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Labrum

The labrum (from Latin labrum, meaning "lip") is a ring of fibrocartilage that lines the rim of certain joint sockets in vertebrates, deepening the articular surface to enhance stability and load distribution.^[1] It is prominently featured in ball-and-socket joints, such as the glenoid labrum surrounding the glenoid cavity of the scapula in the shoulder and the acetabular labrum encircling the acetabulum of the pelvis in the hip.^[2]^[3] The term is also used in zoology for a lip-like mouthpart in arthropods, but these structures are analogous rather than homologous.^[4]

Etymology and overview

Definition

The labrum is a flap-like, unpaired structure forming the anterior margin of the mouth in arthropods, serving as an upper lip that aids in food manipulation and containment during feeding.^[5] It is typically a simple, sclerotized plate hinged to the clypeus, distinct from the paired appendages like mandibles and maxillae that make up the rest of the arthropod mouthparts.^[6] In insects, the labrum is located above the mandibles and helps direct food toward the oral cavity, often bearing sensory setae for detecting food particles.^[7] The labrum is composed of chitinous cuticle, providing rigidity and flexibility, with a layered structure including epicuticle, exocuticle, and endocuticle that contributes to its sclerotization.^[8] Sensory structures such as setae and campaniform sensilla may be present on its surface for mechanoreception.^[9] The arthropod labrum's features have been documented in zoological literature since the 18th century, with systematic descriptions in 19th-century entomology texts and advancing through 20th-century morphological studies that elucidated its developmental origins and evolutionary significance.^[10] In vertebrate anatomy, the term "labrum" is analogously applied to fibrocartilaginous rims that deepen joint sockets, such as in the shoulder and hip, but these structures are not homologous to the arthropod labrum.^[4]

Etymology

The term "labrum" originates from the Latin labrum, meaning "lip," a designation that reflects its anatomical appearance as a projecting, lip-like edge bordering the mouth or socket.^[11]^[12] In zoological nomenclature, "labrum" was adopted for arthropod anatomy in the 18th century, with early uses in entomological works such as those by Jacob Sturm (1800).^[10] By the 19th century, the term was established in studies of insect and arthropod morphology, as seen in comprehensive texts on invertebrate anatomy. The earliest English usage dates to 1578 in human anatomical contexts, but its application to arthropods underscores its broader utility for lip-like structures across taxa.^[12] Related to the Latin labium ("lip"), from which terms like "labia" derive for paired folds in vertebrates, "labrum" in arthropods emphasizes its singular, unpaired nature as a mouthpart.

Anatomy

Macroscopic structure

The labrum is a fibrocartilaginous structure that typically exhibits a triangular or wedge-shaped cross-section when viewed macroscopically.^[13] In adults, it possesses an average thickness of 3-5 mm and height of 4-6 mm, though these dimensions vary regionally and between joints.^[14] For instance, the superior glenoid labrum measures approximately 6 mm in both thickness and height, while the anterior portion is thinner at around 4 mm.^[15] The labrum attaches firmly to the bony rim of the joint socket through a transitional zone of calcified cartilage, which provides a secure interface between the fibrocartilage of the labrum and the underlying hyaline cartilage.^[16] On its outer surface, it blends seamlessly with the joint capsule and associated ligaments, such as the glenohumeral ligaments in the shoulder.^[15] This attachment pattern enhances the structural continuity of the joint. Morphological variations exist across joint types, with the acetabular labrum in the hip presenting a more uniform, C-shaped ring that encircles the acetabulum, averaging 6 mm in width and 5 mm in height.^[17] In contrast, the glenoid labrum in the shoulder shows greater variability, particularly in the superior region, which is often loosely attached and may appear meniscoid or rounded rather than firmly triangular.^[15] These differences reflect adaptations to the distinct biomechanical demands of each joint, as detailed in subsequent sections on specific locations.

