The femoral head is the smooth, ball-shaped upper end of the thighbone (femur) that sits inside the hip socket, forming one half of the body’s largest and most mechanically demanding joint. Covered almost entirely in articular cartilage, it bears the full weight of the upper body during standing, walking, and running, routinely absorbing forces several times a person’s body weight with each step. Its anatomy is elegant but precarious: the same design features that allow a wide range of painless motion also leave it uniquely vulnerable to blood-supply disruption, fracture, and degeneration.
What the Femoral Head Looks Like and How It Is Built
From the outside, the femoral head looks roughly spherical, though it is not a perfect ball. It sits atop the femoral neck at an angle, pointing upward and slightly inward toward the center of the pelvis. Nearly all of its surface is covered in a layer of hyaline cartilage that provides a slick, low-friction gliding surface against the cartilage lining the acetabulum (the cup-shaped socket of the pelvis). One small patch on the surface, called the fovea, is bare of cartilage. The fovea sits slightly behind and below the center of the head and serves as the attachment point for the ligamentum teres, a short ligament that tethers the femoral head to the floor of the socket.
Inside, the femoral head is not solid bone. It is a lattice of thin, interconnected struts called trabeculae, organized in patterns that reflect the loads the bone habitually carries. Two main trabecular systems have been identified within the proximal femur. A medial system runs vertically from the inner shaft of the femur up into the head, while horizontal groups of trabeculae arch inward from the outer shaft and greater trochanter, crossing the vertical struts inside the head. The intersection of these groups creates a balanced internal framework: the vertical struts handle compressive forces, and the horizontal struts handle tensile forces, together stabilizing the femoral neck and shaft under load.
1PubMed Central. Functional morphology of trabecular system in human proximal femur: a perspective from P45 sectional plastination and 3D reconstruction finite element analysisThe density of this internal lattice is not uniform. Micro-CT studies show that the first couple of millimeters just beneath the joint surface are distinctly denser and more plate-like than the deeper trabecular bone. And the top of the head, which bears the most weight, has thicker, more closely packed trabeculae than the bottom or sides. This gradient is a classic example of bone remodeling itself in response to the forces it experiences most often.
2PubMed. A micro-computed tomography study of the trabecular bone structure in the femoral headA Precarious Blood Supply
Understanding the femoral head’s blood supply explains much of what can go wrong with it. The head sits entirely inside the hip joint capsule, meaning it is surrounded by synovial fluid rather than by soft tissue packed with blood vessels. This intracapsular position limits the routes blood can take to reach the bone.
3Orthopedic Clinics. Classification systems for osteonecrosis: an overviewThe main blood supply comes from a ring of vessels formed around the base of the femoral neck by branches of the medial and lateral circumflex arteries. From this ring, small retinacular arteries travel upward along the surface of the neck, hugging the bone beneath the joint capsule before entering the head near its margin. Their position on the surface of the neck makes them extremely vulnerable to injury whenever the neck is fractured or the head dislocates. The lateral retinacular vessels are considered the most important group because they feed the lateral weight-bearing portion of the head.
4Orthopedic Clinics. Classification systems for osteonecrosis: an overviewMeasurement of the bony channels these arteries pass through gives a sense of scale. The canals carrying the superior retinacular arteries have a combined cross-sectional area of roughly 15.6 square millimeters, dwarfing the canals of the inferior retinacular arteries (about 3.6 mm²), the anterior arteries (about 4.3 mm²), and the artery running through the ligamentum teres (about 1.6 mm²). The superior retinacular system clearly supplies the most blood.
5PubMed Central. Total cross-sectional area of the femoral neck nutrient foramina measured to assess arterial vascular beds in the femoral headA small additional supply reaches the head through the ligamentum teres, fed by a branch of the obturator artery. This vessel is important during childhood, when the head is still growing, but in adults the contribution from the ligamentum teres to femoral head perfusion is considered negligible.
6European Society of Radiology. Ligamentum teres of the hip: Anatomy, pathology, and treatmentAfter a femoral neck fracture, one critical question is how much of the blood supply survives. Angiography studies of patients with hip fractures have found that the inferior retinacular arterial system remained intact in all nondisplaced fractures and in about 60% of moderately displaced fractures, suggesting it can serve as a salvage route for keeping the head alive when other vessels are damaged.
