No abstract

Predictions from biomechanical models of gait may be sensitive to joint center locations. Most often, the hip joint center (HJC) is derived from locations of reflective markers adhered to the skin. Here, predictive techniques use regression equations of pelvic anatomy to estimate the HJC, whereas functional methods track motion of markers placed at the pelvis and femur during a coordinated motion. Skin motion artifact may introduce errors in the estimate of HJC for both techniques. Quantifying the accuracy of these methods is an area of open investigation. In this study, we used dual fluoroscopy (DF) (a dynamic x-ray imaging technique) and three-dimensional reconstructions from computed tomography (CT) images, to measure HJC locations in-vivo. Using dual fluoroscopy as the reference standard, we then assessed the accuracy of three predictive and two functional methods. Eleven non-pathologic subjects were imaged with DF and reflective skin marker motion capture. Additionally, DF-based solutions generated virtual markers placed on bony landmarks, which were input to the predictive and functional methods to determine if estimates of the HJC improved. Using skin markers, functional methods had better mean agreement with the HJC measured by DF (11.0 ± 3.3 mm) than predictive methods (18.1 ± 9.5 mm); estimates from functional and predictive methods improved when using the DF-based solutions (1.3 ± 0.9 and 17.5 ± 8.6 mm, respectively). The Harrington method was the best predictive technique using both skin markers (13.2 ± 6.5 mm) and DF-based solutions (10.6 ± 2.5 mm). The two functional methods had similar accuracy using skin makers (11.1 ± 3.6 and 10.8 ± 3.2 mm) and DF-based solutions (1.2 ± 0.8 and 1.4 ± 1.0 mm). Overall, functional methods were superior to predictive methods for HJC estimation. However, the improvements observed when using the DF-based solutions suggest that skin motion artifact is a large source of error for the functional methods.

Our results suggest that clinicians should consider using the Meyer lateral and 90° Dunn views to evaluate patients in whom cam FAI is suspected. However, the α angle and head-neck offset measurements from these and other plain film views could describe no more than half of the overall variation in the shape of the proximal femur and cam lesion. Thus, caution should be exercised when evaluating femoral head anatomy using the α angle and head-neck offset measurements from plain film radiographs. Given these findings, we believe there is merit in pursuing research that aims to develop the framework necessary to integrate statistical shape modeling into clinical evaluation, because this could aid in the diagnosis of cam FAI.

The proximal femur is abnormally shaped in patients with cam-type femoroacetabular impingement (FAI). Impingement may elicit bone remodeling at the proximal femur, causing increases in cortical bone thickness. We used correspondence-based shape modeling to quantify and compare cortical thickness between cam patients and controls for the location of the cam lesion and the proximal femur. Computed tomography images were segmented for 45 controls and 28 cam-type FAI patients. The segmentations were input to a correspondence-based shape model to identify the region of the cam lesion. Median cortical thickness data over the region of the cam lesion and the proximal femur were compared between mixed-gender and gender-specific groups. Median [interquartile range] thickness was significantly greater in FAI patients than controls in the cam lesion (1.47 [0.64] vs. 1.13 [0.22] mm, respectively; p<0.001) and proximal femur (1.28 [0.30] vs. 0.97 [0.22] mm, respectively; p<0.001). Maximum thickness in the region of the cam lesion was more anterior and less lateral (p<0.001) in FAI patients. Male FAI patients had increased thickness compared to male controls in the cam lesion (1.47 [0.72] vs. 1.10 [0.19] mm, respectively; p<0.001) and proximal femur (1.25 [0.29] vs. 0.94 [0.17] mm, respectively; p<0.001). Thickness was not significantly different between male and female controls. Clinical significance: Studies of non-pathologic cadavers have provided guidelines regarding safe surgical resection depth for FAI patients. However, our results suggest impingement induces cortical thickening in cam patients, which may strengthen the proximal femur. Thus, these previously established guidelines may be too conservative.

