Magnetic Resonance Imaging of the Globe-Tendon Interface for Extraocular Muscles: Is There an "Arc of Contact"?

Functional anatomy of the orbit in strabismus surgery: Connective tissues, pulleys, and the modern surgical implications of the "arc of contact" paradigm

Oculomotricity is a multidimensional domain characterised by a delicate interplay of anatomical structures and physiological processes. This manuscript meticulously dissects the nuances of this interplay, bringing to the fore the integral role of the extraocular muscles (EOMs) and their intricate relationship with the myriad orbital connective tissues as it harmoniously orchestrates binocular movements, ensuring synchronised and fluid visual tracking. Historically, the peripheral oculomotor apparatus was conceptualised as a rudimentary system predominantly driven by neural directives. While widely accepted, this perspective offered a limited view of the complexities inherent in ocular movement mechanics. The twentieth century heralded a paradigm shift in this understanding. With advances in anatomical research and imaging techniques, a much clearer picture of the gross anatomy of the EOMs emerged. This clarity challenged traditional viewpoints, suggesting that the inherent biomechanical properties of the EOMs, coupled with their associated tissue pulleys, play a pivotal role in dictating eye movement dynamics. Central to this revised understanding is the "arc of contact" paradigm. This concept delves deep into the mechanics of eye rotation, elucidating the significance of the point of contact between the EOMs and the eyeball. The arc of contact is not just a static anatomical feature; its length and orientation play a crucial role in determining the effective torque generated by a muscle, thereby influencing the amplitude and direction of eye rotation. The dynamic nature of this arc, influenced by the position and tension of the muscle pulleys, offers a more comprehensive model for understanding ocular kinematics. Previously overlooked in traditional models, muscle pulleys have now emerged as central players in the biomechanics of eye movement. These anatomical structures, formed by dense connective tissues, guide the paths of the EOMs, ensuring that their pulling angles remain optimal across a range of gaze directions. The non-linear paths resulting from these pulleys provide a more dynamic and intricate understanding of eye movement, challenging two-dimensional, linear models of orbital anatomy. The implications of these revelations extend beyond mere theoretical knowledge. The insights garnered from this research promise transformative potential in the realm of strabismus surgery. Recognising the pivotal role of muscle pulleys and the "arc of contact" paradigm allows for more precise surgical interventions, ensuring better post-operative outcomes and minimising the risk of complications. Surgical procedures that previously relied on basic mechanical principles now stand to benefit from a more nuanced understanding of the underlying anatomical and physiological dynamics. In conclusion, this manuscript serves as a testament to the ever-evolving nature of scientific knowledge. Challenging established norms and introducing fresh perspectives pave the way for more effective and informed clinical interventions in strabismus surgery.

Revisiting Posterior Fixation Sutures Surgery: Unveiling Novel Approaches for Primary Management of Diverse Esotropia Cases

INTRODUCTION

Strabismus, specifically esotropia, presents a significant challenge in ophthalmic surgery, while several treatment options exist. This study aims to evaluate the results of posterior fixation sutures (PFS) on the medial rectus as a primary approach for some types of esotropia.

METHODS

The medical records of consecutive patients who underwent surgery for esotropia over 11 years and had at least 1 year of follow-up were reviewed retrospectively. Patients were classified into one of three types of deviation: infantile (IE), partially accommodative (PAE) and basic (BE) esotropias. An alignment within 16 prism diopters (PD) of orthotropia was a successful outcome.

RESULTS

A total of 404 patients were included: 67 IE, 180 PAE and 157 BE. Before surgery, a deviation greater than 30 PD was present in 88.1% and 80.1%, and a deviation greater than 50 PD was present in 66.5% and 52.9% of patients (near and distance, respectively). In the BE group, PFS was the baseline surgery in a smaller number of cases (75%) compared to the other two groups (versus 86.6% [IE] and 88.3% [PAE], p = 0.002). The need for an additional procedure was significantly higher in the infantile esotropia group (44.8% vs. 18.9% and 24.8%, p < 0.001). Final surgical success was achieved in 95.3% of all patients. Orthotropia was achieved in 19.4% (IE), 29.6% (PAE) and 25.5% (BE) of cases.

CONCLUSION

PFS of the medial rectus without recession proved successful as a first-line procedure for esotropia in the subtypes of patients evaluated in this study.

Human orbital muscle in adult cadavers and near-term fetuses: its bony attachments and individual variation identified by immunohistochemistry

PURPOSE

To compare fetal and adult morphologies of the orbital muscle (OM) and to describe the detailed topographical anatomy in adults.

METHODS

Using unilateral orbits from 15 near-term fetuses and 21 elderly cadavers, semiserial horizontal or sagittal paraffin sections were prepared at intervals of 20-100 µm. In addition to routine histology, we performed immunohistochemistry for smooth muscle actin.

RESULTS

At near term, the OM consistently extended widely from the zygomatic bone or the greater wing of the sphenoid to the maxilla or ethmoid. Thus, it was a large sheet covering the future inferior orbital fissure. In contrast, the adult OM was a thin and small muscle bundle connecting (1) the greater wing of the sphenoid to the maxilla (11/19 cadavers), (2) the lesser wing of the sphenoid to the maxilla (5/19) or the greater wing (3/19). The small OM was likely to be restricted within the greater wing (5/19 cadavers) or the maxilla (3/19). Two of these five types of OM coexisted in eight orbits. OM attachment to the lesser wing was not seen in fetuses, whereas ethmoid attachment was absent in adults.

