Hip joint capsule & mechanics
This work is under construction and is currently incomplete
Hip joint capsule
The hip joint capsule is strong and dense being attached to:
Acetabular margin 5-6 mm beyond its labrum, outer labral aspect and near the acetabular notch.
Transverse acetabular ligament (straddles the inferior limit of the acetabulum) and the adjacent rim of the obturator foramen.
Surrounds the femoral neck attaching to the anterior intertrochanteric line, base of the femoral neck, 1 cm above the posterior intertrochanteric crest and the femoral neck near the lesser trochanter.
The hip joint capsule is reinforced by three primary fibrous capsular ligaments that has distinct functional roles to stabilise the joint (Ng et al 2019):
Iliofemoral ligament. transverse and descending fibrous branches. They form an inverted ‘Y’ shape as they both attach to the AIIS (which is located in the anterosuperior region of the acetabular margin) —> intertrochanteric line (transverse part: superolateral end of the intertrochanteric line attaching to the gluteus minimus tendon; descending part: inferomedial end of the intertrochanteric line attaching to the deep aponeurosis of the iliopsoas) (Tsutsumi et al 2022). Reinforces the capsule during external rotation and extension. The transverse arm predominately resists external rotation in flexion and 0 degs at internal rotation in extension (Ng et al 2019). As the iliofemoral ligament is a ‘joint capsule complex’ with connections to the gluteus minimus tendon and deep aponeurosis of the iliopsoas, it may also dynamically coordinate the hip position during movement by transmitting the contraction force of the gluteus minimus and iliopsoas to the joint (Tsutsumi et al 2022).
Ischiofemoral ligament. Ischium (posteroinferior to the acetabular rim) —> posterior intertrochanteric line. Reinforces the capsule during internal rotation at 0 degs and in FADIR (Ng et al 2019). It also acts as the primary contributor to hip stability by restricting internal rotation of the hip at < 30° flexion as it pulls the femoral head into the acetabulum (Baba et al 2022). This stabilising function is augmented by the external rotators (Parvaresh et al 2019) (refer below ‘hip joint compression: external rotators and ischiofemoral ligament’).
Pubofemoral ligament. Superior pubic ramus —> converges with the medial iliofemoral and inferior ischiofemoral ligaments to insert into the femur. Reinforces the inferior capsule during hip extension/abduction/external rotation (Ng et al 209).
Zona orbicularis. Composed of circular fibers that form a stability-inducing collar, which closes around the femoral neck. During hip extension, the posteroinferior aspect of the zona orbicularis overlaps to medialise and secure the femoral head anteriorly, while during deep flexion, the anteroinferior aspect of the zona orbicularis medialises and secures the femoral head posteriorly. The zona orbicularis (along with the capsule and labrum) are crucial for rotational and distraction stability. The zona orbicularis may have a role in circulating synovial fluid between the central and peripheral compartments of the hip joint capsule (Ng et al 2019).
The hip joint capsule can be implicated in the pathogenesis of pain from being too loose, too tight, too irregular, too thick, too thin or from too much adhesion formation (capsulolabral adhesions or peripheral compartment adhesions to the femoral neck). Also, overlying musculotendinous units (iliopsoas, rectus femoris, gluteus minimus, iliocapsularis, obturator externus and conjoint tendon) may also induce a nerve stretch instigating pain (Meheux et al 2021).
Other important ligamentous structures are:
Ligamentum teres. Triangular-shaped. Peripheral inferior acetabular notch —> femoral head (fovea). Provides a conduit for small vessels and innervation to the femoral head. It plays a critical role in proprioception and structural stability acting as secondary restraint (along with the labrum) in wider external rotation (Ng et al 2019).
Proximal iliofemoral ligament is larger in cross-sectional area and stiffer than the posterior capsule. This means the anterior capsule is stronger than the posterior region and the iliofemoral ligament is dependent on age-related changes (Ng et al 2019).
The capsular ligaments undergo changes in thickness and length, becoming progressively tighter, in response to mechanical stress and progressive arthritis or FAI (cam/pincer/mixed). Thicker regions of the capsule corresponded to shorter capsule length, which may explain anterior hip tightness and pain (Ng et al 2019).