7PubMed Central. Epiphyseal Arterial Network and Inferior Retinacular Artery Seem Critical to Femoral Head Perfusion in Adults With Femoral Neck FracturesForces During Walking and Standing
The hip joint is a deep ball-and-socket design, and the femoral head experiences enormous loads during everyday activity. During a normal walking cycle, contact stress on the head is not evenly distributed. It concentrates mainly at the top and slightly toward the front and outer edge, beneath the dome of the acetabulum. The contact area between the head and socket shifts throughout the gait cycle, ranging from roughly 294 to 998 mm², with the largest contact area appearing around the middle of the stance phase when the leg is fully bearing weight.
8PubMed Central. Changes in hip joint contact stress during a gait cycle based on the individualized modeling method of “gait-musculoskeletal system-finite element”Peak contact stress hits the femoral head at terminal stance, just before the foot pushes off the ground. Modeling studies have recorded peak stresses in the range of roughly 7 MPa at that moment. Direct measurements in cadaver hips, using tiny pressure sensors embedded in the cartilage, found that at a joint load of about 2,700 newtons (roughly three times body weight for an average person), the peak local stress averaged around 8.8 MPa, while the average stress across the full contact area was about 2.9 MPa. The pressure pattern was not smooth; it typically formed an irregular ridge running roughly from front to back across the head.
9PubMed. In vitro contact stress distributions in the natural human hipThese forces explain why the superior pole of the femoral head has the densest bone: it sits directly under the region of highest habitual stress. The match between loading pattern and bone architecture is a textbook case of Wolff’s law, the principle that bone adapts its structure to the demands placed on it.
When the Blood Supply Fails: Osteonecrosis
Osteonecrosis of the femoral head (sometimes called avascular necrosis) is the condition most intimately tied to the head’s vulnerable blood supply. When blood flow is disrupted for long enough, bone cells die. The dead bone initially looks normal on X-ray, but over time the structural integrity of the internal lattice deteriorates, and the head can collapse under the loads of normal activity.
Collapse does not happen randomly. Research shows that the area where collapse begins is the margin of the dead zone, specifically the lateral column of the head. Histologically, fractures appear at the junction between thickened necrotic trabeculae and the still-living bone at the edge of the lesion. This makes sense mechanically: the boundary between stiff dead bone and more elastic living bone creates a stress concentration, like a crack propagating along the seam between two materials of different stiffness.
10Journal of Bone and Joint Surgery. Relationship Between Blood Flow and Collapse of Nontraumatic Osteonecrosis of the Femoral HeadFinite element models support this picture, showing that stress is highest at the surface of the necrotic zone rather than at its base. When peak stress exceeds the already weakened bone’s threshold, tiny fractures occur in the cancellous bone just below the cartilage surface. Accumulation of these microfractures is likely the direct cause of visible collapse.
11PubMed Central. Predicting the collapse of the femoral head due to osteonecrosis: From basic methods to application prospectsRisk Factors for Osteonecrosis
Corticosteroid use and heavy alcohol consumption are the two best-known non-traumatic risk factors for femoral head osteonecrosis. A Japanese case-control study found that the odds of developing osteonecrosis were roughly 31 times higher in people who had used corticosteroids compared to those who had not, even without any alcohol use. Among people who had never used steroids, heavy long-term drinkers had about 11 times the odds of non-drinkers. The study found that when both exposures were present, the risk did not climb beyond the steroid-only level, suggesting that the steroid effect so thoroughly dominates the picture that alcohol adds little on top.
12PubMed. The effect of alcohol intake and the use of oral corticosteroids on the risk of idiopathic osteonecrosis of the femoral head: a case-control study in JapanA separate analysis looking at the combined effect of tobacco, alcohol, and corticosteroid use found a different and somewhat alarming pattern when all three are present. Patients with all three risk factors showed a risk ratio of roughly 12.5 compared to those with none. The combination of any two risk factors also elevated risk in a multiplicative fashion: corticosteroids plus alcohol carried a risk ratio around 10.2, corticosteroids plus tobacco about 8.7, and tobacco plus alcohol about 5.3.
13PubMed. The Multiplicative Effects of Individual Risk Factors in the Development of Osteonecrosis of the Femoral HeadOther known triggers include sickle cell disease, radiation therapy, certain chemotherapy drugs, and organ transplant immunosuppression. Trauma-related osteonecrosis is a separate category, typically occurring after a femoral neck fracture that disrupts the retinacular arteries.
Femoral Head Fractures
Fractures of the femoral head itself (as opposed to the more common femoral neck fracture) are rare and almost always the result of high-energy trauma such as car crashes or falls from height. They typically occur alongside posterior dislocation of the hip, when the head is driven backward out of the socket and strikes the rim of the acetabulum hard enough to shear off a piece of the articular surface.
