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Syndesmosis
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A syndesmosis (“fastened with a band”) is a type of fibrous joint in which two bones are united to each other by fibrous connective tissue. The gap between the bones may be narrow, with the bones joined by ligaments, or the gap may be wide and filled in by a broad sheet of connective tissue called an interosseous membrane.[1] The syndesmoses found in the forearm and leg serve to unite parallel bones and prevent their separation.
Examples
[edit]In the forearm, the wide gap between the shaft portions of the radius and ulna bones are strongly united by an interosseous membrane. Similarly, in the leg, the shafts of the tibia and fibula are also united by an interosseous membrane. In addition, at the inferior tibiofibular joint, the articulating surfaces of the bones lack cartilage and the narrow gap between the bones is anchored by fibrous connective tissue and ligaments on both the anterior and posterior aspects of the joint. Together, the interosseous membrane and these ligaments form the tibiofibular syndesmosis.
However, a syndesmosis does not prevent all movement between the bones, and thus this type of fibrous joint is functionally classified as an amphiarthrosis. In the leg, the syndesmosis between the tibia and fibula strongly unites the bones, allows for little movement, and firmly locks the talus bone in place between the tibia and fibula at the ankle joint. This provides strength and stability to the leg and ankle, which are important during weight bearing. In the forearm, the interosseous membrane is flexible enough to allow for rotation of the radius bone during forearm movements. Thus in contrast to the stability provided by the tibiofibular syndesmosis, the flexibility of the antebrachial interosseous membrane allows for the much greater mobility of the forearm.[1]
Pathology
[edit]The interosseous membranes of the leg and forearm also provide areas for muscle attachment. Damage to a syndesmotic joint, which usually results from a fracture of the bone with an accompanying tear of the interosseous membrane, will produce pain, loss of stability of the bones, and may damage the muscles attached to the interosseous membrane. If the fracture site is not properly immobilized with a cast or splint, contractile activity by these muscles can cause improper alignment of the broken bones during healing.[1]
References
[edit]- ^ a b c Betts, J. Gordon (2013). "9.2 Fibrous joints". Anatomy & physiology. Houston, Texas: OpenStax. ISBN 978-1-947172-04-3. Retrieved 14 May 2023.
Source text
[edit]
This article incorporates text from a free content work. Licensed under CC BY 4.0. Text taken from Anatomy and Physiology, J. Gordon Betts et al, Openstax.
Syndesmosis
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Overview
Definition
A syndesmosis is a type of fibrous joint in which two adjacent bones are connected by a ligament or dense fibrous connective tissue, rather than through direct osseous contact. This connection typically involves an interosseous ligament or membrane that spans between parallel or nearly parallel bones, providing structural integrity while permitting limited motion. The term originates from the Ancient Greek syndesmos, meaning "bond" or "ligament," combined with the suffix -osis denoting a condition or state, reflecting its role as a binding articulation.[6][7] Unlike other fibrous joints such as sutures, which are synarthrotic and essentially immovable to protect the developing skull, or gomphoses, which feature a peg-in-socket arrangement like the periodontal ligament securing teeth to alveolar bone, syndesmoses are classified as amphiarthrotic, allowing slight gliding or rotational movement due to the inherent elasticity and length of the intervening fibrous tissue. This subtle mobility distinguishes syndesmoses within the broader category of fibrous joints, where the degree of separation between bones and the composition of the connective tissue determine the extent of permitted motion.[8][9] The concept of syndesmosis as a distinct joint type emerged in early modern anatomy, with the term first appearing in English anatomical literature in 1726, as recorded by the Scottish anatomist Alexander Monro in his descriptions of articulations. It was further integrated into systematic joint classifications during the 19th century, contributing to the foundational taxonomy of synovial, cartilaginous, and fibrous joints that remains standard in anatomical studies today.[10]Classification
