The Hip — Form & Function
The hip joint is one of the largest and most mechanically demanding joints in the human body. It must simultaneously bear enormous loads — up to four to five times body weight during walking, and significantly more during running or stair climbing — while permitting the wide range of movement needed for everyday life.
This combination of high load-bearing and multi-directional mobility is achieved through a sophisticated arrangement of bony architecture, articular cartilage, a fibrocartilaginous labrum, strong capsular ligaments, and powerful surrounding musculature. Understanding each of these components helps explain both how the hip functions normally and what happens when it is affected by conditions such as osteoarthritis.
The Bones of the Hip Joint
The hip joint is formed by the articulation of two bones: the femur (the thigh bone) and the pelvis. More specifically, the rounded head of the femur fits into the cup-shaped socket of the pelvis called the acetabulum.
The Femoral Head
The femoral head is a near-perfect sphere forming approximately two-thirds of a ball. It is covered in smooth articular cartilage which, in a healthy joint, is 2–3 mm thick and provides an almost frictionless bearing surface. At its centre is a small pit — the fovea — where the ligamentum teres attaches. The femoral head is connected to the femoral shaft by the femoral neck, which projects at an angle that positions the leg optimally for load transfer during walking.
The Acetabulum
The acetabulum is a hemispherical socket in the pelvis formed by the fusion of three pelvic bones — the ilium, ischium and pubis — which meet at the centre of the acetabulum. It faces outward, forward and downward, orientated to provide optimal coverage of the femoral head during the weight-bearing phase of the gait cycle. Only the superior and peripheral portions of the socket are lined with articular cartilage in a horseshoe-shaped pattern; the central floor is non-articular and occupied by the cotyloid fossa, filled with a fat pad and the ligamentum teres.
The Labrum
The acetabular labrum is a ring of fibrocartilage that deepens the acetabulum by approximately 20%, significantly increasing the surface area available for femoral head coverage and improving joint stability. It acts as a seal around the joint, maintaining a thin film of synovial fluid between the articular surfaces under load — critical to cartilage nutrition and lubrication. The labrum also contributes to proprioception (joint position sense). Labral tears are a recognised cause of hip pain, particularly in younger, active individuals.
The Femoral Neck & Trochanters
The femoral neck connects the femoral head to the shaft at an angle called the neck-shaft angle (normally 120–135°) and is angled forward relative to the femoral shaft — a property called femoral anteversion. These angular relationships are important in determining hip biomechanics. The greater trochanter, projecting laterally, is the attachment point for the powerful abductor muscles. The lesser trochanter, on the inner aspect of the proximal femur, is where the iliopsoas (the primary hip flexor) attaches.
Cartilage, Synovium & Joint Lubrication
The remarkable load-bearing capacity of the hip joint is made possible by the properties of articular cartilage and the synovial fluid that lubricates it. Together, these create a bearing surface with a coefficient of friction lower than that of ice on ice — an engineering achievement that no artificial bearing has yet fully replicated.
Articular Cartilage
Articular cartilage is a highly specialised tissue that lines the weight-bearing surfaces of the femoral head and the acetabulum. It is composed predominantly of water (approximately 70–80%), collagen fibres (primarily type II collagen) and proteoglycans — large molecules that attract and retain water, giving cartilage its remarkable ability to resist compressive loads. Under loading, water is squeezed out of the cartilage and onto the surface, contributing to lubrication; during unloading, the water is reabsorbed.
Articular cartilage has no blood supply — it receives its nutrition through diffusion from the synovial fluid and, to a lesser extent, from the underlying bone. This avascularity means cartilage has very limited capacity for self-repair. Once damaged significantly — as in osteoarthritis — the repair process is inadequate and the damage is progressive.
The Synovial Joint & Joint Capsule
The hip is a synovial joint — meaning its cavity is enclosed by a joint capsule and lined by a specialised membrane called the synovium. The synovial membrane secretes synovial fluid, which serves as both lubricant and nutrient medium for the avascular articular cartilage. Synovial fluid contains hyaluronic acid and lubricin, which are responsible for its characteristic viscosity and lubricating properties.
The joint capsule is a strong, fibrous sleeve that surrounds the joint and contributes significantly to its stability. It is reinforced by three major ligaments — the iliofemoral, pubofemoral and ischiofemoral ligaments — which are arranged in a spiral pattern such that they tighten in extension (providing stability when standing) and relax in flexion. This ingenious arrangement allows the joint to "lock" passively in the extended position with minimal muscular effort, reducing the energy cost of standing.
Why Cartilage Cannot Repair Itself
Unlike most body tissues, articular cartilage contains no blood vessels, lymphatics or nerves in its deeper layers. The chondrocytes (cartilage cells) that maintain the cartilage matrix are sparsely distributed and have a very limited capacity for division and repair. When cartilage is damaged — by injury, by the inflammatory process of arthritis, or by cumulative wear — the body's repair response produces fibrocartilage rather than the original hyaline cartilage. Fibrocartilage is mechanically inferior and less durable, meaning that damage tends to progress rather than resolve. This is why arthritis, once established, is a progressive condition.
Ligaments & Joint Stability
The hip joint achieves its stability through a combination of bony congruence (the deep socket providing inherent mechanical stability), the labrum, and a series of powerful ligaments that reinforce the joint capsule.
Posterior capsule repair in hip replacement: The posterior capsule and short external rotators — divided during the posterior surgical approach — are meticulously repaired and reattached at the end of surgery. Restoring these structures is a key factor in reducing the risk of post-operative dislocation and is an important technical step in Mr. Lodge's operative technique.
