Kroeber Anthropological Society Papers, Nos. 71-72, 1990

Humeral Morphology of AchondroplasiaRina Malonzo and Jeannine Ross

Unique humeral morphologicalfeatures of two prehistoric achondroplastic adult individuals are des-cribed. Thesefeatures are compared to the humerus ofa prehistoric non-achondroplastic dwarfand tothe humeri ofa 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 createdby the authors. These guidelines will be useful as an aid in diagnosing achondroplastic individualsfromthe archaeological record.

INTRODUCTION

For several decades dwarfism has been aprominent topic within the study of paleopathol-ogy. It has been represented directly by skeletalevidence and indirectly by artistic representationin the archaeological record (Hoffman andBrunker 1976). Several prehistoric Egyptian andNative American dwarfed skeletons have beenrecorded, indicating that this pathology is notlinked solely with modem society (Brothwell andSandison 1967; Hoffman and Brunker 1976;Niswander et al. 1975; Snow 1943). Artifactssuch as paintings, tomb illustrations and statuesof dwarfed individuals have been discovered invarious parts of the world. However, interpreta-tions of such artifacts are speculative, for it isnecessary to allow artistic license for individualis-tic expression and interpretation. Paintings andsculptures may represent children, for instance.Thus, dwarfism in the prehistoric archaeologicalrecord can only be definitively diagnosed throughthe study of skeletal remains.

Among the several forms of dwarfism,achondroplasia is the most common, both inpresent and prehistoric times (Brothwell and San-dison 1967; Roberts 1988). Achondroplasia isa specific type of dwarfism classified as rhizome-lic micromelia (proximal limb segments are moreeffected than distal segments). Although all ofthe limbs are shortened, the humerus and femurare more effected than the distal limb segments.This is due to the fact that in normal individualsthe distal limbs grow to a lesser degree and,therefore, appear to be less involved (Rubin1964). The mechanical effects of normal bodymass on shortened limbs leads to a distinct hu-meral morphology.

The tern achondroplasia has often been mis-used as a general diagnosis for dwarfism (Hall1986). Some dwarfed specimens have temporar-ily been diagnosed as achondroplastic, such as inthe Hrdlicka Paleopathology Collection in the SanDiego Museum of Man (specimens 1915-2-382

and 1915-2-463) (Merbs 1980). The followingpaper describes a set of humeral morphologicalcharacteristics which can be used as a guide toidentifying achondroplastic individuals from thearchaeological record.

MATERIALS AND METHODS

A comparative population sample, housed bythe Lowie Museum of Anthropology (LMA) atthe University of California at Berkeley, wasderived from a random sample forming a total ofsixty adult individuals (thirty males and thirtyfemales) from six different prehistoric ar-chaeological sites within California. Twoachondroplastic adult individuals from similarcontexts, specimen number 6670 (spc. 6670) andspecimen number 9199 (spc. 9199), were ana-lyzed. Spc. 6670 is a complete female skeletonfrom the Augustine site in Sacramento, California(CA-Sac-127). Spc. 9199 is a left humerusrecovered from the Tank site (also called the To-panga or Topanga Canyon site) in Los Angeles,California (CA-LAn-1). A non-achondroplasticdwarf, specimen number 3854 (spc. 3854), wasincluded for comparison in the analysisl (Figure1). Spc. 3854 is from the Mosher Mound site inSacramento, California (CA-Sac-?). All the indi-viduals are housed by LMA.

Measurements were taken in millimetersusing a digital caliper and an osteometic board.Sexing of the individuals was determined byanalysis of the pelvis and/or the cranium follow-ing the methods outlined by Phenice (1969) andBass (1987).

DESCRIPTION

The two achondroplastic specimens shareseveral unique features. They both exhibit shor-tening in the proximal and distal limbs, and bothhumeral heads exhibit wasting under the inferior

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border, resulting in distinct mushroom shapednecks. Their humeral lengths range within 155-160 mm, and their deltoid tuberosities are largeand rectangular in shape. A tubercle is present onthe inferior border of the crest of the lesser tuber-osity, at the insertion of teres major muscle(Figure 1). This tubercle is only faintly presentin the non-achondroplastic specimen and is ab-sent in the normal humerus (Figure 2).

