Recreation of eyelid mechanics using the sling concept✰

Sling concept for restoring eye closure

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Recreation of eyelid mechanics using the sling concept Shaheen Hasmat MD BMed , Shaun McPherson BMed , Gregg J. Suaning BSc MSc PhD , Nigel H. Lovell BE PhD , Tsu-Hui (Hubert) Low MBBS BSc Med FRACS , Jonathan R. Clark MBBS BSc (Med) FRACS MBiostat PII: DOI: Reference:

S1748-6815(20)30002-4 https://doi.org/10.1016/j.bjps.2019.12.007 PRAS 6377

To appear in:

Journal of Plastic, Reconstructive & Aesthetic Surgery

Received date: Accepted date:

27 April 2018 30 December 2019

Please cite this article as: Shaheen Hasmat MD BMed , Shaun McPherson BMed , Gregg J. Suaning BSc MSc PhD , Nigel H. Lovell BE PhD , Tsu-Hui (Hubert) Low MBBS BSc Med FRACS , Jonathan R. Clark MBBS BSc (Med) FRACS MBiostat , Recreation of eyelid mechanics using the sling concept, Journal of Plastic, Reconstructive & Aesthetic Surgery (2020), doi: https://doi.org/10.1016/j.bjps.2019.12.007

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Title: Recreation of eyelid mechanics using the sling concept Shaheen Hasmat, MD BMed 1,2,6; Shaun McPherson, BMed3; Gregg J. Suaning, BSc MSc PhD4; Nigel H. Lovell, BE PhD5; Tsu-Hui (Hubert) Low, MBBS BSc Med FRACS2,6,7; Jonathan R. Clark, MBBS BSc (Med) FRACS MBiostat1,2,6,7 1

Faculty of Medicine, University of Sydney; Camperdown, NSW 2006

2

Department of Head and Neck Surgery, The Chris O’Brien Lifehouse; Camperdown, NSW

2050 3

Faculty of Medicine, University of New South Wales; Sydney, NSW 2052

4

School of Aerospace Mechanical & Mechatronic Engineering, University of Sydney;

Camperdown, NSW 2006 5

Graduate School of Biomedical Engineering, UNSW Sydney, NSW 2052

6

Sydney Facial Nerve Service, The Chris O’Brien Lifehouse; Camperdown, NSW 2050

7

Central Clinical School, University of Sydney; Sydney, Camperdown, NSW 2050

Corresponding author: Shaheen Hasmat, MD BMed, The Chris O’Brien Lifehouse, 119143 Missenden Road, Camperdown, New South Wales 2050, Australia, P: +61405364760 E: [email protected] Short title: Sling concept for restoring eye closure Abstract Background: Paralytic lagophthalmos causes major functional, aesthetic and psychological problems in patients with facial paralysis. The Bionic Lid Implant for Natural Closure (BLINC) project aims to restore eyelid function using an implanted electromagnetic actuator combined with an eyelid sling. The authors performed a preliminary study using cadaveric 1

heads to investigate optimal application of an eyelid sling in various configurations around the orbit. Methods: The sling was tested in a cadaveric sheep head using two medial anchor points and four lateral ostectomy points. An impulse was generated using gravitational force to test each combination of medial and lateral sling insertion sites using weights between 10-50g. Each generated blink was recorded and analysed. The final result was validated in human cadaveric model. Results: The maximum amount of eye-closure and closure speed displayed in sheep were 83.7±9.4% of total closure and 70.6±6.9mm/s at a maximum force of 490mN, respectively. The two inferior lateral attachments performed better at displacing the eyelid than the superior attachments. The position with the highest degree of eye-closure (improvement of 21.6%, p < 0.001) and speed (improvement of 30.4mm/sec, p < 0.001) was the combination of a posterior medial attachment and an inferior-posterior lateral attachment which resulted in a near-physiological closure in human cadaver Conclusion: Closure improved with an inferior lateral position due to increased force acting in the direction of closure. Posterior positioning increases force acting radially, towards the centre of eyelid movement. The latter directs the closure force to effectively move the eyelid around the curved globe.

