REFERENCE: Rogers NL, Flournoy LE, McCormick WF. The rhomboid fossa of the clavicle as a sex and age estimator. J Foren- sic Sci 2000;45(1):61–67. ABSTRACT: The costoclavicular (rhomboid) ligament connects the first rib to the clavicle, stabilizing the pectoral girdle. It produces skeletal traits that may be tubercles, roughened impressions, shal- low groove-like fossae, deep fossae, or leave no trace. A pit or de- pression at this site is often called a “rhomboid fossa.” While these markings may appear pathological, they are normal variants of the clavicle. Using a large contemporary sample (N = 344: 113 fe- males, 231 males), we evaluated the presence of a rhomboid fossa as a sex and age indicator for unidentified skeletal remains. Logistic regression found significant relationships between the presence of a rhomboid fossa and sex and between presence of a rhomboid fossa and age. Fossae were more common in males (36% left, 31% right) than in females (3% left, 8% right). Posterior prob- abilities suggest that a fossa on the right clavicle is indicative of a male with 81.7% probability; a fossa on the left is indicative of a male with 92.2% probability. Younger individuals more commonly exhibited rhomboid fossae than older individuals, and the largest fossae were most common in males 20–30 years of age. However, the age effect was not conclusive and must be corroborated by other methods. A test of the sex estimation method on an independent sample (26 males, 23 females) found nine males and only one fe- male with fossae present on the left clavicle. When the costoclavicular attachment exhibits an impression, a tubercle, or leaves no trace, this method cannot be used for sex esti- mation. When a clavicle exhibits a rhomboid fossa, it is likely from a male. The greater difference in fossa expression between the sexes on the left clavicle makes use of the left bone preferable. This tech- nique can corroborate other sex estimates or provide an estimate for unknown individuals in the absence of other skeletal indicators. KEYWORDS: forensic science, sex estimation, age estimation, rhomboid fossa, costoclavicular ligament, rhomboid ligament, clav- icle, costal tuberosity, costal impression The costoclavicular ligament, also known as the rhomboid liga- ment because of its shape, connects the superior portion of the first rib to the inferior sternal end of the clavicle (Fig. 1). This ligament stabilizes the pectoral girdle and can produce skeletal landmarks on the inferior border of the clavicle including tubercles, impressions, all sizes and depths of grooves or fossae at the site of attachment, or may leave no mark. While White (1) describes landmarks at this site as costal impressions or costal tuberosities, depressed or pitted landmarks at this attachment site have been called “rhomboid fos- sae” in the anatomical, radiological, and anthropological literature throughout the 20th century (2–10). Inexperienced observers may speculate that these traits are pathological, but they are “normal variants” of the clavicle (11). Previous studies suggest that rhom- boid fossae are more common in males than females in populations of European ancestry and are most often clinically insignificant (2–10). In this study, we evaluated the relationship between rhom- boid fossae, age, and sex. Specifically, we were interested in using the presence of this trait for age and/or sex estimation of unidenti- fied individuals. Based upon the literature, the anatomy of the cos- toclavicular syndesmosis, and our personal experience with au- topsy cases, we hypothesized that fossae would be more common: 1) in males, and 2) in younger individuals. Several studies of the rhomboid fossa state that its presence in males is related to muscularity and mechanical usage. In an early study, Pendergrass and Hodes (3) state that fossae are present only in muscular males. However, other studies suggest that muscular mass is not necessarily implicated in rhomboid fossa etiology. For exam- ple, Shauffer and Collins (9) describe three male patients with rhom- boid fossae: two were of “average musculature” and the third was 11 years of age. While the link between the trait and mechanical stress has not been confirmed, many investigators operate under the as- sumption that these fossae are correlated with mechanical usage. For example, Stirland (12,13) describes rhomboid fossae as “lesions and bony buildup” at the costoclavicular syndesmosis and cites them as evidence of occupational stress. However, she cautions that effects due to sex, age, and asymmetry may confound use of such indicators to estimate occupational activity patterns from skeletal remains. Studies of the frequencies of rhomboid fossae have used both ra- diological and osteological observations. While Parsons (2) re- ported that 10% of the 286 excised clavicles studied exhibited rhomboid fossae, Shauffer and Collins (9) state that fossae were present in only 59 of 10,000 chest photoflourograms examined (0.59%). This discrepancy in results is likely due to methodology. The more detailed observation possible in studies of excised clavi- cles gives rise to questions concerning the definition of landmarks at this site. Traits at this site can take the form of a raised tubercle, a rough- ened impression, a shallow groove, a deep pit, or no trace at all. Some clavicles exhibit ambiguous traits that reduce the probability of replicable classification. Most studies of excised bones have classified all variants in the category of “rhomboid fossa.” Such skeletal observations are not comparable to radiographic studies because the latter can distinguish only deep pits at the site. Jit and Kaur’s (10) study of excised clavicles found that 59% of males and 54% of females in their North Indian sample exhibited some form of skeletal landmark at this site. However, many factors associated with sex and gender (hormone levels, activity patterns, etc.) likely cause different morphological changes. By classifying all skeletal traits exhibited at this site as “rhomboid fossae,” their 61 Nikki L. Rogers, 1 Ph.D.; Lori E. Flournoy, 1 M.A.; and William F. McCormick, 1 M.S., M.D. The Rhomboid Fossa of the Clavicle as a Sex and Age Estimator 1 Department of Anthropology, The University of Tennessee, Knoxville, TN. Received 12 Jan. 1999; and in revised form 19 April 1999; accepted 20 April 1999. Copyright © 2000 by ASTM International