14PubMed Central. Fractures of the femoral head: a narrative reviewClassification of these injuries, historically grouped by the Pipkin system developed in 1957, depends on the size and location of the fragment and whether the fracture is accompanied by a broken acetabular wall or femoral neck. More recent systems have incorporated CT scanning to better capture the full picture in an emergency setting. A series of 55 cases with CT scans and an average follow-up of nine years provided the basis for one updated classification, which factors in fragment size and associated injuries to guide urgent treatment decisions.
15PubMed. Fracture-dislocations of the femoral headWhen the femoral neck also breaks alongside a head fracture, the prognosis worsens considerably because the blood supply to the head is jeopardized from two directions at once. In those cases, salvaging the native head becomes much less likely, and hip replacement is often the best option.
Childhood Conditions Affecting the Femoral Head
Two pediatric conditions specifically target the femoral head during growth. Legg-Calvé-Perthes disease is a childhood form of osteonecrosis in which blood supply to the developing femoral head is disrupted, leading to bone death and eventual deformation. The most supported theory for why some children develop it centers on mechanical ischemia: in certain susceptible children, the blood vessels supplying the head are compressed or kinked during normal hip motion, starving the bone of oxygen.
16PubMed Central. Aetiology of Legg-Calvé-Perthes disease: A systematic reviewSlipped capital femoral epiphysis (SCFE) is a different problem. In adolescents, the femoral head is still connected to the neck through a cartilaginous growth plate. In SCFE, the head slips off the neck like a scoop of ice cream sliding off a tilted cone. Biomechanical research has identified elevated shear stress across the growth plate as a key risk factor. Hips on the opposite side from an already-slipped hip showed significantly higher shear stress (about 0.81 MPa versus 0.51 MPa in healthy controls) and a more vertical growth-plate angle, suggesting these hips were mechanically primed to slip as well.
17Journal of Pediatric Orthopaedics. Shear Stress in Epiphyseal Growth Plate is a Risk Factor for Slipped Capital Femoral EpiphysisMulti-scale modeling of SCFE has shown that damage concentrates in the medial (inner) region of the growth plate in both healthy and affected hips, but is significantly greater in the affected hip at both macro and micro levels.
18PubMed. Multi-scale finite element model of growth plate damage during the development of slipped capital femoral epiphysisShape Variations and Femoroacetabular Impingement
Not everyone’s femoral head is the same shape, and deviations from a smooth sphere can cause problems. In cam-type femoroacetabular impingement (FAI), a bump of extra bone forms at the junction where the head meets the neck. During hip flexion, this bump jams into the rim of the acetabulum, pinching the labrum (the ring of cartilage lining the socket’s edge) and grinding the articular cartilage.
Three-dimensional shape analysis has quantified how much extra bone we are talking about. Normal femoral heads deviate from a perfect sphere by up to about 2.5 mm. Hips with cam-type FAI showed maximum deviations of roughly 4 to 5 mm, about double the normal variation. The difference sounds small, but in a tight-fitting joint that moves through wide arcs hundreds of times a day, a few extra millimeters of bone in the wrong place creates repetitive mechanical damage that can progress to osteoarthritis if untreated.
19PubMed Central. Three-dimensional quantification of femoral head shape in controls and patients with cam-type femoroacetabular impingementOsteoarthritis and Bone Changes Beneath the Cartilage
Osteoarthritis of the hip follows a well-recognized sequence visible on X-ray: the joint space narrows as cartilage wears thin, bone spurs (osteophytes) develop around the margins of the head and acetabulum, the bone beneath the cartilage becomes dense and sclerotic, and eventually cysts form within the bone.
20PubMed. Hip osteoarthritis: what the radiologist wants to knowThe sclerotic bone that forms in osteoarthritic femoral heads is not simply old bone that got compressed. Research examining those sclerotic regions has found a nine-fold accumulation of mesenchymal stem cells compared to non-sclerotic areas, alongside dense clusters of immature osteocytes. About 76% of osteocytes in sclerotic zones were morphologically immature, compared to roughly 15% in non-sclerotic areas. This suggests the sclerosis is an active, somewhat frantic remodeling process: the bone is churning out new cells in response to abnormal mechanical signals, but the result is structurally inferior bone rather than a functional repair.
