Sc joint

BIOMECHANICS

1. Mechanics of Sterno Clavicular Joint K.Soundararajan MPT (Ortho)

2. INTRODUCTION

3. The shoulder region is a complex of 20 muscles, 3 bony articulations, and 3 soft tissue moving surfaces (functional joints) T hat permit the greatest mobility of any joint area found in the body (approximately 180 degrees of flexion, abduction and rotation and 60 degrees of hyperextension). The shoulder complex not only provides a wide range for hand placement but also carries out the important functions of stabilization for hand use, lifting and pushing , elevation of the body, forced inspiration and expiration, and even weight bearing as in crutch-walking or handstands .

4. The extensive mobility is provided by the six moving areas: 1 . Bony articulations a . Sternoclavicular b . Acromioclavicular c . Glenohumeral 2. Functional joints a . Scapulothoracic b . Suprahumeral (or subacromial ) c . Bicipital groove

5. Mobility, however, is at the expense of structural stability The only attachment of the upper extremity to the trunk is at the sternoclavicular joint, and the head of the humerus hangs loosely on the inclined plane of the glenoid fossa Thus, support and stabilization of the shoulder primarily depend on muscles and ligaments.

6. The contradictory requirements on the shoulder complex for both mobility and stability are met through active forces, or dynamic stabilization, a concept for which the shoulder complex is considered a classic example. In essence, dynamic stability exists when a moving segment or set of segments is limited very little by passive forces such as articular surface configuration,capsule , or ligaments and instead relies heavily on active forces or dynamic muscular control . Dynamic stabilization results in a wide range of mobility for the complex and provides adequate stability when the complex is functioning normally. However, the competing mobility and stability demands on the shoulder girdle

7. STERNOCLAVICULAR JOINT

8. The sternoclavicular joint is the only joint that connects the upper extremity directly with the thorax. The shoulder girdle, together with the entire upper extremity, is suspended from the skull and the cervical spine by muscles, ligaments, fascia. The position of this hanging structure is determined partly by the action of gravity and partly by the clavicle, which restricts shoulder girdle movements in all directions, particularly in a forward direction.

9. Cases of the clavicle's being absent have been reported in the medical literature- These individuals were able to move their shoulders so far forward that the tips of the shoulders almost met in front of the body. With complete surgical removal of the clavicle, Lewis and associates (1985) found shoulder ranges of motion to be the same as those on the uninvolved side. The maximum isokinetic torques for shoulder extension, as well as internal and external rotation, were also the same . The shoulder flexors,abductors , and adductors on the involved side, however, had a 50 percent loss of isokinetic torque.

10. The only point of skeletal attachment of the upper extremity to the trunk occurs at the sternoclavicular joint. At this joint, the clavicle is joined to the manubrium of the sternum. The clavicle serves four roles By serving as a site of muscular attachment, P roviding a barrier to protect underlying structures, Acting as a strut to stabilize the shoulder P revent medial displacement when the muscles contract, and preventing an inferior migration of the shoulder girdle

11. Movement of the clavicle at the SC joint inevitably produces movement of the scapula under conditions of normal function, because the scapula is attached to the lateral end of clavicle . In order for the scapula to not move with the clavicle during SC motion , equal and opposite motions would have to occur at the AC joint; this is not typical with an intact claviculoscapular linkage. Similarly , any motions of the scapula must result in motion at the SC joint ( unless scapular motions are isolated to the AC joint—which is , again, unlikely under normal circumstances).

12. The SC joint is a plane synovial joint with 3 rotatory and 3 translatory degrees of freedom. This joint has a synovial capsule, a joint disk, and three major ligaments . ■ Sternoclavicular Articulating Surfaces The SC articulation consists of two saddle-shaped surfaces, one at the sternal or medial end of the clavicle and one at the notch formed by the manubrium of the sternum and first costal cartilage

13. The articular surfaces are covered with fibrocartilage  (as opposed to hyaline cartilage, present in the majority of synovial joints). The joint is separated into two compartments by a fibrocartilaginous articular disc The ligamentous reinforcements of this joint are very strong, often resulting a fracture of the clavicle before a dislocation of the SC Joint

14. Because tremendous individual differences exist across people and the saddle shape of these surfaces is very subtle, the SC joint is often classified as a plane synovial joint. The sternal end of the clavicle and the manubrium are incongruent ; that is, there is little contact between their articular surfaces . The superior portion of the medial clavicle does not contact the manubrium at all;

15. Instead it serves as the attachment for the SC joint disk and the interclavicular ligament. At rest, the SC joint space is wedge-shaped and open superiorly. Movements of the clavicle in relation to the manubrium result in changes to the areas of contact between the clavicle, the SC joint disk , and the manubriocostal cartilage .