Microscopic structure

The labrum exhibits a fibrocartilaginous composition with distinct zonal organization that varies slightly between the glenoid and acetabular variants but shares core histological features. The outer fibrous zone, adjacent to the joint capsule, is dominated by dense bundles of type I collagen fibers oriented parallel to the labral rim, conferring tensile strength and structural integrity to withstand peripheral stresses.^[18] Inner zones transition to a more cartilaginous structure, featuring type II collagen fibers that mimic the architecture of hyaline cartilage, enabling effective load distribution and compressive resistance.^[18] At the bone-labrum interface, a thin calcified layer anchors the structure to the underlying osseous tissue, similar to the tidemark in articular cartilage, facilitating stable attachment while minimizing shear at the junction.^[19] Cellular components within the labrum include a mix of chondrocytes, responsible for matrix maintenance in the inner zones, and fibroblasts, predominant in the outer fibrous region for collagen production.^[20] Overall cellularity remains low relative to articular cartilage, with sparse cell distribution that limits regenerative potential and contributes to the tissue's avascular character in deeper layers.^[21] Vascular supply to the labrum is limited, rendering most of the tissue avascular except for peripheral capillary loops originating from the joint capsule, which provide nourishment primarily to the outer zones.^[22] Inner regions, particularly the chondral-facing aspects, rely on diffusion of nutrients from synovial fluid, a mechanism that supports metabolic needs but impairs healing after injury.^[20]

Specific locations

Glenoid labrum

The glenoid labrum is a fibrocartilaginous ring that circumferentially surrounds the glenoid cavity of the scapula, attaching to the rim of the articular cartilage and deepening the shallow socket of the shoulder joint. It increases the effective depth of the glenoid fossa by approximately 50%, with contributions varying by direction: an addition of 9 mm superoinferiorly and 5 mm anteroposteriorly.^[23]^[19] This enhancement is achieved through the labrum's variable morphology, which generally measures 3-6 mm in thickness and 4 mm in width, though these dimensions are highly variable across individuals.^[24] Regional variations in the glenoid labrum reflect its adaptation to the shoulder's biomechanics, with the anterior-inferior portion exhibiting a rounded shape, firmer bony attachment, and greater lateral extension for enhanced structural integrity. In contrast, the superior glenoid labrum is often meniscoid in appearance, particularly in some individuals, featuring a looser attachment to the glenoid rim and a more mobile, triangular cross-section that allows for greater flexibility. Thickness also varies regionally, with the superior region typically the tallest and thickest (mean height 5.96 mm, thickness 6.02 mm), while the anterior region is shorter and thinner (mean height 3.63 mm, thickness 3.94 mm); the inferior labrum tends to be more consistent in form but narrower radially (around 2 mm).^[19]^[25]^[2] The glenoid labrum serves as an attachment site for key glenohumeral ligaments, including the superior glenohumeral ligament (anterosuperiorly at approximately 12:15-1:10 o'clock position), the middle glenohumeral ligament (superiorly to anteriorly at 12:50-3:10 o'clock), and the inferior glenohumeral ligament (anteroinferiorly and posteroinferiorly from 2-9 o'clock). Additionally, the superior labrum anchors the long head of the biceps brachii tendon, with 40-60% of its fibers blending into this region, contributing to the overall capsulolabral complex.^[19]^[24] These attachments underscore the labrum's role in integrating with the joint's soft tissue envelope, though its composition as fibrocartilage is detailed elsewhere in anatomical overviews.^[2]

Acetabular labrum

The acetabular labrum is a fibrocartilaginous ring that encircles the acetabulum of the pelvis, attaching securely to its bony rim and spanning the acetabular notch inferiorly.^[3] This structure deepens the acetabular socket by approximately 21%, enhancing femoral head containment, while also increasing the articulating surface area by about 28%.^[16] By providing these enhancements, the labrum contributes to the overall stability of the hip joint without altering its fundamental ball-and-socket configuration. In cross-section, the acetabular labrum typically exhibits a uniform triangular shape with a thickness of 3-4 mm, though it may vary slightly in different regions.^[26] Its free anterior and posterior edges are positioned to form a fluid-tight seal around the femoral head, helping to maintain synovial fluid pressure and lubricate the articular surfaces during motion.^[27] The labrum's attachments integrate it closely with surrounding structures: inferiorly, it fuses with the transverse acetabular ligament, bridging the acetabular notch; anteriorly and superiorly, it blends seamlessly with the hip joint capsule and the iliofemoral ligament, reinforcing the joint's anterior stability.^[28]^[29]^[30]

