Normal growth and development

Introduction

Growth patterns and development in children unique to the individual and therefore defining 'normal' has potential ramifications. Statistically, 'normal' is defined as '95% of a population that falls within two standard deviations of the mean from any given measurement. The challenge for the orthopedic surgeon caring for children is to understand which deviations from normal are likely to result in impairment of function, progressive deformity, or premature degenerative arthritis and pain.
Terminologies to describe deviations from normal are enumerated below:

Congenital anomalies can be categorized into:

• Production problems: These include abnormalities caused by malformation, dysplasia or disruption, that will not spontaneously resolve.
• Packaging problems: These include deformations caused by mechanical causes including in utero positioning and molding, and usually resolve with time. The effects of in utero positioning are physiologic in origin but may produce parental concerns. The child may be 3-4 yr old before the effects of the in utero position completely resolve.

Growth and Development

Consideration of growth and development differentiates pediatric from adult orthopedics, and helps to formulate treatment strategies designed to preserve or restore normal growth potential. Growth is not a constant feature and is subject to many variables including genetics, nutrition, general health, endocrine status, mechanical forces and physiological age. The application of forces to the growing children can improve or worsen deformities in children and is commonly explained by 'Wolff's law' and 'Heuter-Volkmann's law'. Growth also varies between two anatomic regions and even between two bones of the same region.
Bone formation or ossification occurs in two different ways:

• Endochondral ossification where mesenchymal cells condense and undergo chondrogenesis to form cartilage, which matures and hyoertrophies, gets calcified and is then replaced by bone. Most bones in the axial and appendicular skeleton are formed in this way.
• Intramembranous ossification where osteoblasts are formed by direct differentiation of mesenchymal cells with no cartilage precursor or model. Flat bones of the skull and clavicle are formed in this way.

Centers of ossification

• Primary centers of ossification: At the beginning of the fetal period the chondrocytes in the midshaft of long bones from the primary centers, growth from which eventually lengthens the bone.
• Secondary centers of ossification: These appear in the chondroepiphysis and mostly appear postnatally. They direct the formation of bone throughout growth.

The ossification centers that are typically present at birth are; diatal femur, proximal tibia, calcaneus and talus.

Typical long bone is divided into:

1. Physis, which is the growth plate located at the end of bone
2. Epiphysis, which is typically the secondary ossification center
3. Metaphysis, which is the bone adjacent to the physis on the side away from the joint
4. Diaphysis, which is the central part or shaft of long bones

The long bones of the extremities (humerus, radius-ulna, femur, and tibia-fibula) have growth plates or physes at each end. The ends of each long bone are composed of the epiphyses. These are covered by articular cartilage and form the associated joints. Epiphyses are almost entirely cartilaginous in the beginning and become progressively more ossified during growth. The articular cartilage also contributes to the growth of the epiphysis. The perichondrial ring, which surrounds the physes, as well as the perichondrium around the epiphyses and periosteum, which surrounds the metaphysis and diaphyseal regions of the bone, contributes to appositional or circumferential growth. Bones without physes, such as the pelvis, scapulae, carpals, and tarsals, grow by appositional bone growth from their surrounding perichondrium and periosteum. Other bones, such as the metacarpals, metatarsals, phalanges, and spine, grow by a combination of both appositional and endochondral ossification.

Important Growth and Developmental Milestones

The growth considerations that are important for clinicians from musculoskeletal standpoint are summarized below:

1. Abnormal stature can be assessed as 'proportionate' or 'disproportionate' based on comparing the ratio of sitting height with subischial height (lower limbs).
2. Normally the arm span is almost equal to standing height.
3. The head is disproportionately large at birth and ratio of head height to total height is approximately 1:4 at birth, which changes to 1:7.5 at skeletal maturity.
4. Lower extremities account for about 15% of height at birth and 30% at skeletal maturity.
5. The rate of height and growth increase is not constant and varies with growth spurts.
6. By age 5, birth height usually doubles and the child is approximately 60% of adult height. The child is about 80% of final height at 9 years. During puberty the standing height increases by approximately 1 cm per month.
7. Bone age is more important than chronological age in determining future growth potential.

Growth patterns in upper and lower extremities

The upper extremity grows longitudinally primarily from physes away from the elbow, with the proximal humeral physis and the distal radial and ulnar physes contributing a greater amount than the physes close to the elbow. This is opposite to the lower extremity growth pattern in which most of the longitudinal growth occurs around the knee, in the distal femoral and the proximal tibial physes.

In the hip joint, the acetabulum forms with the convergence of three primary ossification centers (ischium, ilium and the pubis), while in the proximal femur there is a confluence of the proximal femoral physis and the greater trochanteric physis in the first few years of life.

Maturation of Gait

Understandably the maturation of CNS contributes significantly to the development of gait. In the beginning of ambulation the child has a wide based gait with hyperflexion of hips and knees, no reciprocal arm swing and an initial contact with heel. By the age of two years, the wide gait diminishes, reciprocal arm swing begins and the initial contact is with the heel with increased step length and velocity. Adult kinematic patterns start developing by 3 years and the time-distance parameters reach adult values by 7 years, with development of a fully mature gait pattern.