16. ■ Sternoclavicular Disk As is generally true at an incongruent joint, the SC joint has a fibrocartilage joint disk, or meniscus, that increases congruence between joint surfaces . The upper portion of the SC disk is attached to the posterosuperior clavicle. The lower portion is attached to the manubrium and first costal cartilage, as well as to the anterior and posterior aspects to the fibrous capsule

17. *The disk diagonally transects the SC joint space and divides the joint into two separate cavities cavities . * Given its attachments, the disk acts like a hinge or pivot point during clavicle motion.

18. In elevation and depression of the clavicle, the medial end of the clavicle rolls and slides on the relatively stationary disk, with the upper attachment of the disk serving as a pivot point . In protraction/retraction of the clavicle, the SC disk and medial clavicle roll and slide together on the manubrial facet, with the lower attachment of the disk serving as a pivot point . The disk , therefore, is considered part of the manubrium in elevation/depression and part of the clavicle in protraction/retraction .

19. As the disk switches its participation from one articular segment to the other during clavicular motions, mobility between the segments is maintained and stability is enhanced. The resultant movement of the clavicle in both elevation/depression and protraction/retraction is a fairly complex set of motions, with the mechanical axis for these two movements located not at the SC joint itself but at the more laterally located costoclavicular ligament

20. It can be seen that the unique diagonal attachment of the SC disk will check medial movement of the clavicle that might otherwise cause the large medial articular surface of the clavicle to override the shallow manubrial facet. The disk also has substantial contact with the medial clavicle, permitting the disk to dissipate the medially directed forces that would otherwise cause high pressure at the small manubrial facet .

21. Although one might think that medially directed forces on the clavicle are rare, we shall see that this is not the case when we examine the function of the AC joint, the upper trapezius muscle, and the coracoclavicular ligament

22. Three-Compartment SC Joint Anatomic examination of the SC articulation has led to the proposal that there are three, rather than two, functional units of the SC joint: lateral compartment between the disk and clavicle for elevation and depression ; medial compartment between the disk and manubrium for protraction and retraction; costoclavicular joint for anterior and posterior long axis rotation. Anterior and posterior rotation are thought to occur between a portion of the disk over the first rib and a “ conus ” on the anteroinferior edge of the articular surface of the medial clavicle

23. Joint Capsule The SC Joint capsule is fairly strong but is dependent on the ligaments noted above for the majority of its support. Joint Disk  The SC Joint is surprisingly incongruent, and because of this incongruency a joint disk is present to enhance joint curvature and contact of the joint surfaces.  Similar to meniscus of the knee, the SC Joint disk increases joint congruence and acts to absorb forces that may be transmitted along the clavicle. The disk is attached to the upper and posterior margin of the the clavicle, and to the cartilage of the first rib, which functions to help prevent medial displacement of the clavicle. This orientation divides the joint into seperate cavities. Greater movement occurs between the disk and the clavicle than between the disk and the manubrium .

24. The costoclavicular ligament has two segments or laminae . The anterior lamina has fibers directed laterally from the first rib to the clavicle, whereas the fibers of the posterior lamina are directed medially from the rib to the clavicle. Both segments check elevation of the lateral end of the clavicle and, when the limits of the ligament are reached , may contribute to the inferior gliding of the medial clavicle that occurs with clavicular elevation . The costoclavicular ligament is also positioned to counter the superiorly directed forces applied to the clavicle by the sternocleidomastoid and sternohyoid muscles . The medially directed fibers of the posterior lamina will resist medial movement of the clavicle,absorbing some of the force that would otherwise be imposed on the SC disk.

25. Ligaments

26. The interclavicular ligament resists excessive depression of the distal clavicle and superior glide of the medial end of the clavicle. The limitation to clavicular depression is critical to protecting structures such as the brachial plexus and subclavian artery that pass under the clavicle and over the first rib . In fact, when the clavicle is depressed and the interclavicular ligament and superior capsule are taut, the tension in the interclavicular ligament can support the weight of the upper extremity

27. KINEMATICS

28. Sternoclavicular Motions The three rotatory degrees of freedom at the SC joint are most commonly described as elevation/depression, protraction/retraction , and anterior/posterior rotation of the clavicle. Motions of any joint are typically described by identifying the direction of movement of the portion of the lever that is farthest from the joint. The horizontal alignment of the clavicle (rather than the vertical alignment of most of the appendicular levers of the skeleton) can sometimes create confusion and impair visualization of the clavicular motions.