Syndesmoses are classified structurally as a type of fibrous joint, connected by dense collagen-rich connective tissue without a joint cavity, distinguishing them from cartilaginous joints.[11] This places syndesmoses alongside other fibrous joint types, including sutures, gomphoses, and schindyleses.[9] Subtypes of syndesmoses are differentiated primarily by the nature and extent of the fibrous connections between bones, particularly the length and type of ligaments or membranes involved. Those with short ligaments attaching directly to bone surfaces provide near-immovability, as seen in distal attachments where minimal separation occurs.[12] In contrast, syndesmoses featuring longer interosseous membranes allow for greater flexibility and slight movement between parallel bones, such as in the forearm.[13] Compared to other fibrous joints, syndesmoses involve bones separated by a ligamentous band or membrane, permitting limited motion, whereas sutures feature interlocking bony edges with minimal intervening tissue for rigid cranial stability.[2] Gomphoses, like the periodontal ligament securing teeth to alveolar sockets, provide peg-in-socket fixation without significant movement.[11] Schindyleses, in which a thin bone plate fits into a groove of another bone, offer interlocking support with even less separation than syndesmoses.[14] Functionally, syndesmoses are classified as amphiarthrotic joints, meaning they are slightly movable, in contrast to synarthrotic (immovable) joints like sutures or diarthrotic (freely movable) synovial joints.[2] This intermediate mobility supports load distribution while maintaining structural integrity.[15]Anatomy
General Structure
A syndesmosis is classified as a type of fibrous joint in which two parallel bones are connected by dense fibrous connective tissue, forming a stable articulation without a synovial cavity.[2] This overall architecture features a narrow interosseous space between the bones, typically filled by ligaments or a broad interosseous membrane composed of tough, collagen-rich tissue that binds the skeletal elements together.[16] The absence of a joint capsule or articular cartilage distinguishes it from more mobile synovial joints, emphasizing its role in providing rigid or slightly flexible union primarily through tensile resistance rather than sliding or hinging mechanisms.[17] At the microscopic level, the connective tissue in a syndesmosis consists predominantly of type I collagen fibers arranged in dense, parallel bundles within the interosseous ligament or membrane, which imparts high tensile strength to withstand forces that might separate the bones.[2] These fibers are embedded in a matrix of proteoglycans and fibroblasts, forming dense regular connective tissue without involvement of hyaline or fibrocartilage, ensuring minimal elasticity and maximal durability.[18] The orientation of these collagen fibers is typically longitudinal or oblique to the bone axes, optimizing resistance to shear and distraction stresses.[19] Structural variations exist among syndesmoses based on the degree of mobility required, with tight forms exhibiting narrow gaps of 1-2 mm or less to limit movement, while more mobile variants allow slightly wider separations filled by broader fibrous sheets that permit limited translation or rotation.[16] In all cases, the fibrous components are oriented to primarily resist diastasis (bone separation), with fiber density and layering adapting to functional demands without compromising overall stability.[2] Developmentally, syndesmoses originate from mesenchymal condensations in the embryonic limb buds during the 6th to 8th weeks of gestation, where undifferentiated mesenchyme differentiates into fibroblasts that synthesize the initial collagenous framework.[2] This primitive tissue gradually matures into dense regular connective tissue through postnatal growth, achieving full structural integrity by adolescence as collagen fibers thicken and align under mechanical influences from weight-bearing and movement.[2]Key Components
