Kroeber Anthropological Society Papers, Nos. 71-72, 1990
Humeral Morphology of Achondroplasia
Rina Malonzo and Jeannine Ross
Unique humeral morphologicalfeatures of two prehistoric achondroplastic adult individuals are des-
cribed. These features are compared to the humerus of a prehistoric non-achondroplastic dwarf and to
the humeri of a normal human population sample. A set of unique, derived achondroplastic characteris-
tics is presented. The non-achondroplastic individual is diagnosed as such based on guidelines created
by the authors. These guidelines will be useful as an aid in diagnosing achondroplastic individuals from
the archaeological record.
INTRODUCTION
For several decades dwarfism has been a
prominent topic within the study of paleopathol-
ogy. It has been represented directly by skeletal
evidence and indirectly by artistic representation
in the archaeological record (Hoffman and
Brunker 1976). Several prehistoric Egyptian and
Native American dwarfed skeletons have been
recorded, indicating that this pathology is not
linked solely with modem society (Brothwell and
Sandison 1967; Hoffman and Brunker 1976;
Niswander et al. 1975; Snow 1943). Artifacts
such as paintings, tomb illustrations and statues
of dwarfed individuals have been discovered in
various parts of the world. However, interpreta-
tions of such artifacts are speculative, for it is
necessary to allow artistic license for individualis-
tic expression and interpretation. Paintings and
sculptures may represent children, for instance.
Thus, dwarfism in the prehistoric archaeological
record can only be definitively diagnosed through
the study of skeletal remains.
Among the several forms of dwarfism,
achondroplasia is the most common, both in
present and prehistoric times (Brothwell and San-
dison 1967; Roberts 1988). Achondroplasia is
a specific type of dwarfism classified as rhizome-
lic micromelia (proximal limb segments are more
effected than distal segments). Although all of
the limbs are shortened, the humerus and femur
are more effected than the distal limb segments.
This is due to the fact that in normal individuals
the distal limbs grow to a lesser degree and,
therefore, appear to be less involved (Rubin
1964). The mechanical effects of normal body
mass on shortened limbs leads to a distinct hu-
meral morphology.
The tern achondroplasia has often been mis-
used as a general diagnosis for dwarfism (Hall
1986). Some dwarfed specimens have temporar-
ily been diagnosed as achondroplastic, such as in
the Hrdlicka Paleopathology Collection in the San
Diego Museum of Man (specimens 1915-2-382
and 1915-2-463) (Merbs 1980). The following
paper describes a set of humeral morphological
characteristics which can be used as a guide to
identifying achondroplastic individuals from the
archaeological record.
MATERIALS AND METHODS
A comparative population sample, housed by
the Lowie Museum of Anthropology (LMA) at
the University of California at Berkeley, was
derived from a random sample forming a total of
sixty adult individuals (thirty males and thirty
females) from six different prehistoric ar-
chaeological sites within California. Two
achondroplastic adult individuals from similar
contexts, specimen number 6670 (spc. 6670) and
specimen number 9199 (spc. 9199), were ana-
lyzed. Spc. 6670 is a complete female skeleton
from the Augustine site in Sacramento, California
(CA-Sac-127). Spc. 9199 is a left humerus
recovered from the Tank site (also called the To-
panga or Topanga Canyon site) in Los Angeles,
California (CA-LAn-1). A non-achondroplastic
dwarf, specimen number 3854 (spc. 3854), was
included for comparison in the analysisl (Figure
1). Spc. 3854 is from the Mosher Mound site in
Sacramento, California (CA-Sac-?). All the indi-
viduals are housed by LMA.
Measurements were taken in millimeters
using a digital caliper and an osteometic board.
Sexing of the individuals was determined by
analysis of the pelvis and/or the cranium follow-
ing the methods outlined by Phenice (1969) and
Bass (1987).
DESCRIPTION
The two achondroplastic specimens share
several unique features. They both exhibit shor-
tening in the proximal and distal limbs, and both
humeral heads exhibit wasting under the inferior
sulting in distinct mushroom shaped
ieir humeral lengths range within 155-
and their deltoid tuberosities are large
gular in shape. A tubercle is present on
r border of the crest of the lesser tuber-
the insertion of teres major muscle
. This tubercle is only faintly present
-achondroplastic specimen and is ab-
normal hunmerus (Figure 2).
