Ontario and McMaster Universities Osteoarthritis Index score (0 is best, 96 is worst) for each patient in the kinematically and mechanically aligned treatment group. The line across the middle of the box identifies the median. The ends of the box indicate the 25th and 75th quantiles. The whiskers extending from either end of the box indicate the 10% and 90% quantiles. The kinematically aligned knee with patient-specific guides has a narrower distribution with a lower mean score than the mechanically aligned knee with conventional instruments. There are 4 outliers in the kinematically aligned total knee arthroplasty. We were unable to determine the cause of these outliers. The same distribution was observed for the Oxford, Knee Society, and Knee Function Scores between the 2 alignment methods. DISCUSSION The purpose of our study was to evaluate an alternative alignment method, kinematic alignment with patient-specific cutting guides, and compare the alignment results with mechanical alignment with conventional instruments. For the primary outcome variables, our study showed that kinematically aligned TKA aligns the hip knee ankle angle and the anatomic angle of the knee similar to mechanically aligned TKA. The obliquity of the preoperative joint line was more closely reproduced in the kinematically aligned group and was not reproduced in the mechanically aligned group because the joint line was perpendicular to the mechanical axis. For the secondary outcome measures, the kinematically aligned knees showed no deterioration of knee scores compared with mechanical alignment; rather, they showed improved clinical outcome scores and flexion at 6 months postoperatively. The explanation for why a collateral ligament release and a PCL recession was not needed to balance the knees in the kinematically aligned group relates to the relationship between the natural variability in limb alignment and how a change in limb alignment to neutral changes the kinematics of the knee. Because 98% of normal limbs are not straight, 1 and because 32% of men and 17% of women have a natural mechanical alignment of 3 varus, 5 the procedures performed to straighten the limb change the coronal angle of the femoral and tibial joint line from normal, which changes the location of the 3 kinematic axes of the knee. 1,6,7 Placing the femoral component along the altered joint line changes the location of the kinematic axes of the knee from normal, and depending on the magnitude of the change, may require collateral ligament, PCL, and lateral retinacular releases to restore ROM and patella tracking. 1,15,17 e166 ORTHOPEDICS ORTHOSuperSite.com

8 KINEMATICALLY VS MECHANICALLY ALIGNED TKA DOSSETT ET AL Study Current study Dunbar et al 23 Dutton et al 24 Matziolis et al 25 Stulberg et al 26 Table 4 Comparison of 5 Studies With 6-month Follow-up Alignment Method Kinematic (patientspecific guides) (n 41) (conventional) (n 41) (trabecular metal tibia) (n 28) (cemented tibia) (n 21) (conventional) (n 52) (computerassisted) (n 56) (conventional) (n 28) (computerassisted) (n 32) (conventional) (n 40) (computerassisted) (n 38) In the kinematically aligned treatment group, the distal and posterior femoral cuts were made at an equal distance from the transverse axis in the femur about which the tibia flexes and extends and were not made to the highly variable transepicondylar axis, which is a reference axis commonly used in mechanical alignment. The only intraoperative morphologic reference to guide the surgeon to the transverse axis in the femur is the articular surface of the femoral condyles. 1,7,18 WOMAC Score a Oxford Score b Combined Knee Society Score c Flexion, deg NA NA NA 18 NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA Abbreviations: NA, not available; WOMAC, Western Ontario and McMaster Universities Osteoarthritis Index. a 0 is best, 96 is worst. b 0 is best, 48 is worst. c 200 is best, 0 is worst. No other morphologic feature of the knee, including the transepicondylar axis and externally rotating the femoral component relative to the posterior condylar line, will lead the surgeon reliably to the transverse axis in the femur about which the tibia flexes and extends. 1,7,16 Of all of the femoral references for aligning the femoral component, the use of the transepicondylar axis consistently malaligns the femoral component off the transverse axes of the femur about which the tibia and patella flex and extend because the average error between the transverse axes and the transepicondylar axis is 5 with a maximum error exceeding 11 when measured in 3-D space. 