Background Eyelids are complex structures that serve to maintain homeostasis in one of the main sense organs, the eye. The intricate design of the eyelid apparatus ensures that blinking is a rapid movement which serves to protect the eye from trauma and at the same time provides a unique lubricating mechanism to prevent ocular irritation. Aesthetically, the eyes and the 2

surrounding structures are perceived as central to facial beauty and symmetry is of great importance in this regard1. The eyes also reflect our level of consciousness, mood and age1. Loss of these functions in facial nerve paralysis results in a disfiguring appearance with enormous psychological consequences2. In cases where the facial nerve is sacrificed for tumour clearance, various reconstructive approaches have been introduced to restore eyelid function3-8. Dynamic function can be restored through muscle transfer and nerve grafting but the outcomes are delayed and unpredictable6,8. More common options include lateral tarsorrhaphy and lid loading with a gold or platinum weight9. However, these can be disfiguring and fail to achieve synchronous eye closure or effective lubrication.10,11 A closely coordinated pair of antagonist muscles, the orbicularis oculi (OOc) and levator palpebrae superioris (LPS), work to close and open the eyelid. LPS is continuously in a tonic state to keep the eye open while OOc is the only protractor that actively closes the eye. Mechanical forces from the two muscles centre on the tarsal plate that supports the eyelid through its rigid structure1,12,13. Blink initiates once the LPS activity ceases followed by activation of the OOc which rapidly closes the palpebral fissure14. The lower lid contributes only 1 mm of upward movement during blink15, however its apposition to the globe is essential for tear drainage.15 Although a blink is often described as vertical motion, the eyelid largely follows the curve of the globe. This movement is essential for tear film formation and is achieved through OOc’s unique muscle fibre arrangement and attachments.13,14 Figure 1 shows a simplified representation of forces involved in eyelid movement. The OOc has two heads medially with the superficial head arising from the insertion of the medial canthal tendon onto the anterior lacrimal crest while the deep head inserts near the posterior lacrimal crest. Laterally, the fibres condense to form the lateral canthal tendon, which inserts onto Whitnall’s tubercle (WT), located approximately 4 mm deep to the lateral orbital rim12. The tarsus moves in a near-circular motion along an arc defined by the surface of the eyeball. 3

To address the challenges in restoring eyelid function, the Bionic Lid Implant for Natural Closure (BLINC) project was commenced to investigate the application of a fully implanted electromagnetic actuator applied to an eyelid sling to achieve rapid, synchronous eyelid closure with effective lubrication16. The concept of a sling is inspired by the temporalis transposition technique, which is a well-established method of creating eye closure in paralytic lagophthalmos17. Tollefson and Senders18 were the first to suggest using this technique for creating artificial eye closure. We believe that if applied correctly, the sling method can closely match natural eyelid dynamics. The primary aim of this study was to investigate the optimal medial and lateral insertion points of the eyelid sling in cadaveric models that will achieve complete eyelid closure with maximal efficiency. Method Figure 2 illustrates the mechanism of the sling action. The sling is tunnelled through the upper eyelid between the OOc muscle and tarsal plate approximately 5 mm superior to the upper eyelid margin, placed through a small incision in the central portion of the eyelid. To recreate natural lid dynamics the sling should follow the natural points of OOc insertion and hence medial insertion points included the anterior and posterior lacrimal crest. Laterally, however, the sling must remain mobile to allow tension in the sling. In order to avoid disrupting the lower lid, lateral OOc insertion, i.e. Whitnall’s tubercle, must be preserved. A good model was found in the sheep for accurately anchoring the sling at various positions in relationship to the Whitnall’s tubercle. This was determined by 3D construction of sheep head CT imaging (Figure 3).our holes were drilled through the lateral orbital wall around Whitnall’s tubercle designated as anterior-superior (A), posterior-superior (B), anteriorinferior (C) and posterior-inferior (D) (Figure 4). A human cadaveric model was used to validate the results.

