One of the distinctions between a genetic man and a genetic woman is the characteristics of their skeleton. The skeleton obviously sets or heavily influences the body size and its proportions.
It's thought that genes (female "XX" or male "XY") mainly determine the basic size and shape of the skeleton, however in the male the high levels of testosterone at puberty help enlarge and ruggedized bones, while the high levels of progesterone help enhance and develop other male skeletal characteristics such greater height and narrower pelvic width. But overall, the differences between the skeletons of male and female bodies are actually surprisingly small compared with the similarities.
Skeletal Sex Differentiation
It is actually quite difficult to distinguish between male and female skeletons as there is a clear range of overlap between the sexes for many measures. Indeed, there is in fact no certain, 100% guaranteed, method of telling the sex of a skeleton from simple examination and inspection alone, as criminologists who have misidentified the sex of murder victims, and archaeologists who have been unable to determine from skeleton X-Rays whether Egyptian Pharaoh Tutankhamun was actually male or female, can confirm.
On average a male skeleton is larger than the female skeleton so this is a differentiation, but there is also a considerable overlap in skeletal size between the sexes - e.g. there are short men and tall women - so this is hardly a reliable method alone. Other significant differences between male and female skeletons are that female bones are usually lighter and thinner than more robust male bones; female head bones are smaller and more lightly built; and the female pelvis is shallower and wider than the male's. This latter difference makes childbirth easier. The pelvis is considered the best area to determine the sex of a skeleton and the skull is the second best area - an estimation of sex can often be determined through analysis of the skull (cranium and mandible) and post-cranial skeleton.
Skull and bone features vary from male to female and differentiation is usually based on the generalization that typically male features are more pronounced and marked than the female features.
By observing all the possible differentiating features of skeleton in a cumulative manner, it's possible to correctly identify the sex of a Caucasoid skeleton in about 90% of cases. [Krogman ranks accuracy of sex determination using the pelvis at 95%, followed by the skull at 92%, the mandible alone at 90%, and long bone measures at 80%. Stewart indicates slightly lower yields, however the order of accuracy was the same with the long bones again the least accurate.] Success rates are somewhat lower for Negroid and Mongoloid skeletons.
Note that it is often impossible to place absolute metric value on what constitutes a male feature, and what constitutes a female. Krogman addressed the difficulty of sex determination from skeletons when he states: "here is the problem of subjectivity versus objectivity, of description versus measurement, of �experience� versus statistical �standardization.� "
Skull
The adult female skull retains gracile attributes seen in the pre-pubescent skull, but the male cranium becomes markedly rougher in adulthood at the sites of muscle attachment.
Major differences between the female and male skill include the posterior of the cranium (the occipital), robustest of the brow-ridge, mastoid process, nuchal crest, temporal lines, and mandible. Although distinct, the ability to quantify measures of the skull for sex determination has met with limited success and successful sex identification based purely on a skull is a very subjective process based experience in identifying and assessing non-metric characteristics.
Above the orbits (eye sockets), the male cranium tends to have "blunt" superior margins and larger supraorbital (brow) ridges. The female cranium tends to have "sharp" superior margins of the orbits and no discernible supraorbital ridges.
The occipital of males tend to have a well-defined nuchal crest. In some cases, the nuchal crest and nuchal line are very rugged and sharp. In very gracile females, there is almost no nuchal crest and the nuchal line is completely absent.
The mandible of a female cranium tends to have a "pointed" chin. The area around the gonial angle is smooth and does not project. The male mandible tends to have a "square" shape and in extreme case the area around the gonial angle is "flared". The dentition (teeth) of males is frequently larger.
Finally, the area of the temporal in the female cranium is smoother and less rugged than that of the male cranium. In the female cranium the zygomatic arch normally does not extend, as a ridge, posterior of the external auditory meatus. In male crania the zygomatic arch typically extends, as a ridge, posterior to the external auditory meatus. In females the mastoid process is small and smooth. In males the mastoid process is large and rugged.
Face
Its worth relating the skull differences just described to the face. Unfortunately for the transsexual woman, the human eye and brain is amazingly able to distinguish between a "male" face and a "female" face on the basis of very minor differentiations and indications.
However, as ever, there is great variance - many handsome actors on close examination have some feminine facial characteristics, while many supermodels have some very male characteristics.
In absolute measures almost all dimensions of the female skull and face are smaller compared to the male features. The facial width is relatively larger in women than in men. Resulting contours are therefore more rounded in females, especially in the orbital area, with more prominent malar (cheek) bones and less prominent mandibular (chin/jaw) angles.
Forehead
In the upper part of the face, the forehead is quite different, most noticeably women have less sloping mid-foreheads and the position of their eyebrows is higher and has a stronger curvature.
