Femoral anatomy described in cylindrical coordinates using computed tomography

Measurement of tibial tuberosity-trochlear groove distance by MRI: assessment and correction of knee positioning errors

OBJECTIVE

The tibial tuberosity-trochlear groove (TTTG) distance varies with the position of the knee in the MR or CT scanner. We present and assess a simple method for adjustment of adduction or abduction of the knee.

MATERIALS AND METHODS

MRI of the knee encompassing a three-dimensional (3D) sagittal sequence including ≥ 8 cm of the proximal tibia was analyzed (29 females, 17 males; median age 45 years). Using 3D visualization software, the central longitudinal axis of the proximal tibia (TA) was constructed, and the TTTG distance was measured before and after alignment of the TA. Observer reliability was assessed with inter- and intra-class correlation coefficient (ICC) and Bland-Altman plots.

RESULTS

Adduction of the knee occurred in 26 examinations, mean 2.7° (range 0.0° to 9.4°), and abduction in 20 examinations, mean 2.6° (range 0.0° to 7.2°). Following adjustment, the mean TTTG distance increased 2.4 mm (range 0.0 to 6.7 mm) in the knees positioned in adduction and decreased 2.3 mm when in abduction (range 0.0 to 5.5 mm). The correlation coefficient (r²) between the deviation in adduction and abduction and the difference between TTTG unadjusted and adjusted was r² = 0.96. ICCs were excellent, but limits of agreement were close to ± 3 mm.

CONCLUSION

Measurement of the TTTG distance by MRI is influenced by a systematic technique-dependent error caused by knee positioning in adduction or abduction. We suggest a simple method for adjusting the positioning.

Femoral anteversion: significance and measurement

Femoral neck anteversion (FNA) is the angle between the femoral neck and femoral shaft, indicating the degree of torsion of the femur. Differences in FNA affect the biomechanics of the hip, through alterations in factors such as moment arm lengths and joint loading. Altered gait associated with differences in FNA may also contribute to the development of a wide range of skeletal disorders including osteoarthritis. FNA varies by up to 30° within apparently healthy adults. FNA increases substantially during gestation and thereafter decreases steadily until maturity. There is some evidence of a further decrease at a much lower rate during adulthood into old age, but the mechanisms behind it have never been studied. Development of FNA appears to be strongly influenced by mechanical forces experienced during everyday movements. This is evidenced by large differences in FNA in groups where movement is impaired, such as children born breech or individuals with neuromuscular conditions such as cerebral palsy. Several methods can be used to assess FNA, which may yield different values by up to 20° in the same participant. While MRI and CT are used clinically, limitations such as their cost, scanning time and exposure to ionising radiation limit their applicability in longitudinal and population studies, particularly in children. More broadly, applicable measures such as ultrasound and functional tests exist, but they are limited by poor reliability and validity. These issues highlight the need for a valid and reliable universally accepted method. Treatment for clinically problematic FNA is usually de-rotational osteotomy; passive, non-operative methods do not have any effect. Despite observational evidence for the effects of physical activity on FNA development, the efficacy of targeted physical activity remains unexplored. The aim of this review is to describe the biomechanical and clinical consequences of FNA, factors influencing FNA and the strengths and weaknesses of different methods used to assess FNA.

Femoral and tibial torsion measurements in children and adolescents: comparison of MRI and 3D models based on low-dose biplanar radiographs

OBJECTIVE

The aim of our study was to evaluate the reliability and interchangeability of femoral (FT) and tibial torsion (TT) measurements in children using magnetic resonance (MR) imaging compared to measurements on 3D models based on biplanar radiographs (BPR).

MATERIALS AND METHODS

FT and TT were measured in 60 children (mean age 10.1 years; range 6.2-16.2 years; 28 female) using axial MR images by two readers. MR measurements were compared to measurements based on BPR-3D models by two separate independent readers. Interreader and intermethod agreements were calculated using descriptive statistics, the intraclass correlation coefficient (ICC), and Bland-Altman analysis.