)th achondroplastic specimens the
id lesser tuberosities are prominent
by a broad, shallow intertubercular
he shafts are bowed, presenting a me-
ivty in anterior view. The epicondylar
If the hurni fall within the nonmal
)le 1). A groove is present in the mid-
troFhlea, located postero-inferiorly.
inon fossa is deep and well-developed
it is blocked by the m edial supra-
ridge and possesses a narrow, oval
In achondroplastic spe. 6670, humeroradio
ces lie within the average of the eomparative
sample (Table 2). The humeroulnar mdices range
from 95 to 98. The maximum length of the nght
huTnerus of spe. 6670 (158 mm) is nearly equal
to the naximum length of the right ulna (155
rnn).
Non-achondroplastic spc 3854 shares some
siilar morphological features with the achondro-
plastic individuals. There is a lack of full elbow
extension, and, although the proximal limbs are
shorter than the distal limbs, the maximum length
of the distal limbs falls within the normal range
(Table 1). Since the distal limb length exceeds
the humeral limb length (Table 2) the humero-
radio and humeroulnar indices are greater than
00
Another similarity spc. 3854 shares with the
achondroplastic individuals is that the shafts of
the limbs are gracile, particularly on the posterior
surface of the senilunar notch of the ulnae and
the superior shaft of the radii, Furthermore, ar-
thritis is present in all the limbsh Ebuhation is
evident on the posterior portion of the left hu-
oeral head, while the right huineral head is nearly
Figure I. Achondroplastic skeletal remains, from left to right: spc 6670 right hurmeruso spe. 6670 left
hurerus; spc. 9199 left htumerus.
border, re
F or
necks. Ttn
1e60 mt
and rectan
the inferio
osity, at
(Figure 1)
ill the nor
sent in the
era
a n T
greater at
separated
groove. IF
dial conca
breadths (
range (Tat
The olecra
however,
condylar
and
wh:
prc
onf.
Ac,1
44
to grow beyond the width of the anatomical neck,
producing a lip over the anatomical neck and
giving it the mushroom appearance. The altered
humeral head morphology does not disturb nor-
mal shoulder function (Bailey 1971).
Another peculiarity of achondroplastic mor-
phology is that the humeri have often been
described as possessing large rectangular shafts.
However, the diaphyses remain normal in width
and thickness (Rubin 1964). What gives the
achondroplastic humerus its stocky, rectangular
appearance is the large, rectangular deltoid tuber-
osity. When comparing the deltoid tuberosity of
the achondroplastic dwarf to the non-achondro-
plastic dwarf and to normal humeri, it is apparent
that, in general, the shorter the limb length the
greater the size of the deltoid tuberosity (Figure
2). This is expressed by the width of the deltoid
tuberosity (Table 1).2 Since muscle development
among achondroplasts is not effected, muscles
grow to their normal size and attach to the smaller
bones. The effects of muscles on bones varies
proportionally with changing limb lengths. A
tubercle usually develops or grows larger at
points where there is a greater amount of local-
ized strain. This increases surface area which, in
turn, decreases the concentration of stress or
strain within the localized area (Hildebrand
1988). Thus, the appearance of additional or
larger tubercles on the achondroplastic humerus
than on nonral humeri may be due to an increase
in localized strain, resulting from a larger muscle
mass acting on a smaller area of bone.
Achondroplastic humeri are also distinct in
that trabeculae are present throughout the shaft
(Figure 3). Generally, trabeculae are located at
the ends of long bones and are not present past
the surgical neck in normal adult humeri; that is,
trabeculae are not found within the humeral shaft.
Their function is to increase the surface area for
Table 1. Measurements of the comparative sample and the dwarfed specimens.
Max. Length
COMPARATIVE SAMPLE
Total Average                  322
Male Average                   320
Female Average                 303
Total Standard Deviation       18.8
Male Standard Deviation        18.
Female Standard Deviation      153
NON-ACHONDROPLASTIC
SPECIMEN #3854
Left                           228
Right                          212
ACHONDROPLASTIC
INDIVIDUALS
SPECIMEN #6670
Left                           154
Right                          158
SPECIMEN #9199
Left                           160
ULNA
.. Max. Length
Deltoid Tubr.
Width
2
2
I
269
270
252
17.0
14.9
14.1
250
248
147
155
19
19
19
6
2
12
10
10
The epicondylar breadth measurement is the maximum distance between the medial and lateral epicondyle of the humerus.