Femoroacetabular impingement syndrome (FAIS) may alter the kinematic function of the hip, resulting in pain and tissue damage. Previous motion analysis studies of FAIS have employed skin markers, which are prone to soft tissue artifact and inaccurate calculation of the hip joint center. This may explain why the evidence linking FAIS with deleterious kinematics is contradictory. The purpose of this study was to employ dual fluoroscopy (DF) to quantify in vivo kinematics of patients with cam FAIS relative to asymptomatic, morphologically normal control participants during various activities. Eleven asymptomatic, morphologically normal controls and seven patients with cam FAIS were imaged with DF during standing, level walking, incline walking, and functional range of motion activities. Model‐based tracking calculated the kinematic position of the hip by registering projections of three‐dimensional computed tomography models with DF images. Patients with FAIS stood with their hip extended (mean [95% confidence interval], −2.2 [−7.4, 3.1]°, flexion positive), whereas controls were flexed (5.3 [2.6, 8.0]°; p = 0.013). Male patients with cam FAIS had less peak internal rotation than the male control participants during self‐selected speed level‐walking (−0.2 [−6.5, 6.1]° vs. −9.8 [−12.2, −7.3]°; p = 0.007) and less anterior pelvic tilt at heel‐strike of incline (5°) walking (3.4 [−1.0, −7.9]° vs. 9.8 [6.4, 13.2]°; p = 0.032). Even during submaximal range of motion activities, such as incline walking, patients may alter pelvic motion to avoid positions that approximate the cam lesion and the acetabular labrum. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:823‐833, 2020

Hip joint center (HJC) measurement error can adversely affect predictions from biomechanical models. Soft tissue artifact (STA) may exacerbate HJC errors during dynamic motions. We quantified HJC error and the effect of STA in 11 young, asymptomatic adults during six activities. Subjects were imaged simultaneously with reflective skin markers (SM) and dual fluoroscopy (DF), an x-ray based technique with submillimeter accuracy that does not suffer from STA. Five HJCs were defined from locations of SM using three predictive (i.e., based on regression) and two functional methods; these calculations were repeated using the DF solutions. Hip joint center motion was analyzed during six degrees-of-freedom (default) and three degrees-of-freedom hip joint kinematics. The position of the DF-measured femoral head center (FHC), served as the reference to calculate HJC error. The effect of STA was quantified with mean absolute deviation. HJC errors were (mean±SD) 16.6±8.4 mm and 11.7±11.0 mm using SM and DF solutions, respectively. HJC errors from SM measurements were all significantly different from the FHC in at least one anatomical direction during multiple activities. The mean absolute deviation of SM-based HJCs was 2.8±0.7 mm, which was greater than that for the FHC (0.6±0.1 mm), suggesting that STA caused approximately 2.2 mm of spurious HJC motion. Constraining the hip joint to three degrees-of-freedom led to approximately 3.1 mm of spurious HJC motion. Our results indicate that STA-induced motion of the HJC contributes to the overall error, but inaccuracies inherent with predictive and functional methods appear to be a larger source of error.

Weightbearing computed tomography (WBCT) enables visualization of the foot and ankle as patients stand under load. Clinical measurements of WBCT images are generally limited to two‐dimensions, which reduces the ability to quantify complex morphology of individual osseous structures as well as the alignment between two or more bones. The shape and orientation of the healthy/normal subtalar joint, in particular, is not well‐understood, which makes it very difficult to diagnose subtalar pathoanatomy. Herein, we employed statistical shape modeling to evaluate three‐dimensional (3D) shape variation, coverage, space, and congruency of the subtalar joint using WBCT data of 27 asymptomatic healthy individuals. The four most relevant findings were: (A) talar and calcaneal anatomical differences were found regarding the presence of (a) the talar posterior process, (b) calcaneal pitch, and (c) curvature of the calcaneal posterior facet; (B) the talar posterior facet articular surface area was significantly greater than the calcaneal posterior facet articular surface area; (C) the posterior facet varied in joint space distance, whereas the anteromedial facet was even; and (D) the posterior and anteromedial facet of the subtalar joint was consistently congruent. Despite considerable shape variation across the population, the posterior and anteromedial articular facets of the subtalar joint were consistently congruent. Results provide a detailed 3D analysis of the subtalar joint under a weightbearing condition in a healthy population which can be used for comparisons to pathological patient populations. The described SSM approach also shows promise for clinical evaluation of the subtalar joint from 3D surface reconstructions of WBCT images.

The minimum IFS during dynamic activities was smaller than axial MRI measurements. Compared with male participants, the IFS in female participants was reduced during standing and walking, despite a lack of kinematic differences between the sexes. The relationship between the IFS and hip joint angles suggests that the hip should be placed into greater extension, adduction, and external rotation in clinical examinations and imaging, as the IFS measured from static images, especially in a neutral orientation, may not accurately represent the minimum IFS during dynamic motion. Nevertheless, this statement must be interpreted with caution, as only asymptomatic participants were analyzed herein.