4

Each comparison was tested three times in three sheep heads (n=3), using an anterior medial anchor on one side of the head and a posterior medial anchor in the other (Figure 3). The sling utilised was a nylon polymer strand, selected for its rigidity and ability to transmit the total test force into the blink. The anchor used for the medial insertion site was a 5mm diameter self-drilling screw secured to the bone. For each medial insertion site, four lateral insertion sites (as above) were tested. An impulse was generated using a weight of 10g, 20g, 30g, 40g and 50g three times for each of the eight potential combinations in three sheep heads giving a total of (3 x 5 x 8 x 3) 360 trials. This was transmitted through a pulley system attached to the sling designed to reduce friction from contact with the bone. A camera with a frame-rate of thirty frames per second was used to determine the position of the eyelid during each generated blink to determine its magnitude. For measurements of eyelid displacement at each applied weight force, percentage closure was also calculated. This mode of measurement is similar to displacement, but corrects for variations in the vertical dimension of the palpebral fissure, which in the sheep heads tested varied from 11-12 mm. The positions of maximum efficiency were tested in a human cadaveric specimen. Statistical analysis Statistical analysis was performed using STATA version 12.0 (StataCorp, College Station, TX). A three-factor Analysis of Variance (ANOVA) was conducted to evaluate the effects of medial and lateral insertion points and the sling weight on eye closure and the speed of closure. Data were tested for the assumptions underlying the application of ANOVA. Statistically significant interactions on the ANOVA model were further analysed under pairwise comparison using the Bonferroni post hoc test. Deviation from normality for both eye closure and closure speed were excluded using normal probability plots and Shapiro-Wilks test. Homogeneity of variance was confirmed using Levene’s test, to ensure that the 5

assumptions underlying ANOVA were met. The results were confirmed using a linear regression model. Results The greatest closure and closure speed achieved were 83.7±9.4% and 70.6±9.9 mm/s, respectively, using a maximum force of 490 mN. The means and standard deviations for eye closure and closure speed as a function of weight and the combinations of medial and lateral sling attachments are presented in Tables 1 and Table 2. The results for three-way ANOVA indicated a significant main effect on closure for each variable: medial attachment, lateral attachment and the sling weight used (p < 0.001, p < 0.001 and p < 0.001, respectively) (Table 3). In addition, the results showed a significant interaction between the medial and lateral insertions (p < 0.001), indicating any differences between insertion points on one end of the sling were dependent upon the insertion point used at the opposite end (see Figure 5 for this interaction). However only 6.9% (Ω2) of the total variance in eye closure was attributed to the interaction between medial and lateral attachments. Similar effects were observed for speed of closure, with the medial and lateral attachment interaction accounting for 7.2% (Ω2) of total variance in speed (Table 4). Given the interaction between medial and lateral attachments was significant, simple main effects were examined, that is, the differences between posterior medial and anterior medial for each of the four lateral attachments and vice versa. To control for Type I error rate across the four simple effects, the alpha level was set at 0.0125 (a/4 = 0.05/4). On pair-wise comparison, closure improved by 13.0% (p < 0.001) and 17.5% (p < 0.001) by moving the lateral attachment inferiorly in combination with anterior and posterior medial insertion points, respectively, at maximum sling weight. Speed improved in a similar fashion. The greatest improvement in closure (21.6%) and speed (30.5 mm/s) was seen by using a

6

combination of posterior medial attachment and a posterior-inferior lateral attachment (p < 0.001 and p < 0.001, respectively). On a regression model controlling for all variables the greatest improvement was confirmed to be a combination of posterior attachment medially and posterior-inferior attachment laterally (21.6%, p < 0.001) followed by a combination of posterior medial and anteriorinferior lateral attachment (16.1%, p < 0.001). Closure speed showed a similar improvement on the model (30.4 mm/s, p < 0.001 and 28.7 mm/s, p < 0.001 for the aforementioned attachment combinations, respectively). The combination of attachments for greatest improvement was applied to the human cadaver by positioning the sling medially in the same way. Laterally the sling was anchored in a posterior-inferior position by creation of a passage inferior to Whitnall’s tubercle, closer to the deep surface of the lateral orbit. The sling was tensioned by activation of the BLINC device to observe a complete and rapid eye closure (Figure 6). Displacement of the actuated eye closure was plotted to closely match physiological blink although this was achieved at a much faster rate (Figure 7). Discussion Eyelid closure is usually described in a single plane. However more sophisticated methods such as magnetic search coils report eyelid displacement as torsional movement to indicate it is truly an angular motion14. This consideration has implications on the approach to restoring eyelid closure. We hypothesised that the eyelid moves along the curve of the globe through the action of a centripetal force (Figure 1). Reinforcement of this force should theoretically result in improved closure19. This is the first study to evaluate the optimal sling insertion sites for reanimation of paralytic lagophthalmos. This study has found that a combination of the posterior lacrimal crest 7