In both achondroplastic specimens thegreater and lesser tuberosities are prominent,separated by a broad, shallow intertuberculargroove. The shafts are bowed, presenting a me-dial concavity in anterior view. The epicondylarbreadths of the humeri fall within the normalrange (Table 1). A groove is present in the mid-line of the trochlea, located postero-inferiorly.The olecranon fossa is deep and well-developed;however it is blocked by the medial supra-condylar ridge and possesses a narrow, ovalopening

in achondroplastic spc. 6670, humeroradioindices lie within the average of the comparative

Figure I. Achondroplastic skeletal remains, fromhumerus; spc. 9199 left humerus.

samnple (Table 2). The humeroulnar indices rangefrom 95 to 98. The maximum length of the righthumerus of spc. 6670 (158 mm) is nearly equalto the maximum length of the right ulna (155mnm).

Non-achondroplastic spc. 3854 shares somesimilar morphological features with the achondro-plastic individuals. There is a lack of full elbowextension, and, although the proximal limbs areshorter than the distal limbs, the maximum lengthof the distal limbs falls within the normal range(Table 1). Since the distal limb length exceedsthe humeral limb length (Table 2), the humero-radio and humeroulnar indices are greater than1(0.

Another similarity spc. 3854 shares with theachondroplastic individuals is that the shafts ofthe limbs are gracile, particularly on the posteriorsurface of the semilunar notch of the ulnae andthe superior shaft of the radii. Furthermore, ar-thritis is present in all the limbs. Eburnation isevident on the posterior portion of the left hu-meral head, while the right humeral head is nearly

left to right: spc. 6670 right humerus; spc. 6670 left

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obliterated by arthritis. Arthritis is also present inthe distal metaphysis of the right humerus and onthe heads of the distal limbs, especially on theradial heads.

DISCUSSION

Dwarfism is broadly defined as the underde-velopment of the body. There are several typesand classifications of dwarfism (Roberts 1988).Classifications are based on the parts of the bodywhich are effected and the types of shorteningwhich occur, such as proportional versus dis-proportional shortening. The most common formof dwarfism is achondroplasia (Roberts 1988).Achondroplasia is a congenital (autosomal domi-nant) hereditary disturbance in which cartilagegrowth rate slows while periostial activity re-mains normal (Rimoin et al. 1970). The result ofthe decreased growth rate is a quantitative shor-tening of the humerus (since, in the normalcondition, the humerus grows at a greater rate

than the distal limbs). The humerus, therefore,appears to be more effected than the distal limbs.

As mentioned earlier, a major distinguishingcharacteristic of achondroplasia is rhizomelic mi-cromelia. Spc. 6670 reflects this morphology,especially when comparing the humeri with theulnae. The humeroulnar indices range around100, indicating that the limbs are of nearly equallength (Table 2). The right humerus exhibits ex-treme rhizomelic micromelia in that it is only 3mm longer than the right ulna.

The humeri of achondroplasts present sev-eral distinct peculiarities in their morphology.The humeral heads are mushroomed shapedwhich reflects the developmental pattern of thecartilage growth plate. When the cartilage diskgrows appositionally (at the periphery of thedisk) instead of interstitially (at the center of thedisk), the result is that the humeral head growslaterally (increasing in diameter) instead of supe-riorly (increasing in height), thereby widening thebase of the humeral head (Rubin 1964). Thiscauses the proximal portion of the humeral head

Figure 2. From left to right: normal human humerus; non-achondroplastic humerus (spc. 3854); achon-droplastic humerus (spc. 6670); achondroplastic humerus (spc. 9199).

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to grow beyond the width of the anatomical neck,producing a lip over the anatomical neck andgiving it the mushroom appearance. The alteredhumeral head morphology does not disturb nor-mal shoulder function (Bailey 1971).