The primary components of a syndesmosis include interosseous ligaments, which are short, strong fibrous bands that directly connect adjacent bones, and interosseous membranes, which are thinner, sheet-like structures that span longer distances between bones to provide broader stabilization.[2] These elements form the core ligamentous and membranous framework, with the ligaments acting as discrete reinforcements and the membranes offering continuous fibrous linkage; variations occur depending on the specific syndesmosis, such as more prominent ligaments in some locations versus membrane dominance in others.[2] In syndesmotic complexes, ligaments are composed predominantly of collagen type I, accounting for 80-90% of their organic content, alongside fibroblasts that synthesize and maintain the extracellular matrix through collagen production and remodeling.[20] The vascular supply to these structures is relatively sparse compared to other tissues, which contributes to their tensile strength and durability.[21] Attachments of these components occur in proximal, middle, and distal zones along the bone surfaces, where the collagen fibers insert directly into the periosteum of the adjacent bones, ensuring secure anchorage without penetrating the cortical bone.[3] This zonal organization allows for graduated load distribution across the syndesmosis.[22]Locations and Examples
Lower Limb
In the lower limb, the primary syndesmotic joint is the distal tibiofibular syndesmosis, also referred to as the inferior tibiofibular joint, which unites the distal ends of the tibia and fibula to form a stable lateral component of the ankle mortise. This fibrous articulation is essential for weight-bearing stability, allowing limited motion while preventing excessive separation of the bones during locomotion. The syndesmosis spans the distal portion of the interosseous space, with its key ligamentous attachments concentrated in the region immediately proximal to the tibiotalar joint.[5] The anatomical components of the distal tibiofibular syndesmosis include four main ligaments that bind the bones: the anterior inferior tibiofibular ligament (AITFL), which originates from the anterior distal tibia and inserts on the anterior fibula; the posterior inferior tibiofibular ligament (PITFL), connecting the posterior distal tibia to the posterior fibula; the interosseous ligament, a distal thickening of the interosseous membrane that fills the space between the bones; and the inferior transverse tibiofibular ligament, which courses horizontally behind the PITFL and functions as the fourth ligament stabilizing the ankle. The osseous interface features the convex lateral surface of the distal fibula articulating within the concave fibular notch (incisura fibularis) of the tibia, with the notch's apex located approximately 6-8 cm above the talocrural joint line. The normal tibiofibular clear space measures approximately 4 mm (range 2-6 mm), with values up to 6 mm considered within normal limits on imaging.[5][3][23][24] Proximal to the distal syndesmosis, the middle tibiofibular syndesmosis is formed by the interosseous membrane of the leg, a fibrous sheet that extends along the shafts of the tibia and fibula from near the proximal tibiofibular joint at the knee level to the distal syndesmosis at the ankle. This membrane, approximately 1-2 mm thick in its central portion, transmits forces between the bones and separates the anterior and posterior compartments of the leg, contributing to overall lower limb stability during weight transfer.[5][25]Upper Limb
The syndesmotic joints of the upper limb are primarily exemplified by the radioulnar syndesmosis, which encompasses the proximal (superior), middle, and distal (inferior) radioulnar joints, facilitating forearm rotation through a fibrous connection between the radius and ulna. This structure is classified as a fibrous syndesmosis, a subtype of fibrous joints where bones are united by dense ligaments without direct synovial articulation. The syndesmosis enables the essential movements of pronation and supination, allowing the hand to adopt various orientations for manipulative tasks, in contrast to the weight-bearing stability emphasized in lower limb syndesmoses.[26][27] At the core of the radioulnar syndesmosis lies the interosseous membrane (IOM), a broad fibrous sheet that spans approximately 10-15 cm between the interosseous margins of the radius and ulna, originating near the radial tuberosity and extending distally toward the wrist. The IOM's fibers exhibit an oblique proximal-to-distal orientation, angled at about 21 degrees relative to the ulna's longitudinal axis, which enhances load transfer and stability during forearm dynamics. With a thickness of 1-2 mm in its central band, the membrane permits minimal interosseous separation—typically up to a few millimeters—during rotation, maintaining overall forearm integrity without bone-to-bone contact, as the connection is purely ligamentous.[28][29][30] Distally, the syndesmosis incorporates the triangular fibrocartilage complex (TFCC), a ligamentous and cartilaginous structure that reinforces the inferior radioulnar joint by attaching to the ulnar fovea and providing additional stability against translational forces during rotation. The TFCC acts as a key stabilizer, absorbing shock and limiting excessive ulnar deviation while integrating with the IOM to ensure coordinated forearm motion. Although minor syndesmotic connections may occur between certain carpal bones in rare anatomical variants, the radioulnar syndesmosis remains the dominant fibrous joint in the upper limb, underscoring its role in flexibility and precision. Another syndesmotic connection in the upper limb is provided by the coracoclavicular ligaments, which link the clavicle to the coracoid process of the scapula, stabilizing the acromioclavicular joint.[31][28][32]Function and Biomechanics