(medial) with a posterior-inferior (lateral) insertion achieves a 22% improvement in upper eyelid displacement and 30 mm/sec improvement in closure speed. We appreciate that the sheep head anatomy is different than that of a human, however its orbital region has been validated for human comparisons20 and its lateral orbital wall provides a uniform plane where the sling position can be accurately moved. As the aim of the study was identifying the position of maximum efficiency, the relative difference in moving the lateral anchor point is more important than the actual values. Whilst the position of sling was studied in the sheep, the final results were verified in a human model to produce eye closure similar to natural blink. To date, various surgical modalities have been developed to recreate eyelid dynamics in paralytic lagophthalmos3-8. Lid-loading is the most commonly adopted approach for restoring eye closure as it is simple and reversible, however it is a gravity dependent procedure that does not restore any dynamic eyelid function and as a result it is relatively ineffective at restoring corneal lubrication9,10. Quantitative evaluation of the efficacy of gold weight implants with a magnetic search coil shows no improvement in the down-phase peak velocity of blink (mean pre-operative value of 40.1+/-9.5% of control eye versus 40.7+/-7.0% postoperatively, p = 0.90), confirming that the effect is essentially passive10. The sling concept originates from the temporalis transposition technique. Originally proposed by Gillies21 in 1934 and later developed by Anderson22, the technique provides dynamic eye closure but is conditioned to conscious clenching. Eyelid dynamics are improved in several ways using the sling method. By combining a posterior attachment at the inner canthus and a posterior-inferior attachment at the lateral canthus, the eyelid velocity and displacement is maximised, apposition of the eyelid to the globe is maintained, and the direction of force facilitates the dispersion of tears from medial to lateral. Figure 8 illustrates dynamics of the top lid using the sling. Moving the insertion 8

points posteriorly help facilitate circular motion of the top lid through reinforcing the centripetal force. This centre seeking force functions to move the lid along the curved globe through the entire closure phase. Theoretically this force should not change the speed of the motion but only define its circular direction19. Kinematic data on blink however shows that during down-phase the eyelid increasingly accelerates before rapidly decelerating around the globe23. This sequence was seen when the sling was applied to the human eye (Figure 7) – eyelid rapidly displaces before slowing down. Such non-uniform radial movement is the effect of tangential force components19. By virtue of placement of the sling there is always a component of tension in the sling acting downwards (Figure 9). In the temporalis transposition technique the fascia passes over the lateral orbital rim. Not unexpectedly, the anterior position of the sling diminishes the centripetal force leading to the low rate of complete eye closure seen using this technique (78%)17. Whilst complete eye closure was not achieved in the sheep in this study, the eye was completely closed in the human cadaver. Previous studies using an electromagnetic actuator reported generating 654 mN of force18 to close human eye, compared to a maximum of 490 mN in the sheep in this study. Sheep eyelid is comparatively thicker with less tissue compliance and hence the need for larger force to displace the eyelid tissue. 15While the sling mechanism should be capable of achieving complete closure, it is not necessary for every blink as naturally complete eye closure occurs less than 50% of the times when young healthy adults spontaneously blink24. Conclusion The sling method is capable of effectively restoring eye closure through reproducing the natural eyelid mechanics. Maximum eye closure and closure speed are achieved by a posterior medial sling attachment in combination with a posterior-inferior lateral attachment. Downward force from the inferior lateral attachment is guided around the globe by the radial

9

force produced as a result of posterior attachments. Investigations where sling is applied to a greater number of human cadaveric heads and compared to physiological data may help to confirm that the sling mechanism can reproduce natural blink dynamics. Financial disclosure: None of the authors has a financial interest in any of the products, devices or drugs mentioned in this manuscript. Acknowledgement All the authors confirm that the presented manuscript conforms to the Declaration of Helsinki. Presented at Australian Society of Otolaryngology and Head and Neck Surgery Annual Scientific Meeting 2018 Contributors’ Statement Shaheen Hasmat: conceptualized the study, drafted the initial manuscript, performed the initial data analysis, drafted the manuscript, and approved the final manuscript as submitted. Shaun McPherson: led the data collection, reviewed and revised the manuscript and approved the final manuscript as submitted Gregg Suaning: reviewed and revised the manuscript and approved the final manuscript as submitted Nigel Lovell: reviewed and revised the manuscript and approved the final manuscript as submitted Hubert Low: reviewed and revised the manuscript and approved the final manuscript as submitted

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Jonathan Clark: reviewed the data analysis, reviewed and revised the manuscript and approved the final manuscript as submitted Conflict of interest The authors have no conflicts of interest relevant to this article to disclose.