Another peculiarity of achondroplastic mor-phology is that the humeri have often beendescribed as possessing large rectangular shafts.However, the diaphyses remain normal in widthand thickness (Rubin 1964). What gives theachondroplastic humerus its stocky, rectangularappearance is the large, rectangular deltoid tuber-osity. When comparing the deltoid tuberosity ofthe achondroplastic dwarf to the non-achondro-plastic dwarf and to normal humeri, it is apparentthat, in general, the shorter the limb length thegreater the size of the deltoid tuberosity (Figure2). This is expressed by the width of the deltoidtuberosity (Table 1).2 Since muscle developmentamong achondroplasts is not effected, muscles

grow to their normal size and attach to the smallerbones. The effects of muscles on bones variesproportionally with changing limb lengths. Atubercle usually develops or grows larger atpoints where there is a greater amount of local-ized strain. This increases surface area which, inturn, decreases the concentration of stress orstrain within the localized area (Hildebrand1988). Thus, the appearance of additional orlarger tubercles on the achondroplastic humerusthan on nonral humeri may be due to an increasein localized strain, resulting from a larger musclemass acting on a smaller area of bone.

Achondroplastic humeri are also distinct inthat trabeculae are present throughout the shaft(Figure 3). Generally, trabeculae are located atthe ends of long bones and are not present pastthe 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 SAMPLETotal Average 322Male Average 320Female Average 303

Total Standard Deviation 18.8Male Standard Deviation 18.Female Standard Deviation 153

NON-ACHONDROPLASTICSPECIMEN #3854Left 228Right 212

ACHONDROPLASTICINDIVIDUALS

SPECIMEN #6670Left 154Right 158

SPECIMEN #9199Left 160

ULNA.. Max. LengthDeltoid Tubr.

Width

22I

269270252

17.014.914.1

250248

147155

191919

62

1210

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

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contact with bone marrow and vascular tissues Spc. 3854 is not classified as an achon-(Marin and Burr 1989). Trabeculae may also be droplastic dwarf for several reasons, the primaryindicative of the amount and the direction of reason being its distal limb length. Spc. 3854'sstress placed on the skeleton (Hildebrand 1988). humerus is shorter than its distal limbs and ap-Construction of excess trabeculae allows the pears to be the only foreshortened limb, while itsenergy or strain to be maximally absorbed in or- ulna and radius fall within the average range ofder to avoid skeletal fatigue or breakage of the maximum distal limb length (Table 1) and are notbone (Martin and Burr 1989). Thus, the appear- shortened. In other words, spc. 3854 does notance of trabeculae throughout the shaft probably exhibit rhizomelic micromelia and this is one rea-reflects the greater amount of total strain placed son for not classifying it as achondroplastic.on the achondroplastic humerus. Another reason for not classifying spc. 3854

Achondroplastic humeri have often been as achondroplastic is that arthritis is present indescribed as possessing flared metaphyses. most of its metaphyses. The left humeral head isHowever, the epicondylar breadths fall within the eburnated and the right humeral head is nearlynormal range (Table 1). Due to the shortened obliterated by arthritis, disrupting normal shoul-maximum limb length, the metaphyses appear to der function. In contrast, shoulder function inbe flared when in fact their maximum epicondylar achondroplastics, generally, is uneffected, al-breadths fall within the normal range. Further- though the chance of arthritic disruption increasesmore, the medial supracondylar ridge is postero- with age (Hall 1986). The dysfunction of theinferiorly constricted which accentuates the flared shoulder in spc. 3854 led to atrophy of the shoul-appearance of the metaphyses. The downward der (deltoid) muscle. This is reflected by theconstriction of the medial supracondylar ridge small deltoid tuberosity on the right humerus.blocks the opening of the olecranon fossa. This A final reason for classifying spc. 3854 asprevents the olecranon process of the ulna from non-achondroplastic is that the superior shaft offully entering the olecranon fossa, thus causing the radii is gracile. As stated earlier, the widththe disruption of full elbow extension. In addi- and cortical thickness of achondroplastic limbston, a groove is present in the midline of the are uneffected.tochlea. The trochlear notch of the ulna is divi- In sum, the overall morphology of spc. 3854ded by a median wedge-like projection which and the occurrence of arthritis in both the left andarficulates with and rides on the trochlear groove right limbs indicate that spc. 3854 is not achon-of the humerus. droplastic and that its arthritis is not due to

Table 2. Humeroradio and humeroulnar indices.