RESULTS

FT/TT was -8.4°-54.1°/0°-45.9° on MR images and -13°-63°/4°-52° for measurements on BPR-3D models. The median of difference between the two methods was -0.18° (range -13.6°-19.1°) for FT and -0.20° (range -18.4°-9.5°) for TT, respectively. Interreader agreement (ICC) of FT/TT measurements was 0.98/0.96 on MR images and 0.98/0.94 on BPR 3D models. Intermethod agreement (ICC) for MR measurements was 0.95 [95% confidence interval (CI), 0.93-0.96] for FT and of 0.86 (CI, 0.24-0.95) for TT. Mean interreader differences at MR were 3.1° (0.0°-8.0°) for FT and 3.2° (0.1°-9.5°) for TT. On Bland-Altman plots all measurements were within the 95% limit of agreement (-10.8°; 11.5° for FT; -14.6°; 4.2°) for TT-except for five measurements of FT and six measurements of TT.

CONCLUSION

FT measurements on MR images are comparable to measurements using BPR-3D models. TT measurements differ between the two modalities, but the discrepancy is comparable to measurement variations between CT and BPR.

Pitfalls in Ultrasonography of Hip Joint Synovitis in the Child

The normal and abnormal anatomy of the hip joint in childhood was evaluated by ultrasonography (US) with reference to examination technique and errors. When compared with computed tomography (CT), it was confirmed that the two sonographic echoes from the hip joint represent the femoral neck and the external surface of the capsule and that the thickness of the echo received from the capsule depends on the frequency of the transducer. Partial volume effects may simulate or increase the sonographic appearance of hip joint effusion. The normal relationship between joint capsule and femoral neck was established.

Measurement of femoral neck anteversion in 3D. Part 1: 3D imaging method

Femoral neck anteversion is the torsion of the femoral head with reference to the distal femur. Conventional methods that use cross-sectional computed tomography (CT), magnetic resonance or ultrasound images to estimate femoral anteversion have met with several problems owing to the complex, three-dimensional (3D) structure of the femur. These problems include not only the difficulty of defining the direction of the femoral neck axis and condylar line but also the dependency upon patient positioning. In particular, the femoral neck axis, the direction of the femoral head, known as the major source of error, is difficult to determine from either a single or several two-dimensional (2D) cross-sectional images. A new method has been devised for the measurement of femoral anteversion using the 3D imaging technique. 3D reconstructed CT images from the femoral head and trochanter to the distal femur are used to measure the anteversion. It is necessary to remove the soft tissue from the CT images and extract just the bone part. Then, the femoral anteversion is measured from a computer-rendered femur image. The 3D imaging method is compared with both the conventional 2D method and the physical method using 20 dried femurs. For the physical method, which is used as a reference value, a special apparatus is devised. The average difference between the results of the physical method and those of the 2D CT method is 5.33 degrees. The average difference between the results of the physical method and those of the 3D imaging method is 0.45 degrees. Seventy-four patients, who suffer from toe-in-gait disease, are tested to compare the 3D imaging method with the conventional 2D CT method. The average difference between the 2D and 3D methods is 8.6 degrees, and the standard is 7.43 degrees. This method provides a very accurate and reliable measurement of femoral anteversion, as it is virtually equivalent to the direct measurement of bisected dried femur in vitro.

The effect of femoral rotation on the projected femoral neck-shaft angle

Femoral neck-shaft angle (NSA) was measured in a series of anteroposterior (AP) hip radiographs of a cadaveric femur in varying degrees of rotation. A mathematical model was developed to predict NSA on an AP radiograph in varying degrees of femoral rotation. The predictions of the model were found to correlate well with the experimental data (correlation coefficient = 0.94). Based on the mathematical model, a wide range of patient positioning was found to result in a <10 degrees error in the measurement of femoral NSA. Coxa vara, increased femoral anteversion, cerebral palsy, and developmental dislocation of the hip resulted in a more restricted range of acceptable femoral positioning. External rotation of the femur should be avoided during patient positioning because as little as 7 degrees can cause a > 10 degrees change in the apparent NSA. For all patient populations, internally rotating the femur will allow for determination of the femoral NSA to within 10 degrees .

Computed tomography and ultrasonography for diagnosis of hip joint effusion in children

Nineteen consecutive children with transient synovitis of the hip were examined for intracapsular joint effusion using computed tomography (CT) and ultrasonography (US). The two techniques were highly correlated in measurements of the anterior displacement of the joint capsule, indicating both to be reliable diagnostic tools. Such an effusion is clinically important because of the increase in intracapsular pressure with ensuing disturbance in the vascular supply to the proximal femoral epiphysis, as demonstrated experimentally and clinically in transient synovitis.