The pelvic belt consists of the sacrum, coccyx, iliac, ischia, and pubic bone (the latter three are collectively referred to as the "hip bone"). The adult hip consists of 4 joints: the left sacroiliac joint, the right sacroiliac joint (SIJ for short), the sacrococcygeal joint, and the pubic joint joint (SPJ for short).
The bones of the pelvic belt, forming 4 joints
The iliac, sciatic, and pubic bones are separated at birth and are connected by transparent cartilage, and by the end of puberty, these bones naturally fuse and are usually not fully ossified until the age of 20-25 years. These three bones are collectively referred to as "nameless bones" (hip bones) when fused. The lateral side of the hip bone is the acetabular, which connects to the femoral head to form the iliofmoral joint, also known as the hip joint.
Iliofria (hip)
Nameless bone (hip bone)
ilium
The iliac bone is fan-shaped and is the uppermost and largest of the three bones that make up the hip bone. The iliac bone is connected to the sacroiliac bone to form the sacroiliac joint, which is "L" shaped, located above the posterior part of the hip bone, and consists of a vertically oriented "short arm" (vertical plane) and a relatively transverse "long arm" (anteroposterior plane).
Short arm (1. vertical) and long arm (2. horizontal plane) of the L-shaped joint surface above the iliac bone
When you put your hand on the hip, you can feel the curved upper edge of the iliac bone, which is the iliac crest. The finger travels slightly down the iliac crest to the anterior edge of the iliac bone, where it can touch a bony protrusion, the anterior superior iliac spine, which is the attachment point of many soft tissues (eg, the seamstress muscle). Slightly downward in the anterior superior iliac spine, another bony protrusion can be touched, the anterior inferior iliac spine, to which part of the rectus femoris muscle is attached. The bony protrusion touched down the posterior margin of the iliac is the posterior superior iliac spine, which is also the attachment point of the soft tissue. The superior anterior iliac spine and posterior superior iliac spine are often used as bony markers to assess the location of the pelvic belt.
Anatomy of the iliac crest, anterior superior iliac spine, anterior inferior iliac spine, and posterior superior iliac spine
ischium
The sciatic bone is narrower than the iliac bone and is located below the iliac bone and behind the pubic bone. The sciatic bone has a very easily accessible area called a "ischial nodule", often referred to as the "bone used to sit", which is an important bony sign, and the rope muscle is attached to it. The sciatic bone is the strongest of the three skeletons in the hip, and nearly two-fifths of the hip fossa is made up of sciatic bones.
Ischia and sciatic nodules
pubis
The pubic bone is the earliest and smallest of the three skeletons of the hip, and nearly 1/5 of the acetabulum is made up of it. The pubic body is hard and flattened, and the two pubic bones on the left and right form a pubic joint, which is connected by a piece of fibrochondrite and is a micro-moving joint. There is a bony protrusion at the upper margin of the pubic bone called a "pubic nodule", which is the attachment point of the inguinal ligament and is also a bony marker on palpation.
Pubic bone, pubic nodule and pubic joint
sacrum
The sacral bone is triangular and sits at the base of the lumbar spine, forming the posterior part of the pelvic cavity. The sacral bone is 5 separate bones at birth, which begin to fuse at the age of 16-18 and are completely fused into one bone at the age of 34. Studies have shown that the structure of the left and right sides of the sacral bone is also different. The sacroiliac joint is formed at the junction of the sacroiliac bone and the iliac bone.
The flanks of the sacrum are located at the S1 to S3 levels, called "sacral wings" (wings of the sacrum), and these auricular "L"-shaped areas form joints with the iliac bone, such as the sacroiliac joint. The iliac bone has a vertically oriented short arm and an anteroposterior horizontal long arm, which coincide with the sacroeal short arm and long arm shown below, and are connected like a puzzle.
Short and long arms (vertical) and long arms (horizontal) of the sacral bone and angle of the sacral bone (lateral)
From another point of view, the sacrum can be regarded as a continuation of the lumbar spine, the bilateral sacroiliac joint resembles an "atypical articular process joint", the sacrum can be used as a separate vertebral body, the left and right sacroiliac joints are equivalent to the articular process joints, the iliac part is the superior articular process joint, and the sacrum part is the inferior articular process joint.
Anatomy of the sacroiliac joint: horizontal plane
Anatomy of the sacroiliac joint: lateral view
tailbone
The coccyx is the terminal of the entire spine, so it is called the "tail vertebrae" and is formed by 3-5 (usually 4) tail vertebrae. Most accounts say that these bones are fused together, and a few scholars believe that the tail vertebrae are separated from each other.
Coccyx and single coccyx stages
Pubic joint joints
The pubic joint is a non-synovial fibrochondond micromotor joint that connects the left and right pubic bones, and the adult has only 2 mm of movement, which may rotate by 1°; the mobility of women during pregnancy and the postpartum period will increase; and the shape of the joint surface and the contraction of the adductor and abdominal muscles may also affect the movement of the pubic joint. Transparent cartilage covers the end of the pubic bone and is connected to the fibrocartilage in the middle of the pubic joint, which has strong upper and lower ligaments, but the posterior ligament is weak.
Pubic joints and their ligaments
Sacroiliac joint
The sacroiliac joint is located between the sacroeum and the iliac bone, and belongs to the true synovial joint, which includes the joint capsule, synovial fluid, joint cartilage, and synovium.
Ligaments of the sacroiliac joint
The sacroiliac joint stability is provided in part by the ligament attached to it, which guarantees the integrity of the joint and the shear resistance. Ligaments directly connected to the sacrum: sacral nodular ligament, sacral spinous ligament, interosseous ligament, and dorsal long ligament of the sacroiliac iliac. The ilioflus lumbar ligament affects the stability of the sacroiliac joint and also affects the stability of the lumbar spine.
The stability relationship between the ligament and the sacroiliac joint: a) anterior view, b) posterior view
Source: Rehabilitation Xiao Nanning