21Rheumatology. The simultaneous analysis of mesenchymal stem cells and early osteocytes accumulation in osteoarthritic femoral head sclerotic boneTreatment When the Femoral Head Is Failing
For early-stage osteonecrosis, before the head has collapsed, a procedure called core decompression is the most widely used joint-preserving intervention. A surgeon drills one or more channels into the head to relieve intraosseous pressure, promote new blood vessel growth, and sometimes pack the channels with bone graft. Patients treated before collapse have significantly better outcomes than those managed with symptom relief alone, and the complication rate is low.
22PubMed. Core decompression with bone grafting for osteonecrosis of the femoral headA network meta-analysis of randomized trials comparing various core decompression techniques with nonsurgical treatment found no statistically significant difference in the rate of conversion to total hip replacement overall, though core decompression combined with cell therapy showed a relatively better result in slowing radiographic progression of the disease.
23PubMed Central. Efficacy of various core decompression techniques versus non-operative treatment for osteonecrosis of the femoral head: a systemic review and network meta-analysis of randomized controlled trialsWhen the head has already collapsed or advanced arthritis has destroyed the joint surface, total hip arthroplasty (hip replacement) is the definitive treatment. In this procedure, the damaged femoral head is removed and replaced with a prosthetic ball on a metal stem that fits inside the femur. The acetabulum is also resurfaced with a prosthetic cup. Debate over what material makes the best artificial femoral head has been ongoing for decades. Ceramic and metal heads are the two main options, both typically paired with a polyethylene socket liner. Long-term data show that ceramic heads produce slightly less wear on the polyethylene, with mean wear rates of about 0.086 mm per year compared to about 0.137 mm per year for metal heads.
24Journal of Bone and Joint Surgery. Long-Term Performance of Ceramic and Metal Femoral Heads on Conventional Polyethylene in Young and Active PatientsWhether that wear difference translates into fewer revision surgeries is a separate question. A systematic review and meta-analysis comparing the two materials found that the wear advantage of ceramic on modern cross-linked polyethylene was statistically real but almost negligibly small. For revision rates and clinical hip scores, no meaningful difference emerged. The authors concluded that the higher cost of ceramic heads is not justified by the available evidence.
25PubMed Central. Comparison between ceramic-on-polyethylene versus metal-on-polyethylene prostheses in Total Hip Arthroplasties: a systematic review and meta-analysisThe Evolutionary Story Behind Its Shape
The femoral head’s current form is a product of millions of years of adaptation to upright walking. When early human ancestors shifted from quadrupedal locomotion to habitual bipedalism, the hip joint had to accommodate a fundamentally different range of motion. The default working position of the hip moved from a flexed posture (as seen in apes) to a more extended one, and the femoral head had to allow for full extension during the push-off phase of gait without jamming against the rim of the socket.
Evolutionary analysis points to the head-neck junction as the key area of adaptation. “Posterior concavity,” meaning the depth of the waist between the ball and the neck on the back side, appears critical for normal human gait. Loss of concavity on the front and top of the head is tolerated well, but inadequate concavity at the back and bottom compromises the ability to walk with a fully extended hip.
26Journal of Hip Preservation Surgery. Evolution of the human hip. Part 1: the osseous frameworkFossil evidence from early hominins shows a femoral head morphology consistent with a more vertical excursion of the femur during walking, in contrast to the adducted (inward-angled) pattern of modern humans and the abducted (outward-angled) pattern of chimpanzees. In other words, the transition to modern human-style walking was not a single leap but a series of incremental changes in how the femoral head related to the rest of the hip, and traces of those intermediate stages survive in the fossil record.
27PubMed. Chimpanzee bipedalism: cineradiographic analysis and implications for the evolution of gaitSex Differences and Implant Design
The femoral head is not the same size or shape in everyone, and one of the most consistent differences is between men and women. Three-dimensional morphological analysis of proximal femurs from CT scans has confirmed that sex is the most important factor in determining how well a prosthetic component fits, while age plays a bigger role in the shape of the inner medullary canal of the femoral shaft.
28PubMed. Three-Dimensional Morphological Analysis of Sex, Age, and Symmetry of Proximal Femurs from Computed Tomography: Application to Total Hip ArthroplastyThis matters practically because most hip implant systems offer a range of head sizes (typically 28, 32, or 36 mm in diameter) and stem sizes, but the proportional relationships between head diameter, neck length, and femoral offset differ systematically between male and female anatomy. A mismatch can alter the mechanics of the reconstructed hip, leading to instability, leg-length discrepancy, or accelerated wear. Manufacturers have increasingly moved toward sex-specific or at least broader ranges of implant geometries, though truly individualized prostheses remain more aspiration than routine practice.