Humeroradio Index # Humeroulnar Index #

SAMPLE POPULATIONAVERAGE 77 84

SPECIMEN #3854Left 1W110Right 03109SPECIMEN #6670Left ....95Right 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. Humeroradio index is the product of radius maximum length times onehundred, 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 themaximum length of the radius and the maximum length of the humerus. The greater the number, the longer the distal limbin comparison to the humerus, where a score of one hundred means that the limbs are of equal length.

is of achondroplastic skeletal remaiis; sp. 9199 left humerus.

n left to right: spe. 6670 right hureru

defect other than be used to diagnose achondroplasia in the archaolo6ical record.

ACKNOWLEDGMENTS

Achondroplastic huneri possess distinctmorphological features. The mrajor classificationof achondroplasia is rhizomelic mricromelia inrhich all of the limbs are effected and shortened,rhe achondroplastic hurerus possesses a mush-oor shaped head, a large, rectangular deltoidtubercle, and a tubercle on the crest of the lessertuberosity. Generally, there is an inverse rela-tionship between lirb length and the width of thedeltoid tuberosity, Trabeculae are found through-out the shaft. The distal end is flared and agroove is present in the nidline of the trochlea.Blockage of the olecranon fossa by the medialsupracondylar ridge limits full elbow extension,rhese traits are generally found among achondro-plastic individualsand are guidelines which can

We would like to give special thanks to GaryRichards and Susan Anton for their inspirationguidance, patience and invaluable comments.Also to Dr. F. Clark Howell of the AnthropologyDepartment, University of California at Berkeleyfor his faculty sponsorship, to the Lowie Mu-seum for allowing the authors access to theskeletal raterial, to Jill Duke of Cowell Hospitalfor the radiographs and to Christian Lynch fortaking and producing the photographs.

NOTES

A case study of specimen number 3854 wasdone concurrently with this paper by Ross and

MARY

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Malonzo.2 It is important to note that the deltoid tuberositywidth measurement is extremely variable. This isdue to the positive correlation between the mid-shaft width and the deltoid tuberosity width: thewider the midshaft, the larger the deltoid tuberos-ity. Hence, the deltoid tuberosity width is cor-related with two variables (midshaft width andmaximum limb length), which results in its highvariability.

REFERENCES CITED

Bailey, Joseph A. (1971) Elbow and otherupper limb deformities in achondroplasia.Clinical Orthopaedics and Related Research80:75-78.

Bass, William (1987) Human Osteology, ALaboratory and Field Manual, 3rd edition.Columbia, Missouri: Missouri ArchaeologicalSociety.

Brothwell, Don and A.T Sandison (1967) Dis-eases in Antiquity. Illinois: BannerstoneHouse.

Hall, J.G. (1986) Human achondroplasia: Amultidisciplinary approach. Basic Life Scien-ces 48:4.

Hildebrand, Milton (1988) Analysis of Verte-brate Structure, 3rd edition. New York: JohnWiley and Sons, Inc.

Hoffman, J. Michael and Lynda Brunker (1976)An achondroplastic dwarf from the Augustine

site (CA-Sac-127). Contributions of the Uni-versity ofCalifornia Archaeological ResearchFacility, Studies in California Paleopathology30:67-76.

Martin, Bruce R. and David B. Burr (1989)Structure, Function, and Adaptation of Com-pact Bone. New York: Raven Press.

Merbs, Charles (1980) Pathologies from theHrdlicka Collection. San Diego, CaliforniaMuseum ofMan.

Niswander, Jerry, Mark V. Barrow and Glenn J.Bingle (1975) Congenital malformations inthe American Indian. Social Biology 22:203-215.

Phenice, T.W. (1969) A newly developedvisual method of sexing in the os pubis.American Journal of Physical Anthropology30:279-301.

Rimoin, David L., Gary N. Hughes, Robert L.Kaufman, Ronald E. Rosenthal, William H.McAlister and Ruth Silber (1970) Endochon-dral ossification in achondroplastic dwarfism.The New England Journal of Medicine 283:728-735.

Roberts, Charlotte A. (1988) A rare case ofdwarfism from the Roman period. Journal ofPaleopathology 2:9-21.

Rubin, Philip (1964) Dynamic Classification ofBone Dysplasias. Chicago: Year Book Medi-cal Publishers.

Snow, Charles E. (1943) Two Prehistoric In-dian Dwarf Skeletons from Moundville.Alabama State Geologist Museum, paper 21.

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