Function

Role in joint stability

The glenoid labrum deepens the shallow glenoid cavity of the scapula, increasing its concavity and thereby enhancing the containment of the humeral head to resist subluxation and translation during shoulder motion.^[31] Similarly, the acetabular labrum augments the acetabulum's depth in the hip joint, providing static stability by improving femoral head coverage and limiting excessive motion.^[32] This structural enhancement contributes approximately 10-20% to overall glenohumeral joint stability through concavity-compression mechanisms.^[19] The labrum also serves as a critical attachment site for key ligaments, facilitating force distribution across the joint. In the shoulder, it anchors the superior, middle, and inferior glenohumeral ligaments, which reinforce static stability and resist anterior-posterior translation.^[14] In the hip, the acetabular labrum integrates with the iliofemoral ligament attachments along the capsular rim, aiding in the restraint of external rotation and anterior subluxation.^[30] Additionally, the labrum contributes to dynamic stability by maintaining negative intra-articular pressure, forming a vacuum seal that resists joint distraction. This suction effect requires up to 50% greater force for humeral or femoral head dislocation compared to scenarios without intact labral sealing.^[33] In the hip, the labrum acts as the primary stabilizer against small distraction forces (1-2 mm), further bolstering overall joint integrity.^[34]

Biomechanical contributions

The labrum contributes significantly to load transmission in synovial joints by absorbing and distributing compressive forces, thereby reducing peak pressures on articular cartilage. In the glenohumeral joint, the glenoid labrum deepens the shallow glenoid fossa by up to 50%, which expands the effective contact area for the humeral head and enhances load distribution during dynamic activities.^[35]^[36] In the hip, the acetabular labrum similarly increases acetabular depth by approximately 21%, facilitating load transfer from the femoral head to the pelvis; finite element analyses indicate it bears 1–2% of the total joint load in normal morphology, rising to 4–11% in dysplastic hips where greater reliance on soft tissues occurs.^[37]^[38] By providing a sealing mechanism and buffering shear stresses at the joint periphery, the labrum enables enhanced range of motion without excessive translation. In the shoulder, this supports greater abduction and adduction, permitting up to 90° of motion while maintaining centralization of the humeral head.^[19] In the hip, it accommodates increased flexion and extension by distributing tangential forces, preserving fluid pressurization for smooth articulation.^[16] Under physiological loads, the labrum undergoes 5–10% strain deformation, reflecting its fibrocartilaginous composition that balances rigidity and flexibility. Its viscoelastic properties, characterized by an elastic modulus of approximately 23 MPa and notable stress relaxation (10–21 MPa post-cycling), allow for energy dissipation and shock absorption, preventing abrupt force transmission to subchondral bone.^[31]^[39]

Clinical significance

Common injuries

Labral injuries commonly affect the glenoid labrum in the shoulder and the acetabular labrum in the hip, with superior labrum anterior-posterior (SLAP) tears representing a primary pathology in the shoulder. SLAP tears are classified into up to ten types (I to X) based on extent and involvement of surrounding structures, with type II being the most prevalent, involving detachment of the superior labrum and biceps tendon anchor from the glenoid rim.^[40] These injuries often result from acute trauma, such as falls on an outstretched arm, or repetitive overhead motions in throwing athletes, leading to shear forces on the labral-biceps complex.^[41] Prevalence among athletes, particularly those in overhead sports like baseball, ranges from 10-20%, with one study reporting 18% in high school baseball players.^[42] In the hip, acetabular labral tears frequently manifest as radial flap or bucket-handle types, where the labrum detaches in a flap-like manner or displaces into the joint like a bucket handle, disrupting the seal of the acetabulum.^[16] These tears are primarily caused by structural abnormalities such as femoroacetabular impingement (FAI), where abnormal contact between the femur and acetabulum stresses the labrum, or hip dysplasia, which alters joint mechanics and increases shear loads.^[43] Incidence in patients presenting with hip pain is reported at 22-55%, highlighting their role as a common source of intra-articular pathology.^[44] Risk factors for labral injuries in both joints include age-related degeneration, particularly in individuals over 40, where progressive wear weakens the fibrocartilaginous tissue; acute trauma from falls or impacts; and repetitive microtrauma from occupational or athletic activities involving joint loading.^[41] Gender differences are notable, with hip labral tears occurring more frequently in females due to higher rates of pelvic anatomy variations and dysplasia predisposing to instability.^[45] In contrast, shoulder SLAP tears show a predilection for younger males engaged in high-velocity sports.^[46]

