Femoral pulse as a guide to the mechanical axis in total knee arthroplasty

Mechanical axis-derived femoral component rotation in extramedullary total knee arthroplasty: a comparison between femoral transverse axis and transepicondylar axis

PURPOSE

Correct rotational alignment of the femoral component is paramount to the success of total knee arthroplasty, but debate continues as to which method is the most reliable. The purpose of this study was to evaluate mechanical axis-derived rotational axis of the femoral component using an extramedullary femoral alignment guide system.

METHODS

Between 2009 and 2010, 60 patients (120 knees) underwent simultaneous bilateral total knee arthroplasty. Postoperative CT scans were performed on all 120 knees. The mechanical axis-derived rotational axis of the femoral component was compared with the surgical transepicondylar axis on CT scans.

RESULTS

The femoral component was externally rotated compared with the surgical transepicondylar axis (mean = 1.6°, SD = 2.2°, range: from 4.8° of internal rotation to 7.9° of external rotation). One hundred and nine of the 120 knees (90.8%) were rotated by less than 5° from the surgical transepicondylar axis. All inter-observer observations on CT scans were within a range of ±2.1° and showed no significant differences.

CONCLUSION

Mechanical axis-derived rotational axis of the femoral component was found to be closely related to the surgical transepicondylar axis, to an extent which suggests that the mechanical axis-derived rotational axis could be a new alternative option in determining optimal rotational alignment of the femoral component during total knee arthroplasty.

LEVEL OF EVIDENCE

II.

How precise is the identification of the center of the femoral head during total knee arthroplasty?

BACKGROUND AND PURPOSE

Identification of the center of the femoral head in the coronal plane is essential during total knee arthroplasty. We evaluated a new method for localization of the center of the hip, thereby detecting the neutral mechanical axis using inter-femoral head center distances (X) measured from a radiograph. Our proposed method was compared with 3 commonly used methods using landmarks that are estimated to be 2 finger-breaths medial to the anterosuperior iliac spine (method A), 2.5 cm perpendicular to the mid-inguinal point (method I), and 1.5 cm lateral to the femoral artery (method F).

METHODS

114 patients undergoing total knee arthroplasty were prospectively enrolled in the study. Four landmarks were marked and conventional anterior-posterior pelvic radiographs were taken. On the radiograph, the distance between the estimated FHC and the neutral mechanical axis was measured.

RESULTS

The median value (mm) of the measured distance was 9 in A, 7 in I, 8.5 in F, and 5 in X. When an error of more than 3° from neutral alignment was defined as an outlier, 15% of measurements in A, 6% of measurements in I, 14% in F, and 2% in X would fall in the outlier zone.

INTERPRETATION

The method detecting the neutral mechanical axis using inter-femoral head center distances (X) showed the least variability and the lowest percentage of outliers.

Alignment of computer-assisted total knee arthroplasty in patients with altered hip center

This study aimed to determine limb and component alignment after computer-assisted total knee arthroplasty in 30 patients (32 limbs) with an altered hip center due to a prior hip implant or deformed femoral head. There were no outliers greater than ±3° in the postoperative coronal alignment of the limb and the femoral component in relation to the altered hip center. Two limbs (8%) were more than ±3° for coronal alignment of the femoral component in relation to the anatomical hip center and 96% of limbs had less than 2° deviation in relation to the altered hip center. Computer-navigated total knee arthroplasty results in accurate restoration of lower limb and component alignment in patients with prior hip implants or deformed femoral heads where accurate restoration of alignment may be challenging due to altered hip center.

Effect of frontal plane tibiofemoral angle on the stress and strain at the knee cartilage during the stance phase of gait

Subject-specific three-dimensional finite element models of the knee joint were created and used to study the effect of the frontal plane tibiofemoral angle on the stress and strain distribution in the knee cartilage during the stance phase of the gait cycle. Knee models of three subjects with different tibiofemoral angle and body weight were created based on magnetic resonance imaging of the knee. Loading and boundary conditions were determined from motion analysis and force platform data, in conjunction with the muscle-force reduction method. During the stance phase of walking, all subjects exhibited a valgus-varus-valgus knee moment pattern with the maximum compressive load and varus knee moment occurring at approximately 25% of the stance phase of the gait cycle. Our results demonstrated that the subject with varus alignment had the largest stresses at the medial compartment of the knee compared to the subjects with normal alignment and valgus alignment, suggesting that this subject might be most susceptible to developing medial compartment osteoarthritis (OA). In addition, the magnitude of stress and strain on the lateral cartilage of the subject with valgus alignment were found to be larger compared to subjects with normal alignment and varus alignment, suggesting that this subject might be most susceptible to developing lateral compartment knee OA.

