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. 2008 Dec;213(6):733-42.
doi: 10.1111/j.1469-7580.2008.01000.x.

Morphology of deltoid origin and end tendons--a generic model

J N A L Leijnse  1 , S-H Han, Y H Kwon
Affiliations

Morphology of deltoid origin and end tendons--a generic model

J N A L Leijnse et al. J Anat. 2008 Dec.

Abstract

This study provides a model of the complex deltoid origin and end tendons, as a basis for further anatomical, biomechanical and clinical research. Although the deltoid is used in transpositions with upper limb paralysis, its detailed morphology and segmentation has not been object of much study. Morphologically, the deltoid faces two distinct challenges. It closely envelops a ball joint, and it reduces its width over a short distance from a very wide origin along clavicle, acromion and spina scapula, to an insertion as narrow as the humerus. These challenges necessitate specific morphological tendon adaptations. A qualitative model for these tendons is developed by the stepwise transformation of a unipennate muscle model into a functional deltoid muscle. Each step is the solution to one of the mentioned morphological challenges. The final model is of an end tendon consisting of a continuous succession of bipennate end tendon blades centrally interspaced by unipennate tendon parts. The origin tendon consists of lamellae that interdigitate with the end tendon blades, creating a natural segmentation. The model is illustrated by qualitative dissection results. In addition, in view of a proliferation of terms found in the literature to describe deltoid tendons, tendon concepts are reviewed and the systematic use of the unique and simple terminology of 'origin and end tendons' is proposed.

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Figures

Fig. 1
Fig. 1
Deltoid overview. (A) (Specimen S3) Anterior deltoid (d) and pectoralis major. cl: clavicle. at: anterior deltoid ET blade. pc, pt: clavicular and thoracic parts of pectoralis major. ch: chiasma of vena cephalica. br: brachialis (biceps has been removed). cb: coracobrachialis. (B) (specimen S5) Posterior deltoid. sp: spina scapula (the supra-spinal scapula was covered by black background). ps: posterior spinal deltoid segment, mobilized from anterior spinal deltoid (as). ac: acromial deltoid. if: infraspinal fascia, covering infraspinatus and spinal deltoid OT sheet. White dotted line: origin line of spinal deltoid OT. Black dotted line: posterior deltoid edge. (C) Cross-section through humerus heads (h), left and right shoulders (from: ‘Visible Human’). Top is posterior. Deltoid (d) transected just beneath spina scapula, showing the great origin width compared to humerus width. White dotted line: outline of deltoid in right shoulder; left shoulder was left unmarked. s: scapula. i: infraspinatus. su: subscapularis.
Fig. 2
Fig. 2
Muscle-tendon models. (a) Elementary tendon-muscle-tendon fiber (TMTF) units. (b) TMTF units in unipennate arrangement. OT, ET: origin and end tendon sheets. F: fascia. (c) Two unipennate muscles, not necessarily of the same size, stacked back-to-back, form a bipennate muscle. The unipennate ET fuse into a bipennate ET. (d) Model of (c) reversed: bipennate OT gives rise to two unipennate muscles. (e) Multipennate or multisegmented muscle, formed by stacking unipennate models back to back.
Fig. 3
Fig. 3
Generic model of deltoid. A unipennate muscle (a) is stepwise transformed into a deltoid model (i). Left branch: generic model of ET. Right branch: OT. For further explanations, see text. The final model (i) shows bipennate OT lamellae interdigitating with bipennate ET blades. cl, ac, sp: clavicular, acromial and spinal parts, resp. j: 2D rendition of muscle fiber structure in a deltoid with seven ET blades and one clavicular, three acromial and one spinal OT lamellae. The tendon structure is detailed in Fig. 6. k: The inter-lamella spaces are natural pathways for segmental neurovascularization.
Fig. 6
Fig. 6
Summary model of dissection results. C, A, S: clavicle, acromion, spina. Origin tendons. Proximal at the clavicle, one (lc) or two superimposed (lc, lcd) lamellae are present. More distally, a short, often flimsy superficial unipennate OT sheet arises (tcm) that variably expands (tcd) near the clavicular–acromial joint and is continuous with the first acromial lamella la1. tcd, white dotted line: transition from unipennate OT to bipennate acromial OT. lai: bipennate acromial lamellae, counting three or four. ls1 (dotted outline): single spinal lamella, shorter than the acromial lamellae, found in 50% of cases. ss: superficial spinal OT sheet, continuous with and shorter than acromial OT. la4, white dotted line: transition from bipennate acromial to unipennate spinal OT, if no spinal OT lamella is present. ls1, white striped line: bi- to unipennate transition when a spinal lamella is present. sdp (black striped line): consistently present deep posterior OT sheet, degenerating into a flimsy structure near the acromion. sc: common OT sheet, dividing in superficial and deep posterior OT sheets at some distance from the spinal line of origin S. End tendons. ET model of specimen 6 (Fig. 5A). Blades b1–b3 split from the anterior ET trunk, inserting at line AT (thick black line). Two central single blades b4 and b5, inserting at lines CT1 and CT2, were continuous with unipennate ET sheets u1, u2 and u3, inserting in the humerus surface between the ET blade insertion lines (gray areas within black dotted lines). The posterior ET trunk, inserting in line PT, splits at some distance from its insertion line in two ET blades b6 and b7.
Fig. 4
Fig. 4
Acromial deltoid segment structure. (A–D) Lateral deltoid (anterior = left). (A,B) (specimen S0) Deltoid after removal of subdermal fat. (B) Interdigitating muscle fiber triangles eased apart. White dotted lines: ET blades towards which muscle fibers bipennately converge. White striped lines: OT lamellae, from which muscle fibers bipennately diverge. (C,D) (specimen S5) OT lamellae in situ after gradual removal of muscle fibers. (E–G) (specimen S1) Dissection of a single acromial segment. (E) Superficial aspect. (F) Superficial muscle fibers removed from OT lamella. (G) Muscle fiber segment removed from ET blades and retracted. OT lamella and ET blades have approximately the same length and have considerable overlap (double white arrow). (H) (specimen S2) OT lamella and its ET blades, all muscle fibers removed. Shoulder in adduction to minimize OT and ET overlap for photograph. The left ET blade has the same length (white arrows) and width (black arrows) as the OT lamella. The same width relationship is not visually evident in the right ET blade, which is spread wider than in the anatomical position.
Fig. 5
Fig. 5
Overview of deltoid origin and end tendons. (A) (specimen S6) ET blades, superimposed, anterior view. This specimen had seven ET blades. The anterior five ET blades are visible (b1–b5); the posterior two blades are hidden beneath the others (see Fig. 6). (B) (specimen S1) Origin tendons, front view, amputated in one piece at origin line after removal of muscle fibers. cl, ac, sp: clavicular, acromial and spinal parts, respectively, marked by white dashed lines. This specimen had two superimposed clavicular OT lamellae (superficial lcs, deep lcd); four acromial lamellae (la1–la4); and one spinal lamella (ls). The posterior spinal OT sheet (ss) is not lamellated. It is superficial to the muscle, with muscle fibers arising unipennately from the backside, as shown by the posterior deltoid muscle mass left in situ. The black dashed line marks the posterior border of the bipennate part of the spinal lamella, which is posterior continuous with the superficial spinal OT sheet.
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