Nose
In the middle part of the face, the angles of the nose differ substantially, especially at the tip portion. Females generally have a more pointed, narrow, and vertically shortened less nasal prominence than males.
Chin
In the lower part of the face the most dominant differences are found in the chin region, which varies markedly between the male and female. The male chin is larger in every dimension, the manible symphysis (upper chin) is generally wide and vertically high, while the female is more rounded, and the male mental eminence (point of the chin) tends to be square and the female more more pointed. The degree of perceived masculinity/femininity due to the chin can vary tremendously.
Pelvis
The pelvic girdle is formed by the sacrum, coccyx, and the two coxae. A coxa is formed by the fusion of three bones, the ilium, ischium, and pubis, which meet in the acetabulum or hip socket. At the back each coxa is attached by strong ligaments to the sacrum (base of spinal cord), and in front to each other at the pubic symphysis joint. This joint allows only slight bending movement, but it softens and becomes more flexible in a female giving birth. [Note: Other names for the pelvic bone are innominate bone and coxal bone.]
Factors contributing to the overall shape of the pelvis are constrained by both the demands of bipedal locomotion, as well as those for perpetuating the species. Of all the bones, the pelvis shows the greatest sexual differentiation, principally in relation to the requirements of childbirth. On average the male pelvis is much heavier and narrower than that of the female. In comparison the female pelvis is broad and shallow, the geometry is designed with a greater outlet for passage through its bony openings of a baby's head and shoulders during birth. The female pelvis is also less massive and more delicate and its muscular impressions are slightly marked.
In the female pelvis the ilia are less sloped, and the anterior iliac spines more widely separated; hence the greater lateral prominence of the hips. The pelvic inlet of females is larger and has a greater absolute circumference. The body of the pubis is longer, thereby increasing the size of the pelvic outlet. The size of the pelvis varies not only in the two sexes, but also in different members of the same sex, and does not appear to be influenced in any way by the height of the individual. Women of short stature, as a rule, have broad pelvises. The characteristic differences between the male and female pelvis are distinctly indicated as early as the fourth month of fetal life.
In the female, the superior ramus of the pubic bone is longer, increasing the pubic/ischium ratio. The greater sciatic notch is wider and forms a longer angle. The increased pubic length and laterally displaced ischia result in a wider subpubic angle. The growth and remodelling of the pubis produces extra bone at the symphysis, leaving a concave inferior ramus, a ventral arc that represents a previous border of the symphysis, and a narrow inferior pubic ramus. The female pubic symphysis is likely to be longer in its superior-inferior diameter and smaller in its dorsal-ventral diameters than is that of a male. Females are more likely to have a well-developed preauricular sulcus, and those who have borne children may have pits or guttering along the dorsal border of the pubic symphysis. Since they have smaller femurs, females have smaller acetabula, with males showing greater robusticity in this feature corresponding to the generally greater size of the male femur head with which it articulates.
The greater sciatic notch is an often-used preliminary tool for sex determination, though not the most accurate.
A triangular shaped pubis with a broad medial aspect and no evidence of a ventral arc is a characteristically male pattern. The female pattern for these features is a rectangular pubis, pronounced ventral arc, and sharp, narrow medial aspect of the ischiopubic ramus. According to Bass, the presence of a ventral arc is the most diagnostic of the female pubic features.
Sacrum
As a rule, the female sacrum is wider and flatter than that of the male, permitting a greater outlet for the birth canal. However, this is a rather subjective observation, and should only be used in conjunction with other techniques of sex determination.
Feminization of the Skeleton
As described there are several basic differences between male and a female facial skeleton, and also in the subcutaneous tissues (depth, type) and the skin quality - differences long appreciated by anthropologists studying skulls, and by artists as well. It is the combination of all these factors that differentiate the appearance of men and women. While hormones may help feminise subcutaneous tissue (redistribution, less muscle, more fat) they will not affect the skeleton of any individual already past puberty when treatment is commenced. However in some cases surgery can help feminise the the appearance of the underlying skeleton.
Pelvis
Although the greatest male and female skeleton differentiation can be found in the pelvic girdle, unfortunately there is nothing that can be done to directly feminise the pelvis of an adult male to female transsexual woman, however an increase in subcutaneous fat deposits due to hormone use will often feminise the hip contours. Although not recommended, this can be supplemented by silicone and other implants in the hips and buttocks.
Skull and Face
Relating the absolute measures of the male and female face, several surgically correctable differences which will make a dramatic difference in the perceived gender can often be identified to the benefit of a male-to-female transsexual.
[Note: An excellent article by Dr. D.K. Ousterhout on the differences between male and female skulls, and the feminization of the face of a transsexual woman can be found here.]
