Internal and external malrotation of the femoral and tibial components is associated with poor function after total knee arthroplasty (TKA). We determined the degree of malrotation for both components in kinematically aligned TKA and whether this malrotation compromised function. Seventy-one patients (mean age 68 years) were followed after TKA. Malrotation was measured. Simple regression determined the association between malrotation and function. Even though the range of malrotation of the tibial component can be greater than that of the femoral component, the malrotation of the femoral and tibial components bounded by the ranges reported in this study is compatible with a well-functioning TKA.
Key points
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Association between femoral component rotation and function.
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Association between tibial component rotation and function.
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WOMAC and Oxford Knee Score determined function.
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High function after kinematic aligned total knee arthroplasty.
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Weak association between component rotation bounded by the ranges reported in the present study and function.
Introduction
Internal and external (I-E) malrotation of the femoral and tibial components is associated with poor function after total knee arthroplasty (TKA). In mechanically aligned TKA, there are several reference lines in use to minimize I-E malrotation of the femoral and tibial components. Three reference lines used to set I-E rotation of the femoral component are
- 1.
The line parallel to the anterior-posterior (A-P) axis of the trochlear groove,
- 2.
The line parallel to the transepicondylar axis, or
- 3.
The line 3° externally rotated to the posterior condylar line of the femur.
Four reference lines used to set I-E rotation of the tibial component are
- 1.
The line between the most medial and most lateral points of the plateau,
- 2.
The line between the medial one-third of the tubercle and the center of the PCL attachment,
- 3.
The line between the medial border of the tubercle and the PCL, and
- 4.
The line between the projection of the anterior crest and the PCL.
However, the range of I-E malrotation of the femoral component (−13° internal to 16° external; SD ± 7°) and of the tibial component (−44° internal to 46° external; SD ± 28°) reported for these reference lines is high, which indicates the placement of these lines is not reproducible.
Kinematic alignment is a new method that has gained interest because 2 studies showed that patients with a kinematically aligned TKA reported better pain relief, better function, better flexion, and a more normal-feeling knee than patients with a mechanically aligned TKA. The goal of kinematic alignment is to correct the arthritic deformity of the limb to the constitutional alignment of the patient with the intent of positioning the femoral and tibial components so that the natural tibial-femoral articular surface, alignment, and laxities of the knee are restored. This is accomplished in part by setting the A-P axes of the femoral and tibial components parallel to the flexion-extension (F-E) plane of the extended knee ( Fig. 1 ). The F-E plane of the extended knee is aligned perpendicular to the F-E axis of the tibia that connects the 2 centers of the circular portion of the posterior femoral condyles from about 20° to 120° and parallel to the F-E axis of the patella and the natural distal and posterior femoral joint lines.
Surgically, the A-P axis of the femoral component is set parallel to the F-E plane of the extended knee by placing a 0° rotation posterior referencing guide in contact with the posterior femoral condyles at 90° and removing posterior femoral resections within ± 0.5 mm of the thickness of the condyles of the femoral component after compensating for cartilage wear and kerf. Surgically, the A-P axis of the tibial component is set parallel to the F-E plane of the extended knee by aligning the A-P axis of the tibial component parallel to the major axis of the elliptical-shaped boundary of the lateral tibial condyle ( Fig. 2 ). However, there are no data reporting the range of I-E malrotation of the femoral and tibial components when these methods of rotational alignment are used to perform a kinematically aligned TKA.
The objectives of the present study were to determine the range of I-E malrotation for both components in a case series of patients treated with a kinematically aligned TKA and then determine whether the degree of I-E malrotation of the femoral and tibial components compromised function as measured by the Oxford Knee Score (OKS) and the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) score.
Introduction
Internal and external (I-E) malrotation of the femoral and tibial components is associated with poor function after total knee arthroplasty (TKA). In mechanically aligned TKA, there are several reference lines in use to minimize I-E malrotation of the femoral and tibial components. Three reference lines used to set I-E rotation of the femoral component are
- 1.
The line parallel to the anterior-posterior (A-P) axis of the trochlear groove,
- 2.
