A clinical method for locating the femoral head centre during total knee arthroplasty

Easy identification of mechanical axis during total knee arthroplasty

PURPOSE

We devised an intraoperatively identifiable mechanical axis (IIMA) as a reference of alignment in total knee arthroplasty (TKA).

MATERIALS AND METHODS

Between February 2010 and January 2011, primary TKAs were consecutively performed on 672 patients (1007 knees) using an IIMA as a reference in the coronal plane.

RESULTS

The alignment of the lower extremity improved from a mean of 11.4±6.7° (-10.3-34.4°) of varus preop. to 0.7±3.5° (-5.2-8.6°) immediately after surgery. Mean alignment of the femoral component in the coronal plane was 89.3±2.3° (83.4-97.2°) postop. and mean alignment of the tibial component was 90.4±2.2° (85.1-94.2°) postop.

CONCLUSION

This study showed that IIMA could be of considerable value as a new guider of alignment that is easily accessible and highly effective during total knee arthroplasty.

The position of the centre of the femoral head relative to the midline of the pelvis: a consistent landmark in total knee replacement surgery

BACKGROUND

Femoro-tibial malalignment in excess of 3° is a recognised contributor of early mechanical failure in total knee replacement (TKR). Knowledge of the location of the centre of the femoral head is a pre-requisite to identification of the mechanical axis of the femur and can facilitate optimal component orientation. We investigated variation in the location of the centre of the femoral head relative to the midline of the pelvis.

METHODS

We analysed the pelvic radiographs of 150 patients with unilateral total hip replacements. The perpendicular distance from the centre of the femoral head of the non-operated hip to the centre of pubic symphysis was measured.

RESULTS

The mean distance from the centre of the femoral head to the pubic symphysis was 89.2mm (standard deviation, 5.7 mm). Patient height strongly correlated with this distance (r=0.53, p<0.01), as did the diameter of the femoral head (r=0.59, p<0.01). The latter was significantly larger in men than in women (50.9 mm vs. 44.5mm, p<0.01).

CONCLUSION

The results demonstrate that the position of the centre of the femoral head has very little variability, irrespective of patient age or body-mass index. If the gender-specific mean femoral head to midline distance is used to estimate the location of the femoral head centre, a line from this point to the centre of the femoral condyles will deviate from the true mechanical axis by no more than 1.5°, in 98% of cases.

An alternative method to create extramedullary references in total knee arthroplasty

PURPOSE

Historically, extramedullary techniques have focused on finding the femoral head center and referred mainly to the anterosuperior iliac spine (the FHC method) in total knee arthroplasty (TKA). The purpose of this study was to evaluate a new extramedullary alignment system that uses neutral overall limb mechanical alignment as an extramedullary reference (the MA method).

METHODS

A retrospective review of 1018 TKAs (508 FHC group, 510 MA group) was performed to compare the radiographic results obtained using these two methods. There were 75 male patients and 577 female patients with varus osteoarthritis of the knee. Limb and prosthesis alignments in the coronal plane were investigated at 2 months after surgery.

RESULTS

The accuracies of postoperative alignments of implanted prostheses were better in the MA group than in the FHC group, as assessed by (1) overall limb alignment (MA group, varus 1.4° ± 2.7°; FHC group, varus 2.2° ± 2.9°), and (2) femoral component coronal alignment (MA group, 89.4° ± 2.3°; FHC group, 88.0° ± 2.7°). Overall limb alignment was within 0° ± 5° of the mechanical axis in 96.1% of the MA group and in 86.6% of the FHC group. Mean femoral component medial angle was within 0° ± 5° in 98.4% of the MA group and in 92.5% of the FHC group.

CONCLUSION

The accuracies of the postoperative radiographic alignments of implanted prostheses were found to be improved when the mechanical axis of the overall lower limb was used as an extramedullary reference. This new reference system would help to achieve correct limb and prostheses alignments during TKA.

LEVEL OF EVIDENCE

III.

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.

The analysis of risk factors in no thumb test in total knee arthroplasty

Background

We would like to analyze the risk factors of no thumb test among knee alignment tests during total knee arthroplasty surgery.

Methods

The 156 cases of total knee arthroplasty by an operator from October 2009 to April 2010 were analyzed according to preoperative indicators including body weight, height, degree of varus deformity, and patella subluxation and surgical indicators such as pre-osteotomy patella thickness, degree of patella degeneration, no thumb test which was evaluated after medial prepatella incision and before bone resection (1st test), no thumb test which was evaluated with corrective valgus stress (2nd test, J test), and the kind of prosthesis. We comparatively analyzed indicators affecting no thumb test (3rd test).

Results

There was no relation between age, sex, and body weight and no thumb test (3rd test). Patellar sulcus angle (p = 0.795), patellar congruence angle (p = 0.276) and preoperative mechanical axis showed no relationship. The 1st no thumb test (p = 0.007) and 2nd test (p = 0.002) showed significant relation with the 3rd no thumb test. Among surgical indicators, pre-osteotomy patella thickness (p = 0.275) and degeneration of patella (p = 0.320) were not relevant but post-osteotomy patellar thickness (p = 0.002) was relevant to no thumb test (3rd test). According to prosthesis, there was no significance with Nexgen (p = 0.575). However, there was significant correlation between Scorpio (p = 0.011), Vanguard (p = 0.049) and no thumb test (3rd test). Especially, Scorpio had a tendency to dislocate the patella, but Vanguard to stabilize the patella.

Conclusions

No thumb test (3rd test) is correlated positively with 1st test, 2nd test, and post-osteotomy patella thickness. Therefore, the more patella osteotomy and the prosthesis with high affinity to patellofemoral alignment would be required for correct patella alignment.

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.

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.