Translatory hip kinematics measured with optoelectronic surgical navigation

Association Between Hip Translation and Hip Rotation and Anatomy: A Pilot Quasi-static MRI Study

Background

There is little known about translation of the hip and the relationship with hip rotation and morphology in asymptomatic patients.

Hypotheses

(1) Femoral head would exhibit significant translations in asymptomatic hips, (2) femoral head translations would correlate to femoral rotations, and (3) range of femoral head translations would correlate to hip morphology.

Study Design

Cross-sectional study; Level of evidence, 3.

Methods

A total of 11 individuals (age, 23-47 years; 64% female) with asymptomatic hips underwent hip magnetic resonance imaging (MRI) in the following postures: neutral (supine), midflexion, maximum-flexion, internal rotation, internal rotation + midflexion, internal rotation + maximum-flexion, adduction, flexion-abduction-external rotation (FABER), extension, and lateral abduction. All rotations were passive. MRI-generated 3-dimensional hip models were used to quantify femoral rotations and translations. Femoral head diameter, acetabular diameter, lateral center-edge angle, alpha angle, femoral anteversion, acetabular version and inclination, and neck-shaft angle were measured from MRI. A t test was used if measured translations were statistically significant. Linear regression was used to assess the associations between translation and rotation. Pearson correlation was used to assess the relationships between hip anatomy and range of femoral head translations.

Results

In all tested positions, the femoral head translated anteriorly by 2 ± 1 mm (maximum 5 mm, P < .001), posteriorly by 1 ± 1 mm (maximum 6 mm, P < .001), superiorly by 2 ± 2 mm (maximum 7 mm, P < .001), inferiorly by 2 ± 2 mm (maximum 6 mm, P < .001), laterally by 1 ± 1 mm (maximum 4 mm, P < .001), and medially by 2 ± 1 mm (maximum 5 mm, P < .001), relative to the rested supine position. Femoral flexion was associated with posterior translation of the femoral head (P = .038). Femoral abduction was associated with medial translation of the femoral head (P = .042). Higher femoral anteversion and smaller alpha angle were associated with a higher total magnitude of femoral head translation in the anterior-posterior direction (P < .04). Smaller femoral anteversion, higher acetabular inclination, smaller lateral center-edge angle, and lower neck-shaft angle were associated with a higher total magnitude of femoral head translation in the superior-inferior direction (P ≤ .03).

Conclusion

Our study demonstrated that, during passive physiologic movement, asymptomatic hips on average translated up to 2 mm (with up to 7 mm maximum translation in some positions), which is potentially related to hip rotations and morphology. Further investigations are warranted to understand the normal and pathologic hip translations and their impact on hip function (ie, instability and impingement).

Experimental modification of morphology reveals the effects of the zygosphene-zygantrum joint on the range of motion of snake vertebrae

Variation in joint shape and soft tissue can alter range of motion (ROM) and create trade-offs between stability and flexibility. The shape of the distinctive zygosphene-zygantrum joint of snake vertebrae has been hypothesized to prevent axial torsion (twisting), but its function has never been tested experimentally. We used experimental manipulation of morphology to determine the role of the zygosphene-zygantrum articulation by micro-computed tomography (μCT) scanning and 3D printing two mid-body vertebrae with unaltered shape and with the zygosphene digitally removed for four species of phylogenetically diverse snakes. We recorded the angular ROM while manipulating the models in yaw (lateral bending), pitch (dorsoventral bending) and roll (axial torsion). Removing the zygosphene typically increased yaw and dorsal pitch ROM. In the normal vertebrae, roll was <2.5 deg for all combinations of pitch and yaw. Roll increased in altered vertebrae but only for combinations of high yaw and ventral pitch that were near or beyond the limits of normal vertebra ROM. In the prairie rattlesnake and brown tree snake, roll in the altered vertebrae was always limited by bony processes other than the zygosphene, whereas in the altered vertebrae of the corn snake and boa constrictor, roll ROM was unconstrained when the pre- and post-zygapophyses no longer overlapped. The zygosphene acts as a bony limit for yaw and dorsal pitch, indirectly preventing roll by precluding most pitch and yaw combinations where roll could occur and potentially allowing greater forces to be applied across the vertebral column than would be possible with only soft-tissue constraints.