Variability in the range of inter-anterior superior iliac spine distance and its correlation with femoral head centre. A prospective computed tomography study of 200 adults

PURPOSE

The anterior superior iliac spine (ASIS) is commonly used to estimate the centre of the femoral head and assess limb alignment during surgical procedures. This study aimed to determine the range of inter-anterior superior iliac spine distances (IADs) and inter-femoral head centre distances (IFDs) among individuals and ascertain whether there is correlation between the IFD and the IAD. We also sought to determine whether gender, height and body mass index (BMI) had any influence on IAD and IFD.

MATERIALS AND METHODS

We prospectively measured IAD and IFD in 200 adults, using transverse computed tomography (CT) scans done for medical causes. We also calculated the distance between the pelvic midline and the centre of the femoral head (XY distance) from the measured IFD. The influence of gender, height and body-mass index on IAD and IFD, and the correlation of IAD with IFD, were also studied.

RESULTS

The overall mean IAD, IFD and XY distances were 22.7+/-1.6 cm, 16.0+/-0.8 cm and 8.0+/-0.4 cm, respectively. There was wide variation within the IAD range with 50% (100/200) of the subjects having their IAD within +/-10 mm of the mean compared to 75.5% (151/200) of the subjects with IFD within +/-10 mm of the mean. The probability that the mean XY distance would fall within 10 mm of the true femoral head centre in all subjects was 100%. The gender difference in IAD and IFD was statistically significant (P=0.03 and P<0.001, respectively), height and BMI had no influence, and the correlation of IAD with IFD was weak (0.35).

CONCLUSION

Although the range of IADs showed wide variation among subjects, this study clearly demonstrated the narrow range of the XY distance and IFD in the study population and provides a useful and accurate basis for a new method to determine the femoral head centre clinically and intraoperatively.

Placement of a palpable marker adjacent to the mid-inguinal point: assessment of a clinical method for detecting the femoral head centre during knee arthroplasty

Localization of the centre of the femoral head is vital when using extra-medullary femoral alignment systems in knee arthroplasty. This study investigated the accuracy of a palpable marker placed in the groin for detecting the centre of the femoral head. A table tennis ball, 38 mm in diameter, was taped 2.5 cm perpendicular to the mid-inguinal point in patients having a plain radiograph of the pelvis. The mean horizontal distance between the centre of the table tennis ball and a vertical line passing through the centre of the femoral head was 8.62 mm (range 0-28.6, SD 6.76). In 100% of cases the horizontal distance between the centre of the table tennis ball and a vertical line passing through the centre of the femoral head was less than one diameter of the table tennis ball. Using a theoretical mechanical axis model our clinical method would confer an error equal to or less than 2 degrees from the weight-bearing axis of the knee in up to 98% of cases. In conclusion, our results suggest that the clinical method reported here provides a simple and reliable way of localizing the centre of the femoral head in knee arthroplasty.

Computerized tomography scout view for determining distal femoral resection angle in intramedullary instrumentation of total knee arthroplasty

This study aims to investigate the results of distal femoral resection by determining the difference between mechanical and anatomical axes of femur using computerized tomography (CT) scout views in pre-operative planning of total knee arthroplasty. CT scout view of the lower extremities was taken before and after the operation in 16 patients undergoing total knee arthroplasty. Distal femoral resection was performed according to the previously determined ideal resection angle (IRA) using intramedullary instrumentation. At post-operative scanogram, femoral component deviation (FCD) was measured. The results were statistically analyzed. The average IRA was 6.95 (5-9) degrees. At post-operative measurements, the average FCD was 0.63 (0-3) degrees. CT scout films improve the accuracy in distal femoral resection and femoral component alignment.

Scoring systems in total knee arthroplasty

For assessment of total knee arthroplasty outcome, various scoring systems have been introduced. The current study assessed the interobserver correlation of four commonly used total knee arthroplasty outcome scores. One hundred eighteen total knee arthroplasties were investigated by two independent observers, using the Hungerford score, the Hospital for Special Surgery score, the Knee Society score, and the Bristol score. Each score consisted of three subscores: pain, knee, and function. For the highest interobserver correlation was computed for the Bristol score (interobserver correlation coefficient, 0.88). For knee range of motion, flexion contracture, and extension lag there was high interobserver correlation (interobserver correlation coefficient > 0.8 each). For walking distance and walking aids, there also was a high interobserver correlation (interobserver correlation coefficient > 0.7 each). For clinical assessment of total knee arthroplasty, pain should be measured on a four-step system, the knee should be assessed by measurement of range of motion, extension lag, and flexion contracture, and function should be measured on a separate score assessing walking distance and walking aids.