29. The motions of elevation/depression and protraction/retraction should be visualized by referencing movement of the lateral end of the clavicle. Clavicular anterior/posterior rotation are long axis rolling motions of the entire clavicle

30. Three degrees of translatory motion at the SC joint can also occur , although they are very small in magnitude. Translations of the medial clavicle on the manubrium are usually defined as occurring in anterior/posterior, medial/lateral , and superior/inferior directions

31. Elevation and Depression of the Clavicle The motions of elevation and depression occur around an approximately anteroposterior (A-P) axis between a convex clavicular surface and a concave surface formed by the manubrium and the first costal cartilage. With elevation, the lateral clavicle rotates upward,and with depression, the lateral clavicle rotates downward .

32. The cephalocaudal shape of the articular surfaces and the location of the axis indicate that the convex surface of the clavicle must slide inferiorly on the concave manubrium and first costal cartilage, in a direction opposite to movement of the lateral end of the clavicle . The SC joint axis is described as lying lateral to the joint at the costoclavicular ligament . The location of this functional (rather than anatomic) axis relatively far from the joint reflects a large intraarticular motion of the medial clavicle.

33. The range of available clavicular elevation has been described as up to 48, whereas passive depression is limited, on average, to less than 15.9 The full magnitude of the available range of elevation is generally not utilized during functional ranges of arm elevation.

34. Protraction and Retraction of the Clavicle Protraction and retraction of the clavicle occur at the SC joint around an approximately vertical ( superoinferior )axis that also appears to lie at the costoclavicular ligament. With protraction, the lateral clavicle rotates anteriorly , and with retraction, the lateral clavicle rotates posteriorly .

35. The configuration of joint surfaces in this plane is the opposite of that for elevation/depression ; the medial end of the clavicle is concave , and the manubrial side of the joint is convex. During protraction, the medial clavicle is expected to slide anteriorly on the manubrium and first costal cartilage. There is about 15 to 20 protraction and 20 to 30 retraction of the clavicle available

36. Anterior and Posterior Rotation of the Clavicle Anterior/posterior , or long axis, rotation of the clavicle occurs as a spin between the saddle shaped surfaces of the medial clavicle and manubriocostal facet . Unlike many joints that can rotate in either direction from resting position of the joint, the clavicle rotates primarily in only one direction from its resting position. The clavicle rotates posteriorly from neutral,bringing the inferior surface of the clavicle to face anteriorly .

37. This has also been referred to as backward or upward rotation rather than posterior rotation . From its fully rotated position, the clavicle can rotate anteriorly again to return to neutral . Available anterior rotation past neutral is very limited, generally described as less than 10.1 The range of available clavicular posterior rotation is cited to be as much as 50.10 The axis of rotation runs longitudinally through the clavicle, intersecting the SC and AC joints.

38. ■ Sternoclavicular Stress Tolerance The bony segments of the SC joint, its capsuloligamentous structure , and the SC disk combine to produce a joint that meets its dual functions of mobility and stability well . The SC joint serves its purposes of joining the upper limb to the axial skeleton, contributing to upper limb mobility, and withstanding imposedstresses . Although the SC joint is considered incongruent, the joint does not undergo the degree of degenerative change common to the other joints of the shoulder complex. Strong force-dissipating structures such as the SC disk and the costoclavicular ligament minimize articular stresses and also prevent excessive intra- articular motion that might lead to subluxation or dislocation. Dislocations of the SC joint represent only 1% of joint dislocations in the body

39. KINETICS

40. Neurovascular Supply Arterial supply to the sternoclavicular joint is from the internal thoracic artery and the suprascapular artery . The joint is supplied by the medial supraclavicular nerve (C3 and C4) and the nerve to subclavius (C5 and C6).

41. Closed Packed Position Maximum Shoulder Elevation Open Packed Position Arm is resting by Side

42. Pathology / Injury The Sternoclavicular Joint is vulnerable to the same disease processes as other synovial joints, the most common of which are instability from injury, osteoarthritis, infection and rheumatoid disease. Patients may also present with other conditions, which are unique to the joint, or are manifestations of a systemic disease process. The most common clinical presentation is pain and swelling in the area of the Sternoclavicular Joint, either after an injury to the shoulder, or insidiously, with no history of trauma. A Sternoclavicular Joint sprain is a relatively rare sporting injury, which can range from a small sprain resulting in minimal pain and allowing ongoing activity, to a severe sprain resulting in significant pain, deformity and disability

43. REFERENCE BOOKS