2.3 Arthrology and Syndesmology of the Appendicular Skeleton
Key Takeaways
- Joints are fibrous (suture, syndesmosis, gomphosis), cartilaginous (synchondrosis of hyaline cartilage, symphysis of fibrocartilage), or synovial (plane, hinge, pivot, condyloid, saddle, ball-and-socket).
- When a convex surface moves on a concave surface, roll and glide are opposite; glenohumeral abduction is superior roll and inferior glide of the humeral head.
- The anterior band of the inferior glenohumeral ligament is the primary anterior stabilizer of the abducted, externally rotated shoulder.
- The iliofemoral ligament (Y of Bigelow) is the strongest hip ligament and limits extension; the anterior band of the ulnar collateral ligament is the primary elbow valgus restraint.
- Closed-pack positions: glenohumeral abduction plus external rotation; knee full extension; talocrural full dorsiflexion. The anterior talofibular ligament is the most commonly sprained ankle ligament.
Classification of joints
Structural classification is the language of Part I stems. Fibrous joints are united by dense connective tissue. Cartilaginous joints are united by cartilage. Synovial joints have a cavity, hyaline articular cartilage, a capsule lined by synovial membrane, and synovial fluid. Functional labels still appear: synarthrosis (immobile), amphiarthrosis (slightly mobile), diarthrosis (freely mobile)—almost all diarthroses are synovial.
| Structural class | Subtype | Tissue | Appendicular example |
|---|---|---|---|
| Fibrous | Suture | Collagen, interlocking bone | None on the limbs (skull) |
| Fibrous | Syndesmosis | Interosseous ligament or membrane | Radioulnar and tibiofibular interosseous membranes; distal tibiofibular syndesmosis |
| Fibrous | Gomphosis | Periodontal ligament | Tooth in socket (not a limb joint, but the third fibrous type) |
| Cartilaginous | Synchondrosis (primary) | Hyaline cartilage | Epiphyseal growth plates; first sternochondral joint |
| Cartilaginous | Symphysis (secondary) | Fibrocartilage | Pubic symphysis; manubriosternal joint after ossification of the disc is still classified historically as a symphysis |
| Synovial | Plane (gliding) | Hyaline + cavity | Acromioclavicular, intercarpal, proximal tibiofibular, tarsometatarsal |
| Synovial | Hinge (ginglymus) | Uniaxial | Humeroulnar, talocrural, interphalangeal |
| Synovial | Pivot (trochoid) | Uniaxial rotation | Proximal and distal radioulnar joints |
| Synovial | Condyloid (ellipsoid) | Biaxial | Radiocarpal, metacarpophalangeal |
| Synovial | Saddle (sellar) | Biaxial | First carpometacarpal; sternoclavicular |
| Synovial | Ball-and-socket (spheroid) | Multiaxial | Glenohumeral, hip |
| Synovial | Bicondylar / modified hinge | Dual condyles | Knee |
Synovial articular cartilage is hyaline, aneural and avascular, nourished by synovial fluid and subchondral bone. The fibrous capsule is lined by synovial membrane (intima plus subintima) that does not cover articular cartilage. Some joints add a fibrocartilage disc or labrum (sternoclavicular disc, triangular fibrocartilage, glenoid and acetabular labra, menisci).
Syndesmology is the study of ligaments. Ligaments may be capsular thickenings (glenohumeral ligaments, iliofemoral) or extracapsular (LCL, coracoclavicular) or intra-articular but extrasynovial (ACL and PCL are intra-articular yet covered by synovial folds).
Articulations of the appendicular skeleton
Shoulder complex
Sternoclavicular (SC) is a saddle synovial joint with an articular disc. It is the only true synovial link from the upper limb to the axial skeleton. Ligaments: anterior and posterior sternoclavicular, interclavicular, and costoclavicular (primary stabilizer). Motion here is obligatory during elevation of the arm.
Acromioclavicular (AC) is a plane synovial joint, often with a partial disc. Horizontal stability comes from the AC capsule and the trapezoid ligament (anterolateral). Vertical stability comes from the conoid ligament (posteromedial). Together they are the coracoclavicular ligament—the structure that fails in higher-grade AC separations.