CONCLUSIONS

The lesser wing attachment of the OM seemed to establish after birth. A growing common origin of the three recti was likely involved in "stealing" the near-term OM attachment from the ethmoid. The strong immunoreactivity of remnant-like OM in the elderly suggests that OM contraction is still likely to occur against the increased flow through a thin vein. However, the contraction might have no clinical significance.

MR-Eye: High-Resolution Microscopy Coil MRI for the Assessment of the Orbit and Periorbital Structures, Part 2: Clinical Applications

In the first part of this 2-part series, we described how to implement microscopy coil MR imaging of the orbits. Beyond being a useful anatomic educational tool, microscopy coil MR imaging has valuable applications in clinical practice. By depicting deep tissue tumor extension, which cannot be evaluated clinically, ophthalmic surgeons can minimize the surgical field, preserve normal anatomy when possible, and maximize the accuracy of resection margins. Here we demonstrate common and uncommon pathologies that may be encountered in orbital microscopy coil MR imaging practice and discuss the imaging appearance, the underlying pathologic processes, and the clinical relevance of the microscopy coil MR imaging findings.

The Effect of Axial Length on Extraocular Muscle Leverage

PURPOSE

Magnetic resonance imaging was used to determine the effect of axial length (AL) on globe rotational axis and horizontal extraocular muscle leverage during horizontal duction.

DESIGN

Prospective observational case series.

METHODS

At a single academic center, 36 orthophoric adults with a wide range of ALs underwent high-resolution axial orbital magnetic resonance imaging in target-controlled adduction and abduction. ALs were measured in planes containing maximum globe cross-sections. Area centroids were calculated to determine globe centers. Rotational axes in orbital coordinates were calculated from displacements of lens centers and globe-optic nerve attachments. Lever arms were calculated as distances between published extraocular muscle insertions and rotational axes.

RESULTS

ALs averaged 26.3 ± 0.3 mm (standard error [range 21.5-33.4 mm]). Rotational axes from adduction to abduction averaged 1.1 ± 0.2 mm medial and 1.1 ± 0.2 mm anterior to the globe's geometric center in adduction. Linear regression demonstrated no significant correlation between AL and rotational axis horizontal (R² = 0.06) or anteroposterior (R² = 0.07) position. Medial rectus (MR) lever arms averaged 12.0 ± 0.2 mm and lateral rectus (LR) lever arms averaged 12.8 ± 0.2 mm. Both MR (R² = 0.24, P < .001) and LR (R² = 0.32, P < .001) lever arms significantly increased by about 0.3 mm per 1.0-mm of increased AL, with a corresponding reduction in predicted per-millimeter effect of surgical repositioning of their insertions.

CONCLUSIONS

Regardless of AL, the globe rotates about a point nasal and anterior to its geometric center, giving the LR more leverage than the MR. This eccentricity may diminish the effect of tendon repositioning in moderate to highly myopic patients, with reductions in per-mill imeter dose/response predicted with longer AL.

Binocular discrepancy in lateral rectus muscle attachment in intermittent exotropia with eye dominance

PURPOSE

To determine whether there is asymmetry in the lateral rectus (LR) muscle attachment between both eyes in patients with intermittent exotropia (IXT) with a non-dominant eye, but without amblyopia or anisometropia.

METHODS

In total, 109 patients who underwent bilateral lateral rectus recession for IXT were included, 81 with and 28 without eye dominance. The limbus-insertion distance and tendon width of the LR muscle were measured intraoperatively using callipers. The insertion-equator distance (presumed arc of contact), area of contact (arc of contact × tendon width), and torque value (radius of globe × arc of contact) of the LR muscle were calculated based on intraoperative measurements and axial length measured using a partial interferometer. Parameters regarding LR muscle attachment were compared between fellow eyes and between groups.

RESULTS

Mean measurements in all parameters related to LR muscle attachments other than tendon width were not different between the two eyes or between groups. The mean tendon width of the non-dominant eye was 9.2 ± 0.7 mm, narrower than the 9.4 ± 0.5 mm width in either eye of patients without dominance (p = 0.020). However, there was no difference in all parameters in 21 pairs of patients after matching. The proportion of patients who showed binocular discrepancies in attachment measurements beyond that attributable to potential measuring errors did not differ between the two groups.

CONCLUSIONS

Structural parameters related to LR muscle attachments did not differ based on eye dominance, suggesting that the anatomic structure of LR muscle attachments is not responsible for eye dominance in IXT.

MR-Eye: High-Resolution Microscopy Coil MRI for the Assessment of the Orbit and Periorbital Structures, Part 1: Technique and Anatomy

Microscopy coil MR imaging of the orbits has been described previously as a technique for anatomic depiction. In the first part of this 2-part series, the improvement in spatial resolution that the technique offers compared with conventional MR imaging of the orbits is demonstrated. We provide a guide to implementing the technique, sharing pearls and pitfalls gleaned from our own practice to make implementation of microscopy coil MR imaging at your own center easy. As a quick reference guide to the small-scale structures encountered when reading the studies, a short anatomy section is included, which doubles as a showcase for the high-quality imaging that can be obtained. In the second part, our experience of microscopy coil MR imaging in day-to-day clinical practice takes it far beyond being a useful anatomic educational tool. Through a series of interesting cases, we highlight the added benefit of microscopy coil MR imaging compared with standard orbital MR imaging.