The line parallel to the transepicondylar axis, or
- 3.
The line 3° externally rotated to the posterior condylar line of the femur.
Four reference lines used to set I-E rotation of the tibial component are
- 1.
The line between the most medial and most lateral points of the plateau,
- 2.
The line between the medial one-third of the tubercle and the center of the PCL attachment,
- 3.
The line between the medial border of the tubercle and the PCL, and
- 4.
The line between the projection of the anterior crest and the PCL.
However, the range of I-E malrotation of the femoral component (−13° internal to 16° external; SD ± 7°) and of the tibial component (−44° internal to 46° external; SD ± 28°) reported for these reference lines is high, which indicates the placement of these lines is not reproducible.
Kinematic alignment is a new method that has gained interest because 2 studies showed that patients with a kinematically aligned TKA reported better pain relief, better function, better flexion, and a more normal-feeling knee than patients with a mechanically aligned TKA. The goal of kinematic alignment is to correct the arthritic deformity of the limb to the constitutional alignment of the patient with the intent of positioning the femoral and tibial components so that the natural tibial-femoral articular surface, alignment, and laxities of the knee are restored. This is accomplished in part by setting the A-P axes of the femoral and tibial components parallel to the flexion-extension (F-E) plane of the extended knee ( Fig. 1 ). The F-E plane of the extended knee is aligned perpendicular to the F-E axis of the tibia that connects the 2 centers of the circular portion of the posterior femoral condyles from about 20° to 120° and parallel to the F-E axis of the patella and the natural distal and posterior femoral joint lines.
Surgically, the A-P axis of the femoral component is set parallel to the F-E plane of the extended knee by placing a 0° rotation posterior referencing guide in contact with the posterior femoral condyles at 90° and removing posterior femoral resections within ± 0.5 mm of the thickness of the condyles of the femoral component after compensating for cartilage wear and kerf. Surgically, the A-P axis of the tibial component is set parallel to the F-E plane of the extended knee by aligning the A-P axis of the tibial component parallel to the major axis of the elliptical-shaped boundary of the lateral tibial condyle ( Fig. 2 ). However, there are no data reporting the range of I-E malrotation of the femoral and tibial components when these methods of rotational alignment are used to perform a kinematically aligned TKA.
The objectives of the present study were to determine the range of I-E malrotation for both components in a case series of patients treated with a kinematically aligned TKA and then determine whether the degree of I-E malrotation of the femoral and tibial components compromised function as measured by the Oxford Knee Score (OKS) and the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) score.
Methods
An institutional review board approved the analyses of 101 consecutive patients (101 knees) treated with a primary kinematically aligned TKA from June to September, 2012, by an inventor surgeon (S.M.H.) who were prospectively followed for 6 months. When feasible, patients were scheduled for a preoperative MRI scan to evaluate cartilage wear and plan the thickness of the posterior resections from the femur. Thirty patients were excluded because they were unable to have our protocol preoperative MRI because of a pacemaker, hardware about the knee, insurance refused to authorize, or an MRI had been performed with a different protocol. The indications for performing kinematically aligned TKA were
- 1.
Disabling knee pain and functional loss unresolved with standard of care, nonoperative, treatment modalities;
- 2.
Radiographic evidence of advanced arthritis indicated by a Kellgren-Lawrence grade of 3 or 4; and
- 3.
Any severity of varus and valgus deformity and flexion contracture.
Seventy-one patients (71 knees) with an average (SD) age of 68 ± 8.6 years, of whom 30 were men, met our inclusion criterion and were included in the analysis ( Table 1 ). The number (percent) of patients with a 5° to 10° varus deformity was 22 (31%), 11° to 15° varus deformity was 18 (26%), greater than 16° varus deformity was 9 (12%), 10° to 15° valgus deformity was 12 (17%), 16° to 20° valgus deformity was 7 (10%), and greater than 20° valgus deformity was 3 (4%). Patient-reported OKS and WOMAC score at 6 months determined function. Function was measured at 6 months because the New Zealand Joint Registry 2014 showed that 6-month OKSs predict the revision rate at 2 years and the function at 5 years.
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