Glenohumeral (GH) is a ball-and-socket joint. The glenoid covers about one-third of the humeral head; the glenoid labrum deepens the socket (the long head of biceps anchors at the supraglenoid tubercle/labrum—the SLAP region). Capsule is redundant inferiorly (axillary pouch). Named capsular thickenings:
| Ligament | When it tightens | Role |
|---|---|---|
| Superior GH ligament (SGHL) with coracohumeral ligament | Adduction | Inferior restraint at the side; rotator-interval roof |
| Middle GH ligament | Mid-abduction, external rotation | Anterior restraint in mid-range |
| Inferior GH ligament, anterior band | Abduction plus external rotation | Primary anterior stabilizer in the throwing position |
| Inferior GH ligament, posterior band | Abduction plus internal rotation | Posterior-inferior restraint |
| Coracoacromial ligament | With acromion, forms the coracoacromial arch | Secondary superior restraint; subacromial space roof |
Static stabilizers are labrum, capsule, and negative intra-articular pressure; dynamic stabilizers are the rotator cuff (myology chapter). Closed pack is abduction and external rotation. Loose (open) pack is about 55° of abduction and 30° of horizontal adduction.
Elbow and forearm
The elbow is three joints in one capsule: humeroulnar (hinge), humeroradial (limited ball-and-socket morphologically, functions with the hinge), and proximal radioulnar (pivot). The annular ligament wraps the radial head. The ulnar (medial) collateral ligament has anterior, posterior, and transverse bands; the anterior band is the primary valgus restraint (about 30–90° of flexion). The radial (lateral) collateral complex, including the lateral ulnar collateral ligament, resists varus and posterolateral rotatory instability. Carrying angle is typically about 5–15° of valgus, often slightly greater in females.
Proximal and distal radioulnar joints are pivots. The interosseous membrane is a syndesmosis that transfers compressive load from radius to ulna. Distally the triangular fibrocartilage complex (TFCC)—articular disc, ulnocarpal ligaments, and the extensor carpi ulnaris sheath—stabilizes the distal radioulnar joint and cushions the ulnar-sided wrist.
Wrist and hand
Radiocarpal is condyloid: the distal radius articulates mainly with scaphoid and lunate; the ulna does not usually touch the carpals directly (TFCC intervenes). Midcarpal joint (proximal row on distal row) adds motion. About 80% of axial load goes through the radius. First carpometacarpal is the textbook saddle joint (trapezium–first metacarpal), allowing opposition. MCP joints are condyloid; IP joints are hinges with volar plates that resist hyperextension. Deep transverse metacarpal ligaments unite the palmar plates of digits 2–5.
Hip
The hip is a deep ball-and-socket with an acetabular labrum and the transverse acetabular ligament spanning the acetabular notch. Ligamentum teres carries the foveolar artery and is a secondary stabilizer. Capsular ligaments:
| Ligament | Attachments | Limits |
|---|---|---|
| Iliofemoral (Y of Bigelow) | AIIS / acetabular rim to intertrochanteric line | Strongest; limits extension and external rotation; allows energy-efficient standing |
| Pubofemoral | Pubic acetabular rim to femoral neck | Abduction and extension |
| Ischiofemoral | Posterior acetabulum to neck / greater trochanter region | Internal rotation and extension |
Zona orbicularis is a circular capsular collar around the neck. Ligamentous closed pack is full extension, abduction, and internal rotation. Resting (open) pack is about 30° flexion, 30° abduction, and slight external rotation. Maximum bony congruence is actually flexion–abduction–external rotation (frog-leg); do not confuse bony congruence with the ligamentous closed-pack position.
Knee
The knee is a bicondylar modified hinge with a patellofemoral articulation. Cruciate ligaments are named for their tibial attachments.
| Structure | Attachments (conceptual) | Primary restraint |
|---|---|---|
| ACL | Anterior tibia to medial aspect of the lateral femoral condyle | Anterior tibial translation and tibial internal rotation; posterolateral bundle taut in extension, anteromedial in flexion |
| PCL | Posterior tibia to lateral aspect of the medial femoral condyle | Posterior tibial translation |
| MCL (tibial collateral) | Medial epicondyle to tibia; attaches to medial meniscus | Valgus |
| LCL (fibular collateral) | Lateral epicondyle to fibular head; no meniscal attachment | Varus |
| Medial meniscus | C-shaped, less mobile, MCL attachment | Load distribution; unhappy-triad partner |
| Lateral meniscus | More circular, more mobile, popliteus relation | Load distribution |
Unhappy (O'Donoghue) triad: ACL, MCL, and medial meniscus after a valgus-plus-rotation injury. Meniscofemoral ligaments of Humphry (anterior to PCL) and Wrisberg (posterior to PCL) tether the posterior horn of the lateral meniscus. Oblique popliteal ligament (semimembranosus expansion) reinforces the posterior capsule; arcuate popliteal ligament arches over popliteus. Coronary (meniscotibial) ligaments attach menisci to the tibia.