Forehead
Forehead re-contouring involves rounding the forehead to approximate a female shape, often by shaving bone from the brow of filling in flat areas to round them out. Reducing brow ridges, if excessive, can considerably improve the feminine appearance.
Common task include:
Scalp reduction: involves advancing forward the scalp between 1.0 cm - 2.5 cm to correct for the receding masculine pattern hairline and to approximate, when possible, a more feminine 'oval' pattern hairline.
SMAS/platysma (deep plane) rhytidectomy: involves lifting the cheeks, jowls, jawline, and neck and their underlying muscles.
Forehead contouring: involves shaving the brow bossing and, if necessary, contouring via shaving across the forehead.
Forehead augmentation: involves the use of 2, 3, or 4 implants (Gore-Tex Subcutaneous Augmentation Material).
Nasal Surgery (Rhinoplasty)
[Not strictly skeletal, but worth listing here!]
The nose is the most prominent feature of the face. Many transwomen find nasal refinement to make one of the most marked improvements in their female facial appearance and it's the most common procedure after breast augmentation.
Nasal surgery or rhinoplasty alters the appearance of the nose by shaping and repositioning the nasal bones and trimming the bone and cartilage as needed to create the desired appearance. As example, rhinoplasty is used to correct a large hump, or a broad or round nasal tip. Small changes, such as subtly lifting the tip of the nose will often create a more youthful and feminine appearance.
Commons task include:
Septal Surgery - The strip of cartilage which separates the two sides of the nose is called the septum. It is sometimes bent and interferes with breathing; if so, it may need to be straightened. If it is too long, it is often shortened.
Shaping the Profile - Many women complain of a hump on the bridge (dorsum) of the nose. This is usually made of both bone and cartilage. It is removed with scissors or a saw. When the nose is flat, the profile may be augmented with grafts of bone or cartilage.
Refining the Tip - If the tip of the nose is large or boxy, much of the tip cartilage may be removed or reshaped to refine the shape.
Narrowing the Nasal Bones - If the bridge of the nose is wide, the surgeon will narrow it by fracturing the bone on each side and moving it closer to the centre.
Reducing Nostril Flare - If the nostrils are flared, they may be reduced by removing a small wedge of tissue from the base of the nose.
Cheeks
Cheek implants, also called submalar implants, can make the face look less flat, which can enhance its feminine appearance. Like any implant, there is a danger of rejection.
Common tasks include:
Cheekbone reduction: involves an intra-oral incision which requires an extended recovery period (with severe facial swelling/bruising for about 10 days).
Cheekbone augmentation: involves an intra-oral incision to augmenting the bone with a hard silicone implant.
Lip contouring: involves lifting the upper lip via a shallow 'v' incision below the nose or by an incision inside the lip. If necessary, the lower lip can be made fuller via an incision inside the lip.
Chin and Jaw
The chin is an extremely important area in gender recognition. Although the potential benefits can be great, the difficulties involved in re-shaping a "robust" lower jaw sufficiently to make a significant improvement in overall facial feminizing prevents this being a very common surgery.
Possible tasks include:
Jaw reduction: involves a 1.0cm extra-oral incision under the jaw line to cut the bone (on average, by 1.0cm depth) and thus reduce the sharp angle of the back corner of the jaw where it rises up to the ear.
Alternatively, an intra-oral incision can shave (but not cut) the bone. Although this technique prevents external scaring, it is less effective, has more risk of infections, and requires an extended recovery period due to severe facial swelling/bruising.
Chin reduction: involves an intra-oral incision to shorten and narrow the chin (if necessary, including contouring with a hard silicone implant). The technique involves: (1) shaving the bone; (2) cutting the bone; or (3) sectioning out the mid-section of chin and joining the upper and lower sections via steel screws.
Sliding genioplasty: involves an intra-oral incision and sliding forward or backward the lower section of chin (if necessary, including contouring with a hard silicone implant).
Chin augmentation: involves an intra-oral incision to advance a short or/and receding chin with a hard silicone implant.
Neck
A trachea shave is a common procedure undertaken in order to reduce the woman's "Adams's Apple".
Other
Rib Removal
This is fairly rare procedure undertaken to improve the figure. One of two of the floating ribs at the bottom of the rib cage are taken out to have a smaller waist, typically by 1 - 2 inches. Corsets also become far more effective.
The resulting back scars, the need to protect internal organs, and the possibility for rib regrowth make this a problematic procedure.
Long Bone Shortening
It is possible to shorten the bones of the legs (most commonly the femur) and arms by resection in order to reduce physical size and height. This procedure is considered dangerous and is very rarely performed on transsexual women. Excessive reduction of limb length will destroy the proportions of the body.