Diagnosis and imaging

Diagnosis of labral pathology typically begins with a thorough clinical history and physical examination to assess for symptoms such as pain, instability, or mechanical symptoms in the affected joint.^[47] For the shoulder glenoid labrum, particularly superior labrum anterior to posterior (SLAP) lesions, the O'Brien's active compression test is commonly used, where the patient flexes the shoulder to 90 degrees with the elbow extended and internally rotates the thumb downward against resistance; pain or weakness indicates a positive test with a sensitivity of 67% for detecting SLAP tears.^[48] In the hip, the flexion-adduction-internal rotation (FADIR) test, involving passive hip flexion to 90 degrees, adduction, and internal rotation, reproduces pain in acetabular labral tears with a sensitivity of 94%.^[47] Imaging modalities play a crucial role in confirming labral pathology, with magnetic resonance imaging (MRI) arthrography being the preferred method due to its higher diagnostic accuracy compared to standard MRI. For both shoulder and hip labral tears, MR arthrography demonstrates a sensitivity of approximately 90-95% in detecting tears by allowing contrast to outline the labral defects and associated intra-articular structures.^[49]^[50] Standard MRI, while non-invasive, has lower sensitivity for subtle tears (around 66% for hip labral tears, missing approximately 34% of cases), particularly in the hip where joint distension enhances visualization with arthrography.^[51] Ultrasound serves as a useful adjunct for dynamic assessment of the hip labrum, enabling real-time evaluation during maneuvers like traction or rotation to detect tears or impingement with a sensitivity of up to 94% in experienced hands.^[52] Differential diagnosis of labral tears requires distinguishing them from conditions such as rotator cuff tears in the shoulder, which present with similar overhead pain but more pronounced weakness on external rotation, or osteoarthritis, characterized by progressive stiffness and crepitus rather than acute mechanical symptoms.^[53] In the shoulder, SLAP lesions are often graded using the Snyder classification system, which categorizes tears into four types based on extent and detachment: type I (fraying), type II (detachment), type III (bucket-handle tear), and type IV (extension into biceps tendon), aiding in prognostic and management decisions.^[54]

Treatment and management

Conservative approaches

Conservative management of labral injuries prioritizes non-invasive strategies to reduce pain, inflammation, and joint stress while promoting natural healing and functional recovery, particularly for stable tears or patients with contraindications to surgery. These approaches are tailored to the specific joint involved—glenoid labrum in the shoulder or acetabular labrum in the hip—but share core principles of symptom control and progressive rehabilitation. The choice of conservative treatment is influenced by injury severity, with milder lesions responding more favorably to these methods. Rest and activity modification form the cornerstone of initial care, aiming to protect the labrum from further irritation during the acute phase. Patients are instructed to avoid provocative motions, such as overhead reaching or throwing for shoulder injuries and deep hip flexion or pivoting for acetabular tears, to minimize pain and allow tissue stabilization. For acute tears, this typically involves a 4-6 week period of relative rest, potentially including sling immobilization for glenoid labral injuries to limit shoulder motion, though hip management focuses more on weight-bearing adjustments without formal bracing. This phase helps prevent exacerbation while maintaining gentle mobility to avoid stiffness. Physical therapy follows rest and emphasizes strengthening the surrounding musculature to enhance joint stability and proprioception. Protocols generally span 6-12 weeks of supervised sessions, starting with low-load exercises like isometrics and progressing to dynamic strengthening. For glenoid labral tears, rotator cuff and scapular stabilizer exercises, such as side-lying external rotations and scapular push-ups, are prioritized to improve shoulder girdle control. In acetabular cases, hip abductor and core strengthening—via clamshells, bridges, and single-leg balances—targets stabilizers to reduce labral loading and enhance neuromuscular control. Proprioceptive training, including balance exercises on unstable surfaces, is integrated throughout to restore joint awareness and prevent reinjury. Pharmacotherapy complements rehabilitation by addressing pain and inflammation. Nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen or naproxen, are commonly prescribed to alleviate symptoms and support early mobility in both shoulder and hip labral injuries. For persistent cases, intra-articular corticosteroid injections provide short-term relief by reducing localized inflammation, though their use is judicious to avoid potential joint weakening with repeated administration.

