Lower Limb Bones and Joints

Lower Limb Bones and Joints

The Pelvic Girdle

The hip bone (os coxae) is formed by three bones that fuse at the acetabulum by the late teens: the ilium (superior), ischium (posteroinferior), and pubis (anteroinferior). The two hip bones articulate anteriorly at the pubic symphysis (secondary cartilaginous joint, fibrocartilage disc) and posteriorly with the sacrum at the sacroiliac joints (synovial + syndesmosis). The sacrum itself comprises five fused vertebrae; the coccyx consists of three to four fused segments.

The pelvis is divided into the greater (false) pelvis above the pelvic brim and the lesser (true) pelvis below. The pelvic inlet is bounded by the promontory, arcuate lines, pectineal lines, and pubic crest. Female pelves tend to have a wider sub-pubic angle (>90°), a larger oval inlet, and a shallower cavity compared with the narrower, heart-shaped male inlet.

Hip Joint

The hip is a multiaxial ball-and-socket joint between the spherical femoral head and the cup-shaped acetabulum. The acetabular labrum (fibrocartilage rim) deepens the socket and increases stability. The joint capsule attaches to the acetabular rim proximally and to the intertrochanteric line (anteriorly) and neck (posteriorly) of the femur distally.

Capsular (iliofemoral, pubofemoral, ischiofemoral) ligaments reinforce the capsule and are taut in extension, locking the hip. Blood supply to the femoral head comes primarily from the medial circumflex femoral artery (MCFA) and lateral circumflex femoral artery (LCFA), which give rise to retinacular arteries that ascend the femoral neck. The artery to the head of the femur (from the obturator artery, acetabular branch) contributes a small additional supply — important after femoral neck fractures where retinacular arteries may be disrupted, risking avascular necrosis.

Knee Joint

The knee is the largest and most complex joint in the body — a bicondylar synovial joint between the femoral condyles, tibial plateau, and patella. Two C-shaped fibrocartilaginous menisci (medial and lateral) sit on the tibial plateau, deepening the joint surface, distributing load, and acting as shock absorbers. The medial meniscus is more firmly attached (to the MCL and joint capsule) and therefore less mobile and more commonly injured than the lateral meniscus.

Cruciate ligaments: the anterior cruciate ligament (ACL) runs from the anterior intercondylar area of the tibia to the lateral femoral condyle — it prevents anterior tibial translation. The posterior cruciate ligament (PCL) runs from the posterior tibial intercondylar area to the medial femoral condyle — it prevents posterior tibial translation. Collateral ligaments: the medial (tibial) collateral ligament (MCL) resists valgus stress; the lateral (fibular) collateral ligament (LCL) resists varus stress. The knee is most stable in full extension (screw-home mechanism: tibial lateral rotation locks the joint). Multiple bursae surround the knee; clinically relevant ones include the prepatellar bursa (housemaid's knee) and superficial infrapatellar bursa (clergyman's knee).

Ankle and Foot

The talocrural (ankle) joint is a hinge joint formed by the tibia, fibula, and talus. The medial malleolus (tibia) and lateral malleolus (fibula) form a mortise around the talus. The deltoid ligament (medial, strong) and lateral ligaments (ATFL, CFL, PTFL) stabilise the joint. Forced eversion can cause a Pott's fracture (fibular shaft fracture + medial malleolus avulsion/deltoid ligament tear).

The subtalar (talocalcaneal) joint allows inversion/eversion. The foot has three arches: medial longitudinal (highest, most important), lateral longitudinal, and transverse. These arches are maintained by the plantar aponeurosis (from calcaneus to flexor sheaths), the spring (plantar calcaneonavicular) ligament, and intrinsic foot muscles. Plantar fasciitis is inflammation of the plantar aponeurosis at its calcaneal attachment, causing heel pain on first weight-bearing in the morning.