The Muscles of the Hip
The hip is surrounded by some of the largest and most powerful muscles in the body. These muscles do not merely move the hip — they are essential to maintaining pelvic stability, controlling posture, absorbing impact during walking and running, and protecting the joint from adverse loading patterns.
Flexors
Iliopsoas (iliacus + psoas major) is the primary hip flexor — responsible for bringing the thigh forward during walking and climbing stairs. It attaches to the lesser trochanter.
Extensors
Gluteus maximus is the body's largest muscle and the primary hip extensor — powerful during climbing, rising from a chair and running. It also contributes to external rotation.
Abductors
Gluteus medius and gluteus minimus are the critical abductors — they stabilise the pelvis during single-leg stance, preventing the opposite side from dropping (Trendelenburg gait).
Adductors
The adductor group (adductor longus, brevis, magnus, gracilis and pectineus) brings the leg towards the midline. Important during walking and generating power in many sporting movements.
External Rotators
The short external rotators (piriformis, obturator internus and externus, gemelli, and quadratus femoris) rotate the thigh outward and contribute to posterior capsular stability — the "cuff" of the hip.
Internal Rotators
Gluteus medius and minimus also contribute to internal rotation, as does tensor fasciae latae via the iliotibial band — though internal rotation is the weakest directional motion at the hip.
The Abductors — A Critical Muscle Group
The hip abductors — primarily gluteus medius — are arguably the most functionally important muscle group for hip joint health and rehabilitation. During single-leg stance (which constitutes approximately 60% of the walking cycle), the abductors must generate a force of approximately three times body weight to prevent the pelvis from tilting to the unsupported side. Weakness in the abductors — from disuse, pain inhibition, or surgical disruption — results in a characteristic lateral lurch (Trendelenburg gait) and significantly increases loading through the hip joint itself.
Strengthening the abductors is a central goal of physiotherapy both before and after hip replacement surgery. The posterior surgical approach preserves the abductors entirely — an important advantage that contributes to the typically good gait pattern seen after a well-performed posterior approach hip replacement.
Blood Supply & Nerve Supply
Blood Supply to the Femoral Head
The blood supply to the femoral head is of considerable clinical importance — it is vulnerable to disruption in fractures of the femoral neck (hip fractures), which can result in avascular necrosis (death of the bone from loss of blood supply). The femoral head receives its blood primarily from the medial and lateral circumflex femoral arteries, which are branches of the profunda femoris artery. These vessels run along the femoral neck in a retinacular pattern, entering the femoral head superiorly and posteriorly. The small vessel in the ligamentum teres contributes meaningfully in childhood but less so in adults.
This anatomy explains why displaced femoral neck fractures in elderly patients so frequently require hip replacement rather than fixation — the retinacular vessels are torn at the time of fracture, and the femoral head cannot be reliably reperfused.
Nerve Supply & Pain Referral
The hip joint is innervated by branches of several nerves — the femoral nerve, the obturator nerve, the nerve to quadratus femoris (sciatic plexus) and the superior gluteal nerve. This multi-source innervation explains the varied distribution of pain that hip pathology can produce. Hip pain is characteristically felt in the groin, the anterior thigh and the lateral hip region — but can also be referred to the knee, which is a well-recognised phenomenon that can lead to delayed or missed diagnosis when a patient presents with knee pain that actually originates from the hip.
The obturator nerve — which supplies the medial aspect of the thigh — is responsible for the pattern of hip pain that radiates down the inner thigh. Pain perceived in the buttock region is more commonly referred from the lumbar spine, sacroiliac joint or gluteal structures rather than from the hip joint itself — an important distinction in the assessment of patients presenting with hip and buttock pain.
Clinical implication: The fact that hip pain can be referred to the knee, and that lumbar spine pain can mimic hip pain, is one of the reasons why a careful clinical examination and — where needed — a diagnostic injection are so important in confirming the hip joint as the true source of a patient's symptoms before recommending surgery. This is discussed in more detail on the Management Options page.
How the Hip Moves
The hip joint is capable of movement in all three planes — flexion and extension (forward and backward), abduction and adduction (sideways), and internal and external rotation. The combination of these movements allows circumduction — the circular motion seen when drawing circles with the foot.
- Flexion (0–120°): Bringing the thigh towards the chest. Essential for sitting, stair climbing and putting on footwear. Often one of the first movements restricted in hip arthritis.
- Extension (0–20°): Moving the thigh behind the body. Important during the propulsive phase of walking. Restricted by the strong iliofemoral ligament anteriorly.
- Abduction (0–45°): Moving the leg away from the midline. Critical for gait stability and essential for stepping sideways.
- Adduction (0–30°): Moving the leg towards and across the midline. Important in many sports movements and contributes to gait.
- Internal rotation (0–35°): Rotating the foot inward relative to the body. Reduced internal rotation — detected on examination with the patient lying flat — is often one of the earliest and most reliable signs of hip joint disease.
- External rotation (0–45°): Rotating the foot outward. Restricted in advanced arthritis along with internal rotation.
Loading Through the Hip Joint
The forces transmitted through the hip joint during everyday activity are substantially greater than body weight alone — because the abductor muscles must generate a counter-force to balance the body's centre of mass over the single stance leg. During level walking, the joint reaction force is approximately 2.5–4 times body weight. During stair climbing this rises to around 5 times body weight, and during activities such as running it can exceed 7 times body weight.
This is why body weight has such a profound effect on hip joint symptoms and on the wear of both the natural and artificial joint. Every kilogram of body weight lost reduces the peak force across the hip by approximately 3–4 kg with each step taken — a compelling argument for weight management as a cornerstone of hip arthritis treatment.