Is distal femoral torsion the same in both of a patient's legs? Morphometric CT study

Femoral condyle bone mineral density in osteoarthritis differs significantly between knees with valgus vs. varus deformity

BACKGROUND

The objective of this study was to measure bone mineral density (BMD) of the cancellous bone in both femoral condyles and to compare the results according to the hip-knee-ankle (HKA) angle in patients with knee osteoarthritis.

HYPOTHESIS

BMD of cancellous bone in the medial condyle is markedly lower in valgus knees compared to that in the lateral condyle in varus knees.

METHODS

Consecutive patients with computed tomography (CT) of the knee and long-leg radiographs obtained in preparation for total knee arthroplasty were included. The 189 knees were divided into five groups based on whether the hip-knee-ankle angle was<170° (major varus deformity), 171°-177° (varus deformity), 178°-182° (normal alignment), 183°-189° (valgus deformity), and>190° (major valgus deformity). A protocol for CT measurement of BMD values at the femoral condyles was developed. Correlations between the HKA angle and BMD were assessed using the ratio of medial-to-lateral condyle BMD values (M/L).

RESULTS

M/L was lower for knees with valgus deformity than for normally aligned knees (0.7 vs. 1, p<0.001). This difference was larger in the group with major valgus deformity, with a mean M/L value of 0.5 (p<0.001). M/L was higher for knees with major varus (mean, 1.2; p=0.035). The correlation coefficients showed excellent intra-observer and inter-observer agreement for the BMD measurements.

CONCLUSION

The BMD values of the femoral condyles correlate with the HKA angle. BMD is lower at the medial femoral condyle of valgus knees, particularly when the deformity exceeds 10°. This finding may deserve consideration when planning total knee arthroplasty.

LEVEL OF EVIDENCE

IV; retrospective study.

Patellar medial-lateral position can be used to correct the effect of leg rotation on preoperative planning in total knee arthroplasty for varus knees

BACKGROUND

Lower limb malrotations can be observed in long leg radiographs, affecting the measurement of the angle between the mechanical and anatomical axes. The purposes were to analyze the effect of limb rotation and to evaluate the accuracy of the corrected angle between the mechanical and anatomical axes based on the patellar ML position.

HYPOTHESIS

The hypothesis was that the correction of the angle between the mechanical and anatomical axes according to the patellar ML position can reduce the error from the angle in the true AP view in most of the knees.

PATIENTS AND METHODS

A total of 100 consecutive knees with varus deformity undergoing primary total knee arthroplasty were included. Computed tomography images were digitally reconstructed in the neutral position, and internally and externally rotated at 10° and 20°, respectively. The patellar ML position relative to the medial (0%) and lateral (100%) epicondyles and the angle between the mechanical and anatomical axes of the femur were measured. The corrected angle between the mechanical and anatomical axes was calculated using the averaged translational ratio.

RESULTS

In the neutral position, the patellar center position was 56.1% (standard deviation [SD]=4.7%), which was 31.4% (SD=7.2%) and 80.2% (SD=5.4%) in the 20° internal and external rotation, respectively. The angle between the mechanical and anatomical axes was 2.6° (SD=2.0°) and 8.1° (SD=2.1°) in the 20° internal and external rotation, respectively. On average, if the patellar center shifted 10%, the change of the angle between the mechanical and anatomical axes of the femur was 1.13°. Applying the corrected angle, a discrepancy from the neutral position decreased.

CONCLUSION

The method to correct the angle between the mechanical and anatomical axes according to the patellar ML position can be used to reduce the measurement error for preoperative planning using a long leg radiograph.

LEVEL OF EVIDENCE

III.

Is lateral femoral condyle hypoplasia a feature of genu valgum? A morphological computed tomography study of 200 knees

BACKGROUND

Young surgeons are taught that genu valgum deformity is associated with hypoplasia of the lateral femoral condyle, despite the absence of supporting evidence. The objective of this study was to determine whether the lateral condyle was hypoplastic in genu valgum, by assessing the morphological features of the distal femur and their variations according to the severity of the coronal deformity.

HYPOTHESIS

The lateral femoral condyle is not hypoplastic in genu valgum deformity.

MATERIAL AND METHODS

The 200 included patients treated by unilateral total knee arthroplasty were divided into five groups based on their preoperative hip-knee-ankle (HKA) angle. The HKA angle, valgus cut angle (VCA), and anatomical lateral distal femoral angle (aLDFA) were measured on long-leg radiographs. Computed tomography images were then used to measure the medial and lateral anterior-posterior condylar lengths (mAPCL and lAPCL) and condylar thicknesses (mCT and lCT), distal femoral torsion (DFT), medial and lateral posterior condylar heights (mPCH and lPCH), and medial and lateral condylar volumes (mCV and lCV).

