Timepoint
Milestone
4 weeks
Limb buds form from ectodermal layer of ventrolateral wall
7 weeks
Cartilaginous models of femur and acetabulum have developed from mesodermal layer. Apoptosis creates cleft between acetabulum and femur which is the site of future hip joint
8 weeks
Shift from cell differentiation to cell growth and maturation. Appearance of femoral ossification center. Appearance of blood supply at nutrient artery site with capillary invasion into cartilage model of the femur
11 weeks
Hip fully formed in appearance
12 weeks
Vascular ring of vessels formed at the base of femoral neck
16 weeks
Femur ossified to the level of lesser trochanter. Femoral head and acetabulum covered in mature hyaline cartilage
Femoral anteversion is first defined at 11 weeks’ gestation at which time it measures 5–10°. As the fetus develops, femoral anteversion increases to maximum of 45° at the time of birth. Subsequently, in the normally developing femur, femoral anteversion gradually decreases to 15° by 16 years of age [4, 5].
The relationship between the neck-shaft angles of the proximal femur also varies through development starting in the fetal stage. At 15 weeks’ gestation the neck-shaft angle is 145° and gradually decreases to 130° by 36 weeks’ gestation [3]. A range of normal neck-shaft angles throughout childhood development has been established in a cohort of 400 children (800 hips), and the authors found that by age 18 the mean neck-shaft angle was 127.3° [6].
Blood Supply to the Femoral Head
As the blood supply to the proximal femur matures through gestation, it develops into three distinct arterial systems, the capsular (retinacular), foveal, and intraosseous [7–12]. The foveal blood supply through the ligamentum teres has consistently been shown to provide minimal blood supply to the femoral head. In fact, resection of the ligamentum teres during open hip reduction procedures in patients with dysplastic hips has shown no increased incidence of osteonecrosis of the femoral head further supporting the notion of minimal contribution to femoral head vascularity [2].
The capsular blood system originates with the medial and lateral femoral circumflex arteries which branch off the profunda femoris in 79% of cases. In 20% of cases 1 of these arteries branches off the femoral artery, and in 1% of cases both arteries arise directly from the femoral artery [13]. The medial and lateral femoral circumflex arteries form an anastomotic extracapsular ring around the base of the femoral neck. The medial circumflex artery is the main contributor of blood supply to the femoral neck, and the deep branch of the medial circumflex artery is the conduit for a majority of the blood flow and comprises the majority of this anastomotic ring. Branching off the extracapsular ring are the ascending cervical (retinacular) arteries which penetrate the joint capsule at the base of the femoral neck along the intertrochanteric line. From here, there are four main groups of ascending cervical arteries of which the lateral (superior) cervical artery is the most important to provide perfusion to the femoral head [7–12]. There is new literature to suggest that the inferior retinacular artery may also provide a significant amount of perfusion to the femoral head [14]. The ascending cervical arteries form a secondary vascular ring at the subcapital region of the femoral neck termed the subsynovial vascular ring of which the terminal branches of the deep branch of the medial circumflex vessels penetrate the posterosuperior aspect of the femoral head 2–4 mm proximal to the start of the articular surface (Fig. 1.1).
Fig. 1.1
(a) Photograph showing the perforation of the terminal branches into the bone (right hip, posterosuperior view). The terminal subsynovial branches are located on the posterosuperior aspect of the neck of the femur and penetrate the bone 2–4 mm lateral to the bone-cartilage junction. (b) Diagram showing: (1) the head of the femur, (2) the gluteus medius, (3) the deep branch of the MFCA, (4) the terminal subsynovial branches of the MFCA, (5) the insertion and tendon of gluteus medius, (6) the insertion and tendon of piriformis, (7) the lesser trochanter with nutrient vessels, (8) the trochanteric branch, (9) the branch of the first perforating artery, and (10) the trochanteric branches (Figure and Caption copyright Gautier et al. [12].)
Anatomy of the Proximal Femur
Proximal Femoral Geometry
Normal constant relationships between the femoral head, femoral neck, greater trochanter, and femoral shaft exist in the grown adult. These relationships are important to define as they are the normal relationships that should be established in the course of operative treatment of a fracture about the proximal femur. As described by Dror Paley, the normal tip of the trochanter to the center of femoral head line orientation to the mechanical or anatomical axis is 90° ± 5° (lateral proximal femoral angle [LPFA]) and 84° ± 5° (medial proximal femoral angle [MPFA]). Another reference line is the neck anatomic axis or medial neck-shaft angle (MNSA) which is 130° ± 10° ([15]; Fig. 1.2).
Fig. 1.2
(a–c) Angle measurements of the proximal femur. The normal tip of the trochanter to the center of femoral head line orientation to the mechanical or anatomical axis is 90° ± 5° (lateral proximal femoral angle [LPFA]) and 84° ± 5° (medial proximal femoral angle [MPFA]). Another reference line is the neck anatomic axis or medial neck-shaft angle (MNSA) which is 130° ± 10°
Internal Geometry of the Femoral Neck
The internal geometry of the femoral neck was defined in 1838 by Ward [16]. He described a trabecular network of which there were compression trabeculae medially along the femoral neck and tensile trabeculae laterally along the femoral neck. Secondary trabeculae are oriented throughout the rest of the proximal femur in accordance with Wolff’s law which states that living bone will react to mechanical loading and unloading of bone segment. In the case of repetitive loading, the bone will remodel overtime to become stronger (i.e., increased trabeculae in the femoral neck) to accommodate the increased load. The area of the femoral neck deficient in trabeculae is termed Ward’s triangle ([16, 17]; Fig. 1.3).