Stability Role
Syndesmoses serve a critical role in maintaining skeletal integrity by preventing excessive separation of adjacent parallel bones, particularly during weight-bearing activities and exposure to torsional forces, which helps preserve overall alignment and joint congruency.[33] This function is essential for distributing mechanical loads across the skeleton without compromising structural stability.[34] In load transmission, syndesmoses facilitate the sharing of axial forces between bones; for example, the interosseous membrane of the tibiofibular syndesmosis transfers up to 30% of the tibial load to the fibula, reducing stress concentration on individual elements.[35] This mechanism enhances the body's ability to handle compressive and shear stresses efficiently during dynamic activities. The ligaments within syndesmoses resist diastasis, or widening of the joint space, with separations greater than 2-4 mm signaling potential disruption to stability, depending on the measurement method.[36] Complementing this, interosseous membranes distribute shear forces, further bolstering resistance to lateral or rotational displacements.[37]Permitted Movements
Syndesmoses are classified as amphiarthrotic joints, allowing only slight gliding or rotational movements, typically limited to 2-5 degrees, in contrast to the greater mobility of true synovial articulations.[2][38] This minimal motion supports functional stability while preventing excessive displacement between the connected bones. The mechanisms enabling this limited mobility involve the elastic deformation of the syndesmotic ligaments under physiological tension, which permits subtle stretching without rupture, and the inherent flexibility of the interosseous membrane that accommodates micromotion.[39] In the forearm, for example, the interosseous membrane facilitates rotational micromotion during pronation and supination by distributing loads and allowing controlled longitudinal shifts between the radius and ulna.[40] These movements are tightly constrained by the fixed length of the ligaments and the precise geometry of the articulating bones, ensuring that displacements remain within physiological norms. Widening of the syndesmosis beyond 2 mm, for instance, exceeds normal limits and signals potential instability or injury.[41][42] Physiological examples illustrate this controlled mobility: during normal gait, external rotation at the ankle widens the tibiofibular syndesmosis by approximately 1 mm, aiding shock absorption without compromising alignment. In the forearm, the interosseous membrane's flexibility enables up to 180 degrees of overall pronation-supination through incremental syndesmotic adjustments.[43][40]Clinical Significance
Common Injuries
Syndesmotic sprains, commonly known as high ankle sprains, represent the primary injury to syndesmotic joints and involve damage to the ligaments stabilizing the syndesmosis, most notably the anterior inferior tibiofibular ligament (AITFL), posterior inferior tibiofibular ligament (PITFL), and interosseous ligament.[44] These injuries are classified into three grades based on the severity of ligament involvement: grade I consists of a mild stretch without significant instability; grade II involves partial tearing with moderate instability; and grade III features complete ligament tears leading to diastasis, or widening, of the syndesmotic joint space.[45] The mechanisms of syndesmosis injuries typically arise from high-impact forces such as excessive external rotation of the foot combined with dorsiflexion, often occurring in collision sports like American football or during falls with the foot planted.[46] These forces disrupt the syndesmotic ligaments sequentially, starting with the AITFL and progressing to more severe involvement of deeper structures if the energy is sufficient.[46] Syndesmosis injuries account for 10-20% of all ankle sprains, though they are less frequent than lateral ligament sprains but associated with longer recovery times.