RESULTS

No significant differences were demonstrated across the five mechanical-axis groups for mAPCL, lAPCL, mCT, lCT, mPCH, or lPCH. The groups differed significantly regarding the VCA (p<0.0001), aLDFA (p<0.0001), DFT (p<0.0001), and mCV/lCV ratio (p<0.0001). VCA and aLDFA were smaller when the valgus exceeded 10°. DFT was similar in all varus knees (2.2°-2.6°) but significantly greater for knees with moderate (4.0°) or severe (6.2°) valgus. Finally, lCV was greater than mCV in valgus knees compared to varus knees.

CONCLUSION

The presence of lateral condyle hypoplasia in knees with genu valgum seems open to question. Apparent hypoplasia noted during the standard physical examination may be chiefly ascribable to distal valgus of the femoral epiphysis in the coronal plane and, with the knee flexed, to distal epiphyseal torsion, whose severity increases with the degree of valgus deformity. These considerations should be taken into account when performing distal femoral cuts for TKA, to ensure the restoration of normal anatomy in patients with genu valgus.

LEVEL OF EVIDENCE

IV.

The use of an asymmetrical tibial tray in TKA optimises tibial rotation when fitted to the posterior tibial plateau border

PURPOSE

The aim of this study was to evaluate the suitability of positioning an asymmetrical tibial tray relative to the posterior tibial edge and to analyse the relationship between the posterior fit and tibial rotation after computer-assisted total knee arthroplasty (TKA). It was hypothesised that an asymmetrical tray would adjust to the posterior border of the tibial plateau with proper tibial rotation.

METHODS

Ninety-three consecutive knees underwent total knee arthroplasty using a Persona fixed-bearing system (63 varus deformities and 30 valgus deformities) and a 3-month follow-up CT scan. An independent examiner measured different variables: the femoral angle between the clinical epicondylar axis and the posterior condylar line of the femoral component, the tibial angle between the posterior borders of the tibial tray and the tibial plateau, and the tibial rotation with respect to the femoral component. These measurements were also compared between varus and valgus subgroups.

RESULTS

For the varus and valgus subgroups, the mean postoperative femoral angle was 2.1º ± 1.2º and 2.5º ± 1.0º, respectively (n.s.). The mean posterior fitting angle of the tibial tray was 0.1º ± 2.4º and 1.4º ± 3.2º for the varus and valgus subgroups, respectively, with a significant difference between groups (p = 0.03). The tibial rotations with respect to the femoral component for the varus and valgus groups were 0.9º ± 3.3º and 2.2º ± 3.1º of external rotation, respectively (n.s.).

CONCLUSIONS

This study demonstrated that fitting an asymmetrical tibial tray to the posterior border of the tibial plateau could optimise tibial rotation. The posterior border was considered to be a reliable and easily identifiable landmark for proper tibial rotation and coverage during a primary TKA.

LEVEL OF EVIDENCE

IV.

The effect of using different coordinate systems on in-vivo hip angles can be estimated from computed tomography images

Measurements of hip kinematics inherently depend on the coordinate system in which they are derived, yet the effect of the coordinate system definition on calculations of hip angles is not well-understood. Herein, hip angles calculated during dynamic activities were compared using coordinate systems described in the literature. In-vivo kinematic data of 24 participants (13 males) were analyzed during gait and the anterior impingement test using dual fluoroscopy and model-based tracking. Two coordinate systems for the pelvis (anterior pelvic plane, International Society of Biomechanics [ISB]) and three coordinate systems for the femur (table top plane with two definitions of the superior-inferior axis, ISB) were evaluated. Bony landmarks visible on computed tomography (CT) images were identified to establish each coordinate system and used as the basis to calculate differences in hip angles between coordinate system pairs. In the analysis during gait, the maximum differences derived from various coordinate system definitions were 6.7° ± 5.5° for flexion, 7.7° ± 2.1° for rotation, and 5.5° ± 0.7° for adduction. For the anterior impingement test, the differences were 8.1° ± 5.9°, 7.1° ± 1.2°, and 5.3° ± 0.7°, respectively. Landmark-based analysis using CT images could estimate these dynamic differences with errors less than 1.0°. Our results indicate that hip angles can be accurately transformed to angles calculated in different coordinate systems by accounting for the inherent bony anatomy. This information may aid in the interpretation of results across biomechanical studies of the hip.