Innervation of the hip joint capsule
The hip joint capsule is innervated by:
Anterior capsule (and the superior labrum): femoral and obturator nerves.
Posterior capsule: nerve to the quadratus femoris posteroinferiorly (15/18 hips), superior gluteal nerve posterosuperiorly in (6/18 hips), sciatic nerve posterosuperiorly (1/16 hips) (Nagpal et al 2021) and inferior gluteal nerve posteroinferiorly (Meheux et al 2021).
The femoral and obturator nerves have the higher density of nociceptors and mechanoreceptors (< anterosuperior capsule and the labrum has the densest nociceptive innervation (Laumonerie et al 2021) and the superolateral capsule has the greatest density of mechanoreceptors and sensory fibers (Meheux et al 2021). Although, the anterosuperior capsule between 1:00 and 2:30 o’clock is void of any innervation (Meheux et al 2021). This greater density of mechanoreceptors reflects the high tensile stress from dense connective tissues (joint capsule and iliofemoral ligament) at their AIIS attachment (Tsutsumi et al 2022).
Hip pain results from the stimulation of free endings of nociceptors of slow-conducting (unmyelinated axons, C-fibers) and faster conducting (thinly myelinated axons, A-delta fibers) nerve fibers. The perception of pain in the hip may be transmitted by autonomic innervation of the intraosseous blood vessels, innervation of the synovium and capsule, and periarticular muscles (Meheux et al 2021).
Aging is associated with decreased pain fiber expression (and increased fibrous tissue formation), therefore greater expression of nerve fibers in, for example, OA hips, does not correlate with pain or disability. However, in pediatric patients, with increased pain fibers expression, greater expression of nerve fibers more significantly affects pain levels. Therefore, in elderly patients it is the increased release of inflammatory mediators that are likely play a major role in pain among patients with OA hips and FAI (Meheux et al 2021).
Muscular attachments to the joint capsule
Muscular attachments to the anterior joint capsule include (Tsutsumi et al 2020):
The deep aponeurosis of the iliopsoas and iliocapsularis muscle. Iliocapsularis: AIIS and strongly attached to the anteromedial hip capsule —> just distal to the lesser trochanter. It reduces capsular impingement during walking and hip flexion (Macdermott et al 2022) and is an important contributor to hip stability, especially in dysplastic hips (Ng et al 2019). As the deepest portion of the iliopsoas, the iliocapsularis is surrounded by, and connected to, the deep aponeurosis of the iliopsoas. The iliocapsularis therefore attaches to the joint capsule via the deep aponeurosis of the iliopsoas.
The deep aponeurosis of the iliopsoas, that attaches on to the anterosuperior joint capsule, descends to the inferomedial end of the intertrochanteric line. Here it shares an attachment with the descending part of the iliofemoral ligament. Where the deep aponeurosis of the iliopsoas and the gluteus minimus tendon attaches to the anterosuperior joint capsule it creates a relative thickening of the capsule.
Gluteus minimus. The deep aponeurosis of the gluteus minimus (which emerges from the gluteus minimus tendon) and the gluteus minimus tendon attaches on to the anterior joint capsule. The connection of the gluteus minimus tendon and the anterior border of the deep aponeurosis of the iliopsoas to the anterosuperior joint capsule creates a relative thickening of the capsule. The gluteus minimus tendon attachment to the tubercle at the superolateral end of the intertrochanteric line gives it a shared attachment with the transverse part of the iliofemoral ligament.
The transverse and descending parts of the iliofemoral ligament are the joint capsule. Due to the shared attachments of the “so‐called iliofemoral ligament” and the iliopsoas and gluteus minimus, and the intimate attachments of the iliopsoas deep aponeurosis and gluteus minimus tendon to the joint capsule, the iliofemoral ligament could be regarded as a dynamic stabiliser. Being able to transmit muscular power to the joint via the capsular complex means it may be able to maintain its tension via the contraction force of the gluteus minimus and iliopsoas, even in hip positions in which the ligament is often considered to be loose (Tsutsumi et al 2020).