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3.

Arion HG. Dynamic closure of the lids in paralysis of the orbicularis muscle. International surgery 1972;57(1):48-50.

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Morel-Fatio D, Lalardrie JP. Palliative Surgical Treatment of Facial Paralysis. The Palpebral Spring. Plast Reconstr Surg 1964;33:446-456.

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Wolfgang D. Mühlbauer, Heinz Segeth, Viessmann A. Restoration of Lid Function in Facial Palsy with Permanent Magnets. Chirurgia plastica. 1973;1(4):295-304.

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Owusu JA, Truong L, Kim JC. Facial Nerve Reconstruction With Concurrent Masseteric Nerve Transfer and Cable Grafting. JAMA Facial Plast Surg 2016;18(5):335-339.

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Thompson N. Autogenous free grafts of skeletal muscle. A preliminary experimental and clinical study. Plast Reconstr Surg 1971;48(1):11-27.

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Frey M, Giovanoli P, Tzou CH, Kropf N, Friedl S. Dynamic reconstruction of eye closure by muscle transposition or functional muscle transplantation in facial palsy. Plast Reconstr Surg 2004;114(4):865-875.

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Tucker SM, Santos PM. Survey: management of paralytic lagophthalmos and paralytic ectropion. Otolaryngol Head Neck Surg 1999;120(6):944-945.

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Abell KM, Baker RS, Cowen DE, Porter JD. Efficacy of gold weight implants in facial nerve palsy: quantitative alterations in blinking. Vision research 1998;38(19):3019-3023.

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Sibony PA, Evinger C, Manning KA. Eyelid movements in facial paralysis. Arch Ophthalmol 1991;109(11):1555-1561.

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Sand JP, Zhu BZ, Desai SC. Surgical Anatomy of the Eyelids. Facial Plast Surg Clin North Am 2016;24(2):89-95.

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Evinger C, Manning KA, Sibony PA. Eyelid movements. Mechanisms and normal data. Invest Ophthalmol Vis Sci 1991;32(2):387-400.

14.

Cruz AA, Garcia DM, Pinto CT, Cechetti SP. Spontaneous eyeblink activity. Ocul Surf 2011;9(1):29-41.

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Ellis DA, Kleiman LA. Assessment and treatment of the paralyzed lower eyelid. Arch Otolaryngol Head Neck Surg 1993;119(12):1338-1344.

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Hasmat S, Lovell NH, Suaning GJ, Low TH, Clark J. Restoration of eye closure in facial paralysis using implantable electromagnetic actuator. J Plast Surg Hand Surg 2016;69(11):1521-1525.

17.

Miyamoto S, Takushima A, Okazaki M, Momosawa A, Asato H, Harii K. Retrospective outcome analysis of temporalis muscle transfer for the treatment of paralytic lagophthalmos. J Plast Surg Hand Surg 2009;62(9):1187-1195.

18.

Tollefson TT, Senders CW. Restoration of eyelid closure in facial paralysis using artificial muscle: preliminary cadaveric analysis. Laryngoscope 2007;117(11):19071911.

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19.

Serway RA. Mechanics. In Serway RA, Jewett JW, ed. Physics for Scientists and Engineers with Modern Physics. 9th ed. Belmont, CA: Thomson-Brooks/Cole, 2014:150-158.

20.

Altunrende ME, Hamamcioglu MK, Hicdonmez T, Akcakaya MO, Birgili B, Cobanoglu S. Microsurgical training model for residents to approach to the orbit and the optic nerve in fresh cadaveric sheep cranium. J Neurosci Rural Pract 2014;5(2):151-154.

21.

Gillies H. Experiences with Fascia Lata Grafts in the Operative Treatment of Facial Paralysis: (Section of Otology and Section of Laryngology). Proc R Soc Med 1934;27(10):1372-1382.