Ankle, hindfoot, and tibiofibular syndesmosis
Talocrural is a hinge: mortise (tibial plafond plus malleoli) on the talar trochlea. Lateral ligaments: anterior talofibular (ATFL) (most commonly sprained, plantarflexion plus inversion), calcaneofibular (CFL) (inversion in more dorsiflexion), posterior talofibular (PTFL) (strongest lateral). Medial deltoid ligament (tibionavicular, tibiocalcaneal, anterior and posterior tibiotalar) is strong; eversion more often avulses bone than tears deltoid in isolation.
Subtalar (talocalcaneal) allows inversion and eversion. Transverse tarsal (Chopart) is talonavicular plus calcaneocuboid. Tarsometatarsal (Lisfranc) joints are plane joints with a critical Lisfranc ligament from medial cuneiform to the second metatarsal base. Plantar ligaments: spring (plantar calcaneonavicular) supporting the talar head, long and short plantar ligaments, and the bifurcate ligament (calcaneonavicular and calcaneocuboid bands) on the dorsolateral midfoot.
Proximal tibiofibular is plane synovial. Distal tibiofibular is a syndesmosis: anterior and posterior inferior tibiofibular ligaments, interosseous ligament, and inferior transverse ligament. Forced external rotation of the foot can produce a high ankle sprain of this syndesmosis.
Pubic symphysis is a secondary cartilaginous joint of the pelvic girdle with a fibrocartilage disc and superior and arcuate pubic ligaments.
Biomechanics
Concave-convex (Kaltenborn) rule
Treat the moving partner as the one you name.
- If a convex surface moves on a fixed concave surface, roll and glide are opposite.
- If a concave surface moves on a fixed convex surface, roll and glide are the same direction.
Roll is always in the direction of the osteokinematic swing (the bone's distal end). Glide is the arthrokinematic slide at the joint surface.
| Motion | Moving surface | Roll | Glide |
|---|---|---|---|
| GH abduction (open chain) | Convex humeral head | Superior | Inferior |
| GH external rotation | Convex humeral head | Posterior | Anterior |
| Knee extension, open chain | Concave tibial plateau | Anterior | Anterior |
| Knee extension, closed chain | Convex femoral condyles | Anterior | Posterior |
| Radiocarpal extension | Convex proximal carpals | Dorsal | Palmar |
| Hip abduction | Convex femoral head | Superior / lateral | Inferior / medial |
| Talocrural dorsiflexion | Convex talus | Anterior | Posterior |
Inferior glide of the humeral head during abduction is why a superiorly migrated head (weak cuff, tight inferior capsule) produces subacromial impingement. Posterior talar glide is the accessory motion of dorsiflexion—relevant when a restricted ankle lacks closed-chain knee-forward travel.
Open-pack versus closed-pack
Closed-pack: maximum ligament tautness, maximal congruency for most joints, least joint play—used for stability testing and as an end-range to avoid during early mobilization. Open- (loose-) pack: capsule lax, maximal joint play—used for joint-play assessment and mobilization.
| Joint | Closed pack | Open (resting) pack |
|---|---|---|
| Glenohumeral | Abduction and external rotation | About 55° abduction, 30° horizontal adduction |
| Humeroulnar | Full extension and supination | About 70° flexion, 10° supination |
| Radiocarpal | Full extension and radial deviation | Slight flexion and ulnar deviation |
| Hip (ligamentous) | Full extension, abduction, internal rotation | 30° flexion, 30° abduction, slight external rotation |
| Knee | Full extension and tibial external rotation | About 25° flexion |
| Talocrural | Full dorsiflexion | About 10° plantarflexion, midway between inversion and eversion |
Kinetic chain, rhythm, and screw-home
In an open kinetic chain, the distal segment is free (seated knee extension). In a closed kinetic chain, the distal segment is fixed (squat). Arthrokinematics reverse which bone is moving, so the concave-convex rule still holds if you correctly name the moving surface.
Scapulohumeral (scapulothoracic) rhythm: after the first ~30° of abduction, roughly 2° of glenohumeral motion per 1° of scapulothoracic upward rotation (a 2:1 ratio) through full elevation. The clavicle elevates and posteriorly rotates at SC and AC joints to allow that scapular rotation.
Screw-home mechanism of the knee: in open chain, the tibia externally rotates on the femur in terminal extension (longer medial femoral condyle). In closed chain, the femur internally rotates on the tibia. Popliteus unlocks the fully extended knee: it internally rotates the tibia in open chain, or externally rotates the femur in closed chain.
The trapeziometacarpal joint of the thumb is classified as which type of synovial joint?
During open-chain glenohumeral abduction, arthrokinematics of the convex humeral head on the concave glenoid follow the concave-convex rule as which pairing?
Which position is the closed-packed position of the glenohumeral joint?