1,7 Because we evaluated a primarily male population treated by 2 surgeons at a Veterans Administration medical center and used 1 brand of cruciate-retaining component with a near single-radius design, any generalizations of our findings should be made carefully Patients with prior fracture of the femur or tibia were excluded, which means that more complex, multiple-level deformities were not evaluated. Finally, our results are preliminary, as they represent alignment, clinical outcomes, and ROMs within the first 6 months postoperatively. With these limitations in mind, further double-blind randomized trials are needed to determine the long-term effects that kinematic alignment with patient-specific cutting guides have on wear, survivorship, clinical outcomes in women, and different implant designs (eg, posterior substituting, anterior cruciate retaining, or rotating platform). An important question is whether kinematic alignment, planned from an MRI of the knee, and with bone cuts designed specifically for each patient, significantly deviates from the principle of aligning the limb as close to a straight line as possible. Our study showed that both the kinematically and mechanically aligned knees were very close to this target, with a hip knee ankle angle of for kinematic alignment and a for mechanical alignment, of which the difference between the mean and variance were not significant (P.693 and P.066, respectively) (Figure 3; Table 2). Assuming the principle of aligning the limb on the mechanical axis is important for longevity of the prosthesis, then kinematic and mechanical alignment methods should have similar durability with time. One study of 4 patients reported that kinematic alignment placed the compo- FEBRUARY 2012 Volume 35 Number 2 e167

9 Feature Article nents, not the alignment of the limb or knee, in 3 off the mechanical axis of the femur and tibia in the coronal plane. This deviation of the components from perpendicular to the mechanical axes of the femur and tibia was thought to indicate malalignment of the components and place the TKA at a high risk for early failure. 19 Our study showed the kinematically aligned tibial component in 2.3 more varus and the femoral component in 2.4 more valgus than in the mechanically aligned group. The obliquity of the joint line in the kinematically aligned TKA replicates the anatomic alignment of the joint line in the normal limb noted by Hungerford and Krackow 20 and Krackow. 21 A study of an anatomically aligned prosthesis with a varus alignment of the tibial component of 3 3, similar to the alignment of the tibial component in the current study, showed a 96% survivorship of the prosthesis at 10 years. 22 One explanation for the better clinical outcome scores and flexion at 6 months of the kinematically aligned group in the current study may be that the clinical outcome scores and flexion of the mechanically aligned group in the current study were poorer than other mechanical alignment surgical techniques. To put the clinical outcome scores and flexion of the mechanically aligned group in the current study into perspective, we compared these results to 4 randomized control trials that used different mechanical alignment surgical techniques with similar clinical followup at 6 months (Table 4) These 4 studies, which used mechanical alignment with conventional instruments and mechanical alignment with computer-assisted instruments, had similar clinical outcomes scores and flexion to the mechanically aligned group in the current study. This indicates that the better clinical outcome scores and motion in the kinematically aligned group than in the mechanically aligned group in the current study was not due to a poor execution of the mechanically aligned surgical technique CONCLUSION We compared a new method of aligning a TKA called kinematic alignment, performed with preoperative computer planning and patient-specific guides, with mechanical alignment with conventional instruments. Alignment of the components with this new method showed obliquity of the joint line was more anatomic, with better clinical outcomes. Based on our results, kinematic alignment with patient-specific guides warrants further study. REFERENCES 1. Eckhoff DG, Bach JM, Spitzer VM, et al. Three-dimensional mechanics, kinematics, and morphology of the knee viewed in virtual reality. J Bone Joint Surg Am. 2005; 87(suppl 2): Collier MB, Engh CA Jr, McAuley JP, Engh GA. Factors associated with the loss of thickness of polyethylene tibial bearings after knee arthroplasty. J Bone Joint Surg Am. 2007; 89(6): Fang DM, Ritter MA, Davis KE. Coronal alignment in total knee arthroplasty: just how important is it [published online ahead of print June 24, 2009]? J Arthroplasty. 2009; 24(6 suppl): Jeffery RS, Morris RW, Denham RA. Coronal alignment after total knee replacement. J Bone Joint Surg Br. 1991; 73(5): Bellemans J, Colyn W, Vandenneucker H, Victor J. 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