The deltoid tuberosity width measurement is the width of the midshaft subtrated from the maximum medio-lateral measure-
ment of the shaft with the deltoid tuberosity (from the lateral edge of the deltoid tuberosity to the medial edge of the shaft).
The maximum lengths of the radii and ulnae were measured from head to styloid process.
I                                              I
45
contact with bone marrow and vascular tissues
(Marin and Burr 1989). Trabeculae may also be
indicative of the amount and the direction of
stress placed on the skeleton (Hildebrand 1988).
Construction of excess trabeculae allows the
energy or strain to be maximally absorbed in or-
der to avoid skeletal fatigue or breakage of the
bone (Martin and Burr 1989). Thus, the appear-
ance of trabeculae throughout the shaft probably
reflects the greater amount of total strain placed
on the achondroplastic humerus.
Achondroplastic humeri have often been
described as possessing flared metaphyses.
However, the epicondylar breadths fall within the
normal range (Table 1). Due to the shortened
maximum limb length, the metaphyses appear to
be flared when in fact their maximum epicondylar
breadths fall within the normal range. Further-
more, the medial supracondylar ridge is postero-
inferiorly constricted which accentuates the flared
appearance of the metaphyses. The downward
constriction of the medial supracondylar ridge
blocks the opening of the olecranon fossa. This
prevents the olecranon process of the ulna from
fully entering the olecranon fossa, thus causing
the disruption of full elbow extension. In addi-
ton, a groove is present in the midline of the
tochlea. The trochlear notch of the ulna is divi-
ded by a median wedge-like projection which
arficulates with and rides on the trochlear groove
of the humerus.
Spc. 3854 is not classified as an achon-
droplastic dwarf for several reasons, the primary
reason being its distal limb length. Spc. 3854's
humerus is shorter than its distal limbs and ap-
pears to be the only foreshortened limb, while its
ulna and radius fall within the average range of
maximum distal limb length (Table 1) and are not
shortened. In other words, spc. 3854 does not
exhibit rhizomelic micromelia and this is one rea-
son for not classifying it as achondroplastic.
Another reason for not classifying spc. 3854
as achondroplastic is that arthritis is present in
most of its metaphyses. The left humeral head is
eburnated and the right humeral head is nearly
obliterated by arthritis, disrupting normal shoul-
der function. In contrast, shoulder function in
achondroplastics, generally, is uneffected, al-
though the chance of arthritic disruption increases
with age (Hall 1986). The dysfunction of the
shoulder in spc. 3854 led to atrophy of the shoul-
der (deltoid) muscle. This is reflected by the
small deltoid tuberosity on the right humerus.
A final reason for classifying spc. 3854 as
non-achondroplastic is that the superior shaft of
the radii is gracile. As stated earlier, the width
and cortical thickness of achondroplastic limbs
are uneffected.
In sum, the overall morphology of spc. 3854
and the occurrence of arthritis in both the left and
right limbs indicate that spc. 3854 is not achon-
droplastic and that its arthritis is not due to
Table 2. Humeroradio and humeroulnar indices.
Humemradin Tndex #
SAMPLE POPULATION
AVERAGE
SPECIMEN #3854
Left
Right
SPECIMEN #6670
Left
Right
|Humeroulnar Index#
m~~~8
, ' '11
:::::10
95
101
The average of the sample population's humeroradio indices and humeroulnar indices is calculated. In addition, the indivi-
diual indices of spc. 3854 and spc. 6670 are listed. Hurneroradio index is the product of radius maximum length times one
hundred, divided by humerus maximum length. Humeroulnar index is the product of ulna maximum length times one hun-
dred, divided by humerus maximum length. The humeroradio index reflects the proportional relationship between the
maximum length of the radius and the maximum length of the humerus. The greater the number, the longer the distal limb
in comparison to the humerus, where a score of one hundred means that the limbs are of equal length.
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47
Malonzo.
2 It is important to note that the deltoid tuberosity
width measurement is extremely variable. This is
due to the positive correlation between the mid-
shaft width and the deltoid tuberosity width: the
wider the midshaft, the larger the deltoid tuberos-
ity. Hence, the deltoid tuberosity width is cor-
related with two variables (midshaft width and
maximum limb length), which results in its high
variability.
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48