Surgical interventions

Surgical interventions for glenoid labral tears, particularly superior labrum anterior to posterior (SLAP) lesions, commonly involve arthroscopic techniques. Arthroscopic debridement is used for stable, degenerative tears, where frayed tissue is trimmed to create a smooth edge, while repair is indicated for unstable or acute tears involving detachment from the glenoid rim. In repair procedures, suture anchors are placed into the glenoid rim to secure the labrum, often using 1-3 anchors depending on tear extent, with knotless anchors preferred to minimize intra-articular prominence and reduce complications. Clinical outcomes for arthroscopic SLAP repair demonstrate success rates of 85-95%, with good to excellent results in up to 97% of cases and return to sport in approximately 84% of patients.^[55] For acetabular labral tears in the hip, arthroscopic repair focuses on refixation of detachable segments, typically suitable for repairable tears to preserve native tissue integrity and joint suction-seal function. Suture anchors are anchored into the acetabular rim to reattach the labrum, with techniques emphasizing circumferential coverage and avoidance of over-tensioning. Success rates for hip labral repair range from 85-90%, with significant improvements in patient-reported outcomes such as pain reduction and function restoration. For irreparable tears, such as those with hypoplastic or calcified labrum, labral reconstruction employs autografts, including iliotibial band or capsular tissue, to recreate labral anatomy and restore joint stability; autografts are favored for their biocompatibility and lower cost compared to allografts. Reconstruction yields comparable or superior outcomes to repair in select cases, with failure rates reduced in older patients or extensive defects.^[56]^[57]^[58] Postoperative rehabilitation protocols for both shoulder and hip labral repairs follow a phased approach lasting 4-6 months to promote healing and restore function. Initial immobilization with a sling (shoulder) or brace (hip) for 3-6 weeks protects the repair, followed by passive range-of-motion exercises progressing to active strengthening and proprioceptive training. Return to sport is generally anticipated at 6-9 months, contingent on achieving full strength and stability milestones. Complications occur in 5-10% of cases, with postoperative stiffness being prominent, often managed through aggressive physical therapy but occasionally requiring manipulation under anesthesia.^[59]^[60]^[61]

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Overall, MRI had an accuracy of 76 %, a PPV of 24 %, and a NPV of 95 %. Sensitivity was 66 %, and specificity was 77 %. MR arthrography had an accuracy of 69 %, ...
[57]
Chronic Shoulder Pain: Part I. Evaluation and Diagnosis - AAFP
Feb 15, 2008 · Common conditions that can result in chronic shoulder pain include rotator cuff disorders, adhesive capsulitis, shoulder instability, and shoulder arthritis.
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SLAP lesions of the shoulder - PubMed
We have labeled this injury a "SLAP lesion" (Superior Labrum Anterior and Posterior). There were 23 males and four females with an average age of 37.5 years.Missing: classification | Show results with:classification
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SLAP lesions: current controversies in - EFORT Open Reviews
Jan 28, 2019 · Several provocative tests for SLAP tears have been described: O'Brien (i.e. active compression test); anterior slide; biceps load (I and II); ...
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Hip Labral Tear Surgery Success Rate | Benjamin Domb M.D.
The hip arthroscopy success rate ranges from 85 to 90 percent. The procedure is considered the safest and most effective method for repairing a labral tear.Missing: refixation | Show results with:refixation
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The Efficacy of Labral Reconstruction: A Systematic Review - PMC
As such, labral repair techniques have evolved, and studies have demonstrated increasingly high success rates. However, because of the relative ...
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Graft Options in Hip Labral Reconstruction - PMC - NIH
Jan 27, 2021 · Autografts. The major advantages of autografts are maintaining patient biology, no risk of disease transmission, availability, and lower cost.
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[PDF] Arthroscopic Labrum Repair of the Shoulder (SLAP)
Rehabilitation exercises to strengthen the rotator cuff muscles may also be recommended. If these conservative measures are insufficient, your doctor may.
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[PDF] Rehabilitation Protocol for Hip Arthroscopy for Femoroacetabular ...
This protocol is intended to guide clinicians through the post-operative course for Hip Arthroscopy for Femoroacetabular.
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Complications After Arthroscopic Shoulder Surgery: A Review ...
Stiffness/arthrofibrosis was the most commonly recorded surgical complication (2.2%).