22.

Andersen JG. Surgical treatment of lagophthalmos in leprosy by the Gillies temporalis transfer. Br J Plast Surg 1961;14:339-345.

23.

VanderWerf F, Brassinga P, Reits D, Aramideh M, Ongerboer de Visser B.. Eyelid movements: behavioral studies of blinking in humans under different stimulus conditions. J Neurophysiol 2003;89(5): 2784-96.

24. Sforza C, Rango M, Galante D, Bresolin N, Ferrario VF. Spontaneous blinking in healthy persons: an optoelectronic study of eyelid motion. Ophthalmic Physiol Opt2008;28(4):345-353.

Figure legends.

13

Figure 1. Top eyelid mechanics. Rapid eye closure occurs through the action of pre-tarsal orbicularis oculi (OOc). It arises medially from the insertion of the medial canthal tendon onto the lacrimal crest. Laterally it meets the Whitnall’s tubercle (WT) through the lateral canthal tendon. OOc meets the levator palpebrae superioris (LPS) fibres at the superior tarsus (A). Forces from the OOc and LPS centre at the tarsal plate to move the top lid around the convex curve of the globe (B). Blink initiates with cessation of LPS activity and simultaneous activation of OOc. The tension in the OOc results in a radially acting force, or centripetal force (Fc), centred around the Whitnall’s tubercle laterally and lacrimal crest medially. The force is responsible for the near-circular motion of the eyelid around the globe.

Figure 2. Sling action. Fixed medially, the sling tunnels along the margin of the top lid. When tensioned, the sling lowers the top-lid to close the eye.

14

Human

Sheep

Front

Side

Figure 3. 3D reconstructed models comparing human and sheep orbital anatomy. The zygomatic bone in human forms the lateral orbital wall where the apex of a V-shaped structure (red dot) forms the rim of the orbit on the axial plane. This complex shape is replaced by a uniform plane of bone in the sheep orbit.

15

Figure 4. Positions for testing the lateral insertion sites of the sling; anterior and superior (A), posterior and superior (B), anterior and inferior (C) and posterior and inferior (D).

Figure 5. ANOVA cell means plot showing interaction between medial and lateral sling attachments. The effects of medial and lateral attachments become more pronounced with increasing weight. Inferior lateral attachments perform consistently better compared to superior lateral attachments. Anterior lateral attachments (as opposed to posterior attachments) perform better in combination with anterior medial attachment. Likewise, posterior lateral attachments perform better in combination with posterior lateral attachments. Greatest closure occurs with a combination of a posterior attachment medially and a posterior-inferior attachment laterally.

16

Figure 6. Sling application to human cadaveric head. a. The sling (nylon suture) is tunnelled above the margin of the top eyelid. Laterally, the orbital wall is drilled to create a passage (black arrow) which runs inferior to the insertion of lateral canthus and closer to the deep surface of the orbital wall. b. Top: The device is fixed to the zygomatic arch. The sling is fixed medially to the medial canthus and tunnelled through the top eyelid to enter the passage created in the lateral orbital wall before inserting into the device laterally. Bottom: upon activation of the device, the sling is tensioned resulting in a complete closure of the eye.

a.

b.

Figure 7. Eyelid displacement in natural blink versus BLINC-actuated eye closure (downphase). Natural down-phase blink duration varies between 72-133 ms in the literature (69 ms shown here) where the rate of eyelid displacement rises exponentially before slowly plateauing23. BLINC applied to the sling mechanism displaces the eyelid to achieve a near 17

complete eye closure within 35 ms with the kinematics closely matching the physiological blink.

Eye Closure (% of total)

100 80 60 40 20

N…

0 0

20

40 Time (ms)

60

80

Figure 8. Effects of moving sling fixation along the transverse plane. Tensioning the sling creates a radially acting force at the point where the sling meets the globe. Anterior fixation (yellow) reduces the radial component (FA) while increasing forces acting away from the globe. Positioning the sling posteriorly (green) increases the radial component (FP). The radially acting force is responsible for the angular motion of the eyelid as it moves during closure.

Figure 9. Effects of moving lateral fixation of the sling. Magnitude of the force in the vertical plane changes with moving the sling position along the plane. A superior position (yellow) increases force in the transverse plan while reducing it in the vertical plane (Fs). Moving the position inferiorly (green) increases the force component acting in the plan (Fi) and as a result increased eye closure occurs.