[47] The distal tibiofibular syndesmosis is the most commonly affected site, particularly in the context of ankle trauma.[46] Injuries to the radioulnar syndesmosis are less frequent but occur in high-energy axial loading scenarios, such as the Essex-Lopresti injury, which combines radial head fracture with disruption of the interosseous membrane and distal radioulnar joint instability.[48] Acute effects of syndesmosis injuries include severe pain localized to the anterior ankle or forearm (depending on the site), significant swelling, and functional instability that impairs weight-bearing or push-off activities.[44] These injuries are associated with syndesmotic disruption in 20-40% of Weber B and 50-100% of Weber C ankle fractures, where the fibular fracture extends to or above the syndesmosis level, exacerbating joint instability.[49]Diagnosis and Treatment
Diagnosis of syndesmosis injuries, particularly of the distal tibiofibular syndesmosis, begins with clinical evaluation using specific provocation tests to assess pain and instability. The squeeze test involves compressing the tibia and fibula at mid-calf level, eliciting pain at the syndesmosis if positive, while the external rotation test applies external rotation and dorsiflexion to the foot with the knee flexed at 90 degrees, reproducing pain in affected cases.[43][50] The Cotton test, or hook test, detects instability by applying a lateral force to the fibula, causing widening of the syndesmosis greater than 5 mm compared to the contralateral side.[43] These tests have high sensitivity for detecting ligamentous disruption, though their specificity varies.[50] Imaging confirms clinical suspicion and evaluates injury extent. Standard ankle radiographs, including anteroposterior, mortise, and lateral views, assess for syndesmotic widening; a tibiofibular clear space exceeding 6 mm on the anteroposterior view or medial clear space greater than 4 mm indicates diastasis.[43] Stress radiographs, such as external rotation views, further evaluate dynamic instability by measuring increased overlap or clear space under load.[43] Computed tomography (CT) provides superior detail for subtle diastasis or rotational malalignment, while magnetic resonance imaging (MRI) visualizes ligament integrity, such as the lambda sign indicating anterior inferior tibiofibular ligament tears.[43][51] Ultrasound offers dynamic assessment during stress maneuvers, detecting gaps in the syndesmosis with high accuracy for grades I and II injuries.[51] Arthroscopy serves as the gold standard for direct visualization and confirmation of ligament tears or instability, particularly in ambiguous cases, though it is invasive.[52] Injuries are graded based on ligament involvement and stability: grade I involves partial tears of the anterior inferior tibiofibular ligament with no diastasis; grade II features complete anterior ligament tear and partial interosseous membrane disruption, potentially unstable; grade III includes complete tears of multiple ligaments with diastasis and instability.[51] Treatment is tailored to injury grade and stability, emphasizing early intervention to prevent chronic instability. For grade I and stable grade II injuries, conservative management employs the RICE protocol (rest, ice, compression, elevation) initially, followed by immobilization in a non-weight-bearing cast or brace for 1-3 weeks (grade I) to 4-6 weeks (grade II), allowing ligament healing without surgery.[51][43] Unstable grade II or grade III injuries require surgical stabilization to restore syndesmotic alignment. Syndesmotic screw fixation, using 3.5- or 4.5-mm screws placed 2-5 cm above the joint line, achieves reduction in 87.9% of cases but necessitates non-weight-bearing for 6-8 weeks and hardware removal to avoid malreduction.[43][53] Tightrope or suture-button devices provide dynamic fixation, permitting earlier weight-bearing and motion with comparable or superior outcomes, including higher American Orthopaedic Foot and Ankle Society scores (mean 93 vs. 88 at 12 months) and reduced reoperation rates (10% vs. 52%).[53][43] Rehabilitation focuses on progressive weight-bearing and functional restoration, typically spanning 3-6 months post-treatment. Initial phases involve protected mobilization in a boot, advancing to full weight-bearing by 6-12 weeks for surgical cases, followed by strengthening, proprioception exercises, and sport-specific training.[43] Early intervention improves outcomes, reducing risks of osteoarthritis and enabling return to activity in 70-80% of cases without residual instability.[51]References
- https://en.wiktionary.org/wiki/syndesmosis