The proximal aponeurosis of the rectus femoris.
The muscular attachments to the posterior joint capsule include (Tsutsumi et al 2020):
Deep aponeuroses of the gluteus minimus.
Obturator internus and gemelli superior and inferior complex.
Obturator externus.
Note: the the piriformis does not have a capsular attachment (Walters et al 2014) although the conjoint tendon (piriformis + obturator internus/gemelli complex), that has attachments to the gluteus medius and obturator externus, does attach to the hip joint capsule (Solomon et al 2010).
Hip joint compression: external rotators & ischiofemoral ligament
The ischiofemoral ligament is the primary contributor to hip stability by restricting internal rotation of the hip at < 30° flexion as it pulls the femoral head into the acetabulum (Baba et al 2022). This stabilising function is augmented by the external rotators. Whilst each individual external rotator’s cross sectional areas has minimal impact on hip joint compression, when looked upon as a collective, they accumulatively exceed that of the gluteus minimus (Parvaresh et al 2019).
The superior gemellus, obturator internus, inferior gemellus, and obturator externus are essentially fused. With the addition of quadratus femoris and piriformis, the collective “short external rotators” become a substantial force-producing unit. Considering them as a unit, with a large physiological cross sectional area and short fiber length, enables to have a stabilising function (Parvaresh et al 2019) < the quadratus femoris (Takao et al 2018). This stabilising effect occurs by their accumulated external rotator function combining with their rotational antagonists (gluteus minimus, pectineus, and adductors), to provide a simultaneous internal and external rotational contraction, that creates a medial compressive force that facilitates dynamic stabilisation of the hip joint (Parvaresh et al 2019).
References
Ng KCG, Jeffers JRT, Beaulé PE. (2019). Hip Joint Capsular Anatomy, Mechanics, and Surgical Management
MacDermott KD, Venter RG, Bergsteedt BJ, Pękala PA, Henry BM, Keet K. (2022). Anatomical features of the iliocapsularis muscle: a dissection study
Laumonerie P, Dalmas Y, Tibbo ME, Robert S, Durant T, Caste T, Vialla T, Tiercelin J, Gracia G, Chaynes P. (2021). Sensory Innervation of the Hip Joint and Referred Pain: A Systematic Review of the Literature.
Nagpal AS, Brennick C, Occhialini AP, Leet JG, Clark TS, Rahimi OB, Hulk K, Bickelhaupt B, Eckmann MS (2021). Innervation of the Posterior Hip Capsule: A Cadaveric Study.
Meheux CJ, Hirase T, Dong D, Clyburn TA, Harris JD (2021). Healthy Hip Joints Have Different Macroscopic and Microscopic Capsular Nerve Architecture Compared With Hips With Osteoarthritis, Femoroacetabular Impingement Syndrome, and Developmental Dysplasia of the Hip: A Systematic Review.
Parvaresh KC, Chang C, Patel A, Lieber RL, Ball ST, Ward SR. (2019). Architecture of the Short External Rotator Muscles of the Hip.
Baba K, Chiba D, Mori Y, Kuwahara Y, Kogure A, Sugaya T, Kamata K, Oizumi I, Suzuki T, Kurishima H, Hamada S, Itoi E, Aizawa T. (2022). Impacts of external rotators and the ischiofemoral ligament on preventing excessive internal hip rotation: a cadaveric study.
Takao M, Otake Y, Fukuda N, Sato Y, Armand M, Sugano N. The Posterior Capsular Ligamentous Complex Contributes to Hip Joint Stability in Distraction.
Tsutsumi M, Nimura A, Akita K (2020). New insight into the iliofemoral ligament based on the anatomical study of the hip joint capsule.
Tsutsumi M, Nimura A, Akita K (2022). Clinical anatomy of the musculoskeletal system in the hip region.
Walters BL, Cooper JH, Rodriguez JA (2014). New findings in hip capsular anatomy: dimensions of capsular thickness and pericapsular contributions.
Solomon L, Lee Y, Callary S, Beck M (2010). Anatomy of piriformis, obturator internus and obturator externus: Implications for the posterior surgical approach to the hip