- Discussion:
- malrotation of femoral component may lead to patellofemoral dislocation or subluxation;
- slight external rotation of femoral component on femur places patellofemoral groove more laterally, enhances
patellofemoral stability (external rotation moves the femoral component further lateral to the tibial tubercle);
- slight external rotation of AP cutting jig may also be necessary in patients with a severe fixed
valgus deformity;
- AP Axis Method: method of choice:
- Posterior Condyles as a Rotational Guide:
- generally the most reliable guide is the posterior condyles;
- w/ normal femoral anatomy, a slight relative external rotation (3 deg) of a line along the posterior aspect of the femoral condyles
will orient the AP cuts perpendicular to the resected tibial surface;
- the medial condyle extends below the epicondylar axis further than the lateral condyle and therefore, more of the medial condyle will be resected than the lateral;
-
pitfalls:
- this technique, however, requires special care in any knee that has a large AP asymmetry in the femoral condyles;
- in such cases rotational alignment only w/ respect to posterior condyles may result in significant
undercutting of the anterior cortex laterally, severely notching the cortex;
- when the posterior condyles are not available as a rotational reference (such as in AVN or severe
valgus deformity), the epicondyles & form of the trochlea as seen in the skyline view can be used;
-
effects of tibial varus deformity on posterior condylar angle: (
tibial varus deformity);
- in 60 consecutive total knee arthroplasties done in 52 patients with primary osteoarthritis and varus or neutral tibiofemoral alignment,
the posterior condylar angle was calculated intraoperatively and averaged 3.98° (range, 0°-9°);
- 18 knees had a posterior condylar angle value less than 3° whereas 27 knees had a posterior condylar angle value of 5° or greater;
- final rotational alignment of the femoral component was set parallel to the transepicondylar axis;
- only one of these 60 knees required a lateral retinacular release for proper patellar tracking during the knee arthroplasty;
- when compared with three previously defined angles measured on the radiographs taken preoperatively, only the tibial plateau
tibial shaft angle values were correlated significantly with the value of the posterior condylar angle;
- as the tibial varus joint line obliquity increased, there was a distinct tendency for the transepicondylar axis to be rotated
more externally relative to the posterior condylar axis;
- this variance suggests that the use of the posterior condylar axis as a rotational reference
is inappropriate in many knees with arthritis with varus or neutral tibiofemoral alignment;
- in particular, varus tibial joint line obliquity of more than 4 deg increases the likelihood of femoral component
malrotation when the posterior femoral condyles are used to reference femoral component rotation;
- ref: Varus Tibial Joint Line Obliquity
A Potential Cause of Femoral Component Malrotation. Mark W. Pagnano MD.
CORR 2001;2001:68-74
- Transepicondylar Axis Method:
- connects the lateral epicondylar prominence and the medial sulcus of the medial epicondyle;
- medial sulcus is a more accurate landmark than the medial prominence;
- site of insertion of the deep part of the MCL;
- this method is probably most indicated in revision TKR;
- can be accurate method, but can be difficult to use do to overlying soft tissues;
- when measured carefully, the epicondylar axis most often will place the AP cutting guide in 4 deg of external rotation (which is ideal);
- note, however, that it may be common to error by placing the AP cutting guide in too much external rotation;
- in the report by Mark C. Miller et al., the authors evaluated the validity of this rotational
landmark and its effect on the patellofemoral and tibiofemoral articulations.
- TKR was done in 11 knees from cadavers;
- knees were tested with various femoral component rotations from 5° internal rotation to 5° external rotation referenced to the epicondylar axis
and to the posterior femoral condyles;
- knees were actively ranged from 0° to 100° by a force on the quadriceps tendon in an Oxford knee simulator;
- femoral component rotation parallel to the epicondylar axis resulted in the most normal patellar tracking and minimized
patellofemoral shear forces early in flexion;
- this optimal rotation also minimized tibiofemoral wear motions;
- these beneficial effects of femoral rotation were less reproducibly related to the posterior condyles;
- rotating the femoral component either internal or external to the epicondylar axis worsened knee function by increasing tibiofemoral wear
motion and significantly worsening patellar tracking with increased shear forces early in flexion;
- based on the current study, the femoral component should be rotationally aligned parallel to the epicondylar axis to avoid
patellofemoral and tibiofemoral complications;
- The Ranawat Award Paper: Optimizing Femoral Component Rotation in Total Knee Arthroplasty
Mark C. Miller, PhD et al.
CORR 2001;2001:38-45
- in the report by Barrack et al, the authors sought to correlate anterior knee pain with TKR component malrotation;
- significant anterior knee pain rating at least 3 of 10 on the visual analog scale was present in 16 knees (13 patients);
- 11 patients with 14 symptomatic knees agreed to undergo CT scanning to accurately determine the rotation of the tibial and femoral components;
- epicondylar axis and tibial tubercle were used as references using a previously validated technique;
- there was a highly significant difference in tibial component rotation between the two groups w/ the patients w/
anterior knee pain averaging 6.2° internal rotation compared with 0.4° external rotation in the control group;
- there also was a significant difference in combined component rotation with the patients w/ anterior knee pain avg
4.7° internal rotation compared with 2.6° external rotation in the control group;
- there was no significant difference in the degree of radiographic patellar tilt or patellar subluxation between the two groups;
- patients with combined component internal rotation were more than five times as likely to experience anterior knee pain after
total knee arthroplasty compared with those with combined component external rotation;
- ref: Component Rotation and Anterior Knee Pain After Total Knee Arthroplasty.
Robert L. Barrack, MD. CORR 2001;2001:46-55
- Effects of Internal Rotation on Femoral Component:
- in no case should the AP femoral jig be internally rotated, since this places the prosthetic trochlear surface away from the patella
and predisposes to postop patellofemoral dislocation;
- small degree of internal rotation may be attempt to prevent undercutting of the anterior cortex;
- significant internal rotation, however, must be avoided because it has adverse effect of increasing
Q angle;
- internal rotation of
femoral component by resection of excessive amounts of posterior lateral femoral condyle or insufficient resection of the posterior
medial femoral condyle moves anterior femoral patellar groove portion of the femoral component medially, making it more difficult for
the relatively laterally placed patella to be captured by the patellofemoral groove;
- internal rotation & medial translation of femoral component increases medial displacement, tilting,
& subluxatin of patella, whereas external rotation of
femoral component has produced x-ray patterns
of patellar tracking most closely reproducing those of knee;
The Variability of Femoral Rotational Alignment in Total Knee Arthroplasty.
The Clinical Consequences of Flexion Gap Asymmetry in Total Knee Arthroplasty.