18

Tables. Table 1. Means and standard deviations (brackets) of eye closure (% of total closure) as functions of weight and sling attachment positions. Lateral sling insertions: anterior-superior (A), posterior-superior (B), anterior-inferior (C) and posterior-inferior (D). Anterior Medial Position Weight (g)

10

20

30

40

50

A

B

24.07

25.00

(2.78)

Posterior Medial Position

C

D

A

B

C

D

40.74

29.63

25.76

30.56

36.36

36.28

(9.32)

(6.51)

(10.30)

(7.76)

(8.32)

(5.58)

(5.14)

35.19

36.11

54.63

47.22

40.57

44.21

46.21

49.24

(6.94)

(12.50)

(6.05)

(8.33)

(10.32)

(9.80)

(6.54)

(6.39)

48.15

44.44

62.96

57.41

52.36

55.07

61.87

63.13

(10.02)

(15.02)

(7.35)

(7.73)

(11.10)

(10.46)

(2.65)

(11.46)

58.33

51.85

72.22

61.11

56.31

62.12

71.80

75.76

(7.22)

(12.34)

(8.33)

(4.17)

(11.89)

(9.91)

(7.40)

(7.42)

60.19

57.41

75.00

68.52

62.12

65.15

78.54

83.67

(10.02)

(12.11)

(7.22)

(5.56)

(9.91)

(12.24)

(3.95)

(9.39)

Table 2. Means and standard deviations (brackets) of speed as functions of weight and sling attachment positions. Lateral sling insertions: anterior-superior (A), posterior-superior (B), anterior-inferior (C) and posterior-inferior (D). Anterior Medial Position

Posterior Medial Position

Weight (g)

A

B

C

D

A

B

C

D

10

8.45

13.87

16.96

13.45

16.03

25.36

28.93

37.97

(5.50)

(6.94)

(6.41)

(8.94)

(7.98)

(11.10)

(13.96)

(13.71)

19

20

30

40

50

17.09

19.39

33.59

24.85

22.04

31.26

34.75

37.74

(5.96)

(4.77)

(9.42)

(4.78)

(7.46)

(13.97)

(5.14)

(7.25)

26.48

25.66

45.90

40.54

31.13

40.29

50.72

53.61

(9.83)

(4.81)

(7.38)

(8.82)

(8.07)

(8.89)

(10.30)

(11.34)

39.29

32.14

49.75

46.37

33.53

44.40

59.41

66.03

(6.85)

(3.73)

(7.36)

(9.20)

(6.51)

(10.44)

(10.66)

(7.65)

32.50

38.17

56.16

58.63

40.14

49.34

68.85

70.62

(8.38)

(6.35)

(7.43)

(16.52)

(4.48)

(14.21)

(9.72)

(6.94)

Table 3. Three-way Analysis of Variance (ANOVA) for eye closure.

Source

SS

df

MS

Medial position

1699.40737 1

1699.40737 21.51

<0.001

Lateral position

13071.3372 3

4357.11241 55.16

<0.001

Medial x Lateral position

1885.46697 3

628.488991 7.96

<0.001

Weight

67031.6054 4

16757.9014 212.13 <0.001

Medial position x Weight

358.653968 4

89.6634919 1.14

0.340

Lateral position x Weight

597.772746 12

49.8143955 0.63

0.816

69.34059

0.570

Medial x Lateral positions x Weight 832.08708

12

Total

359 308.510752

110755.36

Table 4. Three Three-way Analysis of Variance for closure speed.

20

F

0.88

p

Source

SS

Medial position

9264.15681 1

9264.15681 114.98 <0.001

Lateral position

22668.068

7556.02268 93.78

<0.001

Medial x Lateral position

1988.97568 3

662.991893 8.23

<0.001

Weight

49342.5342 4

12335.6335 153.10 <0.001

Medial position x Weight

407.023165 4

101.755791 1.26

0.285

Lateral position x Weight

3114.13685 12

259.511405 3.22

<0.001

Medial x Lateral positions x Weight 1153.74554 12

96.1454617 1.19

0.287

Total

df

3

MS

113722.579 359 316.775986

21

F

p